Measurements of branching ratios of tau-leptons to charged and neutral kaons at OPAL

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PROCEEDINGS SUPPLEMENTS

ELSEVIER Nuclear Physics B (Proc. Suppl.) 40 (1995) 361-370

Measurements of Branching Ratios of Tau-Leptons to Charged and Neutral Kaons at OPAL

Harold Evans a *

a Enrico Fermi Institute, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA

We present recent preliminary measurements of branching ratios of tau-leptons to kaons using 1990--1993 data collected with the OPAL detector at LEP. From samples of approximately 50 000 e+e---+r+r - candidates we find B(r---~K-u~) = 0.594-0.06+0.05%, B(r---~K->lrr°u~) = 0.55=h0.07=h0.07%, B(r---+K->0h°ur) = 1.434-0.08-4-0.08%, B(r---4K~rr->0h°ur) = 0.724-0.074-0.04%, B(~'---~K~K-:>0h°u~) = 0.20+0.04+0.03%, and B(r----~K°X-u~) = 0.884-0.074-0.04%, where the first error is statistical and the second systematic, and where h ° is taken to include rr ° and K ° and X - is any combination of charged and neutral particles with total charge -1. For completeness we also include out published measurement of the inclusive K*- branching ratio using 1990--1992 data, B(r---4K*->0h°u~) = 1.944-0.27-4-0.15%. These measurements are consistent with existing theoretical predictions and also with recent measurements by the ALEPH, CLEO, and DELPHI groups.

1. I N T R O D U C T I O N

With the expanding statistics of tau-lepton da ta sets at LEP and CESR studies of rare decay modes of the tau are becoming feasible. Among these are decays to final states containing one or more strange mesons. Due to lack of events, strange decay modes of the tau, apart f rom decays to charged kaons and K* (892) have not been widely studied up to now. Measurements of many tau decay modes to strange particles, however, have recently been published by the ALPEH [1,2], CLEO [3], DELPHI [4], and OPAL [5] collabora- tions.

On the theoretical side, the branching ratio for the exclusive decay r - - + K - u T has recently been calculated to O(a) [6] in the con- text of the Standard Model. Precise measurements of B ( r - - + K - u ~ ) , therefore, serve as rig- orous tests of the model. Accurate theoretical predictions for the decay ~-----~K*-u~ are also possible using asymptot ic SU(3) flavor symme- try to relate B ( r - - + K * - u r ) to B(r--gp- u~). The r - - - 4 K ° K - u r branching ratio can also be est imated theoretically, al though with limited ac- curacy, by using the Conserved Vector Current

*Address: CERN, PPE Division, CH-1211, Geneva 23, Switzerland.

0920-5632/95/$09.50 © 1995 Elsevier Science B.V. SSDI 0920-5532(95)00162-X

(CVC) hypothesis and various SU(3) assumptions to relate the isovector part of the e+e---+~r+zr - cross section to r - - - 4 K ° K - u r [7]. Other strange decay modes are described by phenomenological models using low-mass resonance dominated form factors [8].

Apar t from testing theoretical predictions of branching ratios, tau decays to kaons are inter- esting in that they must be subtracted from inclusive tau to charged hadron measurements to give estimates of tau to pion decays. Decays in- volving two kaons in the final state could also be used to detect effective second class currents [9] as these are expected to be much less suppressed in the strange sector than in multi-pion final states due to the large u-s quark mass difference. Mea- surements of the tau-neutrino mass can be made with multi-kaxm final states as well because of the large invariant mass of the decay products.

In this note preliminary OPAL measurements of branching ratios of the tau to charged, neutral and two kaon fined states are presented.

2. T H E O P A L D E T E C T O R A N D M O N T E C A R L O

Charged particle tracking and electromagnetic calorimetry in the OPAL detector, which are cru-

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362 H. Evans~Nuclear Physics B (Proc. Suppt) 40 (1995) 361-370

cial to the measurements presented here, are described briefly in this section. Details of the detector as a whole can be found in [10].

The central tracking chambers at OPAL are contained in a 4 bar pressure vessel and immersed in a 0.435 T axial magnetic field. At small ra- dius (<0.235 m) a precision vertex drift chamber provides up to 18 space point measurements - - 12 with axial wires and 6 with small angle stereo wires. Outside the vertex chamber is a 4m long 3.7 m diameter jet chamber with 159 lay- ers of axial wires which is surrounded by drift chambers giving up to 6 precise measurements of the z-coordinates (along the beam direction) of charged particles as they leave the tracking system. A high precision silicon micro-vertex detector is also present with two planes of silicon at radii of 6.1 and 7.5 cm. This detector, however, was not used in the analyses presented here. The momentum resoultion of the entire OPAL tracking system is ((T(pt)/pt) 2 ..~ 0.022 + (0.0015. pt) 2 where Pt is the track momentum in the r-¢ plane (transverse to the beam direction).

The OPAL tracking system is well suited for studies of the decays of taus to strange mesons. The jet chamber has a spatial resolution in r-¢ of 135 pm and a two-hit resolution of--~2.5 ram, allowing high efficiency reconstruction of K ~ r + z r - decay vertices. Note that the typical ~r + zr- opening angle for K ° 's from the decay r - - + K * - - + K ° z r - at LEP ( (P(K°)) = 17 GeV)is 1.50 .

Charged particle energy loss ( d E / d x ) is also measured accurately with the OPAL jet chamber [11]. The d E / d x resolution found in muon- pair events is 3.0% for tracks with hits on all 159 wires (possible in the polar angle range I cos 0[<0.73). This corresponds to a 2-sigma ~r/K separation over the momentum range 1.5-40.5 GeV, which is well matched to the r - - -~K-u~ decay at LEP where kinematics constrain P ( K - ) to be greater than 3.5 GeV. In the dense tracking environment of multihadronic events the resolution is degraded to 3.5%.

Electromagnetic energy is measured in OPAL using a calorimeter consisting of lead-glass blocks and a presampler located outside the magnet coil. In the barrel region each of the 9440 lead-glass

blocks subtends an angular region of --~40×40 mrad 2, is 24.6 radiation lengths deep, and is ar- ranged in an approximately pointing geometry.

Efficiency estimates for the decay modes measured here were obtained using the KORALZ 4.0 Monte Carlo event generator [12] with the TAUOLA 2.5 decay package [13]. Generated events were passed through a GEANT-based simulation of the OPAL detector [14] and were sub- jected to the same analysis chain as the data.

3. B R A N C H I N G R A T I O M E A S U R E - M E N T S

3 .1 . P r e - S e l e c t i o n o f T a u - P a i r E v e n t s Before applying decay-mode specific cuts a

general e+e---+v+r - selection was made. This "pre-selection" is described in detail in [15]. Briefly, events were selected if they contained a pair of back-to-back, narrow jets with low charged particle and electromagnetic cluster mul- tiplicities, suppressing multihadronic background (e+e---+q~l). Background from two-photon processes (e+e- -+(e+e- )X) was removed by requiring a minimum total energy and signifi- cant missing transverse energy in the event. Electron- and muon-pair events (e+e- -+e+e - and e + e - - + p + # - ) were rejected by cuts on the total energy in each jet and its distribution throughout the detector.

These simple topological and total energy cuts resulted in an efficiency for e+e - - ~ v + v - events, estimated using the Monte Carlo described above, of ~93% within the geometrical acceptance of [cos0[<0.68. The total background in the e+e---~v+~ -- pre-selection from non-tau modes was estimated to be 1.8+0.4% using Monte Carlo simulations of each of the processes corrected with data control samples.

3 .2 . r - -+K- M o d e s

One-prong tau decays were selected in the polar angle region [cos0[<0.68 from 95 232 pre- seclected tau candidates in the 1990-1993 data sample by requiring exactly one good track in a tau jet with momentum at least 5% of the beam energy which was inconsistent with being an electron or muon. In order to ensure a good d E / d x

tl. Euans/Nuclear Physics B (Proc. Suppl.) 40 (1995) 361-370 363

measurement the track was required to have at least 100 hits (out of 159 possible) in the jet chamber. The efficiencies for selecting various charged kaon decay modes of the tau using these cuts, es- t imated from Monte Carlo, are given in table 1

The 40 032 one-prong candidates selected using these criteria (the r - - -+h- >0h°u~ data set) were then divided into two mutually exclusive samples depleted and enriched in rr°'s. The v-----~h-u~ sample was defined by requiring that the total energy of the tau jet, measured in the electromagnetic calorimeter, divided by the track momentum (Eje~/P) be less than 0.8, and that the tan jet mass (Mjet) be less than 0.5 GeV. Ejet was defined using tracks and electromagnetic clusters and Mjet included reconstructed rr°'s using the first of the two algorithms described in refer- ence [16]. The complementary r - -+h->lTr° t ,~ sample was defined using only Mjet > 0.5 GeV. The efficiencies of these selections and the size of the events samples are given in table 1.

The fraction of charged kaons in each of the three event samples was found in bins of the track momentum with a maximum likelihood fit to the track's dE/dz. The form of the likelihood function which was minimized to give the best values for the fractions of each particle type, fi(i=e,/t,Tr,K), was

Nevts

C : H E fi exp ( -D~ /2 ) j = l i=e,#,rc,K X/~Cri

D~ = dE/dx- (dE/dz)i (1) o" i

where dE/dz is the measured dE/dx of the track, (dE/dz)~ is the expected dE/dz for particle type i, and cq is the error on (dE/dx)i. In the fit the sum of f~ was constrained to unity.

Because of the precise nature of this measurement small deviations of Di from Gaussian dis- tributions of zero mean and unit width had to be taken onto account. Scaling factors on (dE/dx)i for all particle species, and for kaons alone in each momentum bin of the fit were added as variable parameters. Typical mometum-averaged values found for the global and kaon scaling factors were 0.995 and 0.994 respectively. An additional momentum independent scaling factor applied to cri

was also used. This factor (1.097) was found from studies of f---+#-~--~u, which were also used to verify that Di was consistent with a single Gaus- sian distribution.

An example of the results of the fits (momentum bin 20-25 GeV in the v---+K->0h°u~ sample) is shown if figure 1 where the difference between the measured and expected dE/dx under a pion hypothesis is compared to the prediction of the likelihood fit. All of the fits have acceptable values of X 2 for this comparison.

Entries 4013

x~/v=30. ~/35 P r o b , =

lO

i J i . . . . i , , ,

- ~ - 2 - I 0 2 dE/a. - (dE/dx). (keY/era)

Figure 1. The difference in the measured and expected dE/dz (in keV/cm) under a pion hypothesis (points with errors) in the momentum bin 20-25 GeV of the r - - + K - > 0 h ° u ~ sample compared to the distribution obtained from the results of the likelihood fit (curve). The hatched region corresponds to the kaon contribution to the fit.

Branching ratios for each of the two exclusive modes ( r - ~ K - u ~ and v---+K-_>lrr°u~) were calculated from the kaon fractions fitted in each momentum bin (fg), the number of events in the

364 H. Evans~Nuclear Physics B (Proc. Suppl.) 40 (1995) 361-370

Table 1 Values of the efficiency and number of candidate events for the three selections where the quantities represent a combination over all momentum bins and are not used as such in the analysis. The efficiencies are expressed as a fraction of decays after the barrel tau-pair selection has been applied. Also included are the expected number of v--+K-K°_>0rc°ur events for each selection: NK-K0 .

Quantity " r - --+h- u~ r - --+h- _> 1 ~r ° u~ r - --+h- >Oh ° u~ e(7-- --+K- u¢) 0.72 0.15 0.90 e ( r - --+K- > 17r° uT) 0.06 0.68 0.83 ~(v- --+K- K ° _> 07r° u~) 0.25 0.33 0.72 Size of sample (N) 10156 26907 40032 Kaons (fK N) 523 527 1142 N K - K 0 70 93 200

data samples, and the number of pre-selected tau candidates using an efficiency migration matr ix determined from Monte Carlo which expresses the efficiency for selecting decay mode i with selection j. The branching ratios were then corrected for non-tau contamination in the pre-selected data set and for the presence of v---+K- K°>07r°u~ decays.

The inclusive branching ratio, B ( r - - + K - > O h ° u ~ ) , was calucu- lated in a similar manner, using the efficiency for r---+K-t,~, and was then corrected for the dif- ferences in the inclusive sample selection efficiencies for r---+K->l~r°u~ and r---+K-K°>0~r°u~ decays from r - - -+K- u¢.

The contributions from v--+K-K°_>0~r°u~ to the various modes were subtracted using Monte Carlo to describe the decay kinematics and using measured branching ratios to set the abso- lute number. Preliminary OPAL (see next section) and CLEO [17] results and the published ALEPH [2] number were averaged to obtain the value of B(r-- -+K-K°>0zr°u~)=0.30±0.05% used in the calculations. Estimates of the total number of r - - -~K-K°>0zr°u r decays in all momentum bins are given in table 1.

A breakdown of the sources of systematic error for each measurement and their relative sizes are given in table 2. The largest systematics in these measurements come from uncertainties in Monte Carlo modelling of the electromagnetic calorimeter mostly for the Mje t cuts ("Exclusive Selec- tions"), errors in the estimation of the residual

multihadronic contamination in the final event samples ( "MH Background"), and the experimen- tal uncertainty on B(v--+K-K°_>07r°~,~) which was discussed previously. The systematic due to Mjet modelling was estimated by changing the Mjet cuts within limits dictated by the agreement of data and Monte Carlo and contains a small contribution due to a discrepancy between data and Monte Carlo at Mj~t=0. A conserva- tive error of 100% of the Monte Carlo estimate of the expected number of multihadronic events in the one-prong sample was used to reflect the uncertainty on the multihadronic contamination in the final data samples. It should be noted that the statistical error on the measurements contains a large contribution (---50%) from using dE/dx scaling factors as fit parameters.

Including the systematics listed above, the OPAL measurements for exclusive and inclusive branching ratios of tau-leptons to charged kaons a r e

B ( r - - + K - u r ) = 0.59 + 0.06 + 0.05%

B(r---+K-_>17r°u~) = 0 . 5 5 + 0 . 0 7 ± 0 . 0 7 %

B(r - - -+K->0h°u~) = 1.43 ± 0.08 ± 0.08%.

3.3. v--+K ° M o d e s Tau jets containing K~-mesons were selected

from a total of 107 624 pre-selected tau candidates in the 1990-1993 data sets within the polar angle region [ cos 0[<0.72. K ° candidates were selected by identifying vertices consistent with coming from K°--+~r+Ir - decays using a vertex finding algorithm described in detail in [5] and briefly

1-L Evans~Nuclear Physics B (Proc. Suppl.) 40 (1995) 361-370 365

Table 2 Relative systematic errors (in %) on the charged K branching ratio measurements.

(%) Source r - --+K- Vr r - --+K- > 17r°vr v- --+K- _>OhOv~-

Exclusive Selections 5.6 MH Background 4.2

dE /dx Resolution 2.2 e+e--+7"+T - and 1-prong Selection 1.5

Efficiency (MC stats) 0.7 B(r---+K-K°>_OTr°v~) 4.4

B( r - - -+K- > Dr°ur)

10.0 1.8

4.5 3.6 5.5 2.7 1.8 1.6 0.9 0.5 5.1 0.6 - - 0.8

T o t a l 8.8 12.9 5.2 S t a t i s t i c a l 9.7 12.9 5.2

summarized in the following. Vertices were found by looking for intersections

in the r-¢ plane of two oppositely charged tracks. Only high quality tracks, with P, greater than 150 MeV, were considered in the vertex finding algorithm. Track crossings consistent with coming from photon conversions were rejected using the measured d E / d x of the tracks. Good quality vertices from the decay of neutral hadrons were selected by cuts on the sum of the radial distances of each track from the primary vertex, the separation of the tracks in z at the intersection point, and the angle between the momentum vector of the neutral particle which decayed to form the vertex and its flight path (a vector from the primary to secondary vertex positions). In addition, vertex candidate were only considered between the radii of 1 cm and 150 cm.

Using this algorithm, 5 578 vertex candidates (with conversions removed) were found in the pre- selected e+e---+r+7 - events. Residual contamination of the vertex events by multihadrons and conversions was then reduced further by demand- ing that the invariant mass of the tracks (assumed to be pions) in each hemisphere (defined with respect to the vertex candidate momentum vector) be less than 2.0 GeV, and that the invariant mass of the two tracks in the vertex under an electron assumption be greater than 150 MeV. Using these background subtracted vertex events several final data samples were defined from which B ( r - - - ~ K ° X - v r ) , B( r - - -+K%r->0h°v~) , B ( r - - + K ~ K - _>0h°v~), and B ( r - --+g*- _>0h°v~)

were measured. The numbers of events and the estimated signals as well as efficiencies for each of the selections are shown in table 3

The inclusive K ° sample, r---+K°sX-ur, was selected by identifying good K°--+lr+lr - candidates in the vertex sample. These were defined as vertices with an angle between the vertex momentum and flight path vectors less than 4 mrad and with the invariant mass of the two vertex tracks (assumed to be pions) in the window 475- 520 MeV. Extra track quality cuts were also applied to the vertex tracks - - the track transverse momentum was required to be less than 25 GeV, and more than 3 (out of a possible 6) z-chamber hits were demanded on each track.

The distribution of invariant masses of the two vertex tracks, Mv(Tr+~--), for all the cuts above except the mass window cut is shown in figure 2. A clear K°-+~r%r - signal is apparent over a small background from mis-identified vertices mostly coming from random crossings of oppositely charged tracks. This mis-identification background was estimated using sidebands in the regions 400-450 MeV and 550-600 MeV to be 10.04-1.5% of the 243 events in the signal region.

In order to calculate B( r - - -+K~X-ur ) the inclusive K~ data sample was divided into bins in the K°-candidate momentum. This reduced the dependence of the results on the assumed K ° momentum spectrum to a negligable level. Background subtraction in each of the bins was performed as described above. The efficiencies of the selection for r - - - + K ° X - v r events were

366 H. Evans~Nuclear Physics B (Proc. Suppl.) 40 (1995) 361 370

Table 3 The total numbers of events in the various r -~K ° samples and the total numbers of signal events estimated from sideband subtraction or fits as described in the text summed over all momentum bins. Also shown are the branching ratio weighted sums of exclusive efficiencies, averaged over momentum, for the inclusive decay modes to pass the selections.

Source v---+K°sX-v~ ~---+K~Tr-_>0h0v~ v:-cK~K-_>0h0u~ r--+K*-_>0h0u~ N(sample) 243 191 191 120

N(signal) 2194-4 138=t=13 374-7 744-8 Ave. Efficiency (%) 20.9 17.9 16.7 6.2

calculated from Monte Carlo as the branching ratio weighted sum of the efficiencies for the modes r----~K*-ur--+K°Tr-v~, r--+K%r-~r0vT, 7---+K°K -vT, r - ~ K ° K -Tr°u~. Published ALEPH measurements of these branching ratios [2] were used in the calculations and contributions from decay modes containing two neutral kaons were neglected as they are expected to be small. Values for the total number of background subtracted signal events and for the momentum averaged inclusive efficiency in the r - -+K ° X- v~ sample are given in table 3.

Sources of systematic error in this measurement are listed in table 4. The major components are uncertainties in estimating the efficiency of the vertex finder ("Vertex Finder Eft."), uncertainties related to signal and background estimation ("SignM Estimation"), and the statistics of the Monte Carlo data samples used. Comparisons between data and Monte Carlo for K~-+~r+~r - and A --+pTr- decays in multihadronic events were used to set a limit on the uncertainty of the vertex finder efficiency. Systematic effects due to the method of calculating signal and background were estimated by changing the signal and sideband mass windows and by using a fit of a Gaussian plus linear background to the Mv(Tr+~r - ) distribution to estimate the signal.

Including all of the systematics listed above, the preliminary OPAL measurements for the inclusive branching ratio of tau-leptons to K °- mesons is

B ( r - - + K ° X - u ~ ) = 0 . 8 8 + 0 . 0 7 + 0 . 0 4 % .

Branching ratios of taus to K~ plus charged pions and charged kaons were measured using an inclusive r - - - + K ° h - > 0 h ° v r data set defined by

requiring, beyond the criteria used to select the inclusive K ° sample, that there be exactly one good quality track, besides the vertex tracks, in the same hemisphere as the vector defined by the K ° momentum. To be counted as "good quality", tracks were required to come from the primary vertex, to have more than 3 z-chamber hits, and to have more than 80 hits in the jet chamber of which at least 40 had to be of sufficient quality to be used in dE/dx measurements.

To determine the fraction of pions and kaons in these "third" tracks a fit, of the same type as was used in the charged kaon analyses, to the measured dE/dx of the tracks was performed in bins of the third track momentum. Variable parameters in the fit were the fractions of pions, kaons, and electrons (constrained to sum to unity). Mo- mentum dependent global and kaon dE/dx scaling factors were fixed at the values found in the r - - -+K->0h°v~ analysis while the momentum independent dE/dx error scaling factor was found from studies of of K~-+r+~r - decays in tau events to be 1.082.

The total number of signal events found using these fits is shown in table 3. Also shown are the mometum averaged efficiencies for inclusive K~Tr- and K°K - events to be included in the v--+K~h-_>0h°v~ selection. As in the r - - -+K°X-v~ selection, these efficiencies are the branching ratio weighted sum of the relevant exclusive modes using ALEPH measurements [2].

The sources of systematic error in these measurements are presented in table 4. The main systematic, aside from those discussed previously, comes from uncertainties in the dE/dx parame- terization used in the fits. An upper limit on its size was found by repeating the fits with all scal-

11. Evans~Nuclear Physics B (Proc. Suppl.) 40 (1995) 361-370 367

Table 4 Relative systematic errors (in %) on the K~ branching ratio measurements.

(%) Source r--+K} >0h-°u >_0h°ur r - - -+K*- E0h°uT

Vertex Finder Eft. 3.4 3.4 3.4 3.0 dE/dx param. - - 2.5 13.1 - -

Signal Est imation 2.5 2.5 2.5 4.8 MC Statistics 1.7 1.7 7.0 1.9

Tracking 1.0 1.0 1.0 1.3 MH Rejection 0.8 1.0 1.0 3.9

Excl. Bs 0.4 1.2 1.2 - - Conversions 0.6 0.6 0.6 1.5

Non-tau Bgrd 0.5 0.5 0.5 0.5 P-Modelling - - - - - - 1.3

T o t a l 4.8 5.5 15.5 7.6 St a t i s t i ca l 8 .0 9.3 18.6 13.9

ing factors set equal to one. Preliminary OPAL measurements for the inclu-

sive branching ratios of tau-leptons to K ° and either ~r- or K - are

B(r--+K°r->_Oh°u~) = 0.724-0.074-0.04%

B(v - - -+K°K -_>0h°u~) = 0.20 4, 0.04 4- 0.03%.

Finally, OPAL has recently published results on the inclusive braching ratio of taus to charged K*(892) [5]. For these measurements a v - - + K ~ h - > 0 h ° u ~ data set with the same cuts as described above, but using only the 1990-1992 data, was used. Events containing K*--mesons were separated from mis-identified vertex background and r -+K ° decays where the K ° did not come from the decay of a K*- using a fit to the K°h - invariant mass distribution. The K°h - mass was calculated assuming that the h - was a ~r- and constraining the vertex mass to be equal to M(K°) . The fit function used was com- posed of a spin-1 Breit-Wigner signal with K* parameters plus a background term parameter- ized as a second order polynomial. The fit in- dicated that 61.94,6.4% of the 120 events in the r - -+K~h-_>0h°v~ sample were associated with K*- production.

The number of fitted signal events and the efficiency of the selection for r--+K*-uT-.+K°r~-uT events are shown in table 3. Systematic errors are again summarized in table 4, the main source

of which comes from the K*- signal est imation method. This error was est imated by comparing the results obtained using a sideband subtraction technique to find the K*- signal with the standard results. The published result [5] for the inclusive branching ratio of tau-leptons to charged K*(892) is

B(v- - -+K*->0h°u~) = 1.944,0.274, 0.15%.

4. D I S C U S S I O N

In conclusion, we have made preliminary measurements of the following branching ratios of tau decays to strange mesons using data collected with the OPAL detector during the 1990-1993 r u n s

B(r--+K-u,) B ( r - - ~ K - _ > l ~ ° u r )

B ( r - - + K - >0h° u~)

B(r- - -~K~K-_>0h°v~)

= 0.59 + 0.06 4- 0.05%

= 0 .55+ 0 .07+ 0.07%

= 1.43 4- 0.08 + 0.08%

= 0 .72± 0 .07+ 0.04%

: 0.20 + 0.04 + 0.03%.

= 0.88 4- 0.07 4- 0.04%

Using 1990-1992 data we have published a measurement of the inclusive branching ratio of tau to charged K* (892)

B ( v - - + K * - > 0 h ° p ~ ) = 1 . 9 4 + 0 . 2 7 + 0 . 1 5 %

368 14. Evans~Nuclear Physics B (Proc. Suppl.) 40 (1995) 361-370

F I I I q I I ( $

9o I , O P A L

N 7 0

411

20 i i I ~ I]

°0.3 035 o.4 o.45 015 0.55 0.6 0.65 0.7 0.75 o.s M(~) W.ev)

Figure 2. The 7r+~r - invariant mass distribution of the vertex tracks in the inclusive K~ data sample showing the 1990-1993 data (points), the Gaussian plus linear background fit to the data used for systematic studies (dashed line), and the prediction of a MC sample containing all r decay modes (solid histogram). Also shown are the peak and sideband regions used for signal and background estimation.

where the first errors above are statistical and the second systematic.

All of these measurements are in generally good agreement with other recent experimen- tal results [1-5,17] although our branching ratios for the K ° modes tend to be higher than other measurements. The r---+K-u~ branching ratio also agrees reasonably well (within two sigma) with the Standard Model expec- tation [6] of 0.737+0.005%. Finally, our measurements B(r---+K°K -_>0h°u~) are signif- icantly larger than the CVC-based prediction for B(r - -+K°K-u~) of 0.06+0.02% [7] suggesting the presence of v--+K°K - lr°u~ decays at a larger level than previously expected.

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15. OPAL Collaboration, G.Alexander, et al., Phys. Lett. B266 (1991) 201.

16. OPAL Collaboration, R. Akers et al., Phys. Lett. B328 (1994) 207.

17. CLEO Collaboration, Contributed Paper, ICHEP Glasgow (1994), preliminary result.

H. Evans~Nuclear Physics B (Proc. Suppl.) 40 (1995) 361-370 369

Discussion

G. Rolandi, CERN Do you see sy s t ema t i c effects in dE/dx in the 1-versus 3- t rack env i ronmen t?

H. Eyan8 No, compar ing the scal ing factors of the (dE/dx)expected we see they are consis tent in the 1- and 3-prong env i ronment .

M. Davier, LAL Orsay W h a t is the ac tua l K-Tr s epa ra t i on in O P A L over the m o m e n t u m range?

H. Evans We have 2or K-~r s epa ra t i on over the m o m e n t u m range ~ 1.5-25 GeV and 1.5c~ sepa ra t i on to 45 GeV.

J. Smith, Colorado Your B ( K ° K - ) for C L E O should also be d iv ided by two since CLEO quotes B ( K ° K - ) not B ( K , K - ) . B = 0.12 + 0.02 -t- 0.02 ::¢, B(KsKuT) 0.06 4- 0.01 4- 0.01.

21. Eva?t8 In my plot , I add the K ° K - and the I£°K-Tc ° branch ing ra t ios , which at C L E O are bo th s imilar . The ' C V C ' p red ic t ion is only for K ° K - . The C L E O exclusive K ° K - is consis tent wi th the ' C V C ' pred ic t ion , and the inclusive ( summed) K ° K - _> 0~r ° is consis tent wi th the O P A L measu remen t .

Measurements of branching ratios of tau-leptons to charged and neutral kaons at OPAL

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