Abstract: The inclusive cross-section for the associated production of a W boson and top quark is measured using data from proton-proton collisions at root s = 13TeV. The dataset corresponds to an integrated luminosity of 3.2 fb(-1), and was collected in 2015 by the ATLAS detector at the Large Hadron Collider at CERN. Events are selected requiring two opposite sign isolated leptons and at least one jet; they are separated into signal and control regions based on their jet multiplicity and the number of jets that are identified as containing b hadrons. The Wt signal is then separated from the t ($) over bar background using boosted decision tree discriminants in two regions. The cross-section is extracted by fitting templates to the data distributions, and is measured to be sigma(Wt) = 94 +/- 10 (stat:)(-22)(+28) (syst:) +/- 2 (lumi:) pb. The measured value is in good agreement with the SM prediction of sigma(theory) = 71: 7 +/- 1: 8 (scale) +/- 3: 4 (PDF) pb [1].

Abstract: We study the appearance of structures in the decay of the B- into K-pi D--(s0)+ (2317) and K-pi D--(s1)+ (2460) final states by forming invariant mass distributions of pi D--(s0)+ and pi D--(s1)+ pairs, respectively. The structure in the distribution is associated to the kinematical triangle singularity that appears when the B- -> K- K*(0) D-0 (B- -> K- K*(0) D*(0)) decay process is followed by the decay of the K*(0) into pi(-) K+ and the subsequent rescattering of the K+ D-0 (K+ D*(0)) pair forming the D-s0(+) (2317) (D-s1(+) (2460)) resonance. We find this type of non-resonant peaks at 2850MeV in the invariant mass of pi D--(s0) pairs from B- -> K- pi(-) D-s0(+) (2317) decays and around 3000MeV in the invariant mass of pi D--(s1)+ pairs from B- -> K- pi(-) D-s1(+)(2460) decays. By employing the measured branching ratios of the B- -> K- K*(0) D-0 and B- -> K- K*(0) D*(0) decays, we predict the branching ratios for the processes B- into K-pi D--(s0)+ (2317) K-pi D--(s1)+ (2460), in the vicinity of the triangle singularity peak, to be about 8 x 10(-6) and 1 x 10(-6), respectively. The observation of this reaction would also give extra support to the molecular picture of the D-s0(+)(2317) and D-s1(+)(2460).

Abstract: Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to an integrated luminosity of 36.1 fb(-1) at a centre-of-mass energy of 13 TeV collected in 2015 and 2016 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons (e or mu). Several signal regions are considered with increasing requirements on the missing transverse momentum above 250 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, and supersymmetric particles in several compressed scenarios.

Abstract: Massive neutrinos demand to ask whether they are Dirac or Majorana particles. Majorana neutrinos are an irrefutable proof of physics beyond the Standard Model. Neutrinoless double electron capture is not a process but a virtual Delta L = 2 mixing between a parent (A)Z atom and a daughter (A)(Z – 2) excited atom with two electron holes. As a mixing between two neutral atoms and the observable signal in terms of emitted two-hole X-rays, the strategy, experimental signature and background are different from neutrinoless double beta decay. The mixing is resonantly enhanced for almost degeneracy and, under these conditions, there is no irreducible background from the standard two-neutrino channel. We reconstruct the natural time history of a nominally stable parent atom since its production either by nature or in the laboratory. After the time periods of atom oscillations and the decay of the short-lived daughter atom, at observable times the relevant 'stationary" states are the mixed metastable long-lived state and the non-orthogonal short-lived excited state, as well as the ground state of the daughter atom. We find that they have a natural population inversion which is most appropriate for exploiting the bosonic nature of the observed atomic transitions radiation. Among different observables of the atom Majorana mixing, we include the enhanced rate of stimulated X-ray emission from the long-lived metastable state by a high-intensity X-ray beam: a gain factor of 100 can be envisaged at current XFEL facilities. On the other hand, the historical population of the daughter atom ground state can be probed by exciting it with a current pulsed optical laser, showing the characteristic absorption lines: the whole population can be excited in a shorter time than typical pulse duration.

Abstract: A search is performed for new phenomena in events having a photon with high transverse momentum and a jet collected in 36.7 fb(-1) of proton-proton collisions at a centre-of-mass energy of root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The invariant mass distribution of the leading photon and jet is examined to look for the resonant production of new particles or the presence of new high-mass states beyond the Standard Model. No significant deviation from the background-only hypothesis is observed and cross-section limits for generic Gaussian-shaped resonances are extracted. Excited quarks hypothesized in quark compositeness models and high-mass states predicted in quantum black hole models with extra dimensions are also examined in the analysis. The observed data exclude, at 95% confidence level, the mass range below 5.3 TeV for excited quarks and 7.1 TeV (4.4 TeV) for quantum black holes in the Arkani-Hamed-Dimopoulos-Dvali (Randall-Sundrum) model with six (one) extra dimensions.