- Email: [email protected]
Search for CP violation in the decay Z → τ+τ−
9 March 1995
PHYSICS
ELSPYIER
LETTERS
6
Physics Letters B 346 (1995) 371-378
Search for CP violation in the decay Z -+ T+TALEPH Collaboration D. Buskulic a, D. Casper a, I. De Bonis a, D. Decamp a, P. Ghez a, C. Goy a, J.-P. Lees a, M.-N. Minard a, P. Odier a, B. Pietrzyka, F. Ariztizabal b, M. Chmeissani b, J.M. Crespo b, V. Gaitanb, Ll. Garridoby15, I. Efthymiopoulos b, E. Fernandez b, M. Fernandez-Bosmanb, M. Martinez b, S. Orteu b, A. Pacheco b, C. Padilla b, F. Palla b, A. Pascual b, J.A. Perlas b, F. Sanchez b, F. Teubert b, D. Creanza ‘, M. de PalmaC, A. Farilla ‘, G. Iaselli ‘, G. Maggi ‘, N. MarinelliC, S. Natali ‘, S. NuzzoC, A. RameriC, G. Raso ‘, F. RomanoC, F. Ruggieri ‘, G. Selvaggi c, L. Silvestris ‘, P. Tempesta c, G. Zito ‘, X. Huang d, J. Lin d, Q. Ouyang d, T. Wang d, Y. Xie d, R. Xu d, S. Xue d, J. Zhang d, L. Zhang d, W. Zhao d, G. Bonvicini e, P. Comas e, P. Coyle e, H. Drevermanne, A. Engelhardt e, R.W. Forty e, M. Frank”, G. Ganis e, C. Gayev3, M. Gironee, R. Hagelberge, J. Harveye, R. Jacobsen e, B. Joste, J. Knobloche, I. Lehrause, M. Maggie, C. Markoue, E.B. Martin e, P. Mato e, H. Meinharde, A. Minten e, R. Miquel e, P. Palazzi e, J.R. Pater e, P. Perrodo e, J.-F. Pusztaszeri e, F. Ranjard e, L. Rolandi e, D. Schlatter”, M. Schmellinge, W. Tejessy e, I.R. Tomaline, R. Veenhof e, A. Venturi e, H. Wachsmuthe, W. Wiedenmanne, W. Witzelinge, J. Wotschacke, Z. Ajaltouni f, M. Bardadin-Otwinowskaf, A. Barresf, C. Boyer f, A. Falvardf, P. Gay f, C. Guicheney f, P. Henrardf, J. Jousset f, B. Michel f, S. Monteil f, J-C. Montret f, D. Pallin f, P. Perretf, F. Podlyski f, J. Proriol f, J.-M. Rossignol f, F. Saadi f, T. Fearnleys, J.B. Hansens, J.D. Hansen a, J.R. Hansen s, P.H. Hansen s, S.D. Johnson s, R. Mollerud s,B.S. Nilsson s, A. Kyriakis h, E. Simopoulou h, I. Siotis h, A. Vayaki h, K. Zachariadou h, A. Blondel’, G. Bonneaud’, J.C. Brient i, P. Bourdon’, L. Passalacqua’, A. Rouge’, M. Rumpf’, R. Tanaka i, A. Valassi i, M. Verderi i, H. Videau i, D.J. Candlin j , M.I. Parsons j, E. Veitch j, E. Focardi k, G. Parrini k, M. Corden ‘, M. Delfino ‘I’~, C. Georgiopoulos e, D.E. Jaffe e, A. Antonelli m, G. Bencivenni m, G. Bolognam*4, F. Bossi m, P. Campanam, G. Capon m, F. Cerutti”, V. Chiarellam, G. Felicim, P. Laurelli”, G. Mannocchi m~5,F. Murtasm, G.P. Murtas m, M. Pepe-Altarellim, S. Salomonem, P. Colrain n, I. ten Have n,6, I.G. Knowles n, J.G. Lynch”, W. Maitland”, W.T. Morton”, C. Raine”, P. Reeves”, J.M. Starr”, K. Smith”, M.G. Smith”, AS. Thompson”, S. Thorn”, R.M. Turnbull”, U. Becker O, 0. Braun’, C. Geweniger O, G. Graefe”, P. Hanke O, V. Hepp O, E.E. Kluge O, A. Putzer O, B. Rensch O, M. Schmidt O, J. Sommer O, H. Stenzel’, K. Tittel’, M. WunschO, R. Beuselinckr, D.M. Binnie r, W. Cameron P, M. Cattaneo P, D.J. Colling P, PJ. Dornan P, J.F. Hassard P, 0370-2693/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSDI 0370-2693(95)00093-3
372
ALEPH Collaboration/Physics Letters B 346 (1995) 371-378
N. Konstantinidis P, L. Moneta P, A. Moutoussi r, J. Nash p, D.G. Payne P, G. San Martin P, J.K. Sedgbeerr, A.G. Wright p, G. Dissertori q, P. Girtlerq, E. Kneringerq, D. Kuhn q, G. Rudolph 9, C.K. Bowdery ‘, T.J. Brodbeck r, A.J. Finch’, F. Foster r, G. Hughes r, D. Jackson r, N.R. Keemer r, M. Nuttall r, A. Pate1r, T. Sloan r, S.W. Snow r, E.P. Whelan r, A. GallaS, A.M. Greene”, K. KleinknechtS, J. Raabs, B. RenkS, H.-G. Sanders, H. SchmidtS, S.M. Walther ‘, R. War&e ‘, B. Wolf ‘, A.M. Bencheikh t, C. Benchouk t, A. Bonissent t, D. Calvet t, J. Carr t, C. Diaconu t, F. Etienne t, D. Nicod f, P Payre t, L. Roos t, D. Rousseau t, M. Talby t, I. Abt “, S. Adlung “, R. Assmann “, C. Bauer “, W. Blum”, D. Brown “, P Cattaneo u,23,B. Dehning “, H. Diet1 “, F. Dydak “12*,A.W. Halley “, K. Jakobs “, H. Kroha “, J. Lauber “, G. Ltitjens “, G. Lutz”, W. Manner”, H.-G. Moser”, R. Richter”, J. Schroder”, A.S. Schwarz”, R. Settles “, H. Seywerd”, U. StierlinUy2,U. Stiegler “, R. St. Denis “, G. Wolf “, R. Alemany “, J. Boucrot “, 0. Callot “, A. Cordier “, F. Courault “, M. Davier “, L. Duflot “, J.-F. Grivaz “, Ph. Heusse “, M. Jacquet “, P. Janot ‘, D.W. Kim “,19, F. Le Diberder “, J. Lefraqois ‘, A.-M. Lutz “, G. Musolino “, I. Nikolic “, H.J. Park “, I.C. Park “, M.-H. Schune “, S. Simion “, J.-J. Veillet “, I. Videau “, D. Abbaneo w, G. Bagliesi w, G. Batignani w, S. Bettarini w, U. Bottigli w, C. Bozzi w, G. Calderini w, M. Carpinelli w, M.A. Ciocci w, V. Ciulli w, R. Dell’Orso w, I. Ferrante w, F. Fidecaro w, L. Foaw”, F. Forti w, A. Giassi w, M.A. Giorgi w, A. Gregorio w, F. Ligabue w, A. Lusiani w, P.S. Marrocchesi w, A. Messineo w, G. Rizzo w, G. Sanguinetti w, A. Sciaba w, l? Spagnolo w, J. Steinberger w, R. Tenchini w, G. Tonelli w,26,G. Triggiani w, C. Vannini w, PG. Verdini w, J. Walsh w, AI? Betteridge ‘, G.A. BlairX, L.M. Bryant”, Y. Gao”,M.G. Green”, D.L. Johnson x, T. Medcalf ‘, L1.M. Mir x, J.A. Strong x, V. Bertin Y,D.R. Botterill Y, R.W. Clifft Y,T.R. Edgecock Y,S. Haywood Y,M. Edwards Y,P. Maley Y,P.R. Nortony, J.C. Thompson Y,B. Bloch-Devaux z, l? Colas ‘, H. Duarte ‘, S. Emery ‘, W. Kozanecki ‘, E. Langon ‘, M.C. Lemaire ‘, E. Locci ‘, B. Marx z, P Perez ‘, J. Rander ‘, J.-F. Renardy ‘, A. Rosowsky ‘, A. Roussarie ‘, J.-P Schuller ‘, J. Schwindling”, D. Si Mohand ‘, A. Trabelsi ‘, B. Vallage z, R.P. Johnson aa, A.M. Litke aa, G. Taylor aa, J. Wear aa, A. Beddall ab, C.N. Boothab, C. Boswellab, S. Cartwrightab, F. Combley ab,I. Dawsonab, A. Koksalab, M. Letho ab, W.M. Newtonab, C. Rankin ab, L.F. Thompson ab, A. Btihrer ac, S. Brandt ac, G. Cowan ac, E. Feigl ac, C. Grupen ac, G. Lutters ac, J. Minguet-Rodriguez ac, F. Riveraacs2’, P. SaraivaaC, U. Sch3ferac, L. SmolikaC, L. Bosisio ad, R. Della Marinaad, G. Giannini ad, B. Gobboad, L. Pitisd, F. Ragusaad,20,H. Kimae, J. Rothbergae, S. Wasserbaechae, S .R. Armstrong af, L. Bellantoni af, J.S. Conway af,24,P Elmer af, Z. Feng af, D.P.S. Ferguson af, Y.S. Gao af, S. Gonzales af, J. Grahl af, J.L. Harton af, O.J. Hayes af, H. HUaf, P.A. McNamara IIIaf, J.M. Nachtmanaf, W. Orejudosaf, Y.B. Panaf, Y. Saadiaf, M. Schmitt af, I. Scott af, V. Sharmaaf, J.D. Turkaf, A.M. Walshaf, F.V. Weberaf*‘, T. Wildish&, Sau Lan WU af, X. Wu af, J.M. Yamartino &, M. Zheng af, G. Zobernig af a hboratoire de Physique des Pardcules (LAPP), IN2P3-CNRS. 74019 Annecy-le-Vieux Cedex, France b Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spain7 c Dipartimento di Fisica. INFN Sezione di Bari, 70126 Bari, Italy d Institute of High-Energy Physics, Academia Sinica. Beijing, China8
ALEPH Collaboration/Physics Letters B 346 (1995) 371-378
313
e European Laboratory for Parricle Physics (CERN), 1211 Geneva 23, Switzerland ’ Luboratoire de Physique Corpusculaire, Universite Blaise Pascal, lN2P3-CNRS, Clemwnt-Ferrand, 63177 AubiPre, France g Niels Bohr Institute, 2100 Copenhagen, Denmark9 h Nuclear Research Center Demokritos (NRCD), Athens, Greece
i Laboratoire de Physique Nucldaire et des Hautes Energies, Ecole Polytechnique, lN2P3-CNRS, 91128 Palaiseau Cedex, France j Department of Physics, Vniversiv of Edinburgh, Edinburgh EH9 3JZ, UK lo k Dipartimento di Fisica, Vniversitd di Firenze, INFN Sezione di Firenze, 50125 Firenze, Italy e Supercomputer Computations Research Institute, Florida State University. Tallahassee, FL 32306-4052, USA 13*14 m Laboratori Nazionaii dell’lNFN (LNF-lNFN), 00044 Frascati, Italy D Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK lo 0 lnstitut fiir Hochenergiephysik, VniversiUtt Heidelberg, 69120 Heidelberg, Germany’6 Q Department of Physics, Imperial College, London SW7 2BZ UK lo 9 Institut fiir Experimentalphysik, Vniversitiit Innsbruck, 6020 Innsbruck, Aust?Ya‘8 r Department of Physics, University of Lancaster, Lancaster LA1 4YB. UK I0 ’ lnstitut jiIr Physik, Vniversitift Mainz, 55099 Mainz, Germany I6 ’ Centre de Physique des Particules, Facultt! des Sciences de Luminy, lN2P3-CNRS 13288 Marseille, France ’ Max-Plunck-lnstitut fr Physik, Werner-Heisenberg-lmtitut, 80805 Miinchen, Germany’6 v Laboratoire de l’Acct%+ateur Linkaire, Vniversitb de Paris-Sud, IN2P3-CNRS, 91405 Orsay Cedex, France w Dipartimento di Fisica dell’llniversitci, INFN Sezione di Piss, e Scuoia Normale Superiore, 56010 Pisa, Italy x Department of Physics, Royal Holloway & Bedford New College, University of London, Surrey TW20 OEX, VKLO Y Particle Physics Dept.. Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, UK lo ’ CEA, DAPNlAIService de Physique des Particules, CE-Saclay, 91191 Gif-sur-Yvette Ceder France’? &1Institute for Particle Physics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA== ab Department of Physics, University of Shefleld, Shefield S3 7RH. UK lo ac Fachbereich Physik, Vniversitiit Siegen, 57068 Siegen, Germany lb ad Dipartimento di Fisica, Vniversitci di Trieste e INFN Sezione di Trieste, 34127 Trieste, Italy ae Experimental Elementary Particle Physics, University of Washington, WA 98195 Seattle, USA af Department of Physics, University of Wisconsin. Madison, WI 53706, USA”
Received 23 November 1994 Editor: K. Winter
Abstract Data collected by ALEPH in the years 1990, 1991 and 1992 have been used to update a previous search for CP violation in the decay of the Z into T+T-. The measurement of the weak dipole form factor of the 7 lepton has been performed by studying correlations between the T leptons. No signal of CP violation was found. The weak dipole form factor is found to be (i, = (+0.15 f 0.58Sbt f 0.38,,) lo-“e . cm, obtained with 19628 identified T+T- events. This gives an upper limit on the weak dipole form factor of l&l < 1.5 . lo-“e - cm at the 95% confidence level.
’ Now at CERN, 1211 Geneva 23, Switzerland. 2 Deceased. 3 Now at Harvard University, Cambridge, MA 02138, USA. 4 Also Istituto di Fisica Generale, Universiti di Torino, Torino, Italy. 5 Also Istituto di Cosmo-Geofisica de1 C.N.R., Torino, Italy. 6 Now at TSM Business School, Enscbede, The Netherlands. ’ Supported by CICYT, Spain. ’ Supported by the National Science Foundation of China. g Supported by the Danish Natural Science Research Council. lo Supported by the UK Science and Engineering Research Council.
I1 Supported by the US Department of Energy, contract DEAC02-76EROO881. I* On leave from Universitat Autonoma de Barcelona, Barcelona, Spain. l3 Supported by the US Department of Energy, contract DEFGOS-92ER40742. I4 Supposed by the US Department of Energy, contract DE-FCOS85ER250000. I5 Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain. l6 Supported by the Bundesministerium ftir Forschung und Technologie, Germany.
ALEPH Collaboration/ Physics Letters B 346 (1995) 371-378
374
1. Introduction The decay of the 2 boson into r+r- is an appropriate process to search for signals of CP violation caused by new interactions. Since in the Standard Model CP violation occurs only in the couplings of the charged current as described in the CKM scheme, CP odd contributions to the reaction Z --f r+r- are at most of the order of IO-’ [ 1] compared to the electroweak amplitude at the Born level. On the other hand extensions of the Standard Model, such as models with leptoquarks or with extended Higgs sectors, but also left-right symmetric or supersymmetric interactions can generate CP violating effects at the Zrr vertex [ 21. The parametrization of these contributions by the weak dipole form factor &(q* = m$)of the r lepton at the Z resonance gives a model independent description of the on-shell amplitude, which in this case consists of the coherent sum of the electroweak amplitude and an amplitude proportional to the weak dipole form factor &. An indirect limit on a dipole form factor can be derived from the measurement of the Z partial width IYr, since there would be an additional contribution from the weak dipole form factor Al?, M (&(*m$/(24r) [ 11. A contribution of an electric dipole form factor is expected to be small [ 51. From the comparison of the value measured at LEP, I, = (84.26&0.34)MeV [ 31, and the theoretical prediction of the Standard Model, IsM = (83.7 f 0.4)MeV [4], one obtains an indir&t upper limit on the weak dipole form factor of ]&I < 2.3. 10-17e.cm (95 % c.1.). However, if there are new physics effects, there may also be CP even
I7Supported 1s Supported
by the Direction des Sciences de la Matiere, C.E.A. by Fonds zur Fiirderung der wissenschafthchen
Forschung, Austria. I9 Permanent address: Kangnung National University, Kangnung, Korea. 2oN~~ at Dipartimento di Fisica, Universita di Milano, Milano, Italy. 2L Also at CERN, 1211 Geneva 23, Switzerland. 22 Supported by the US Department of Energy, grant DEFG0392ER40689. 23 Now at Universita di Pavia, Pavia, Italy. 24 Now at Rutgers University, Piscataway, NJ 08854, USA. 25 Partially supported by Colciencias, Colombia. 26 Also at lstituto di Matematica e Fisica, Universim di Sassari, Sassari,
Italy.
contributions to the partial width, which can cancel partially the contribution from &. In order to measure the real part of the dipole form factor directly from CP odd correlations between the r leptons, we use the following CP odd tensor observables [ 561:
cj=(8+- iqi. i,j=l,2,3
(fi+’ ,$+
x
fi-)j
B_I
+
(i -
j> (1)
$+ ($-) denotes the normalized momentum vector of one of the decay particles of the r+ (r- ), i, j are Cartesian coordinate indices. The relation between the observable and the real part of the dipole form factor & is given by [5,6]:
A and B are the decay modes of r- and r+, respectively. The constants 2~s describe the sensitivity of the angular distributions of the decay particles used to build ? to the spin states of the r leptons. Therefore these constants depend in magnitude and sign explicitly on the decay mode of r- and r+. This analysis uses the major r decay modes evy, ,WV, TV, pv, and al v, with at -+ rn+n-- and al -+ ~29. This results in 18 different classes, the three classes al-at are not used due to low statistics and high backgrounds. The matrix Sij = diag( -i, -h, i) is a diagonal matrix representing the tensor polarisation of the Z boson in the coordinate system in which the z axis is determined by the direction of the incident positron beam. As can be seen from the matrix sij, i@s;3 is the most sensitive component and will give the largest signal if & # 0. Since in addition the three diagonal elements are strongly correlated due to the fact that the trace of ? must be zero and since the systematic checks are easier to perform for ?ss, only psp33 is used in this analysis. The analysis described in this paper improves upon and supersedes the result published by the ALEPH Collaboration (I&] < 3.7 . lo-“e . cm at the 95 % c.1.) [ 71. This is due to a better selection, yielding about 30% more acceptance, the inclusion of the r decays into ai and a doubling of the collected data. A limit on the weak dipole form factor
ALEPH Collaboration/
Physics Letters B 346 (1995) 371-378
Table 1 Efficiencies and background with statistical errors for the event classesused in the analysis. All numbers are given in percent. Background fractions from
Event
Selection-
class
efficiency
7 events
other processes
e-e
36.4 f 0.6
2.8 f 0.3
3.6f
e-p
67.9 f 0.4
3.2 f 0.2
l.O& 0.3
e-a
39.3 f 0.5
9.9 f 0.5
0.3 & 0.3
e-p
33.3 f 0.4
7.4 * 0.4
0.2 f 0.2
e-7777+7r-
41.7 f 0.7
5.5 f 0.5
e-7r2v"
25.2 f 0.6
26.8 f
P-P
60.3 f 0.6
3.2 f 0.2
2.5 & 0.7
1.0
1.2
0.2 * 0.2 0.4 f 0.3
P-T
55.5 f 0.5
8.9 f 0.4
0.3 zt 0.2
CL-P
42.3 f 0.4
7.2 f 0.3
< 0.1
p-7797+rr-
50.4 f 0.7
5.1 f 0.4
0.3 f 0.3
/L--?r27ro
32.8 f 0.6
27.9 f 0.9
0.3 f 0.3
lr-*
47.1 f 1.0
14.3 f 0.9
1.o It 0.7
T-P
36.4 f 0.5
13.6 zh 0.5
0.2 f 0.2
n-mrflr-
42.8 f 0.8
13.7 f 0.8
0.2 zt 0.2
?r-?r2?ro
27.6 f 0.7
32.3 f 1.2
0.5 f 0.5
P-P
28.3 f 0.4
12.4 f 0.6
0.2 f 0.2
p-m+v-
30.9 f 0.5
10.8 f 0.4
0.3 f 0.2
p-7r2P
22.2 f 0.5
30.3 f 0.9
< 0.1
of the r has also been published by the OPAL Collaboration (]&I < 7.0.10-t7e.cmat the95%c.1.)[8].
2. ALEPH detector and event selection The ALEPH detector is described in detail elsewhere [ 91. The main components used to measure the energies and the momenta are the electromagnetic calorimeter (ECAL) and the tracking devices, namely the vertex detector (VDET) , the inner tracking chamber (ITC) and the time projection chamber (TPC). The particle identification relies on the dE/dx measurement in the TPC, on the ECAL, on the hadronic calorimeter (HCAL) and the muon chambers. This analysis employs data collected by ALEPH in the years 1990, 1991 and 1992, corresponding to 57000 produced r pairs. The event selection is based on a method developed for the analysis of the r polar-
375
isation [lo], and which is therein referred to as the neural net method. This includes particle identification and the cuts applied to classify the r decays as well as to reject the background from non-r events. In contrast to the polarisation analysis, events with both r leptons decaying into electrons are retained and the decay of the at into +r2?r” is included. Event classes involving the latter decays can be used safely despite the large background and the uncertainty in the description of the multipion decays of the 7, because the sensitivity of these classes comes mainly from the lepton, n- or p in the opposite hemisphere. The decay at -+ ~r25-o is identified by cutting on the reconstructed masses of the at, of the intermediate p and of the reconstructed ?r”‘s. In the case of two-electron events, severe cuts on the energy and ~os.f&,,~~ are applied to reduce background from Bhabha events as much as possible, because these events could fake a CP violating effect due to the forward-backward asymmetry of the t-channel contribution and to bremsstrahlung in the detector material, which leads to a systematic mismeasurement of the momenta. The resulting efficiencies and the background fractions for all classes are listed in Table 1.
3. Data analysis The constants EABare determined for each class separately using a Monte Carlo generator for the process Z 4 r+r- with CP violating couplings [ 111. The CAB’Scan be determined from the generated distributions with known & using relation (2). Since there are 18 different classes, which all have to be treated separately, several million events must be generated. Therefore, in order to save computing time a simplified detector simulation was used. Kinematic cuts were implemented at generator level, whereas acceptance variations in the kinematic variables were taken care of by a weighting procedure based on acceptances for single event hemispheres including factorized event acceptances. These weights have been determined from the Monte Carlo including the full detector simulation. To test the effects of the detector resolution, events with the full detector simulation were generated for three classes. It was found that the differences between the constants EATcomputed that way and using the simplified detector simulation
316
ALEPH Collaboration /Physics Letters B 346 (1995) 371-378
were negligible. Misidentification of r decay modes and background events change the value of the constants ?AB in the various event classes. This is considered computing effective values by making the sum of the ?A8 of the event class and the contributing background channels weighted by their relative fraction. The uncertainties on the ?ABof the background channels due to the cuts applied to reject that background were estimated from the effects of the cuts on the tAB of the event classes, where the cuts have been implemented in the Monte Carlo. These uncertainties together with the ones on the amount of background resulted in an additional error on the effective sensitivities. The main source of the error on the sensitivities is the statistical one. For the r decays into p or at mesons one can either use the reconstructed total momentum vector of the p or at to build ?ss, or the momentum vector of a single pion from the decay, which is the charged pion in the case of the p and the pion with unique charge in the case of the al. It was pointed out in [ 121 that the sensitivity in the cIasses with a leptonic decay on the other side will be higher if a single pion is taken to build ?sf;3,whereas for a hadronic decay it is better to use the momentum of the p or UI meson. The resulting values of the effective 2AB’sare shown in Table 2. Even though all steps of the selection are built to be C and P blind, the CP invariance of the selection was checked. Further checks concern the radiation of photons, which affects the measurement of the momenta, and the event reconstruction from the subdetectors used in the analysis. In order to avoid any systematic uncertainty from possible misalignments of the tracking devices a method is applied which ensures that the determination of d; is not affected by a possible shift in ?sp33 due to uncertainties in the track measurement. From Eq. (2) it follows that the influence of a systematic shift in i$s on & depends on the value of ZAB.The measured JAB for each individual class can be written as the sum of the physical dipole form factor & and a part originating from a detector shift A?, as discussed in [ 71:
Posed. The AABare then determined by minimising the square of the error on & with the two constraints x.AAB= 1 and ZAA~/?AB = 0. It can be noted that compared to the combined result of all classes with no constraint, this solution has practically the same statistical precision; this is due to the fact that the constants ZIABhave different signs. No effect on A? from the different momentum spectra of the decay particles of the 7 was found. The result is: & = (i-O.15 f 0.58,& lo-“e A? = -l-0.004 f 0.008 The ECAL, used for the reconstruction of TO’S, enters the measurement only in a few classes. Therefore a shift from the ECAL. cannot be eliminated with the above method. To test it, an event mixing method is used. The observable ?ss is built from combinations of p’s and al’s, respectively, originating from different events and having an acollinearity between the charged pions greater than 170° for the p and 160° for the at. Detector effects are then still visible in the mean value (fss), but a shift due to a true CP violation will vanish. In all these cases no systematic effect was found, and the statistical errors of the methods are used as systematic uncertainties on (fsij3).
4. Results and conclusion Using the data collected in the years 1990, 1991 and 1992 no signal of CP violation was found. Table 2 shows the results for & in the individual classes with the following errors: _ A@+ statistical error of the data. - AZ”: systematic error, resulting from the checks concerning the CP invariance of the selection and the radiation of photons. The contribution of AT is not propagated, because it is negligible and cancels out in the combined result of all classes. - AL$: error from the combined errors of the constants :AB. -
The shift can be eliminated in the combined result 2 = ZX,AAJA~if the constraint X.AAB/EAB = 0 is im-
. cm
AdTcAL: error originating from the uncertainty of
the event mixing. Correlations in the systematic errors between the classes have been taken into account for the combination of the individual measurements. The result ob-
ALEPH Collaboration/ Physics Letters B 346 (1995) 371-378
311
Table 2 The dipole form factors and the errors (in units of 10-l’ e. cm) and the constants PAa for the event classes used in the analysis. The specification of the errors is given in the text. Event class e-e
e-p
Number of events 668
f0.59
+5.1
4.2
1.5
0.6
-
2449
iO.72
-3.5
2.0
1.1
0.3
-
e-r
1101
-0.89
-0.5
2.3
I.1
< 0.1
e-p
1801
+0.36
+1.5
4.7
2.6
0.3
-0.9
5.2
2.2
0.2
e--7rlr+7r-
804
+0.49
e-r2?r0
560
+0.44
+2.5
6.9
1.9
0.7
P-P
1260
+0.90
+2.2
2.2
0.4
0.2
P-T
1463
-0.57
-5.8
3.3
0.6
0.7
P-P
2291
+0.39
+3.8
3.8
0.4
0.6
p-w+?r-
931
+0.49
-3.9
4.8
1.1
0.3
/.L-?r2?ro
730
+0.63
+0.9
4.3
0.6
0.2
?r-?r
479
-1.88
-1.4
1.7
0.2
0.1
-
=-P
1352
-1.53
fO.0
1.2
0.2
< 0.1
0.4
rr-rrrr+rr-
564
-1.54
+1.3
1.9
0.4
0.2
-
7r-7r27ro
461
-1.45
+1.1
2.2
0.4
0.2
1.1
P-P
1100
-0.95
+1.9
2.1
0.6
0.2
0.6
p?rP+T-
889
-0.70
+6.2
3.4
1.1
1.6
0.8
p-?r2lro
713
-0.72
-2.7
3.7
1.3
0.6
2.2
mined with 19628 identified T+T- events considering the r decay modes evv, ,uvv, TV, pv and ulv is:
d; = (+0.15 f 0.5f& f 0.38,,,) lo-t7 e . cm [cl,\ < 1.5. 10-17e.cm
(95Sc.l.)
Acknowledgements
We wish to thank our colleagues from the accelerator division for the operation of LEI? We are indebted to the engineers and technicians in all our institutions for their contribution to the good performance of ALEPH. Those of us from non-member states thank CERN for its hospitality.
-
References [ 11 W. Bemreuther, U. Liiw, J. P Ma, O.Nachtmann, Z. Phys. C43 (1989) 117 12] W. Bemreuther, T. Schroder, T. N. Pham, Phys. Lett. B 279, (1992) 389 S. M. Barr, A. Masiero, Phys. Rev. Len 187B ( 1987) 187 A. Barroso, J. Maalampi, Phys. Lett. 187B ( 1987) 85 W. Bemmuther in Proceedings of the LP-HBP91 conference edited by S. Hegarty, K. Potter, E. Quercigh, World Scientific, Singapore (1992) 211 R. N.Mohapatra in CP Violation - Advanced Series on Directions in High Energy Physics Vol.3 edited by C. Jarlskog, World Scientitic, Singapore, 1989 J.C.Pati, A. Salam, Phys. Rev. DlO (1975) 275 R.N. Mohapatra, J. C. Pati, Phys. Rev. Dl 1 ( 1975) 566.2558 G.Bcker, W. Grimus, H. Neufeld, Nucl. Phys. B247 ( 1984) 70 [3] D. Schaile, Rappotteur talk given at the 27th International Conference on High Energy Physics, Glasgow, Scotland, 20-27 Jul. 1994 [4] W. Hollii in Proceedings of the XVI International Symposium on Lepton and Photon Interactions, Ithaca, 1993,
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ALEPH Collaboration /Physics Letters B 346 (1995) 371-378
edited by P Drell and D. Rubin, AIP, New York ( 1994) 352 [5] W. Bemreuther, 0. Nachtmann, Phys. Rev. Lett. 63 (1989) 2787 [ 61 W. Bemreuther, G. W. Botz, 0. Nachtmann, P Overmann, 2. Phys. C52 (1991) 567 [7] D.Buskulic et al., ALEPH Collaboration, Phys. Lett. B297 ( 1992) 459 181 P. D. Acton et al., OPAL Collaboration, Phys. L&t. B281 (1992) 405 In a recent preprint, P D. Acton et al., OPAL Collaboration, CERN-PPE/94-171, an improved limit has been given.
[9] D. Decamp et al., ALEPH Collaboration, Nucl. Inst. and Meth. A294 (1990) 121 [ 101 D.Buskulic et al., ALEPH Collaboration, Z. Phys. C59 (1993) 369 [ II] Program written by W. Bemreuther, P Overmann. private communication [ 121 W. Bernreuther, 0. Nachtmann, P.Overmann, Phys. Rev. D48 (1993) 78
PHYSICS
ELSPYIER
LETTERS
6
Physics Letters B 346 (1995) 371-378
Search for CP violation in the decay Z -+ T+TALEPH Collaboration D. Buskulic a, D. Casper a, I. De Bonis a, D. Decamp a, P. Ghez a, C. Goy a, J.-P. Lees a, M.-N. Minard a, P. Odier a, B. Pietrzyka, F. Ariztizabal b, M. Chmeissani b, J.M. Crespo b, V. Gaitanb, Ll. Garridoby15, I. Efthymiopoulos b, E. Fernandez b, M. Fernandez-Bosmanb, M. Martinez b, S. Orteu b, A. Pacheco b, C. Padilla b, F. Palla b, A. Pascual b, J.A. Perlas b, F. Sanchez b, F. Teubert b, D. Creanza ‘, M. de PalmaC, A. Farilla ‘, G. Iaselli ‘, G. Maggi ‘, N. MarinelliC, S. Natali ‘, S. NuzzoC, A. RameriC, G. Raso ‘, F. RomanoC, F. Ruggieri ‘, G. Selvaggi c, L. Silvestris ‘, P. Tempesta c, G. Zito ‘, X. Huang d, J. Lin d, Q. Ouyang d, T. Wang d, Y. Xie d, R. Xu d, S. Xue d, J. Zhang d, L. Zhang d, W. Zhao d, G. Bonvicini e, P. Comas e, P. Coyle e, H. Drevermanne, A. Engelhardt e, R.W. Forty e, M. Frank”, G. Ganis e, C. Gayev3, M. Gironee, R. Hagelberge, J. Harveye, R. Jacobsen e, B. Joste, J. Knobloche, I. Lehrause, M. Maggie, C. Markoue, E.B. Martin e, P. Mato e, H. Meinharde, A. Minten e, R. Miquel e, P. Palazzi e, J.R. Pater e, P. Perrodo e, J.-F. Pusztaszeri e, F. Ranjard e, L. Rolandi e, D. Schlatter”, M. Schmellinge, W. Tejessy e, I.R. Tomaline, R. Veenhof e, A. Venturi e, H. Wachsmuthe, W. Wiedenmanne, W. Witzelinge, J. Wotschacke, Z. Ajaltouni f, M. Bardadin-Otwinowskaf, A. Barresf, C. Boyer f, A. Falvardf, P. Gay f, C. Guicheney f, P. Henrardf, J. Jousset f, B. Michel f, S. Monteil f, J-C. Montret f, D. Pallin f, P. Perretf, F. Podlyski f, J. Proriol f, J.-M. Rossignol f, F. Saadi f, T. Fearnleys, J.B. Hansens, J.D. Hansen a, J.R. Hansen s, P.H. Hansen s, S.D. Johnson s, R. Mollerud s,B.S. Nilsson s, A. Kyriakis h, E. Simopoulou h, I. Siotis h, A. Vayaki h, K. Zachariadou h, A. Blondel’, G. Bonneaud’, J.C. Brient i, P. Bourdon’, L. Passalacqua’, A. Rouge’, M. Rumpf’, R. Tanaka i, A. Valassi i, M. Verderi i, H. Videau i, D.J. Candlin j , M.I. Parsons j, E. Veitch j, E. Focardi k, G. Parrini k, M. Corden ‘, M. Delfino ‘I’~, C. Georgiopoulos e, D.E. Jaffe e, A. Antonelli m, G. Bencivenni m, G. Bolognam*4, F. Bossi m, P. Campanam, G. Capon m, F. Cerutti”, V. Chiarellam, G. Felicim, P. Laurelli”, G. Mannocchi m~5,F. Murtasm, G.P. Murtas m, M. Pepe-Altarellim, S. Salomonem, P. Colrain n, I. ten Have n,6, I.G. Knowles n, J.G. Lynch”, W. Maitland”, W.T. Morton”, C. Raine”, P. Reeves”, J.M. Starr”, K. Smith”, M.G. Smith”, AS. Thompson”, S. Thorn”, R.M. Turnbull”, U. Becker O, 0. Braun’, C. Geweniger O, G. Graefe”, P. Hanke O, V. Hepp O, E.E. Kluge O, A. Putzer O, B. Rensch O, M. Schmidt O, J. Sommer O, H. Stenzel’, K. Tittel’, M. WunschO, R. Beuselinckr, D.M. Binnie r, W. Cameron P, M. Cattaneo P, D.J. Colling P, PJ. Dornan P, J.F. Hassard P, 0370-2693/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSDI 0370-2693(95)00093-3
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N. Konstantinidis P, L. Moneta P, A. Moutoussi r, J. Nash p, D.G. Payne P, G. San Martin P, J.K. Sedgbeerr, A.G. Wright p, G. Dissertori q, P. Girtlerq, E. Kneringerq, D. Kuhn q, G. Rudolph 9, C.K. Bowdery ‘, T.J. Brodbeck r, A.J. Finch’, F. Foster r, G. Hughes r, D. Jackson r, N.R. Keemer r, M. Nuttall r, A. Pate1r, T. Sloan r, S.W. Snow r, E.P. Whelan r, A. GallaS, A.M. Greene”, K. KleinknechtS, J. Raabs, B. RenkS, H.-G. Sanders, H. SchmidtS, S.M. Walther ‘, R. War&e ‘, B. Wolf ‘, A.M. Bencheikh t, C. Benchouk t, A. Bonissent t, D. Calvet t, J. Carr t, C. Diaconu t, F. Etienne t, D. Nicod f, P Payre t, L. Roos t, D. Rousseau t, M. Talby t, I. Abt “, S. Adlung “, R. Assmann “, C. Bauer “, W. Blum”, D. Brown “, P Cattaneo u,23,B. Dehning “, H. Diet1 “, F. Dydak “12*,A.W. Halley “, K. Jakobs “, H. Kroha “, J. Lauber “, G. Ltitjens “, G. Lutz”, W. Manner”, H.-G. Moser”, R. Richter”, J. Schroder”, A.S. Schwarz”, R. Settles “, H. Seywerd”, U. StierlinUy2,U. Stiegler “, R. St. Denis “, G. Wolf “, R. Alemany “, J. Boucrot “, 0. Callot “, A. Cordier “, F. Courault “, M. Davier “, L. Duflot “, J.-F. Grivaz “, Ph. Heusse “, M. Jacquet “, P. Janot ‘, D.W. Kim “,19, F. Le Diberder “, J. Lefraqois ‘, A.-M. Lutz “, G. Musolino “, I. Nikolic “, H.J. Park “, I.C. Park “, M.-H. Schune “, S. Simion “, J.-J. Veillet “, I. Videau “, D. Abbaneo w, G. Bagliesi w, G. Batignani w, S. Bettarini w, U. Bottigli w, C. Bozzi w, G. Calderini w, M. Carpinelli w, M.A. Ciocci w, V. Ciulli w, R. Dell’Orso w, I. Ferrante w, F. Fidecaro w, L. Foaw”, F. Forti w, A. Giassi w, M.A. Giorgi w, A. Gregorio w, F. Ligabue w, A. Lusiani w, P.S. Marrocchesi w, A. Messineo w, G. Rizzo w, G. Sanguinetti w, A. Sciaba w, l? Spagnolo w, J. Steinberger w, R. Tenchini w, G. Tonelli w,26,G. Triggiani w, C. Vannini w, PG. Verdini w, J. Walsh w, AI? Betteridge ‘, G.A. BlairX, L.M. Bryant”, Y. Gao”,M.G. Green”, D.L. Johnson x, T. Medcalf ‘, L1.M. Mir x, J.A. Strong x, V. Bertin Y,D.R. Botterill Y, R.W. Clifft Y,T.R. Edgecock Y,S. Haywood Y,M. Edwards Y,P. Maley Y,P.R. Nortony, J.C. Thompson Y,B. Bloch-Devaux z, l? Colas ‘, H. Duarte ‘, S. Emery ‘, W. Kozanecki ‘, E. Langon ‘, M.C. Lemaire ‘, E. Locci ‘, B. Marx z, P Perez ‘, J. Rander ‘, J.-F. Renardy ‘, A. Rosowsky ‘, A. Roussarie ‘, J.-P Schuller ‘, J. Schwindling”, D. Si Mohand ‘, A. Trabelsi ‘, B. Vallage z, R.P. Johnson aa, A.M. Litke aa, G. Taylor aa, J. Wear aa, A. Beddall ab, C.N. Boothab, C. Boswellab, S. Cartwrightab, F. Combley ab,I. Dawsonab, A. Koksalab, M. Letho ab, W.M. Newtonab, C. Rankin ab, L.F. Thompson ab, A. Btihrer ac, S. Brandt ac, G. Cowan ac, E. Feigl ac, C. Grupen ac, G. Lutters ac, J. Minguet-Rodriguez ac, F. Riveraacs2’, P. SaraivaaC, U. Sch3ferac, L. SmolikaC, L. Bosisio ad, R. Della Marinaad, G. Giannini ad, B. Gobboad, L. Pitisd, F. Ragusaad,20,H. Kimae, J. Rothbergae, S. Wasserbaechae, S .R. Armstrong af, L. Bellantoni af, J.S. Conway af,24,P Elmer af, Z. Feng af, D.P.S. Ferguson af, Y.S. Gao af, S. Gonzales af, J. Grahl af, J.L. Harton af, O.J. Hayes af, H. HUaf, P.A. McNamara IIIaf, J.M. Nachtmanaf, W. Orejudosaf, Y.B. Panaf, Y. Saadiaf, M. Schmitt af, I. Scott af, V. Sharmaaf, J.D. Turkaf, A.M. Walshaf, F.V. Weberaf*‘, T. Wildish&, Sau Lan WU af, X. Wu af, J.M. Yamartino &, M. Zheng af, G. Zobernig af a hboratoire de Physique des Pardcules (LAPP), IN2P3-CNRS. 74019 Annecy-le-Vieux Cedex, France b Institut de Fisica d’Altes Energies, Universitat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spain7 c Dipartimento di Fisica. INFN Sezione di Bari, 70126 Bari, Italy d Institute of High-Energy Physics, Academia Sinica. Beijing, China8
ALEPH Collaboration/Physics Letters B 346 (1995) 371-378
313
e European Laboratory for Parricle Physics (CERN), 1211 Geneva 23, Switzerland ’ Luboratoire de Physique Corpusculaire, Universite Blaise Pascal, lN2P3-CNRS, Clemwnt-Ferrand, 63177 AubiPre, France g Niels Bohr Institute, 2100 Copenhagen, Denmark9 h Nuclear Research Center Demokritos (NRCD), Athens, Greece
i Laboratoire de Physique Nucldaire et des Hautes Energies, Ecole Polytechnique, lN2P3-CNRS, 91128 Palaiseau Cedex, France j Department of Physics, Vniversiv of Edinburgh, Edinburgh EH9 3JZ, UK lo k Dipartimento di Fisica, Vniversitd di Firenze, INFN Sezione di Firenze, 50125 Firenze, Italy e Supercomputer Computations Research Institute, Florida State University. Tallahassee, FL 32306-4052, USA 13*14 m Laboratori Nazionaii dell’lNFN (LNF-lNFN), 00044 Frascati, Italy D Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK lo 0 lnstitut fiir Hochenergiephysik, VniversiUtt Heidelberg, 69120 Heidelberg, Germany’6 Q Department of Physics, Imperial College, London SW7 2BZ UK lo 9 Institut fiir Experimentalphysik, Vniversitiit Innsbruck, 6020 Innsbruck, Aust?Ya‘8 r Department of Physics, University of Lancaster, Lancaster LA1 4YB. UK I0 ’ lnstitut jiIr Physik, Vniversitift Mainz, 55099 Mainz, Germany I6 ’ Centre de Physique des Particules, Facultt! des Sciences de Luminy, lN2P3-CNRS 13288 Marseille, France ’ Max-Plunck-lnstitut fr Physik, Werner-Heisenberg-lmtitut, 80805 Miinchen, Germany’6 v Laboratoire de l’Acct%+ateur Linkaire, Vniversitb de Paris-Sud, IN2P3-CNRS, 91405 Orsay Cedex, France w Dipartimento di Fisica dell’llniversitci, INFN Sezione di Piss, e Scuoia Normale Superiore, 56010 Pisa, Italy x Department of Physics, Royal Holloway & Bedford New College, University of London, Surrey TW20 OEX, VKLO Y Particle Physics Dept.. Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, UK lo ’ CEA, DAPNlAIService de Physique des Particules, CE-Saclay, 91191 Gif-sur-Yvette Ceder France’? &1Institute for Particle Physics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA== ab Department of Physics, University of Shefleld, Shefield S3 7RH. UK lo ac Fachbereich Physik, Vniversitiit Siegen, 57068 Siegen, Germany lb ad Dipartimento di Fisica, Vniversitci di Trieste e INFN Sezione di Trieste, 34127 Trieste, Italy ae Experimental Elementary Particle Physics, University of Washington, WA 98195 Seattle, USA af Department of Physics, University of Wisconsin. Madison, WI 53706, USA”
Received 23 November 1994 Editor: K. Winter
Abstract Data collected by ALEPH in the years 1990, 1991 and 1992 have been used to update a previous search for CP violation in the decay of the Z into T+T-. The measurement of the weak dipole form factor of the 7 lepton has been performed by studying correlations between the T leptons. No signal of CP violation was found. The weak dipole form factor is found to be (i, = (+0.15 f 0.58Sbt f 0.38,,) lo-“e . cm, obtained with 19628 identified T+T- events. This gives an upper limit on the weak dipole form factor of l&l < 1.5 . lo-“e - cm at the 95% confidence level.
’ Now at CERN, 1211 Geneva 23, Switzerland. 2 Deceased. 3 Now at Harvard University, Cambridge, MA 02138, USA. 4 Also Istituto di Fisica Generale, Universiti di Torino, Torino, Italy. 5 Also Istituto di Cosmo-Geofisica de1 C.N.R., Torino, Italy. 6 Now at TSM Business School, Enscbede, The Netherlands. ’ Supported by CICYT, Spain. ’ Supported by the National Science Foundation of China. g Supported by the Danish Natural Science Research Council. lo Supported by the UK Science and Engineering Research Council.
I1 Supported by the US Department of Energy, contract DEAC02-76EROO881. I* On leave from Universitat Autonoma de Barcelona, Barcelona, Spain. l3 Supported by the US Department of Energy, contract DEFGOS-92ER40742. I4 Supposed by the US Department of Energy, contract DE-FCOS85ER250000. I5 Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain. l6 Supported by the Bundesministerium ftir Forschung und Technologie, Germany.
ALEPH Collaboration/ Physics Letters B 346 (1995) 371-378
374
1. Introduction The decay of the 2 boson into r+r- is an appropriate process to search for signals of CP violation caused by new interactions. Since in the Standard Model CP violation occurs only in the couplings of the charged current as described in the CKM scheme, CP odd contributions to the reaction Z --f r+r- are at most of the order of IO-’ [ 1] compared to the electroweak amplitude at the Born level. On the other hand extensions of the Standard Model, such as models with leptoquarks or with extended Higgs sectors, but also left-right symmetric or supersymmetric interactions can generate CP violating effects at the Zrr vertex [ 21. The parametrization of these contributions by the weak dipole form factor &(q* = m$)of the r lepton at the Z resonance gives a model independent description of the on-shell amplitude, which in this case consists of the coherent sum of the electroweak amplitude and an amplitude proportional to the weak dipole form factor &. An indirect limit on a dipole form factor can be derived from the measurement of the Z partial width IYr, since there would be an additional contribution from the weak dipole form factor Al?, M (&(*m$/(24r) [ 11. A contribution of an electric dipole form factor is expected to be small [ 51. From the comparison of the value measured at LEP, I, = (84.26&0.34)MeV [ 31, and the theoretical prediction of the Standard Model, IsM = (83.7 f 0.4)MeV [4], one obtains an indir&t upper limit on the weak dipole form factor of ]&I < 2.3. 10-17e.cm (95 % c.1.). However, if there are new physics effects, there may also be CP even
I7Supported 1s Supported
by the Direction des Sciences de la Matiere, C.E.A. by Fonds zur Fiirderung der wissenschafthchen
Forschung, Austria. I9 Permanent address: Kangnung National University, Kangnung, Korea. 2oN~~ at Dipartimento di Fisica, Universita di Milano, Milano, Italy. 2L Also at CERN, 1211 Geneva 23, Switzerland. 22 Supported by the US Department of Energy, grant DEFG0392ER40689. 23 Now at Universita di Pavia, Pavia, Italy. 24 Now at Rutgers University, Piscataway, NJ 08854, USA. 25 Partially supported by Colciencias, Colombia. 26 Also at lstituto di Matematica e Fisica, Universim di Sassari, Sassari,
Italy.
contributions to the partial width, which can cancel partially the contribution from &. In order to measure the real part of the dipole form factor directly from CP odd correlations between the r leptons, we use the following CP odd tensor observables [ 561:
cj=(8+- iqi. i,j=l,2,3
(fi+’ ,$+
x
fi-)j
B_I
+
(i -
j> (1)
$+ ($-) denotes the normalized momentum vector of one of the decay particles of the r+ (r- ), i, j are Cartesian coordinate indices. The relation between the observable and the real part of the dipole form factor & is given by [5,6]:
A and B are the decay modes of r- and r+, respectively. The constants 2~s describe the sensitivity of the angular distributions of the decay particles used to build ? to the spin states of the r leptons. Therefore these constants depend in magnitude and sign explicitly on the decay mode of r- and r+. This analysis uses the major r decay modes evy, ,WV, TV, pv, and al v, with at -+ rn+n-- and al -+ ~29. This results in 18 different classes, the three classes al-at are not used due to low statistics and high backgrounds. The matrix Sij = diag( -i, -h, i) is a diagonal matrix representing the tensor polarisation of the Z boson in the coordinate system in which the z axis is determined by the direction of the incident positron beam. As can be seen from the matrix sij, i@s;3 is the most sensitive component and will give the largest signal if & # 0. Since in addition the three diagonal elements are strongly correlated due to the fact that the trace of ? must be zero and since the systematic checks are easier to perform for ?ss, only psp33 is used in this analysis. The analysis described in this paper improves upon and supersedes the result published by the ALEPH Collaboration (I&] < 3.7 . lo-“e . cm at the 95 % c.1.) [ 71. This is due to a better selection, yielding about 30% more acceptance, the inclusion of the r decays into ai and a doubling of the collected data. A limit on the weak dipole form factor
ALEPH Collaboration/
Physics Letters B 346 (1995) 371-378
Table 1 Efficiencies and background with statistical errors for the event classesused in the analysis. All numbers are given in percent. Background fractions from
Event
Selection-
class
efficiency
7 events
other processes
e-e
36.4 f 0.6
2.8 f 0.3
3.6f
e-p
67.9 f 0.4
3.2 f 0.2
l.O& 0.3
e-a
39.3 f 0.5
9.9 f 0.5
0.3 & 0.3
e-p
33.3 f 0.4
7.4 * 0.4
0.2 f 0.2
e-7777+7r-
41.7 f 0.7
5.5 f 0.5
e-7r2v"
25.2 f 0.6
26.8 f
P-P
60.3 f 0.6
3.2 f 0.2
2.5 & 0.7
1.0
1.2
0.2 * 0.2 0.4 f 0.3
P-T
55.5 f 0.5
8.9 f 0.4
0.3 zt 0.2
CL-P
42.3 f 0.4
7.2 f 0.3
< 0.1
p-7797+rr-
50.4 f 0.7
5.1 f 0.4
0.3 f 0.3
/L--?r27ro
32.8 f 0.6
27.9 f 0.9
0.3 f 0.3
lr-*
47.1 f 1.0
14.3 f 0.9
1.o It 0.7
T-P
36.4 f 0.5
13.6 zh 0.5
0.2 f 0.2
n-mrflr-
42.8 f 0.8
13.7 f 0.8
0.2 zt 0.2
?r-?r2?ro
27.6 f 0.7
32.3 f 1.2
0.5 f 0.5
P-P
28.3 f 0.4
12.4 f 0.6
0.2 f 0.2
p-m+v-
30.9 f 0.5
10.8 f 0.4
0.3 f 0.2
p-7r2P
22.2 f 0.5
30.3 f 0.9
< 0.1
of the r has also been published by the OPAL Collaboration (]&I < 7.0.10-t7e.cmat the95%c.1.)[8].
2. ALEPH detector and event selection The ALEPH detector is described in detail elsewhere [ 91. The main components used to measure the energies and the momenta are the electromagnetic calorimeter (ECAL) and the tracking devices, namely the vertex detector (VDET) , the inner tracking chamber (ITC) and the time projection chamber (TPC). The particle identification relies on the dE/dx measurement in the TPC, on the ECAL, on the hadronic calorimeter (HCAL) and the muon chambers. This analysis employs data collected by ALEPH in the years 1990, 1991 and 1992, corresponding to 57000 produced r pairs. The event selection is based on a method developed for the analysis of the r polar-
375
isation [lo], and which is therein referred to as the neural net method. This includes particle identification and the cuts applied to classify the r decays as well as to reject the background from non-r events. In contrast to the polarisation analysis, events with both r leptons decaying into electrons are retained and the decay of the at into +r2?r” is included. Event classes involving the latter decays can be used safely despite the large background and the uncertainty in the description of the multipion decays of the 7, because the sensitivity of these classes comes mainly from the lepton, n- or p in the opposite hemisphere. The decay at -+ ~r25-o is identified by cutting on the reconstructed masses of the at, of the intermediate p and of the reconstructed ?r”‘s. In the case of two-electron events, severe cuts on the energy and ~os.f&,,~~ are applied to reduce background from Bhabha events as much as possible, because these events could fake a CP violating effect due to the forward-backward asymmetry of the t-channel contribution and to bremsstrahlung in the detector material, which leads to a systematic mismeasurement of the momenta. The resulting efficiencies and the background fractions for all classes are listed in Table 1.
3. Data analysis The constants EABare determined for each class separately using a Monte Carlo generator for the process Z 4 r+r- with CP violating couplings [ 111. The CAB’Scan be determined from the generated distributions with known & using relation (2). Since there are 18 different classes, which all have to be treated separately, several million events must be generated. Therefore, in order to save computing time a simplified detector simulation was used. Kinematic cuts were implemented at generator level, whereas acceptance variations in the kinematic variables were taken care of by a weighting procedure based on acceptances for single event hemispheres including factorized event acceptances. These weights have been determined from the Monte Carlo including the full detector simulation. To test the effects of the detector resolution, events with the full detector simulation were generated for three classes. It was found that the differences between the constants EATcomputed that way and using the simplified detector simulation
316
ALEPH Collaboration /Physics Letters B 346 (1995) 371-378
were negligible. Misidentification of r decay modes and background events change the value of the constants ?AB in the various event classes. This is considered computing effective values by making the sum of the ?A8 of the event class and the contributing background channels weighted by their relative fraction. The uncertainties on the ?ABof the background channels due to the cuts applied to reject that background were estimated from the effects of the cuts on the tAB of the event classes, where the cuts have been implemented in the Monte Carlo. These uncertainties together with the ones on the amount of background resulted in an additional error on the effective sensitivities. The main source of the error on the sensitivities is the statistical one. For the r decays into p or at mesons one can either use the reconstructed total momentum vector of the p or at to build ?ss, or the momentum vector of a single pion from the decay, which is the charged pion in the case of the p and the pion with unique charge in the case of the al. It was pointed out in [ 121 that the sensitivity in the cIasses with a leptonic decay on the other side will be higher if a single pion is taken to build ?sf;3,whereas for a hadronic decay it is better to use the momentum of the p or UI meson. The resulting values of the effective 2AB’sare shown in Table 2. Even though all steps of the selection are built to be C and P blind, the CP invariance of the selection was checked. Further checks concern the radiation of photons, which affects the measurement of the momenta, and the event reconstruction from the subdetectors used in the analysis. In order to avoid any systematic uncertainty from possible misalignments of the tracking devices a method is applied which ensures that the determination of d; is not affected by a possible shift in ?sp33 due to uncertainties in the track measurement. From Eq. (2) it follows that the influence of a systematic shift in i$s on & depends on the value of ZAB.The measured JAB for each individual class can be written as the sum of the physical dipole form factor & and a part originating from a detector shift A?, as discussed in [ 71:
Posed. The AABare then determined by minimising the square of the error on & with the two constraints x.AAB= 1 and ZAA~/?AB = 0. It can be noted that compared to the combined result of all classes with no constraint, this solution has practically the same statistical precision; this is due to the fact that the constants ZIABhave different signs. No effect on A? from the different momentum spectra of the decay particles of the 7 was found. The result is: & = (i-O.15 f 0.58,& lo-“e A? = -l-0.004 f 0.008 The ECAL, used for the reconstruction of TO’S, enters the measurement only in a few classes. Therefore a shift from the ECAL. cannot be eliminated with the above method. To test it, an event mixing method is used. The observable ?ss is built from combinations of p’s and al’s, respectively, originating from different events and having an acollinearity between the charged pions greater than 170° for the p and 160° for the at. Detector effects are then still visible in the mean value (fss), but a shift due to a true CP violation will vanish. In all these cases no systematic effect was found, and the statistical errors of the methods are used as systematic uncertainties on (fsij3).
4. Results and conclusion Using the data collected in the years 1990, 1991 and 1992 no signal of CP violation was found. Table 2 shows the results for & in the individual classes with the following errors: _ A@+ statistical error of the data. - AZ”: systematic error, resulting from the checks concerning the CP invariance of the selection and the radiation of photons. The contribution of AT is not propagated, because it is negligible and cancels out in the combined result of all classes. - AL$: error from the combined errors of the constants :AB. -
The shift can be eliminated in the combined result 2 = ZX,AAJA~if the constraint X.AAB/EAB = 0 is im-
. cm
AdTcAL: error originating from the uncertainty of
the event mixing. Correlations in the systematic errors between the classes have been taken into account for the combination of the individual measurements. The result ob-
ALEPH Collaboration/ Physics Letters B 346 (1995) 371-378
311
Table 2 The dipole form factors and the errors (in units of 10-l’ e. cm) and the constants PAa for the event classes used in the analysis. The specification of the errors is given in the text. Event class e-e
e-p
Number of events 668
f0.59
+5.1
4.2
1.5
0.6
-
2449
iO.72
-3.5
2.0
1.1
0.3
-
e-r
1101
-0.89
-0.5
2.3
I.1
< 0.1
e-p
1801
+0.36
+1.5
4.7
2.6
0.3
-0.9
5.2
2.2
0.2
e--7rlr+7r-
804
+0.49
e-r2?r0
560
+0.44
+2.5
6.9
1.9
0.7
P-P
1260
+0.90
+2.2
2.2
0.4
0.2
P-T
1463
-0.57
-5.8
3.3
0.6
0.7
P-P
2291
+0.39
+3.8
3.8
0.4
0.6
p-w+?r-
931
+0.49
-3.9
4.8
1.1
0.3
/.L-?r2?ro
730
+0.63
+0.9
4.3
0.6
0.2
?r-?r
479
-1.88
-1.4
1.7
0.2
0.1
-
=-P
1352
-1.53
fO.0
1.2
0.2
< 0.1
0.4
rr-rrrr+rr-
564
-1.54
+1.3
1.9
0.4
0.2
-
7r-7r27ro
461
-1.45
+1.1
2.2
0.4
0.2
1.1
P-P
1100
-0.95
+1.9
2.1
0.6
0.2
0.6
p?rP+T-
889
-0.70
+6.2
3.4
1.1
1.6
0.8
p-?r2lro
713
-0.72
-2.7
3.7
1.3
0.6
2.2
mined with 19628 identified T+T- events considering the r decay modes evv, ,uvv, TV, pv and ulv is:
d; = (+0.15 f 0.5f& f 0.38,,,) lo-t7 e . cm [cl,\ < 1.5. 10-17e.cm
(95Sc.l.)
Acknowledgements
We wish to thank our colleagues from the accelerator division for the operation of LEI? We are indebted to the engineers and technicians in all our institutions for their contribution to the good performance of ALEPH. Those of us from non-member states thank CERN for its hospitality.
-
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edited by P Drell and D. Rubin, AIP, New York ( 1994) 352 [5] W. Bemreuther, 0. Nachtmann, Phys. Rev. Lett. 63 (1989) 2787 [ 61 W. Bemreuther, G. W. Botz, 0. Nachtmann, P Overmann, 2. Phys. C52 (1991) 567 [7] D.Buskulic et al., ALEPH Collaboration, Phys. Lett. B297 ( 1992) 459 181 P. D. Acton et al., OPAL Collaboration, Phys. L&t. B281 (1992) 405 In a recent preprint, P D. Acton et al., OPAL Collaboration, CERN-PPE/94-171, an improved limit has been given.
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