A search for the narrow structure, X(5568), reported by the D0 Collaboration in the decay sequence X→B_{s}^{0}π^{±}, B_{s}^{0}→J/ψϕ, is presented. The analysis is based on a data sample recorded with the ATLAS detector at the LHC corresponding to 4.9  fb^{-1} of pp collisions at 7 TeV and 19.5  fb^{-1} at 8 TeV. No significant signal was found. Upper limits on the number of signal events, with properties corresponding to those reported by D0, and on the X production rate relative to B_{s}^{0} mesons, ρ_{X}, were determined at 95% confidence level. The results are N(X)<382 and ρ_{X}<0.015 for B_{s}^{0} mesons with transverse momenta above 10 GeV, and N(X)<356 and ρ_{X}<0.016 for transverse momenta above 15 GeV. Limits are also set for potential B_{s}^{0}π^{±} resonances in the mass range 5550 to 5700 MeV.

Abstract:The electroweak production and subsequent decay of single top quarks in the t-channel is determined by the properties of the Wtb vertex, which can be described by the complex parameters of an effective Lagrangian. An analysis of a triple-differential decay rate in t-channel production is used to simultaneously determine five generalised helicity fractions and phases, as well as the polarisation of the produced top quark. The complex parameters are then constrained. This analysis is based on 20.2 fb −1 of protonproton collision data at a centre-of-mass energy of 8 TeV collected with the ATLAS detector at the LHC. The fraction of decays containing transversely polarised W bosons is measured to be f 1 = 0.30 ± 0.05. The phase between amplitudes for transversely and longitudinally polarised W bosons recoiling against left-handed b-quarks is measured to be δ − = 0.002π +0.016π +0.017π , giving no indication of CP violation. The fractions of longitudinal or transverse W bosons accompanied by right-handed b-quarks are also constrained. Based on these measurements, limits are placed at 95% CL on the ratio of the complex coupling parameters Re [g R /V L ∈ [−0.12, 0.17] and Im [g R /V L ∈ [−0.07, 0.06]. Constraints are also placed on the ratios |V R /V L | and |g L /V L |. In addition, the polarisation of single top quarks in the t-channel is constrained to be P > 0.72 (95% CL). None of the above measurements make assumptions about the value of any of the other parameters or couplings and all of them are in agreement with the Standard Model. Keywords: Electroweak interaction, Hadron-Hadron scattering (experiments), Top physicsArXiv ePrint: 1707.05393Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. Article funded by SCOAP 3 .https://doi.org/10.1007/JHEP12(2017)017 JHEP12(2017)017 The ATLAS collaboration 43 IntroductionThe top quark is the heaviest known fundamental particle, making the measurement of its production and decay kinematic properties an important probe of physical processes beyond the Standard Model (SM). Within the SM, the top quark decays predominantly through the electroweak interaction to an on-shell W boson and a b-quark. Due to its large mass [1], its lifetime O(10 −25 s) is smaller than its hadronisation time-scale O(10 −24 s), allowing this quark to be studied as a free quark. Since the top-quark lifetime is also shorter than the depolarisation timescale O(10 −21 s) [2] and the W boson is produced onshell in the top-quark decay, the top-quark spin information is directly transferred to its decay products. Comparing angular measurements of the decay products of polarised top quarks with precise SM predictions provides a unique way to study the non-SM couplings in the Wtb vertex [3]. The normalised triple-differential cross-section (to be defined in section 2) is the joint probability distribution in all three of the angles determining the -1 - JHEP12(2017)017kinematics of the decay t → W b from a polarised initial state. Its analysis is the most complete investigation of ...

Measurement of the Zγ → ννγ production cross section in p p collisions at √ s = 13 TeV with the ATLAS detector and limits on anomalous triple gauge-boson couplingsThe ATLAS CollaborationThe production of Z bosons in association with a high-energy photon (Zγ production) is studied in the neutrino decay channel of the Z boson using pp collisions at √ s = 13 TeV. The analysis uses a data sample with an integrated luminosity of 36.1 fb −1 collected by the ATLAS detector at the LHC in 2015 and 2016. Candidate Zγ events with invisible decays of the Z boson are selected by requiring significant transverse momentum (p T ) of the dineutrino system in conjunction with a single isolated photon with large transverse energy (E T ). The rate of Zγ production is measured as a function of photon E T , dineutrino system p T and jet multiplicity. Evidence of anomalous triple gauge-boson couplings is sought in Zγ production with photon E T greater than 600 GeV. No excess is observed relative to the Standard Model expectation, and upper limits are set on the strength of Z Zγ and Zγγ couplings.

This paper presents updated Monte Carlo configurations used to model the production of single electroweak vector bosons (W, Z/γ∗) in association with jets in proton-proton collisions for the ATLAS experiment at the Large Hadron Collider. Improvements pertaining to the electroweak input scheme, parton-shower splitting kernels and scale-setting scheme are shown for multi-jet merged configurations accurate to next-to-leading order in the strong and electroweak couplings. The computational resources required for these set-ups are assessed, and approximations are introduced resulting in a factor three reduction of the per-event CPU time without affecting the physics modelling performance. Continuous statistical enhancement techniques are introduced by ATLAS in order to populate low cross-section regions of phase space and are shown to match or exceed the generated effective luminosity. This, together with the lower per-event CPU time, results in a 50% reduction in the required computing resources compared to a legacy set-up previously used by the ATLAS collaboration. The set-ups described in this paper will be used for future ATLAS analyses and lay the foundation for the next generation of Monte Carlo predictions for single vector-boson plus jets production.

Abstract. The search for tt resonances allows the investigation of a wide range of physics beyond the Standard Model. For a high mass resonance, the top quark is produced with a transverse momentum that is large, compared to its mass, and the decay of such a highly boosted top often leads to a topology that di↵ers in several respects from that encountered when the top quarks are produced approximately at rest. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 14 fb 1 of proton-proton collisions data collected at center-of-mass energy p s = 8 TeV. No evidence for a tt resonance is found and 95% CL limits on the production rate are determined for massive states in two benchmark models. A narrow leptophobic topcolor Z' boson with a mass below 1.8 TeV is excluded and a Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.0 TeV. IntroductionThis note reports on an ATLAS experiment [1] search for the production of top quark pair resonances, such as a leptophobic Z' or a Kaluza-Klein gluon, produced in protonproton collisions at a center-of-mass energy of 8 TeV using data collected in 2012 with an integrated luminosity of 14.3 fb 1 [2]. This search is carried out in the lepton plus jets decay channel, where one of the W bosons from a top quark decays leptonically (to an electron or a muon and a neutrino) and the other decays hadronically. The tt invariant mass spectrum is tested for any local excess of events that may stem from a resonance decaying into tt. The events are reconstructed using a combination of resolved and boosted reconstruction methods. In the former, the hadronically decaying top quark is identified by two or three distinct small-radius jets. In the latter, the hadronically decaying top quark is identified by reconstructing one large-radius jet with a substructure consistent with the decay products of a W boson and a b quark. High momentum top quark decays are indeed reconstructed more e ciently using boosted reconstruction techniques. For both reconstruction methods, the semileptonically decaying top quark is identified by a lepton, one small-radius jet and missing transverse momentum.Both ATLAS [2] and CMS [3] have used two specific theoretical models as benchmarks to test the production of narrow and broad resonances as compared to the detector resolution, which is of the order of 7%. The narrow resonance benchmark is a topcolor, leptophobic Z' given by model IV of Harris et al. [4] with a width of a e-mail: dechenaux@lpsc.in2p3.fr Z 0 /m Z 0 = 1.2%. The broad resonance benchmark is related to Kaluza-Klein (KK) excitation states of the gluon as predicted in Randall-Sundrum models with a warped extra-dimension and where all the standard model fields can propagate in the five dimensions. A resonance width of g KK /m g KK = 15.3% is used. Details on the tested models can be found in previous ATLAS studies [5]. Data and Monte Carlo samplesOnly data recorded under stable beam conditions and operationnal ATLAS subdetect...