This document presents the status of the searches for supersymmetric particles with √ the ATLAS detector at LHC, in proton proton collisions at √s = 8TeV in the context of natural Supersymmetry. This covers an extensive list of scenarios including the production of third generation squarks (both direct and gluino mediated), charginos and neutralinos. No deviation from the background expectations has been observed in data and it has been possible to put stringent limits on the masses of gluinos, third-generation squarks and light gauginos in a wide range of models

Meloni, F.

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Meloni, F

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The quest for Natural SUSY: ATLAS searches

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DOI 10.1393/ncc/i2014-11661-2Colloquia: IFAE 2013IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014The quest for natural SUSY: ATLAS searchesF. MeloniDipartimento di Fisica, Universita` di Milano e INFN, Sezione di MilanoVia Celoria 16, 20133 Milano, Italyricevuto l’1 Ottobre 2013Summary. — This document presents the status of the searches for supersym-metric particles with the ATLAS detector at LHC, in proton proton collisions at√s = 8TeV in the context of natural Supersymmetry. This covers an extensive listof scenarios including the production of third generation squarks (both direct andgluino mediated), charginos and neutralinos. No deviation from the backgroundexpectations has been observed in data and it has been possible to put stringentlimits on the masses of gluinos, third-generation squarks and light gauginos in awide range of models.PACS 12.60.Jv – Supersymmetric models.1. – IntroductionSupersymmetry (SUSY) provides an extension of the Standard Model (SM) by intro-ducing supersymmetric partners of the known bosons and fermions. In the frameworkof an R-parity conserving minimal supersymmetric extension of the SM, SUSY particlesare produced in pairs and the lightest supersymmetric particle (LSP), in many modelsthe lightest neutralino χ01, is stable and a possible candidate for dark matter.SUSY can naturally resolve the hierarchy problem by preventing a large fine-tuningin the Higgs sector: a typical natural SUSY spectrum contains light third generationsquarks (stops and sbottoms), charginos and neutralinos. This spectrum is motivatedby the fact that the dominant radiative corrections to the Higgs boson mass arise fromthe top quark in the SM. Thus a relatively light stop can cancel out these corrections.Furthermore the SM Z boson mass, the neutralino and chargino masses are related tothe μ parameter in SUSY, implying gaugino masses as low as the electroweak scale.This document reports several ATLAS searches in the context of natural supersym-metry, carried out using LHC proton proton collision data collected at√s = 8TeV.c© CERN on behalf of the ATLAS CollaborationCreative Commons Attribution 3.0 License (http://creativecommons.org/licenses/by/3.0) 2728 F. MELONI [GeV]g~m500 600 700 800 900 1000 1100 1200 1300 [GeV]10 m2004006008001000 forbidden10t tg~ not included.theorySUSY95% CL limits.  = 8 TeVs, 10t tg~ production, g~-g~ Status: Moriond QCD 2013 PreliminaryATLASExpectedObservedExpectedObservedExpectedObservedExpectedObserved0-lepton, 6-9 jets 3 b-jets0-lepton,  4 jets3-leptons, 2-SS-leptons, [0-3] b-jetsATLAS-CONF-2012-103ATLAS-CONF-2012-145ATLAS-CONF-2012-151ATLAS-CONF-2013-007]-1 = 5.8 fbint[L]-1 = 12.8 fbint[L]-1 = 12.8 fbint[L]-1 = 20.7 fbint[L [GeV]1b~m300 350 400 450 500 550 600 650 [GeV]1± χ∼m100200300400500600 forbidden1±χ∼ t→b~)10χ∼) = 2 m(1±χ∼, m(1±χ∼ t→1b~ production, 1b~-1b~2 same-charge leptons + jets=8 TeVs,-1 L dt = 20.7 fb∫244 186227111120161102122911077771807380617365706470157997971Numbers give 95% CL excluded model cross sections [fb])theorySUSYσ1±Observed limit ()expσ1±Expected limit (=8TeVs,-1 4 jets, 13.0fb≥3-leptons,ATLAS PreliminaryAll limits at 95% CLFig. 1. – Left: Exclusion limits at 95% CL for analyses using pp collision data at√s = 8TeVin the (m(g˜)-m(χ˜01)) plane. A simplified model is assumed where a pair of gluinos decays viaoff-shell stop to four top quarks and two neutralinos LSPs. Right: 95% CL exclusion limits onthe cross section for the direct production of bottom squarks in a model assuming b˜1 → tχ˜±1with m(χ˜±1 ) = 2×m(χ˜01) obtained with 20.7 fb−1 at√s = 8TeV [1].2. – Search for direct sbottom and gluino productionIf gluinos are light enough (up to about 1TeV), the production of third generationsquarks is expected to be found first through gluino decay. A large number of b-taggedjets, large missing transverse momentum EmissT and possibly leptons (coming from topquark’s leptonic decays) are expected in the final state. If instead the top and bottomsquarks are heavier than the gluino, the latter can still decay to a similar final statethrough virtual third generation squarks.The analysis presented in [1] addresses the channel g˜ → ttχ˜01, which is expected topresent a very rich final state, requiring two isolated leptons (e or μ) with the samecharge and multiple energetic jets. This analysis is also sensitive to the direct productionof bottom squarks decaying into a top quark and a chargino. Three event classes aredistinguished depending on the number of jets identified as originating from b-quarkdecays: zero (b-jet veto), at least one and three b-jets. The data sample consists of anintegrated luminosity of 20.7 fb−1.No deviation from the SM expectation is observed. In particular, focusing on processesthat involve third generation SUSY particles, limits are set in the m(g˜)-m(χ˜01) and m(b˜1)-m(χ˜±1 ) planes (fig. 1) assuming a 100% branching ratio towards the targeted final state.3. – Search for direct stop productionIf the gluino is too heavy, third generation squarks can only be produced directly.Final states of third generation squark production are typically extremely rich: theymight include leptons, multiple jets (possibly b-tagged) and EmissT . ATLAS has a set ofanalyses covering many complementary search channels (fig. 2): the main results in thesearch for direct stop production are documented in the following.3.1. Fully hadronic final state. – The analysis presented in [2] assumes top squarksdecaying exclusively into a top quark and the LSP. The analysis uses 20.7 fb−1 of data.THE QUEST FOR NATURAL SUSY: ATLAS SEARCHES 29 [GeV]1t~m200 300 400 500 600 70010+mt < m1t~m200 300 400 500 600)10 m = 2 1 ( m1+mb < m1t~m < 106 GeV 1 m+5 GeV)10 = m1 ( m1+mb < m1t~m < 103.5 GeV1m [GeV]10 m0100200300400500600Observed limits)theoObserved limits (-1Expected limits10 m = 2 1m -1 = 20.7 fbintL - 10 GeV1t~ = m1m-1 = 13 fbintL = 150 GeV1m-1 = 20.7 fbintL = 106 GeV1m-1 = 4.7 fbintL + 5 GeV10 =  m1m-1 = 12.8 fbintL-1 = 4.7 fbintL -1 21 fb intLATLAS Preliminary production1t~1t~Status: March 26, 2013=8 TeVs -1 = 13-21 fbintL =7 TeVs -1 = 4.7 fbintL0L ATLAS-CONF-2013-0241L ATLAS-CONF-2013-037-0L ATLAS-CONF-2012-171-1L ATLAS-CONF-2013-0372L ATLAS-CONF-2012-1671L ATLAS-CONF-2013-0370L [1208.1447]1L [1208.2590]2L [1209.4186]-2L [1208.4305], 1-2L [1209.2102]--1-2L [1209.2102]10+(*) W1, 1 b+ 1t~ 10 t 1t~10 t 1t~0L, 10 t 1t~1L, 10 t 1t~2L,  + 5 GeV10 = m10L, m = 106 GeV1, m1 b+ 1t~1-2L,  = 150 GeV1, m1 b+ 1t~1L,  - 10 GeV1t~ = m1, m1 b+ 1t~2L, 10 m = 2 1, m1 b+ 1t~1-2L, Fig. 2. – Summary of the dedicated ATLAS searches for top squark (stop) pair productionbased on up to 20.7 fb−1 of pp collision data taken at√s = 8TeV and 4.7 fb−1 of pp collisiondata taken at√s = 7TeV. Exclusion limits at 95% CL are shown in the t˜1-χ˜01 mass plane.The dashed and solid lines show the expected and observed limits, respectively, including alluncertainties except the theoretical signal cross section uncertainty (PDF and scale). The dottedlines represent the results obtained when reducing the nominal signal cross section by 1σ of itstheoretical uncertainty. Two decay modes are considered separately with 100% branching ratio:t˜1 → χ˜01, where the t˜1 is mostly t˜R and t˜1 → b+ χ˜±1 , χ˜±1 →W(∗)+ χ˜01. In the latter case, severalhypotheses on the t˜1, χ˜±1 , χ˜01 mass hierarchy are assumed: fixed χ˜±1 mass, m(χ˜±1 ) ∼ 2×m(χ˜01),fixed m(t˜1)−m(χ˜±1 ) = 10GeV and fixed m(χ˜±1 )−m(χ˜01) = 5GeV.The search is conducted in events with large missing transverse momentum and six ormore jets, targeting the fully hadronic top final state. The final discriminating variablebetween signal and background is EmissT . Three signal regions have been defined, tar-geting different ranges of the top squark mass, with increasingly tighter requirements onthe event EmissT : 200GeV, 300GeV and 350GeV.No evidence is found for physics beyond the SM and results are interpreted in a R-parity-conserving minimal supersymmetric scenario of direct top squark pair production.Top squarks in this model with masses between 320 and 660GeV are excluded at 95% CLfor a nearly massless LSP. For a LSP mass of 150 GeV the exclusion interval is between400 and 620GeV.3.2. One-lepton final state. – A search is presented for direct top squark pair produc-tion in final states with one isolated electron or muon, jets, and EmissT [3] . The analysisis based on 20.7 fb−1 of data. The stop is assumed to decay to a top quark and the LSPor to a bottom quark and a chargino, where the chargino decays to an on- or off-shell Wboson and to the LSP. Six signal regions are defined in order to optimize the sensitivityto different stop and LSP masses, as well as the two considered stop decay scenarios.Three signal regions target the t˜1 → b + χ˜±1 decay: the loosest one relies on the EmissTand the transverse mass mT (from the lepton pT and EmissT ) to discriminate signal from30 F. MELONI [GeV]1±χ∼, 20χ∼m100 200 300 400 500 600 700 [GeV]0 1χ∼m010020030040050060001χ∼ < m02χ∼m10χ∼ = 2m20χ∼m02χ∼ = m±1χ∼m)/202χ∼ + m01χ∼m = ( Ll~ m)ν ν∼l (Ll~ ν∼), l ν ν∼l(Ll~ ν Ll~ → 02χ∼ ±1χ∼01χ∼) ν ν l l (01χ∼ ν l →ATLAS Preliminary=8 TeVs, -1 L dt = 20.7 fb∫)theorySUSYσ1 ±Observed limit ()expσ1 ±Expected limit ( = 8 TeVs, -1ATLAS 13.0 fbAll limits at 95% CL [GeV]1±χ∼, 20χ∼m100 150 200 250 300 350 400 [GeV]0 1χ∼m05010015020025030001χ∼ < m02χ∼mZ = m10χ∼ - m20χ∼m10χ∼ = 2m20χ∼m02χ∼ = m±1χ∼m10χ∼ (*) Z10χ∼ (*) W→ 02χ∼ ±1χ∼ATLAS Preliminary=8 TeVs, -1 L dt = 20.7 fb∫)theorySUSYσ1 ±Observed limit ()expσ1 ±Expected limit ( = 8 TeVs, -1ATLAS 13.0 fbAll limits at 95% CLFig. 3. – Observed and expected 95% CL limit contours for chargino and neutralino productionin the simplified model scenario with decay via sleptons (left) and decay via gauge bosons(right) [5]. The band around the median expected limit shows the ±1σ variations on the medianexpected limit, including all uncertainties except theoretical uncertainties on the signal cross-section. The dotted lines around the observed limit indicate the sensitivity to ±1σ variationson these theoretical uncertainties. The blue lines correspond to the 8TeV, 13 fb−1 limits fromthe ATLAS three lepton analysis. The limits are calculated using the statistical combination ofall signal regions for each of the model points.background. The other two signal regions have been designed for models with high stopmasses and medium-to-high mass difference between the stop and chargino, leading tob-jets with a transverse momentum significantly higher than the main SM backgrounds.Consequently, two or more b-jets are required in these signal regions, each with large pT.The remaining three signal regions target the t˜1 → tχ˜01 decay: in order to enhance theirsensitivity, they all require three jets with an invariant mass compatible with the topquark mass. The loosest selection exploits a shape fit of EmissT and mT, that targets theparameter space where the stop and its decay products are nearly mass degenerate. Thetwo tighter selections rely mostly on tight cuts on both EmissT and mT. The data arefound to be consistent with SM expectations. Assuming both top squarks decay to a topquark and the LSP, top squark masses between 200 and 610GeV are excluded at 95%confidence level for massless LSP, and top squark masses around 500GeV are excludedfor LSP masses up to 250GeV. Assuming both top squarks decay to a bottom quarkand the lightest chargino, top squark masses up to 410GeV are excluded for masslessLSPs and an assumed chargino mass of 150GeV.3.3. Two leptons final state. – The analysis presented in [4] shows the results of asearch for direct top squark pair production with 13.0 fb−1 of integrated luminosity. Thescalar tops are assumed to decay into a b quark and a chargino in events with two leptonsin the final state. To separate the signal from the large SM background contributionsdominated by top pair and W pair production, the mT2 variable is used. It is defined asmT2 = (p1T ,p2T ,pmissT ) = minqT+rT=pmissT{max[mT(p1T ,qT),mT(p2T , rT)]},THE QUEST FOR NATURAL SUSY: ATLAS SEARCHES 31where mT indicates the transverse mass, p1T and p2T are the transverse momenta of thetwo leptons, and qT and rT are vectors which satisfy qT+rT = pmissT . The minimizationis performed over all the possible decompositions of pmissT .Three signal regions are defined with increasingly tighter mT2 lower cuts: 90GeV,100GeV and 110GeV. No excess above the SM expectation is observed. A scalar topquark with mass between 150 and 450GeV is excluded at 95% confidence level for achargino approximately degenerate in mass with the stop and a massless lightest neu-tralino.4. – Search for direct gaugino productionA search for the direct production of charginos and neutralinos in final states withthree leptons (e or μ) and EmissT is presented in [5]. The analysis is based on 20.7 fb−1of data. The presence of at least one same-flavour opposite-sign lepton pair (SFOS)is required in the event, as the decays of χ˜02 → χ˜01 through sleptons or Z bosons yieldsuch pairs. Six mutually exclusive signal regions are defined: three Z-depleted regionstargeting neutralino decays via sleptons, with no SFOS pairs having an invariant masswithin 10GeV of the Z-boson mass; and three Z-enriched regions targeting neutralinodecays via on-shell Z bosons, where the invariant mass cut is inverted.The final selection based on the EmissT and mT (computed using EmissT and the leptonwhich is not included in the SFOS lepton pair with invariant mass closest to the Z-bosonmass) of the event.No excess above the SM expectation is observed and limits are placed at the 95%CL on the masses of the charginos and neutralinos in simplified supersymmetric models.First chargino and second neutralino masses are excluded up to 600GeV if these particlesdecay through sleptons and up to 315GeV in cases where they decay via gauge bosonsto a massless lightest neutralino (fig. 3).5. – ConclusionsATLAS is conducting a comprehensive set of searches aiming at the natural SUSYmodels. No excess in data with respect to the SM expectation has been observed so far.However, much of the parameter space remains to be probed, and the search for SUSYat the LHC will continue to be pursued as the study of new channels will bring newopportunities for the discovery of a potential excess.REFERENCES[1] ATLAS Collaboration, ATLAS-CONF-2013-007, https://cds.cern.ch/record/1522430.[2] ATLAS Collaboration, ATLAS-CONF-2013-024, https://cds.cern.ch/record/1525880.[3] ATLAS Collaboration, ATLAS-CONF-2013-037, https://cds.cern.ch/record/1532431.[4] ATLAS Collaboration, ATLAS-CONF-2012-167, https://cds.cern.ch/record/1497733.[5] ATLAS Collaboration, ATLAS-CONF-2013-035, https://cds.cern.ch/record/1532426.

The quest for natural SUSY: ATLAS searches

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The quest for natural SUSY: ATLAS searches

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