The topologies arising from the production of supersymmetric particles at the
LEP collider are briefly reviewed recalling detector requirements, simulation
and other experimental issues.Comment: 4 pages, 3 figures. To be published in Proceedings of 'XIV IFAE -
  Incontri sulla Fisica delle Alte Energie (Meetings on High Energy Physics)',
  3-5 April 2002, Parm

Sguazzoni, Giacomo

English

arXiv

The topologies arising from the production of supersymmetric particles at the LEP collider are briefly reviewed recalling detector requirements, simulation and other experimental issues

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THE TOPOLOGIES OFSUPERSYMMETRY SIGNALS AT LEPGiacomo SguazzoniEuropean Laboratory for Particle Physics (CERN)CH-1211 Geneva 23, SwitzerlandAbstractThe topologies arising from the production of supersymmetric par-ticles at the LEP collider are briefly reviewed recalling detectorrequirements, simulation and other experimental issues.1 IntroductionTwo years after the dismantling of LEP, most of the results of the dedicatedsearches for production of supersymmetric particles have been published orare close to publication [1]. The analysed topologies are here briefly re-viewed focusing on the experimental issues faced by SUSY hunters. Thehuge experience gained on this eld is an important part of the LEP legacy.Hopefully, it will turn out to be very useful for SUSY searches at the up-coming experiments.2 Overview of topologiesR-parity conservation is assumed: the LSP (Lightest Supersymmetric Par-ticle) is stable, and, most probably, also neutral and weakly interacting;at LEP the sparticles are pair produced; the decay processes bring to nalstates containing at least one LSP.1Evisible / MP˜/ P˜Heavy stable chargedparticlesKinksImpact parameterAcoplanar particlesP˜ LDET 1 mP˜ LDET 0:1 mmP˜ LDETISRphotonTriggerThresholdFigure 1: A schematic diagram of the P˜vs. Evisible plane reporting the relevanttopologies within the R-parity conserving scenario.The variety of supersymmetric models leads to a very wide phenomenolo-gy. For each type of nal state (hadronic jets, leptons, γ’s) many topologiesexist, depending on Evisible, the energy of the visible system, and on P˜, thesparticle decay length. At a given collision energy, Evisible is proportionalto the mass dierence between the sparticle and the escaping LSP (M =MP˜−MLSP) and P˜ is related to the couplings and to M by means of thesparticle lifetime P˜.An exhaustive search for SUSY at LEP has been pursued by using severaldierent analyses, which, however, can be roughly grouped by experimentaltopologies, as illustrated in Figure 1: energy of the visible system above the detector sensitivity ( GeV triggerthreshold): acoplanar particles, impact parameter or kinks and heavy stablecharged particles, respectively for P˜smaller, comparable or greater thanthe typical detector dimension; energy of the visible system below the detector sensitivity: ISR photon,the hard initial state radiation photon being used for trigger.The analyses for the R-parity violating scenario, also widely faced atLEP, search for a dierent set of topologies, not discussed here.Acoplanar particles. The main signature of this topology, arisingin case of pair production of sparticles decaying promptly, is the missingenergy and momentum from escaping LSP’s, hence the hermeticity is themain detector requirement. The selections often implement ducial cuts toavoid missing energy and momentum to be faked by cracks or dead zones in2.6Gev HC.69Gev EC2.6Gev HC Run 50892 Event 4932ℵ ALEPH(a) (b)Figure 2: (a) Simulated event with long-lived sparticle pair leading to a kink and atrack with large impact parameter. (b) Very small M sparticle signal candidatewith the triggering hard ISR photon.calorimeters and luminosity monitors.Signal event shape and backgrounds, and thus selections, heavily dependon M . The small M region is the most problematic since the visiblesystem is soft and the dominant background, e+e− ! γγ, has a huge crosssection (> 10nb) to be compared with the typical SUSY cross sections (0:1pb). The strong γγ rejection factor required ( 105) results in degradedsignal eciencies. Large eorts have been spent to get the backgroundestimation reliable despite the large samples needed and the complex physicsunderlying the elementary γγ processes.Powerful anti-γγ criteria exist; requiring small energy deposition in theforward directions (jj< 10) is one example. Unfortunately, the beam pipeand other passive materials (as radiation shields) make this region intrinsi-cally not hermetic, complicated in geometry and aected by beam-relatednoise and backgrounds so that the evaluation of the detector response isoften dicult. These issues are taken into account within the systematicuncertainties or overcome by applying cuts to safely over-reject γγ’s.Impact parameter or kinks. This peculiar signature, illustrated bythe simulated event in Figure 2(a), can be identied at LEP by means of thepowerful tracking, even if the reconstruction algorithms had in some cases tobe suitably modied. The non-negligible source of background due to ma-terial eects (splash-backs and nuclear interactions), for which simulationsare normally not accurate enough, is kept under control by using topologicalcuts or by requiring low vertex multiplicity. Cosmic rays recorded as e+e−interaction events can also fake tracks not coming from the primary vertex.Good timing capability are usually useful to reject these accidentals.Heavy stable charged particles. Heavy stable charged particles, pro-duced with relatively low momentum, can be easily tagged by means of par-ticle identication capabilities. Most commonly dE/dx measurements areused, proting of the large specic ionization loss.In case of heavy stable hadrons, a big issue is represented by the simu-lation of the aspects related to the hadronization and to the strong inter-action into the calorimeters. The simulations are based on some reasonableextrapolation of the behaviour of known particles; however, as an extrasafety margin, the selections have been designed to make minimal use of thecalorimetric informations.ISR photon. If M falls below few hundreds of MeV’s, the triggeringon the sparticle signal relies only on hard photons from initial state radiation.As an example, Figure 2(b) reports a candidate event selected in such type ofanalyses. The method allows to extend the sensitivity down to  150MeV;reliable ISR simulations are crucial for a correct evaluation of the resultingtiny eciencies ( < 1%).3 ConclusionDespite the frustrating negative outcome, hunting for Supersymmetry atLEP turned out to be a challenging search for topologies not present or rarein the Standard Model. The experimental issues required a stimulating anddeep comprehension of the detectors and the development of new techniquesand dedicated algorithms.4 AcknowledgmentsI would like to thank P. Azzurri and G. Ganis for their valuable help andmany useful discussions.References[1] F. Mazzucato, Search for Supersymmetry at LEP2, and referencestherein; these proceedings.

The topologies of Supersymmetry signals at LEP

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