Nuclear Instruments and Methods in Physics Research A325 (1993) 92-108North-Holland

Calibration and performance of the CHARM-II detectorCHARM-II CollaborationD. Geiregat ', P. Vilain 2 and G. Wilquet 2

Inter-University Institute for High Energies (ULB-VUB), Brussels, Belgium

U. Binder 3, H. Burkard, W. Flegel, H . Grote, T. Mouthuy 4, H. Overâs, J. Panman,A. Rozanov 5, K. Winter, G. Zacek and V. ZacekCERN Genet a, Switzerland

R. Beyer ", F.W. Büsser, C. Foos, L. Gerland, T. Layda, F. Niebergall, G. Rädel', P. Stähelin,A. Tadsen ' and T. VossII. Institut für Experimentalphysik *, Umrersttàt Hamburg, Hamburg, Germany

D. Favart, G. Grégoire, E. Knoops ' and V. LemaitreUniversité Catholique de Loui ain, Louvain-la-Neuue, Belgium

P. Gorbunov, E . Grigoriev, V. Khovansky and A. MaslennikovInstitute for Theoretical and Experimental Physics, Moscow, Russian Federation

W. Lippich, A. Nathaniel, H. Neumeyer, A. Staude and J. VogtSekttort Physik * der Unicersitat Munchen, Munich, Germany

M. Caria', C. Cicalô 9, B . Eckart l° , A.G . Cocco, A. Ereditato, G . Fiorillo, S . Mennella,V. Palladino, P. Paolucci and P. StrolinUnit ersita e Isiituto Nazionale dt Fisica Nucleare (INFN), Naples, Italy

A. Capone, D. De Pedis, E. Di Capua ' 1 , U. Dore, A. Frenkel-Rambaldi, P.F . Loverre,D . Macina, G. Piredda, R. Santacesaria and D. ZanelloUniuersity 'La Sapienza' e Istituto Nazionale di Fisica Nucleare (INFN), Rome, Italy

Received 15 September 1992

The neutrino detector CHARM-11 at CERN, consisting of a large target calorimeter equipped with streamer tubes and a muonspectrometer, has been exposed to a test beam of electrons, pions and muons with momenta between 2 and 60 GeV/c in order tostudy the digital response of the streamer tubes, the analog response of the pickup strips and the momentum resolution of the

i Inter-University Institute for Nuclear Science, Belgium .2 National Foundation for Scientific Research . Belgium .Now at University of Freiburg, Germany .Now at Centre de Physique des Particules, Faculté deLuminy, France .

5 On leave of absence from ITEP, Moscow, Russian Federa-tion

a Now at CERN, Geneva, Switzerland .

0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V All rights reserved

NUCLEARINSTRUMENTS& METHODSIN PHYSICSRESEARCH

SectionA

7 Now at DESY, Hamburg, Germany .s Now at University of Neuchatel, Switzerland .e Now at University di Perugia . Italy

'° Now at University of Basel, Switzerland .i i

Dipartimento di Fisica, Umversità di Ferrara, Italy.* Supported by the German Bundesministerium für

Forschung und Technologie under contract numbers 05-4HH22P and 05-4MU12P .

muon spectrometer . Methods have been developed to carry the calibration results in space from the impact point of the test beamto the bulk of the detector and in time from the test beam period to all the data taking periods between 1987 and 1991 . Thenumber of hits along muon tracks was used to parametrize the dependence on fluctuations of temperature, tube voltage and gascomposition . Curves and formulae are presented showing the energy response and the energy resolution for the different beamparticles . The test beam data have been used to optimize and calibrate algorithms for the kinematical reconstruction of electronshowers and for their extraction from the vast background of hadronic showers.

1. Introduction

Neutrino physics continues to play an importantand unique role in the investigation of the basic ques-tions of elementary particle physics . In addition to theimpressive successes of the Standard Model obtainedin experiments performed at high energy pp and e +e -colliders, neutrino experiments offer the possibility of acomplementary study of the neutral current sector ofthe Standard Model.

The CHARM-II Collaboration, profiting from theexperience gained in the earlier CHARM experiment[1,2], has designed and built a new detector for opera-tion in the wide-band neutrino beam of the CERNSPS. The main aim of the experiment is the determina-tion of the weak mixing angle sine Ow using the scatter-ing of moon neutrinos and muon antineutrinos offelectrons . Signature for a neutrino-electron scatteringevent is a single recoiling electron which is, because ofits low mass, kinematically constrained to a stronglyforward peaked cone with an opening angle Be given bythe condition

Eeee < 2rrfeC2 .

At the mean energy Ee y 10 GeV of recoiling elec-trons, B e is less than = 10 mrad .

One major challenge of the experiment is to extractneutrino-electron scattering events out of a 10 000-foldlarger background which is due to neutrino scatteringoff nuclei . To distinguish hadronic and electromagneticshowers, different characteristics of the shower devel-opment, such as the lateral width and the energydensity, can be exploited . By an appropriate choice ofthe target material and of the granularity of sensitivedetector elements, the required selection power can beachieved . The remaining electromagnetic part of thebackground due to neutrino-nucleon scattering canlargely be rejected by a precise measurement of thekinematically invariant quantity E0 2 which, being gov-erned by the nucleon mass, shows a much broaderdistribution . Furthermore, showers induced by thescattering of neutrinos on electrons start with onesingle electron at the vertex, which provides an addi-tional constraint on the selection of events . Thus, for areliable recognition of neutrino electrons scatteringevents, the detector design has to provide means formeasuring shower energy and shower direction and foridentifying the event vertex.

CHARM-II Collaboration / Calibration of CHARM-II detector

93

To extract physics results from the measurements ofneutrino interactions in the CHARM-II detector, theresponse to electrons and hadrons over a wide energyrange and the distinct properties of the showers in-duced by these particles have to be studied. The expo-sure of the calorimeter to electron and pion test beamsof known energy allowed us to obtain the absoluteenergy response of the calorimeter and the characteris-tics of electron induced showers in comparison tohadronic showers.

For practical reasons only the front part of the longCHARM-II detector could be exposed to the testbeams. Moreover, the test beam was only available fora few weeks, which is short compared to the five yearsof data collection in the neutrino beam . A method wastherefore required to apply the test beam calibration toother parts of the detector and to other data takingperiods. An ideal source to continuously monitor thedetector response are cosmic ray muons which wererecorded, in parallel to neutrino data taking, in be-tween two successive neutrino bursts . This referencesample can be used to correct globally for space andtime dependent sensitivity variations . Because of en-ergy dependent saturation effects this method, how-ever, turned out to be unsatisfactory when applied toelectromagnetic showers. Thus, in addition, also neu-trino-induced electromagnetic showers were used tocarry the calibration result in space from the front endto the full detector and in time from the test beamexposure to other periods.A first calibration of the CHARM-II detector had

been performed in 1986 . The analysis presented here isbased on data taken in a test beam of electrons, pionsand moons during October and November 1989 . Thetest beam data have been used to optimize the meth-ods which were developed to distinguish electromag-netic and hadronic showers and to determine the en-ergy and the direction of showers. Together with adetailed description of the CHARM-11 detector [3],this paper is intended to help in the critical under-standing of published [4-6] and of forthcoming physicsresults .

2. The CHARM-II neutrino detector

The design of the CHARM-II detector is optimizedto obtain good angular resolution and efficient elec-

94

Fig . 1 . Overview of the CHARM-11 detector . The veto system was removed during the test beam operation .

tron-hadron separation . This led to the realization of ahighly granular structure of active detector elements,giving the possibility of frequent shower sampling andthe choice of low Z target material to reduce theeffects of multiple scattering . In addition a large fidu-cial mass is mandatory to collect a statistically signifi-cant sample of neutrino-electron scattering events de-spite the very small reaction cross section o-/E - 2 X10 -42 cm2/GeV .The detector layout is shown in fig . 1 . It consists of

a target calorimeter, 36 m long, with a square crosssection of 3.7 m X 3.7 m and with a total mass of 692tons . It is centered along the neutrino beam axis and isfollowed by a magnetized iron spectrometer used for

a)

b)

Fig . 2. Display of two test beam events in one out of twoavailable projections : (a) 10-GeV electron and (b) 10-GeVpion . The small dots represent tube hits, and the area of thesquares is proportional to the pulse height on the pick strips .

CHARM-11 Collaboration / Calibration of CHARM-11 detector

the momentum analysis of penetrating muons neededfor the purpose of neutrino flux normalization. Thetarget calorimeter is constructed out of 420 modules.Each module consists of a 4.8 cm thick glass targetplate (Z = 11), followed by a layer of plastic streamertubes with 1 em wire spacing. The tubes layers areinstrumented with 2 em wide pickup strips orientedorthogonally to the direction of the wires. The wiresignals are registered digitally, i .e . any signal exceedinga fixed discriminator level is counted and will subse-quently be referred to as one hit . The charges inducedon the pickup strips are read out as analog signals . Theorientation of the streamer tubes alternates in subse-quent modules between vertical and horizontal . Thus,in one module, depending on the projection, either thehit distribution from the tubes or the charge distribu-tion from the pickup strips is observed . Every fifthmodule contains a layer of scintillators which is 3 cmthick and covers an area of 3 m X 3 m. In total thecalorimeter contains 154 560 digital channels of thestreamer tube system, 73 920 cathode pickup stripsmultiplexed into 9 240 analog channels and 1540 ana-log channels of the scintillator system .

The muon spectrometer * ' consists of six magne-tized iron modules, each of which is equipped withfour scintillator counter tube layers . Nine drift cham-bers, each containing three layers of wires in differentorientations ( - 60°, 0°, + 60°), allow the reconstructionof the muon momentum from the track curvature . Amore detailed description of the individual detectorelements can be found in ref. [3] .

Fig. 2 shows, in one of the two available projections,(a) a purely electromagnetic shower originating from asingle incident electron and (b) a hadronic showerwhich is due to an incident pion from a test beam .

The main components of the muon spectrometer havekindly been lent to us by the CDHS collaboration .

Tube hits are visible as small dots in every secondmodule . The analog signals from the interleaving tubelayers are represented by small squares with an areaproportional to the signal amplitude. It is evident thatthe differences between the two showers can easily berecognized . Electromagnetic showers are narrow andshow a more or less regular evolution, while hadronicshowers are wider and show a highly irregular struc-ture .

3. Test beam calibration

3.1 Layout of the test beam

To produce electrons and pions needed for thecalibration of the detector a secondary particle beamfrom the SPS, containing electrons and pions with anenergy of 135 GeV, is sent onto a copper or leadtarget . Depending on the target material chosen, atertiary beam predominantly consisting of electrons orpions is produced . The low intensity tertiary test beamX7 used for the present calibration covered a momen-tum range from 1 to 63 GeV/c. For beam momentafrom 3 to 40 GeV/c the central beam momentum wasknown to better than ±1% and the momentum spreadwas 0.75% at the smallest opening of the momentumslits, The particle beam was ejected in a spill of 2.4 sduration, followed by the short 6 ms burst of theneutrino beam . The beam intensity was kept below 100particles per spill in order to allow for the readout ofvarious beam monitoring devices in the available time .

--__>.--_-ADz7-BEq, IN

H2

WBSv - BEAM AXIS

NOTTOSCALE

CHARM-11 Collaboration / Calibration of CHARM-I1 detector

MWC 1+2

259m

Fig. 3 shows the layout of the test beam . The testbeam passed through vacuum up to the last bendingmagnet (BM) . This magnet allows to steer the beam totwo different impact points (PI and P2) on thecalorimeter front face . This is important for studyingthe response of the detector to incident particles ofdifferent directions and to test at least two differentpositions of the detector . In position P1 the beam ranparallel to the calorimeter axis which coincided withthe axis of the neutrino beam . In position P2, whichwas 20 cm below the level of position P1, the beamformed an angle of 30 mrad with respect to thecalorimeter axis . The quadrupole OP upstream of thebending magnet allowed horizontal focusing or defo-cusing of the beam, compromising thereby between thedesirable maximization of the beam width and theminimization of scattering on limiting apertures.

Most of the distance between the bending magnetand the calorimeter is occupied by a 21 .9 m longhelium filled threshold Cherenkov counter which couldbe moved together with the beam, depending on thechosen impact point. Two More Cherenkov counters,filled with helium and nitrogen gas, respectively, areinstalled further upstream of the bending magnet . De-pending on the selected beam energy the pressure ofthe Cherenkov counters was adjusted to keep goodefficiency for the identification of electrons. A coinci-dence signal of all three Cherenkov counters reliablytagged electrons up to = 40 GeV/c . At higher ener-gies, electrons were distinguished from pions by theirshower profile observed in the CHARM-11 detector(cf . section 8) .

C-COUNTER

MWC 3.4

TI

CONCRETE

TARGET GLASS PLANES

Fig. 3. Layout of the test beam as seen from the top. P1 and P2 are the two points of impact of the beam on the calorimeter .

95

96

A set of three halo counter hodoscopes (Hl, H2,H3) encircled the beam line at different positions . Oneof these lead scintillator counters (H3) was mounteddirectly in front of the calorimeter while the remainingtwo counters were located further upstream in thebeam line . Signals registered in the halo counters servedto reject particles having experienced an interaction inthe beam line which may have affected their energy ordirection.

Two beam defining scintillators (TA, TB), installedat the entrance and at the exit of the long Cherenkovcounter were used in the trigger logic to indicate thecrossing of a particle . A valid beam event was definedby a coincidence signal from both scintillators being manticoincidence with the signals from the halo coun-ters .

To define the point of impact of the beam particleand its direction with high precision, the beam line wasequipped with four multiwire chambers . These beamprofile counters (BPC1 to BPC4) were mounted infront of and behind the 22 m long Cherenkov counterand achieved a precision of about 2 mm. The informa-tion from the beam profile counters was used to de-velop and optimize algorithms for the reconstruction ofthe shower vertex and of the shower direction.

3.2 . Measurement conditions and test beam parameters

The calibration of the target calorimeter was per-formed in parallel to neutrino data taking. For thispurpose the veto planes (see fig . 1), which precede thecalorimeter, were moved sideways to allow the particletest beam to impinge onto the front end of the targetcalorimeter. The first glass plate of the target calorime-ter had a hole of 20 cm diameter at the impact pointPl . The gap between the first and the second targetplate had been increased in order to leave room,behind the streamer tube layer, for a scintillator whichcould be moved in and out of the beam . This scintilla-tor was used to study the energy deposition E,,,,, at thebeginning of an electron-induced shower, with the aimto distinguish it from a shower due to the decay of a -rr °into two gammas . From the second plate onwards thecalorimeter remained in its regular structure.

With the hole in position PI open, showers startedin a tube layer with vertical tubes. In order to study theeffect of possible asymmetries between the perfor-mance of vertical and horizontal detector elements, anadditional streamer tube layer with horizontal wirescould be mounted in front of the first glass plate. Inthis case the hole in the plate was refilled with its glasscore .

To simulate the uniform probability of neutrinointeractions at any depth within a glass target plate, anarray of triangular aluminum wedges was mountedbetween the beam scintillator TB and the halo counter

CHARM-II Collaboration / Calibration of CHARM-77 detector

H3 . The height of the wedges was chosen to be equiva-lent to the thickness of a target plate when expressedm units of radiation lengths. The base width of thewedges, 5 mm, was small compared to the beam size .Incoming beam particles, therefore, had uniform prob-ability of traversing between 0 mm and 42 mm ofaluminum before reaching the the first tube layer .When the particle beam was directed to the impact

points PI or P2 Cherenkov-flagged electrons or pionswith beam momenta l, 2, 2.5, 3, 4, 6, 8, 10, 15, 20, 25,30, 35 and 40 GeV/c were measured . These momen-tum values safely included the energy range of 3-24GeV used in the analysis of neutrino-electron scatter-ing. For higher momenta the bending power of themagnet was insufficient to steer the beam to positionPl ; therefore measurements with beam momenta 50,55 and 63 GeV/c were taken only at position P2 .

Variations in the applied streamer tube voltageinfluence the determination of the shower energy andthe event selection efficiencies . To study these effectstest beam data were taken for which the nominalvoltage of 4300 V was systematically varied by ±30 Vand _+60 V.

The muon spectrometer was calibrated using muonswith a momentum of 60 GeV/c. The muon beam wasobtained by selecting in the X7 tertiary beam the 60GeV decay products of 100 GeV secondary pions. Itscalculated momentum spread was Ap/p = 3% and theuncertainty in the absolute scale was ±1%.

4. Calibration of the streamer tube system

4.1 . A new method to monitor the streamer tube response

Cosmic-ray muons provide a perfect source formonitoring the long-time performance of the detector .During normal data taking, 10 cosmic-ray events wererecorded per accelerator cycle, yielding 25 000 eventsper ten hours. During test beam operations, when thedetector conditions were changed frequently, the rateof cosmic-ray unions recorded was increased to allowmonitoring of the behaviour of the detector in steps ofabout three hours.

Only events for which the muon track could bereconstructed in both projections have been used forcalibration and monitoring purposes . Events showingseveral tracks or a bremsstrahlung shower along thetrack were rejected . The cosmics trigger required apenetration of at least = I10 modules and thus se-lected rather flat cosmics with a direction deviating atmost 22° from the calorimeter axis .

These cosmic rays and high-energy muons accompa-nying the neutrino beam have been used to measurethe precise alignment of the streamer tubes. For theposition of the streamer tubes a precision of ±0.3 mmhas been obtained .

The voltage applied to the streamer tubes has directimpact on their response : with increasing voltage moreand more particles traversing only a small part of atube, e .g . delta rays crossing a corner only, get achance to induce a complete streamer . Moreover,cross-talk effects simulating additional hits becomemore important with increasing voltage. In addition,the response of the streamer tubes is affected by thegas pressure (correlated with the atmospheric pres-sure), by the gas temperature and by the gas composi-tion . In order to compensate variations of these param-eters a feed-back-loop system had been installed (seeref. [3], p. 675) . While the correction for pressurevariations proved to be fully satisfactory, the compen-sation for temperature fluctuations worked for a spe-cific location only . Over the full length of the detector,temperature differences were unavoidable and changedwith time . As a result of these variations and becauseof manual adjustments of the nominal tube voltagebetween data-taking periods, the response of thestreamer tube system showed a dependence on time aswell as on the longitudinal calorimeter coordinate .

This led to the development of a method to correctfor position and time dependent fluctuations of thetube response. As a first step we had to find anobservable which allowed continuous and reliable mon-itoring of the tube sensitivity . Among several possibili-ties, the average multiplicity a,,,ult of hits along cosmicmuon tracks was chosen as the observable to be moni-tored. The probability that a short delta-ray or a pri-mary track crossing only a corner of a tube triggers adetectable streamer depends on the gas composition,the gas pressure, the temperature and the tube voltageand is a useful indicator of tube sensitivity .

Before this multiplicity can be used, two correctionsmust be applied. The first correction is necessary be-

Z,Û

75E

0 .90 0 .1 0 .2 0 .3 0 .4

SlopeFig. 4. Dependence of the hit multiplicity on the absolutevalue of the slope of the muon track, measured in a planeperpendicular to the wire . No angular dependence is observed

m the other projection .

CHARM-II Collaboration / Calibration of CHARM-II detector

y

97

Fig . 5. Dependence of the hit multiplicity on the coordinatealong the wire . The positive end of the abscissa corresponds

to the read out end of the wire .

cause the directional distribution of accepted cosmicrays is not exactly the same in all parts of the detector.The probability of one or more hits being registered ina given tube layer depends on the direction of thetrack. Tracks which are orthogonal to a tube layer haveabout 11% probability of being invisible in this particu-lar layer, because they can traverse the layer entirelyinside a wall separating two tubes. If the slope of atrack (as measured in plane which is orthogonal to thedirection of the tubes) increases, the probability of itbeing invisible decreases and the probability of pene-trating two adjacent tubes grows. This dependence onthe the track slope has been measured (fig . 4) and isused to reduce the observed hit multiplicities to anominal hit multiplicity referring to tracks which arenormal to the tube layers .

The second correction compensates variations ofthe hit multiplicity as a function of the position alongthe tubes. In comparision to the multiplicity at the readout end of the tube, the multiplicity was found to beabout 10% higher at the far end of the tube . Thiseffect is mostly due to cross-talk via the high-resistivitycover which is common to eight adjacent streamertubes. Hence, it depends on the distance the signaltravels along the wire towards the read out end #z . Thecorrection is performed on the basis of the curveshown in fig . 5.

The result of the two corrections is a normalized hitmultiplicity amult (x, t) which characterizes the sensitiv-ity of the tubes as a function of space and time . Thevalues of amid, happen to be close to 1.0 because theloss due to "invisible" tracks is approximately compen-sated by extra hits due to delta rays .

#z A posteriori, it was found that the effect could have beenreduced to a negligible level by an improvement of theground return .

9 8 CHARM-II Collaboration / Calibration of CHARM-11 detector

U (Volt)

Fig . 6. Dependence of the normalized hit multiplicity on thetube voltage . The straight line represents eq . (2).

4.2. Parametrization of the tube sensitivity by an effectivetube voltage

For practical applications the normalized hit multi-plicity was converted to a variable which is accessibleto a more direct control during test beam operation.An "effective tube voltage" Ueff has been defined as afunction of the normalized hit multiplicity by the equa-tion

Ueff= 4300 V + (amu,l - 1 .006) 1808 V.

(2)

The effective voltage is, to a close approximation, thetube voltage which produces at some reference condi-tion of the detector (pressure 1019 mbar and tempera-ture 23°C) the normalized multiplicity amult . Fig. 6shows for test beam muons the observed relation be-tween hit multiplicity and tube voltage under referenceconditions of the detector . In the remainder of thisreport, instead of amid, the dependent variable Ueff

(amu,t) will be used as a measure of tube sensitivity .An average value of the "effective tube voltage" Ueff

was determined for each group of 20 modules for eachperiod of about one week . Shorter periods wereadopted if necessary. Typically about 25 000 cosmic-raymuons entered in the determination of one set ofeffective voltages . Fig. 7 shows the effective tube volt-age averaged over the full detector as a function oftime. The variation of Ueff along the detector axis isshown fig . 8 for one period (middle of June 1988).

The tube sensitivity, which is parametrized by meansof the effective voltage, will enter the determination ofthe shower energy performed on the basis of therecorded number of hits . In addition it has to beincluded in the evaluation of event selection efficien-cies. Variations of the tube voltage during the testbeam operation allowed one to study how the energy

46004500

44004300

4200

4100

4000

Fig . 7 . Effective streamer tube voltage as a function of time .Each entry corresponds to one calibration period . Only data-taking periods are shown. The distance between marks on theabscissa is one month . (The detailed study of the responsebehaviour discussed in section 4 1 had been triggered by the

discovery of the high fluctuations in 1988.)

response and the selection efficiencies depended onthe tube voltage. The knowledge of the effective tubevoltage for each group of detector modules in eachperiod led to a determination of energy and selectionefficiencies which were independent of time and spaceover the five years.

4.3. Energy of electromagnetic showers from the numberof active tubes

The determination of the energy of eletromagneticshowers using the streamer tubes is based on theobserved number of hits in the first 30 tube layers ofthe shower . Laterally, only tubes within a distance ofless than 25 cm from the shower axis are included .

For test beam electrons of different energies, therelation between the number of hits N observed andthe energy EC of the incident electron is shown in fig . 9and is parametrized as :

Ee-1N+36.7 ) 1 83I1

GeV.57 .5

To transport this calibration to other parts of thedetector and to other periods of time, the then ob-served number of hits N,b , has to be corrected to

4600

4500

4400

4300

4200

4100

400O L-2

198-8, " L1989

'1990 ~ ' '1991,Year

F-T - rT i 1

1

I

1

1

1

i

0019 090 9 6**a

4 6 8 10 12 14 16 18 20 22Group of planes

Fig. 8 . Effective streamer tube voltage as a function of thecoordinate along the beam axis . Each entry represents a

group of twenty modules

match the standard operating conditions of the testbeam period . This is done by using for each group of20 modules the corresponding specific value of theeffective voltage previously determined with the helpof cosmic ray muons. The resulting correction factorwhich has to be applied to the observed number of hitsshows the following dependence on Ueff :

Fe(Ue,) =1 .00 - 0.14(Ueff - 4300 V)/(100 V) .

It has been found that this correction is to a goodapproximation independent of the shower energy .

In addition, one has to correct for cross-talk effects .Due to cross talk between individual streamer tubes,the number of active tubes in a shower depends on thedistance v of the shower from the read out end.However, the number of active profiles is not sensitiveto the shower position . (A profile is a group of eightadjacent streamer tubes under a common high-resistiv-ity cover; it is said to be active when there is at leastone tube hit.) The average ratio of active tubes toactive profiles can therefore be used to derive a correc-tion factor P(v) which takes care of cross-talk effects .Referring to a shower in a central position, the correc-tion amounts to about +10% and -10% respectivelyat both ends of the tubes.The combination of the two corrections converts the

observed number of hits to the number N expectedunder standard conditions of the detector and accountsfor cross-talk effects :N-FelUeff) PlL)Nobs,

Inserting this value into eq. (3) yields the energy ofthe electromagnetic shower . In the energy range 2-40GeV which includes the energies used in the analysisof neutrino-electron scattering, the absolute scale er-ror of this energy determination is estimated to be

Fig . 9. Number of tubes hit by an electromagnetic shower as afunction of energy. The curve represents the fit function given

by eq . (3) .

CHARM-II Collaboration / Calibration of CHARM-II detector

24

20

16

1210o

10 1E (GeV)

4.4. Energy ofpion-induced hadronic showers

F(Ueff) =1 .0-0.10(Ueff -4300 V)/(100 V) .

99

Fig . 10. The number of active tubes per GeV as a function ofthe pion energy .

c 5%. A fit to the energy resolution in the range 3-40GeV yields

o,(E,)/Ee=0.09+0.15/ Ee/GeV .

(4)

For a safer selection of pions, beyond the tagging inthe Cherenkov counters, only pions were acceptedwhich showed a clearly visible incoming track in thefirst modules of the calorimeter . The number N ofaccepted tube hits was not restricted by any geometri-cal cut and did not include the hits caused by theincoming pion before the hadronic interaction . To ob-tain the energy of the hadronic shower E n , the energyloss of the pion between its point of entrance into thecalorimeter and the shower vertex was subtracted fromthe nominal beam energy .

In the energy range 2-60 GeVthe relation observedbetween the number of hits generated by test beampions and the hadronic energy is shown in fig. 10 andcan be described by :

N = 27 .7(E,,/GeV)° 863- 16 .8 .

(5)

As in the case of electromagnetic showers a correc-tion factor depending on the effective voltage has to beapplied to the observed number of hits when the testbeam calibration is transported to different time peri-ods and to other parts of the detector . The correctionfactor is given by :

The correction which is expressed in this factor issmaller than in the case of electrons because electro-magnetic showers are denser and, due to higher chargesin the shower core, more likely to produce extra hitsvia cross talk .

The observed energy resolution resolution in theenergy range from 2 to 60 GeV can be parametrized as

o,(E,)/E, = 0.07 + 0 .31/ En/GeV .

(6)

10 0

4.4.1 . Neutrino-induced hadronic showersOne aim of the CHARM-II experiment is the inves-

tigation of neutrino-induced events with two outgoingmuons. Most of these events are generated via theproduction and the decay of D-mesons . For the analy-sis of these dimuon events, one needs to know theresponse of the detector to hadronic showers producedby semileptonic neutrino interactions . Hadronic show-ers due to neutrino interactions differ from pion-in-duced showers in their particle composition and intheir opening angle.

The Lund Monte Carlo program #3 was used togenerate dimuon events. The response of the detectorto each secondary particle was simulated and superim-posed using the LEANT Monte Carlo program, whichhad been extended and fine-tuned to reproduce theresponse to test beam electrons and pions, includingposition dependent crosstalk effects (see section 4.1)and "satellite" hits due to soft photons below thecut-off energy used in the simulation .

To check the validity of this procedure, test beamevents, pions and/or electrons, were superimposed byORing their hits . The same combinations were gener-ated with the Monte Carlo program. Good agreementwas found, and the results gave confidence in theMonte Carlo simulation .

The result is a formula which yields, as a function ofthe number N of tube hits, the energy E,c of hadronicshowers produced in neutrino interactions . While thisrelation has been derived for the special case of dimuonevents, we believe that it holds to a good approxima-tion for any hadronic showers produced in "chargedcurrent" reactions i.e . in reactions which change theincoming neutrino into an outgoing muon :Ece

N

N

Ueff - 4300 VGeV

18 .69(1 + 2980 -0.056-

100 V

)(7)

Again a correction has been included to take intoaccount time and position dependent sensitivity fluctu-ations which are parametrized by Ueff .

The energy resolution is independent of the tubevoltage:

o,(E cc )/E,, = 0.02 + 0.52/

E,,/GeV .

(8)

5. Calibration of the pickup strip system

5. I. Response of the pickup strips to cosmic ray muons

Cosmic rays were used to study the response of thepickup strips to the charge induced by the streamers.

#3 LEPTO version 5 .2, July 31 1987, in combination withJETSET version 6 .3 .

CHARM-11 Collaboration /Calibration of CHARM-II detector

Û 400a

cn 35LU

45

30

1

I

I

I I0 4 8 12 16

Wire numberFig . 11 . Variation of the charge from the pickup strips as afunction of the relative position within the modular structure

of the streamer tube layers .

Clean tracks traversing a streamer tube layer inducethe highest charge directly on the strip the track tra-verses, and smaller satellite signals on neighbouringstrips . For monitoring purposes, the "cluster charge",i .e . the sum of the charges on three neighbouring stripsclosest to the track, was used .

It was found that the cluster charge shows a peri-odic dependence on the coordinate along the strips,reflecting the modular structure of the underlyingstreamer tubes (fig . 11). Each group of eight tubes hasa common high -resistivity cover between the gas vol-ume and the pickup strips which run across the tubes.Tracks passing through a tube near the center of agroup gave a charge smaller by = 25% than trackspassing through an edge tube . Superimposed is a slightmodulo-16 periodicity, because two adjacent groups ofeight tubes are mounted in a common envelope . Theseobserved periodictties were used to apply correctionsfor the calibration of the average strip response .

For the analysis of our neutrino data in previouspublications, only one average calibration constant perstreamer tube layer was used . A more detailed map-ping of the strip response within the tube layers led toa slight improvement in vertex and direction resolu-tion . The response varies between individual strips (i)and depends on the underlying group of eight tubes. Itturns out that the response function can be factorizedinto a strip-dependent part s, and a part t, dependingon the tubes. For each strip a separate calibrationconstant was determined, yielding a total of 73 920calibration constants s, for the full detector . The de-pendence of the response function on the underlyingtubes is superimposed . Each group of eight tubes waslongitudinally subdivided into four sectors, and foreach sector (j) the average response was measured .For the full calorimeter, this gave 73 920 additionalconstants t,

The multiplication of the raw cluster charge withthe two calibration factors gives the corrected chargewhich enters the data analysis : Q=s, tjQraw .

Fig. 12 compares, for one particular tube layer, theraw cluster charge and the resulting corrected chargeas a function of the strip number i, averaged over allunderlying tubes. The fact that the distribution ofcorrected pulse heights is nearly flat gives an a posteri-ori justification for the factorization of the calibrationconstants. A study of the long-time stability, usingcosmic rays taken during the five years of data taking,has shown that it is sufficient to determine a new set ofconstants s, and t, every two months .

Penetrating cosmic rays have also been used todetermine the exact position of the pickup strips. Aslight curvature of the strips was found. In some fewcases the extreme values of the sagitta amounted to 5mm. As a consequence of the fabrication process,neighbouring strips showed a similar curvature, and itwas sufficient to fit a common parabola to every groupof eleven adjacent strips . For the full calorimeter thedescription of the strip shapes required = 20000 con-stants. With this parametrization it was possible toreduce the uncertainty of the strip positions from about+_ 4 mm to less than +_ 2 mm. These corrections wereused in the analysis of electromagnetic showers inorder to improve the precision of the vertex location .

5.2 . Energy of electromagnetic showers from the chargeon the pickup strips

The energy of an electromagnetic shower can alsobe determined from the total charge induced on thepickup strips in the first 30 tube layers following theevent vertex . Fig. 13 shows the dependence of thischarge on the energy of test beam electrons. Thecharge scale (ADC-channels per GeV) contains a fac-tor A which cancels when neutrino induced showersare compared (this is the purpose of the calibration) to

40

20

mc 20U

10

0 20 40 60 80 100 120 140 160Strip number

Fig. 12 . Variation as a function of the current strip numberwithin one selected tube layer : (a) raw charge from the pickupstrips before and (b) corrected charge after the strip-by-strip

and tube-by-tube calibration .

CHARM-H Collaboration / Calibration of CHARM-II detector

N

0

U0amroÛ

8000

6000

4000

2000

0 8 16 24 32 40E (GeV)

Fig . 13 . Total charge (sum of ADC-counts) on the pickupstrips for electromagnetic showers as a function of the testbeam energy . The curve fitted to the data is given by eq . (9) .

test beam showers. Test beam data, in the range from2 to 40 GeV, can be fittet by the relation

Q(E) =AE(I -E/(220 GeV)) +0.16 GeV .

(9)

The last term, a constant offset, is a consequence ofthe noise on the pickup strips . The energy resolutionfound in test beam showers is

(o,(Ee)/E,)TB=0.034+0 .24/ Ee/GeV .

(10)

To transport this calibration result in space andtime, cosmic muons are not useful as a referencestandard because they are not affected by charge satu-ration effects which can be present in the core ofshowers. Instead, neutrino-induced events were usedwhich passed all the selection criteria for electromag-netic showers (see section 8) . Only the digital informa-tion of the streamer tubes was used for this selection.A careful study confirmed that these reference eventswere evenly distributed throughout the detector.

For each group (k) of 20 successive detector mod-ules and for time slices (m) of about 1 month, theaverage charge Qk,m of these reference events wasevaluated. The ratio Qk,m/Q1,TB was then used tocarry the energy calibration from the first group oftube layers (k = 1) and the test beam period (m = TB)to other parts of the detector and to other data-takingperiods.

This procedure suffers from statistical errors due tothe limited number of available neutrino-induced ref-erence showers per detector subdivision and time slice .The size of the statistical errors was estimated from thefluctuations which were observed when a finer subdivi-sion of the sample was used . This adds a contribution(o,(Ee)/E,)transport = 0.042 to the energy resolution .

10 2

20

5

0 s 16 24 32 40E (GeV)

Fig . 14 . Overall resolution function for the energy determina-tion of electromagnetic showers using the charges on thepickup strips. Intrinsic errors due to shower fluctuations inthe test beam are combined with the errors due to thepropagation of the calibration from the test beam to the rest

of the detector . The curve represents eq . (11)

This contribution is largely independent of energy.Since both error sources are independent, their squarescan be added in each energy bin.

The overall energy resolution determined from thecharges induced on the pickup strips is finally given by(fig . 14):

o- (E,)IEe = 0.05 + 0.23/VE~/GeV .

The main contribution (±6%) to the relative erroron the absolute energy scale arises from the limitedstatistics of neutrino-induced reference showers in thefront part of the calorimeter where the test beamevents were located. Combined with other minorsources, the systematic error on the energy scale addsup to about 7% .

6. Calibration of the scintillators

The scintillators of the target calorimeter had beenin use since 1978 in an earlier CHARM experiment [1]where their main purpose was the determination ofshower energies in sampling steps of one radiationlength . In the larger CHARM-II detector, the scintilla-tor layers were separated by 2.5 radiation lengths, andtheir use was restricted to the separation, in a sampleof events, of electron-induced showers starting with asingle electron track from -rr°-induced backgroundshowers which start with an even number of tracks .

The response of the scintillator system to cosmic

CHARM-11 Collaboration / Calibration of CHARM-II detector

rays was monitored in roughly monthly intervals. Itturned out that the average light attenuation factor, asmeasured over a length of 2.4 m, increased linearlywith time from a factor 1.6 when the scintillators werefirst used in 1978 [2] to 4.5 at the start of CHARM-IIin 1986, and to 6.5 at the end of 1990 .

The original plan to use the shower response inscintillators as a reference to transport calibration re-sults from the front end to the bulk of the calorimeterwas dropped, because the methods based on the nor-malized hit multiplicity of the streamer tube system(section 4.1) and the use of neutrino-induced showersfor the system of pickup strips (section 5 .2) gave supe-rior results . The inferiority of scintillators was due tothe fact that the sampling steps, one scintillator layerper 2.5 radiation lengths, were too coarse compared tothe shower evolution in the region covered by the testbeam. Since the test beam setup did not forsee thepossibility of a longitudinal shift of the impact point bymore than half a radiation length, it was not possible toaverage over all vertex positions modulo the samplingsteps.

7. Angular resolution for electron showers

The determination of the shower direction is closelyconnected to the determination of the event vertex .Two algorithms are used :

The maximized road algorithm combines the searchfor the event vertex with the reconstruction of theshower axis . A first approximation for the axis is ob-tained from the digital data . After removal of isolatedhits far away from the shower, this axis is obtained as astraight line fitted to the mean positions of hits in eachtube layer and the vertex is defined as the intersectionof this axis with the first tube layer of the shower . In asecond step, the tube hits are weighted with the pulseheight on the corresponding pickup strip. In order tosuppress noisy analog channels, the analog informationis validated by comparison with the hit pattern inneighbouring tube layers . Combining both the digitaland the analog data improves the vertex and directionestimate . Finally, the shower axis is determined in bothprojections by using the vertex as a pivot to rotate theaxis through the shower, maximizing the number ofhits in a narrow band ("road") centered on the axis . Inthis procedure each hit is attributed a weight accordingto a Gaussian function depending on the lateral dis-tance from the axis .

This determination of the shower axis has beenshown to give the best resolution for the shower direc-tion . However, the algorithm is not fully efficient ; itfails for 0.3% of the events . In this case the overallmedian algorithm is used . It makes use of the vertexdefined by the first step of the maximized road algo-

rithm. Digital and analog data are used independently .Only tubes and pickup strips closer than 10 cm to thefirst estimate of the shower axis are used . Furthermorein the longitudinal direction only activity within 90% ofthe longitudinal containment of the deposited energy istaken into account. For all activity inside this box, theangle between the line connecting it to the vertex andthe detector axis is calculated . Weight factors depend-ing on longitudinal and lateral position have beenadjusted with the help of electron showers from thetest beam . In the case of pickup strips, the positiondependent weight is multiplied with the correspondingpulse height . The computed axis is the one which splitsthe shower, i .e . the sum of all weights, into halves . Thefinal result is the average of the directions found fromdigital and from analog data . This algorithm is fullyefficient, but the resolution is slightly worse than thatof the maximized road.

The angular and the vertex resolutions, displayed infig. 15 are obtained from the test beam . The improve-ment of the vertex resolution at higher energies, is aconsequence of combining the vertex search with thedirection finding. Above 12 GeV a resolution of ±2.3mm was found.

The resolution for the projected angle of electronshowers m the energy range from 2 to 40 GeV can beparametrized by combining three terms:

Q(Bproi ) -

2.25

14.5

~

mrad

J ' - [ log,0(Ee/GeV) 12+ [

Ee/GeV

l2

The first

CHARM-II Collaboration / Calibration of CHARM-TI detector

+ [0 .03E

GeV ]-

(12)

term is due to finite vertex resolution . Itdepends on the distance between vertex and showercenter, and thus shows a logarithmic dependence onthe shower energy . The second term is due to particle

EE

32

24

os

Fig. 15 The angular resolution (left) for test beam electrons(solid curve, eq (12) and for neutrino induced electrons(dashed curve, eq. (13)). The right side shows the vertex

resolution .

statistics in the shower and the third term accounts forsaturation effects in the shower core .

The angular resolution measured in the test beamneeds a small correction before it can be applied toneutrino-electron scattering events . Due to variationsof the response function along the calorimeter, theangular resolution averaged over the full calorimeter isslightly worse. To account for this, a random fluctua-tion was superimposed to the strip charges which weremeasured in the test beam . The amount of this varia-tion was deduced from a study of the distribution ofthe pickup charges when exposed to cosmic-ray muons.This procedure led to a slightly modified resolutionfunction (dashed curve in fig. 15):

0- ( eproi )

2

2.64

2

15 .0[

mrad

1 - [ log, 0(Ee/GeV), + [

Ee/GeV 1

At 10 GeV, the mean energy of electronsexperiment, the resolution is op,,, = 5.4 mrad .

8. Electron-hadron separation

8.1. Event selection

10 3

in this

The initial step in the selection of neutrino-elec-tron scattering candidates is a fast event recognitionand hadron rejection without reconstruction of kine-matic variables. It uses digital information of thestreamer tubes only .A shower is accepted if at least 30 hits are found in

a range of at least 10 active tube layers, with not morethan 7 consecutive non-active tube layers in between.Events with an outgoing muon are rejected by therequirement that there are not more than 60 activetube layers . These selection criteria are fully efficientfor electrons with energies above 1.5 GeV.

In a first step of electron-hadron separation, fouralgorithms are applied:

1) The digital road algorithm is based on a compari-son between the energy deposited in a narrow regionaround the shower axis and the total shower energy .For a preliminary determination of the shower-axis,the medians of the deposited energy in each tube layerare used . Only activity within the 5th plane and the20th plane after the event vertex is included :

20

20Rdg= Y_ (N±,,, mJ,/ Y (Ntot)i~

=5

t=5(14)

where N+,O,m is the number of active streamer tubeswithin +10 cm of the median and Ni, is the numberof all active streamer tubes in the same layer (i) .

104

2) In order to calculate the 80%o-width separator,one determines in each individual tube layer the hitdistribution with respect to the median position of hitsin this particular layer . All these distributions are thensuperimposed and the width which remains after cut-ting 10% of the hits from each tail is the 80%-widthseparator.

3) The holicity algorithm makes use of the fact thatelectromagnetic showers are more regular and densethan hadronic showers. Holicity 1s defined as

12

( n hole - (nhole\ ~2 'r

/tXhoie -

0-1hole

CHARM-II Collaboration / Calibration of CHARM-II detector

(15)

where nho" is the number of non-active streamer tubes(holes) detected between the two outermost streamertubes hits in tube layer (i). The quantities (nhole) anda,

hot' denote the average number of holes and itsstandard deviation respectively and are determinedfrom test beam electron events .

4) The fourth algorithm is based on the graphtheoretical concept of a minimal spanning tree . It triesto interconnect a given set of points (nodes) in amultidimensional space with the shortest possible net-work of direct links. This approach is described indetail in ref. [7] .

All four selection algorithms are strongly corre-lated . Each one alone can give a very good electron-hadron separation if the cuts are sufficiently strong.Only the combination of the four separators allows,however, to loosen the cuts to such an extent thatelectronmagnetic showers are retained with an effi-ciency of 99.6%, while hadronic showers are rejectedwith rejection factor of 27.

More refined algorithms reduce the background ofhadronic events further . They make use of vertex anddirection determinations described above. The selec-tion is performed in three steps:

1) Rejection because of tracks or of back scattering :a) Single tracks emerging from the vertex with an anglelarger than 18° with respect to the shower directionsuggest a hadronic process; b) Long tracks emergingfrom the shower core give evidence for hadronic activ-ity inside the shower ; c) More than two streamer tubeshit upstream of the vertex are regarded as backscatter-ing from a semileptonic reaction .

2) The combined road algorithm: this algorithm issimilar to the digital road algorithm. Here, the showeris longitudinally divided into a vertex region and ashower core region, and laterally into six subregionswhich allow the formation of a narrow road and of abroad road . Tube hits and strip charges are summedup for each region . The selection of the events is basedon the symmetry of the lateral energy deposition andon the fractions of the deposited energy contained inthe two roads. All quantities are calculated separately

100

80

60

40

20

0

10

20

30

40

50

60

70E (GeV)

Fig . 16 . Trigger efficiency as a function of the electron showerenergy .

for digital and for analog data, for the two projections,and for the vertex and the shower core region . Com-parison with results from test beam data leads to adecision of rejecting or retaining the event.

3) The hit multiplicity in the vertex plane (1-hit cut) :The most severe cut applied in the selection of elec-tron events is the requirement of exactly one streamertube hit in the first active tube layer of the shower .This cut rejects about 16% of the neutrino-electronscattering events, but more than twice this fraction ofbackground events .

8.2 . Electron efficiency determination

The first selection step for neutrino-electron scat-tering candidates is the trigger decision . For the deter-mination of the nominal trigger efficiency, i.e . assum-ing perfect hardware performance, a software simu-lated trigger is applied to test beam electron events .Test beam particles hit always the same part of thedetector . To avoid a bias, the observed showers havebeen artificially moved through the detector beforeapplying the software trigger. The trigger is fully effi-cient above 3 GeV, and the efficiency drops slowly atenergies above 30 GeV (fig . 16) .

To study the effect of hardware failures, a sample ofevents taken with lower trigger thresholds was used toderive effective threshold functions for each of thesingle trigger input signals . These threshold functionswere then used and combined m the application of thesoftware trigger to test beam events .

The selection efficiency for neutrino-electron scat-tering events includes all selection algorithms appliedin the off-line analysis, except the 1-hit cut, which will

100

40

200

i

iCP0 0 0 0 0 0 0 0

00 0

60 7010 20 30 40 50E (GeV)

Fig . 17 Selection efficiency as a function of the electronshower energy .

be treated separately . A study with test beam electronsof different energies showed that the selection effi-ciency was essentially independent of energy (see fig.17). Weighted with the energy spectra of neutrino-electron scattering events, the average selection effi-ciency is

Esel = 0.947 + 0 .003,a, ± 0.017,ys, .

(16)

The systematic error arises from a small dependence ofEsel on the vertex position in the calorimeter due tocross talk effects in the streamer tubes and from a verysmall dependence on the applied tube voltage.

The selection efficiency of the 1-hit cut depends onthe early development of the shower, on the geometri-cal acceptance, on the response of the streamer tubes,on the tube voltage used during data taking and on thelateral position of the vertex . The first streamer tubelayer following the glass plate in which the neutrinoreaction has taken place can be inefficient . This hap-pens if the electron traverses a tube layer entirelyinside a wall which separates two streamer tubes, andis therefore not detected . The vertex algorithm willthus only find the vertex one plane later . The 1-hitefficiency accounts for this effect . Fig. 18 shows the hitmultiplicity in the first streamer tube layer for testbeam electrons.

For all selection efficiencies one also has to con-sider a possible dependence on tube sensitivity and oncross-talk effects . In the case of the 1-hit cut botheffects are significant and contribute to the systematicerror. The resulting error of the 1-hit efficiency, ob-tained from test beam data, contains an explicit depen-dence on the effective tube voltage:

DUEl h,t = 0 .804 ± 0 .003,t, ± 0.022,yst + 0 .03

(2)100 V '

with t1U= (Uett ) - 4300 V, where (Ueff ) is the effec-tive voltage defined in eq.(2) and averaged over oneyear's data . The result is independent of the showerenergy .

CHARM-II Collaboration / Calibration of CHARM-II detector

w

0

10 ° I

1

1

1

1 I I0 2 4 6 8 10

Hit multiplicityFig. 18 . Distribution of the hit multiplicity in the vertex plane

of an electromagnetic shower.

To apply the value of the 1-hit efficiency to neutrinodata a small difference in the material traversed by theelectron has to be accounted for . On average, there are0.23 radiation lengths between a neutrino-electronscattering vertex and the first streamer tube layer . Dueto the monitoring equipment in the beam line, thisquantity is increased to 0.26 radiation lengths for testbeam events . To correct for this difference, test beamdata taken with additional material in the beam lineare used . From fig. 19 one can read of the correctionwhich has to applied to the 1-hit selection efficiency :

einlt =si hit + 0 .034 ± 0 .012 .

(18)

With the single contributions added in quadrature, thetotal systematic error of the electron selection effi-ciency amounts to 3.4% of the final 1-hit efficiency .

9. Calibrations for muons

For the interpretation of the CHARM-II data, it isimportant to know the energy spectrum of the incom-

Lw

88

84

80

76

72

105

0 0.02 0.04 0.06 0.08Additional material (X 0)

Fig. 19. Dependence of the 1-hit efficiency on the material infront of the first streamer tube layer .

106

ing neutrinos . This can be calculated from the mo-menta of muons which have been created by quasielas-tic neutrino scattering off nuclei . In this way, thecalibration of the muon spectrometer sets, for theanalysis of our data, the scale of neutrino energies .

9.1 . Angular resolution for muon tracks in the calorime-ter

The direction of muon tracks is determined in thetarget calorimeter from the digital information of thestreamer tubes. A straight line fit, extending over 30consecutive tube layers, takes into account space reso-lution and multiple scattering .

The resolution for the projected angle can be esti-mated to be

'010,PFOJ

2_

18

+0.42 ,

(19)mrad I

_[ (p/(GeV/c))j/4 Ju

where p refers to the muon momentum .This has been experimentally verified with the use

of long muon tracks, which were cut into two segments,and the direction of both segments was compared atthe location of the cut. The resolution, determinedfrom the observed differences, is in good agreementwith this estimate (fig . 20).

9.2. Efficiency of the muon-spectrometer trigger system

The trigger for events with muons requires theresponse from at least 8 out of the first 16 scintillatorlayers in the muon spectrometer . Due to the large

mÉ0,3

0 25 50 75pp (GeV/c)

Fig . 20 . Momentum dependence of the angular resolution formuon tracks in the target calorimeter . The line shows theMonte Carlo prediction (eq (19)) . The agreement betweenthe data points for different years shows the stability of the

detector.

CHARM-II Collaboration / Calibration of CHARM-II detector

100

0.20

0.15

0.05

o Data

MC

00 0 a " a 01(os

1

0 4 8 12 16n

Fig. 21 . Probability distribution for the number n of activescintillator layers, averaged over all muon events in the 1988data. The Monte Carlo prediction is compared with the data

which have been taken with the trigger condition n>_ 8.

redundancy of the trigger, its efficiency could be de-duced from the triggering events themselves . For eachof the 768 scintillation counters, the pulse height distri-bution as a function of muon impact position and theminimum pulse height to produce a logic signal weredetermined from ADC and pattern unit information .This allows one to calculate for a given track theprobability that less than 8 discriminators gave signalsand hence no trigger occurred .

The model was checked by comparing the predictedand the observed distributions for the number of activescintillator layers in a trigger (fig . 21).

The performance of the scintillators has graduallydeteriorated with time . Compared with data taken in1988, an increase of the effective thresholds by 12% forthe 1989 data and by 28% for the 1990 data has beenobserved . Because of that, the trigger requirementshave been relaxed from 1989 onwards to only at least 6active scintillator layers .

The calculated inefficiencies, caused by the muonspectrometer trigger, is 2.16% before 1989, 1 .56% in1989 and 2.20% in 1990 . The error of the efficiency ismostly systematic and estimated to be ±0.2%.

9.3. Efficiency of track reconstruction in the spectrome-ter

The reconstruction efficiency for muons in the spec-trometer has been studied with Monte Carlo events .The results have been corroborated with test beammuons.

The Monte Carlo description of the data containschamber efficiency, multiple scattering and energy losswith its fluctuations . Background hits, including theircorrelations with the muon track and within adjacentdrift chamber gaps, are added according to distribu-

6 F_ I

5 0 19880 19890 1990

4

tions observed in the data . The reconstruction effi-ciency of Monte Carlo events varies slightly with muonmomentum : it increases from 95% at 5 GeV/c to aconstant value of 98.5% for momenta above 20 GeV/c.

Test beam muons entering the calorimeter with 60GeV/c lose on average 14 .3 GeV/c before they reachthe spectrometer with 45 .7 GeV/c. The observed re-construction efficiency for these muons is 97 .4 ± 0.3%.

9.4. Determination of muon momenta

Crucial for the reconstruction of the muon momen-tum in the magnetic spectrometer is an exact knowl-edge of the drift chamber positions. The chambers hadbeen aligned by surveyers with 2 mm precision alongthe beam line and with 0.5 mm precision perpendicularto it . Using high energy muons accompaning the neu-trino beam the perpendicular alignment was improvedto +_0.2 mm.

As for the determination of the reconstruction effi-ciency, mainly Monte Carlo events were used to studythe momentum determination . The result has beenchecked at 46 GeV/c with the use of test beamrations .

For the spatial distribution of the magnetic field, aparametrisation derived by the CDHS group [8] wasused, which locally reproduces the field to about 1% .The integral in radial direction was checked experi-mentally with +0.6% precision by measuring the timeintegral of the voltage induced in a pickup loop whenreversing the excitation current.

Fig. 22 shows the relative momentum resolution andthe relative bias as a function of muon momentumderived from Monte Carlo events, the bias being thedifference of the reconstructed minus the true momen-tum. For the bulk of our data, which have a meanmomentum around 25 GeV/c, the resolution is 13%with a bias of -2%.

0

.------------------------------------

-005,0

50

100

150pp (GeV/c)

Fig 22 . Momentum resolution (a) and momentum determina-tion bias (b) for reconstructed Monte Carlo events .

CHARM-II Collaboration / Calibration of CHARM-II detector

80

70

60

(n 50

w 40

30F-

201--

10

Acknowledgements

10 7

Fig. 23 . Distribution of momenta reconstructed at the en-trance of the target calorimeter . The muons in the test beamhad an average momentum of 60 GeV/c. The momentum

bite of the beam was 3% .

The predicted bias and the resolution have beenverified with test beam muons of 60 GeV/c. Data havebeen taken with both signs of the excitation current(focussing or defocussing the tracks in the spectrome-ter) . In the focussing mode, the data are reconstructedwith 46 .4 GeV/c mean energy at the entrance of thespectrometer, corresponding to 60.7 GeV/c at theupstream end of the target calorimeter, while for MonteCarlo muons of 60 GeV/c the values 45 .7 and 60 .0GeV/c are obtained . The corresponding numbers forthe defocussing mode are 46 .0 and 60 .3 GeV/c for thedata and 46.3 and 60.6 GeV/c for Monte Carlo. Theobserved differences are consistent with the uncertaini-ties of the test beam momentum (±]%), the fielddistribution (±I%), the field integral (±0.6%), andthe statistical errors of the measurements (±0.3% forthe data and ±0.4%o for Monte Carlo) .

We conclude that the scale of the momentum deter-mination is known within ±2%.

The observed momentum resolution for test beammuons is reproduced to better than 10% by the MonteCarlo calculation, as shown in fig . 23 .

We gratefully acknowledge the skill and dedicationof our many technical collaborators who have con-tributed to the realization and the operation of thedetector and of its associated systems. The experiment

0 .20

0 .15 (a) -

0 .10

005

i(b) _

108

has been made possible by grants from the Inter-Uni-versity Institute for Nuclear Sciences (Belgium), CERN(Geneva, Switzerland), the Bundesministerium fürForschung and Technologie (FRG), the Institute ofTheoretical and Experimental Physics (Moscow, Rus-sian Federation), and the Istituto Nazionale di FisicaNucleare (Italy); we gratefully acknowledge their sup-port . We should like to thank L. Gatignon and hiscollaborators for his competent assistance ensuring theexcellent performance of the CERN X7 test beam .

References

[1] CHARM Collaboration, J. Dorenbosch et al ., Nucl . Instrand Meth . A253 (1987) 203

CHARM-II Collaboration / Calibration of CHARM-II detector

[2] CHARM Collaboration, M. Jonker et al ., Nucl . Instr. andMeth . 200 (1982) 183.

[3] CHARM-II Collaboration, K. De Wmter et al ., Nucl .Instr. and Meth . A278 (1989) 670.

[4] CHARM-II Collaboration, D. Geiregat et al ., Phys . Lett.B259 (1991) 499.

[51 CHARM-11 Collaboration, P. Vilain et al, Nuel. Instrand Meth . A277 (1989) 83

[6] CHARM-II Collaboration, P. Vilain et al ., Phys . LettB281 (1992) 159.

[7] CHARM-II Collaboration, K. De Winter et al ., Nucl .Instr and Meth . A277 (1989) 170.

[8] CERN-Dortmund-Heidelberg-Saclay Collaboration, pri-vate communication