Suchergebnisse

Ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC and NSRF, European Union. Funded by SCOAP3. ; Results of a search for new phenomena in events with at least three photons are reported. Data from proton-proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb, were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95 % confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay , as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles (a), limits are found to be for 10 GeV 62 GeV. Limits are also presented for Higgs boson-like scalars (H) for 125 GeV, and for a decaying to three photons via . Additionally, the observed limit on the branching ratio of the Z boson decay to three photons is found to be BR, a result five times stronger than the previous result from LEP.

Ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union. Funded by SCOAP3. ; This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of . A method utilising the production of leptons via decays is used. This method is applied to the sample of 20.3 fb of pp collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the lepton branching fraction into three muons, , is () at 90 % confidence level.

Ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC, FP7, Horizon 2020 and Marie Sk?odowska-Curie Actions, European Union. Funded by SCOAP3. ; The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles with inside high-momentum jets in dijet events using 20.3 fb of data recorded with the ATLAS detector in pp collisions at collisions at the LHC. The jets considered have transverse momenta from 50 up to and beyond 1.5 . The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Furthermore, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.

We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union. Funded by SCOAP3. ; This paper presents measurements of distributions of charged particles which are produced in proton-proton collisions at a centre-of-mass energy of and recorded by the ATLAS detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of 160 was used. A minimum-bias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and charged-particle multiplicity, including measurements of final states at high multiplicity. The results are corrected for detector effects and are compared to the predictions of various Monte Carlo event generator models which simulate the full hadronic final state.

We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union. Funded by SCOAP3. ; The performance of the jet trigger for the ATLAS detector at the LHC during the 2011 data taking period is described. During 2011 the LHC provided proton-proton collisions with a centre-of-mass energy of 7 TeV and heavy ion collisions with a 2.76 TeV per nucleon-nucleon collision energy. The ATLAS trigger is a three level system designed to reduce the rate of events from the 40 MHz nominal maximum bunch crossing rate to the approximate 400 Hz which can be written to offline storage. The ATLAS jet trigger is the primary means for the online selection of events containing jets. Events are accepted by the trigger if they contain one or more jets above some transverse energy threshold. During 2011 data taking the jet trigger was fully efficient for jets with transverse energy above 25 GeV for triggers seeded randomly at Level 1. For triggers which require a jet to be identified at each of the three trigger levels, full efficiency is reached for offline jets with transverse energy above 60 GeV. Jets reconstructed in the final trigger level and corresponding to offline jets with transverse energy greater than 60 GeV, are reconstructed with a resolution in transverse energy with respect to offline jets, of better than 4 % in the central region and better than 2.5 % in the forward direction.

Ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union. Funded by SCOAP3. ; The algorithms used by the ATLAS Collaboration to reconstruct and identify prompt photons are described. Measurements of the photon identification efficiencies are reported, using 4.9 fb of pp collision data collected at the LHC at and 20.3 fb at . The efficiencies are measured separately for converted and unconverted photons, in four different pseudorapidity regions, for transverse momenta between 10 and 1.5 . The results from the combination of three data-driven techniques are compared to the predictions from a simulation of the detector response, after correcting the electromagnetic shower momenta in the simulation for the average differences observed with respect to data. Data-to-simulation efficiency ratios used as correction factors in physics measurements are determined to account for the small residual efficiency differences. These factors are measured with uncertainties between 0.5% and 10% in 7 data and between 0.5% and 5.6% in 8 data, depending on the photon transverse momentum and pseudorapidity.

Ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union. Funded by SCOAP3. ; The dijet production cross section for jets containing a b -hadron (b -jets) has been measured in proton-proton collisions with a centre-of-mass energy of TeV, using the ATLAS detector at the LHC. The data used correspond to an integrated luminosity of . The cross section is measured for events with two identified b -jets with a transverse momentum GeV and a minimum separation in the - plane of . At least one of the jets in the event is required to have GeV. The cross section is measured differentially as a function of dijet invariant mass, dijet transverse momentum, boost of the dijet system, and the rapidity difference, azimuthal angle and angular distance between the b -jets. The results are compared to different predictions of leading order and next-to-leading order perturbative quantum chromodynamics matrix elements supplemented with models for parton-showers and hadronization.

Ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union and Generalitat de Catalunya. Funded by SCOAP3. ; A study of the decays and has been performed using data corresponding to an integrated luminosity of 25 fb of 7 and 8 TeV proton-proton collisions collected with the ATLAS detector during the LHC Run 1. For the dimuon decay, an upper limit on the branching fraction is set at at 95 % confidence level. For , the branching fraction is measured. The results are consistent with the Standard Model expectation with a p value of 4.8 %, corresponding to 2.0 standard deviations.

Altres ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of EPLANET, ERC, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union and MINECO, PIC, Spain. Funded by SCOAP3. ; The large rate of multiple simultaneous proton-proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 data sample collected at a centre-of-mass energy of . The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.

Altres ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of PLANET, ERC and NSRF, European Union and MINECO, PIC, Spain. Funded by SCOAP3.

Ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO and PIC, Spain and EPLANET, ERC and NSRF, European Union. Funded by SCOAP3. ; A search for the production of single-top-quarks in association with missing energy is performed in proton-proton collisions at a centre-of-mass energy of with the ATLAS experiment at the large hadron collider using data collected in 2012, corresponding to an integrated luminosity of fb. In this search, the boson from the top quark is required to decay into an electron or a muon and a neutrino. No deviation from the standard model prediction is observed, and upper limits are set on the production cross-section for resonant and non-resonant production of an invisible exotic state in association with a right-handed top quark. In the case of resonant production, for a spin- resonance with a mass of GeV, an effective coupling strength above is excluded at 95 confidence level for the top quark and an invisible spin- state with mass between and GeV. In the case of non-resonant production, an effective coupling strength above is excluded at 95 confidence level for the top quark and an invisible spin- state with mass between and GeV.

Ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of EPLANET, ERC and NSRF, European Union and MINECO and PIC, Spain. Funded by SCOAP3. ; A search is presented for the direct pair production of a chargino and a neutralino , where the chargino decays to the lightest neutralino and the boson, , while the neutralino decays to the lightest neutralino and the 125 GeV Higgs boson, . The final states considered for the search have large missing transverse momentum, an isolated electron or muon, and one of the following: either two jets identified as originating from bottom quarks, or two photons, or a second electron or muon with the same electric charge. The analysis is based on 20.3 of proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations, and limits are set in the context of a simplified supersymmetric model.

Altres ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of EPLANET, ERC and NSRF, European Union and MINECO, PIC, Spain. Funded by SCOAP3. ; The production of a boson in association with a meson in proton-proton collisions probes the production mechanisms of quarkonium and heavy flavour in association with vector bosons, and allows studies of multiple parton scattering. Using of data collected with the ATLAS experiment at the LHC in collisions at , the first measurement of associated production is presented for both prompt and non-prompt production, with both signatures having a significance in excess of . The inclusive production cross-sections for boson production (analysed in or decay modes) in association with prompt and non-prompt are measured relative to the inclusive production rate of bosons in the same fiducial volume to be and respectively. Normalised differential production cross-section ratios are also determined as a function of the transverse momentum. The fraction of signal events arising from single and double parton scattering is estimated, and a lower limit of at confidence level is placed on the effective cross-section regulating double parton interactions.

Altres ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of EPLANET, ERC and NSRF, European Union and MINECO, PIC, Spain. ; Studies of the spin and parity quantum numbers of the Higgs boson in the final state are presented, based on proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb at a centre-of-mass energy of TeV. The Standard Model spin-parity hypothesis is compared with alternative hypotheses for both spin and CP. The case where the observed resonance is a mixture of the Standard-Model-like Higgs boson and CP-even () or CP-odd () Higgs boson in scenarios beyond the Standard Model is also studied. The data are found to be consistent with the Standard Model prediction and limits are placed on alternative spin and CP hypotheses, including CP mixing in different scenarios.

Altres ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of EPLANET, ERC and NSRF, European Union and MINECO, PIC, Spain. Funded by SCOAP3. ; A search for a new resonance decaying to a W or Z boson and a Higgs boson in the final states is performed using 20.3 fb of pp collision data recorded at 8 TeV with the ATLAS detector at the Large Hadron Collider. The search is conducted by examining the WH / ZH invariant mass distribution for a localized excess. No significant deviation from the Standard Model background prediction is observed. The results are interpreted in terms of constraints on the Minimal Walking Technicolor model and on a simplified approach based on a phenomenological Lagrangian of Heavy Vector Triplets.