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Altheimer, A.

Publications and source records attributed to Altheimer, A..

At least 127 records · Page 7
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DOE OSTI2015

Search for New Phenomena in Dijet Angular Distributions in Proton-Proton Collisions at s = 8 TeV Measured with the ATLAS Detector

A search for new phenomena in LHC proton-proton collisions at a center-of-mass energy of √s=8 TeV was performed with the ATLAS detector using an integrated luminosity of 17.3 fb⁻¹. The angular distributions are studied in events with at least two jets; the highest dijet mass observed is 5.5 TeV. All angular distributions are consistent with the predictions of the standard model. In a benchmark model of quark contact interactions, a compositeness scale below 8.1 TeV in a destructive interference scenario and 12.0 TeV in a constructive interference scenario is excluded at 95% C.L.; median expected limits are 8.9 TeV for the destructive interference scenario and 14.1 TeV for the constructive interference scenario.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Observation and measurements of the production of prompt and non-prompt ${\text {J}\psi }$ mesons in association with a ${Z}$ boson in ${pp}$ p p collisions at ${\sqrt{s}= 8\,\text {TeV}}$ with the ATLAS detector

The production of a Z boson in association with a J/ψ meson in proton–proton collisions probes the production mechanisms of quarkonium and heavy flavour in association with vector bosons, and allows studies of multiple parton scattering. Using 20.3 fb -1 of data collected with the ATLAS experiment at the LHC in pp collisions at √s=8TeV, the first measurement of associated Z + J/ψ production is presented for both prompt and non-prompt J/ψ production, with both signatures having a significance in excess of 5σ. The inclusive production cross-sections for Z boson production (analysed in μ + μ - or e + e - decay modes) in association with prompt and non-prompt J/ψ( → μ + μ - ) are measured relative to the inclusive production rate of Z bosons in the same fiducial volume to be (36.8 ± 6.7 ± 2.5) × 10 -7 and (65.8 ± 9.2 ± 4.2) × 10 -7 respectively. Normalised differential production cross-section ratios are also determined as a function of the Jψ transverse momentum. As a result, the fraction of signal events arising from single and double parton scattering is estimated, and a lower limit of 5.3 (3.7) mb at 68 (95)% confidence level is placed on the effective cross-section regulating double parton interactions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Determination of spin and parity of the Higgs boson in the $WW^*\rightarrow e \nu \mu \nu $ decay channel with the ATLAS detector

Research of the spin and parity quantum numbers of the Higgs boson in the WW* → eνμν final state are presented, based on proton–proton collision data collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb –1 at a centre-of-mass energy of √s=8 TeV. The Standard Model spin-parity J CP =0 ++ hypothesis is compared with alternative hypotheses for both spin and CP. The case where the observed resonance is a mixture of the Standard-Model-like Higgs boson and CP-even (J CP =0 ++ ) or CP-odd (J CP =0 +– ) Higgs boson in scenarios beyond the Standard Model is also studied. The data are found to be consistent with the Standard Model prediction and limits are placed on alternative spin and CP hypotheses, including CP mixing in different scenarios.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Measurement of the charge asymmetry in dileptonic decays of top quark pairs in pp collisions at $ \sqrt{s}=7 $ TeV using the ATLAS detector

A measurement of the top-antitop (t$\bar{t}$) charge asymmetry is presented using data corresponding to an integrated luminosity of 4.6 fb -1 of LHC pp collisions at a centreof-mass energy of 7 TeV collected by the ATLAS detector. Events with two charged leptons, at least two jets and large missing transverse momentum are selected. Two observables are studied: A C ℓℓ based on the identified charged leptons, and A$t\bar{t}\atop{C}$, based on the reconstructed tt final state. The asymmetries are measured to be A $ℓℓ\atop{C}$ =0.024 ± 0.015 (stat.) ± 0.009 (syst.), A $t\bar{t}\atop{C}$ = 0.021 ± 0.025 (stat.) ± 0.017 (syst.). The measured values are in agreement with the Standard Model predictions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for $W' \rightarrow tb \rightarrow qqbb$ decays in $pp$ p p collisions at $\sqrt{s}$ TeV with the ATLAS detector

A search for a massive W' gauge boson decaying to a top quark and a bottom quark is performed with the ATLAS detector in pp collisions at the LHC. The dataset was taken at a centre-of-mass energy of √s=8 TeV and corresponds to 20.3 fb –1 of integrated luminosity. This analysis is done in the hadronic decay mode of the top quark, where novel jet substructure techniques are used to identify jets from high-momentum top quarks. This allows for a search for high-mass W' bosons in the range 1.5–3.0 TeV. b-tagging is used to identify jets originating from b-quarks. The data are consistent with Standard Model background-only expectations, and upper limits at 95% confidence level are set on the W' → tb cross section times branching ratio ranging from 0.16 pb to 0.33 pb for left-handed W' bosons, and ranging from 0.10 pb to 0.21 pb for W' bosons with purely right-handed couplings. As a result, upper limits at 95% confidence level are set on the W'-boson coupling to tb as a function of the W' mass using an effective field theory approach, which is independent of details of particular models predicting a W' boson.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Measurement of the top-quark mass in the fully hadronic decay channel from ATLAS data at $\sqrt{s}=7\mathrm{\,TeV}$

In this study, the mass of the top quark is measured in a data set corresponding to 4.6 fb -1 of proton–proton collisions with centre-of-mass energy √s=7 TeV collected by the ATLAS detector at the LHC. Events consistent with hadronic decays of top–antitop quark pairs with at least six jets in the final state are selected. The substantial background from multijet production is modelled with data-driven methods that utilise the number of identified b-quark jets and the transverse momentum of the sixth leading jet, which have minimal correlation. The top-quark mass is obtained from template fits to the ratio of three-jet to dijet mass. The three-jet mass is calculated from the three jets produced in a top-quark decay. Using these three jets the dijet mass is obtained from the two jets produced in the W boson decay. The top-quark mass obtained from this fit is thus less sensitive to the uncertainty in the energy measurement of the jets. A binned likelihood fit yields a top-quark mass of m t =175.1±1.4(stat.) ±1.2(syst.) GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at $\sqrt{s}=8 $ TeV with the ATLAS detector

The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. The search is based on proton-proton collision data at a centre-of-mass energy √ = 8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb -1 . No significant excess above the Standard Model expectation is observed. Limits are set on supersymmetric particle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. Limits are also set on the parameters of a minimal universal extra dimension model, excluding a compactification radius of 1/R c = 950 GeV for a cut-off scale times radius (ΛR c ) of approximately 30.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Measurement of Spin Correlation in Top-Antitop Quark Events and Search for Top Squark Pair Production in p p Collisions at s = 8 TeV Using the ATLAS Detector

A measurement of spin correlation in tt¯ production is presented using data collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb⁻¹. The correlation between the top and antitop quark spins is extracted from dilepton tt¯ events by using the difference in the azimuthal angle between the two charged leptons in the laboratory frame. In the helicity basis the measured degree of correlation corresponds to A helicity =0.38±0.04, in agreement with the standard model prediction. A search is performed for pair production of top squarks with masses close to the top quark mass decaying to predominantly right-handed top quarks and a light neutralino, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 191 GeV are excluded at the 95% confidence level.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for charged Higgs bosons decaying via H ± → τ ± ν in fully hadronic final states using pp collision data at s $$ \sqrt{s} $$ = 8 TeV with the ATLAS detector

The results of a search for charged Higgs bosons decaying to a τ lepton and a neutrino, H ± → τ ± ν, are presented. The analysis is based on 19.5 fb –1 of proton-proton collision data at √s = 8 TeV collected by the ATLAS experiment at the Large Hadron Collider. Charged Higgs bosons are searched for in events consistent with top-quark pair production or in associated production with a top quark, depending on the considered H ± mass. The final state is characterized by the presence of a hadronic τ decay, missing transverse momentum, b-tagged jets, a hadronically decaying W boson, and the absence of any isolated electrons or muons with high transverse momenta. The data are consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the product of branching ratios Β(t → bH ± ) × Β(H ± → τ ± ν), between 0.23% and 1.3% for charged Higgs boson masses in the range 80-160GeV. It also leads to 95% confidence-level upper limits on the production cross section times branching ratio, σ(pp → tH ± + X) × Β(H ± → τ ± ν), between 0.76 pb and 4.5 fb, for charged Higgs boson masses ranging from 180 GeV to 1000 GeV. In the context of different scenarios of the Minimal Supersymmetric Standard Model, these results exclude nearly all values of tan β above one for charged Higgs boson masses between 80 GeV and 160 GeV, and exclude a region of parameter space with high tan β for H ± masses between 200 GeV and 250 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for anomalous production of prompt same-sign lepton pairs and pair-produced doubly charged Higgs bosons with $ \sqrt{s}=8 $ TeV pp collisions using the ATLAS detector

A low-background inclusive search for new physics in events with same-sign dileptons is presented. The search uses proton-proton collisions corresponding to 20.3 fb -1 of integrated luminosity taken in 2012 at a centre-of-mass energy of 8 TeV with the ATLAS detector at the LHC. Pairs of isolated leptons with the same electric charge and large transverse momenta of the type e ± e ± , e ± μ ± , and μ ± μ ± are selected and their invariant mass distribution is examined. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the cross-sections for processes beyond the Standard Model. Limits are placed as a function of the dilepton invariant mass within a fiducial region corresponding to the signal event selection criteria. Exclusion limits are also derived for a specific model of doubly charged Higgs boson production.[Figure not available: see fulltext.]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗