ATLAS

Abstract:

This note provides an interim summary of the current recommendations of the PDF4LHC working group for the use of parton distribution functions (PDFs) and of PDF uncertainties at the LHC, for cross section and cross section uncertainty calculations. It also contains a succinct user guide to the computation of PDF uncertainties and correlations using available PDF sets. A companion note (the PDF4LHC Working Group Interim Report) summarizes predictions for benchmark cross sections at the LHC (7 TeV) at NLO using modern PDFs currently available from 6 PDF fitting groups.

Abstract:

A search for excited leptons is carried out with the CMS detector at the LHC, using 36 pb-1 of pp collision data recorded at s=7 TeV. The search is performed for associated production of a lepton and an oppositely charged excited lepton pp→ℓℓ*, followed by the decay ℓ*→ℓγ, resulting in the ℓℓγ final state, where ℓ=e,μ. No excess of events above the standard model expectation is observed. Interpreting the findings in the context of ℓ* production through four-fermion contact interactions and subsequent decay via electroweak processes, first upper limits are reported for ℓ* production at this collision energy. The exclusion region in the compositeness scale Λ and excited lepton mass Mℓ* parameter space is extended beyond previously established limits. For Λ=Mℓ*, excited lepton masses are excluded below 1070 GeV/c2 for e* and 1090 GeV/c2 for μ* at the 95% confidence level. © 2011 CERN.

Abstract:

A search for new interactions and resonances produced in LHC proton-proton (pp) collisions at a centre-of-mass energy √s = 7 TeV was performed with the ATLAS detector. Using a dataset with an integrated luminosity of 36 pb -1, dijet mass and angular distributions were measured up to dijet masses of ∼3.5TeV and were found to be in good agreement with Standard Model predictions. This analysis sets limits at 95% CL on various models for new physics: an excited quark is excluded for mass between 0.60 and 2.64 TeV, an axigluon hypothesis is excluded for axigluon masses between 0.60 and 2.10 TeV and quantum black holes are excluded in models with six extra space-time dimensions for quantum gravity scales between 0.75 and 3.67 TeV. Production cross section limits as a function of dijet mass are set using a simplified Gaussian signal model to facilitate comparisons with other hypotheses. Analysis of the dijet angular distribution using a novel technique simultaneously employing the dijet mass excludes quark contact interactions with a compositeness scale A below 9.5 TeV. © 2011 CERN for the benefit of the ATLAS Collaboration.

Abstract:

Measurements of primary charged hadron multiplicity distributions are presented for non-single-diffractive events in proton-proton collisions at centre-of-mass energies of √s = 0.9, 2.36, and 7TeV, in five pseudorapidity ranges from |η| < 0.5 to |η| < 2.4. The data were collected with the minimum-bias trigger of the CMS experiment during the LHC commissioning runs in 2009 and the 7TeV run in 2010. The multiplicity distribution at √s = 0.9 TeV is in agreement with previous measurements. At higher energies the increase of the mean multiplicity with √s is underestimated by most event generators. The average transverse momentum as a function of the multiplicity is also presented. The measurement of higher-order moments of the multiplicity distribution confirms the violation of Koba-Nielsen-Olesen scaling that has been observed at lower energies.

Abstract:

The charged particle transverse momentum (pT) spectra are presented for pp collisions at √s = 0:9 and 7TeV. The data samples were collected with the CMS detector at the LHC and correspond to integrated luminosities of 231 μb-1and 2.96 pb-1, respectively. Calorimeter-based high-transverse-energy triggers are employed to enhance the statistical reach of the high-pT measurements. The results are compared with leading and next-toleading order QCD and with an empirical scaling of measurements at different collision energies using the scaling variable xT - 2pT=ps over the pT range up to 136 GeV/c. Using a combination of xT scaling and direct interpolation at fixed pT, a reference transverse momentum spectrum at √s = 2:76TeV is constructed, which can be used for studying high-pT particle suppression in the dense QCD medium produced in heavy-ion collisions at that centre-of-mass energy. Copyright CERN.