ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 1 Raluca Muresan NBI Copenhagen...

ISMD 2004, Sonoma State University

R. Muresan NBI BEC studies 1

Raluca Muresan

NBI

Copenhagen

Bose-Einstein Correlation Studies at HERA-B



Outline

HERA-B experiment BEC – introduction Why BEC at HERA-B Main ingredients for BEC analysis Preliminary results in pA with A=C, Ti, W On-going studies Summary



HERA-B experiment

The HERA-B Experimentat DESY

Ring Imaging Cherenkov Counter

250 mrad

220 mrad

Magnet

Si-StripVertexDetector

Calorimeter

TRD

Muon Detector

TargetWires

0 m5101520

T op V iew

ProtonBeam

ElectronBeam

Vertex Vessel

Inner / Outer Tracker

high-pt

Al BeamPipe

Located at the proton-electron collider HERA at DESY.Fixed target experiment: 920 GeV proton collisions on various targets. Large acceptance at mid-rapidity x(15, 220) mrad, y (15-160) mrad



Target wires

C, Ti, W, Pd, Al wires inserted in to the halo of HERA proton beam

Mounted in 2 stations separated by 4 cm

Each wire can be inserted independently



VDS

64 double-sided silicon microstrip detectors aranged in 8superlayers, divided in 4 quadrants

quadrants located in Roman pots allowing the detectorsto be retracted during injection (except SI08)



OTR

Superlayers (MC’s, PC’s, TC’s) are made of few modules containing several layers with angles of 0 and 5 degrees.The layers are made of honeycomb drift cells.



RICH

Large vessel filled with C4F10

radiator gas. The photons are reflected, by a serie of mirrors,to the photon detectors placed outside the detector acceptance.



BEC

Symmetric wave functions of bosons, no exclusion principle; Enhanced probability for the identical bosons to be emitted with

small relative momenta; Quantum statistical correlations between pairs of identical

particles.

Presuming that only particles emitted from the same or very close sources exhibit this behaviour from studies of BEC one can obtain information about the size, shape and space–time development of the particle emitting source.



BEC

For two identical bosons the Bose-Einstein correlation is defined as:

))P(pP(p

),pP(p),pC(p

21

2121

),( 21 ppP

)(),( 21 pPpP

probability density of two particles to be produced with 4-momentum p1 and p2 ;

probability densities for a single particle to be produced with 4-momentum p1 or p2 , difficult to build in practice reference sample



BEC function parametrisation

)1()1()(22

2QeNQ

RQC

21 ppQ

N

R

invariant four-momentum difference,

related to the fraction of identical bosons whichdo interfere,

interpreted as the geometrical radius of the presumablyspherical boson emitting source (just an approximation),

overall normalization,

linear background.



Why BEC at HERA-B ?

Opportunity to measure BEC parameters at sqrt(s)=41,6 GeV, intermediary value between SPS and RHIC; Possibility of studying the BEC parameters dependence on the target material (C, Ti, W). BEC can be studied for both pion and kaon pairs (RICH selection) . Large minimum bias sample(single wire runs) 103 million pC events, 74 million pW events, 28 million pTi events. large enough for differential studies (directional dependence, multiplicity, transverse mass dependence ).

This talk is presenting only a feasibility study, made on a small sample (few million events).



Reference sample

Mixed sample – tracks from different events, some other kind of correlations are also disappearing (long range correlations , energy-momentum ...)

mixMCrecnobe,

MCrecnobe

data,mix

data

dQ

dN

dQ

dN

dQ

dN

dQ

dN

C

2

To correct for the effects introduced by mixing double ratio.



221

212

2

212 )(

)cos1(*2*pp

ppmppQ

Remove from the analysed sample the pairs of tracks too close in space : abs(tx1-tx2)<0.0008, tx=px/pz; abs(ty1-ty2)<0.0008, ty=py/pz; and momentum abs(p1-p2)<0.5 GeV p=abs( ) to be resolved.

The C2(Q) distributions for pions corrected for Coulomb interaction (Gamow), only small variation in the parameter values occured.

Event, track, pair selection

p

Pion and kaon tracks - RICH likelihood.



MC studies -pC

BEC JETSET: MSTJ(51)=2 the shape of correlation function – Gaussian MSTJ(52)=9 BEC for , K , PARJ(92)=1 meaning that particles that can be subject to BEC are subject to BEC. PARJ(93)= 2 fm =R. PARJ(91)= 0.020 GeV minimum particle width above which the particle decays are assumed to take place before the stage when BEeffects are introduced. Particles with broader width than 0.020 are assumed to have time to decay before BE effects are to be considered

Not all the pions are correlated and PARJ(92) and themeasured lambda (obtained by fitting) are different



MC study -Influence of the non-belonging pairs- pC

Q(GeV)

C2 (Q)

Non-belonging pairs = pairs in which, at least one particle is not a pion (based on MC truth).

Is a junk, a kaon, a proton etc...

The junks and the possible pion contamination do not affect our result.

P1=N, P2=P3=R, P4=



MC studies – pC

C2 (Q)

Q(GeV)

P1=N, P2=P3=R, P4=

C input

Radius 2 fm

All range

R(fm)

0.294±0.039

2.014±0.161

To test the procedure

Studies of systematic errorsare necessary but so far it seemsthat the procedure gives the correct results.

The radius used as MC example is bigger than the radius we measure.



C2(Q) - Data

C

C2 (Q)

Q(GeV)

P1=N, P2=P3=R, P4=

C2 (Q)

Q(GeV)W

0.276 ± 0.016R = 1.057±0.056 (fm)

0.293± 0.021R = 1.307±0.076 (fm)



C2(Q) - data

Ti Q(GeV)

C2 (Q)

P1=N, P2=P3=R, P4=

0.253± 0.020R = 1.298±0.010 (fm)



Fit results- Dependence on the fit range

Q<1.2 GeV Q<0.72GeV Q<0.36 GeV

C

12

R(fm)χ2

0.288±0.013

1.003±0.042

106/95

0.275±0.016

1.057±0.058

74/55

0.183±0.035

1.347±0.143

36/25

Ti

48

R(fm)

χ2

0.207±0.014

0.903±0.050

127/95

0.253±0.020

1.298±0.097

68/55

0.223 ±0.032

1.849±0.259

24/25

W

184

R(fm)

χ2

0.305±0.018

1.132±0.055

130/95

0.293±0.021

1.307±0.076

62/55

0.242±0.042

1.525±0.177

35/25

About 1.000.000 events for each sampleWeak dependence, if any, on the target material ( same as Na44- S on S, Ag, Pb).Radius value smaller than the Na44 one, pPb R=2.89±0.30, but pt larger at HERA-B, source size decreases….



LCMS

The spherical source shapejust an approximation, difficult to interpret.

LCMS - the spatial dimension of the source couples to all components - the temporal component couples only to Qt,out.

beam



Multidimensional correlation function

2,

2,

2sidetouttT QQQ

)exp(1 2222 longlongTT QRQRC

)exp(1 222,,

22,

2,2 longlongsidetsidetouttoutt QRQRQRC

2-dim

3-dim



Even more preliminary results 2-dim

0.261±0.002

RT(fm) 0.885±0.034

RL (fm) 1.053±0.040

About 5.5 million pCevents



On-going studies - Kaons

First results (about 18 million pC events) indicate, as it was expected that the BEC radius for kaons is smaller than for pions (about one half).



Plans - Multiplicity dependence

Code ready, results onthe way



Plans - mt dependence of the 3-dim BEC parameters

Code ready, results onthe way

2

22,

221,

2tt

t

pmpmm



Summary

HERA-B is the place to do interesting studies of BEC correlations More results are expected very soon

It still a lot to be done in terms of running over all the data set, careful study of systematic errors, undestanding the results and comparingthem with other experimental results.

BEC correlations were observed both for and KK pairs.Preliminary results, , were presented for both 1-dim BEC parameters (pA - A=C, Ti, W) and two-dim BEC (pC).Studies on-going: 3-dim, multiplicity and mt dependence; KK.



Back-up sildes



HERA-B detector



Gamow correction

To measure the undisturbed BEC we have to substract all other correlation effects from our distributions. For the correlated distributions of charged particles one has to correct for Coulomb interaction by weighting the Q distribution with the inverse Gamow factor.

1)exp(

lG

Qm /2

)(GeVQ

lG



Gamow correction

Before:

λ=0.247± 0.012

R=0.958 ± 0.043 fm

After:

λ=0.288± 0.013

R=1.003± 0.042 fm

P1=N, P2=P3=R, P4=



Cut abs(p1-p2)>0.5 GeV. abs(tx1-tx2)<0.0008 abs(ty1-ty2)<0.0008



Pion selection

p(GeV)medium soft

The identification probability is not constant over the momenta range in the medium selection. In the soft selection is constant for 5<p<40GeV

p(GeV)



Only the pairs with 0<Q<1.2 GeV

We must not worry about momenta > 40 GeV and <2.6 GeV



Purity of the sample –MC

0<Q<1.2 and particles that pass the soft pion selection

Here the truthid of the track is the LUND code of the truth.Tracks for which no MCtruthwas found were considered junk and their truthid was set to 0

We need to look around 2000and in the region whereThe code is smaller than 500



Purity of the sample

1 pion and

1 junktrack

2 junktracks

1 junk trackand

1 pion

Twopions

Junk tracks= tracks without MCGEN correspondent



Purity of the sample

Twoprotons



target all range 5<p<40 GeV

C R(fm)

0.288±0.013

1.003±0.042

0.339±0.020

1.017±0.065

Data

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 1 Raluca Muresan NBI Copenhagen...

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