Search results

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 ARTEMIS and ERC, European Union and MICINN, PIC, Spain. ; The top quark mass has been measured using the template method in the channel based on data recorded in 2011 with the ATLAS detector at the LHC. The data were taken at a proton-proton centre-of-mass energy of and correspond to an integrated luminosity of 1.04 fb −1. The analyses in the e +jets and μ +jets decay channels yield consistent results. The top quark mass is measured to be m =174.5±0.6±2.3 GeV.

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 and ERC, European Union and MICINN, PIC, Spain. ; In this paper, a measurement of τ polarization in W → τν decays is presented. It is measured from the energies of the decay products in hadronic τ decays with a single final state charged particle. The data, corresponding to an integrated luminosity of 24 pb −1, were collected by the ATLAS experiment at the Large Hadron Collider in 2010. The measured value of the τ polarization is , in agreement with the Standard Model prediction, and is consistent with a physically allowed 95 % CL interval [−1,−0.91]. Measurements of τ polarization have not previously been made at hadron colliders.

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 and ERC, European Union and MICINN, Spain. ; This paper presents a search for the t-channel exchange of an R-parity violating scalar top quark (t~) in the e ± μ ∓ continuum using 2.1 fb−1 of data collected by the ATLAS detector in s√=7 TeV pp collisions at the Large Hadron Collider. Data are found to be consistent with the expectation from the Standard Model backgrounds. Limits on R-parity-violating couplings at 95 % C.L. are calculated as a function of the scalar top mass (mt~). The upper limits on the production cross section for pp→eμX, through the t-channel exchange of a scalar top quark, ranges from 170 fb for mt~=95 GeV to 30 fb for mt~=1000 GeV.

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 and ERC, European Union and MICINN, PIC, Spain. ; A measurement of the jet activity in events produced in proton-proton collisions at a centre-of-mass energy of 7 TeV is presented, using 2.05 fb −1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. The events are selected in the dilepton decay channel with two identified b -jets from the top quark decays. Events are vetoed if they contain an additional jet with transverse momentum above a threshold in a central rapidity interval. The fraction of events surviving the jet veto is presented as a function of this threshold for four different central rapidity interval definitions. An alternate measurement is also performed, in which events are vetoed if the scalar transverse momentum sum of the additional jets in each rapidity interval is above a threshold. In both measurements, the data are corrected for detector effects and compared to the theoretical models implemented in , , and . The experimental uncertainties are often smaller than the spread of theoretical predictions, allowing deviations between data and theory to be observed in some regions of phase space.

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 and ERC, European Union and MICINN, Spain. ; This letter reports on a search for hypothetical heavy neutrinos, N, and right-handed gauge bosons, W , in events with high transverse momentum objects which include two reconstructed leptons and at least one hadronic jet. The results were obtained from data corresponding to an integrated luminosity of 2.1 fb −1 collected in proton-proton collisions at with the ATLAS detector at the CERN Large Hadron Collider. No excess above the Standard Model background expectation is observed. Excluded mass regions for Majorana and Dirac neutrinos are presented using two approaches for interactions that violate lepton and lepton-flavor numbers. One approach uses an effective operator framework, the other approach is guided by the Left-Right Symmetric Model. The results described in this letter represent the most stringent limits to date on the masses of heavy neutrinos and W bosons obtained in direct searches.

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. ; The jet energy scale (JES) and its systematic uncertainty are determined for jets measured with the ATLAS detector using proton-proton collision data with a centre-of-mass energy of TeV corresponding to an integrated luminosity of . Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells using the anti- algorithm with distance parameters or , and are calibrated using MC simulations. A residual JES correction is applied to account for differences between data and MC simulations. This correction and its systematic uncertainty are estimated using a combination of in situ techniques exploiting the transverse momentum balance between a jet and a reference object such as a photon or a boson, for and pseudorapidities . The effect of multiple proton-proton interactions is corrected for, and an uncertainty is evaluated using in situ techniques. The smallest JES uncertainty of less than 1 % is found in the central calorimeter region () for jets with . For central jets at lower , the uncertainty is about 3 %. A consistent JES estimate is found using measurements of the calorimeter response of single hadrons in proton-proton collisions and test-beam data, which also provide the estimate for TeV. The calibration of forward jets is derived from dijet balance measurements. The resulting uncertainty reaches its largest value of 6 % for low- jets at . Additional JES uncertainties due to specific event topologies, such as close-by jets or selections of event samples with an enhanced content of jets originating from light quarks or gluons, are also discussed. The magnitude of these uncertainties depends on the event sample used in a given physics analysis, but typically amounts to 0.5-3 %.

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 measurement is presented of the ϕ×BR(ϕ→K+K−) production cross section at s√ = 7 TeV using pp collision data corresponding to an integrated luminosity of 383 μb−1, collected with the ATLAS experiment at the LHC. Selection of ϕ(1020) mesons is based on the identification of charged kaons by their energy loss in the pixel detector. The differential cross section is measured as a function of the transverse momentum, pT,ϕ, and rapidity, yϕ, of the ϕ(1020) meson in the fiducial region 500 230 MeV and kaon momentum pK< 800 MeV. The integrated ϕ(1020)-meson production cross section in this fiducial range is measured to be σϕ×BR(ϕ→K+K−) = 570 ± 8 (stat) ± 66 (syst) ± 20 (lumi) μb.

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. ; This paper presents a study of the performance of the muon reconstruction in the analysis of proton-proton collisions at TeV at the LHC, recorded by the ATLAS detector in 2010. This performance is described in terms of reconstruction and isolation efficiencies and momentum resolutions for different classes of reconstructed muons. The results are obtained from an analysis of meson and boson decays to dimuons, reconstructed from a data sample corresponding to an integrated luminosity of 40 pb. The measured performance is compared to Monte Carlo predictions and deviations from the predicted performance are discussed.

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 MICINN, PIC, Spain. ; A measurement of splitting scales, as defined by the k clustering algorithm, is presented for final states containing a W boson produced in proton-proton collisions at a centre-of-mass energy of 7 TeV. The measurement is based on the full 2010 data sample corresponding to an integrated luminosity of 36 pb −1 which was collected using the ATLAS detector at the CERN Large Hadron Collider. Cluster splitting scales are measured in events containing W bosons decaying to electrons or muons. The measurement comprises the four hardest splitting scales in a k cluster sequence of the hadronic activity accompanying the W boson, and ratios of these splitting scales. Backgrounds such as multi-jet and top-quark-pair production are subtracted and the results are corrected for detector effects. Predictions from various Monte Carlo event generators at particle level are compared to the data. Overall, reasonable agreement is found with all generators, but larger deviations between the predictions and the data are evident in the soft regions of the splitting scales.

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 MICINN, PIC, Spain. ; The inclusive jet cross-section has been measured in proton-proton collisions at s√=2.76 TeV in a dataset corresponding to an integrated luminosity of 0.20 pb−1 collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-k t algorithm with two radius parameters of 0.4 and 0.6. The inclusive jet double-differential cross-section is presented as a function of the jet transverse momentum p T and jet rapidity y, covering a range of 20≤p T<430 GeV and |y|<4.4. The ratio of the cross-section to the inclusive jet cross-section measurement at s√=7 TeV, published by the ATLAS Collaboration, is calculated as a function of both transverse momentum and the dimensionless quantity xT=2pT/s√, in bins of jet rapidity. The systematic uncertainties on the ratios are significantly reduced due to the cancellation of correlated uncertainties in the two measurements. Results are compared to the prediction from next-to-leading order perturbative QCD calculations corrected for non-perturbative effects, and next-to-leading order Monte Carlo simulation. Furthermore, the ATLAS jet cross-section measurements at s√=2.76 TeV and s√=7 TeV are analysed within a framework of next-to-leading order perturbative QCD calculations to determine parton distribution functions of the proton, taking into account the correlations between the measurements.

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 MICINN, Spain and EPLANET, ERC and NSRF, European Union. ; A search for supersymmetric particles in final states with zero, one, and two leptons, with and without jets identified as originating from b -quarks, in 4.7 fb −1 of pp collisions produced by the Large Hadron Collider and recorded by the ATLAS detector is presented. The search uses a set of variables carrying information on the event kinematics transverse and parallel to the beam line that are sensitive to several topologies expected in supersymmetry. Mutually exclusive final states are defined, allowing a combination of all channels to increase the search sensitivity. No deviation from the Standard Model expectation is observed. Upper limits at 95 % confidence level on visible cross-sections for the production of new particles are extracted. Results are interpreted in the context of the constrained minimal supersymmetric extension to the Standard Model and in supersymmetry-inspired models with diverse, high-multiplicity final states.

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 MICINN, PIC, Spain. ; A search for a charged Higgs boson (H +) in tt¯ decays is presented, where one of the top quarks decays via t→H + b, followed by H +→ two jets (cs¯). The other top quark decays to Wb, where the W boson then decays into a lepton (e/μ) and a neutrino. The data were recorded in pp collisions at s√=7 TeV by the ATLAS detector at the LHC in 2011, and correspond to an integrated luminosity of 4.7 fb−1. With no observation of a signal, 95 % confidence level (CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons varying between 5 % and 1 % for H + masses between 90 GeV and 150 GeV, assuming B(H+→cs¯)=100 %.

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 and ERC, European Union and MICINN, Spain. ; The measurement of the jet energy resolution is presented using data recorded with the ATLAS detector in proton-proton collisions at s√=7 TeV. The sample corresponds to an integrated luminosity of 35 pb−1. Jets are reconstructed from energy deposits measured by the calorimeters and calibrated using different jet calibration schemes. The jet energy resolution is measured with two different in situ methods which are found to be in agreement within uncertainties. The total uncertainties on these measurements range from 20 % to 10 % for jets within |y|<2.8 and with transverse momenta increasing from 30 GeV to 500 GeV. Overall, the Monte Carlo simulation of the jet energy resolution agrees with the data within 10 %.

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 MICINN, Spain. ; A measurement of the top quark pair production cross section in the final state with a hadronically decaying tau lepton and jets is presented. The analysis is based on proton-proton collision data recorded by the ATLAS experiment at the LHC, with a centre-of-mass energy of 7 TeV. The data sample corresponds to an integrated luminosity of 1.67 fb −1. The cross section is measured to be and is in agreement with other measurements and with the Standard Model prediction.

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 MICINN, PIC, Spain. ; The luminosity calibration for the ATLAS detector at the LHC during pp collisions at s√=7 TeV 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 s√=7 TeV. A luminosity uncertainty of δL/L=±3.5 % is obtained for the 47 pb−1 of data delivered to ATLAS in 2010, and an uncertainty of δL/L=±1.8 % is obtained for the 5.5 fb−1 delivered in 2011.