BSM searches (SUSY and Exotic) from ATLAScds.cern.ch/record/2117088/files/ATL-PHYS-PROC-2015-196.pdf · 2015-12-23 · BSM searches (SUSY and Exotic) from ATLAS Marco Schioppa 3

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BSM searches (SUSY and Exotic) from ATLAS

Marco Schioppa∗†

University of Calabria - INFNE-mail: [email protected]

Searches for new physics beyond the Standard Model (SM) at the LHC are mainly driven by twoapproaches: a signature-based search where one looks for a deviation from the SM predictionin event yield or kinematic properties, and a more theory-oriented approach where the search isdesigned to look for specific signatures/topologies predicted by certain beyond standard model(BSM ) scenarios. Typical examples for the latter are searches for Supersymmetry and otherBSM theories with extended symmetries. Supersymmetry predicts a new partner for every SMparticle. An extension to the SM by introducing new gauge or global symmetries (including inHidden Sector) usually leads to the presence of new heavy gauge bosons. Extensive searches forsuch particles have been performed in ATLAS at LHC in the context of Supersymmetry, ExtendedGauge models, Technicolor, Little Higgs, Extra Dimensions, Left-Right symmetric models, andmany other BSM scenarios. Highlights from these searches are presented.

Proceedings of the HEPMAD 2015 "7th High Energy Physics International Conference",September 17-22, 2015Antananarivo, Madacascar

∗Speaker.†On behalf of the ATLAS Collaboration

c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/

mailto:[email protected]

BSM searches (SUSY and Exotic) from ATLAS Marco Schioppa

1. Introduction

The Standard Model (SM) of elementary particles and their interactions successfully describesa wide range of phenomena that can be investigated experimentally with the current generation ofparticle accelerators. It is a general opinion between physicists that the SM is only an effective the-ory that must break down above a certain energy scale. There are strong theoretical arguments tobelieve that it breaks down at the electroweak scale. The unexplained phenomena and problems likethe nature of dark matter and dark energy, matter-antimatter asymmetry, neutrino oscillations, in-consistency with the general relativity and hierarchy problem, suggests that the universe we observerequires theoretical developments beyond the Standard Model (BSM). Supersymmetry (SUSY) isproposed as a simple and elegant extension of the SM. Other theoretical approaches providing so-lutions to some of the problems mentioned above, and often offering a rich phenomenology, aregrouped under the term of exotic physics. The ATLAS experiment [1] at the Large Hadron Col-lider (LHC) has run extensive physics searches to cover as many scenarios as possible. Theseproceedings review a selection of results among the many searches for supersymmetry and exoticsignatures based on the LHC proton-proton collision data collected in Run 1 at

√s = 7 and 8 TeV.

2. ATLAS detector and data taking

The ATLAS detector consists of four major parts: the Inner Detector to track precisely chargedparticles and reconstruct common vertices; the Calorimeter system to measure the energy of easilystopped particles; the muon system to make additional standalone measurements of highly pene-trating muons; the two magnet systems to bend charged particles in the Inner Detector and in theMuon Spectrometer, allowing their momenta to be measured.

The detector operates with close to 100% efficiency and provides performance characteristicsslightly better than its design values. A three level trigger system provides information to identify,in real time, the most interesting events to retain for detailed analysis. Figure 1 shows the totalintegrated luminosity recorded in 2011 (

√s = 7 TeV) and 2012 (

√s = 8 TeV).

Figure 1: Cumulative luminosity as function of time delivered to (green), recorded by (yellow) and good forphysics for (blue) ATLAS during stable beams for LHC proton-proton collisions in 2011 and 2012. Takenfrom [https://twiki.cern.ch/twiki/bin/view/AtlasPublic].

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3. Searches for SUSY physics signals

The collider experiments can give the direct confirmation of the existence of the superpartnersof bosons and fermions predicted by SUSY theory [2, 3, 4]. The sparticle searches are generallyconducted by selecting promising and as clear as possible signature of interest expected from aSUSY model. The signal selection is made choosing good discriminant kinematics variables ableto disentangle the signal events from the SM background. If agreement is seen between the SMprediction and the observed data, 95% CL exclusion limits are set, ruling out scenarios that areincompatible with observations. These limits are usually given as a function of parameters in themodel. The searches covered in these proceedings refer to a selection of sparticles production bothvia strong and electroweak interactions. A complete list of public results on SUSY searches arereported in [5].

3.1 Electroweak produced SUSY

In the limit where the strongly produced sparticles are most often produced off-shell, produc-tion of the electroweakinos may be the dominant SUSY process at the LHC, and will most likelybe found via multilepton final states. Direct production of charginos (χ̃±) and neutralinos (χ̃0

i ), con-sidering the χ̃0

1 as LSP, is expected to have a clean signature due to the presence of SM bosons andleptons in the decay chains. Depending on the mass splitting, a heavier neutralino can decay to theLSP via a Z or the SM Higgs, and the charginos via a W. The final states in direct pair production ofcharginos and neutralinos are characterized by a large lepton multiplicity ( ≥ 2). Depending on thedecay, a dilepton invariant mass compatible with the Z mass can be vetoed or required. Charginomasses below 350 GeV are excluded for the decay via SM bosons and 700 GeV for the decays viasleptons [6, 7]. The summary of all ATLAS electroweakinos searches can be seen in Figure 2. Theanalysis uses 20.3 fb−1 of

√s = 8 TeV pp collision data recorded in 2012.

3.2 Strongly produced SUSY

Direct production of gluinos and first and second generation squarks have high cross-sectionsfor masses up to around 1 TeV. The majority of models consider the decay of these SUSY particlesto the LSP (χ̃0

i ). This will escape the experiment undetected, provided it is stable. In these cases,the LSP provides a good candidate for a weakly interacting dark matter particle. The decay chainleading from the strongly produced sparticles to the LSP will involve the production of a numberof SM particles, predominantly hadronic jets and, depending on the scenario, sometimes leptons.The escaping LSP provides a source of missing transverse energy, a requirement on which is alsocommonly included in searches. Results of the ATLAS searches [8, 9, 10, 11] are summarized inthe two plots of Figure 3. On the left are shown the limits for the photon + j analysis set in the two-dimensional plane of the general-gauge-mediated supersymmetry breaking model (GGM) [12, 13]parameters µ and M3 for the higgsino-bino GGM model with a positive value of the µ parameter.On the right are shown the exclusion limits in simplified models with squark-pair production andsubsequent direct squark decays to a quark and the lightest neutralino.

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Figure 2: Summary of ATLAS search for electroweak production of charginos and neutralinos. Limits areset in the chargino/neutralino mass planes. The dashed and solid lines show the expected and observed limits,respectively, including all uncertainties except the theoretical signal cross section uncertainties. Taken from[7].

3.3 Third generation produced SUSY

Direct production of the third generation strongly-interacting sparticles, i.e. the stops andsbottoms, are considered separately from other SUSY ATLAS searches [14, 15, 16]. They have amore moderate production cross-section than the gluinos and first and second generation squarks,up to masses of around 0.5 TeV. The decays of these particles also produce a large number ofjets, possibly leptons, and missing transverse energy. Figure 4 summarises ATLAS results from anumber of analyses in the χ̃0

1 mass versus t̃1 mass parameter space. Different regions of this spaceare accessed using different decay modes of the stop, as these are dependent on the mass differencebetween the t̃1 and χ̃0

1. Lines are included to mark the limits of a given decay mode, as labelled.The analysis contributes to three regions of the parameter space via these different decay modes, ascan be seen by the three distinct exclusion regions displayed. At stop mass values below 210 GeVthe use of the azimuthal angle distribution between the two leptons in the dileptonic events helpsto probe the difficult region where the mass of the stop is closed to that of the top [17].

3.4 R-parity violating searches

R-parity violating supersymmetry scenarios can lead to long-lived particles (LLPs) if the cou-pling is weak enough, giving a displaced decay of the metastable LSP. The ATLAS long-livedslepton search [18] for charged LLPs with cτ > 1 m has results interpreted in the context of a τ̃1

in models with Gauge Mediated SUSY Breaking [19]. LLPs with cτ > 1 m are likely to decayoutside ATLAS and, if charged, leave a track similar to a muon throughout the detector systems.LLPs are differentiated from muons using track β values calculated from dE/dx measurements inthe Inner Tracker pixel detectors and time-of-flight measurements in the Calorimeters and MuonSpectrometer. Tracks are considered to be LLP candidates if they have transverse momentum > 50GeV and β < 0.95. The background to this analysis is overwhelmingly high transverse momentum

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Figure 3: On the left exclusion limits imposed by the photon+ j analysis in the two-dimensional plane ofthe GGM parameters M3 and µ, for the higgsino-bino GGM model with µ > 0. The observed limits areshown for the nominal SUSY model cross-section expectation. Values of M3 below 1100 GeV are excludedfor most values of the µ parameter, although a significant region corresponding to the case for which thegluino mass is close to that of the lightest neutralino masses remains unexcluded due to the requirements ofone or more jets arising from the gluino decay. The top and right axes represent the corresponding valuesof the lightest neutralino and gluino masses, respectively. On the right exclusion limits in the mass plane ofthe lightest neutralino and first- and second-generation squarks assuming squark-pair production and directdecays q̃→ qχ̃0

1. The solid red line and the dashed red line show respectively the combined observed andexpected 95% CL exclusion limits. Taken from [8] and [11], respectively.

muons with mismeasured β values. Background is reduced by requiring two LLP candidates in anevent and the signal region is set by calculating candidate masses from their transverse momentumand β, before applying a model dependant mass cut. For the model parameter range tanβ = 5-50,limits excluding mτ̃1 < 347-402 GeV can be set.

4. Searches for Exotic physics signals

As well as for the SUSY searches, the Exotic one are conducted identifying the most promisingand as much as possible background free signatures of interest expected from some Exotic models.The data analysis starts from a series of selection criteria able to disentangle the signal eventsfrom the SM background. The result of the signal selection is presented using kinematics variablescapable to well discriminate the signal from the SM Monte Carlo background. If data are in goodagreement with the MC background the exclusion limits are set on the parameters of the benchmarkmodels. A complete list of public results on Exotic searches are reported in [20].

4.1 Search for high-mass dilepton resonances

The dilepton final state has been one of the first channel studied at ATLAS because it benefitsfrom a clear signature and a relatively low background. The data analysis has been performed in thecases where the two leptons are well isolated electrons and muons [21], for proton-proton collisionsat√

s = 8 TeV corresponding to an integrated luminosity of about 20 fb−1.The main challenge ofthis analysis is the transverse momentum resolution of the two leptons. Only in the electron case,no charge identification is required. The main backgrounds for dielectron and dimuon final states

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Figure 4: Summary of 95 % CL exclusion limits in strong production scenarios in the χ̃01 mass versus t̃1

mass plane. The dashed and solid lines show the expected and observed limits, respectively. Four decaymodes are considered separately with a branching ratio of 100%: t̃ → tχ̃0, where the t̃1 is mostly t̃R, for∆m(t̃, χ̃0

1) > mt; t̃1 → Wbχ̃01 (three-body decay) for mW + mb < ∆m(t̃, χ̃0

1) < mt; t̃1 → cχ̃01 and t̃1 → b f f ′χ̃0

1(four-body decay) for ∆m(t̃, χ̃0

1) <mW +mb. The latter two decay modes are superimposed. Taken from [16].

originate from Drell-Yan, diboson, top and jet productions. Misidentified jets are an importantbackground for the electron final states for which the fake rate is measured with data.

The dilepton invariant mass distributions are shown in Figure 5. A good agreement betweendata and SM model predictions is found in all channels. Exclusion limits have been set on vari-ous model interpretation like the grand-unification model based on the E6 gauge group [22, 23],Z∗ bosons, Minimal Z′ model, the Sequential Standard Model Z′S S M boson [24], the spin-2 gravi-ton excitation from Randall-Sundrum models [25], quantum black holes and a Minimal WalkingTechnicolor model with a composite Higgs boson. The narrow resonance with SM Z coupling tofermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron chanel,2.53 TeV in the dimuon channel and 2.90 TeV in the two channels combined. The exclusion limitsare shown in Figure 6.

4.2 Search for vector diboson resonances: WZ, WW, ZZ

The search of diboson resonances helps to investigate about the source of electroweak sym-metry breaking. Extensions of the SM, like Grand Unified Theory, Technicolor, extra dimensionsmodel, etc., predict diboson resonances at high masses. Diboson searches have been performedin all the possible final states [26, 27] at

√s = 8 TeV proton-proton collisions for an integrated

luminosity of about 20 fb−1.In this proceedings only the WZ diboson resonance search is presented. The fully leptonic

channel lνl′l′ (l, l′ = e,µ), with exactly 3 charged leptons reconstructed at the same primary vertex,is used to search for a new resonance decaying to WZ. It shows a better sensitivity at low massthanks to its higher mass resolution and its smaller backgrounds that come mainly from SM diboson

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Figure 5: Dielectron (left) and dimuon invariant mass distributions (right), with two selected SSM Z′ signalsoverlaid, compared to the stacked sum of all expected backgrounds, and the ratios of data to backgroundexpectation. The green band in the ratio plot shows the systematic uncertainties. Taken from [21].

Figure 6: Observed cross-section times branching ratio upper limits for various models in the dilepton finalstate. Taken from [21].

production as well as top-antitop production associated with a W or a Z. The invariant mass of theWZ→ lνl′l′ system is reconstructed from the four-vectors of the candidate W and Z bosons and isused as the discriminating variable for the signal. The main uncertainties are those associated withthe PDFs and the normalization and factorization scales for the backgrounds. A good agreement isfound between the observed and predicted WZ invariant mass distribution for events in the high-mass signal region, leading to the exclusion limits on the Extended Gauge Model (EGM) W′ below1.52 TeV and on the Heavy Vector Triplet (HVT) [28] benchmark models, as shown in Figure 7.

4.3 Search for high mass states in event with one lepton and missing energy

Many new-physics scenarios propose the existence of new high mass states like the additionalheavy gauge bosons [24] proposed by the SSM (the charged ones are denoted W′), the electroweakdoublet vectors (Z∗,W∗) predicted by theories [29] based on the existence of U(3)W = S U(3)W xU(1)W gauge extension of the S U(2)W x U(1)Y electroweak group which is spontaneously brokendown to TeV scale and the direct production of weakly interacting candidate dark matter particles[30]. This proceedings describe such a search [31] in events containing only one lepton (muon

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Figure 7: Observed cross-section times branching ratio upper limits as a function of the signal mass m,where X stands for the signal resonance. The expected limits (red dashed line) are shown together with the±1 and ±2 standard deviation uncertainty bands. Theoretical cross sections for the EGM W′ and the HVTbenchmark models are also shown. Taken from [26].

or electron) and missing transverse momentum using 8 TeV pp collision data corresponding to atotal integrated luminosity of 20.3 fb−1. The kinematic variable used to identify the signal is the

transverse mass mT =

√2pT Emiss

T (1− cosφlν) where pT is the lepton transverse momentum, EmissT

is the missing transverse momentum and φlν is the angle between the pT and EmissT vectors. The

main background comes from the so called electroweak background (W→ lν, W and Z productionwith decays to τ→ ντlνl, diboson production). The data distributions of pT , Emiss

T and mT afterselection well agree with the expected background and the exclusion limits on the high mass statespredicted by the considered theoretical models are set.

4.4 Non-prompt lepton jets

Several models of physics beyond the Standard Model predict neutral particles called γd (dark-photons) that decay into final states consisting of collimated jets of light leptons and hadrons (so-called "lepton jets"). These particles can also be long-lived with decay length comparable to,or even larger than, the LHC detectors’ linear dimensions. ATLAS has searched for non-promptmuonic lepton jets at

√s = 7 TeV [32] and for lepton jets in all the possible final states: ≥ 2µ and no

e, ≥ 2(e/π) and no µ, ≥ 2µ and ≥ 2(e/π) at√

s = 8 TeV [33]. The high-resolution, high-granularitymeasurement capability of the ATLAS Muon Spectrometer is crucial for this type of search. Inaddition, the ATLAS Inner Detector can be used to define isolation criteria to significantly reduce,for decay vertices far from the interaction point, the otherwise overwhelming SM background fromproton-proton collisions. No excess of events has been observed over the estimated backgroundlevel, and limits have been set on the Falkowski-Ruderman-Volansky-Zupan (FRVZ) benchmarkmodels [34] which predict non-SM Higgs boson decays to lepton jets. The limits are set on theHiggs boson decay branching fraction to lepton jets as a function of the dark photon mean lifetime.In the case of a dark photon which kinetically mixes with the SM photon, these limits can beconverted into exclusion limits on the kinetic mixing parameter ε. For H → 2γd + X with a darkphoton mass = 0.4 GeV, the interval that is excluded at 95% CL is 7.7 x 10−7 ≤ ε ≤ 2.7 x 10−6.The final results of the search and their contribution to the parameter space exclusion plot for dark

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photons are shown in Figure 8.

Figure 8: Parameter space exclusion plot for dark photons as a function of the γd mass and of the kineticmixing parameter [33]. Shown are existing 90% CL exclusion regions from beam dump experiments E137,E141, and E774, Orsay, U70, CHARM, LSND, A1, the electron and muon anomalous magnetic moment,HADES, KLOE, the test run results reported by APEX, an estimate using a BaBar result, and constraintsfrom astrophysical observations. The 90% CL exclusion limits from the ATLAS search, assuming the FRVZmodel H → 2γd + X with decay branching fraction to dark photon of 5/10/20/40% and the NNLO gluonfusion Higgs production cross section, are shown. Taken from [33].

5. Conclusions

ATLAS is conducting a wide range of searches for SUSY particles and other exotic new parti-cles, exploiting different signatures and using various detection techniques. These proceedings givea summary of the results of some of these searches. Despite a huge effort, no new signal has beenobserved in the data to date and exclusion limits on cross sections and masses are given, continuingto constrain the parameter space of many physics models. While the Run 1 physics program isbeing completed, the emphasis on the data at

√s = 13 TeV grows more and more.

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BSM searches (SUSY and Exotic) from ATLAScds.cern.ch/record/2117088/files/ATL-PHYS-PROC-2015-196.pdf · 2015-12-23 · BSM searches (SUSY and Exotic) from ATLAS Marco Schioppa 3