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Altheimer, A.

Publications and source records attributed to Altheimer, A..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Erratum to: Search for single top-quark production via flavour-changing neutral currents at 8 TeV with the ATLAS detector

One correction is noted for the paper. The branching fraction ($\mathcal{B}$($W →{ℓv}$) = 0.3246 was not included in the conversion of the observed cross-section limit, σ x $\mathcal{B}$( $t →Wb$) x ($\mathcal{B}$($W →{ℓv}$) < 2.9to the coupling constants $κ_{ugt}$ and $κ_{cgt}$ and the branching fractions $\mathcal{B}$( $t →ug$) and $\mathcal{B}$( $t →cg$). The inclusion leads to weaker observed exclusion limits on the coupling constants divided by the scale of new physics of $k_{ugt}$/ Λ <10 x 10 -3 TeV -1 and $k_{cgt}$/Λ <23 x 10 -3 TeV -1 and on the branching fractions $\mathcal{B}$( $t →ug$) < 1.2 x10 -4 and $\mathcal{B}$( $t →cg$) < 6.4 x 10 -4 . The predicted exclusion limits on the coupling constants divided by the scale of new physics are $k_{ugt}$/ Λ < 9.5 x 10 -3 TeV -1 and $\mathcal{B}$( $t →cg$)/ Λ < 22 x 10 -3 TeV -1 and on the branching fractions $\mathcal{B}$( $t →ug$) < 1.1 x 10 -4 and $\mathcal{B}$( $t →cg$) < 5.7 x 10 -4 . Updated distributions of the observed upper limits on the coupling constants for combinations of cgt and ugt channels are shown in Figure 10a and on the branching fractions in Figure 10b.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1

The reconstruction of the signal from hadrons and jets emerging from the proton–proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Lastly, topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

Addendum to ‘Measurement of the $t\bar{t}$ production cross-section using $e\mu $ events with b-tagged jets in pp collisions at $\sqrt{s}$ = 7 and 8 $\,\mathrm{TeV}$with the ATLAS detector’

The ATLAS measurement of the inclusive top quark pair ($t\bar{t}$) cross-section σ$t\bar{t}$ in proton-proton collisions at √s = 8 TeV has been updated using the final 2012 luminosity calibration. The updated cross-section result is: σ $t\bar{t}$ = 242.9 ± 1.7 ± 5.5 ± 5.1 ± 4.2 pb, where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, knowledge of the integrated luminosity and of the LHC beam energy. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. The measurement of the ratio of $t\bar{t}$ cross-sections at √s = 8 TeV and √s = 7 TeV, and the √s = 8 TeV fiducial measurement corresponding to the experimental acceptance of the leptons, have also been updated.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Performance of pile-up mitigation techniques for jets in pp collisions at √s=8 TeV using the ATLAS detector

The large rate of multiple simultaneous proton–proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 fb -1 data sample collected at a centre-of-mass energy of $\sqrt{s}$ = 8TeV. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at $\sqrt{s}=8\,~{\mathrm {TeV}}$ using the ATLAS detector

Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, t$\bar{t}$ system and event-level kinematic observables in proton–proton collisions at a centre-of-mass energy of √s=8TeV. The observables have been chosen to emphasize the t$\bar{t}$ production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb - 1 , recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Identification of high transverse momentum top quarks in pp collisions at $ \sqrt{s}=8 $ TeV with the ATLAS detector

This study presents studies of the performance of several jet-substructure techniques, which are used to identify hadronically decaying top quarks with high transverse momentum contained in large-radius jets. The efficiency of identifying top quarks is measured using a sample of top-quark pairs and the rate of wrongly identifying jets from other quarks or gluons as top quarks is measured using multijet events collected with the ATLAS experiment in 20.3 fb -1 of 8 TeV proton-proton collisions at the Large Hadron Collider. Predictions from Monte Carlo simulations are found to provide an accurate description of the performance. The techniques are compared in terms of signal efficiency and background rejection using simulations, covering a larger range in jet transverse momenta than accessible in the dataset. Finally and additionally, a novel technique is developed that is optimized to reconstruct top quarks in events with many jets.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Measurement of D*±, D± and D s ± meson production cross sections in pp collisions at s = 7 TeV with the ATLAS detector

The production of D *± , D ± and D $±\atop{s}$charmed mesons has been measured with the ATLAS detector in pp collisions at √s = 7 TeV at the LHC, using data corresponding to an integrated luminosity of 280 nb -1 . The charmed mesons have been reconstructed in the range of transverse momentum 3.5 T (D)<100 GeV and pseudorapidity |η(D)|<2.1. The differential cross sections as a function of transverse momentum and pseudorapidity were measured for D *± and D ± production. The next-to-leading-order QCD predictions are consistent with the data in the visible kinematic region within the large theoretical uncertainties. Using the visible D cross sections and an extrapolation to the full kinematic phase space, the strangeness-suppression factor in charm fragmentation, the fraction of charged non-strange D mesons produced in a vector state, and the total cross section of charm production at √s = 7 TeV were derived.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Reconstruction of hadronic decay products of tau leptons with the ATLAS experiment

This document presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of Z → ττ and Z(→ μμ)+jets events selected from proton–proton collisions at a centre-of-mass energy √s = 8 TeV, corresponding to an integrated luminosity of 5 fb -1 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Measurement of the transverse momentum and Φ$*\atop{η}$ distributions of Drell–Yan lepton pairs in proton–proton collisions at √s=8 TeV with the ATLAS detector

Distributions of transverse momentum p$ℓℓ\atop{T}$ and the related angular variable Φ$*\atop{η}$ of DrellΓÇôYan lepton pairs are measured in 20.3$\perp$áfb -1 of protonΓÇôproton collisions at √s=8$\perp$áTeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in protonΓÇôproton collisions at √s=7$\perp$áTeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of Φ$*\atop{η}$<1 the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of Φ$*\atop{η}$ this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of pℓℓTpTℓℓ while the fixed-order prediction of Dynnlo falls below the data at high values of p$ℓℓ\atop{T}$ . ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the Φ$*\atop{η}$ and p$ℓℓ\atop{T}$ distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Measurement of the differential cross-sections of prompt and non-prompt production of J / ψ and ψ (2S) in pp collisions at √s = 7 and 8 TeV with the ATLAS detector

The production rates of prompt and non-prompt J/ψ and ψ(2S) mesons in their dimuon decay modes are measured using 2.1 and 11.4 fb -1 of data collected with the ATLAS experiment at the Large Hadron Collider, in proton–proton collisions at √s=7 and 8 respectively. Production cross-sections for prompt as well as non-prompt sources, ratios of ψ(2S) to J/ψ production, and the fractions of non-prompt production for J/ψ and ψ(2S) are measured as a function of meson transverse momentum and rapidity. Lastly, the measurements are compared to theoretical predictions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Search for the standard model Higgs boson produced in association with a vector boson and decaying into a tau pair in p p collisions at s = 8 TeV with the ATLAS detector

A search for the standard model Higgs boson produced in association with a vector boson with the decay H → τ τ is presented. The data correspond to 20.3 fb -1 of integrated luminosity from proton-proton collisions at √ s = 8 TeV recorded by the ATLAS experiment at the LHC during 2012.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

A new method to distinguish hadronically decaying boosted Z bosons from W bosons using the ATLAS detector

The distribution of particles inside hadronic jets produced in the decay of boosted W and Z bosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W or Z boson, this paper presents a technique for further differentiating Z bosons from W bosons. The variables used are jet mass, jet charge, and a b-tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of W' → WZ for bosons in the transverse momentum range 200 GeV < p T < 400 GeV in s= 8 TeV pp collisions with the ATLAS detector at the LHC. For Z-boson tagging efficiencies of ϵ Z = 90 , 50, and 10 % , one can achieve W + -boson tagging rejection factors (1 / ϵ W+ ) of 1.7, 8.3 and 1000, respectively. It is not possible to measure these efficiencies in the data due to the lack of a pure sample of high p T , hadronically decaying Z bosons. However, the modelling of the tagger inputs for boosted W bosons is studied in data using a tt¯ -enriched sample of events in 20.3 fb - 1 of data at √s= 8 TeV. The inputs are well modelled within uncertainties, which builds confidence in the expected tagger performance.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗