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Hunt for new phenomena using large jet multiplicities and missing transverse momentum with ATLAS in 4.7 fb$^{-1}$ of $\sqrt{s}$ = 7 TeV proton-proton collisions

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  EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN) CERN-PH-EP-2012-141 Submitted to: Journal of High Energy Physics Hunt for new phenomena using large jet multiplicities andmissing transverse momentum with ATLAS in  4 . 7 fb − 1 of √  s  = 7 TeV  proton-proton collisions The ATLAS Collaboration Abstract Results are presented of a search for new particles decaying to large numbers of jets in associ-ation with missing transverse momentum, using 4.7fb − 1 of  pp  collision data at √  s  = 7 TeV  collectedby the ATLAS experiment at the Large Hadron Collider in 2011. The event selection requires miss-ing transverse momentum, no isolated electrons or muons, and from  ≥ 6 to  ≥ 9 jets. No evidenceis found for physics beyond the Standard Model. The results are interpreted in the context of aMSUGRA/CMSSM supersymmetric model, where, for large universal scalar mass  m 0 , gluino massessmaller than  840 GeV  are excluded at the 95% confidence level, extending previously published limits.Within a simplified model containing only a gluino octet and a neutralino, gluino masses smaller than 870 GeV  are similarly excluded for neutralino masses below  100 GeV .   a  r   X   i  v  :   1   2   0   6 .   1   7   6   0  v   2   [   h  e  p  -  e  x   ]   4   A  u  g   2   0   1   2  Prepared for submission to JHEP Hunt for new phenomena using large jet multiplicitiesand missing transverse momentum with ATLAS in 4 . 7 fb − 1 of   √  s  = 7 TeV  proton-proton collisions The ATLAS Collaboration Abstract:  Results are presented of a search for new particles decaying to large numbersof jets in association with missing transverse momentum, using 4.7fb − 1 of   pp  collisiondata at  √  s  = 7 TeV collected by the ATLAS experiment at the Large Hadron Collider in2011. The event selection requires missing transverse momentum, no isolated electrons ormuons, and from  ≥ 6 to  ≥ 9 jets. No evidence is found for physics beyond the StandardModel. The results are interpreted in the context of a MSUGRA/CMSSM supersymmetricmodel, where, for large universal scalar mass  m 0 , gluino masses smaller than 840 GeVare excluded at the 95% confidence level, extending previously published limits. Withina simplified model containing only a gluino octet and a neutralino, gluino masses smallerthan 870 GeV are similarly excluded for neutralino masses below 100 GeV.  Contents 1 Introduction 12 The ATLAS detector and data samples 23 Object reconstruction 34 Event selection 45 Monte Carlo simulations 56 Multi-jet backgrounds 6 6.1 Systematic uncertainties on multi-jet backgrounds 7 7 ‘Leptonic’ backgrounds 8 7.1 Systematic uncertainties on ‘leptonic’ backgrounds 11 8 Results, interpretation and limits 119 Summary 1310 Acknowledgments 20A Event displays 23 1 Introduction Many extensions of the Standard Model of particle physics predict the presence of TeV-scale strongly interacting particles that decay to lighter, weakly interacting descendants.Any such weakly interacting particles that are massive and stable can contribute to thedark matter content of the universe. The strongly interacting parents would be produced inthe proton-proton interactions at the Large Hadron Collider (LHC), and such events wouldbe characterized by significant missing transverse momentum  E  missT  from the unobservedweakly interacting daughters, and jets from emissions of quarks and/or gluons.In the context of   R -parity conserving [1–5] supersymmetry [5–10], the strongly inter- acting parent particles are the squarks ˜ q   and gluinos ˜ g , they are produced in pairs, and thelightest supersymmetric particles can provide the stable dark matter candidates [11, 12]. Jets are produced from a variety of sources: from quark emission in supersymmetric cas-cade decays, production of heavy Standard Model particles ( W  ,  Z   or  t ) which then decayhadronically, or from QCD radiation. Examples of particular phenomenological interest– 1 –  include models where squarks are significantly heavier than gluinos. In such models thegluino pair production and decay process˜ g  + ˜ g  →  t + ¯ t + ˜ χ 01  +  t + ¯ t + ˜ χ 01  can dominate, producing large jet multiplicities when the resulting top quarks decayhadronically. In the context of MSUGRA/CMSSM models, a variety of different cascadedecays, including the ˜ g ˜ g  initiated process above, can lead to large jet multiplicities.A previous ATLAS search in high jet multiplicity final states [13] examined data taken during the first half of 2011, corresponding to an integrated luminosity of 1 . 34fb − 1 . Thispaper extends the analysis to the complete ATLAS 2011  pp  data set, corresponding to4.7fb − 1 , and includes improvements in the analysis and event selection that further increasesensitivity to models of interest.Events are selected with large jet multiplicities ranging from ≥ 6 to ≥ 9 jets, in associ-ation with significant  E  missT  . Events containing high transverse momentum (  p T ) electronsor muons are vetoed in order to reduce backgrounds from (semi-leptonically) decaying topquarks or  W   bosons. Other complementary searches have been performed by the ATLAScollaboration in final states with  E  missT  and one or more leptons [14, 15]. Further searches have been carried out by ATLAS using events with at least two, three or four jets [16], or with at least two  b -tagged jets [17]. Searches have also been performed by the CMS collaboration, including a recent analysis in fully hadronic final states [18]. 2 The ATLAS detector and data samples The ATLAS experiment [19] is a multi-purpose particle physics detector with a forward- backward symmetric cylindrical geometry and nearly 4 π  coverage in solid angle. 1 The lay-out of the detector is dominated by four superconducting magnet systems, which comprisea thin solenoid surrounding inner tracking detectors, and a barrel and two end-cap toroidssupporting a large muon spectrometer. The calorimeters are of particular importance tothis analysis. In the pseudorapidity region  | η |  <  3 . 2, high-granularity liquid-argon (LAr)electromagnetic (EM) sampling calorimeters are used. An iron/scintillator-tile calorime-ter provides hadronic coverage for  | η |  <  1 . 7. The end-cap and forward regions, spanning1 . 5  <  | η |  <  4 . 9, are instrumented with LAr calorimetry for both EM and hadronic mea-surements.The data sample used in this analysis was taken during April – October 2011 withthe LHC operating at a proton-proton centre-of-mass energy of   √  s  = 7 TeV. Applicationof beam, detector and data-quality requirements resulted in a corresponding integratedluminosity of 4 . 7 ± 0 . 2fb − 1 [20]. The analysis makes use of dedicated multi-jet triggers thatrequired either at least four jets with  p T  >  45 GeV or at least five jets with  p T  >  30 GeV, 1 ATLAS uses a right-handed coordinate system with its srcin at the nominal interaction point in thecentre of the detector and the  z  -axis along the beam pipe. Cylindrical coordinates ( r,φ ) are used in thetransverse plane,  φ  being the azimuthal angle around the beam pipe. The pseudorapidity  η  is defined interms of the polar angle  θ  by  η  = − lntan( θ/ 2). – 2 –  where the energy is measured at the electromagnetic scale 2 and the jets must have | η | <  3 . 2.In all cases the trigger efficiency was greater than 98% for events satisfying the offline jetmultiplicity selections described in Section 4. 3 Object reconstruction The jet, lepton and missing transverse momentum definitions are based closely on those of Ref. [13], with small updates to account for evolving accelerator and detector conditions. Jet candidates are reconstructed using the anti- k t  jet clustering algorithm [21, 22] with radius parameter of 0 . 4. The inputs to this algorithm are clusters of calorimetercells seeded by cells with energy significantly above the noise level. Jet momenta arereconstructed by performing a four-vector sum over these topological clusters of calorimetercells, treating each as an ( E,  p ) four-vector with zero mass. The jet energies are correctedfor the effects of calorimeter non-compensation and inhomogeneities by using  p T - and η -dependent calibration factors based on Monte Carlo (MC) simulations validated withextensive test-beam and collision-data studies [23]. Only jet candidates with  p T  >  20 GeVand | η | <  4 . 9 are retained. Further corrections are applied to any jet falling in problematicareas of the calorimeter. The event is rejected if, for any jet, this additional correctionleads to a contribution to  E  missT  that is greater than both 10 GeV and 0 . 1 E  missT  . Thesecriteria, along with selections against non-collision background and calorimeter noise, leadto a loss of signal efficiency of   ∼ 8% for the models considered. When identification of  jets containing heavy flavour quarks is required, either to make measurements in controlregions or for cross checks, a tagging algorithm exploiting both impact parameter andsecondary vertex information is used. Jets are tagged for  | η | <  2 . 5 and the parameters of the algorithm are chosen such that 70% of   b -jets and  ∼ 1% of light flavour or gluon jets,are selected in  t ¯ t  events in Monte Carlo simulation [24]. Jets initiated by charm quarks are tagged with about 20% efficiency.Electron candidates are required to have  p T  >  20 GeV and  | η |  <  2.47, and to sat-isfy the ‘medium’ electron shower shape and track selection criteria of Ref. [14]. Muon candidates are required to have  p T  >  10 GeV and  | η |  <  2.4. Additional requirements areapplied to muons when defining leptonic control regions. In this case muons must havelongitudinal and transverse impact parameters within 1mm and 0.2mm of the primaryvertex, respectively, and the sum of the transverse momenta of other tracks within a coneof ∆ R  = 0 . 2 around the muon must be less than 1 . 8 GeV, where ∆ R  =   (∆ η ) 2 + (∆ φ ) 2 .The measurement of the missing transverse momentum two-vector    p missT  and its magni-tude (conventionally denoted  E  missT  ) is then based on the transverse momenta of all electronand muon candidates, all jets with  | η |  <  4 . 5 which are not also electron candidates, andall calorimeter clusters with  | η | <  4 . 5 not associated to such objects [25]. 2 The electromagnetic scale is the basic calorimeter signal scale for the ATLAS calorimeters. It hasbeen established using test-beam measurements for electrons and muons to give the correct response forthe energy deposited in electromagnetic showers, although it does not correct for the lower response of thecalorimeter to hadrons. – 3 –
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