SUSY Searches : lessons from the first LHC Run

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1 SUSY Searches : lessons from the first LHC Run P. Pralavorio, On behalf of the ATLAS and CMS Collaborations. CPPM, Aix-Marseille Univ. and CNRS/INP3, 63 avenue de Luminy, case 9, 388 Marseille cedex 9, France A review of direct searches for new particles predicted by electroweak scale Supersymmetry after the first run of the LHC is presented, with an emphasis on the latest results provided by ATLAS and CMS by the time of the conference. ATL-PHYS-PROC-4-3 September 4 Introduction Bruno Zumino, one of the founding fathers of supersymmetry (SUSY), said in 974 : I am sure we all agree that a giraffe is truly beautiful, but she doesn t seem to serve any purpose. With around fb of data from the first run of the LHC at 7-8 TeV in the center of mass, the ATLAS 3 and CMS 4 experiments had an unique opportunity to probe thoroughly natural SUSY models. In this short report, the main results of the direct searches for particles predicted by ElectroWeak (EW) scale SUSY are presented as well as how these results change our current understanding of particle physics. The full list of public results are available in these public websites 5,6. Direct SUSY searches at LHC: framework and strategy SUSY is the leading theory for physics beyond the Standard Model (SM) since it provides a solution to most of its shortcomings. The minimal SM extension that realizes N= supersymmetry, called MSSM, was proposed at the beginning of the 8 s 7 and is still used as a test bench for SUSY exploration at colliders. This model predicts new particles shown in Fig. left. First, the Higgs sector contains five bosons instead of one in the SM. The lightest one (h ) is SM-like and bounded from above 8 such that m h 3 GeV. These constraints perfectly match with the properties of the particle discovered in by the ATLAS and CMS experiments 9,. Then a long list of sparticles follows, composed of SUSY partners of SM particles in chiral multiplets. Each sparticle only differs from the SM by half a unit of spin and a negative R-Parity (P R ), a new quantum number 8. As a result the superpartners of the SM fermions, called squarks and sleptons, are scalars, whereas fermionic partners of the SM bosons called neutralinos, charginos

2 Figure SUSY particles in MSSM 8 (left) and natural SUSY particle mass spectra giving less than % finetuning (right) and gluinos, are Majorana fermions. Finally the gravitino G is the supersymmetric partner of the graviton. With equal number of fermions and bosons in the MSSM, the hierarchy problem, i.e. the fine-tuning of the Higgs mass parameter, can be solved naturally provided the sparticle masses are close enough to the SM ones a. With up to % fine-tuning, the superpartners of the most massive SM particles ( H, W, B, t, b L ), the most strongly coupled to the Higgs, are constrained to be in the mass range -6 GeV, see Fig. right. The gluino mass is expected to be around TeV. This natural spectrum was almost completely uncovered before the start of the LHC in and was therefore the central focus of the LHC searches for the first run. The standard framework for LHC searches, called plain vanilla MSSM, is based on three additional assumptions. First, R-Parity must be conserved, implying that the SUSY particles are pair-produced and the lightest one is stable. Second, the nature of this Lightest SUSY Particle (LSP) is a massive neutralino (noted Ñ or ) or an almost massless G. Third, the SUSY spectrum is open, i.e. the mass difference ( M) between the highest particle produced at LHC (M SUSY ) and the LSP mass (M LSP ) is large, at least GeV, allowing highly energetic objects to be produced in the sparticle decay. In this framework, sparticles are assumed to decay promptly. Fig. shows the different discriminating variables adopted for discovering neutralinos/charginos, third generation squarks and gluinos/other squarks from left to right. Direct searches of these sparticles are discussed in Sec. 3, 4 and 5. Other scenarios (R-Parity violated, compressed spectra, long-lived particles...) and beyond MSSM frameworks are treated in Sec. 6. Discovering SUSY at LHC is an extremely challenging task. First, a high number of final states is possible with different mixtures of reconstructed objects (photon, electron, muon, tau, b-jet) and global variables like missing transverse energy (ET miss ) which estimate the undetected LSP transverse energy. Second, the new scalars and weakly interacting particles have production cross-sections orders of magnitude below the SM ones. Finally, the few expected signal events are generally located in tails of the kinematic distribution, requiring an accurate background modelling in a complicated region of phase space. As no signal has been seen, the results are interpreted in topological or simplified models 3. Limits are shown in a mass plane constructed with the highest accessible SUSY particle at the LHC and the LSP while all other sparticles are generally assumed unreachable at LHC. These model independent limits can then be recast in realistic SUSY models. a A fully natural model will have equal masses for sparticles and particles, but this is ruled out experimentally.

3 Figure Strategy for direct searches at LHC : charginos and neutralinos (left), third generation squarks (center) and gluinos and first/second generation squarks (right). Figure 3 Three possible EW SUSY mass spectra depending on the relative values of M, M and µ parameters (see text) which correspond to three flavors of e (a) bino-like, (b) wino-like and (c) higgsino-like. 3 Direct searches in the EW SUSY sector Weakly interacting SUSY particles are split into three categories : five neutral and charged Higgs bosons with positive R-parity, eight neutralinos/charginos (EWKinos) and nine sleptons with negative R-parity. The Higgs searches are discussed in another report 4. In natural SUSY, the EWKino masses are expected to be close to the EW scale as seen in Fig. right, whereas the slepton masses are largely unconstrained. In the case of the EWKinos, the gauge eigenstates bino ( B), wino ( W ) and higgsino ( H) are governed by the M, M and µ mass parameters which are not constrained by the theory. The states then mix to form the charginos ( ± ) and neutralinos ( ). To ease the presentation of results, three scenarios corresponding to different flavors of the (a) bino-like, (b) wino-like and (c) higgsino-like are considered (Fig. 3). Let s first consider as the LSP and scenario (a). The decay of the next-to-lightest EWKinos follows ± W ± ( lν) and Z ( ll). For + and ± production, dilepton and trilepton plus ET miss final states are thus generated. The signal sensitivity is illustrated by the distribution of the transverse mass-like variable m 5,6 T in Fig. 4 left. A SUSY signal will manifest itself by an excess at large m T whereas the SM naturally cuts-off around GeV. Chargino masses below 8 GeV for massless are excluded, beyond the LEP limit (Fig. 4 center). Note that a similar analysis sets limits to the right- and left-handed selectron/smuon of 5 and 3 GeV respectively 7 but no limit exists for staus. In the trilepton channel 8, degenerate ± and are excluded below 43 GeV for masses below GeV, far beyond the previous result (Fig. 4 right). In all cases, LHC results have poor sensitivity to compressed spectra. Depending on SUSY parameters, the process ± h W ± may also be allowed. It is searched for by considering h Z Z, W W, b b decays which give one to four lepton final states. Fig. 5 left shows the limits obtained by combining all three channels, two times worse 9

4 Figure 4 m T distribution in the dilepton+et miss channel (left), chargino mass limit (center) and degenerate e -e ± mass limits (right) 7. [GeV] m CMS ± pp H ± W - s = 8 TeV L = 9.5 fb 95% CL CLs NLO Exclusions Observed ±σ theory Expected ±σ experiment m - m = m H m ±=m [GeV] 3 95% CL upper limit on cross section (fb) 95% CL upper limit on σ/σ ref - CMS Preliminary, L = 9.3 fb, s = 8 TeV m m ± m ; m LSP Expected ± σ exp. Expected ± σ exp. Observed Higgsino mass m (GeV) Figure 5 Degenerate e -e ± mass limits when Higgs appears in the cascade 9 (left). Upper limits on production cross-sections involving EWKinos from scenario (c) of Fig. 3 with a G e LSP (right). than in Fig. 4 right. Scenario (b) and (c) are presently not accessible at LHC. When the massless G is the LSP, the situation changes dramatically since the decays and gives extra photon(s) or extra W ± /Z /h in the final state. This greatly helps the background reduction and mass limits are typically increased by a factor of two for scenario (a). LHC experiments are also sensitive to scenario (b) and (c) where extra W /Z are involved,9 and mass limits on neutralinos are generally in the range -4 GeV. However no limits exist yet when two h are present in scenario (c), as shown in Fig. 5 right. Despite a remarkable effort, it is fair to say that EWKinos masses are presently poorly constrained by the LHC data. 4 Direct searches for third generation of squarks Searches for third generation of squarks are very sensitive probes of new physics at LHC. With the as LSP, four top squark decays are possible : t t, t W ( ) b, t c depending on M = M e t M e or t b ± bw ±( ). Dedicated analyses were set up to scan all possible decays. The most sensitive one is the one-lepton plus four jets plus ET miss (including at least one b-jet) which covers two of the main scenarios t t and t b ± bw ±. Other channels with lepton, leptons or even c-tagged jets help to cover the full phase space of the t - mass plane, as shown in Fig. 6. This ensures a good coverage of the interesting area for natural SUSY, up to 6-7 GeV for massless. These results assume a % branching ratio for the stop decay. A reduction by 6% of this decay with no possibility to detect other

5 ; Figure 6 Stop mass limits 5. decay chains will typically shrink 3 the limits by GeV. In Fig. 6, funnels are still visible for fixed values of M in the case t t. In particular for the case M = M t, analyses were built to look at the t t/h /Z t channels, helping to almost fill the gap 4,5 for M e t < 35 GeV. The case where G is the LSP is also actively pursued and gives similar results in general. This includes the case where is Higgsino-like [scenario (c) of Fig. 3] which has recently been analyzed in several Higgs decay modes 4,6,7. Because the bottom quark is in the same SU() L doublet as the top quark, the left-handed sbottom is expected close to the stop masses. Two decay channels b L b and b L t ± bw + W are searched for in the b-jets 8,9 and same-sign leptons 3,3 final states, respectively. In the first case, a powerful discriminating variable called m 3,33 CT, similar to m T, is used as illustrated in Fig. 7 left. ATLAS and CMS results are shown in Fig. 7 center and right: CMS pushes further the sbottom mass limit for massless to reach 7 GeV and ATLAS is more sensitive to compressed spectra. Somewhat lower limits are obtained for the b L t ± case. The G LSP scenarios are covered by the stop analyses discussed previously. The absence of hints for t and b L in the -5 GeV mass range puts strong constraints on the SUSY natural spectrum. 5 Direct searches for gluinos and first/second generation of squarks If TeV-scale gluinos are produced at the LHC, as suggested by Fig. right, they will decay promptly in long cascades containing many quark and gluon jets and two LSPs. Therefore final states with multiple energetic jets, ET miss and no leptons are searched for. Depending on the spectrum below the gluino mass, one, two, three or even four step decays are possible. If is the LSP, the simplest scenario occurs when the gluino decays via first/second generation off-shell squarks like g q q ( ) qq. With four jets (or more) and high Emiss T, the effective mass 34 m eff as defined in Fig. right peaks at higher values for the signal than for the SM backgrounds, see Fig. 8 left. In this simplest scenario, it is possible to exclude gluino

6 Figure 7 m CT distribution in the two b-jet analysis 9 (left), ATLAS sbottom mass limits 8 (center) and CMS sbottom mass limits 9 (right). LSP mass [GeV] ~ g- ~ g production, - CMS L = fb s = 8 TeV Observed SUSY Observed - σ theory Expected m(gluino) - m(lsp) = m(w) + m(top) m(gluino) - m(lsp) = m(top) ~ g t t miss arxiv:4.477: H T +H T +multijets miss arxiv:35.39: E T +H T +b-jets miss arxiv:3.4937: E T + lep+n 6 jets miss arxiv:3.6736: E T + lep(ss)+b-jets gluino mass [GeV] Figure 8 m eff distribution in the -lepton+ 5jets+ET miss channel (left), gluino mass limits 35 (center) and gluino mediated stop mass limits (right). masses below.4 TeV for masses below GeV, see Fig. 8 center. This analysis can serve to exclude squarks, though limits are less strong 35,36. For compressed spectra and multi-step decays, leptonic analyses 3,3,37,38,39 or low ET miss analysis 4 perform generally better and help to increase the coverage. If the squarks happen to be of the third generation type, four-top, two-top plus two-b or four-b final states could be generated and b-jets as well as lepton(s) could be added to the signature to reach similar limits. As an example, Fig. 8 right shows the limits in four-top final states. When G is the LSP, photons or leptons from W /Z decay or b s from Higgs decay will populate the final state, helping to reduce the background and excluding gluino masses below TeV. Analyses developed for the LSP case can be reused or dedicated analyses are needed. This is in particular the case for scenario (a) of Fig. 3, with bino-like, where the diphoton+emiss T analysis excludes gluino masses beyond.4 TeV as in the LSP case. Similarly a γ+h ( b b) analysis helps scenario (c) of Fig. 3 and excludes gluino masses below TeV 4. A gluino mass above TeV is a strong constraint on the SUSY natural spectrum. 6 Naturalness or something else? The negative results from the LHC searches for plain-vanilla MSSM may suggest that the Standard Model is a fine-tuned theory b. There are three main caveats to this affirmation : i) limits are weaker or nonexistent for compressed scenario, ii) complicated SUSY spectrum with lower branching ratio for each decay weakens these limits and iii) some channels like those including b Accepting a higher level of fine-tuning for the SM allows much higher sparticle masses than the ones of Fig..

7 Figure 9 Limits on long-lived Particles neutralino (left), chargino 4 (center) and gluino 43 (right). Higgs in their decay chain or direct chargino production still have limited sensitivity. These three difficult scenarios will be probed by the time the LHC program will be completed, around 3. However there are also plenty of scenarios lying beyond the plain vanilla one, still within the MSSM framework. For example, some sparticles could have non negligible lifetime before decaying and even be stable within the detector volume. They then generate a signature that is missed by the standard analyses. Dedicated analyses, generally free from SM background and relying on an exquisite understanding of the detector, have therefore been developed. A snapshot of the results obtained can be seen in Fig. 9 in the lifetime-mass plane of a given longlived particle (neutralino, chargino or gluino from left to right). Long-lived sleptons and squarks can also be excluded 44,45 in the range.4- TeV. It is interesting to notice that those longlived sparticles are also expected from unnatural SUSY 46. Another example of scenario going beyond the plain vanilla one is provided by the violation of R-Parity which is not constrained by naturalness arguments. There, spectacular multilepton signatures 47,48 or 3-jet resonances 49,5 are expected and searched for. Finally, beyond the MSSM scenario, where extra scalar particles are added to the gaugino multiplet could also present interesting signature like -jet resonances coming from the decay of the scalar gluon 5,5. 7 Conclusions Particles predicted by natural plain vanilla SUSY scenarios (R-Parity conservation, or G as the LSP and a reasonable open spectra), have not been observed in the first run of the LHC. The strongest limits are put on the gluino mass, above TeV in most of the scenarios. Stop and sbottom masses are also excluded in most of the scenarios below 5 GeV, though it is not excluded that they escape detection because of an intricate decay chain. Benefiting from high statistics of the 8 TeV run, the EW SUSY sector has now been probed beyond the LEP limits, but more data are needed to reach definite conclusions. Other scenarios beyond the plain vanilla one were also probed, but unsuccessfully till now. With the restart of the LHC at a higher energy in 5 and the completion of the physics program with 3 fb of data around 3, ATLAS and CMS will continue to explore an uncharted territory, where hopefully a first sign of physics beyond the Standard Model will be detected 53,54. References. G. Senjanovic, Int.J.Mod.Phys.Conf.Ser. 3, 8 (), arxiv: L. Evans and P. Briant, JINST 3, S8 (8). 3. ATLAS Coll., JINST 3, S83 (8).

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