Aperio Bella, L.

Hal - Université Grenoble Alpes

Searches for Exotics physics states in jets and boosted objects final states

HAL Université de Savoie

Searches for Exotics physics states in jets and boostedobjects final statesL. Aperio BellaTo cite this version:L. Aperio Bella. Searches for Exotics physics states in jets and boosted objects final states.Physics at LHC 2012 - PLHC 2012, Jun 2012, Vancouver, Canada. 2012. <in2p3-00748598>HAL Id: in2p3-00748598http://hal.in2p3.fr/in2p3-00748598Submitted on 5 Nov 2012HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.Searches for Exotics physics states in jets and boostedobjects final states at ATLASLudovica Aperio Bella, on behalf of the ATLAS Collaboration.LAPP (Laboratoire d’Annecy-le-Vieux de Physique des Particules)9 Chemin de Bellevue - BP 110, 74941 Annecy-le-Vieux, FRANCE1 IntroductionDue to the outstanding performance of the Large Hadron Collider [1] (LHC) that in 2011delivered more than 5 fb−1 of proton-proton collision data at center-of-mass energy of 7 TeV,the ATLAS experiment [2] has been able to explore a wide range of exotic models in order toaddress the questions unanswered by the Standard Model (SM) of particle physics. ATLASsearches for new particles decaying to jets, photons and jets, monojet, tt resonances and4th generation particles decaying hadronically are presented in this paper. No evidence forphysics beyond the SM is found and model-independent limits and limits on the parametersof particular models are set.2 New Physics searches with jets in the final stateSearches in Dijet Mass and Angular DistributionThe production of events with two energetic jets of particles (dijet events) is well under-stood within the Standard Model. An enhanced production of dijet final states is expectedin several scenarios of new physics. A variety of models of new physics, including modelswith excited quarks and axigluons, predict the resonant production of states decaying pre-dominantly to two jets. Other models, such as quark contact interactions, predict an excessof events with two jet of central rapidity and forming a high invariant mass. By studyingthe dijet invariant mass (mjj) and the dijet angular distributions, sensitive searches for bothresonant and non-resonant deviations from the Standard Model are performed. In the dijetresonance search, the mjj of the two leading jets is studied using 4.8 fb−1 of data collectedin 2011. In the analysis the BumpHunter [4] algorithm is used to evaluate the presenceof an excess of events in the mjj spectrum on top of a smooth background estimate byfitting the data with the formula f(x) = p1(1− x)p2xp3+p4 lnx (where the pi are the fit pa-rameters and x ≡ mjj/√s). No resonant structure is founded and masses of excited quarkless then 3.35 TeV are excluded at 95% confidence level (C.L.). In the angular analysisconvenient variables that emphasizes the dijet central scattering region are employed. Theχ variable is defined as the exponential of the rapidity difference of the two jets with thehighest transverse momenta (pT ). In Figure 1 (left) the χ distribution for different mjjinterval is shown in data along with background predictions from NLO QCD. The Fχ(mjj)1χ1 10χ/ dσ) dσ(1/00.050.10.150.20.250.30.35 < 1200 GeVjj800 < m < 1600 GeV (+0.04)jj1200 < m < 2000 GeV (+0.08)jj1600 < m < 2600 GeV (+0.12)jj2000 < m > 2600 GeV (+0.16)jjmQCD PredictionTheoretical uncertaintiesTotal Systematics = 4.0 TeVDQBH (n=6), M=7 TeVs, -1dt  = 4.8 fbL∫ATLAS Preliminary [GeV]jjm1000 1500 2000 2500 3000 3500 4000 4500χF00.10.20.30.40.50.6ATLAS Preliminary=7 TeVs, -1dt = 4.8 fbL∫  QCD Prediction  Theoretical uncertainties  Total Systematics  data = 7.5 TeVΛ  Contact Interaction:  = 4.5 TeVD  QBH (n=6) : M = 2.5 TeVq*  q*: m Upper boundary to control region Lower boundary to search regionFigure 1: Left: The χ distributions for different dijet mass bins. Right: Fχ(mjj) distri-bution versus mjj . The QCD predictions are shown with theoretical and total systematicuncertainties (bands) as well as the data points with statistical uncertainties [3].variable is defined as the ratio Fχ(mjj) = Ncentral/Ntotal where Ncentral is the number ofdijet events in a defined central region, and Ntotal is the number of dijet events in the fulldistribution extending to χ < 30. This variable has excellent sensitivity to resonant as wellas non-resonant excesses of central jets. The resulting distribution for different mjj valuesis shown in Figure 1 (right) for the data and the NLO QCD background. Data and QCDprediction agrees in each mjj bin. Limits at 95% C.L. are set on models of contact inter-actions and quantum black holes. The full analysis details, including limits on additionalmodels, are given in Reference [3].Searches in Photon-Jet Mass DistributionThe photon-jet invariant mass distribution (mγj) is shown to be a useful tool for searchingfor resonances indicative of new physics. The most recent ATLAS results [5] are obtainedfrom the analysis of 2.11 fb−1 of 2011 proton-proton collision data. Data events with mγjmasses up to 2 TeV are compared with a smooth background estimate, and no evidence ofresonant production is found. Limits are set on generic Gaussian-shape signals (excludedbelow 2 TeV) and on a benchmark excited-quark model excluding masses below 2.46 TeV.Searches in Monojet plus Missing Transverse Momentum Final StatesThe search for new physics in events with a jet of high transverse energy and large missingtransverse energy constitute one the simplest and most striking signatures that can beobserved at a hadron collider. Different theoretical models for physics Beyond the StandardModel predict the presence of monojet signatures in the final state like, for example, LargeExtra Dimension scenarios. In the analysis presented in Reference [6] 1 fb−1 of data areused to select events with one high pT jet and a large amount of missing transverse energy.A lepton veto is applied to reduce the electroweak background. The shape of the dominant2electroweak background is taken from Monte Carlo normalized using data in control regions,while the multi-jet contribution and the non-collision background are estimated with a data-driven technique. Good agreement is observed between the data and the Standard Modelpredictions. Model-independent upper limits at 95% C.L. are set on the fiducial cross sectionfor the non-Standard Model production of different signal pT -region varying between 2.02pb and 0.045 pb. Additionally, an interpretation is made in terms of the Large ExtraDimensions model. Values of the fundamental Planck scale between 3.2 and 2.0 TeV areexcluded for a number of extra dimensions corresponding to 2 and 6, respectively.3 Searches of High-Scale physics scenarios with Boosted Ob-jects in the Final StateHadronic decays of heavy particles such as W bosons, top quarks and potential hithertounobserved particles may be collimated into a single heavy jet characterized by a distinctsubstructure and large mass. In this section exotic analyses using boosted or mostly resolvedreconstruction topology are presented. mass [GeV]tt0 500 1000 1500 2000 2500 3000 3500events / GeV-310-210-1101ATLAS = 7 TeVs   -1Ldt=2.05 fb∫Datatt W+jetsSingleTopMultijetsZ+jetsDibosonUncertainty (1.3 TeV)KKgEvents10210310410ATLAS-1 L dt =1.04 fb∫ = 7 TeV)sData ( = 300 GeVb’m = 500 GeVb’mOtherW+JetsttEvents(data-SM)/SM-101=6=0=6=1=62≥=7=0=7=1=72≥8≥=08≥=18≥2≥jetsNWN(data-SM)/SMFigure 2: Left: Reconstructed invariant mass distribution of the tt candidates after thelepton plus jets signal selection. The shaded band indicates the uncertainty in the nor-malization of the Standard Model prediction [7]. Right: Distribution of the numbers ofevents observed in the data and expected from SM processes for Njets = 6, 7,≥ 8 withNW = 0, 1,≥ 2. The expected b′ signals for two masses are also shown, stacked on top ofthe backgrounds [9].Search for tt resonances in lepton plus jets events with highly boosted top quarksA search for resonant production of high-mass top-quark pairs is performed on 2.05 fb−1of ATLAS 7 TeV data [7]. The signal is assumed to originate from the resonance of aleptophobic Z’ or a Kaluza-Klein gluon. The analysis focuses on the lepton plus jets finalstate obtained when one W boson decays to a charged lepton and a neutrino, and the other3decays to a quark and an anti-quark pair. The selection and reconstruction are specificallydesigned for the collimated topology that arises from the decay of boosted top quarks.The hadronically decaying top quark candidate is identified as a single fat jet with radiusparameter R = 1. In Figure 2 (left) the reconstructed tt mass spectrum is compared with atemplate for the Standard Model prediction constructed using a combination of Monte Carlosimulations and data-driven measurements using control samples. The data are found to becompatible with the SM within uncertainties. Upper limits at 95% CL on the productioncross section times the branching ratio of narrow Z’ resonances and broad colored resonancesare derived. Leptophobic Z’ with masses between 600 GeV and 1.2 TeV is excluded andKaluza-Klein gluon with a mass smaller than 1.5 TeV is also excluded. The sensitivity ofthis search obtained with highly boosted top quarks is significantly enhanced in the 1-2 TeVregion with respect to a previously published search using the same data set [8].Search for exotic heavy quarksA search for pair production of a fourth generation down-type quark b′ in events with onelepton is performed with 1.04 fb−1 of 7 TeV data [9]. In the model where b′ is a chiral quarkwith mass larger than mt +MW , the predominant decay mode is b′ →Wt→WWb, whichleads to four W bosons and two b quarks in the b′b′ production events. Due to the highexpected b′ mass, the W bosons coming from the b′ →Wt decay are expected to have a highmomentum and the decay products of such W bosons should be closer together than jetsfor background processes, but still resolvable as separate jets (mostly resolved topology).Considering this, the quantity suitable for distinguishing b′ signal from background is thenumber of jet pairs with small opening angle and an invariant mass close to the W bosonmass. The final discriminant consists of nine exclusive bins as a function of the multiplicityof hadronic W decays (NW = 0, 1,≥ 2) and jet multiplicity (Njet = 6, 7,≥ 8), as shown inFigure 2 (right). A binned maximum likelihood fit is performed to derive the most likelycross-section of b′b′ pairs. No evidence of b′ production is observed with 1.04 fb−1 data.The b′ masses below 480 GeV are excluded at 95% confidence level.References[1] L. Evans and P. Bryant (editors) JINST (2008) 3 S08001.[2] ATLAS Collaboration, JINST (2008) 3 S08003.[3] ATLAS Collaboration, ATLAS-CONF-2012-038, http://cdsweb.cern.ch/record/1432206/.[4] Choudalakis, G, arXiv.1101.0390, On hypothesis testing, trials factor, hypertests andthe BumpHunter.[5] ATLAS Collaboration, PRL 108 (2012) 211802.[6] ATLAS Collaboration, ATLAS-CONF-2011-096, http://cdsweb.cern.ch/record/1369187/.[7] ATLAS Collaboration, Submitted to JHEP, arXiv:1207.2409 (2012) [hep-ex].[8] ATLAS Collaboration, ATLAS-CONF-2012-029, https://cdsweb.cern.ch/record/1430738/.[9] ATLAS Collaboration, PRL 109 (2012) 032001.4

Searches for Exotics physics states in jets and boosted objects final states

Abstract

Similar works

Full text

Available Versions

Hal - Université Grenoble Alpes

HAL Université de Savoie

HAL-IN2P3