Search for new phenomena in final states with large jet multiplicities and missing transverse momentum at $ \sqrt{s}=8 $ TeV proton-proton collisions using the ATLAS experiment

Abstract

A search is presented for new particles decaying to large numbers (7 or more) of jets, with missing transverse momentum and no isolated electrons or muons. This analysis uses 20.3 fb −1 of pp collision data at √ s = 8 TeV collected by the ATLAS experiment at the Large Hadron Collider. The sensitivity of the search is enhanced by considering the number of b-tagged jets and the scalar sum of masses of large-radius jets in an event. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of various simplified supersymmetry-inspired models where gluinos are pair produced, as well as an mSUGRA/CMSSM model. The ATLAS collaboration 34 IntroductionMany extensions of the Standard Model of particle physics predict the presence of TeVscale strongly interacting particles that decay to weakly interacting descendants. In the context of R-parity-conserving supersymmetry (SUSY) [1][2][3][4][5], the strongly interacting parent particles are the partners of the quarks (squarks,q) and gluons (gluinos,g), and are produced in pairs. The lightest supersymmetric particle (LSP) is stable, providing a candidate that can contribute to the relic dark-matter density in the universe [6,7]. If they are kinematically accessible, the squarks and gluinos could be produced in the proton-proton interactions at the Large Hadron Collider (LHC) [8].-1 - JHEP10(2013)130Such particles are expected to decay in cascades, the nature of which depends on the mass hierarchy within the model. The events would be characterised by significant missing transverse momentum from the unobserved weakly interacting descendants, and by a large number of jets from emissions of quarks and/or gluons. Individual cascade decays may include gluino decays to a top squark (stop,t) and an anti-top quark, g →t +t (1.1a)followed by the top-squark decay to a top quark and a neutralino LSP,χ 0 1 , t → t +χ 0 1.(1.1b)Alternatively, if the top squark is heavier than the gluino, the three-body decay, g → t +t +χ 0 1 (1.2) may result. Other possibilities include decays involving intermediate charginos, neutralinos, and/or squarks including bottom squarks. A pair of cascade decays produces a large number of Standard Model particles, together with a pair of LSPs, one from the end of each cascade. The LSPs are assumed to be stable and only weakly interacting, and so escape undetected, resulting in missing transverse momentum. In this paper we consider final states with large numbers of jets together with significant missing transverse momentum in the absence of isolated electrons or muons, using the pp collision data recorded by the ATLAS experiment [9] during 2012 at a centre-of-mass energy of √ s = 8 TeV. The corresponding integrated luminosity is 20.3 fb −1 . Searches for new phenomena in final states with large jet multiplicities -requiring from at least six to at least nine jets -and missing transverse momentum have previously been reported by the ATLAS Collaboration using LHC pp collision data corresponding to 1.34 fb −1 [10]...

Rare top-quark decays to Higgs boson in the MSSM

“…There are of course relevant constraints for parameters important for B(t → q h) emerging from direct, mainly LHC, SUSY searches [60,61]. These are shown in Table 1.…”

Boosted top quarks and jet structure

“…The strategy of the analysis in [140] is to look for an excess of events beyond the SM prediction in a final state with at least seven anti-k T R = 0.4 jets (small-R jets), missing energy, and no leptons. This final state is especially sensitive to a SUSY model like that used in Sect.…”

“…Also shown is the distribution of a SUSY model with m(g) = 900 GeV and m(χ 0 1 ) = 150 GeV, generated with MadGraph+HERWIG++. From [140], used under CC BY 4.0 and unchanged from original (a) (b) Fig. 73 The 95 % confidence level exclusion curve (small masses are excluded) for the simplified SUSY modelg → ttχ 0 1 in the gluino-LSP mass plane for a the analysis stream that uses b-tags and b the stream that uses the sum of fat jet masses.…”

The Hunt for Supersymmetry at the Tevatron

“…The early result from ATLAS nicely displays the increase in sensitivity compared to the Tevatron results (as given in Figure 17 for DØ with 2.1 fb −1 and in Figure 19 for CDF with 2.0 fb −1 ). Recently, ATLAS and CMS published updated results with about 20 fb −1 data, each taken at 8 TeV [70,71]. Thus, the lower Tevatron gluino mass limit increased from about 300 GeV (CDF [27], DØ [26]) to a current LHC mass limit above about 1.1 TeV (ATLAS [70], CMS [71]).…”

“…Recently, ATLAS and CMS published updated results with about 20 fb −1 data, each taken at 8 TeV [70,71]. Thus, the lower Tevatron gluino mass limit increased from about 300 GeV (CDF [27], DØ [26]) to a current LHC mass limit above about 1.1 TeV (ATLAS [70], CMS [71]). The LHC has already largely extended the excluded parameter region of supersymmetric models compared to the Tevatron results.…”