SUSY Searches at the LHC Diego Casadei on Behalf of the ATLAS and CMS Collaborations

SUSY searches at the LHC Diego Casadei on behalf of the ATLAS and CMS Collaborations

New York University

Searches for supersymmetry (SUSY) at the LHC are reviewed, to highlight what experimental signatures might lead to a discovery of new physics with early LHC data. AT- miss miss miss LAS and CMS perspectives with jet+pT , lepton+pT , and photon+pT experimental signatures are considered. Both experiments will be sensitive to portions of the parameters space beyond the coverage of the Tevatron already with few hundred pb−1 integrated luminosity.

1 Introduction

This papera reviews the experimental signatures that look most promising for early discovery of supersymmetry (SUSY) in high-energy proton-proton collisions produced by the CERN Large Hadron Collider (LHC). Actually, the huge amount of work done in the last several years by so many people can not be fully accounted here: we will focus on the possible signatures from R-parity conserving SUSY scenarios that could be detected with O(1) fb−1 of LHC integrated luminosity. Public results from the ATLAS and CMS Collaborations mostly come from two references, [2] and [3], and all have been estimated with simulations carried on at 14 TeV, whereas LHC will provide collisions at 10 TeV at the startup phase. Hence their results are not representative of the actual reach of early searches: work is in progress to determine the expected performances at the lower energy. Pile-up could also be important in collisions already at the first year of data taking (with luminosity of 1032 cm−2 s−1), though not all simulations have included it (in ref. [2] 5 interactions per bunch crossing are considered). Before considering searches for new physics, a number of other tasks need to be performed with early data. First, detector simulations must be validated with known Standard Model (SM) processes, which in turn will also be measured with conditions that no other accelerator allowed in the past. Accurate knowledge of the SM background must be attained before any discovery can be claimed. This requires detailed understanding of detector (e.g. noise and calibration) and physics environment in collisions (pile-up, beam-gas, beam-halo, beam-pipe collisions, cosmic rays, etc.), that will also impact on the trigger rates, in addition to the offline event reconstruction. On the other hand, theoretical models of SM processes must also be refined with the data taken at the new energy scales probed by LHC. Because all these studies are interrelated, such work will require a continuous effort that will extend well past the first data taking, although not covered here. Though here we focus on direct SUSY searches with the ATLAS and CMS detectors, it is important to emphasize that indirect evidences may also come from precision measurements of rare SM processes. From this point of view, the LHCb experiment, dedicated to the study of rare B decays, nicely complements the aforementioned ones (see for example [4]). There are many viable SUSY models, that differ in the details of the supersymmetry breaking, and they have many independent parameters that may vary by orders of mag- nitude. However, the categories of the final states in high-energy p-p collisions that are aSlides including several plots are available at [1].

DIS 2009 detectable in practice are not so many, so that experimental signatures may be simulated using only a very few models. Hence, the ATLAS and CMS Collaborations made detailed simulations on a limited set of benchmark points characterized by distinct ﬁnal states (hence providing ﬁgures for their sensitivity that are valid in more general cases), while making scans of the parameter space with fast simulations.

2 Direct searches for R-parity conserving SUSY

Clearly, people will first focus on signatures that are expected to have the best sensitivity, because any early claim for discovery could be made only in case of a clear excess over the SM prediction. This implies that the SM processes will need to be well understood at the LHC energy scales, and that trigger and offline selections need to provide very effective background rejection. The one thing in common to the R-parity conserving models that provide viable dark matter (DM) candidates is the prediction of neutral stable particles in the final states, that are the lightest supersymmetric particles (LSPs). Being undetectable, they always carry away some non negligible fraction of momentum, inducing an imbalance in the measured b miss total transverse momentum . Hence, missing transverse momentum (pT , also named miss ET because it is mainly a calorimetric measurement) characterizes almost all approaches to early datac, though it might not be required at trigger level. Both ATLAS and CMS miss triggers implement two techniques to reconstruct pT : negative vectorial sums over all calorimeter data (called MET) or over already clustered (e.g. jet) energy (called MHT).

2.1 Trigger strategies Both ATLAS and CMS will enable diﬀerent trigger types in parallel, to maximize the sensitivity to new physics and provide redundancy and robustness. For CMS the triggers under consideration are:

1. inclusive jet trigger, with the lowest unprescaled threshold at pT > 100 GeV; 2. inclusive MET trigger with threshold of ∼ 100 GeV; 3. HT + MHT trigger, where HT is the scalar sum of jets pT and MHT the vector sum; 4. inclusive MHT triggers.

The motivation for the last two choices is that HT, giving some measurement of the overall jet activity, gives a too high rate at 1032 cm−2 s−1 if used alone. On the other hand MHT will be more robust against hardware problems (noise, hot towers, etc.) and beam related background than MET. MET, MHT and HT will be available both at Level 1 and at the high-level trigger (HLT). In addition, CMS will also consider single and di-lepton triggers as well as single and diphoton triggers (for GMSB). For ATLAS the strategy for early data taking is similar. Jet and jet + MET (e.g. miss 70 GeV threshold on jet pT and ET > 30 GeV) will be the baseline trigger selections, but

bThe longitudinal projection of the total momentum suffers by large fluctuations, due to the lost fraction in the detector apertures that are necessary to allow the beams to reach the collision point, and to the Lorentz amplification of parton-level fluctuations (parton longitudinal momenta are not known on event-by-event basis). For these reasons, detectors are optimized in the measurement of the transverse component. cA possible exception is the multi-lepton analysis.

DIS 2009 complementary triggers based on electron or muon features will also be active. The trigger menu will contain single-jet (with ∼ 120 GeV threshold) and multi-jet (e.g. 3 jets above 30– 40 GeV) signatures, together with di-lepton signatures (e.g. one electron with pT > 10 GeV and one muon above 6 GeV). Though understanding the MET trigger might take some time, it is also foreseen to have inclusive chains based only on MET selection, to try accepting those events which might fail the aforementioned triggers.

miss 2.2 Jets + pT This is the most inclusive SUSY signature at the LHC, with background coming from QCD jets, tt events, W,Z boson production, and can be triggered by jet + MET or jet + MHT signatures. −1 The ATLAS event selection for the first 1 fb (triggering with jet pT > 70 GeV and MET > 30 GeV) may require different numbers of jets.d To simplify combination of different analyses, leptons are excluded here and included in the analysis described in section 2.4. miss In case ≥ 2, or ≥ 3 jets are required, they must have pT > 150, 100, 100 GeV, ET > miss 100 GeV, and ET > 0.3Meff (2-jet) or > 0.25Meff (3-jet) not aligned with jets, where (j) miss Meff = P pT + ET . The 4-jet analysis has softer pT thresholds (100 GeV for the leading miss miss jet, 50 GeV for the others), the same ET threshold, a looser condition ET > 0.2Meff, and one additional cut on the sphericity (ST > 0.2), where ST = 2λ2/(λ1 + λ2) is built (k) (k) with the eigenvectors of the 2 × 2 tensor Sij = Pk pTi pTj . The idea is that the QCD di-jet topology tends to be back-to-back, whereas SUSY final states are more uniformly distributed. It comes out that all selected SUSY points would produce a clear excess over e the SM background above Meff ≈ 1 TeV, with the low-mass mSUGRA point SU4 being visible already above 0.5 TeV [3]. For the same integrated luminosity, the CMS preselection requires at least 1 primary f g vertex, the e.m. fraction Fem ≥ 0.175, and the charged fraction Fch ≥ 0.1. Further miss requirements are: at least 3 jets with |η| < 1.7 and pT > 180, 110, 110 GeV, ET > 200 GeV not aligned with jets, no isolated tracks, no e.m. jets, and HT > 500 GeV, where (j) miss (1) miss HT = Pj>1 ET + ET = Meff − ET (i.e. HT excludes the leading jet). A plot of ET (or HT) would reveal a clear excess above 200 GeV (or 500 GeV) in the case of the low-mass mSUGRA point LM1 h [2]. Work is in progress to understand whether selections not based miss on ET are feasible [5]. Complementary triggers are based on HT with single or multi-jet.

miss 2.3 Jets + pT + b-jets SUSY processes often produce b or b quarks, hence eﬃcient b-tagging is a powerful tool for selecting events. Both ATLAS and CMS will have b-tagging capability, including trigger [3, 6], though their tuning and optimization may require quite a bit of time. miss ATLAS jet + ET analysis can be improved by requiring that 2 jets are tagged as b-jets: the signal-to-background ratio would be quite high for low-mass points like SU4 (having a large top content in decay chains) [3], allowing for discovery already with 0.1 fb−1.

dAnalyses are inclusive in the number of jets, requiring ≥ 2, ≥ 3, or ≥ 4 jets. e SU4: m0 = 200 GeV, m1/2 = 160 GeV, tan β = 10, sgn(µ) =+, A0 = −400 GeV. f Fem is the ET-weighted jet e.m. fraction over the e.m. calorimeter acceptance, |η| < 3. g Fem is the average ratio of the track-based jet pT (within |η| < 1.7) over the calorimetric jet ET. h LM1: m0 = 60 GeV, m1/2 = 250 GeV, tan β = 10, sgn(µ) =+, A0 = 0 GeV.

DIS 2009 miss For CMS, after a trigger requiring a jet with pT > 88 GeV and ET > 46 GeV, events are selected with 4 jets with pT > 30 GeV and |η| < 2.5, one of which should be tagged as miss b-jet, ET > 150 GeV not aligned with the b-jet, and one isolated lepton with pT > 5 GeV and |η| < 2.5. This analysis would be sensitive to a good portion of the (m0,m1/2) mSUGRA plane already with 1 fb−1, covering larger region as the integrated luminosity increases (see [2], ﬁg. 13.23).

miss 2.4 Jets + pT + leptons

The jet + lepton signature is particularly important to extend the coverage of the (Λ, tan β) miss GMSB plane, but in general it extends the searches only based on jet and ET . If the ATLAS 4-jet selection is reﬁned by requiring 2 same-sign leptons with pT > 20 GeV, the SM background is highly reduced (only W decays and tt processes survive, but are strongly reduced) [3]. The CMS event selection includes a di-muon trigger (with 7 GeV threshold), with a separation of R =0.01 among muons with pT > 10 GeV and track and calorimeter isolation, miss plus ET > 200 GeV and 3 jets with pT > 175, 130, 55 GeV. In case of 2 tau leptons, the miss selection requires central taus with ET > 150 GeV and 2 jets with pT > 150 GeV. A good portion of the mSUGRA parameter space is covered with 1 fb−1 [2]. The thresholds listed above refer to the 14 TeV high-luminosity searches: at LHC startup softer selections will be adopted.

miss 2.5 Multi-leptons + pT

In mSUGRA there are situations in which the next-to-lightest SUSY particles (NLSPs) decays into LSPs plus sleptons or leptons. This topology is complementary to jet-based searches, and the search is based on lepton triggers.

The ATLAS selection requires at least 3 leptons with pT > 10 GeV and |η| < 2.5. Two of the leptons must be of the same flavour but opposite sign (a SFOS pair), and the invariant mass built with the SFOS pair should be 10 GeV away from the Z mass. Leptons should be isolated: no neighboring (within R =0.4) jet or (if MSFOS < 20 GeV) calorimeter and track isolation. Optionally, a jet veto (with pT > 20 GeV) may be applied. Low-mass mSUGRA points like SU4 and SU3 i would be detectable with 1 fb−1. Because the background from the SM is similar in the SFOS and same-flavour same- sign, a common technique to subtract the SM background is to plot the bin-wise difference between the histogram of the SFOS pair invariant mass and the one for the same quantity computed with events without a SFOS pair. An alternative statistical procedure has been adopted by CMS [2], in which the SFOS invariant mass is fitted with a theoretical curve that includes background (including the Z peak). The CMS strategy is to trigger with single or double isolated muons, then select events with a leading muon with pT > 30 GeV and less than 10 GeV deposited in calorimeters within a radius of R = 0.3 in (η, φ) space, and 3 jets with pT > 50 GeV [2]. Again, softer cuts will be used at LHC startup.

i SU3: m0 = 100 GeV, m1/2 = 300 GeV, tan β = 6, sgn(µ) =+, A0 = −300 GeV.

DIS 2009 1000 ∼ τ LSP tanβ =10, A =0, μ >0 CMS 1 0 900 nosystematics -1 = 120 GeV 1fb 800 mh

700 jet+MET 600 μ+jet+MET SS2 μ

(GeV) 500 OS 2 l Figure 1: Accessible mSUGRA region by

1/2 2 τ −1 −1

m 400 Higgs CMS in 1 fb (2 fb for the Higgs anal- Z0 300 top ysis), including statistical errors only. [2]

mh =114GeV 200 m =103GeV 100 χ NOEWSB 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000

m0 (GeV)

miss 2.6 2 photons + pT + jets This signature is important for GMSB with neutralino as NLSP (the gravitino is the LSP). The SM background is practically negligible in the relevant (though somewhat limited) region of the parameter space. For example, one may reﬁne jet-based searches by requiring at least 2 photons: the ATLAS event selection [3] requires 2γ with pT > 20 GeV in |η| < 2.5, miss 4 jets with pT > 100, 50, 50, 50 GeV, and ET > 100 GeV. The trigger can be based on miss photons alone, but also on jets or photons + ET .

3 Summary

Provided that the SM background is known to suﬃcient statistical and systematic precision, SUSY might be discovered with the ﬁrst LHC data in the case of low-mass points. The miss searches will mainly focus on jet + ET signatures, though multi-lepton, photons, taus and b-jets will also be used by redundant analyses.

Acknowledgments

Suggestions and comments by Oliver Buchmueller, Dave Charlton, Paul de Jong, Clara Matteuzzi, George Redlinger, Alex Tapper have been very precious.

References

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