By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

Search for additional heavy neutral Higgs and gauge bosons in the ditau final state produced in 36 fb −1 of pp collisions at √ s = 13 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.ch Abstract: A search for heavy neutral Higgs bosons and Z bosons is performed using a data sample corresponding to an integrated luminosity of 36.1 fb −1 from proton-proton collisions at √ s = 13 TeV recorded by the ATLAS detector at the LHC during 2015 and 2016. The heavy resonance is assumed to decay to τ + τ − with at least one tau lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2-2.25 TeV for Higgs bosons and 0.2-4.0 TeV for Z bosons. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in benchmark scenarios. In the context of the hMSSM scenario, the data exclude tan β > 1.0 for m A = 0.25 TeV and tan β > 42 for m A = 1.5 TeV at the 95% confidence level. For the Sequential Standard Model, Z SSM with m Z < 2.42 TeV is excluded at 95% confidence level, while Z NU with m Z < 2.25 TeV is excluded for the non-universal G(221) model that exhibits enhanced couplings to third-generation fermions. 6 Background estimation 10 6.1 Jet background estimate in the τ had τ had channel 10 6.1. The ATLAS collaboration 37-1 - JHEP01(2018)0551 IntroductionThe discovery of a scalar particle [1, 2] at the Large Hadron Collider (LHC) [3] has provided important insight into the mechanism of electroweak symmetry breaking. Experimental studies of the new particle [4][5][6][7][8] demonstrate consistency with the Standard Model (SM) Higgs boson [9][10][11][12][13][14]. However, it remains possible that the discovered particle is part of an extended scalar sector, a scenario that is predicted by a number of theoretical arguments [15,16]. The Minimal Supersymmetric Standard Model (MSSM) [15,17,18] is the simplest extension of the SM that includes supersymmetry. The MSSM requires two Higgs doublets of opposite hypercharge. Assuming that CP symmetry is conserved, this results in one CPodd (A) and two CP-even (h, H) neutral Higgs bosons and two charged Higgs bosons (H ± ). At tree level, the properties of the Higgs sector in the MSSM depend on only two non-SM parameters, which can be chosen to be the mass of the CP-odd Higgs boson, m A , and the ratio of the vacuum expectation values of the two Higgs doublets, tan β. Beyond tree level, a number of additional parameters affect the Higgs sector, the choice of which defines various MSSM benchmark scenarios. In the m mod+ h scenario [19], the top-squark mixing parameter is chosen such that the mass of the lightest CP-even Higgs boson, m h , is close to the measured mass of the Higgs boson that was discovered at the LHC. A different approach is employed in the hMSSM scenario [20,21] in which the measured value of m h can be used, with certain assumptions, to predict the remaining masses and couplings of the MSSM Higgs bosons without explicit reference to the sof...

The distributions of event-by-event harmonic flow coefficients v n for n = 2-4 are measured in √ s N N = 2.76 TeV Pb+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using charged particles with transverse momentum p T > 0.5 GeV and in the pseudorapidity range |η| < 2.5 in a dataset of approximately 7 µb −1 recorded in 2010. The shapes of the v n distributions suggest that the associated flow vectors are described by a two-dimensional Gaussian function in central collisions for v 2 and over most of the measured centrality range for v 3 and v 4 . Significant deviations from this function are observed for v 2 in mid-central and peripheral collisions, and a small deviation is observed for v 3 in mid-central collisions. In order to be sensitive to these deviations, it is shown that the commonly used multi-particle cumulants, involving four particles or more, need to be measured with a precision better than a few percent. The v n distributions are also measured independently for charged particles with 0.5 < p T < 1 GeV and p T > 1 GeV. When these distributions are rescaled to the same mean values, the adjusted shapes are found to be nearly the same for these two p T ranges. The v n distributions are compared with the eccentricity distributions from two models for the initial collision geometry: a Glauber model and a model that includes corrections to the initial geometry due to gluon saturation effects. Both models fail to describe the experimental data consistently over most of the measured centrality range. The ATLAS collaboration 41 IntroductionHeavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) create hot, dense matter that is thought to be composed of strongly interacting quarks and gluons. A useful tool to study the properties of this matter is the azimuthal anisotropy of particle emission in the transverse plane [1,2]. This anisotropy has been interpreted as a result of pressure-driven anisotropic expansion (referred to as "flow") of the created matter, and is described by a Fourier expansion of the particle distribution in azimuthal angle φ, around the beam direction:where v n and Φ n represent the magnitude and phase of the n th -order anisotropy of a given event in the momentum space. These quantities can also be conveniently represented by the per-particle "flow vector" [2]: ⇀ v n = (v n cos nΦ n , v n sin nΦ n ). The angles Φ n are commonly referred to as the event plane (EP) angles.In typical non-central [2] heavy ion collisions, the large and dominating v 2 coefficient is associated mainly with the "elliptic" shape of the nuclear overlap. However, v 2 in -1 - JHEP11(2013)183central (head-on) collisions and the other v n coefficients in general are related to various shape components of the initial state arising from fluctuations of the nucleon positions in the overlap region [3]. The amplitudes of these shape components, characterized by eccentricities ǫ n , can be estimated via a simple Glauber model from the...

The luminosity calibration for the ATLAS detector at the LHC during pp collisions at in 2010 and 2011 is presented. Evaluation of the luminosity scale is performed using several luminosity-sensitive detectors, and comparisons are made of the long-term stability and accuracy of this calibration applied to the pp collisions at . A luminosity uncertainty of is obtained for the 47 pb−1 of data delivered to ATLAS in 2010, and an uncertainty of is obtained for the 5.5 fb−1 delivered in 2011.

The ATLAS CollaborationDark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for H → invisible decays where H is produced according to the Standard Model via vector boson fusion, Z( )H, and W/Z(had)H, all performed with the ATLAS detector using 36.1 fb −1 of pp collisions at a center-of-mass energy of √ s = 13 TeV at the LHC. In combination with the results at √ s = 7 and 8 TeV, an exclusion limit on the H → invisible branching ratio of 0.26 (0.17 +0.07 −0.05 ) at 95% confidence level is observed (expected). 1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and the z-axis along the beam pipe. The x-axis points to the center of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). The distance between two objects in η-φ space is ∆R = (∆η) 2 + (∆φ) 2 . Transverse momentum is defined by p T = p sin θ.