Measurement of the Z→ττ cross section with the ATLAS detector

The Z→ττ cross section is measured with the ATLAS experiment at the LHC in four different final states determined by the decay modes of the τ leptons: muon-hadron, electron-hadron, electron-muon, and muon-muon. The analysis is based on a data sample corresponding to an integrated luminosity of 36pb-1, at a proton-proton center-of-mass energy of √s=7TeV. Cross sections are measured separately for each final state in fiducial regions of high detector acceptance, as well as in the full phase space, over the mass region 66-116 GeV. The individual cross sections are combined and the product of the total Z production cross section and Z→ττ branching fraction is measured to be 0.97±0.07(stat)±0.06(syst)±0.03(lumi)nb, in agreement with next-to-next-to-leading order calculations.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

Search for a heavy neutral particle decaying into an electron and a muon using 1 fb-1 of ATLAS data

A search is presented for a high mass neutral particle that decays directly to the e±μ∓ final state. The data sample was recorded by the ATLAS detector in √ s = 7 TeV pp collisions at the LHC from March to June 2011 and corresponds to an integrated luminosity of 1.07 fb-1. The data are found to be consistent with the Standard Model background. The high e±μ∓ mass region is used to set 95% confidence level upper limits on the production of two possible new physics processes: tau sneutrinos in an R-parity violating supersymmetric model and Z'-like vector bosons in a lepton flavor violating model.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

Search for pair production of a new b′ quark that decays into a Z boson and a bottom quark with the ATLAS detector

A search is reported for the pair production of a new quark b′ with at least one b′ decaying to a Z boson and a bottom quark. The data, corresponding to 2.0fb-1 of integrated luminosity, were collected from pp collisions at √s=7TeV with the ATLAS detector at the CERN Large Hadron Collider. Using events with a b-tagged jet and a Z boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum b′ candidates is tested for an enhancement. No evidence for a b′ signal is detected in the observed mass distribution, resulting in the exclusion at a 95% confidence level of b′ quarks with masses m b′<400GeV that decay entirely via b′→Z+b. In the case of a vectorlike singlet b′ mixing solely with the third standard model generation, masses mb′<358GeV are excluded.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

A study of the material in the ATLAS inner detector using secondary hadronic interactions

The ATLAS inner detector is used to reconstruct secondary vertices due to hadronic interactions of primary collision products, so probing the location and amount of material in the inner region of ATLAS. Data collected in 7 TeV pp collisions at the LHC, with a minimum bias trigger, are used for comparisons with simulated events. The reconstructed secondary vertices have spatial resolutions ranging from ∼ 200μm to 1 mm. The overall material description in the simulation is validated to within an experimental uncertainty of about 7%. This will lead to a better understanding of the reconstruction of various objects such as tracks, leptons, jets, and missing transverse momentum.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

Search for down-type fourth generation quarks with the ATLAS detector in events with one lepton and hadronically decaying W bosons

This Letter presents a search for pair production of heavy down-type quarks decaying via b ′→Wt in the lepton+jets channel, as b′b' ̄′→W¯tW +t̄→bb̄W +W¯W +W¯ -→l ±νbb̄qq̄qq̄qq̄. In addition to requiring exactly one lepton, large missing transverse momentum, and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum W bosons. In data corresponding to an integrated luminosity of 1.04fb -1 from pp collisions at √s=7TeV recorded with the ATLAS detector, a heavy down-type quark with mass less than 480GeV can be excluded at the 95% confidence level.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

Hunt for new phenomena using large jet multiplicities and missing transverse momentum with ATLAS in 4.7 fb-1 of √s = 7 TeV proton-proton collisions

Results are presented of a search for new particles decaying to large numbers of jets in association with missing transverse momentum, using 4.7 fb-1 of pp collision data at √s = 7 TeV collected by the ATLAS experiment at the Large Hadron Collider in 2011. The event selection requires missing transverse momentum, no isolated electrons or muons, and from ≥ 6 to ≥ 9 jets. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of a MSUGRA/CMSSM supersymmetric model, where, for large universal scalar mass m0, gluino masses smaller than 840 GeV are excluded at the 95% confidence level, extending previously published limits. Within a simplified model containing only a gluino octet and a neutralino, gluino masses smaller than 870 GeV are similarly excluded for neutralino masses below 100 GeV.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

A study of the material in the ATLAS inner detector using secondary hadronic interactions

The ATLAS inner detector is used to reconstruct secondary vertices due to hadronic interactions of primary collision products, so probing the location and amount of material in the inner region of ATLAS. Data collected in 7 TeV pp collisions at the LHC, with a minimum bias trigger, are used for comparisons with simulated events. The reconstructed secondary vertices have spatial resolutions ranging from ∼ 200μm to 1 mm. The overall material description in the simulation is validated to within an experimental uncertainty of about 7%. This will lead to a better understanding of the reconstruction of various objects such as tracks, leptons, jets, and missing transverse momentum.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

Measurement of dijet azimuthal decorrelations in pp collisions at √s = 7 TeV

Azimuthal decorrelations between the two central jets with the largest transverse momenta are sensitive to the dynamics of events with multiple jets. We present a measurement of the normalized differential cross section based on the full data set (Ldt = 36pb-1) acquired by the ATLAS detector during the 2010 √s = 7 TeV proton-proton run of the LHC. The measured distributions include jets with transverse momenta up to 1.3 TeV, probing perturbative QCD in a high-energy regime.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

Search for supersymmetry using final states with one lepton, jets, and missing transverse momentum with the ATLAS detector in √s = 7 TeV pp collisions

This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from √s = 7TeV proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of 35pb-1. No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. Within this framework, for A0 = 0 GeV, tan-β=3, and μ>0 and for equal squark and gluino masses, gluino masses below 700 -GeV are excluded at 95% confidence level.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat

Search for same-sign top-quark production and fourth-generation down-type quarks in pp collisions at √s = 7 TeV with the ATLAS detector

A search is presented for same-sign top-quark production and down-type heavy quarks of charge -1/3 in events with two isolated leptons (e or μ) that have the same electric charge, at least two jets and large missing transverse momentum. The data are selected from pp collisions at ps = 7 TeV recorded by the ATLAS detector and correspond to an integrated luminosity of 1.04 fb-1. The observed data are consistent with expectations from Standard Model processes. Upper limits are set at 95% confidence level on the cross section of new sources of same-sign top-quark pair production of 1.4-2.0 pb depending on the assumed mediator mass. Upper limits are also set on the pair-production cross-section for new heavy down-type quarks; a lower limit of 450 GeV is set at 95% confidence level on the mass of heavy down-type quarks under the assumption that they decay 100% of the time to Wt.La lista completa de autores que integran el documento puede consultarse en el archivo.Instituto de Física La Plat