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Measurements of cross sections and charge asymmetries for e+e−→τ+τ−and e+e−→μ+μ−fors from 52 to 57 GeV
Volume 218, number I
PHYSICS LETTERS B
9 February 1989
M E A S U R E M E N T S OF CROSS SECTIONS A N D CHARGE ASYMMETRIES FOR e÷e--,'~+~ - A N D e÷e--~It+lt-FOR v/s F R O M 52 TO 57 GeV
AMY Collaboration A. BACALA a R.L. M A L C H O W b, K. SPARKS b,1, R. IMLAY a p. K I R K a R.R. McNEIL a, W. M E T C A L F a, Winston KO b, R.L. L A N D E R b, K. MAESHIMA b, J.R. SMITH b, M.C.S. WILLIAMS b, C.P. C H E N G c j. LI c, Y.K. LI c Z.P. MAO c, y . YAN ~, Y.T. XU c, Y.C. Z H U c A. ABASHIAN d, K. G O T O W d, F. KAJINO d, E. LOW d, F. NAITO d, L. PIILONEN d, R. CHILDERS e, C. D A R D E N e, S. LUSIN e, C. ROSENFELD e, A. WANG e, S. WILSON e, M. FRAUTSCHI f, H. KAGAN r, R. KASS f, C.G. T R A H E R N f, K. ABE g, S. CHAKRABARTI g, Y. FUJII g, Y. HIGASHI ~, Y. K U R I H A R A g, A. MAKI g, T. NOZAKI g, T. OMORI g, P. PEREZ g, H. SAGAWA g, Y. SAKAI g, Y. S U G I M O T O g, Y. TAKAIWA g, S. TERADA g, K. TSUCHIYA g, R. P O L I N G h, j. GREEN i, I.H. PARK i, S. SAKAMOTO i, F. SANNES i, S. S C H N E T Z E R i, R. STONE i, S. T R E N T A L A N G E i, D. Z I M M E R M A N i, K. MIYANO J, H. MIYATA J, M. OGAWA J, Y. YAMASHITA k, D. BLANIS ~, A. B O D E K ~, H. B U D D ~, R. COOMBES ~, S. ENO ~, C.A. FRY ~, H. HARADA ~, Y.H. HO ~, Y.K. KIM ~, T. K U M I T A ~, T. MORI ~, S.L. OLSEN ~.m, N.M. SHAW ~, A. SILL ~, E.H. T H O R N D I K E ~, K. U E N O ~, H.W. Z H E N G ~, H. ASAKURA n, K. E G U C H I n, H. I T O H n, S. KOBAYASHI n, A. M U R A K A M I ", K. T O Y O S H I M A n, J.S. K A N G o, H.J. KIM o, S.K. KIM o, M.H. LEE o, S.S. M Y U N G o, E.J. KIM P, G.N. KIM P, D. SON P, H. K O Z U K A q, S. M A T S U M O T O q, T. SASAKI q, T. TAKEDA q, R. TANAKA q, R. CHIBA r, K. HANAOKA r, S. IGARASHI r, H. MURATA r, H. Y O K O T A r, y . ISHI 5, T. ISHIZUKA ~and K. OHTA a b c d
Louisiana State University, Baton Rouge, LA 70803, USA UniversityofCalifornia, Davis, Davis, CA 95616, USA Institute for High Energy Physics, Beijing, P.R. China Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA University of South Carolina, Columbia, SC 29208, USA f Ohio State University, Columbus, OH 43210, USA g KEK, National Laboratory for High Energy Physics, lbaraki 305, Japan h University of Minnesota, Minneapolis, M N 55455, USA Rutgers University, New Brunswick, NJ 08854, USA J Niigata University, Niigata 950-21, Japan k Nihon Dental College, Niigata 951, Japan University of Rochester, Rochester, N Y 14627, USA m Tsukuba University, Ibaraki 305, Japan Saga University, Saga 840, Japan ° Korea University, Seoul 132, Korea o Kyungpook National University, Taegu 635, Korea q Chuo University, Tokyo 112, Japan Tokyo Institute of Technology, Tokyo 152, Japan Saitama University, Urawa 338, Japan
Received 14 November 1988
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0370-2693/89/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 218, number l
PHYSICS LETTERS B
9 February 1989
Measurements of the differential cross sections for e+e---*~t+~t- and e+e-~z+z - at values of ~/s from 52 to 57 GeV are reported. The forward-backward asymmetries and the total cross sections for these reactions are found to be in agreement with predictions of the standard model of the electro-weak interactions. These measurements are used to extract values of the weakcoupling constants g~g~vand g~Ag~,where £= ~t or z.
Since the leptonic reactions e + e - ~ t + p . - and e+e ---,z+z - have only point-like particles in both the initial and final state, they provide sensitive and unambiguous tests of the standard SU (2) X U ( 1 ) theory o f the electro-weak interactions [ 1 ]. The TRIST A N electron-positron storage ring at the National Laboratory for High Energy Physics in Japan ( K E K ) produces e+e - collisions at CM energies in the 5060 GeV range, where the model predicts a large forward-backward charge asymmetry for these processes. This is caused primarily by the interference between the electromagnetic and weak amplitudes. Previous measurements at PEP [2] and P E T R A [ 3 ] are in good agreement with the predictions of the standard model. The first report o f this measurement at T R I S T A N was published recently by the T O P A Z Collaboration [4], using 6.5 pb -~ of data collected at x / s = 5 2 and 55 GeV. This report describes measurements of this asymmetry and the total cross section for the reactions e +e---,g+~t- and e+e - ~ x + x using 17.7 p b - ~of data at CM energies from 52 to 57 GeV collected by the A M Y detector at TRISTAN. The A M Y detector is a compact detector based on a 3 Tesla solenoid and optimized for lepton detection. Electron identification is provided by a highly segmented shower counter located inside the coil. Muon identification is provided by an absorber with a thickness that is equivalent, on average, to 1.65 m of iron. The detector has been described in detail elsewhere [5,6], only those features central to the analysis presented here will be discussed. Charged-particle tracking is done by a four layer, 576 cell tube-type inner tracking chamber ( I T C ) o f inner radius 13 cm followed by a cylindrical drift chamber ( C D C ) with 9048 cells arranged in 25 axial and 15 small-angle stereo layers that extends to an outer radius of 67 cm. In the event reconstruction, a charged track in the C D C is required to have at least Present address University of Rochester, Rochester, NY 14627. USA
eight axial hits and five stereo hits that provide a good three-dimensional fit to a helix. Transverse momenta are determined with a precision ap,/pt -~ [ 0 . 7 % ( G e V ) / c ) - ~ ]Pt. Outside o f the CDC, starting at a radius of 79 cm, is a 15 radiation-length cylindrical electromagnetic calorimeter (SHC) that covers the region Icos01 ~<0.73. The SHC is constructed from twenty alternating layers of lead and gas proportional tubes. The energy resolution is aE/ E~_23%GeVW2/x/-E+6%. Outside of the iron flux return for the magnet is a m u o n identification system consisting of two orthogonal double-layer planar drift chambers (containing a total of l 184 drift cells) followed by scintillation counters to provide timing information (to within 3 ns). The muon detector covers the region Icos 01 ~<0.74. A m i n i m u m transverse m o m e n t u m of 1.9 G e V / c is needed to penetrate the hadron absorber; the penetration efficiency reaches 100% for muons with p,/> 2.5 GeV/c. The detector is triggered by energy deposition in the barrel or end-cap calorimeters, and by a variety of track patterns in the ITC, C D C and the outermost four layers of the SHC. Triggering of~t+~t - events depends exclusively on the sensitivity o f these track patterns to two-track topologies, and several different criteria were implemented. For the ITC and C D C triggers, the efficiency was determined using Bhabha scattering events (e+e - --,e+e - ), which are independently triggered by SHC energy triggers. For the outer layers of the SHC, the efficiency was measured using e+e ---,~t+~t - events that triggered the detector independently o f the SHC. Triggering of x+z - events is accomplished by a variety o f charged-track and energy triggers. The efficiency o f the charged-track triggers was estimated by analyzing those events in the final sample that were triggered by energy triggers. Similarly, the efficiency of SHC total energy triggers were estimated using the sample of events satisfying charged-track triggers. The ~t+~t- and ~+z- triggers were enabled at the 113
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beam crossing time, which occurs every 5 ~ts (Beam Gate), and for equal time intervals starting midway between beam crossings (Cosmic Ray Gate). The triggers in the Cosmic Ray Gate were caused by cosmic rays traversing the detector and provided a direct measurement of the level of cosmic ray induced background that survives each stage o f the analysis. The topology of ~t+la - events consists o f two backto-back tracks in the central detector with m o m e n t a consistent with beam energy, shower counter energy deposition consistent with m i n i m u m ionizing particles, and hits in the m u o n chambers consistent with the observed tracks. All events with at least two reconstructed tracks were considered. Events were required to have at least two tracks that: (i) pass within ___0.3 cm of the interaction point in the plane transverse to the beam direction (l~r~l ~<0.3 cm) and within + 3.0 cm of the interaction point along the beam direction (I~1 < 3 . 0 cm); (ii) have a momentum greater than 0.2 ECM with at least one above 0.25 ECM; (iii) are included in the solid angle of the muon identification system ( Icos 01 ~<0.72 ); and (iv) are within 10 ° of being back-to-back collinear. These requirements reduce cosmic ray backgrounds while selecting m u o n pair and Bhabha scattering events with high efficiency. They also eliminate most of the events due to tau pair and two-photon initiated e+e-~t+~t - final states where only the muons are detected. N o requirement was placed upon the charges of the tracks. Bhabha events were eliminated by requiring the total energy deposited in the shower counter to be less than 0.1 ECM. This is well above the energy deposited by two m i n i m u m ionizing particles ( ~ 800 MeV) and entirely eliminates the Bhabha scattering events that pass the previous cuts. At this point, only events with two charged tracks remained in the sample. We required that both tracks point to a muon detector sextant in which there were hits in at least three of the four m u o n drift chamber layers and that at least one of the two such sextants had a valid hit in a corresponding muon scintillation counter. The counter time was required to be in the interval between 0 and 35 ns o f the beam crossing. For those cases where two counter hits were recorded ( ~ 90% of the final event sample) the time difference between the two hit counters was required to be in the interval - 14 ns to 114
9 February 1989
+ 20 ns. (The time difference for cosmic rays ranges from - 2 1 ns to - 3 0 ns.) The overall efficiency o f our selection criteria (including the geometrical acceptance) for the muon pair reaction was estimated from a full Monte Carlo simulation of the detector. In this simulation, we assigned an efficiency to each of the individual m u o n detector elements as was determined from an analysis of acollinear muon pairs from the reaction e + e - ~ e + e - I ~ + l l - and from cosmic rays; the efficiencies of the individual chamber and counter elements were typically ~ 95%. By requiring only three of four chamber planes for each muon and only one of the two counter planes for the pair of tracks, the overall detection efficiency o f the muon identification system was kept greater than 97%. The efficiency of event reconstruction, including selection cuts, was greater than 94% for Icos 01 ~<0.65. Beyond this, it dropped smoothly to zero by I cos 0l ~0.74. Thus the overall reconstruction efficiency, including geometrical acceptance, was 59_+1.5%. The efficiency for triggering within our acceptance was determined to be 9 4 + 2.6%. Seven events which occurred during the Cosmic Ray Gate passed the final cuts, indicating a cosmic ray background of 2.3_+0.8%. For two events, the track fitting program assigned the same charge to both tracks. These events were used for the total cross section determination but not used in the determination of the forward-backward asymmetry. We estimate the level of backgrounds due to Bhabha scattering by examining the probability that a track from a selected Bhabha event matches a hit in a muon chamber. The results show that the probability of both tracks from the Bhabha event matching corresponding hits in the m u o n chambers is much less than 0.1% and is thus negligible. The backgrounds due to e+e - ~ + x - and e + e - ~ e + e - ~ t + ~ - events are obtained from Monte Carlo simulations of these processes. The results indicate that the backgrounds due to these two processes are each less than 0.2% and are thus negligible. The branching ratio for x decay into a single charged particle is ~ 83%; that for the three charged particles is ~ 17%. Thus, about two-thirds of the e+e - ~ z + x events have two almost back-to-back charged tracks, with most of the rest having a topology of one versus three charged tracks. We accept both topologies but reject those two-track candidates where both tracks
Volume 218, number 1
PHYSICS LETTERS B
are identified as m u o n s or where both are identified as electrons. Cosmic rays and e + e - - , e + e - ~ t + ~ t events are large backgrounds to the sample where both taus decay to muons; Bhabha events and e+e---,e+e-e+e events c o n t a m i n a t e the sample where both taus decay to electrons. In a d d i t i o n to Bhabha scatters and e + e - - ~ e + e - e + e - , i m p o r t a n t sources o f background for the accepted candidates are expected to be two-photon initiated tau pairs (e+e - - - . e + e - ~ + T - ), radiative Bhabha scatters, a n d two-photon multihadronic events. Backgrounds from Bhabha event are o f particular concern, since a small a m o u n t o f c o n t a m i n a t i o n would have an appreciable effect on the m e a s u r e d asymmetry. We searched for tau pair events from a m o n g all o f the events that had two or more reconstructed charged particle tracks with [cos 0[ ~<0.73, the angular region where there is reliable electron and m u o n identification. In the events with only two tracks, we required each track to have [~rol 4 0 . 5 cm and 1~-1 ~<3.0 cm; for events with three or m o r e accepted tracks, this req u i r e m e n t was loosened to lOto[ ~<2.0 cm; [~-1~< 5.0 cm. Events were then selected if they had either two charged tracks or one track recoiling against a cluster o f charged tracks, where a cluster is defined to be a tightly collimated group o f two or more charged tracks with a total invariant mass that is less than 2 G e V / c 2. Only tracks satisfying the [cos 0[ a n d the vertex requirements were included; others were simply ignored. We also required the total visible energy, Eves, to be greater than 0.2 EcM for both event topologies. In the case o f events that c o n t a i n e d clusters, we define the direction o f the cluster to be the vector sum o f the charged tracks that c o m p r i s e it. Tracks were required to be within 45 ° o f the axis o f the cluster in order to be included. Events were required to have opposing tracks or clusters with an opening angle greater than 160 °. This r e q u i r e m e n t rejects two-photon initiated events. We also required all oppositely charged pairs o f tracks within any cluster to have an invariant mass o f Moa~r>~150 M e V / c 2, where each track is assumed to be an electron. This cut eliminates conversions from r a d i a t e d photons. F o r two-prong events, we rejected events in which the energy o f the m a x i m u m shower in the electromagnetic c a l o r i m e t e r exceeds 0.45 EcM. We further rejected events with total shower energy greater than 0.8 EcM, or with m o r e than 0.6 EcM d e p o s i t e d in the T M
9 February 1989
shower counter by charged tracks. If a charged track o f m o m e n t u m p deposited an a m o u n t o f energy in the calorimeter which was greater than 0.4.0, the track was called as electron. I f both tracks in the event were identified as electrons, the event was not kept. These cuts reject Bhabha events and radiative Bhabha events. Backgrounds from e+e - - - , e + e T+~ - were rej e c t e d by requiring the observed longitudinal mom e n t u m [P--I ~<0.4 Evis. The efficiency and acceptance for the z+x - selection was estimated from a sample o f Monte Carlo simulated events to be 24.8 + 3.5%. (This estimate includes the branching ratios o f the accepted decay modes. ) The trigger efficiency for these events is est i m a t e d to be 96.0_+3.0%. Backgrounds were estim a t e d from simulated Bhabha events, two-photon initiated tau pair, electron pair, a n d m u l t i h a d r o n i c events as well as m u l t i h a d r o n i c annihilation events. The b a c k g r o u n d from two-photon m u l t i h a d r o n i c p r o d u c t i o n is estimated to be 0.5%, while that from Bhabha and radiative Bhabha events is found to be 1.1_+0.8%. T w o - p h o t o n electron pair events are found to comprise less than 2.3 + 1.0% o f the final event sample. The contribution from two-photon tau pairs and m u l t i h a d r o n i c annihilation events was found to be negligible. None o f the events in the final tau pair sample originated during the Cosmic R a y G a t e indicating a cosmic ray background that is less than 1.5%. The results for the total cross section for the g+~tand z+x - processes at each center-of-mass energy are summarized in table 1. The total cross section is given by N a = E(1 + ~ R c ) q L '
(1)
where N i s the observed n u m b e r o f events after selection cuts have been applied; L is the integrated luminosity; t/is the trigger efficiency within our acceptance; ( 1 + fiRC) is the radiative correction [ 7,8 ] ~'2' and c is the detection efficiency. This detection effi*~ Ref. [7] gives a complete description of the Monte Carlo technique for determining radiative corrections. However, due to the increased importance of terms involving the Zo at TRISTAN energies the Monte Carlo actually used for our analysis is that ofref. [8 ]. ,2 The difference in the radiative corrections between la+~t- and x+x- is due to the different selection criteria in the two cases. 115
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ciency includes the geometric acceptance as well as various instrumental and software efficencies. It is calculated by a combination of Monte Carlo simulation and analysis of calibration data. Since a statistical subtraction must be made for the fraction of background events remaining after the selection cuts have been applied, e also contains a factor for this correction. The integrated luminosity was determined from the number of Bhabha scattering events detected in the endcap calorimeters, as described in ref. [ 5 ]. The uncertainty in the total cross section measurement is dominated by the statistical error. However, it does include a systematic error of ~ 5%. The main contributions to this error are the systematic uncertainty in the luminosity measurement and the uncertainty in the computer modeling of the various efficiencies and backgrounds. These modeling uncertainties have been estimated from the change in the result when the parameters of the model are varied within acceptable limits. Included in table 1 is the lowest order prediction of the standard model. Our results are consistent with these theoretical expectations. The differential cross section for each of the two reactions was obtained in the same way as the total cross section except that the acceptance, trigger efficiency and radiative corrections were done on a binby-bin basis in cos 0. In the case of the e+e - ~ z + z - reaction, the presence of unobserved neutrinos in the final state made the determination of the original direction of the tau inexact. The vector sum of charged and neutral products was computed for each hemisphere of each event. Only photons within a cone of 30 ° around the charged-track axis were included. All charged tracks in a cluster were required to be within a cone of angle 30 °. In both the ~t+~t- and the x+~- samples, the vector difference of the three-momenta of the two sides was then used as the estimate for the direction of the production angle. In the case of the taus, the error in the production angle estimate was determined from the simulated events to be less than 2°. For the dimuons the resolution in the production angle was much better than 1 ° The differential cross sections for both reactions for the combined data from all energies are shown in fig. 1. We extract the forward-backward charge asymmetry by fitting the angular distributions, which have 116
9 February 1989
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Volume 218, number 1
PHYSICS LETTERS B
(~) A ~ : - 0 3 3 ± 0 08 3t
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9 February 1989
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T h e d o t t e d c u r v e s are t h e s t a n d a r d m o d e l p r e d i c t i o n s .
been adjusted for radiative corrections, at each en, ergy to the function ( ~ = g or z) do
N M
~4 ~4
O~2
dr2 - 4s [Re~( 1 +cos20) + B ~ c o s 0] ,
d d d d d +1 +1 +1 +1 +1 d d d d d ' ~ d d d d d 'g +1 +1 +l +1 +1
(2)
g~NN ~4
with both R~e and B~ allowed to vary. Here, R~ is the ratio of the total cross section for e+e - ~ + t ~ - to that expected from QED alone, and the forward-backward asymmetry A~ is given by (B~/R~). The radiative corrections are calculated to order c~3 in the full electroweak theory. They yield a positive contribution of ~ 4% to the asymmetry in the region Icos01 ~<0.74. Our measurements o f A~ and R~ are summarized in table 2. Again the uncertainties are dominated by the statistical errors. The Re~ measurement also has a systematic error of the same size and from the same sources as the total cross section measurement. The systematic error in the Ae~ measurement is also ~ 5%. It is dominated by the uncertainty in the bin-to-bin variation in the efficiency and radiative correction calculations and in the case of the taus by the uncertainty in the production angle as discussed above. The weighted averages of the measured asymmetries are shown, together with results from previous measurements [ 2 - 4 ] in fig. 2. The experimental results show a large increase in the forward-backward asymmetry between the previously highest available energies and TRISTAN. In the lowest order electroweak model [ 9 ], Ree and A~e can be expressed in terms o f the vector and axialvector weak neutral coupling constants gv and gA, and the Z ° contribution Z, as
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117
PHYSICS LETTERS B
Volume 218, number 1 . . . .
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A~ = 6g~g~z,
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where s
(5)
Z = 16 sinZ0w cos20w ( s - M ~ ) " We used eq. ( 4 ) to d e t e r m i n e the axial-vector couplings, which were then used in eq. ( 3 ) together with the m e a s u r e d values o f R~, to extract the vector couplings. In this procedure, we used sin20w=0.23 for the Weinberg angle and M z = 9 2 . 5 G e V / c 2 for the weak neutral boson. T h e results for the neutral weak coupling constants are presented in table 2. The theoretical predictions for the asymmetries, using the values for sin20w and M z given above, are i n c l u d e d in fig. 2. The m e a s u r e d asymmetries for m u o n s a n d taus are in good agreement with the electroweak predictions. Consequently, the extracted values o f the product g~g~ also agree with the model. The large relative errors on the extracted values o f the p r o d u c t g~eg~, prevent a meaningful c o m p a r i s o n 118
with the theory. However, these values are in general agreement with the s t a n d a r d m o d e l predictions. The A M Y experiment at the T R I S T A N storage ring has observed m u o n pair and tau pair p r o d u c t i o n at center-of-mass energies from 52 to 57 GeV. The total and differential cross sections for both processes are in agreement with the predictions of the s t a n d a r d m o d e l o f electroweak interactions. The angular distributions show significant negative asymmetries, reflecting the interference between the electromagnetic and weak neutral currents. The m e a s u r e d values o f these f o r w a r d - b a c k w a r d a s y m m e t r i e s are in agreem e n t with the lowest order predictions o f this model. We t h a n k the T R I S T A N staff for the excellent operation o f the storage ring. In a d d i t i o n we acknowl"edge the strong support a n d enthusiastic assistance p r o v i d e d by the staffs of our h o m e institutions. This work has been s u p p o r t e d by the J a p a n Ministry o f Education, Science a n d Culture ( M o n b u s h o ) , The US D e p a r t m e n t o f Energy and N a t i o n a l Science F o u n d a t i o n , the K o r e a n Science and Engineering F o u n d a t i o n and Ministry o f Education, and the Acad e m i a Sinica o f the People's Republic o f China. References
R ~ = 1 + 8 "eSv6vz "~ "
1
9 February1989
[ l ] S.L. Glashow, Nucl. Phys. 22 ( 1961 ) 579; S. Weinberg, Phys. Rev. Lett. 19 (1967) 1264; A. Salam, in: Proc. 8th Nobel Symp. (Aspenasgarden, 1968) (Almqvist and Wiksell, Stockholm) p. 367. [ 21 HRS Collab., M. Derrick et al., Phys. Rev. D 31 ( 1985 ) 2352; MAC Collab., W. Ash et al., Phys. Rev. Lett. 55 (1985) 1831; MARK-II Collab., M.E. Levi et al., Phys. Rev. Len. 51 ( 1983 ) 1941. [ 31 MARK-J Collab., B. Adeva et al., Phys. Rev. Lett. 55 ( 1985 665; JADE CoUab., W. Bartel et al., Z. Phys. C 26 (1985) 507; PLUTO Collab., Ch. Berger et al., Z. Phys. C 27 ( 1985 ) 341; CELLO Collab., H.J. Behrend et al., DESY Report 87-005; TASSO Collab., M. Althoffet al., Z. Phys. C 22 (1984) 13. [4] TOPAZ CoUah., L Adaehi et al., Phys. Lett. B 208 (1988) 319. [5l AMY Collab., H. Sagawa et al., Phys. Rev. Lett. 60 (1988) 93. [6] AMY Collah., S. Igarashi et al., Phys. Rev. Lett. 60 (1988) 2359. [7] F.A. Berends, R. Kleiss and S. Jadach, Nucl. Phys. B 202 (1982) 63. 18l M. lgarashi et al., Nucl. Phys. B 263 (t986) 347; S. Kawabata, Comput. Phys. Commun. 41 (1986) 127. 191R. Budny, Phys. Lett. B 55 (1975) 227; J. Ellis and M.K. Gaillard, CERN preprint CERN 76-18.
PHYSICS LETTERS B
9 February 1989
M E A S U R E M E N T S OF CROSS SECTIONS A N D CHARGE ASYMMETRIES FOR e÷e--,'~+~ - A N D e÷e--~It+lt-FOR v/s F R O M 52 TO 57 GeV
AMY Collaboration A. BACALA a R.L. M A L C H O W b, K. SPARKS b,1, R. IMLAY a p. K I R K a R.R. McNEIL a, W. M E T C A L F a, Winston KO b, R.L. L A N D E R b, K. MAESHIMA b, J.R. SMITH b, M.C.S. WILLIAMS b, C.P. C H E N G c j. LI c, Y.K. LI c Z.P. MAO c, y . YAN ~, Y.T. XU c, Y.C. Z H U c A. ABASHIAN d, K. G O T O W d, F. KAJINO d, E. LOW d, F. NAITO d, L. PIILONEN d, R. CHILDERS e, C. D A R D E N e, S. LUSIN e, C. ROSENFELD e, A. WANG e, S. WILSON e, M. FRAUTSCHI f, H. KAGAN r, R. KASS f, C.G. T R A H E R N f, K. ABE g, S. CHAKRABARTI g, Y. FUJII g, Y. HIGASHI ~, Y. K U R I H A R A g, A. MAKI g, T. NOZAKI g, T. OMORI g, P. PEREZ g, H. SAGAWA g, Y. SAKAI g, Y. S U G I M O T O g, Y. TAKAIWA g, S. TERADA g, K. TSUCHIYA g, R. P O L I N G h, j. GREEN i, I.H. PARK i, S. SAKAMOTO i, F. SANNES i, S. S C H N E T Z E R i, R. STONE i, S. T R E N T A L A N G E i, D. Z I M M E R M A N i, K. MIYANO J, H. MIYATA J, M. OGAWA J, Y. YAMASHITA k, D. BLANIS ~, A. B O D E K ~, H. B U D D ~, R. COOMBES ~, S. ENO ~, C.A. FRY ~, H. HARADA ~, Y.H. HO ~, Y.K. KIM ~, T. K U M I T A ~, T. MORI ~, S.L. OLSEN ~.m, N.M. SHAW ~, A. SILL ~, E.H. T H O R N D I K E ~, K. U E N O ~, H.W. Z H E N G ~, H. ASAKURA n, K. E G U C H I n, H. I T O H n, S. KOBAYASHI n, A. M U R A K A M I ", K. T O Y O S H I M A n, J.S. K A N G o, H.J. KIM o, S.K. KIM o, M.H. LEE o, S.S. M Y U N G o, E.J. KIM P, G.N. KIM P, D. SON P, H. K O Z U K A q, S. M A T S U M O T O q, T. SASAKI q, T. TAKEDA q, R. TANAKA q, R. CHIBA r, K. HANAOKA r, S. IGARASHI r, H. MURATA r, H. Y O K O T A r, y . ISHI 5, T. ISHIZUKA ~and K. OHTA a b c d
Louisiana State University, Baton Rouge, LA 70803, USA UniversityofCalifornia, Davis, Davis, CA 95616, USA Institute for High Energy Physics, Beijing, P.R. China Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA University of South Carolina, Columbia, SC 29208, USA f Ohio State University, Columbus, OH 43210, USA g KEK, National Laboratory for High Energy Physics, lbaraki 305, Japan h University of Minnesota, Minneapolis, M N 55455, USA Rutgers University, New Brunswick, NJ 08854, USA J Niigata University, Niigata 950-21, Japan k Nihon Dental College, Niigata 951, Japan University of Rochester, Rochester, N Y 14627, USA m Tsukuba University, Ibaraki 305, Japan Saga University, Saga 840, Japan ° Korea University, Seoul 132, Korea o Kyungpook National University, Taegu 635, Korea q Chuo University, Tokyo 112, Japan Tokyo Institute of Technology, Tokyo 152, Japan Saitama University, Urawa 338, Japan
Received 14 November 1988
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0370-2693/89/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 218, number l
PHYSICS LETTERS B
9 February 1989
Measurements of the differential cross sections for e+e---*~t+~t- and e+e-~z+z - at values of ~/s from 52 to 57 GeV are reported. The forward-backward asymmetries and the total cross sections for these reactions are found to be in agreement with predictions of the standard model of the electro-weak interactions. These measurements are used to extract values of the weakcoupling constants g~g~vand g~Ag~,where £= ~t or z.
Since the leptonic reactions e + e - ~ t + p . - and e+e ---,z+z - have only point-like particles in both the initial and final state, they provide sensitive and unambiguous tests of the standard SU (2) X U ( 1 ) theory o f the electro-weak interactions [ 1 ]. The TRIST A N electron-positron storage ring at the National Laboratory for High Energy Physics in Japan ( K E K ) produces e+e - collisions at CM energies in the 5060 GeV range, where the model predicts a large forward-backward charge asymmetry for these processes. This is caused primarily by the interference between the electromagnetic and weak amplitudes. Previous measurements at PEP [2] and P E T R A [ 3 ] are in good agreement with the predictions of the standard model. The first report o f this measurement at T R I S T A N was published recently by the T O P A Z Collaboration [4], using 6.5 pb -~ of data collected at x / s = 5 2 and 55 GeV. This report describes measurements of this asymmetry and the total cross section for the reactions e +e---,g+~t- and e+e - ~ x + x using 17.7 p b - ~of data at CM energies from 52 to 57 GeV collected by the A M Y detector at TRISTAN. The A M Y detector is a compact detector based on a 3 Tesla solenoid and optimized for lepton detection. Electron identification is provided by a highly segmented shower counter located inside the coil. Muon identification is provided by an absorber with a thickness that is equivalent, on average, to 1.65 m of iron. The detector has been described in detail elsewhere [5,6], only those features central to the analysis presented here will be discussed. Charged-particle tracking is done by a four layer, 576 cell tube-type inner tracking chamber ( I T C ) o f inner radius 13 cm followed by a cylindrical drift chamber ( C D C ) with 9048 cells arranged in 25 axial and 15 small-angle stereo layers that extends to an outer radius of 67 cm. In the event reconstruction, a charged track in the C D C is required to have at least Present address University of Rochester, Rochester, NY 14627. USA
eight axial hits and five stereo hits that provide a good three-dimensional fit to a helix. Transverse momenta are determined with a precision ap,/pt -~ [ 0 . 7 % ( G e V ) / c ) - ~ ]Pt. Outside o f the CDC, starting at a radius of 79 cm, is a 15 radiation-length cylindrical electromagnetic calorimeter (SHC) that covers the region Icos01 ~<0.73. The SHC is constructed from twenty alternating layers of lead and gas proportional tubes. The energy resolution is aE/ E~_23%GeVW2/x/-E+6%. Outside of the iron flux return for the magnet is a m u o n identification system consisting of two orthogonal double-layer planar drift chambers (containing a total of l 184 drift cells) followed by scintillation counters to provide timing information (to within 3 ns). The muon detector covers the region Icos 01 ~<0.74. A m i n i m u m transverse m o m e n t u m of 1.9 G e V / c is needed to penetrate the hadron absorber; the penetration efficiency reaches 100% for muons with p,/> 2.5 GeV/c. The detector is triggered by energy deposition in the barrel or end-cap calorimeters, and by a variety of track patterns in the ITC, C D C and the outermost four layers of the SHC. Triggering of~t+~t - events depends exclusively on the sensitivity o f these track patterns to two-track topologies, and several different criteria were implemented. For the ITC and C D C triggers, the efficiency was determined using Bhabha scattering events (e+e - --,e+e - ), which are independently triggered by SHC energy triggers. For the outer layers of the SHC, the efficiency was measured using e+e ---,~t+~t - events that triggered the detector independently o f the SHC. Triggering of x+z - events is accomplished by a variety o f charged-track and energy triggers. The efficiency o f the charged-track triggers was estimated by analyzing those events in the final sample that were triggered by energy triggers. Similarly, the efficiency of SHC total energy triggers were estimated using the sample of events satisfying charged-track triggers. The ~t+~t- and ~+z- triggers were enabled at the 113
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beam crossing time, which occurs every 5 ~ts (Beam Gate), and for equal time intervals starting midway between beam crossings (Cosmic Ray Gate). The triggers in the Cosmic Ray Gate were caused by cosmic rays traversing the detector and provided a direct measurement of the level of cosmic ray induced background that survives each stage o f the analysis. The topology of ~t+la - events consists o f two backto-back tracks in the central detector with m o m e n t a consistent with beam energy, shower counter energy deposition consistent with m i n i m u m ionizing particles, and hits in the m u o n chambers consistent with the observed tracks. All events with at least two reconstructed tracks were considered. Events were required to have at least two tracks that: (i) pass within ___0.3 cm of the interaction point in the plane transverse to the beam direction (l~r~l ~<0.3 cm) and within + 3.0 cm of the interaction point along the beam direction (I~1 < 3 . 0 cm); (ii) have a momentum greater than 0.2 ECM with at least one above 0.25 ECM; (iii) are included in the solid angle of the muon identification system ( Icos 01 ~<0.72 ); and (iv) are within 10 ° of being back-to-back collinear. These requirements reduce cosmic ray backgrounds while selecting m u o n pair and Bhabha scattering events with high efficiency. They also eliminate most of the events due to tau pair and two-photon initiated e+e-~t+~t - final states where only the muons are detected. N o requirement was placed upon the charges of the tracks. Bhabha events were eliminated by requiring the total energy deposited in the shower counter to be less than 0.1 ECM. This is well above the energy deposited by two m i n i m u m ionizing particles ( ~ 800 MeV) and entirely eliminates the Bhabha scattering events that pass the previous cuts. At this point, only events with two charged tracks remained in the sample. We required that both tracks point to a muon detector sextant in which there were hits in at least three of the four m u o n drift chamber layers and that at least one of the two such sextants had a valid hit in a corresponding muon scintillation counter. The counter time was required to be in the interval between 0 and 35 ns o f the beam crossing. For those cases where two counter hits were recorded ( ~ 90% of the final event sample) the time difference between the two hit counters was required to be in the interval - 14 ns to 114
9 February 1989
+ 20 ns. (The time difference for cosmic rays ranges from - 2 1 ns to - 3 0 ns.) The overall efficiency o f our selection criteria (including the geometrical acceptance) for the muon pair reaction was estimated from a full Monte Carlo simulation of the detector. In this simulation, we assigned an efficiency to each of the individual m u o n detector elements as was determined from an analysis of acollinear muon pairs from the reaction e + e - ~ e + e - I ~ + l l - and from cosmic rays; the efficiencies of the individual chamber and counter elements were typically ~ 95%. By requiring only three of four chamber planes for each muon and only one of the two counter planes for the pair of tracks, the overall detection efficiency o f the muon identification system was kept greater than 97%. The efficiency of event reconstruction, including selection cuts, was greater than 94% for Icos 01 ~<0.65. Beyond this, it dropped smoothly to zero by I cos 0l ~0.74. Thus the overall reconstruction efficiency, including geometrical acceptance, was 59_+1.5%. The efficiency for triggering within our acceptance was determined to be 9 4 + 2.6%. Seven events which occurred during the Cosmic Ray Gate passed the final cuts, indicating a cosmic ray background of 2.3_+0.8%. For two events, the track fitting program assigned the same charge to both tracks. These events were used for the total cross section determination but not used in the determination of the forward-backward asymmetry. We estimate the level of backgrounds due to Bhabha scattering by examining the probability that a track from a selected Bhabha event matches a hit in a muon chamber. The results show that the probability of both tracks from the Bhabha event matching corresponding hits in the m u o n chambers is much less than 0.1% and is thus negligible. The backgrounds due to e+e - ~ + x - and e + e - ~ e + e - ~ t + ~ - events are obtained from Monte Carlo simulations of these processes. The results indicate that the backgrounds due to these two processes are each less than 0.2% and are thus negligible. The branching ratio for x decay into a single charged particle is ~ 83%; that for the three charged particles is ~ 17%. Thus, about two-thirds of the e+e - ~ z + x events have two almost back-to-back charged tracks, with most of the rest having a topology of one versus three charged tracks. We accept both topologies but reject those two-track candidates where both tracks
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are identified as m u o n s or where both are identified as electrons. Cosmic rays and e + e - - , e + e - ~ t + ~ t events are large backgrounds to the sample where both taus decay to muons; Bhabha events and e+e---,e+e-e+e events c o n t a m i n a t e the sample where both taus decay to electrons. In a d d i t i o n to Bhabha scatters and e + e - - ~ e + e - e + e - , i m p o r t a n t sources o f background for the accepted candidates are expected to be two-photon initiated tau pairs (e+e - - - . e + e - ~ + T - ), radiative Bhabha scatters, a n d two-photon multihadronic events. Backgrounds from Bhabha event are o f particular concern, since a small a m o u n t o f c o n t a m i n a t i o n would have an appreciable effect on the m e a s u r e d asymmetry. We searched for tau pair events from a m o n g all o f the events that had two or more reconstructed charged particle tracks with [cos 0[ ~<0.73, the angular region where there is reliable electron and m u o n identification. In the events with only two tracks, we required each track to have [~rol 4 0 . 5 cm and 1~-1 ~<3.0 cm; for events with three or m o r e accepted tracks, this req u i r e m e n t was loosened to lOto[ ~<2.0 cm; [~-1~< 5.0 cm. Events were then selected if they had either two charged tracks or one track recoiling against a cluster o f charged tracks, where a cluster is defined to be a tightly collimated group o f two or more charged tracks with a total invariant mass that is less than 2 G e V / c 2. Only tracks satisfying the [cos 0[ a n d the vertex requirements were included; others were simply ignored. We also required the total visible energy, Eves, to be greater than 0.2 EcM for both event topologies. In the case o f events that c o n t a i n e d clusters, we define the direction o f the cluster to be the vector sum o f the charged tracks that c o m p r i s e it. Tracks were required to be within 45 ° o f the axis o f the cluster in order to be included. Events were required to have opposing tracks or clusters with an opening angle greater than 160 °. This r e q u i r e m e n t rejects two-photon initiated events. We also required all oppositely charged pairs o f tracks within any cluster to have an invariant mass o f Moa~r>~150 M e V / c 2, where each track is assumed to be an electron. This cut eliminates conversions from r a d i a t e d photons. F o r two-prong events, we rejected events in which the energy o f the m a x i m u m shower in the electromagnetic c a l o r i m e t e r exceeds 0.45 EcM. We further rejected events with total shower energy greater than 0.8 EcM, or with m o r e than 0.6 EcM d e p o s i t e d in the T M
9 February 1989
shower counter by charged tracks. If a charged track o f m o m e n t u m p deposited an a m o u n t o f energy in the calorimeter which was greater than 0.4.0, the track was called as electron. I f both tracks in the event were identified as electrons, the event was not kept. These cuts reject Bhabha events and radiative Bhabha events. Backgrounds from e+e - - - , e + e T+~ - were rej e c t e d by requiring the observed longitudinal mom e n t u m [P--I ~<0.4 Evis. The efficiency and acceptance for the z+x - selection was estimated from a sample o f Monte Carlo simulated events to be 24.8 + 3.5%. (This estimate includes the branching ratios o f the accepted decay modes. ) The trigger efficiency for these events is est i m a t e d to be 96.0_+3.0%. Backgrounds were estim a t e d from simulated Bhabha events, two-photon initiated tau pair, electron pair, a n d m u l t i h a d r o n i c events as well as m u l t i h a d r o n i c annihilation events. The b a c k g r o u n d from two-photon m u l t i h a d r o n i c p r o d u c t i o n is estimated to be 0.5%, while that from Bhabha and radiative Bhabha events is found to be 1.1_+0.8%. T w o - p h o t o n electron pair events are found to comprise less than 2.3 + 1.0% o f the final event sample. The contribution from two-photon tau pairs and m u l t i h a d r o n i c annihilation events was found to be negligible. None o f the events in the final tau pair sample originated during the Cosmic R a y G a t e indicating a cosmic ray background that is less than 1.5%. The results for the total cross section for the g+~tand z+x - processes at each center-of-mass energy are summarized in table 1. The total cross section is given by N a = E(1 + ~ R c ) q L '
(1)
where N i s the observed n u m b e r o f events after selection cuts have been applied; L is the integrated luminosity; t/is the trigger efficiency within our acceptance; ( 1 + fiRC) is the radiative correction [ 7,8 ] ~'2' and c is the detection efficiency. This detection effi*~ Ref. [7] gives a complete description of the Monte Carlo technique for determining radiative corrections. However, due to the increased importance of terms involving the Zo at TRISTAN energies the Monte Carlo actually used for our analysis is that ofref. [8 ]. ,2 The difference in the radiative corrections between la+~t- and x+x- is due to the different selection criteria in the two cases. 115
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ciency includes the geometric acceptance as well as various instrumental and software efficencies. It is calculated by a combination of Monte Carlo simulation and analysis of calibration data. Since a statistical subtraction must be made for the fraction of background events remaining after the selection cuts have been applied, e also contains a factor for this correction. The integrated luminosity was determined from the number of Bhabha scattering events detected in the endcap calorimeters, as described in ref. [ 5 ]. The uncertainty in the total cross section measurement is dominated by the statistical error. However, it does include a systematic error of ~ 5%. The main contributions to this error are the systematic uncertainty in the luminosity measurement and the uncertainty in the computer modeling of the various efficiencies and backgrounds. These modeling uncertainties have been estimated from the change in the result when the parameters of the model are varied within acceptable limits. Included in table 1 is the lowest order prediction of the standard model. Our results are consistent with these theoretical expectations. The differential cross section for each of the two reactions was obtained in the same way as the total cross section except that the acceptance, trigger efficiency and radiative corrections were done on a binby-bin basis in cos 0. In the case of the e+e - ~ z + z - reaction, the presence of unobserved neutrinos in the final state made the determination of the original direction of the tau inexact. The vector sum of charged and neutral products was computed for each hemisphere of each event. Only photons within a cone of 30 ° around the charged-track axis were included. All charged tracks in a cluster were required to be within a cone of angle 30 °. In both the ~t+~t- and the x+~- samples, the vector difference of the three-momenta of the two sides was then used as the estimate for the direction of the production angle. In the case of the taus, the error in the production angle estimate was determined from the simulated events to be less than 2°. For the dimuons the resolution in the production angle was much better than 1 ° The differential cross sections for both reactions for the combined data from all energies are shown in fig. 1. We extract the forward-backward charge asymmetry by fitting the angular distributions, which have 116
9 February 1989
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Volume 218, number 1
PHYSICS LETTERS B
(~) A ~ : - 0 3 3 ± 0 08 3t
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9 February 1989
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T h e d o t t e d c u r v e s are t h e s t a n d a r d m o d e l p r e d i c t i o n s .
been adjusted for radiative corrections, at each en, ergy to the function ( ~ = g or z) do
N M
~4 ~4
O~2
dr2 - 4s [Re~( 1 +cos20) + B ~ c o s 0] ,
d d d d d +1 +1 +1 +1 +1 d d d d d ' ~ d d d d d 'g +1 +1 +l +1 +1
(2)
g~NN ~4
with both R~e and B~ allowed to vary. Here, R~ is the ratio of the total cross section for e+e - ~ + t ~ - to that expected from QED alone, and the forward-backward asymmetry A~ is given by (B~/R~). The radiative corrections are calculated to order c~3 in the full electroweak theory. They yield a positive contribution of ~ 4% to the asymmetry in the region Icos01 ~<0.74. Our measurements o f A~ and R~ are summarized in table 2. Again the uncertainties are dominated by the statistical errors. The Re~ measurement also has a systematic error of the same size and from the same sources as the total cross section measurement. The systematic error in the Ae~ measurement is also ~ 5%. It is dominated by the uncertainty in the bin-to-bin variation in the efficiency and radiative correction calculations and in the case of the taus by the uncertainty in the production angle as discussed above. The weighted averages of the measured asymmetries are shown, together with results from previous measurements [ 2 - 4 ] in fig. 2. The experimental results show a large increase in the forward-backward asymmetry between the previously highest available energies and TRISTAN. In the lowest order electroweak model [ 9 ], Ree and A~e can be expressed in terms o f the vector and axialvector weak neutral coupling constants gv and gA, and the Z ° contribution Z, as
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117
PHYSICS LETTERS B
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where s
(5)
Z = 16 sinZ0w cos20w ( s - M ~ ) " We used eq. ( 4 ) to d e t e r m i n e the axial-vector couplings, which were then used in eq. ( 3 ) together with the m e a s u r e d values o f R~, to extract the vector couplings. In this procedure, we used sin20w=0.23 for the Weinberg angle and M z = 9 2 . 5 G e V / c 2 for the weak neutral boson. T h e results for the neutral weak coupling constants are presented in table 2. The theoretical predictions for the asymmetries, using the values for sin20w and M z given above, are i n c l u d e d in fig. 2. The m e a s u r e d asymmetries for m u o n s a n d taus are in good agreement with the electroweak predictions. Consequently, the extracted values o f the product g~g~ also agree with the model. The large relative errors on the extracted values o f the p r o d u c t g~eg~, prevent a meaningful c o m p a r i s o n 118
with the theory. However, these values are in general agreement with the s t a n d a r d m o d e l predictions. The A M Y experiment at the T R I S T A N storage ring has observed m u o n pair and tau pair p r o d u c t i o n at center-of-mass energies from 52 to 57 GeV. The total and differential cross sections for both processes are in agreement with the predictions of the s t a n d a r d m o d e l o f electroweak interactions. The angular distributions show significant negative asymmetries, reflecting the interference between the electromagnetic and weak neutral currents. The m e a s u r e d values o f these f o r w a r d - b a c k w a r d a s y m m e t r i e s are in agreem e n t with the lowest order predictions o f this model. We t h a n k the T R I S T A N staff for the excellent operation o f the storage ring. In a d d i t i o n we acknowl"edge the strong support a n d enthusiastic assistance p r o v i d e d by the staffs of our h o m e institutions. This work has been s u p p o r t e d by the J a p a n Ministry o f Education, Science a n d Culture ( M o n b u s h o ) , The US D e p a r t m e n t o f Energy and N a t i o n a l Science F o u n d a t i o n , the K o r e a n Science and Engineering F o u n d a t i o n and Ministry o f Education, and the Acad e m i a Sinica o f the People's Republic o f China. References
R ~ = 1 + 8 "eSv6vz "~ "
1
9 February1989
[ l ] S.L. Glashow, Nucl. Phys. 22 ( 1961 ) 579; S. Weinberg, Phys. Rev. Lett. 19 (1967) 1264; A. Salam, in: Proc. 8th Nobel Symp. (Aspenasgarden, 1968) (Almqvist and Wiksell, Stockholm) p. 367. [ 21 HRS Collab., M. Derrick et al., Phys. Rev. D 31 ( 1985 ) 2352; MAC Collab., W. Ash et al., Phys. Rev. Lett. 55 (1985) 1831; MARK-II Collab., M.E. Levi et al., Phys. Rev. Len. 51 ( 1983 ) 1941. [ 31 MARK-J Collab., B. Adeva et al., Phys. Rev. Lett. 55 ( 1985 665; JADE CoUab., W. Bartel et al., Z. Phys. C 26 (1985) 507; PLUTO Collab., Ch. Berger et al., Z. Phys. C 27 ( 1985 ) 341; CELLO Collab., H.J. Behrend et al., DESY Report 87-005; TASSO Collab., M. Althoffet al., Z. Phys. C 22 (1984) 13. [4] TOPAZ CoUah., L Adaehi et al., Phys. Lett. B 208 (1988) 319. [5l AMY Collab., H. Sagawa et al., Phys. Rev. Lett. 60 (1988) 93. [6] AMY Collah., S. Igarashi et al., Phys. Rev. Lett. 60 (1988) 2359. [7] F.A. Berends, R. Kleiss and S. Jadach, Nucl. Phys. B 202 (1982) 63. 18l M. lgarashi et al., Nucl. Phys. B 263 (t986) 347; S. Kawabata, Comput. Phys. Commun. 41 (1986) 127. 191R. Budny, Phys. Lett. B 55 (1975) 227; J. Ellis and M.K. Gaillard, CERN preprint CERN 76-18.