[en] In the midst of the intense activity that will arise from the proton-proton collisions at the LHC, muons will be very useful to spot rare events of interest. The good resolution expected for their momentum measurement shall also make them powerful tools in event reconstruction. Muon identification will thus be a crucial issue in the ATLAS experiment at the LHC. Their charged tracks can be reconstructed in the external spectrometer only, but the combination of such '' stand-alone '' tracks with tracks from the inner detector shall increase the precision and rateability of the reconstructed muon. This is particularly true in the lower part of the p_T spectrum, where the inner detector is more performant. We will present here the various strategies for combined muon identification in the ATLAS experiment. The main algorithms, called Staco and Muid, perform the combination of existing tracks in the inner detector and in the muon spectrometer,allowing the best identification of muon tracks. Their efficiency is completed by muon tagging algorithms, which identify inner tracks as muons using raw information from the outer spectrometer; they are very useful for regions with limited detector coverage and for low energy muons. Finally, calorimeter tagging algorithms extract additional muon candidates from their minimum ionization deposits in the calorimeter cells. The performance of all those algorithms both on collisions simulation and cosmic data will be detailed, along with their latest developments aiming at first data at the end of 2009. (author)

[en] Nearly 50 years ago, theoretical physicists proposed that a field permeates the universe and gives energy to the vacuum. This field was required to explain why some, but not all, fundamental particles have mass. Numerous precision measurements during recent decades have provided indirect support for the existence of this field, but one crucial prediction of this theory has remained unconfirmed despite 30 years of experimental searches: the existence of a massive particle, the standard model Higgs boson. The ATLAS experiment at the Large Hadron Collider at CERN has now observed the production of a new particle with a mass of 126 giga-electron volts and decay signatures consistent with those expected for the Higgs particle. This result is strong support for the standard model of particle physics, including the presence of this vacuum field. The existence and properties of the newly discovered particle may also have consequences beyond the standard model itself. (authors)

[en] This Letter presents a search for magnetic monopoles with the ATLAS detector at the CERN Large Hadron Collider using an integrated luminosity of 2.0 fb^-1 of pp collisions recorded at a center-of-mass energy of √s=7 TeV. No event is found in the signal region, leading to an upper limit on the production cross section at 95% confidence level of 1.6 /ε fb for Dirac magnetic monopoles with the minimum unit magnetic charge and with mass between 200 GeV and 1500 GeV, where is the monopole reconstruction efficiency. The efficiency is high and uniform in the fiducial region given by pseudorapidity |η| ≤ 1.37 and transverse kinetic energy 600-700≤E^kin sinθ≤1400 GeV. The minimum value of 700 GeV is for monopoles of mass 200 GeV, whereas the minimum value of 600 GeV is applicable for higher mass monopoles. Therefore, the upper limit on the production cross section at 95% confidence level is 2 fb in this fiducial region. Assuming the kinematic distributions from Drell-Yan pair production of spin-1/2 Dirac magnetic monopoles, the efficiency is in the range 1%-10%, leading to an upper limit on the cross section at 95% confidence level that varies from 145 fb to 16 fb for monopoles with mass between 200 GeV and 1200 GeV. This limit is weaker than the fiducial limit because most of these monopoles lie outside the fiducial region. (authors)