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.

Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb −1 of proton-proton collision data at √ s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider.No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.Keywords: Supersymmetry, Hadron-Hadron Scattering The ATLAS collaboration 33 IntroductionSupersymmetry (SUSY) [1][2][3][4][5][6][7][8][9] is a spacetime symmetry that postulates for each Standard Model (SM) particle the existence of a partner particle whose spin differs by one-half unit. The introduction of these new particles provides a potential solution to the hierarchy problem [10][11][12][13]. If R-parity is conserved [14][15][16][17][18], as is assumed in this paper, SUSY particles are always produced in pairs and the lightest supersymmetric particle (LSP) emerges as a stable dark-matter candidate.-1 - JHEP05(2014)071The charginos and neutralinos are mixtures of the bino, winos and higgsinos that are superpartners of the U(1), SU(2) gauge bosons and the Higgs bosons, respectively. Their mass eigenstates are referred to asχ ± i (i = 1, 2) andχ 0 j (j = 1, 2, 3, 4) in the order of increasing masses. Even though the gluinos and squarks are produced strongly in pp collisions, if the masses of the gluinos and squarks are large, the direct production of charginos, neutralinos and sleptons through electroweak interactions may dominate the production of SUSY particles at the Large Hadron Collider (LHC). Such a scenario is possible in the general framework of the phenomenological minimal supersymmetric SM (pMSSM) [19][20][21]. Naturalness suggests that third-generation sparticles and some of the charginos and neutralinos should have masses of a few hundred GeV [22,23]. Light sleptons are expected in gauge-mediated [24][25][26][27][28][29] and anomaly-mediated [30,31] SUSY breaking scenarios. Light sleptons could also play a role in the co-annihilation of neutralinos, allowing a dark matter relic density consistent with cosmological observations [32,33]. This paper presents searches for electroweak production of charginos, neutralinos and sleptons using 20.3 fb −1 of proton-proton collision data with a centre-of-mass energy √ s = 8 TeV collected at the LHC with the ATLAS detector. The searches target final states with two oppositely-charged leptons (electrons or muons) and missing transverse momentum. Similar searches [34,35] SUSY scenariosSimplified models [42] are considered for optimization of the event selection and interpretation of the results. The LSP is the lightest neutralinoχ 0 1 in all SUSY scenarios considered, except in...

A search for the electroweak production of charginos and sleptons decaying into final states with two electrons or muons is presented. The analysis is based on 139 fb −1 of proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider at √ s = 13 TeV. Three R-parity-conserving scenarios where the lightest neutralino is the lightest supersymmetric particle are considered: the production of chargino pairs with decays via either W bosons or sleptons, and the direct production of slepton pairs. The analysis is optimised for the first of these scenarios, but the results are also interpreted in the others. No significant deviations from the Standard Model expectations are observed and limits at 95% confidence level are set on the masses of relevant supersymmetric particles in each of the scenarios. For a massless lightest neutralino, masses up to 420 GeV are excluded for the production of the lightest-chargino pairs assuming W-boson-mediated decays and up to 1 TeV for slepton-mediated decays, whereas for slepton-pair production masses up to 700 GeV are excluded assuming three generations of mass-degenerate sleptons. Contents

A search for excited states of the Bc(±) meson is performed using 4.9  fb(-1) of 7 TeV and 19.2  fb(-1) of 8 TeV pp collision data collected by the ATLAS experiment at the LHC. A new state is observed through its hadronic transition to the ground state, with the latter detected in the decay Bc(±)→J/ψπ(±). The state appears in the m(Bc(±)π(+)π(-))-m(Bc(±))-2m(π(±)) mass difference distribution with a significance of 5.2 standard deviations. The mass of the observed state is 6842±4±5  MeV, where the first error is statistical and the second is systematic. The mass and decay of this state are consistent with expectations for the second S-wave state of the Bc(±) meson, Bc(±)(2S).