Operation and performance of the ATLAS semiconductor tracker

The semiconductor tracker is a silicon microstrip detector forming part of the inner tracking system of the ATLAS experiment at the LHC. The operation and performance of the semiconductor tracker during the first years of LHC running are described. More than 99% of the detector modules were operational during this period, with an average intrinsic hit efficiency of (99.74±0.04)%. The evolution of the noise occupancy is discussed, and measurements of the Lorentz angle, δ-ray production and energy loss presented. The alignment of the detector is found to be stable at the few-micron level over long periods of time. Radiation damage measurements, which include the evolution of detector leakage currents, are found to be consistent with predictions and are used in the verification of radiation background simulations

Measurement of event-shape observables in Z→ℓ+ℓ− events in pp collisions at √ s=7 TeV with the ATLAS detector at the LHC

Event-shape observables measured using charged particles in inclusive $Z$-boson events are presented, using the electron and muon decay modes of the $Z$ bosons. The measurements are based on an integrated luminosity of $1.1 {\rm fb}^{-1}$ of proton--proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy $\sqrt{s}=7$ TeV. Charged-particle distributions, excluding the lepton--antilepton pair from the $Z$-boson decay, are measured in different ranges of transverse momentum of the $Z$ boson. Distributions include multiplicity, scalar sum of transverse momenta, beam thrust, transverse thrust, spherocity, and $\mathcal{F}$-parameter, which are in particular sensitive to properties of the underlying event at small values of the $Z$-boson transverse momentum. The Sherpa event generator shows larger deviations from the measured observables than Pythia8 and Herwig7. Typically, all three Monte Carlo generators provide predictions that are in better agreement with the data at high $Z$-boson transverse momenta than at low $Z$-boson transverse momenta and for the observables that are less sensitive to the number of charged particles in the event.Comment: 36 pages plus author list + cover page (54 pages total), 14 figures, 4 tables, submitted to EPJC, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2014-0

Measurement of elastic J/#psi# photoproduction at HERA

The reaction #gamma# p #-># J/#psi#p has been studied in ep interactions using the ZEUS detector at HERA. The cross section for elastic J/#psi# photoproduction has been measured as a function of the photon-proton centre of mass energy W in the range 40 < W < 140 GeV at a median photon virtuality Q"2 of 5 x 10"-"5 GeV"2. The photoproduction cross section, #sigma#_#gamma#_p_#->#_J_/_#psi#_p, is observed to rise steeply with W. A fit to the data presented in this paper to determine the parameter #delta# in the form #sigma#_#gamma#_p_#->#_J_/_#psi#_p #propor to# W"#delta# yields the value #delta# = 0.92 #+-# 0.14 #+-# 0.10. The differential cross section d#sigma#/d vertical stroke t vertical stroke is presented over the range vertical stroke t vertical stroke < 1.0 GeV"2 where t is the square of the four-momentum exchanged at the proton vertex. d#sigma#/d vertical stroke t vertical stroke falls exponentially with a slope parameter of 4.6 #+-# 0.4"+"0"."4_-_0_._6 GeV"-"2. The measured decay angular distributions are consistent with s-channel helicity conservation. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Charged-particle distributions in pp interactions at √s = 8 TeV measured with the ATLAS detector at the LHC

This paper presents measurements of charged-particle distributions which are produced in proton\u2013proton collisions at a centre-of-mass energy of 1as = 8 TeV and recorded by the ATLAS detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of 160 \u3bcb 121 was used. A minimum-bias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and chargedparticle multiplicity, including measurements of final states at high multiplicity. The results are presented as particle-level distributions to which predictions of various Monte Carlo event generator models are compared