M-L. Andrieux

Measurement of the cross section for the production of a W boson in association with b-jets in pp collisions at root s=7 TeV with the ATLAS detector

A measurement is presented of the cross section for the produ cti n of aW boson with one or two jets, of which at least one must be a b-jet, in pp collisions at √ s = 7 TeV. Production via top decay is not included in the signal definition. The measurement is based on 35 pb −1of data collected with the ATLAS detector at the LHC. The W+b-jet cross section is defined for jets reconstructed with the anti -kt clustering algorithm with transverse momentum above 25 GeV and rapidity within±2.1. Theb-jets are identified by reconstructing secondary vertices. The fiducial cross section is measured both for the electron and muon decay chan nel of theW boson and is found to be 10.2 ± 1.9 (stat) ± 2.6 (syst) pb for one lepton flavour. The results are compared with next-to-leading order QCD calculations, which predict a cross section smaller than, though consistent wit h, the measured value.

Radiation qualification of the front-end electronics for the readout of the ATLAS liquid argon calorimeters

The ATLAS detector has been built to study the reactions produced by the Large Hadron Collider (LHC). ATLAS includes a system of liquid argon calorimeters for energy measurements. The electronics for amplifying, shaping, sampling, pipelining, and digitizing the calorimeter signals is implemented on a set of front-end electronic boards. The front-end boards are installed in crates mounted between the calorimeters, where they will be subjected to significant levels of radiation during LHC operation. As a result, all components used on the front-end boards had to be subjected to an extensive set of radiation qualification tests. This paper describes radiation-tolerant designs, radiation testing, and radiation qualification of the front-end readout system for the ATLAS liquid argon calorimeters.

Construction and test of the first two sectors of the ATLAS barrel liquid argon presampler

The electromagnetic (e.m.) calorimeter of the ATLAS experiment for the Large Hadron Collider will be a sampling liquid argon accordion calorimeter. To achieve sufficient energy resolution, it is necessary to correct for the energy loss in the material upstream of the calorimeter. For this purpose, a separate presampler detector fixed on the inner face of the e.m. calorimeter, in the same cryostat, is being built by the ATLAS collaboration. Two presampler sectors have already been built and tested in their final version. The geometry of the detector and the various steps of its construction are reviewed. The hardware performance of the detector measured both at CERN and at the ISN-Grenoble (specific test bench) is discussed. It is concluded that the presampler will adequately fulfill its role for future operation at the CERN Large Hadron Collider.

Response of an α source mounted in a liquid argon ionization cell and read out in full charge collection mode

Abstract The total charge response curve of an 241 Am source placed in a liquid argon ionization cell has been measured. In a pure liquid argon sample (O 2 concentration 0.1 ppm), it can be described by a semi-empirical formula which is derived from a model formerly elaborated by Thomas and Imel. The validity of this new formula was also tested in the case of oxygen-polluted liquid argon. The agreement with the data seems to indicate that the initial ion–electron pair recombination factor is not affected when the O 2 concentration is smaller than 7 ppm. Further use of our recombination factor parameterization has been made to analyze data of liquid argon purity monitors utilizing 241 Am sources with a sensitivity to O 2 concentrations of ∼ 0.1 ppm.

Pollution of liquid argon after neutron irradiation

Abstract The purpose of the neutron facility installed at SARA is to investigate the behavior of various materials to be used in the ATLAS liquid argon calorimeter, when submitted to fast neutron radiation. The samples are placed in a liquid argon cryostat a few cm away from the neutron source. Various pieces of the electromagnetic calorimeter have been tested in order to evaluate the rate of pollution of the liquid and consequently the possible signal loss in energy measurements. The average fluence was equivalent to the maximum expected in the calorimeter in about 10 years. The most striking feature of the results is that the pollution is not due to oxygen, at least for most of it. Using a particular value of the absorption length derived from these data, a simulation was carried out and the energy signal loss in the calorimeter could be predicted. Within the limits of our present knowledge, the conclusion is that damages due to this pollution will not be a problem.

Redundancy or GaAs? Two different approaches to solve the problem of SEU (Single Event Upset) in a digital optical link

The fast digital optical links for the ATLAS Liquid Argon Calorimeter must survive in a high radiation environment with a total fluence of 3x10 neutrons (1MeV Si)/cm and 10 kGy (Si). The links based on Agilent Technologies Glink serializer/deserializer set, show a total dose radiation resistance to neutrons and gammas that would allow for 10 years of operation in the ATLAS detector. We have observed, however, an unacceptable rate of single event upsets (SEU) due to neutrons interacting in the silicon-based serializer. In order to solve this problem, we have developed two link systems. The first one, Dual-Glink, is based on a principle of redundancy: data are sent on two independent links. On the reception side, data are analyzed and error recovery is performed without dead time. The second solution uses a GaAs serializer/deserializer set from TriQuint. We observe a very small number of SEU’s. In addition, high speed of 2.5 Gb/s allows for transmission of the data twice during one event period and for error recovery. The design of the 3 types of links, their performance in the laboratory and the results of the radiation tests are presented for all systems.

The ring imaging Cherenkov detectors of DELPHI

A ring imaging Cherenkov (RICH) detector system has been built and is now in full operation within the DELPHI experiment. Large data samples of Z/sup 0/ decays are being collected with good resolution on the observed Cherenkov angles. Several studies of Z/sup 0/ decays using the RICH have already been performed on limited samples. Disturbance of the detector operation caused by shrinkage of polymeric construction materials and by migration of radiator substance is reported. These effects have been counteracted and do not endanger the quality of the data. >

Search for the Standard Model Higgs boson in the decay channel H→ZZ(*)→4ℓ with 4.8 fb-1of pp collision data at √s=7 TeV with ATLAS

This Letter presents a search for the Standard Model Higgs boson in the decay channel H → ZZ → lll ′+l ′ −, where l, l ′ = e or μ, using proton-proton collisions at √ s = 7 TeV recorded with the ATLAS detector and corresponding to an integrated luminosity of 4.8 fb. The four-lepton invariant mass distribution is compared with Standard Model background expectations to derive upper limits on the cross section of a Standard Model Higgs boson with a mass between 110 GeV and 600 GeV. The mass ranges 134−156 GeV, 182−233 GeV, 256−265 GeV and 268−415 GeV are excluded at the 95% confidence level. The largest upward deviations from the background-only hypothesis are observed for Higgs boson masses of 125 GeV, 244 GeV and 500 GeV with local significances of 2.1, 2.2 and 2.1 standard deviations, respectively. Once the look-elsewhere effect is considered, none of these excesses are significant.

Measurement of the W→τντcross section in pp collisions at s=7 TeV with the ATLAS experiment

The cross section for the production of W bosons with subsequent decay W → τντ is measured with the ATLAS detector at the LHC. The analysis is based on a data sample that was recorded in 2010 at a proton-proton center-of-mass energy of √ s = 7 TeV and corresponds to an integrated luminosity of 34 pb. The cross section is measured in a region of high detector acceptance and then extrapolated to the full phase space. The product of the total W production cross section and the W → τντ branching ratio is measured to be σ W→τντ = 11.1± 0.3 (stat)± 1.7 (syst)± 0.4 (lumi) nb.The cross section for the production of W bosons with subsequent decay W→τν_τ is measured with the ATLAS detector at the LHC. The analysis is based on a data sample that was recorded in 2010 at a proton–proton center-of-mass energy of √s = 7TeV and corresponds to an integrated luminosity of 34 pb^(−1). The cross section is measured in a region of high detector acceptance and then extrapolated to the full phase space. The product of the total W production cross section and the W→τν_τ branching ratio is measured to be σ^(tot) _(W→τντ) = 11.1±0.3 (stat)±1.7 (syst)±0.4 (lumi) nb.

Search for diphoton events with large missing transverse momentum in 1 fb-1of 7 TeV proton-proton collision data with the ATLAS detector

CERN-PH-EP-2011-160 A search for diphoton events with large missing transverse momentum has been performed using 1.07 fb of protonproton collision data at √ s = 7TeV recorded with the ATLAS detector. No excess of events was observed above the Standard Model prediction and 95% Confidence Level (CL) upper limits are set on the production cross section for new physics. The limits depend on each model parameter space and vary as follows: σ < (22 − 129) fb in the context of a generalised model of gauge-mediated supersymmetry breaking (GGM) with a bino-like lightest neutralino, σ < (27−91) fb in the context of a minimal model of gauge-mediated supersymmetry breaking (SPS8), and σ < (15−27) fb in the context of a specific model with one universal extra dimension (UED). A 95% CL lower limit of 805GeV, for bino masses above 50GeV, is set on the GGM gluino mass. Lower limits of 145TeV and 1.23TeV are set on the SPS8 breaking scale Λ and on the UED compactification scale 1/R, respectively. These limits provide the most stringent tests of these models to date.