1 SEARCH FOR SINGLE LEPTOQUARK PRODUCTION IN ELECTRON-PHOTON SCATTERING p a AT s = 161 AND 172 GEV STEFAN SOLDNER-REMBOLD for the OPAL collab oration Universitat Freiburg, Hermann-Herder-Str. 3, D-79104 Freiburg i. Br., Germany A search for a rst generation scalar lepto quark (LQ) has b een p erformed using + the data collected by the OPAL detector in 1996 at e e centre-of-mass energies p s of 161 and 172 GeV. It is assumed that a single lepto quark can b e pro duced in the pro cess eq! LQ, where the initial state quark originates from a hadronic uctuation of a quasi-real photon which has b een radiated by one of the LEP b eams. Lower limits at the 95 % con dence level on the mass of a rst generation scalar lepto quark of 131 GeV for =0:5 and = 1, coupling values larger than p 4 and lepto quark charges 1=3or5=3 are obtained. em 1 Kinematics and Monte Carlo simulations Lepto quarks are coloured spin 0 or spin 1 particles carrying b oth baryon and lepton quantum numb ers. Recently it has b een suggested to search for lep- 1 to quarks in electron-photon collisions at LEP . The photon, which has b een radiated by one of the LEP b eams, serves as a source of quarks through its uctuations into hadronic states. The electron-quark interaction pro duces a lepto quark which is assumed to decay subsequently into an electron or a neu- b trino and a quark. In electron-photon scattering rst generation lepto quarks of charge 1=3, 5=3, 2=3 and 4=3 can b e pro duced. The cross-section to pro duce charge 2=3 and 4=3 lepto quarks is suppressed, since there is less d quark content in the photon than u quark. The limits will therefore b e given for lepto quark charges 1=3 or 5=3. The cross-sections in e scattering for b oth charge states are identical, since it is equally probable to nd a u or a u quark in the photon. In principle this search is also sensitive to electron-charm states, since the probability for a photon to split into ccoruu is exp ected to b e ab out equal for lepto quark masses M>>m . Furthermore it has b een assumed that c either left or right handed couplings to fermions vanish. The cross-sections in e scattering for b oth couplings are identical, whereas the branching ratio a To b e published in the pro ceedings of PHOTON'97, Egmond aan Zee b Charge conjugation is implied throughout this pap er and p ositrons are referred to as electrons FREIBURG-EHEP-97-04 2 into eq nal states is 1 for right handed couplings and 1/2 for left handed 2 couplings . The total cross-section for the pro duction of scalar lepto quarks of mass M is a convolution of the Weizsacker-Williams e ective photon distribution f (z), with z b eing the momentum fraction carried by the photon, and the =e 2 parton distribution functions f (x; ) of the photon, evaluated at the scale q= 1 = M : Z 1 2 dz 2 2 2 + f (z; Q )f (M =(zs);M ): (1) (e e ! LQ+X)= =e q= max 2s z 2 M =s 3;4 The Monte Carlo simulation of this pro cess is done with PYTHIA 5.722 . In 2 the simulation the maximum photon virtuality Q used in the Equivalent max Photon Approximation equals s=4, but the simulated photon is always real 2 5 (Q = 0). The GRV parametrisation of the parton distribution functions was used. In the kinematic region relevant for lepto quark pro duction the variations of the cross-section due to the di erent parameterisations are small. Interference e ects with deep-inelastic e scattering are also neglected. The p total cross-section in PYTHIA for s = 172 is ab out 10{20 % lower than the p cross-sections given for s = 175 in Ref. 1. Vector lepto quarks can currently not b e simulated with PYTHIA. The limits are therefore given only for scalar lepto quarks. The standard PYTHIA Monte Carlo has b een mo di ed to include LQ ! d decays in addition to the standard LQ ! eu decays. e 2 Event Analysis Jets were reconstructed using a cone jet nding algorithm with a cone size R = 1 and a cut on the minimum transverse jet energy E of 15 GeV. Tracks T and calorimeter clusters were used as input for the jet nding algorithm and for determining the missing transverse energy E= of the event. A matching algo- T rithm b etween tracks and clusters is applied. The electron was identi ed using 6 the standard OPAL neural net electron identi cation . All relevant Standard Mo del background pro cesses were studied using Monte Carlo generators. The 1 total data sample corresp onds to an integrated luminosity of 20.5 pb . 2.1 The electron plus hadronic jet channel For this channel the identi ed electron with the largest momentum was as- sumed to b e the electron from the lepto quark decay. The electron is usually reconstructed as a jet. Candidate events were selected based on the following cuts: FREIBURG-EHEP-97-04 3 + + In order to reduce background from deep-inelastic e and e e ! events, exactly two jets must have b een found in the event(n = 2). j + + + + A large number of e e ! and e e ! e e events are rejected by requiring a minimum numb er of 5 reconstructed tracks (n 5). In ch p s has to b e less than 0.9, where E is addition, the ratio E = ECAL ECAL the energy in the electromagnetic calorimeter. The missing transverse energy E= must b e less than 15 GeV in order to T + + reduce background from and W W pair pro duction. An isolation cut is applied on the identi ed electron. The jet with the smallest angular distance to the electron is chosen to b e the electron jet. j The di erence between the energy E of this jet and the energy E of e e + the electron must b e less than 4 GeV. Most multihadronic e e ! qq events are removed by this cut. Events where an electron was scattered at a small angle are rejected by requiring for the angle of the electron j cos j < 0:85. e The total multiplicity n of the quark jet must b e n 7, where n is q q q the total numb er of tracks and calorimeter clusters asso ciated to this jet. The cuts on the transverse momenta of the jets and on the angle j cos j of the e electron reduce signi cantly the sensitivity to nd a lepto quark which is lighter than approximately M =2, the region excluded by the LEP1 searches. These Z cuts are necessary to reduce the background from deep-inelastic e events which b ecomes increasingly imp ortant at small masses. After all cuts we exp ect a background of 5:2 0:4events from all Standard Mo del pro cesses. In the data four events are observed with jet-jet invariant masses M of 36, 37, 62 and 98 GeV. In Fig. 1a the M distribution of the four jj jj candidate events is shown together with the sum of all Monte Carlo background p distributions. Also shown is a p ossible lepto quark signal for = 4 and em di erent LQ masses. The mass distribution of the candidate events is consistent with the exp ectation from the background Monte Carlo simulation. 2.2 The neutrino plus hadronic jet channel This search has to b e optimized for a single hadronic jet in the detector. Its j transverse energy E must b e balanced by the neutrino. The cuts are therefore: T In order to reject events with large missing transverse energy due to badly measured tracks, the ratio of the energy E to the total visible ECAL energy E has to b e larger than 20 %. vis FREIBURG-EHEP-97-04 4 j Exactly one jet has to be found (n = 1) with E > 15 GeV. The jet j T direction in the lab oratory frame is required to lie within a pseudorapidity j range j j < 2 to reject events where a single jet, usually due to an electron, was found in the forward detectors. n 5 and n 7. ch q The missing transverse energy E= must b e greater than 15 GeV and it T j should b e mainly due to the jet. Therefore we require jE E= j < 3 T T j GeV and E =E > 0:5. vis Since no additional cuts on electron variables are necessary, the eciency to 0 detect a lepto quark is higher in the q than in the eq channel. For M = e 0 100 GeV the eciency is ab out 61 % in the q and 55 % in the eq channel. e 1 1 - - (a) OPAL prel. (b) OPAL prel. √s=161 and 172 GeV √s=161 and 172 GeV 10 LQ→eq 10 LQ→νq´ Events in 20.5 pb Events in 20.5 pb data data M=45 GeV M=45 GeV M=80 GeV M=80 GeV M=120 GeV M=120 GeV M=140 GeV M=140 GeV 1 1 background background -1 -1 10 10 20 40 60 80 100 120 140 160 180 200 220 20 40 60 80 100 120 140 160 180 200 220 M (GeV) jj MT=2E/ T (GeV) p 0 Figure 1: Numb er of (a) LQ ! eq and (b) LQ ! q events exp ected with = 4 e em 1 in 20.5 pb of data after all cuts for M =45;80; 120 and 140 GeV (histograms) and the candidate events (data p oints).