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]...

This paper reports a measurement of the W +b-jets (W +b+X and W +bb+X) production cross-section in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC. These results are based on data corresponding to an integrated luminosity of 4.6 fb −1 , collected with the ATLAS detector. Cross-sections are presented as a function of jet multiplicity and of the transverse momentum of the leading b-jet for both the muon and electron decay modes of the W boson. The W +b-jets cross-section, corrected for all known detector effects, is quoted in a limited kinematic range. Combining the muon and electron channels, the fiducial cross-section for W +b-jets is measured to be 7.1 ± 0.5 (stat) ± 1.4 (syst) pb, consistent with the next-to-leading order QCD prediction, corrected for non-perturbative and double-parton interactions (DPI) contributions, of 4.70 ± 0.09 (stat) 9 Results without single-top subtraction 21+0 Conclusions 22The ATLAS collaboration 29 IntroductionThis paper reports a measurement of the cross-section for W boson production in association with b-quark jets in proton-proton (pp) collisions at √ s = 7 TeV. The measurement, performed differentially in the b-jet transverse momentum (p T ) for the 1-jet and 2-jet final states, provides an important test of perturbative quantum chromodynamics (QCD) in the presence of heavy quarks.Next-to-leading-order (NLO) perturbative QCD calculations of the W +b-jets process have recently become available in Monte Carlo (MC) simulations, both at the parton level [1] and enhanced with parton shower models [2][3][4][5]. Several processes contribute to W +b-jets production at NLO. In the four-flavour number scheme (4FNS), where only u, d, c, s are considered as initial-state quarks, these are qq → W bb(g) and gq → W bbq. When considering the presence of b-quarks in the initial state (five-flavour number scheme, or 5FNS), the single b-quark processes bq → W bq(g) and bg → W bqq also play a significant role at LHC energies [6][7][8][9]. In addition, double-parton interactions (DPI), in which a W -1 - JHEP06(2013)084boson and b-jets are produced from different parton-parton interactions within the same pp collision, are also expected to contribute to the total observed W +b-jets cross-section [10].The production of W +b-jets events via top-quark decay (from single or pair-produced top-quarks) is not included in the signal definition for the primary measurement. An additional set of measurements is performed including the contribution from single top-quark production. These are of particular relevance for the differential cross-section, since single top-quark and W +b-jets events are difficult to separate and their relative contribution depends strongly on the b-jet transverse momentum.The W +b-jets process is an important background to the Higgs boson associatedproduction process W H with H → bb decays [11]. The associated-production measurements are a substantial ingredient in determining the coupling of the Higgs boson to fermions, through the decay H → bb, and...

A search for a charged Higgs boson (H+) in decays is presented, where one of the top quarks decays via t→H+b, followed by H+→ two jets (). The other top quark decays to Wb, where the W boson then decays into a lepton (e/μ) and a neutrino. The data were recorded in pp collisions at by the ATLAS detector at the LHC in 2011, and correspond to an integrated luminosity of 4.7 fb−1. With no observation of a signal, 95 % confidence level (CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons varying between 5 % and 1 % for H+ masses between 90 GeV and 150 GeV, assuming .