Alderweireld, T

Alemany, R

García, C

Gil-Botella, I

Kluit, P M

Rebecchi, P

Schwickerath, U

Wolf, G

CERN Document Server

OPEN-99-41515/06/1999HEP’99 # 7 373 DELPHI 99-90 CONF 277Submitted to Pa 7 15 June 1999Pl 7, 8Search for chargino pair productionin gravitino LSP and stau NLSPscenarios at LEPPreliminaryDELPHI CollaborationPaper submitted to the HEP’99 ConferenceTampere, Finland, July 15-21DELPHI CollaborationSearch for chargino pair production in gravitino LSPand stau NLSP scenarios at LEPT. Alderweireld(1), R. Alemany(2), C. Garc´ıa(2), I. Gil Botella(2), P. Kluit(3),P. Rebecchi(4), U. Schwickerath(5) and G. Wolf(4)(1) Univ. Instelling Antwerpen, Belgium(2) IFIC (Centro Mixto CSIC-Universidad de Valencia), Spain(3) NIKHEF, The Nederlands(4) CERN, Switzerland(5) Institut fur Experimentelle Kernphysik, Karlsruhe, GermanyAbstractPromptly decaying lightest charginos were searched for in the context of scenarioswith gravitino LSP. It was assumed that the stau is the next to lightest supersym-metric particle (NLSP). Data collected with the DELPHI detector at centre-of-massenergies of 183 and 189 GeV were analysed combining the methods developed inprevious searches. No evidence for the production of these particles was found.Hence, limits were derived at 95% confidence level. The mass of charginos wasfound to be greater than 91.8 GeV/c2 for mχ˜+1 −mτ˜1  0.3GeV/c2, independentlyof the mass of the gravitino.1 IntroductionIn some supersymmetric models, supersymmetry (SUSY) may be broken at a scale below107 GeV, with the ordinary gauge interaction acting as the messenger of supersymmetrybreaking [1, 2]. In this case, the gravitino, ~G, is the lightest supersymmetric particle (LSP)and the lightest Standard Model superpartner is the next to lightest supersymmetricparticle (NLSP). Thus, the NLSP is unstable and decays to its Standard Model (SM)partner and a gravitino.The number of generations of supersymmetry breaking messengers in minimal modelsusually determines which particle is the NLSP [3, 5, 6, 7].In this letter, lightest chargino pair production is searched for under the assumptionthat the ~1, the lightest of the mass eigenstates produced by the mixing of ~R and ~L [8],is the NLSP. If kinematically allowed, charginos could be produced at LEP through theexchange of a photon or a Z in the s-channel, or through the exchange of an electronsneutrino in the t-channel. Each chargino would then promptly decay into a ~1 and a tauneutrino:e+e− ! ~+1 ~−1 ! ~+1 ~−1 ττ (1)Being the NLSP, ~1 is expected to decay in the laboratory frame into  + ~G with amean decay length of:L = 1:76 10−3√E2m2τ˜1− 1(mτ˜1100 GeV=c2)−5 (mG˜1 eV=c2)2cm; (2)which depends strongly on mτ˜ , mG˜ and the energy of the ~1, E.It can be deduced from eq. (2) that for a given stau mass, dierent ranges of gravitinomasses could determine three dierent nal state topologies. For mG˜ . 10 eV=c2 the staudecays at the interaction point. Thus, if chargino pairs would be produced in this context,the nal state topology would correspond to two acoplanar taus and missing energy andmomentum. This signature has already been studied by the DELPHI Collaboration in theso-called leptonic channel of its search for charginos within the Minimal SupersymmetricStandard Model (MSSM) [9].If the mass of the gravitino lies between 10 and 200 eV=c2, the staus would decayinside the tracking devices of the DELPHI detector. Thus, the signature of chargino pairproduction would contain one or two tracks with a kink or a decay vertex. In the case inwhich a stau decays before reaching the active tracking volume, at least one track with abig impact parameter would be observed. This nal state topology is almost identical tothe one that would be produced by a pair of staus for the same range of gravitino masses,and has been studied by the DELPHI collaboration in ref. [10].Finally, if the mass of the gravitino is above 200 eV=c2, the staus would tend to decayoutside the tracking devices, and would appear to be stable particles. A pair of trackswith anomalous ionization and Cherenkov radiation would then be observed. Also thisnal signature has been already studied by the DELPHI Collaboration in the search forstable heavy particles [11].Thus, this letter is intended to make use without modication of the analyses alreadyapplied in the three searches described in refs. [9], [10] and [11]. Real data and simulatedSM samples atps = 183 GeV have already been analysed in this context, and the1comparison between real and simulated data is included in refs. [9, 10, 11]. No excess overthe expected SM background has been observed.The selections developed for these three searches were thus applied to simulated datasamples with dierent gravitino masses, and the results were interpreted in terms ofregions in the (mτ˜1 ; mχ˜+1 ) space which are excluded at 95% condence level (CL).Section 2 describes the samples used for this work and presents the experimentalresults. Section 3 describes the interpretation of the results within one model with agravitino LSP [5].2 Event sample and resultsThe search is based on data collected by the DELPHI collaboration during 1997 at centre-of-mass energies of 183 and 189 GeV. The total integrated luminosities were 53.9 and158 pb−1 respectively. A detailed description of the DELPHI detector can be found in[12] and its performance in [13].The program SUSYGEN [14] was used to generate the chargino pair production anddecay. In order to compute detection eciencies, a total of 45 samples of 500 events eachwere generated with gravitino masses of 1, 100 and 1000 eV=c2, mτ˜1 + 0:3 GeV=c2 mχ˜+1 ps=2 and mτ˜1  68 GeV=c2. Samples with smaller m = mχ˜+1 − mτ˜1 werenot generated because in that region charginos decay into W s and gravitinos with anappreciable branching ratio. In the aforementioned samples, charginos decay exclusivelyinto a stau and a tau neutrino. The dierent background samples and event selectionsare described in references [9, 10, 11].The generated signal and background events were passed through the detailed simu-lation [13] of the DELPHI detector and then processed with the same reconstruction andanalysis programs used for real data. Table 1 shows the range of eciencies, the maincomponents and the total amount of the expected background events, and the numberof observed data events for each sample 1. Figure 1 shows the distributions of the threemain variables used the analysis described in [10] for real data, expected standard modelbackground, and a simulated signal of mχ˜+1 = 90 GeV=c2and mτ˜+1 = 85 GeV=c2.Sample Eciencies (%) Main bgds. Expected bgd. Obs. evts.mG˜ = 1 eV=c2 24 - 36 WW , γγ 38.94:9 36mG˜ = 100 eV=c2 28 - 50 γγ 2.050:91 1mG˜ = 1000 eV=c2 0 - 63 (γ) 1.70:3 1Table 1: Range of eciencies, main sources of background, expected background andobserved data events for the dierent analyses.1In the case of mG˜ = 1 eV/c2, the expected background and observed events correspond to the sumof the degenerate and non-degenerate scenarios in the leptonic channels of [9], with errors added inquadrature.210-111010 20 20 40 60 80 100 120 140 160 180Angle hadr. int. [0]N/3.6010-210-111010 20 20 40 60 80 100 120 140 160 180Angle between tracks [0]N/1.8010-210-111010 20 20 40 60 80 100 120 140 160 180Angle with shower [0]N/3.60Figure 1: (a) angle between the hadronic interaction and the reconstructed vertex, (b)angle between the electromagnetic shower and the direction dened by the dierencebetween the momenta of ~1 and its associated  , dened at the crossing point, and (c)angle between the tracks of the kink, for real data (dots), expected Standard Modelbackground (cross-hatched histogram) and simulated signal for mχ˜+1 = 90 GeV=c2 andmτ˜+1 = 85 GeV=c2 decaying with a mean distance of 50 cm (blank histogram). The arrowsindicate selection criteria imposed as explained in [10].33 InterpretationSince no evidence for a signal was found in the data, a limit on the production cross-sectionfor chargino pairs was derived for each (mG˜,mτ˜1 ,mχ˜+1 ) combination. Figure 2 shows the95% C.L. upper limit on the chargino pair production cross-section atps = 189 GeVas a function of mχ˜+1 and mτ˜1 after combining the results of the searches atps = 183and 189 GeV with the maximum likelihood ratio method [4] for m  0:3 GeV=c2 andmG˜ = 1, 100 and 1000 eV=c2. These limits, which directly reflect the eciencies of theapplied selections, can be understood as follows:mG˜ = 1 eV=c2: The eciency of this analysis is essentially flat over the kinematicallyallowed region. Thus, the main eect on the production cross section limit is givenby the 211.9 pb−1 used for the region mχ˜+1 < 91:5 GeV=c2, and the 158 pb−1 formχ˜+1 > 91:5 GeV=c2.mG˜ = 100 eV=c2: The map of eciencies is the result of the convolution of two factors.First, larger stau masses imply a smaller lifetime, and hence a smaller eciency [10].Second, a larger chargino mass leads to smaller stau momenta, and to smaller decaylengths.mG˜ = 1000 eV=c2: In this case, the map of eciencies is mainly aected by the momen-tum of the stau, because the method used to identify heavy stable particles relieson the lack of Cherenkov radiation in DELPHI’s RICH detectors. To remove SMbackgrounds, low momentum particles are removed, thus reducing the eciency forhigher chargino masses, especially in the region of small m.Limits were derived in the frame of the model described in reference [5]. This isa general model which assumes only radiatively broken electroweak symmetry and nulltrilinear couplings at the scale of the messengers’ masses. The corresponding parameterspace was scanned as follows: 1  n  4, 5 TeV    900 TeV, 1:1  M=  9000,1:1  tan  50, and  > 0, where n is the number of messenger generations in themodel,  is the ratio between the vacuum expectation values of the auxiliary componentsupereld and the scalar component of the supereld and M is the messenger mass scale,tan  and  are dened as for the MSSM.Figure 3 shows the regions excluded at 95% CL in the (mχ˜+1 ,mτ˜1) plane. The positive-slope area is excluded for all gravitino masses. The negative-slope area is excluded formG˜  100 eV=c2. The area below mG˜ = 75:8 GeV=c2 is excluded by the direct searchfor stau pair production [10]. The area of m 0:3 GeV=c2 is not excluded because nosimulated events were generated in this region. Thus, if m0:3GeV=c2, limits at 91.8,93.0 and 93:0 GeV=c2 can be set for mG˜ = 1, 100 and 1000 eV=c2 respectively. The limitat mG˜ =1 eV=c2 is also valid for smaller masses of the gravitino, because they lead to thesame nal state topologies. The same argument is true for mG˜ 1 keV=c2. The charginomass limit decreases with decreasing mτ˜1 because in scenarios with gravitino LSP, smallstau masses correspond to small sneutrino masses (both are proportional to ), and henceto smaller production cross-sections due to the destructive interference between the s- andt-channels.470758085909570 72.5 75 77.5 80 82.5 85 87.5 90 92.5 9570758085909570 72.5 75 77.5 80 82.5 85 87.5 90 92.5 9570758085909570 72.5 75 77.5 80 82.5 85 87.5 90 92.5 95+1~χm (GeV/c  )2+1~χm (GeV/c  )2+1~χm (GeV/c  )2mτ~(GeV/c  )21mτ~(GeV/c  )21mτ~(GeV/c  )21DELPHI (b)(c)σσσσσσσσσ < 0.5< 0.2< 0.08< 0.06< 0.04< 0.09< 0.07< 0.05< 0.04σ < 0.09σ< 0.12σ< 0.17σ< 0.21σ < 0.25Figure 2: Limits in picobarn on the lightest chargino pair production cross-section at95% CL. Limits are shown as functions of mχ˜+1 and mτ˜1 for (a) mG˜ = 1 eV=c2, (b)mG˜ = 100 eV=c2 and (c) mG˜ = 1000 eV=c2.570758085909570 75 80 85 90 95DELPHIall mG~m   > 100 eV/c2G~mτ~(GeV/c  )21+1~χm (GeV/c  )2excluded by the search for τ pair production~Figure 3: Areas excluded at 95% CL in the (mχ˜+1 ,mτ˜1) plane. The positive-slope area isexcluded for all mG˜. The negative-slope area is excluded for mG˜  100 eV=c2. The greyarea is excluded by the direct search for stau pair production [10].4 SummaryLightest chargino pair production was searched for in the context of light gravitino sce-narios. It was assumed that the ~1 is the NLSP. Three dierent searches were used inorder to explore the (mχ˜+1 ; mτ˜1) plane in dierent domains of the gravitino mass.The search in the context of very light gravitinos (the same as for MSSM chargino pairproduction with leptonic nal states) produced 36 candidate events to be compared to38.94:9 events expected from the SM background. An upper limit on the correspondingproduction cross-section between 0.09 and 0.25 pb was derived at 95% CL.The search in the context of medium range gravitino masses (leading to stau produc-tion with impact parameters or kinks), selected one event from the data, to be comparedto 2.050:91 events expected from the SM background. The 95% CL upper limit on thecorresponding production cross-section varies between 0.04 and 0.09 pb.The search in the context of heavier gravitinos (leading to the production of stablestaus) gave one candidate event, with 1.70:3 events expected from the SM background.An upper limit on the corresponding production cross-section between 0.04 and 0.5 pbwas set at 95% CL in the kinematically allowed region.When combined, these results imply a 95% CL lower limit on the mass of the lightestchargino of 91.8 GeV=c2, valid for m  0:3 GeV=c2, independently of the mass of thegravitino.6AcknowledgementsWe are greatly indebted to our technical collaborators, to the members of the CERN-SLDivision for the excellent performance of the LEP collider, and to the funding agenciesfor their support in building and operating the DELPHI detector.We acknowledge in particular the support ofAustrian Federal Ministry of Science and Tracs, GZ 616.364/2-III/2a/98,FNRS{FWO, Belgium,FINEP, CNPq, CAPES, FUJB and FAPERJ, Brazil,Czech Ministry of Industry and Trade, GA CR 202/96/0450 and GA AVCR A1010521,Danish Natural Research Council,Commission of the European Communities (DG XII),Direction des Sciences de la Matiere, CEA, France,Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie, Germany,General Secretariat for Research and Technology, Greece,National Science Foundation (NWO) and Foundation for Research on Matter (FOM),The Netherlands,Norwegian Research Council,State Committee for Scientic Research, Poland, 2P03B06015, 2P03B03311 andSPUB/P03/178/98,JNICT{Junta Nacional de Investigac~ao Cientca e Tecnologica, Portugal,Vedecka grantova agentura MS SR, Slovakia, Nr. 95/5195/134,Ministry of Science and Technology of the Republic of Slovenia,CICYT, Spain, AEN96{1661 and AEN96-1681,The Swedish Natural Science Research Council,Particle Physics and Astronomy Research Council, UK,Department of Energy, USA, DE{FG02{94ER40817.References[1] M. Dine, W. Fischler and M. Srednicki, Nucl. Phys. B189 (1981) 575 ; S. Di-mopoulos and S. Raby, Nucl. Phys. B192 (1981) 353 ; M. Dine and W. Fischler,Phys. Lett. B110 (1982) 227 ; M. Dine and M. Srednicki, Nucl. Phys. B202 (1982)238 ; L. Alvarez-Gaume, M. Claudson and M. Wise, Nucl. Phys. B207 (1982) 96 ;C. Nappi and B. Ovrut, Phys. Lett. B113 (1982) 175 .[2] M. Dine and W. Fischler, Nucl. Phys. B204 (1982) 346 ; S. Dimopoulos and S. Raby,Nucl. Phys. B219 (1983) 479.[3] J. A. Bagger, K. Matchev, D. M. Pierce and R. Zhang, Phys. Rev. D55 (1997) 3188.[4] A.L. Read, Optimal statistical analysis of search results based on the likelihood ratioand its application to the search for the MSSM Higgs boson atps = 161 and 172GeV, DELPHI 97-158 PHYS 737 (1997) and references therein.[5] D. A. Dicus, B. Dutta, S. Nandi, Phys. Rev. D56 (1997) 5748 ; D. A. Dicus, B. Dutta,S. Nandi, Phys. Rev. Lett. 78 (1997) 3055.7[6] F. Borzumati, On the Minimal Messenger Model, hep-ph/9702307 and WIS/96-50-PH, Dec. 1996.[7] G. F. Giudice, R. Rattazzi, Theories with Gauge-Mediated Supersymmetry Breaking,CERN-TH/97-380, Jan. 1998 and hep-ph/980127. Submitted to Phys. Rev. D.[8] A. Bartl et. al., Z. Phys. C73 (1997) 469.[9] DELPHI Collaboration, P. Abreu et al., CERN-EP/98-176, Phys. Lett. B446 (1999)75; Paper 7 213 of this Conference.[10] DELPHI Collaboration, P. Abreu et al., E. Phys.J. C7 (1999) 595; Papers 7 230 and7 372 of this Conference.[11] DELPHI Collaboration, P. Abreu et al., CERN-EP/98-171, Phys. Lett. B444 (1998)491; Paper 7 119 of this Conference.[12] DELPHI Collaboration, P. Aarnio et al., Nucl. Instr. and Meth. 303 (1991) 233.[13] DELPHI Collaboration, P. Abreu et al., Nucl. Instr. and Meth. 378 (1996) 57.[14] SUSYGEN 2.2, S. Katsanevas and S. Melachroinos in Physics at LEP2, CERN 96-01,Vol. 2, p. 328 and http://lyoinfo.in2p3.fr/susygen/susygen.html ; S. Katsanevas andP. Moravitz, Comp. Phys. 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Search for chargino pair production in gravitino LSP and stau NLSP scenarios at LEP

Search for chargino pair production in gravitino LSP and stau NLSP scenarios at LEP

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