Search for long-lived particles in ATLAS and CMS S.Giagu Sapienza Universita` di Roma and INFN Roma, 00185-Roma, IT on behalf of the ATLAS and CMS collaborations The ATLAS and CMS detectors can be used to search for heavy long-lived particles which might signal physics beyond the Standard Model. Such new states can be distinguished from Standard Model particles by exploiting their unique signatures, ranging from multi-leptons and/or jets production anywhere within the detector volume, to minimum ionizing particles with low velocity and high momentum. Here are reviewed the strategies proposed by ATLAS and CMS to search for these signals, with particular emphasis on possible challenges to the trigger and detector operations. 1. CHARGED META-STABLE PARTICLES Heavy stable charged particles are predicted by many models of physics beyond the Standard Model [1], in which one or more new states carry a new conserved, or almost conserved, global quantum number, like for example R- parity or KK-parity. Stable heavy charged sleptons appear for example in Gauge Mediated Supersymmetry Breaking (GMSB) models, when the lightest supersymmetric particle (NLSP), typically the stau slepton, couples weakly with the gravitino LSP. Production of sleptons at the LHC proceeds mainly via decay chains of heavier supersymmetric particles, with typical cross-section at LHC ranging from 100 fb to 1 pb depending on the mass of the slepton particle. Stable leptons are also predicted by the Universal Extra Dimensions model (UED), where for each Standard Model particle exists a corresponding so-called Kaluza-Klein (KK) state in extra dimensions, with the same quantum numbers and spin as the Standard Model partner. KK leptons are directly produced in pairs in p-p collisions, with cross-section at LHC of the order of 20-30 fb. In addition to lepton-like particles, also hadron-like stable particles (R-hadrons) can be produced at LHC. R-hadrons are predicted in Split-SUSY models, or in the framework of the gravitino LSP scenario of SUGRA models, and can be copiously produced at the LHC, with cross-sections up to few nb. A common and largely model independent signature for all these processes is the existence of one or more stable massive charged particles, with low velocity and high transverse momentum, that behave similarly to "massive" muons in the LHC detectors. 1.1. TRIGGER At trigger level a lepton-like heavy stable particle has a high probability of being reconstructed as a muon. However particles with velocity significantly smaller than the speed of light may reach the muon system out of time with respect to the typical relativistic muons and therefore would either be reconstructed in the wrong bunch crossing or fail to be reconstructed at all because of quality cuts imposed by the Level-1 or High Level Trigger algorithms. This effect is ATL-PHYS-PROC-2008-029 07 October 2008 more important for the ATLAS detector due to the larger dimensions compared to CMS. Typically in GMSB models the two sleptons are produced with different velocities (β), and with the β distribution peaking at high values, so that at least one of the two produced sleptons has β > 0:7 in 99% of the events. R-hadrons instead are much more problematic due to a much lower average β, and the possibility of experiencing multiple nuclear interactions with the detector materials, with consequent charge flipping, and the chance to fail the quality requirements usually applied at high level of trigger. 1.2. VELOCITY MEASUREMENT The identification of a heavy stable charged particle relies on the precise determination of its mass through the measurement of both the velocity β and the particle momentum. ATLAS and CMS are able to measure with precision β using time-of-flight techniques, exploiting the excellent time resolution of the muon systems. This can 6 4 Selection and results with early data -1 -1 Tk Tk ! 1.9 ! 2.2 10 CMS Preliminary CMS Preliminary 104 1.8 2 ~ 1.7 3 10 1.8 1.6 1 1.5 1.6 102 1.4 1.4 10-1 1.3 10 1.2 1.2 1 1 1 1.2 1.4 1.6 1.8 2 2.2 2.4 1 1.2 1.4 1.6 1.8 2 2.2 !-1 !-1 DT DT 1 1 Figure 1: Distribution of β− versus β− for background (left) and for the signal sample t˜1 with −1 Tk DT Figure 1: CMS: distributionmass 500of β GeV (right).as measured from dE=dx (y axis) and from time-of-flight (x axis), for background (left) and for the signal sample t~ (right). selection). No background events pass the above selection in a fully-simulated MC sample cor- responding to 1 fb 1. The efficiency of the above selection for signal samples is reported in ∼ − Table 3. be done offline and also at trigger level, exploiting the fast time response of the Resistive Plate Chamber systems, available in both ATLASIn the and case CMS where experiment. the tails of the two In additionβ distributions to theare higher time-of-flight when they are determination, measured on CMS performs also an independent measurementdata than what exploiting is expected the from specific simulation, ionization a tighter selection ( can) measured be used. The in goal the is centralto tracking system. β 1 dE=dx have less than one background event expected for 1 fb− . Several additional cuts can be used The usage of dE=dx allowswhich also drastically to reduce reduce theto abackground, negligible but level have thesmall background effects on the signal. due to They cosmic cannot muonsbe or to muons from different bunch crossings.properly Figure investigated 1 shows on the background distribution for the of availableβ−1 as statistics measured since fromthe simpledE=dx selectionversus the determination proposed above leaves no events. from time-of-flight, for muon background events and for a possible R-hadron signal (stop squark of 500 GeV mass). A clear correlation between the two measurement is visible in the case of the signal. 4.2 Tracker standalone selection A standalone selection using only the tracker for the β measurement is the following: 1 reconstructed muon with p > 100 GeV • T 1.3. DISCOVERY POTENTIALβ < 0.8. • tk The muon system is still used but no time-of-flight information is extracted from it. This anal- ysis has been tested on a sample of events expected to be selected by the muon trigger and cor- 1 In order to maximizeresponding the background to about 1 fbrejection,− of integrated CMS luminosity. uses a combined The resulting selection number of based selected on events the two measurements of β, reducing the backgroundis reported to a in negligible Table 4. The level efficiency without on signal significant samples is slightly loss of higher signal. than Using that of thisthe com- technique CMS is able to bined analysis given that the DT coverage, which is not required for the standalone selection, −1 keep the expected backgroundis limited to below the barrel one region event of CMS. for an integrated luminosity of 1 fb , maintaining reasonable signal efficiencies. The CMS integrated luminosity needed to observe 3 events in the signal region, for different models is 4.3 Systematic uncertainty shown in Figure 2. Heavy Stable Charged Particles can be discovered with early data for different models and in The first source of systematic uncertainty is the trigger efficiency for late particles. The final −1 different mass regions.muon The triggerstable settings gluino for search time gates with and 1 synchronization fb is sensitive can change to gluino the efficiency masses for trigger-above 1 TeV and the GMSB scenarios with stable stauing on can a late be particle discovered in the correct with bunch a few crossing. 100 Thispb− can1 [2]. easily Similar change the sensitivities trigger efficiency are obtained also by the by 50% and can shift the spectrum of the recorded HSCP towards higher values of β, further ATLAS collaboration [3]. 8 4 Selection and results with early data -1 3 CMS Preliminary CMS Preliminary 103 2.5 102 2 Particles in 1 fb 10 ) to observe 3 events 1.5 -1 (pb 1 int Gluino 1 L Stop -1 10 GMSB stau 0.5 KK tau 0 200 400 600 800 1000120014001600 0 200 400 600 800 1000 1200 Mass (GeV) Mass (GeV) 1 Figure−1 2: The left plot shows the integrated luminosity (pb− ) needed for 3 events, for the four Figure 2: CMS: integrated luminosity (pbsignal) needed models (gluino to observe full circles, 3 signal stop events, full squares, for the KK four tau empty signal circles, models stau (gluino, empty squares)stop, KK τ and stau) as a function of the particle mass.as a function The error of HSCP bands mass. correspond The right to plot a systematic shows the reconstructeduncertainty of mass 50% distribution on the estimated with 1 trigger efficiency. 1 fb− for two of the lowest cross section samples (300 GeV KK tau and 800 GeV stop). would be recomputed using more data. 4.4 Discovery and exclusion In the following, we compute the luminosity needed to observe 3 events in the signal region for different models as an estimate for the integrated luminosity needed for exclusion/discovery when no background events are expected. If an excess is observed it is also possible to perform several cross checks. First, one can check the distribution of the reconstructed mass (as shown in the right plot of Figure 4.4). The Poisson probability to observe no events when 3 are expected is 5%.