Search for pair production of sleptons have been performed using the data collected by the ALEPH detector at LEP in the year 2000. The centre-of-mass energy ranges from 204 to 209 GeV, corresponding to an integrated luminosity of 207.3 pb-1. No evidence for the production of these particles was found. The number of candidates observed is consistent with the background expected from four-fermion processes and gamma-gamma interactions. Improved mass limits at 95% C.L. are reported

Clerbaux, B

CERN Document Server

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)ALEPH 00-018CONF 00-015P R E L I M I N A R YSearch for scalar leptons in e+e− collisionsat centre-of-mass energies up to 208 GeVThe ALEPH CollaborationAbstractSearch for pair production of sleptons have been performed using the data collected bythe ALEPH detector at LEP in the year 2000. The centre-of-mass energy ranges from 204to 209 GeV, corresponding to an integrated luminosity of 207.3 pb−1. No evidence for theproduction of these particles was found. The number of candidates observed is consistentwith the background expected from four-fermion processes and γγ interactions. Improvedmass limits at 95 % C.L. are reported.Contributed Paper for LP01 and EPS HEP 2001Contact person: Barbara Clerbaux (Barbara.Clerbaux@cern.ch)1 IntroductionThis note describes preliminary results of searches for sleptons (~‘) obtained using the datacollected by the ALEPH detector at LEP during the year 2000, at centre-of-mass energiesranging from 204 to 209 GeV. The data were divided into three energy bins. The correspondingluminosities are given in Table 1.The theoretical framework is the Minimal Supersymmetric Model (MSSM), with R-parityconservation and the assumption that the lightest neutralino is the Lightest SupersymmetricParticle (LSP).Scalar leptons are pair produced at LEP through s-channel exchange of Z0 or γ and give riseto the reactions e+e− ! ~‘L~‘L or ~‘R~‘R. Selectrons can also be produced through the t-channelexchange of a neutralino, which yields in addition the e+e− ! ~eL~eR process. The sleptonssearch is based on their detection via the decay mode ~‘ ! ‘ ~01. The experimental signatureis a nal state with two leptons of the same family and missing energy, due to the presence oftwo undetected neutralinos.Cross sections and branching ratios are calculated in the framework of the MSSM with theprogram SUSYGEN [1]. The only dependence of the cross section for smuon and stau productionon supersymmetric parameters is through the slepton mass matrix, whereas the selectron crosssection depends on the selectron mass and, via the t-channel, on the MSSM parameters ;M2and tan . The o-diagonal elements of slepton mass matrix are proportional to the leptonmass. Left and right handed sleptons can therefor mix to form mass eigenstates. The mixingis expected to be negligible for smuons and selectrons due to the small masses of their partnerleptons.Energy bin (GeV) Luminosity (pb−1)204 - 206 75.30206 - 207 122.56> 207 9.39Table 1: Luminosities for the 2000 data collected by the ALEPH detector, for three energy bins.2 Selection criteriaSlepton pair production leads to a nal state characterized by two leptons of identical flavour.The twp leptons are acoplanar, due to the missing energy and momentum carried by the neu-tralinos. The selection criteria depend on the flavour of the leptons and on the mass dierencebetween the slepton and the neutralino ( M = m˜`−mχ˜0). The main background for large Mare the W pair production, four-fermion processes and dilepton production, whereas for smallM the dominant background comes from two-photon processes with lepton in the nal state.Therefore two dierent sets of selection are optimized for small and large M . The variousselections are identical to those described in Ref. [2] for a e+e− centre-of-mass energy up to 202GeV, with cut values suitably rescaled to take into account the beam energy increase.A comparison between data and expected backgrounds from standard model processes ispresented in gure 1 when using a pre-selected sample which consists in removing the criteria2against the four-fermion backgrounds. The gures 1a and 1b show the distributions of thetotal mass and lepton momenta, in case of selectron and smuon. The gures 1c and 1d showdistributions of the total mass and of the momenta of identied lepton in the tau jet, in caseof stau. At the pre-selection level, good agreement is observed between data and expectedbackgrounds (the main contribution to the backgrounds is from W pair production).For small M the eciency for the ~eR~eR selection decreases. In this region, t-channel ~eL~eRproduction leads to a nal state characterised by a pair of one high and one low momentumelectron. Two selections are performed, depending on the momentum of the electron stemmingfrom the ~eR decay.The high M selection for acoplanar electrons, described above, is applied with slidingcuts adapted to the kinematical range allowed for ~eL~eR production. For very small M thesingle electron selection, described in [2], is used, based on a well identied electron with hightransversal momentum. Disjunction from acoplanar electron selections is obtained by a cut onthe transversal momentum of the second electron.3 Candidate events and backgroundsThe number of candidate events and expected backgrounds are given in table 2 for selectron andsmuon searches and in table 3 for stau searches. Partial background substraction is performedfollowing the procedure of Ref. [2].Energy ~e ~(GeV) small M high M small M high MNobs Nbkg Nobs Nbkg Nobs Nbkg Nobs Nbkg205.2 0 0.118 15 14.0 0 0.122 8 12.5206.6 0 0.194 24 22.9 0 0.201 28 20.6208.0 0 0.016 0 1.76 0 0.167 3 1.59Table 2: Number of candidate events (Nobs) and background events expected from standardmodel processes (Nbkg), for selectron and smuon searches.Energy ~(GeV) small M high M small or high MNobs Nbkg Nobs Nbkg Nobs Nbkg205.2 5 6.6 4 7.3 8 9.9206.6 12 10.9 13 11.9 17 16.2208.0 2 0.82 0 0.90 2 1.23Table 3: Number of candidate events (Nobs) and background events expected from standardmodel processes (Nbkg), for stau searches.The search for sleptons is performed as a function of both slepton and neutralino masses. Asliding cut is then applied requiring that the lepton momenta fall into the range kinematically305101520250 25 50 75 100 125 150 175 200ww4feeqq2fγγlepγγhadMtot [ GeV/c2 ]051015202530350 20 40 60 80 100 120Plep [ GeV/c ]0510152025300 20 40 60 80 100 120 140 160 180 200Mtot [ GeV/c2 ]051015202530350 10 20 30 40 50 60 70 80 90 100Plep [ GeV/c ]A L E P H P R E L I M I N A R Y(a) (b)(c) (d)Figure 1: Comparison between data (crosses) and expected backgrounds (histograms) in caseof pre-selected selectron and smuon sample: (a) total mass (Mtot) and (b) momentum of theleading lepton (Plep); in case of the pre-selected stau sample: (c) total mass (Mtot) and (d)momentum of the leading lepton identied in the tau jet (Plep).allowed for a signal with the considered values of slepton and neutralino masses. For the selec-tron, smuon and stau searches, the number of candidate events and expected background eventsare displayed in gure 2 as a function of the slepton and the ~01 masses, after the sliding cuts.The background for the single electron selection is dominated by We and Zee events.Reduced by the systematical error, these processes are subtracted. This amount correspondsto 80.3 % of the total background. Candidate events and expected backgrounds for the threeenergy bins are shown in table 4. There is no evidence for selectrons with small M .4 Interpretation in the MSSMThe non-observation of any excess of candidates can be interpreted as lower limits on the massesof the particles searched for. Unless stated otherwise, limits are derived under the assumptionthat only ~‘R~‘R production contributes. This assumption is conservative because of the smaller4Energy(GeV) Nobs Nbkg205.2 5 7.7206.6 7 13.8208.0 0 1.0Table 4: Number of candidate events (Nobs) and background events expected from standardmodel processes (Nbkg), for the single electron selection.025507510060 80 1000510152025303540Me [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼025507510060 80 1000510152025303540Mµ [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼025507510060 80 1000510152025303540Mτ [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼025507510060 80 1000510152025303540Me [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼025507510060 80 1000510152025303540Mµ [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼025507510060 80 1000510152025303540Mτ [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼e˜ µ˜ τ˜e˜ µ˜ τ˜A L E P H P R E L I M I N A R YFigure 2: Number of selected events (upper gures) and expected background events fromstandard model processes (lower gures) in the search for selectrons, smuons and staus. Theshading densities reflect the number of events as indicated on the vertical bar. The kinematicallyaccessible region at 208 GeV is indicated by the full lines.cross section for the production of right-handed sleptons compared to left-handed sleptons forpure s-channel production.Mixing is expected to be negligible for all sleptons except for staus, since the tau is muchheavier than the other leptons. Therefore limits are calculated for stau in mixed and unmixedscenarios. Assuming the mixing eect to be negligible, the most conservative limits are set byconsidering pair production of ~R. In the case where ~L and ~R mix, limits are set on the massof the lightest stau ~1, choosing the mixing angle such that ~1 completely decouples from the Z0boson (corresponding to τ˜ ’ 52).The unication of gaugino masses is assumed for the computation of the slepton branchingratios and of the selectron cross section. Branching ratios for the ~‘R decay are calculated for5Channel M(~‘) > M(~‘) >obtained expected~eR 94 GeV 96 GeV~R 86 GeV 86 GeV~R 75 GeV 78 GeV~(min) 73 GeV 76 GeVTable 5: Values of slepton masses excluded at 95% C.L. for the production of right states, forM > 15 GeV, with the hypothesis of BR(~‘ ! l ~01)=1. For stau, the bound correspondingto the minimum cross section is also given. In case of selectron, the values  = −200 GeV=c2and tan  = 2 are used for the cross section computation. = −200 GeV=c2 and tan  = 2. The branching ratio (~‘ ! ‘ ~01) is dominant except whenthe decay ~‘R ! ‘~02 is kinematically allowed.The 95% C.L. bounds on the mass for selectrons, smuons and staus as function of theneutralino mass are displayed in gure 3. In the exclusion plots for the selectron and smuonsearches, the dashed lines show the limits computed under the conservative assumption that theselection eciency for decay channels other than ~‘ ! ‘ ~01 is zero.The expected and the observed bounds are both drawn in the gure and are in good agree-ment with each other for the selectron search. For the smuon search at high M (stau search),the expected limit is below (above) the observed one, because of a small decit (excess) of eventsobserved, compared to the background expectation. For selectrons and smuons, the eect of cas-cade decays for  = −200 GeV=c2 and tan  = 2, assuming no eciency for the cascade decaysis shown in the gure. The limit obtained for a stau mixing angle τ˜ ’ 52, is given as well.The 95% C.L. bounds on the slepton masses for M > 15 GeV are summarized in table 5,(the values  = −200 GeV=c2 and tan  = 2 are used in case of selectrons), with the hypothesisof BR(~‘ ! l ~01)=1.Observed and expected 95% C.L. bounds in the me˜R versus mχ˜0 plane, obtained by thecombination of all selectron selections are shown in gure 3d. They are calculated for tan=2.0and =-200. The limit given by the ~eL~eR-selections depends on the mass of the lefthandedselectron. The expected limit is below the observed one because of the decit of events observed,compared to the background expectation.The ~eL~eR selection yields a 95% C.L. bound on the selectron mass, independent of M fortan=2.0 and =-200 of 75 GeV (74 GeV expected).References[1] S.Katsanevas, P.Morawitz, "SUSYGEN 2.2 - A Monte Carlo event generator for MSSMsparticle production at e+e− colliders", Comp. Phys. Comm. 112 (1998) 227.[2] R. Barate et al., ALEPH Coll., "Searches for sleptons and quarks in e+e− collisions at 189GeV", CERN EP/1999-140, Phys. Lett. B469 (1999) 303.R. Barate et al., ALEPH Coll., "Search for Supersymmetric Particles in e+e− Collisionsatps up to 202 GeV and Mass Limit for the Lightest Neutralino", CERN EP/2000-139,subm. to Phys. Lett. B.602040608010050 60 70 80 90 100Me [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼ALEPH preliminary∼etanβ= 2µ=-200 GeV/c2expected Limitexcluded atCL=95%02040608010050 60 70 80 90 100Mµ [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼ALEPH preliminary∼µexpected Limitexcluded atCL=95%010203040506070809010050 60 70 80 90 100Mτ [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼ALEPH preliminary∼τexpected Limitexcluded atCL=95%02040608010050 60 70 80 90 100Me [ GeV/c2 ]Mχ [ GeV/c2 ]∼∼ALEPH preliminarytanβ= 2µ=-200 GeV/c2expected Limitexcluded atCL=95%∼eFigure 3: Excluded regions at 95% C.L. in the m˜`Rversus mχ˜01 plane from slepton searches. Theobserved (full lines) and the expected (dotted lines) limits are given. For selectrons and smuons,the dashed lines give the limits in case of cascade decay of the slepton, using the hypothesis = −200 GeV=c2 and tan  = 2. For the staus, the dashed line gives the limit obtainedfor a stau mixing angle τ˜ ’ 52. For the slepton exclusion plot (d) the ~eL~eR production for = −200 GeV=c2 and tan  = 2 is included.7

Search for scalar leptons in e+e- collisions

Abstract

Similar works

Full text

Available Versions

CERN Document Server