Experimental status of particle and astroparticle searches for supersymmetry
EExperimental status of particle and astroparticlesearches for supersymmetry
Vasiliki A Mitsou
Instituto de F´ısica Corpuscular (IFIC), CSIC – Universitat de Val`encia,Parc Cient´ıfic de la U.V., C/ Catedr´atico Jos´e Beltr´an 2, E-46980 Paterna (Valencia), SpainE-mail: [email protected]
Abstract.
An overview of supersymmetry searches is presented, covering collider experiments,direct and indirect searches for supersymmetric dark matter. Recent LHC experimentalresults are reviewed, and the constraints from B -meson decays are reported. Implications forsupersymmetry of the latest direct and indirect searches are thoroughly discussed. The focus ison the complementarity of the various probes — particle and astrophysical — for constrainingSupersymmetry.
1. Introduction
The Standard Model provides the current most accurate description of elementary particlephysics. It has been experimentally tested up to the TeV scale with remarkably successfulresults. Nevertheless, there are pieces of evidence pointing to physics beyond the SM such asthe existence of dark matter (DM), the matter-antimatter asymmetry, the neutrino masses,and the hierarchy problem. Several theories have been proposed to address these issues andSupersymmetry (SUSY) [1] is one of the most favoured.SUSY postulates a discrete symmetry between fermions and bosons, thus assigning a newparticle, a superpartner , to each SM field. Since no SUSY particles have been found yet, SUSYmust be a broken symmetry, leading to numerous proposed models and various symmetry-breaking mechanisms. SUSY is characterised by a large number of free parameters, e.g. morethan 100 parameters are introduced in the Minimal Supersymmetric Standard Model (MSSM).Nonetheless, a variety of experimental and observational handles are at our disposal to searchfor SUSY and constrain it. The different facets of SUSY manifestation and the current statusof the hunt for SUSY is discussed hereupon.This paper is structured as follows. Section 2 provides a brief introduction to dark matterand its implications for SUSY, also discussing alternative SUSY models yielding a non-trivialDM density. It reviews both direct and indirect DM detection methods and recent results.In Section 3 the features of collider experiments that play a central role in exploring SUSYare highlighted. There we concisely discuss the SUSY constraints originating from B -physicsexperiments, whilst strong emphasis is given on recent attempts to discovering supersymmetryat the Large Hadron Collider (LHC) [2] and the ensuing exclusion limits, covering both R -parity conserving and R -parity breaking scenarios — the latter receiving special attention. Theimplications for SUSY arising from the observation of a Higgs-like particle are also addressed.The paper concludes with a summary in section 4.1 a r X i v : . [ h e p - ph ] A p r xperimental status of particle and astroparticle searches for supersymmetry
2. SUSY as dark matter
Unveiling the nature of dark matter [3, 4, 5] is a quest in both Astrophysics and Particle Physics.According to observations over the past two decades — obtained by combining a variety ofastrophysical data, such as type-Ia supernovae [6], cosmic microwave background (CMB) [7],baryon oscillations [8] and weak lensing data [9] —, most of our Universe energy budget consistsof unknown entities: ∼
23% is dark matter and ∼
72% is dark energy, a form of ground-stateenergy. Dark matter existence is inferred from gravitational effects on visible matter, but isundetectable by emitted or scattered electromagnetic radiation. The most precise measurementcomes from anisotropies of the cosmic microwave background [7, 10]; the third peak in thetemperature power spectrum, shown in Fig. 1, is used to extract information about dark matter.
Figure 1.
The WMAP 7-year temperaturepower spectrum, along with the temperaturepower spectra from the ACBAR and QUaDexperiments. The solid line shows the best-fitting 6-parameter flat CDM model to theWMAP data alone. From [7].
Figure 2.
Predicted values of Ω χ h as afunction of the DM particle mass m χ forvarious deviations k from the weak couplingand for different values of foam fluctuations σ . The yellow horizontal band represents theWMAP7 value for Ω χ h [7]. From [11].Weakly-interacting massive particles (WIMPs), where the typical (weak-scale) annihilationcross sections are of the same order of magnitude as the thermally-averaged DM annihilationcross section, constitute the most popular class of dark matter candidates. Supersymmetry is atheoretical scenario that inherently proposes such dark matter candidates. Within SUSY with R -parity conservation, the particle that plays the role of dark matter is the lightest supersymmetricparticle (LSP); LSPs that have the right properties are the axino, the gravitino (as superWIMPs)and the lightest neutralino (as WIMP). Such scenarios — should they indeed explain dark matter— are severely constrained from the Big-Bang nucleosynthesis, the measured DM abundanceand the results of direct DM detection experiments and indirect detection telescopes. WIMPs were created thermally in the early universe, hence their relic abundance, Ω χ , and theirproperties, such as mass and couplings, are closely related through the averaged annihilationcross section. For a specific SUSY model, the predicted neutralino relic density, calculated bytaking into account all possible LSP annihilation processes, is compared to the measured Ω χ ,imposing constraints on the model parameters.The DM relic density estimation is based on the Standard Cosmological Model (ΛCDM) [12],involving cold DM as the dominant DM species, and a positive cosmological constant Λ > xperimental status of particle and astroparticle searches for supersymmetry D -particle) foam in string/brane-theory leads to Finsler-type metric distortions, inducedby interactions of the DM particle(s) with the defects in the foam [11]. These metrics leadto modifications in the pertinent Boltzmann equation and consequently to enhancement of therelic abundances, as displayed in Fig. 2. However stringent constraints on the defect densityin D -foam models can also be imposed by astrophysical telescopes on the arrival times of highenergy cosmic photons [13].The presence of the time-dependent dilaton may also affect the dark matter relic densitycalculation, since it modifies the Boltzmann equations. In this case, a dilution in the neutralinodensity of O (10) is predicted [14], widening the allowed parameter space of supersymmetry atcollider searches. Furthermore, other astrophysical observations such as type-Ia supernovae andgalaxy ages set tight constraints on the parameters of these models through their prediction forthe dark energy contribution [15]. In direct detection low background underground experiments, one attempts to observe thenuclear recoil produced by WIMP scattering off nucleons [16]. The expected signal featuresa recoil spectrum which falls exponentially with energy and extends to a few tens of keV only.Various techniques applied to observe the recoil with uncorrelated systematic uncertainties:scintillator NaI, cryogenic, noble liquids (Ar, Xe), bubble chamber (superheated liquid).The current experimental landscape is shown in Fig. 3 in terms of WIMP-nucleus elastic crosssection versus WIMP mass [17]. The positive results shown represent the annual modulationobserved by DAMA/LIBRA [18] and CoGeNT [19], as well as unidentified excesses of events seenby CoGeNT [19] and CRESST-II [20]. CDMS does not see any hint of annual modulation [21]although their energy threshold is higher than CoGeNT. The XENON100 data [17] lead to thestrongest exclusion limit so far for constraining the DM-nucleon cross section to below 10 − cm for m χ ∼
100 GeV. For comparison, other experimental results are also shown [22, 19, 20],together with the 1 σ/ σ regions preferred by supersymmetric (Constrained MSSM) models [23],still untouched by the XENON limits. Figure 3.
Spin-independent WIMP-nucleon scattering from XENON100: The expectedsensitivity is shown by the green/yellow band (1 σ/ σ ) and the resulting exclusion limit (90%CL) in blue. From [17]. xperimental status of particle and astroparticle searches for supersymmetry WIMPs can annihilate and their annihilation products can be detected; these include neutrinos, γ -rays, positrons, antiprotons, and antinuclei [24]. “Smoking gun” signals for indirect detectionare neutrinos coming from the center of the Sun or Earth, and mono-energetic photons fromWIMP annihilation in space. These techniques could distinguish between different couplingscenarios and the nature of WIMPs: neutralinos, Kaluza-Klein states, etc.The observation of 14 dwarf spheroidal galaxies with the Fermi Gamma-Ray Space Telescopeshowed no significant γ -ray emission above 100 MeV from the candidate dwarf galaxies [25].Therefore upper limits to the γ -ray flux assuming representative spectra from WIMP annihilationwere set, as demonstrated in Fig. 4, also compared with predictions from mSUGRA, MSSM witha reduced set of parameters [26], Kaluza-Klein dark matter in universal extra dimensions (UED)and wino-like dark matter in AMSB. The upper limits on the γ -ray flux are already competitivefor MSSM models, provided that they correspond to low thermal relic density. Furthermore,these flux upper limits already disfavour AMSB models with masses <
300 GeV.
Figure 4. mSUGRA (upper left), MSSM (upper right), Kaluza-Klein UED (lower left) andanomaly mediated (lower right) models in the ( m WIMP , (cid:104) σv (cid:105) ) plane. Red points have a Ω χ compatible with the inferred DM density (blue points have a lower DM density). The linesindicate the Fermi 95% upper limits from likelihood analysis on the selected dwarfs. From [25]. xperimental status of particle and astroparticle searches for supersymmetry E γ (cid:39)
135 GeV [29]. The required annihilation cross section to explain this signal isvery large, however supersymmetric models with such large signals do exist. Further progressto determine if the line signal is robust and to improve sensitivities for both continuum and linesearches is sure to come from continued running of existing and upcoming experiments.To recapitulate, direct and indirect DM detection techniques set severe constraints onsupersymmetric models, yet accelerator experiments are required to provide a measurementof different model parameters exploiting uncorrelated systematics. This is concisely depicted inFig. 5, where the coverage of the direct and indirect detection with photons are complementedby the LHC and neutrino telescopes [30]. In the following years we expect a continuous interplaybetween particle physics experiments and astrophysical/cosmological observations.
Figure 5.
Illustration of the reach of direct and indirect dark matter detection experimentsfor γ -ray detection. The shaded region is the approximate range of WMAP-compatible MSSMmodel space. The smaller hatched region is a rough estimate of where Kaluza-Klein modelsreside [31]. The dashed rectangular indicates the region that may be covered by LHC andneutrino telescopes. The MSSM results are based on Ref. [32], to which the reader is directedfor information on the mass range, composition, etc. From [30].
3. Searches in colliders
The stringent exclusion limits set on supersymmetric models and parameters come from colliderexperiments, such as the LEP, Tevatron and LHC detectors. The LHC currently in operationat CERN in Geneva, Switzerland, is an ideal machine for discovering supersymmetry. Althoughhere we focus on direct searches for SUSY signals, discussed in Section 3.2, SUSY is alsoconstrained indirectly through the (non-)observation of rare decays of B mesons. xperimental status of particle and astroparticle searches for supersymmetry B -physics constraints Precise experimental measurements and theoretical predictions have been achieved for the B -meson systems in the past decade and stringent constraints due to considerable beyond-SM contributions to many observables have been acquired [33]. The rare decay BR ( B s → µ + µ − ) deserves special attention as new results have been recently announced by the LHCbcollaboration [34] using pp collisions at √ s = 8 TeV. An excess of B s → µ + µ − candidates withrespect to the background expectation is observed with a 3 . σ significance [35]. This result isin agreement with the Standard Model expectation, therefore limits on new-physics models areset.Here, we review constraints obtained with the LHCb results using 1 fb − of data at √ s =7 TeV, where a stringent 95% CL limit on the branching ratio BR ( B s → µ + µ − ) < . · − has been obtained [36]. The decay B → K + µ + µ − , on the other hand, provides a varietyof complementary observables as it gives access to angular distributions in addition to thedifferential branching fraction. In addition to the above observables, B → X s γ , B → τ ν , B → Dτ ν τ , B → X s µ + µ − and D s → τ ν τ are also very sensitive to SUSY.A comparison between different flavour observables in the plane ( m / , m ) is given in Fig. 6,where limits from B → X s γ , B → τ ν , R l ( K → µν µ ), B → Dτ ν τ , B → X s µ + µ − and D s → τ ν τ are also shown [37]. The LHCb limit strongly constrains the CMSSM with large tan β = 50,however for tan β = 30, as can be seen from the figure, the flavour constraints and in particular B s → µ + µ − , are inferior to those from direct searches. On the other hand, B → X s γ and B → K + µ + µ − related observables and in particular the forward-backward asymmetry couldplay a complementary role in the intermediate tan β regime. Figure 6.
Constraints from flavour observables in CMSSM in the plane ( m / , m ) fortan β = 30 and A = 0 with the 2011 results. The black line corresponds to the CMS exclusionlimit with 1.1 fb − of data [38] and the red line to the CMS exclusion limit with 4.4 fb − ofdata [39]. From [37]. SUSY searches in ATLAS [40] and CMS [41] experiments typically focus on events with hightransverse missing energy ( E missT ) which can arise from (weakly interacting) LSPs, in the caseof R -parity conserving SUSY, or from neutrinos produced in LSP decays, when R -parity isbroken. Hence, the event selection criteria of inclusive channels are based on large E missT , noor few leptons ( e , µ ), many jets and/or b -jets, τ -leptons and photons. In addition, kinematical xperimental status of particle and astroparticle searches for supersymmetry M T , and the effective mass, M eff , assist in distinguishingfurther SUSY from SM events, whilst the effective transverse energy [42] can be useful to cross-check results, allowing a better and more robust identification of the SUSY mass scale, shoulda positive signal is found. Although the majority of the analysis simply look for an excess ofevents over the SM background, there is an increasing application of distribution shape fittingtechniques [43].Typical SM backgrounds are top-quark production — including single-top —, W / Z inassociation with jets, dibosons and QCD multi-jet events. These are estimated using semi-or fully data-driven techniques. Although the various analyses are motivated and optimised fora specific SUSY scenario, the interpretation of the results are extended to various SUSY modelsor topologies.Analyses exploring R -parity conserving (RPC) SUSY models are currently divided intoinclusive searches for: (a) squarks and gluinos, (b) third-generation fermions, and (c) electroweakproduction ( ˜ χ , ˜ χ ± , ˜ (cid:96) ). Recent results from each category of CMS searches are presented inRef. [44]. It is stressed that, although these searches are designed to look for RPC SUSY,interpretation in terms of R -parity violating models is also possible (cf. Section 3.4).Strong SUSY production is searched in events with large jet multiplicities and large missingtransverse momentum, with and without leptons. Various channels fall into this class of searches;here the 0-lepton plus three jets plus E missT analysis from CMS is highlighted. The results areinterpreted in terms of the CMSSM (Fig. 7) excluding squark masses up to 1400 GeV, gluinomass up to 900 GeV, or m ˜ q ∼ m ˜ g ∼ q ˜ q → q ˜ χ q ˜ χ , assuming that m ˜ g (cid:29) m ˜ q [46].The results in the ( m ˜ q , m LSP ) plane are shown in Fig. 8.
Figure 7.
Observed and expected 95% CLlower limits in the CMSSM ( m ˜ g , m ˜ q ) plane,for tan β = 10, µ >
0, and A = 0. Theyellow-shaded region shows the 1 σ variationin the expected limit, while the dot-dashedcurves show the variation in the observed limitwhen the signal cross section is varied by itstheoretical uncertainties. Limits from earliersearches by other experiments derived withdifferent models or parameter choices are alsoshown. From [45]. Figure 8.
Upper limit on cross sectionat 95%CL as a function of m ˜ q and m LSP for1-step ˜ q ˜ q → q ˜ χ q ˜ χ simplified model. Thesolid thick black line indicates the observedexclusion region assuming NLO+NLL SUSYproduction cross section. The thin black linesrepresent the observed excluded region whenvarying the cross section by its theoreticaluncertainty. The dashed purple lines indicatethe median (thick line) ± σ (thin lines)expected exclusion regions. From [46]. xperimental status of particle and astroparticle searches for supersymmetry Z boson or a photon. Such signatures can be detected by looking for events with a leptonically-decaying Z , high E missT and jets. The ATLAS analysis [48] yielded the exclusion limits depictedin Fig. 9 in the context of the general gauge mediation (GGM) model. Figure 9.
Expected and observed 95% CLexclusion limits for SR1 on the m ˜ g and µ parameters for GGM models with tan β = 30, M = M = 1 TeV, cτ NLSP < . m ˜ q = 1 . Figure 10.
Exclusion limits at 95% CL for8 TeV analyses in the ( m ˜ g , m ˜ χ ) plane for theGtt simplified model where a pair of gluinosdecays via off-shell stop to four top quarks andtwo neutralinos (LSP). From [49].The mixing of left- and right-handed gauge states which provides the mass eigenstates ofthe scalar quarks and leptons can lead to relatively light 3 rd generation particles. Stop (˜ t )and sbottom (˜ b ) with a sub-TeV mass are favoured by the naturalness argument, while thestau (˜ τ ) is the lightest slepton in many models. Therefore these could be abundantly producedeither directly or through gluino production and decay. Such events are characterised by severalenergetic jets (some of them b -jets), possibly accompanied by light leptons, as well as high E missT .Both main LHC experiments, ATLAS and CMS, have embarked in analyses dedicated tounveil third-generation sparticle production covering a wide spectrum of signatures. The resultsextracted by ATLAS with up to 13 fb − of data at √ s = 8 TeV, as far as the gluino-mediatedproduction is concerned, are summarised in Fig. 10. Gluino masses of up to 1240 GeV andlightest neutralino masses up to 500 GeV in the Gtt simplified model have been excluded. Moreupdated results on the third-generation are available at Refs. [50, 49], for CMS and ATLAS,respectively.The electroweak SUSY production at LHC proceeds through weak gaugino (charginos andneutralino) or slepton pair production and is typified by low cross section compared to thestrong processes. These events are characterised by multiple charged leptons and moderately xperimental status of particle and astroparticle searches for supersymmetry E missT . Their kinematics is quite different from the long cascade decays typically foundin SUSY processes through strong interactions. Therefore, kinematic variables other than theeffective mass may be deployed such as the visible transverse energy/momentum , to improvethe power of the analysis and also give way to the determination of a combination of sparticlemasses [51].In certain SUSY breaking scenarios, characteristic signatures are expected involving heavysparticles with a long lifetime. A non-exhaustive list of examples of such (meta-)stable particlesincludes long-lived sleptons in GMSB models and R-hadrons with long-lived gluinos or squarksin split SUSY. These particles traverse the entire detector and, due to their high mass, theymove slowly ( β < E/ d x , and a long timeof flight (TOF). The result of a search for heavy long-lived charged particles produced in pp collisions at √ − of CMS data [52] is shown in Fig. 11. The inner trackingdetectors were used to define a sample of events containing high- p T tracks and high d E/ d x .A second sample of events, which have high-momentum tracks satisfying muon identificationrequirements in addition to meeting high-ionisation and long time-of-flight requirements, wasanalysed independently. In both cases, the results are consistent with the expected backgroundestimated from data, thus establishing cross-section limits as a function of mass within thecontext of models with long-lived gluinos, scalar top quarks and scalar taus. Lower limits at95% confidence level on the mass of gluinos (stops) are found to be 1098 (737) GeV. A limitof 928 (626) GeV is set for a gluino (stop) that hadronizes into a neutral bound state beforereaching the muon detectors. The lower mass limit for a pair produced ˜ τ is found to be 223 GeV.It is worthy to mention that the MoEDAL [53] experiment at LHC is specifically designed toexplore such high-ionisation signatures. Figure 11.
Predicted theoretical crosssections with associated uncertaintiesand observed 95% CL upper limitson the cross section for the differentsignal models considered: production˜ t , ˜ g , and ˜ τ ; different fractions of R-gluonball states, using the tracker andTOF information. From [52]. Figure 12.
95% CL upper limits on the pairproduction cross section for neutral particles, eachof which decays into a photon and invisible particles,as a function of the neutral particle proper decaylength. The observed values as a function of massare shown by the solid line. The dashed lineindicates the expected median of results for thebackground-only hypothesis, while the green (dark)and yellow (light) bands indicate the ranges thatare expected to contain 68% and 95% of all observedexcursions from the median, respectively. From [54]. xperimental status of particle and astroparticle searches for supersymmetry p T photons which mayor may not point back to the primary collision vertex. These final states arise in the decay ofa long-lived ˜ χ decaying to a photon and a gravitino. CMS carried out a search using eventscontaining photons, E missT , and jets with 2.23 fb − of data at √ s = 7 TeV [54]. The impactparameter of the photon relative to the beam-beam collision point can be reconstructed usingconverted photons. The method is sensitive to lifetimes of the order of 0.1 to 1 ns. Cross-sectionlimits on pair production for such particles, each of which decays into a photon and invisibleparticles, are presented in Fig. 12. The observed 95% CL limits vary between 0.11 and 0.21 pb,depending on the neutral particle lifetime.The spectrum of SUSY searches at the LHC is huge both in terms of the variety of testedmodels as well as the explored signatures and it was merely highlighted here. A summary ofthe SUSY-related analysis results from CMS [50] is shown in Fig. 13. A similar synopsis ofresults from the ATLAS experiment can be found in Ref. [49]. In conclusion, LHC probessupersymmetry in scales between 100 GeV and 1 TeV, without having seen yet some evidenceof its existence. Experiments in upcoming e + e − colliders, such as the ILC [55] and CLIC [56],are expected to discover and constrain SUSY in the future. Figure 13.
Exclusion limits for the masses of the mother particles, for m ˜ χ = 0 GeV (darkblue) and m mother − m ˜ χ = 200 GeV (light blue), for each analysis, for the hadronic (left) andleptonic (right) results. In this plot, the lowest mass range is m mother = 0, but results areavailable starting from a certain mass depending on the analyses and topologies. From [50]. xperimental status of particle and astroparticle searches for supersymmetry The recent discovery by ATLAS [57, 58] and CMS [59] of a new boson with properties consistentwith those of a SM-like Higgs boson shed new light in the supersymmetric landscape [60]. Thenewly observed boson having a mass of 125 −
126 GeV is rather heavy for being the lightestsupersymmetric Higss, h , however it still lies in the allowed range. If h is SM-like, i.e. in thedecoupling limit, the heaviest neutral SUSY Higgs bosons, H and A are pushed to the 1 TeVscale. In this case, high values of tan β are favoured, together with high stop mixing and heavysquarks.Some (minimal) models are disfavoured, while other remain viable even with other constraintspresent, e.g. the NMSSM [61]. The singlet that is introduced in this model to evade the µ problem, leads to a richer Higgs sector than the MSSM that can accommodate a relativelyheavy Higgs mass [62]. This is demonstrated in Fig. 14, where the blue points denote themodels with Higgs boson masses compatible with the measured Higgs mass of 125 GeV within ± γγ event rate. Figure 14.
Masses of the Higgs (pseudo-)scalars H , H (left) and H , A (right). Red pointsare ruled out either by HiggsBounds constraints or the ATLAS 1 fb − jets and E missT SUSYsearch. Green points have no Higgs with a mass in the 122 −
128 GeV interval, blue pointshave a Higgs ( H and/or H ) within this mass range, and black points have a Higgs such that R ggγγ > .
4. From [62]. R -parity violation Throughout the previous discussion, we assumed that R -parity — a multiplicative quantumnumber defined as R = ( − B − L )+2 s , where B , L and s are the baryon number, the leptonnumber and the spin, respectively — was conserved. This supposition is hinted but not requiredby proton stability; rapid proton decay is avoided if one of B or L is conserved. Allowing R -parity violating (RPV) terms [63] in the superpotential has decisive implications for SUSYphenomenology in colliders. First of all, single sparticle production is possible. The LSP maybe charged and/or carry colour and, most importantly, decays leaving open the possibility fornew discovery signatures and for the LSP-mass reconstruction through the invariant mass of itsdecay products. Furthermore, the E missT may or may not be high depending on the LSP-decayproducts and the underlying RPV model. xperimental status of particle and astroparticle searches for supersymmetry R -parity, the violation through bilinear terms is especiallyinteresting for its connection with neutrino physics. In this model the spontaneous breaking of R -parity gives vacuum expectation values (vevs) to sneutrinos providing a vev-seesaw mechanismthat leads to neutrino masses [64]. Below the scale of these vevs, R -parity breaking is explicitthrough bilinear L -violating terms and their corresponding soft breaking terms. The sameparameters that induce neutrino masses and mixings are also responsible for the decay of theLSP which induces a relation between some decay modes of the LSP and neutrino mixing angles.The latest exclusion limits set on this model, when the bilinear terms are embedded in mSUGRA,were obtained with 5 fb − of ATLAS data at √ s = 7 TeV by searching for events with high jetmultiplicity, E missT and one lepton [65] and are shown in Fig. 15. Figure 15.
Expected and observed 95%CL exclusion limits in the bilinear R -parityviolating model obtained by combining the e and µ channels. The band around the medianexpected limit shows the ± σ variationson the median expected limit, including alluncertainties except theoretical uncertaintieson the signal. The dotted lines around theobserved limit indicate the sensitivity to ± σ variations on these theoretical uncertainties.The thin solid black contours show the LSPlifetime. The result from the previous ATLASsearch [66] is also shown. From [65]. [mm] (cid:111) c1 10 [ pb ] . B R (cid:109) -4 -3 -2 -1 PROSPINO 700 GeV squark pair productionPROSPINO 1.5 TeV squark pair production
ATLAS -1 Ldt = 4.4 fb (cid:48) = 7 TeVs
MHMLHH
Figure 16.
Upper limits at 95% CL on σ · BR vs. the neutralino lifetime for differentcombinations of squark and neutralino masses,based on the observation of zero eventssatisfying all criteria in a 4.4 fb − datasample. The shaded areas around thesecurves represent the ± σ uncertainty bandson the expected limits. The horizontallines show the cross sections calculated fromPROSPINO for squark masses of 700 GeV and1500 GeV. The shaded regions around theselines represent the uncertainties on the crosssections. From [67].Moreover, the potentially long LSP lifetime may give rise to displaced vertices (DV) in thedetector. For instance, an RPV SUSY scenario, where the non-zero RPV coupling λ (cid:48) ij allowsthe decay ˜ χ → µq ¯ q (cid:48) , would give rise to a multi-track DV that contains a high- p T muon at adistance between millimeters and tens of centimeters from the pp interaction point. The resultsof such a search [67] performed by ATLAS are shown in Fig. 16. Fewer than 0.03 backgroundevents are expected in the data sample of 33 pb − , and no events are observed. Based on thisnull observation, upper limits are set on the supersymmetry production cross-section σ × B ofthe simulated signal decay chain for different combinations of squark and neutralino masses andfor different values of the neutralino lifetime, cτ . xperimental status of particle and astroparticle searches for supersymmetry µ from ν ” supersymmetric standard model( µν SSM) [68], which solves the naturalness problem of the MSSM, the so-called µ -problem,and explains the origin of neutrino masses by simply using right-handed neutrino superfields.The model is characterised by three singlets (as opposed to one in NMSSM) and bilinear RPVcouplings, giving rise to a rich phenomenology with many Higgs bosons and neutralinos. Ithas been demonstrated [69] that a 125-GeV Higgs boson while decaying to a pair of unstablelong-lived neutralinos, can lead to a distinct signal with non-prompt multileptons. This signalprovides an unmistakable signature of the model, pronounced with light neutralinos. Evidenceof this signal is well envisaged with sophisticated displaced vertex analysis, as shown in Fig. 17. Figure 17.
Cylindrical coordinates ρ DV versus | z DV | for the ˜ χ → τ τ ν decay, whena µν SSM Higgs is produced at pp collisions at √ s = 8 TeV and an integrated luminosity of20 fb − . From [69]. Figure 18.
Constraints on ˜ G DM lifetime vs.mass in µν SSM. The region below the magenta(red) line is excluded by γ -ray observations(Fermi). Black lines denote predictions forseveral values of | U ˜ γν | . From [70].As a last remark, we address the issue of (not necessarily cold) dark matter in RPV SUSYmodels. These seemingly incompatible concepts can be reconciled in models with a gravitino [71]or an axino [72] LSP with a lifetime exceeding the age of the Universe. In both cases, RPV isinduced by bilinear terms in the superpotential that can also explain current data on neutrinomasses and mixings without invoking any GUT-scale physics. Decays of the next-to-lightestsuperparticle occur rapidly via RPV interaction, and thus they do not upset the Big-Bangnucleosynthesis, unlike the R -parity conserving case. Such gravitino DM is proposed in thecontext of µν SSM with profound prospects for detecting γ rays from their decay [70]. Theconstraints on ˜ G lifetime versus mass set by Fermi and γ -ray observations are summarised inFig. 18. Values of the gravitino mass larger than 10 GeV are disfavoured, as well as lifetimessmaller than about 3 to 5 × s.
4. Summary
The experimental and observational landscape of SUSY searches is multifaceted. In the darkmatter front, the cosmological evidence helps orientate the SUSY model building, while theexploration through direct and indirect detection continues with intriguing results. Concerningthe high-energy colliders, the LHC is in the forefront: powerful direct searches probe variousmanifestations of SUSY, whilst indirect information is collected. No signal has been observedso far, thus strong limits have been set up to a SUSY mass scale of ∼ xperimental status of particle and astroparticle searches for supersymmetry Acknowledgments
The author is grateful to the DISCRETE2012 organisers for the kind invitation and support.Thanks to them, a warm, friendly and intellectually stimulating atmosphere was enjoyed bythe speakers and participants throughout the Symposium. This work was supported in part bythe Spanish Ministry of Economy and Competitiveness (MINECO) under the project FPA2009-13234-C04-01 and by the Spanish Agency of International Cooperation for Development underthe PCI project A1/035250/11.
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