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Ahmad, A.

Publications and source records attributed to Ahmad, A..

29 records ยท Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Measurements of electroweak W ยฑ Z boson pair production in association with two jets in pp collisions at $ \sqrt{s}$ = 13 TeV with the ATLAS detector

Measurements of integrated and diferential cross-sections for electroweak W ยฑ Z production in association with two jets (W ยฑ Zjj) in proton-proton collisions are presented. The data collected by the ATLAS detector at the Large Hadron Collider from 2015 to 2018 at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV are used, corresponding to an integrated luminosity of 140 fb -1 . The W ยฑ Zjj candidate events are reconstructed using leptonic decay modes of the gauge bosons. Events containing three identifed leptons, either electrons or muons, and two jets are selected. Processes involving pure electroweak W ยฑ Zjj production at Born level are separated from W ยฑ Zjj production involving a strong coupling. The measured integrated fducial cross-section of electroweak W ยฑ Zjj production per lepton favour is ฯƒ W Zjj-EWโ†’โ„“' ฮฝโ„“โ„“jj = 0.368 ยฑ 0.037 (stat.) ยฑ 0.059 (syst.) ยฑ 0.003 (lumi.) fb, where โ„“ and โ„“' are either an electron or a muon. Respective cross-sections of electroweak and strong W ยฑ Zjj production are measured separately for events with exactly two jets or with more than two jets, and in three bins of the invariant mass of the two jets. The inclusive W ยฑ Zjj production cross-section, without separating electroweak and strong production, is also measured to be ฯƒ W Zjjโ†’โ„“ ' ฮฝโ„“โ„“jj = 1.462 ยฑ 0.063 (stat.) ยฑ 0.118 (syst.) ยฑ 0.012 (lumi.) fb, per lepton favour. The inclusive W ยฑ Zjj production cross-section is measured diferentially for several kinematic observables. Finally, the measurements are used to constrain anomalous quartic gauge couplings by extracting 95% confdence level intervals on dimension-8 operators.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSโ†—
DOE OSTI2024

Measurement of jet substructure in boosted $t\overline{t}$ events with the ATLAS detector using 140 fb -1 of 13 TeV $pp$ collisions

Measurements of the substructure of top-quark jets are presented, using 140 fb -1 of 13 TeV pp collision data recorded with the ATLAS detector at the LHC. Top-quark jets reconstructed with the anti-k t algorithm with a radius parameter R = 1.0 are selected in top-quark pair ($t\overline{t}$) events where one top quark decays semileptonically and the other hadronically, or where both top quarks decay hadronically. The top-quark jets are required to have transverse momentum p T > 350 GeV, yielding large samples of data events with jet p T values between 350 and 600 GeV. One- and two-dimensional differential cross sections for eight substructure variables, defined using only the charged components of the jets, are measured in a particle-level phase space by correcting for the smearing and acceptance effects induced by the detector. The differential cross sections are compared with the predictions of several Monte Carlo simulations in which top-quark pair-production quantum chromodynamic matrix-element calculations at next-to-leading-order precision in the strong coupling constant ฮฑ S are passed to leading-order parton shower and hadronization generators. The Monte Carlo predictions for measures of the broadness, and also the two-body structure, of the top-quark jets are found to be in good agreement with the measurements, while variables sensitive to the three-body structure of the top-quark jets exhibit some tension with the measured distributions.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND โ†—
DOE OSTI2024

Search for pair production of higgsinos in events with two Higgs bosons and missing transverse momentum in $\sqrt{๐‘ }$ = 13 TeV ๐‘โข๐‘ collisions at the ATLAS experiment

This paper presents a search for pair production of higgsinos, the supersymmetric partners of the Higgs bosons, in scenarios with gauge-mediated supersymmetry breaking. Each higgsino is assumed to decay into a Higgs boson and a nearly massless gravitino. The search targets events where each Higgs boson decays into $b\bar{b}$, leading to a reconstructed final state with at least three energetic ๐‘-jets and missing transverse momentum. Two complementary analysis channels are used, with each channel specifically targeting either low or high values of the higgsino mass. The low-mass (high-mass) channel exploits 126 (139) fb โˆ’1 of $\sqrt{๐‘ }$ = 13 TeV data collected by the ATLAS detector during Run 2 of the Large Hadron Collider. No significant excess above the Standard Model prediction is found. At 95% confidence level, masses between 130 GeV and 940 GeV are excluded for higgsinos decaying exclusively into Higgs bosons and gravitinos. Exclusion limits as a function of the higgsino decay branching ratio to a Higgs boson are also reported.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSโ†—
DOE OSTI2024

Search for singly produced vectorlike top partners in multilepton final states with 139 fb โˆ’1 of ๐‘โข๐‘ collision data at โˆš๐‘  =13 TeV with the ATLAS detector

A search for the single production of a vectorlike top partner (๐‘‡) with mass greater than 1 TeV decaying into a ๐‘ boson and a top quark is presented, using the full Run 2 dataset corresponding to 139 fb โˆ’1 of ๐‘โข๐‘ collisions at โˆš๐‘  =13 TeV, collected in 2015โ€“2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a pair of electrons or muons with opposite-sign charges which form a ๐‘-boson candidate, as well as by the presence of ๐‘-tagged jets and forward jets. Events with exactly two or at least three leptons are categorized into two independently optimized analysis channels. No significant excess above the background expectation is observed and the results from the two channels are statistically combined to set exclusion limits at 95% confidence level on the masses and couplings of ๐‘‡. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength (๐œ…) of the vectorlike quark. For singlet ๐‘‡ quarks, ๐œ… values between 0.22 and 0.64 are excluded for masses between 1000 and 1975 GeV. For ๐‘‡ quarks in the doublet scenario, where the production cross section is much lower, ๐œ… values between 0.54 and 0.88 are excluded for masses between 1000 and 1425 GeV.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND โ†—
DOE OSTI2024

Search for high-mass resonances in final states with a ฯ„-lepton and missing transverse momentum with the ATLAS detector

A search for high-mass resonances decaying into a ฯ„-lepton and a neutrino using proton-proton collisions at a center-of-mass energy of $\sqrt{s}$ =13 TeV is presented. The full run 2 data sample corresponding to an integrated luminosity of 139 fb -1 recorded by the ATLAS experiment in the years 2015โ€“2018 is analyzed. The ฯ„-lepton is reconstructed in its hadronic decay modes and the total transverse momentum carried out by neutrinos is inferred from the reconstructed missing transverse momentum. The search for new physics is performed on the transverse mass between the ฯ„-lepton and the missing transverse momentum. No excess of events above the Standard Model expectation is observed and upper exclusion limits are set on the W' โ†’ฯ„โขv production cross section. Heavy W' vector bosons with masses up to 5.0 TeV are excluded at 95% confidence level, assuming that they have the same couplings as the Standard Model W boson. For nonuniversal couplings, W' bosons are excluded for masses less than 3.5โ€“5.0 TeV, depending on the model parameters. In addition, model-independent limits on the visible cross section times branching ratio are determined as a function of the lower threshold on the transverse mass of the ฯ„-lepton and missing transverse momentum.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSโ†—
DOE OSTI2024

Search for Nearly Mass-Degenerate Higgsinos Using Low-Momentum Mildly Displaced Tracks in ๐‘โข๐‘ Collisions at $\sqrt{s}$ = 13 TeV with the ATLAS Detector

Higgsinos with masses near the electroweak scale can solve the hierarchy problem and provide a dark matter candidate, while detecting them at the LHC remains challenging if their mass splitting is ๐’ชโก(1 GeV). This Letter presents a novel search for nearly mass-degenerate Higgsinos in events with an energetic jet, missing transverse momentum, and a low-momentum track with a significant transverse impact parameter using 140 fb โˆ’1 of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the ATLAS experiment. For the first time since LEP, a range of mass splittings between the lightest charged and neutral Higgsinos from 0.3 to 0.9 GeV is excluded at 95% confidence level, with a maximum reach of approximately 170 GeV in the Higgsino mass.

Hadron collidersโ†—
DOE OSTI2024

Measurement of t -channel production of single top quarks and antiquarks in pp collisions at 13 TeV using the full ATLAS Run 2 data sample

The production of single top quarks and top antiquarks via the t-channel exchange of a virtual W boson is measured in proton-proton collisions at a centre-of-mass energy of 13 TeV at the LHC using 140 fb -1 of ATLAS data. The total cross-sections are determined to be ฯƒ(tq) = 137$^{+8}_{-8}$ pb and ฯƒ($\bar{t}q$) = 84$^{+6}_{-5}$ pb for top-quark and top-antiquark production, respectively. The combined cross-section is found to be ฯƒ(tq + $\bar{t}q$) = 221$^{+13}_{-13}$ pb and the cross-section ratio is R t = ฯƒ(tq) / ฯƒ($\bar{t}q$) = 1.636$^{+0.036}_{-0.034}$. The predictions at next-to-next-to-leading-order in quantum chromodynamics are in good agreement with these measurements. The predicted value of R t using different sets of parton distribution functions is compared with the measured value, demonstrating the potential to further constrain the functions when using this result in global fits. The measured cross-sections are interpreted in an effective field theory approach, setting limits at the 95% confidence level on the strength of a four-quark operator and an operator coupling the third quark generation to the Higgs boson doublet: -0.37 < C$^{3,1}_{Qq}$ /ฮ› 2 < 0.06 and -0.87 < C$^{3}_{ฮฆQ}$ / ฮ› 2 < 1.42. The constraint |V tb | > 0.95 at the 95% confdence level is derived from the measured value of ฯƒ(tq + $\bar{t}q$), assuming that the Wtb interaction is a left-handed weak coupling and that |V tb | $\gg$ |V td |, |V ts |. In a more general approach, pairs of CKM matrix elements involving top quarks are simultaneously constrained, leading to confdence contours in the corresponding two-dimensional parameter spaces.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSโ†—
DOE OSTI2024

Search for new particles in final states with a boosted top quark and missing transverse momentum in proton-proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

A search for events with one top quark and missing transverse momentum in the final state is presented. The fully hadronic decay of the top quark is explored by selecting events with a reconstructed boosted top-quark topology produced in association with large missing transverse momentum. The analysis uses 139 fb -1 of proton-proton collision data at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV recorded during 2015-2018 by the ATLAS detector at the Large Hadron Collider. The results are interpreted in the context of simplified models for Dark Matter particle production and the single production of a vector-like T quark. Without significant excess relative to the Standard Model expectations, 95% confidence-level upper limits on the corresponding cross-sections are obtained. The production of Dark Matter particles in association with a single top quark is excluded for masses of a scalar (vector) mediator up to 4.3 (2.3) TeV, assuming m ฯ‡ = 1 GeV and the model couplings ฮป q = 0.6 and ฮป ฯ‡ = 0.4 (a = 0.5 and g ฯ‡ = 1). The production of a single vector-like T quark is excluded for masses below 1.8 TeV assuming a coupling to the top quark ฮบ T = 0.5 and a branching ratio for T โ†’ Zt of 25%.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSโ†—
DOE OSTI2024

Azimuthal Angle Correlations of Muons Produced via Heavy-Flavor Decays in 5.02 TeV Pb+Pb and pp Collisions with the ATLAS Detector

Angular correlations between heavy quarks provide a unique probe of the quark-gluon plasma created in ultrarelativistic heavy-ion collisions. Results are presented of a measurement of the azimuthal angle correlations between muons originating from semileptonic decays of heavy quarks produced in 5.02 TeV Pb+Pb and pp collisions at the LHC. The muons are measured with transverse momenta and pseudorapidities satisfying p$^ฮผ_T$ > 4 GeV and |ฮท ฮผ | < 2.4, respectively. The distributions of azimuthal angle separation ฮ”โขฮฆ for muon pairs having pseudorapidity separation |ฮ”ฮท| > 0.8, are measured in different Pb +Pb centrality intervals and compared to the same distribution measured in pp collisions at the same center-of-mass energy. Results are presented separately for muon pairs with opposite-sign charges, same-sign charges, and all pairs. A clear peak is observed in all ฮ”โขฮฆ distributions at ฮ”โขฮฆ ~ ฯ€, consistent with the parent heavy-quark pairs being produced via hard-scattering processes. The widths of that peak, characterized using Cauchy-Lorentz fits to the ฮ”โขฮฆ distributions, are found to not vary significantly as a function of Pb +Pb collision centrality and are similar for pp and Pb +Pb collisions. This observation will provide important constraints on theoretical descriptions of heavy-quark interactions with the quark-gluon plasma.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSโ†—
NASA NTRS1991

The Small Explorer power system electronics

The power system electronics for the NASA Goddard Space Flight Center Small Explorer Satellites are intended to satisfy various planned missions. The selected topology is a direct energy transfer (DET) system with the battery connected directly to the bus. The shunt control technique is a linear sequential full shunt which provides a simple solar array interface and can support both 3 axis stabilized and spinner satellites. In addition, it can meet stringent electromagnetic interference requirements which are expected on some Small Explorer Missions. The Power Systems Electronics (PSE) performs battery charge control using both temperature compensated charge/discharge ratio ampere hour integration and voltage-temperature control. The PSE includes all the circuits needed to perform telemetry and command functions using an optical MIL-STD-1773 interface.

Dakermanji, G.โ†—
NASA NTRS1990

The Small Explorer power system

A direct-energy-transfer (DET) power system design with the battery connected directly to the bus was selected as the generic power system to satisfy the various Small Explorer spacecraft missions. The power system electronics (PSE) are designated to handle power levels up to a 250-W load with minimum changes in the electronics. The command and telemetry interface is through an optical MIL-STD-1773 interface that is contained in the PSE. The system provides high performance, low weight, and low EMI (electromagnetic interference) generation. The design and performance of the power system have been verified through analysis and extensive tests on a breadboard.

Dakermanji, Georgeโ†—