Suchergebnisse

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 MINECO, Spain and EPLANET, ERC, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union. Funded by SCOAP3. ; The distribution of particles inside hadronic jets produced in the decay of boosted W and Z bosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W or Z boson, this paper presents a technique for further differentiating Z bosons from W bosons. The variables used are jet mass, jet charge, and a b -tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of for bosons in the transverse momentum range 200 GeV 400 GeV in TeV pp collisions with the ATLAS detector at the LHC. For Z -boson tagging efficiencies of , 50, and , one can achieve -boson tagging rejection factors () of 1.7, 8.3 and 1000, respectively. It is not possible to measure these efficiencies in the data due to the lack of a pure sample of high , hadronically decaying Z bosons. However, the modelling of the tagger inputs for boosted W bosons is studied in data using a -enriched sample of events in 20.3 fb of data at TeV. The inputs are well modelled within uncertainties, which builds confidence in the expected tagger performance.

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. PIC (Spain). Funded by SCOAP3 ; The production rates of prompt and non-prompt J/ψ and ψ(2S) mesons in their dimuon decay modes are measured using 2.1 and 11.4 fb−1 of data collected with the ATLAS experiment at the Large Hadron Collider, in proton-proton collisions at s√=7 and 8 respectively. Production cross-sections for prompt as well as non-prompt sources, ratios of ψ(2S) to J/ψ production, and the fractions of non-prompt production for J/ψ and ψ(2S) are measured as a function of meson transverse momentum and rapidity. The measurements are compared to theoretical predictions.

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. PIC (Spain). Funded by SCOAP3. ; Distributions of transverse momentum and the related angular variable of Drell–Yan lepton pairs are measured in 20.3 fb of proton–proton collisions at TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton–proton collisions at TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z -boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of the predictions from the Monte Carlo generator are generally consistent with the data within the theoretical uncertainties. However, at larger values of this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of while the fixed-order prediction of falls below the data at high values of . and the parton-shower Monte Carlo generators provide a much better description of the evolution of the and distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.

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. PIC (Spain). Funded by SCOAP3. ; This paper presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of Z→ττ and Z(→μμ)+jets events selected from proton-proton collisions at a centre-of-mass energy s√=8TeV, corresponding to an integrated luminosity of 5 fb−1.

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. ; Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of . The observables have been chosen to emphasize the production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b -quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.

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. ; A search is presented for a high-mass Higgs boson in the , , , and decay modes using the ATLAS detector at the CERN Large Hadron Collider. The search uses proton-proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb. The results of the search are interpreted in the scenario of a heavy Higgs boson with a width that is small compared with the experimental mass resolution. The Higgs boson mass range considered extends up to for all four decay modes and down to as low as 140 , depending on the decay mode. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the four decay modes yields upper limits on the production cross-section of a heavy Higgs boson times the branching ratio to boson pairs. 95 % confidence level upper limits range from 0.53 pb at GeV to 0.008 pb at GeV for the gluon-fusion production mode and from 0.31 pb at GeV to 0.009 pb at GeV for the vector-boson-fusion production mode. The results are also interpreted in the context of Type-I and Type-II two-Higgs-doublet models.

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.

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.

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.

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. ; Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton-proton collision data used in these searches were collected at a centre-of-mass energy TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb. Two leptonic production mechanisms are considered: decays of squarks and gluinos with Z bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the Z -boson mass; and decays of neutralinos (e.g. ), resulting in a kinematic endpoint in the dilepton invariant mass distribution. For the former, an excess of events above the expected Standard Model background is observed, with a significance of three standard deviations. In the latter case, the data are well-described by the expected Standard Model background. The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos.

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. ; Measurements of the ZZ and WW final states in the mass range above the and thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the , and final states. The result is based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb at a collision energy of TeV. Using the method, the observed 95 confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1-8.6, with an expected range of 6.7-11.0. In each case the range is determined by varying the unknown and background K-factor from higher-order quantum chromodynamics corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs boson production, a combination with the on-shell measurements yields an observed (expected) 95 CL upper limit on in the range 4.5-7.5 (6.5-11.2) using the same variations of the background K-factor. Assuming that the unknown background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95 CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.

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 Higgs boson decays to invisible particles is performed using 20.3 of pp collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. The process considered is Higgs boson production in association with a vector boson ( or Z) that decays hadronically, resulting in events with two or more jets and large missing transverse momentum. No excess of candidates is observed in the data over the background expectation. The results are used to constrain VH production followed by H decaying to invisible particles for the Higgs boson mass range GeV. The 95 % confidence-level observed upper limit on varies from 1.6 pb at 115 GeV to 0.13 pb at 300 GeV. Assuming Standard Model production and including the contribution as signal, the results also lead to an observed upper limit of 78 % at 95 % confidence level on the branching ratio of Higgs bosons decays to invisible particles at a mass of 125 GeV.