Search for chargino nearly mass-degenerate with the lightest neutralino is performed with the data collected at a centre-of-mass energy from 189 to 209~GeV by the ALEPH detector at LEP. The analysis uses the detection of an isolated and energetic initial state radiation photon, produced in association with the chargino pair, whose decay products have small visible energy. No evidence of signal is found and lower limits on the chargino mass of 91 and 89~GeV/$c^2$ at 95\% confidence level are derived, for large scalar masses, in gaugino and in higgsino region respectively

De Filippis, N

Grivaz, J F

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ALEPH 2001-004CONF 2001-001July 5, 2001PRELIMINARYSearch for charginos nearly mass-degeneratewith the lightest neutralino in e+e− collisionsup tops = 209 GeVThe ALEPH CollaborationContact person: N. De Filippis (nicola.delippis@ba.infn.it)Contributed Paper for LP01 and EPS HEP 20011 IntroductionThis paper reports on the search for chargino (~1 ) with a mass dierence with respect tothe lightest neutralino(~01) of less than 5 GeV=c2. In the Minimal Supersymmetric extensionof the Standard Model (MSSM) [1], ~1 and ~01 are nearly mass-degenerate for large M1;2(higgsino region); small values of m (m = m˜±1−m˜01 ) are possible in gaugino region if thegaugino unication relation between M1 and M2 is relaxed. Moreover supersymmetric modelswith anomaly mediated supersymmetric breaking mechanism [2] foresee the degeneration ofchargino and neutralino, since M1=M2 >> 1.The analysis is based [3] on the reconstruction of an isolated initial state radiation (ISR)photon together with the low-momentum detectable chargino decay products. The ISR photonsare required to have high transverse momentum with respect to the beam line to ensure hightrigger eciency.Data collected by the ALEPH detector [4] at LEP during 1998{2000, at centre-of-massenergies ranging from 189 to 209 GeV, are used. The total data sample corresponds to anintegrated luminosity of about 627 pb−1.Background events expected from Standard Model processes are generated with a statisticscorresponding to at least 20 times the collected luminosity used in the analysis.Signal events are simulated with the Monte Carlo program SUSYGEN [5]. Charginos areproduced in pairs with masses ranging from 40 GeV=c2 up to the kinematic limit, for mbetween 0.1 to 5 GeV=c2, and with decay length, , up to 80 cm. Corrections to theeciency calculations are applied to take into account the spectral functions of the hadronicdecay [6], which are based on the Ku¨hn-Santamaria parametrisation [7]. Parameters are tunedto describes correctly the semileptonic  decays to pions [8]. The ISR is enabled in the MonteCarlo samples [9]. The photon energy spectrum is dierent in various regions of the MSSMparameter space; it depends on the relative contribution of the s-channel Z exchange, which,in turn, depends on the Z ~+1 ~−1 coupling; this eect becomes as important as the chargino isfar from the kinematic limit. The photon energy and the transverse momentum Monte Carlospectra are shown in Fig. 1a) and b) respectively.2 Event selectionThe signal topology depends on m values Below the threshold of pion production, charginosare long-lived and strongly ionising particles; stable particle analysis [10] selects eciently thisnal state. When m is larger than 5 GeV=c2, missing energy chargino analysis [11] are used.In the intermediate range, m < m < 5 GeV=c2, the analysis described below is tailored tosearch for events with few low-momentum particles accompanied by an ISR photon.Photons are reconstructed by localised energy depositions within groups of neighbouringECAL towers and by the photon conversion nding [12]. In order to optimise the energyreconstruction of photons, which are not well-contained in ECAL, the sum of the localisedenergy depositions in ECAL and HCAL is computed inside a cone with half-opening of 11:5.Events are selected if at least one photon in the geometrical acceptance (j cos γj < 0:95) is101000200030004000500010 20 30 40 50 60 70 80Ö  s = 189 GeVm χ 1± = 50 GeV/c2Eγ (GeV)Nγ / 1.6 GeVALEPH PRELIMINARY1010 210 310 410 50 10 20 30 40 50 60 70 80Ö  s = 189 GeVm χ 1± = 50 GeV/c2pt γ (GeV/c)Nγ / 0.4 GeV/cALEPH PRELIMINARYa) b)Figure 1: Generated energy (a) and transverse momentum (b) distribution of ISR γ atps = 189 GeV for m˜±1 = 50 GeV/c2; dierent colours correspond to the histograms in gauginoor higgsino region.reconstructed with the transverse momentum with respect to the beam axis , pt, greater thanps sin min, where min = 35 mrad is the lowest polar angle covered by the SICAL detector.The energy deposition is then large enough to ensure the trigger.Two or more charged tracks, reconstructed with at least 4 hits in the ITC or 4 hits in theTPC, are required. Photons have to be isolated from the closest charged track by 30.Good tracks are dened requiring the impact parameter in the transverse plane to the beamaxis to be less than 4 cm, if no hits in the TPC are used in the reconstruction. The cut istightened to 2 cm, and the distance from the interaction point along the beam axis is requiredto be less than 10 cm, if at least 4 TPC hits are used in the track nder. Most of the two andfour fermions events are rejected requiring that the number of good tracks is less than or equalten.To prevent from γ(γ) events with one non identied photon conversion in the TPC,at least one track has to be reconstructed with a number of ITC hits greater than zero;secondary vertex reconstruction is enforced for each pair of tracks, the momentum of the tracksis computed at the secondary vertex position and the invariant mass is determined assumingelectron mass for each track and it is asked to be greater than 0:1 GeV/c2. Events with arecoiling mass to the photon smaller than 100 GeV/c2 are also discarded.Candidates with a non vanishing value of the visible energy inside a cone of 14 around thebeam axis are rejected. The total visible transverse momentum of the event pvist is required tobe greater than 3:5%ps. Both these cuts improve the signal to γγ background discrimination.The total energy not associated to the reconstructed photons, is constrained to the maximumvalue of the expectation for charginos decaying promptly, at each chargino mass and m.2Similarly pvist is constrained to the maximum value as determined by the Monte Carlo signalsimulations. Finally the photon energy is required to be less than the maximum energy for anISR photon as computed with the following formula:Emaxγ =s− (2m˜±1 )22ps:The selection criteria have been optimised according to the N95 [13] prescription over thewhole range of m.3 ResultsThe selection eciency of pair-produced charginos with an ISR high transverse momentumphoton is about 40%. Fig. 2 shows the dependence of the average eciency on the generatedtransverse momentum of the photon, pγt . In the range of pγt between 15 and 25 GeV=c, about20% of the events are lost due to the photon acceptance and reconstruction. Photon isolationcriteria reduce the eciency of about 30%.The selection eciency with respect to all chargino pair produced is below 3% and it ismostly due to the small fraction of events with a high pγt photon. The eciency does notdepend on m signicantly, but on the chargino mass through the ISR photon reconstructedpt requirements as shown in Fig. 3a).Ineciency arises when the chargino gets a sizeable decay length, due to the cut on theadditional reconstructed energy in addition to the photon. Signal eciency is shown in Fig. 3b)as a function of  with m˜±1= 71 GeV=c2, m < m < 0:3 GeV=c2 and for 1- nal state.Additional ineciency due to beam related background leading to depositions in the veryforward detectors is estimated using random trigger events. The applied correction to theeciency amounts to about (41)%. The total systematic uncertainty on the selection eciencyis 15 % and it is dominated by the limited Monte Carlo statistics.The number of observed events is in good agreement with the expectation from standardmodel processes as shown in Table 1, for m˜±1> 50; 65; 85 GeV=c2 and m < 2; 1; 0:3 GeV=c2respectively The main background contributions are also reported. The main contributionsto the background after the selection come from +−, γγ ! +− and γ(γ) events. Acandidate event which is contributing up to m˜±1 = 84 GeV=c2 and any m is shown in Fig. 4.4 Interpretation in the MSSMThe non-observation of any excess of candidate events is interpreted in terms of upper limits onthe chargino production cross section in the MSSM framework. The cross section upper limitsare in turn translated into 95% condence level excluded regions in the (m˜±1 ; m) plane. No300.10.20.30.40.50.60 5 10 15 20 25 30 35 40 45 50pt γ (GeV/c)EfficiencyALEPH PRELIMINARYFigure 2: Chargino selection eciency as a function of generated transverse momentum of theISR photon for chargino masses from 46 to 91 GeV=c2 andps = 189 GeV.400.0050.010.0150.020.0250.0345 50 55 60 65 70 75 80 85 90Ö  s = 189 GeV∆m = 0.25 GeV/c2m χ 1± (GeV/c2)EfficiencyALEPH PRELIMINARY00.00250.0050.00750.010.01250.0150.01750.020 20 40 60 80 100λ (cm)mpi ≤ ∆m ≤  0.3 GeV/c2m χ 1± = 71 GeV/c2√ s= 189 GeVALEPH PRELIMINARYEfficiencya) b)Figure 3: a) Signal eciency as a function of chargino mass atps = 189 GeV for m =0:25 GeV/c2 and  = 0; b) Signal eciency as a function of  for m˜±1 = 71 GeV=c2,m < m < 0:3 GeV=c2 and for 1- nal state.ps = 189− 209 GeV Data backgroundmχ˜±1(GeV=c2) ∆m (GeV=c2) γ(γ)¯ γγ ! +− γγ ! e+e− +− 4f Tot. Exp.> 50 < 2 13 1.2 2.0 0.3 3.8 1.3 9.0> 65 < 1 5 0.7 1.8 0.3 1.7 0.6 5.4> 85 < 0:3 1 0.6 1.4 0 0.5 0 2.9Table 1: Number of observed events on data and standard model background expected eventsfor m˜±1> 50; 65; 85 GeV=c2 and m < 2; 1; 0:3 GeV=c2 respectively.background subtraction is performed and systematics are taken into account according to Ref.[14].The excluded regions in gaugino and higgsino scenario are shown in Fig. 5 and 6 and are validfor all tan and in the hypothesis of high scalar masses. The results of the standard missingenergy selection, the ISR analysis and the stable particles search are distinguished. Lower limitson chargino mass of 88 GeV=c2 and 8x GeV=c2 are set in the gaugino and higgsino regionrespectively. These results are improved with respect to those obtained by the DELPHI [15]and L3 [16] collaborations using data collected at energies fromps= 189 GeV up tops =202 GeV.55 ConclusionsSearch for charginos nearly mass degenerate with the lightest neutralino is performed usingdata collected ALEPH detector at LEP at a centre-of-mass energies from 189 GeV to 209 GeV.No excess on candidate events with respect to standard model background prediction is found.In the framework of MSSM, in the high scalar masses scenario and for any tan  the followingmass limits are set at 95% condence level: in gaugino region: m˜±1 > 91 GeV=c2; in higgsino region: m˜±1 > 89 GeV=c2.References[1] H.E. Haber and G.L. Kane, Phys. Rep. C 117 (1985) 75;R. Barbieri, Riv. Nuovo Cimento 11 (1988) 1.[2] G.F.Giudice, M.A. Luty, H. Murayama and R. Rattazzi, J. High Energy Phys. 9812 (1998)027;L. Randall and R. Sundrum, Nucl. Phys. B 557 (1999) 79;T. Gherghetta, G.F. Giudice and J. D. Wells, Nucl. Phys. B 559 (1999) 27.[3] C.H. Chen, M. Drees and J.F. Gunion, Phys. Rev. Lett. 76 (1996) 2002.[4] ALEPH Collaboration, ALEPH: A detector for electron-positron annihilation at LEP.Nucl. Instrum. and Methods A 294 (1990) 121;Performance of the ALEPH detector at LEP. Nucl. Instrum. and Methods A 360 (1995)481.[5] SUSYGEN version 2.2: S. Katsanevas and P. Morawitz, Comp. Phys. Comm. 122 (1998) 227.[6] C.H. Chen, M. Drees and J.F. Gunion, Phys. Rev. D55 (1997) 330, erratum Phys. Lett.82 (1999) 3192.[7] J.H. Ku¨hn and A. Santamaria, Z. Phys. C 48 (1990) 445.[8] ALEPH collaboration, Measurement of the spectral functions of vector current hadronic decays, Z. Phys. C 76 (1997) 15;Measurement of the spectral functions of axial-vector hadronic  decays and determinationof s(M2tau), Eur. Phys. Journal C 4 (1998) 409.[9] REMT: F.A. Berends and R. Kleiss, Nucl. Phys. B 260 (1985) 32.[10] ALEPH Collaboration, Search for gauge mediated SUSY breaking topologies atps 189 GeV Eur. Phys. J. C 16 (2000) 1;Single- and multi-photon production and a search for slepton pair production in GMSBtopologies in e+e− collisions at aps up to 208 GeV, ALEPH 2001-010 CONF 2001-007.6[11] ALEPH collaboration, Search for charginos and neutralinos in e+e− collisions at centre-of-mass energies near 183 GeV and constraints on the MSSM parameter space, Eur. Phys.J. C 11 (1999) 193;Search for charginos and neutralinos in e+e− collisions atps up to 208 GeV and masslimit for the lightest neutralino, ALEPH 2001-009, CONF 2001-009.[12] ALEPH collaboration, Single- and multi-photon production in e+e− collisions at a centre-of-mass energy of 183 GeV, Phys. Lett. B 429 (1998) 201.[13] ALEPH Collaboration, Phys. Lett. B 384 (1996) 427;J.-F. Grivaz and F. Le Diberder, Complementary analyses and acceptance optimization innew particle searches., LAL 92-37 (1992).[14] R.D. Cousins and V.L. Highland, Nucl. Instrum. and Methods A 320 (1992) 331.[15] DELPHI Collaboration, Search for charginos nearly mass-degenerate with the lightestneutralino , Eur. Phys. J. C 11 (1999) 1;Update of the search for charginos nearly mass-degenerate with the lightest neutralino,Phys. Lett. B 485 (2000) 95.[16] L3 Collaboration, Search for charginos with a small mass difference to the lightestsupersymmetric particle atps = 189 GeV , Phys. Lett. B482 (2000) 31;Search for charginos with a small mass difference to the lightest supersymmetric particleat 189 <ps < 202 GeV , L3 Note 2582, July 3,2000, Contribution to the XXXthInternational Conference on High Energy Physics, Osaka, Japan, July 27-August 2, 2000.7DALI_E2     ECM=195.5 Pch=.586 Efl=20.7 Ewi=21.1 Eha=0     dati                                                      Nch=2     EV1=.989 EV2=.938 EV3=.025 ThT=.951           99−08−18 13:10           Detb=  E1FFFRun=50892   Evt=4932  ALEPH13.Gev EC.08Gev HCYX hist.of BA.+E.C.0|−500cm             500cm|X 0 |−500cm             500cm| Y 13.Gev E.08Gev H16. Gev E.32 Gev HRZ0|−600cm             600cZ    0 |−600cm             600cm|ρ Figure 4: Candidate event atps = 195:4 GeV which contributes to the range m˜±1<84 GeV=c2. The reconstructed photon energy is about 21 GeV.80.10.20.30.40.50.60.70.80.912340 50 60 70 80 90 100 110 120LEP 1 limit∆m (GeV/c2)M χ 1± (GeV/c2)StandardStable particlesISR tagALEPH preliminaryFigure 5: Excluded region in the (m˜±1 ; m) plane at 95% condence level, for m0 >500 GeV=c2 in the gaugino region. The green region is excluded by the standard missingenergy chargino selection [11], the red region is the resulting exclusion of the ISR analysis; theblue region is excluded by the the stable particle search [10]. The 95% condence level lowerlimit on chargino mass is 91 GeV=c2.90.10.20.30.40.50.60.70.80.912340 50 60 70 80 90 100 110 120LEP 1 limit∆m (GeV/c2)M χ 1± (GeV/c2)StandardStable particlesISR tagALEPH preliminaryFigure 6: 95% condence level excluded region for m0 > 500 GeV=c2 in the higgsino region. Thegreen region is excluded by the results of the standard missing energy chargino results [11], thered region by the ISR analysis; the blue region by the the stable particle search [10]. Charginomasses below 89 GeV=c2 are excluded at 95% condence level.10

Search for charginos nearly mass-degenerate with the lightest neutralino $e^+e^-$ collisions up to $\sqrt{s}$ = 209GeV

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Search for charginos nearly mass-degenerate with the lightest neutralino e+e−e^+e^-e+e− collisions up to s\sqrt{s}s​ = 209GeV

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Search for charginos nearly mass-degenerate with the lightest neutralino $e^+e^-$ collisions up to $\sqrt{s}$ = 209GeV