Duflot, L

Grivaz, J F

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OPEN-99-12806/09/99Search for scalar top quarks in e+e collisionsat LEP1 energiesThe ALEPH CollaborationLABORATOIRE EUROPEEN POUR LA PHYSIQUE DES PARTICULES (CERN)EPS 0416PA-09PL-12AbstractThe supersymmetric partners of the quarks, called squarks, are commonly be-lieved to be out of the reach of LEP1. However, because of the large top quark mass,the mixing between the \left-handed" and \right-handed" stops can be substantial,and the lighter mass eigenstate could be light enough to be pair-produced at LEP1energies. Moreover, the coupling of the stop to the Z can be suppressed for certainvalues of the mixing angle. The negative chargino searches imply that the decay~t! +b is forbidden. The stop then decays into c via a loop, with a lifetime longenough for the stop to hadronize into a stop-hadron before decaying, in contrast toother squarks. Stop pair production in e+e collisions, where the stop decays intoc, has been searched for in 140 pb 1of data at and around the Z peak collectedby the ALEPH detector until 1994. No signal was observed and exclusion domainsresult, presented as a function of the stop mass, the  mass and the mixing anglein the stop sector.Contribution to the International Europhysics Conference on High Energy PhysicsBrussels, Belgium, July 27-August 2, 199511 IntroductionScalar quarks are excluded by experiments at hadron colliders well beyond the massdomain accessible at LEP [1]. Typically, however, these exclusions apply under hypothesessuch as mass degeneracy of ve avour species and of left and right partners of ordinaryquarks, which in eect means ten degenerate scalar quark species. The mass splittingsbetween the various scalar partners of the ve lightest quarks may indeed be expected tobe negligible for those analyses in the context of minimal supergravity models. However,it is also expected that the scalar partners of the top quark might be substantially lighter,due to the eect of the large top quark Yukawa coupling in the renormalization groupequations. Moreover, while the mass eigenstates are expected to be almost identical tothe electroweak interaction eigenstates in the case of scalar partners of light quarks, thisis no longer true for the two scalar partners of the top. This feature is due to potentiallylarge o-diagonal entries in the scalar top mass matrix related to the large value of thetop quark mass:M =0@m2~tLmtamta m2~tR1Awhere the a parameter is typically expected to be of the same order as the scalar quarkmasses. It could therefore very well be that one of the scalar partners of the top quark,hereafter called \stop", is much lighter than all the other scalar quarks. The stop is amixture of the weak eigenstates~t =~tLcos t+~tRsin t;dened by the mixing angle, t.The fact that the stop is not a weak eigenstate has important consequences for itsproduction cross-section at LEP1 energies. The Z~t~t coupling is proportional to12cos2t 23sin2W;which implies that the stop decouples from the Z for t 0:98 rad. Even in that case,however, stops can be pair produced by photon exchange, but with a much reduced rate.The cross-section used in this analysis is taken from Ref. [2], where the leading order QCDcorrections are given. In addition, QED corrections calculated to second order with softphoton exponentiation are applied.In the stop mass domain accessible at LEP1 energies, the direct decays~t ! t and~t ! b+are kinematically forbidden. The dominant decay mode is thus~t ! c whichproceeds via loop diagrams. The stop lifetime [2] then turns out to be larger than thehadronization time, and this is taken into account in the present analysis in the followingway. Stops rst hadronize as if they were ordinary quarks, using the JETSET model witha Peterson et al. fragmentation function controlled by a parameter scaled from b-quarks~t= bm2bm2~t 0:006m2bm2~t:2Stop hadrons then decay according to~t ! c, following a simple spectator model. Thelightest neutralino, , is assumed to be the lightest supersymmetric particle, stable byR-parity conservation.The results reported here were obtained using a sample of 140 pb 1accumulatedby the ALEPH detector at LEP, at energies at and close to the Z peak, during theyears 1989 to 1994. This data sample contains approximately 3.7 million hadronic Zdecays. Descriptions of the ALEPH detector, of the relevant trigger conditions and ofthe algorithms used in the analysis can be found in Refs. [3], [4] and [5], respectively. Inorder to design the selection criteria, many signal Monte Carlo samples, correspondingto various doublets of stop and  masses, were used. These were obtained with a fastsimulation program, calibrated to reproduce the results of full simulations performed forfewer mass doublets. Full simulations were in particular performed close to the boundariesof the sensitivity domain. In addition, large fully simulated Monte Carlo samples wereused for all standard background processes (two and four-fermion nal states).2 Event selectionsEvents resulting from the production of a stop pair are expected to show as a pair ofacoplanar (c-quark) jets, with missing energy taken away by the two s. Such an acoplanarjet topology has already been investigated [6] in the context of the search for the standardmodel Higgs boson produced in association with a  pair (\H" analysis). The eciencyof that search applied to stops is 24% for m~t= 42 GeV/c2and m= 2 GeV/c2. TheH analysis was optimized for the case of a heavy Higgs boson, leading to not violentlyacollinear hadronic jets with a large visible mass. Here, the situation is easier in a largefraction of the (m~t, m) parameter space where typically more than half of the centre ofmass energy escapes undetected. This allows simpler and, in some congurations, moreecient selection criteria to be designed.Only events with at least four charged particle tracks are considered. Most of the qqbackground is rejected by the requirement that the visible mass should be smaller than50%ps. Events with an angle Tof the thrust axis with respect to the beam axis suchthat j cos Tj > 0:9 are eliminated, and so are events with energy detected within 12fromthe beam axis as in both cases additional energy is likely to have escaped down the beampipe. The bulk of the  background is rejected by the requirement that the componentof the total visible momentum transverse to the beam axis should exceed 5%ps.Each event is divided into two hemispheres by a plane perpendicular to the thrustaxis. The acollinearity angle is dened as the angle between the two hemisphere momenta,and the acoplanarity angle as that between those momenta projected onto the planeperpendicular to the beam axis. The remaining events are then classied as i) highmultiplicity events, with more than seven charged particle tracks, in which the mainbackground is still qq events (mostly with heavy quark semileptonic decays), and ii) lowmultiplicity events where  events and tau pairs dominate.3Masses Eciencies in per centm~tmH High mult. Low mult. total45 42 0 0.1 0.6 0.745 40 0 4.3 12.0 16.345 25 14.0 30.6 9.4 54.045 02 22.2 33.4 5.3 60.942 32 3.0 20.7 13.6 37.330 28 0.2 0.2 0.6 1.030 26 0.2 5.2 9.9 15.330 15 24.1 11.8 6.6 42.520 10 21.6 5.9 5.5 33.010 06 3.3 0.3 0.4 4.05 2 0.7 0 0 0.7Table 1: Analysis eciencies. The eciency of the High multiplicity analysis is to beadded to that of the H one, and that of the Low multiplicity analysis to the sum of thetwo others.In the high multiplicity sample, the acollinearity and the acoplanarity angles arerequired to be smaller than 135and 150, respectively. The eect of the acoplanaritycut is shown in Fig. 1. In addition, after forcing the jet algorithm to reconstruct exactlythree jets, the sum of all three jet-jet angles is required to be greater than 350. (Theselectivity of this variable against the qq background has been demonstrated in the Hanalysis.)In the low multiplicity sample, the thrust axis is required to form an angle of at least45with respect to the beam axis, which takes advantage of the sin2 angular distributionof the signal. For \monojet" events, i.e. events in which one of the hemispheres is empty,it is further required that the visible mass be in excess of 5 GeV/c2and the value ofthe thrust lower than 0.95; these cuts eliminate the  and + backgrounds. For non-monojet events, it is required that each hemisphere contain at least two charged particletracks and that the acoplanarity angle be lower than 150(Fig. 1), thus eliminating theremaining + background.No events were selected in the data, neither by the H analysis nor by any of the\High multiplicity" or \Low multiplicity" analyses described above. Based on studiesof the standard background Monte Carlo samples, less than 1.7 events were expected tobe selected. In Table 1, the cumulative eciencies of these three analyses are given forvarious congurations of the stop and  masses. As soon as the dierence m betweenthe stop and  masses exceeds 5 GeV/c2, the total eciency is larger than 10% andincreases with m and with the stop mass.In order to increase the sensitivity in the region of small m, a complementary analysishas been designed, but this time not expected to be free of standard backgrounds (mostlyfrom  interactions). Here too, the visible mass should be lower than 50%ps and no4energy should be detected within 12from the beam axis. The total visible energy isrequired to be greater than 3 GeV and the missing momentum should point more than55away from the beam axis. Each of the event hemispheres is required to containat least two charged particle tracks and both hemisphere momenta should point morethan 45away from the beam axis. Finally, the acoplanarity angle should not exceed135(Fig. 1). Fourteen events were selected in the data, which is compatible with theexpectation from  processes. The eciency of these criteria is of a few per cent for mas low as 2 GeV/c2.3 ResultsThe accurate Z width measurement at LEP allows limits on stop pair production in Zdecays to be set, which translate into stop mass upper limits as a function of the stopmixing angle as shown in Fig. 2. (A maximum of 23 MeV has been allowed, at the 95%condence level, for a possible contribution from non standard processes to the Z width.)These limits are independent of the stop decay pattern, and in particular of the  mass.These exclusion domains can be extended by the analyses described in the previoussection. In order to accomodate the various systematic errors (of which the largest comefrom the limitedMonte Carlo statisics, from uncertainties in the stop fragmentation modeland from the trigger simulation), the eciencies have been reduced by 10%. A rstexclusion domain is obtained by the combination of the three background free selections(H, High and Low multiplicity). This domain is extended for low m values by theuse of the complementary dedicated selection instead, considering the fourteen candidateevents as possible signal events. The results are shown in the (m~t,m) plane in Fig. 3, fora pure~tL(t= 0), for a pure~tR(t= =2), and for a stop fully decoupled from the Z.They are also shown in Fig. 2 in the (m~t,t) plane for m  2 and m  5 GeV/c2.It can be seen that stops are excluded up to the kinematic limit, unless the stop-mass dierence m is very small and, at the same time, the mixing angle tis suchthat the stop decouples from the Z. These results improve somewhat on those reportedearlier by the OPAL collaboration [7]. Recently, the D0 collaboration at the Tevatron hasexcluded a stop mass region extending approximately from 50 to 100 GeV/c2, i.e. notoverlapping with the LEP1 search domain, and for m exceeding  30 GeV/c2[8].5References[1] See for instance: S.Hagopian in Proc. of the XXVIIthInt. Conf. on High EnergyPhysics, Glasgow, Scotland, 20-27 July 1994, eds. P.J. Bussey and I.G. Knowles,p.809.[2] M. Drees and K. Hikasa, Phys. Lett. B 252 (1990) 127.[3] ALEPH Coll., D. Decamp et al., Nucl. Inst. and Methods A 294 (1990) 121.[4] ALEPH Coll., D. Decamp et al., Phys. Rep. 216 (1992) 253.[5] ALEPH Coll., D. Buskulic et al., Nucl. Inst. and Methods A 360 (1995) 481.[6] ALEPH Coll., \Improved Mass Limit for the Standard Model Higgs Boson",Contribution EPS0414 to the International Europhysics Conference on High EnergyPhysics, Brussels, Belgium, July 27-August 2, 1995.[7] OPAL Coll., R. Akers et al., Phys. Lett. B 337 (1994) 394.[8] D0 Coll., \Search for Light Top Squarks with the D0 detector",presented at the 10thTopical Workshop on Proton Antiproton Collider Physics,Batavia, IL, 9-13 May 1995; andContribution EPS0435 to the International Europhysics Conference on High EnergyPhysics, Brussels, Belgium, July 27-August 2, 1995.611010 2100 120 140 160 18010100 120 140 160 18011010 2100 120 140 160 180110100 120 140 160 18011010 210 3100 120 140 160 1801100 120 140 160 180Figure 1: Distribution of the acoplanarity angle in the High multiplicity analysis (top),in the Low multiplicity analysis (middle) and in the complementary analysis (bottom).On the left side, the data are represented by points with error bars and the backgroundexpected from standard processes by shaded histograms; the cut locations are indicatedby arrows. On the right side, the corresponding expected signal distributions are shownfor m~t=45 and m=25 GeV/c2(top), for m~t=30 and m=27 GeV/c2(middle), and form~t=42 and m=40 GeV/c2(bottom).70 0.2 0.4 0.6 0.8 1 1.2 1.405101520253035404550ALEPHθmix stop mass(a) m  2 GeV/c20 0.2 0.4 0.6 0.8 1 1.2 1.405101520253035404550ALEPHθmix stop mass(b) m  5 GeV/c2Figure 2: 95% C.L. excluded domains in the (m~t; t) plane for two values of m. Thehatched areas are excluded by the Z width constraint.8ALEPH0 5 10 15 20 25 30 35 40 45 5005101520253035404550stop massχ massFigure 3: 95% C.L. excluded domains in the (m~t;m) plane, for three values of t.The outermost (continuous) contour corresponds to t= 0, the intermediate (dashed)one to t= =2, and the innermost (dotted) one to a stop fully decoupled from the Z(cos2t= (4=3) sin2W).9

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