Kluit, P M

Schwickerath, U

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OPEN-99-38415/06/1999HEP’99 # 7 119 DELPHI 99-75 CONF 262Submitted to Pa 7 15 June 1999Pl 7, 8Search for Heavy Stable andLong-lived Charged Particles ine+e− CollisionsPreliminaryDELPHI CollaborationPaper submitted to the HEP’99 ConferenceTampere, Finland, July 15-21Search for Heavy Stable andLong-lived Charged Particles ine+e− CollisionsPreliminaryPeter Kluit (NIKHEF) and Ulrich Schwickerath (Universita¨t Karlsruhe)AbstractA search for stable and long-lived heavy charged particles has been performedusing the data taken by the DELPHI experiment at an energy of 189 GeV. TheCherenkov light detected in the Ring Imaging Cherenkov and the ionisation lossmeasured in the Time Projection Chamber are used to identify heavy particlespassing through the detector. Preliminary limits at 95% confidence level are given onthe masses of left and right handed smuons and staus. Including previous DELPHIresults, left (right) handed stable smuons and staus can be excluded between 2GeV/c2 and 87.9 (87.0) GeV/c2 at 95% CL.1 IntroductionA search has been made for the production of stable and long-lived 1heavy charged par-ticles in events with low multiplicity using the data taken by the DELPHI experimentat energies of about 189 GeV. The search extends previous DELPHI results presented in[1] in the leptonic topology to energies of 189 GeV. In most models of Supersymmetry(SUSY) the supersymmetric partners of standard particles are unstable and have shortlifetimes, except the lightest supersymmetric particle (LSP) which is commonly believedto be neutral and stable. In most of the searches it is therefore assumed that the super-symmetric particles decay promptly. It is, however, possible that a stable or long-livedheavy charged SUSY-particle exists. In the MSSM with a very small amount of R-parityviolation the LSP can be a charged slepton or squark and decay with a long lifetime intoStandard Model particles [2].In gauge mediated supersymmetric models the gravitino is the LSP and the next tolightest supersymmetric particle (NLSP) can obtain a long lifetime in a very natural wayfor large values of the SUSY-breaking scale [3]. This is possible for sleptons if e.g. thestau is the NLSP.Limits on the production cross-section and masses will be given for stable and long-lived sleptons.It is assumed that the long-lived particles decay outside the tracking volume of thedetector, which extends to a typical radius of 1.5 m. It is further assumed that theseparticles do not interact more strongly than ordinary matter particles.Heavy stable particles are selected by looking for high momentum charged particleswith either anomalous ionisation loss dE/dx measured in the Time Projection Chamber(TPC), or the absence of Cherenkov light in the gas and liquid radiators of the BarrelRing Imaging CHerenkov (RICH). The combination of the TPC and RICH detectorsand kinematical cuts provides an ecient detection of new heavy particles with a smallbackground.The data analysed corresponds to 153.9 pb−1 at an energy of about 189 GeV.2 Event selectionA description of the DELPHI apparatus and its performance can be found in ref.[5], withmore details on the Barrel RICH in ref. [6]. Charged particles were selected if their impactparameter was less than 5 cm in the azimuthal and less than 10 cm in the longitudinalplane, and their polar angle lies between 20 and 160 degrees. The relative error on themeasured momentum was required to be less than 100 % and the track length larger than30 cm. The energy of a charged particle was evaluated from its momentum 2 assumingthe pion mass. Neutral particles were selected if their deposited energy was larger than0.5 GeV and their polar angle was between 2 and 178 degrees.Only events with three or less charged tracks were considered. It was further demandedthat at least one charged particle had a momentum above 5 GeV/c reconstructed by the1Throughout the paper stable particles include long-lived particles decaying outside the detector.2In the following, ”momentum” means the apparent momentum, defined as the momentum dividedby the charge |q|, because this is the physical quantity measured from the track curvature in the 1.23 Tmagnetic field.1TPC and was inside the acceptance of the Barrel RICH | cos θ| < 0.68, where θ is thepolar angle. The event should have at least two reconstructed charged particle tracks.Cosmic muons were removed by putting tighter cuts on the impact parameter withrespect to the average beam-spot position. When the event had two charged particleswith at least one identied muon in the muon chambers, the impact parameter in theazimuthal plane was required to be less than 0.15 cm, and in the longitudinal plane lessthan 1.5 cm.Charged particles were selected, if they fulllled a combination of the following criteria:1. the Gas Veto: no photons were observed in the Gas RICH2. the Liquid Veto: four or less photons were observed in the Liquid RICH3. the TPC high ionisation loss: measured normalised energy loss was above 2 unitsi.e. twice the energy loss for a minimum ionising particle4. the TPC low ionisation loss: measured normalised energy loss was at least below0.3 of that expected for protonsThe particle identication using the RICH is described in detail in ref. [7].For the Gas and Liquid Vetoes it was required that the RICH was fully operationaland that for a selected track photons from other tracks or ionisation hits were detectedinside the drift tube crossed by the track. Due to tracking problems electrons often passeda Gas or Liquid Veto. Therefore it was required that particles that deposit more than 5GeV in the electromagnetic calorimeter, had hits included in the outer tracking detector.At least 80 wires were required for the measurement of the normalised energy loss in theTPC.An event was selected if the momentum of the charged particle was above 15 GeV/cand the Gas Veto (1) was conrmed by a Liquid Veto (2) or a low ionisation loss (4) (inboolean notation (1)·(2)+(1)·(4)) or if the momentum of the charged particle was above5 GeV/c and the Gas Veto was conrmed by a high ionisation loss ((1)·(3)). The eventwas also accepted if both hemispheres, dened by the thrust axis, had charged particleswith both momenta above 15 GeV/c, less than 5 GeV deposited in the electromagneticcalorimeter, and a high ionisation or low ionisation loss ((3)·(3’)+(4)·(4’)). This additionalsearch window improves the eciency for large masses up to about 10%, but has littleeects for masses around 50 GeV/c2. For masses below 60 GeV/c2 the detection eciencycan be improved by including a double gas veto[1]. This will be done at a later stage.3 Preliminary Analysis resultsOne event was selected in the data, in agreement with the expected background of 0.96± 0.13 events. The expected background was evaluated from the data. The candidateevent passes both the Gas and Liquid Vetoes. Figure 1 shows the measured normalisedionisation loss and the measured Cherenkov angle in the liquid radiator for the data takenat 189 GeV, after applying the Gas Veto.The eciency for selecting an event was evaluated as a function of the mass for right-handed smuons. Signal events were simulated with SUSYGEN [8], and passed throughthe detector simulation. The resulting eciency curve for a centre-of-mass energy of 189GeV is shown in Figure 2a.2These preliminary results have been combined with previous published results from [1]to obtain experimental limits on the masses of left and right handed stable or long-livedsmuons and staus. The candidate event has been taken into account over the full massrange. Figure 2b shows the cross-section for left and right handed smuons at an energy of189 GeV, calculated with SUSYGEN [8] as a function of the particle mass. The 95 % CLcross-section limit is also shown. Taking into account the results from previous searchesat lower energies [1], left (right) handed stable smuons and staus can be excluded between2 GeV/c2 and 87.9 (87.0) GeV/c2.4 ConclusionsA search is made for stable and long-lived heavy charged particles in leptonic nal statesat energies of 189 GeV, using particles identied by the Cherenkov light in the RICH andthe ionisation loss in the TPC.One event is observed in the data, in agreement with the expectation of 0.96 ± 0.13events. The results are combined with previous results of the DELPHI Collaboration[1],and preliminary limits on the masses of left and right handed stable or long-lived smuonsand staus are derived. Including the new data, left (right) handed stable smuons andstaus can be excluded between 2 GeV/c2 and 87.9 (87.0) GeV/c2 at 95% CL.References[1] P. Abreu et al. (DELPHI Collaboration), Phys. Lett. B444 (1998) 491[2] H. Dreiner, hep-ph/9707435.[3] M. Dine et al., Phys. Rev. Lett. 76 (1996) 3484;S. Dimopoulos et al., Nucl. Phys. B Proc Supl 52A (1997) 38.[4] G.F. Giudice and R. Rattazzi, hep-ph/9801271.[5] P. Aarnio et al., Nucl. Instr. Meth. A 303 (1991) 233;P. Abreu et al., Nucl. Instr. Meth. A 378 (1996) 57.[6] W. Adam et al., Nucl. Instr. Meth. A343 (1994) 1.[7] W. Adam et al., Nucl. Instr. Meth. A343 (1994) 240;M. Battaglia and P. M. Kluit, Particle Identification using the RICH detectors basedon the RIBMEAN package, DELPHI note 96-133 RICH 90.[8] S. Katsanevas and S. Melanchroinos in Physics at LEP2, CERN 96-01 (Vol.2) 328.3DELPHI slepton searches at 189 GeV0123456789105.0                15.                 45.                 135.       þ apparent momentum p/|q| (GeV/c)normalised dE/dxq=±eq=±2/3 e00.20.40.60.815.0                15.                 45.                 135.       þ apparent momentum p/|q| (GeV/c)Cherenkov angle liquid (rad)Figure 1: (a) Normalised energy loss as a function of the apparent momentum p/|q| afterthe Gas Veto for the 183 GeV data. (b) Measured Cherenkov angle in the liquid radiatoras a function of the apparent momentum after the Gas Veto: if four photons or lesswere observed in the liquid radiator, the Cherenkov angle was set equal to zero. Therectangular areas in (a) indicate selections (3) and (4), and that in (b) shows selection(2). The selection criteria are explained in the text.400.10.20.30.40.50.60.70.845 50 55 60 65 70 75 80 85 90mµ (GeV/c2)Efficiency PRELIMINARY s=189 GeV00.10.20.30.40.50.60.70.845 50 55 60 65 70 75 80 85 90left handed µ95% CLmµ (GeV/c2)σ(e+e-->µ+µ-)(pb) PRELIMINARY s=133-189 GeVright handed µ~~~~~~DELPHIDELPHIFigure 2: (a) Eciency for detecting stable and long-lived smuons (staus) as a functionof the smuon mass at a centre-of-mass energy of 189 GeV.(b)Production cross-section for left and right handed smuons and staus as a function of thesmuon mass. The 95 % CL cross-section limit, derived from DELPHI searches between130 GeV and 189 GeV, is also shown.5

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