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At least 1,639 records · Page 91
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DOE OSTI2011

ATLAS and CMS hints for a mirror Higgs boson

ATLAS and CMS have provided hints for the existence of a Higgs-like particle with mass of about 144 GeV with production cross section into standard decay channels which is about 50% that of the standard model Higgs boson. We show that this 50% suppression is exactly what the mirror matter model predicts when the two scalar mass eigenstates, each required to be maximal admixtures of a standard and mirror-Higgs boson, are separated in mass by more than their decay widths but less than the experimental resolution. We discuss prospects for the future confirmation of this interesting hint for nonstandard Higgs physics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Search for anomalous production of prompt like-sign muon pairs and constraints on physics beyond the standard model with the ATLAS detector

An inclusive search for anomalous production of two prompt, isolated muons with the same electric charge is presented. The search is performed in a data sample corresponding to 1.6 fb{sup -1} of integrated luminosity collected in 2011 at {radical}s = 7 TeV with the ATLAS detector at the LHC. Muon pairs are selected by requiring two isolated muons of the same electric charge with p{sub T} > 20 GeV and |{eta}| < 2.5. Minimal requirements are placed on the rest of the event activity. The distribution of the invariant mass of the muon pair m({mu}{mu}) is found to agree well with the background expectation. Upper limits on the cross section for anomalous production of two muons with the same electric charge are placed as a function of m({mu}{mu}) within a fiducial region defined by the event selection. The fiducial cross-section limit constrains the like-sign top-quark pair-production cross section to be below 3.7 pb at 95% confidence level. The data are also analyzed to search for a narrow like-sign dimuon resonance as predicted for e.g. doubly charged Higgs bosons (H{sup {+-}{+-}}). Assuming pair production of H{sup {+-}{+-}} bosons and a branching ratio to muons of 100% (33%), this analysis excludes masses below 355 (244) GeV and 251 (209) GeV for H{sup {+-}{+-}} bosons coupling to left-handed and right-handed fermions, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurement of the parity-violating asymmetry parameter α b and the helicity amplitudes for the decay Λ b 0 → J / ψ Λ 0 with the ATLAS detector

A measurement of the parity-violating decay asymmetry parameter, α b , and the helicity amplitudes for the decay Λ$ ^0_b $→J/ψ(μ + μ - )Λ 0 (pπ - ) is reported. The analysis is based on 1400 Λ$ ^0_b $ and $ \bar{Λ} ^0_b $ baryons selected in 4.6 fb -1 of proton-proton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the Λ$ ^0_b $ and $ \bar{Λ} ^0_b $ samples under the assumption of CP conservation, the value of α b is measured to be 0.30 ± 0.16(stat) ± 0.06(syst). This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Measurements of spin correlation in top-antitop quark events from proton-proton collisions at s = 7 TeV using the ATLAS detector

Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.6 fb⁻¹. Events are selected in final states with two charged leptons and at least two jets and in final states with one charged lepton and at least four jets. Four different observables sensitive to different properties of the top quark pair production mechanism are used to extract the correlation between the top and antitop quark spins. Some of these observables are measured for the first time. The measurements are in good agreement with the Standard Model prediction at next-to-leading-order accuracy.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for new phenomena in events with a photon and missing transverse momentum in p p collisions at s = 8 TeV with the ATLAS detector

Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum with the ATLAS experiment at the LHC are reported. Data were collected in proton-proton collisions at a center-of-mass energy of 8 TeV and correspond to an integrated luminosity of 20.3 fb⁻¹. The observed data are well described by the expected Standard Model backgrounds. The expected (observed) upper limit on the fiducial cross section for the production of events with a photon and large missing transverse momentum is 6.1 (5.3) fb at 95% confidence level. Exclusion limits are presented on models of new phenomena with large extra spatial dimensions, supersymmetric quarks, and direct pair production of dark-matter candidates.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for vectorlike B quarks in events with one isolated lepton, missing transverse momentum, and jets at √s = 8 TeV with the ATLAS detector

A search has been performed for pair production of heavy vectorlike down-type (B) quarks. The analysis explores the lepton-plus-jets final state, characterized by events with one isolated charged lepton (electron or muon), significant missing transverse momentum, and multiple jets. One or more jets are required to be tagged as arising from b quarks, and at least one pair of jets must be tagged as arising from the hadronic decay of an electroweak boson. The analysis uses the full data sample of pp collisions recorded in 2012 by the ATLAS detector at the LHC, operating at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb⁻¹. No significant excess of events is observed above the expected background. Limits are set on vectorlike B production, as a function of the B branching ratios, assuming the allowable decay modes are B → Wt/Zb/Hb . In the chiral limit with a branching ratio of 100% for the decay B → Wt , the observed (expected) 95% C.L. lower limit on the vectorlike B mass is 810 GeV (760 GeV). In the case where the vectorlike B quark has branching ratio values corresponding to those of an SU (2) singlet state, the observed (expected) 95% C.L. lower limit on the vectorlike B mass is 640 GeV (505 GeV). The same analysis, when used to investigate pair production of a colored, charge 5/3 exotic fermion T 5/3 , with subsequent decay T 5/ 3 → Wt , sets an observed (expected) 95% C.L. lower limit on the T 5/3 mass of 840 GeV (780 GeV).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for long-lived, weakly interacting particles that decay to displaced hadronic jets in proton-proton collisions at s = 8 TeV with the ATLAS detector

A search for the decay of neutral, weakly interacting, long-lived particles using data collected by the ATLAS detector at the LHC is presented. This analysis uses the full data set recorded in 2012: 20.3 fb -1 of proton-proton collision data at √s = 8 TeV. The search employs techniques for reconstructing decay vertices of long-lived particles decaying to jets in the inner tracking detector and muon spectrometer. Signal events require at least two reconstructed vertices. No significant excess of events over the expected background is found, and limits as a function of proper lifetime are reported for the decay of the Higgs boson and other scalar bosons to long-lived particles and for Hidden Valley Z' and Stealth SUSY benchmark models. The first search results for displaced decays in Z' and Stealth SUSY models are presented. The upper bounds of the excluded proper lifetimes are the most stringent to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for type-III seesaw heavy leptons in p p collisions at s = 8 TeV with the ATLAS detector

A search for the pair production of heavy leptons (N⁰,L ± ) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels N⁰→W ± l ∓ (ℓ=e,μ,τ) and L ± →W ± ν (ν=ν e ,ν μ ,ν τ ) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying W boson and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of 20.3 fb⁻¹ of pp collisions at s√=8 TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair production is observed. Heavy leptons with masses below 325–540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for high-mass diphoton resonances in p p collisions at s = 8 TeV with the ATLAS detector

This article describes a search for high-mass resonances decaying to a pair of photons using a sample of 20.3 fb -1 of pp collisions at s√=8 TeV recorded with the ATLAS detector at the Large Hadron Collider. The data are found to be in agreement with the Standard Model prediction, and limits are reported in the framework of the Randall-Sundrum model. This theory leads to the prediction of graviton states, the lightest of which could be observed at the Large Hadron Collider. A lower limit of 2.66 (1.41) TeV at 95% confidence level is set on the mass of the lightest graviton for couplings of k/M¯¯ Pl =0.1 (0.01).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Search for long-lived, massive particles in events with displaced vertices and missing transverse momentum in s = 13 TeV p p collisions with the ATLAS detector

A search for long-lived, massive particles predicted by many theories beyond the Standard Model is presented. Here, the search targets final states with large missing transverse momentum and at least one high-mass displaced vertex with five or more tracks, and uses 32.8 fb -1 of $\sqrt s = 13$ TeV pp collision data collected by the ATLAS detector at the LHC. The observed yield is consistent with the expected background. The results are used to extract 95% C.L. exclusion limits on the production of long-lived gluinos with masses up to 2.37 TeV and lifetimes of O (10 - 2 ) - O (10) ns in a simplified model inspired by split supersymmetry.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Search for long-lived particles in final states with displaced dimuon vertices in p p collisions at s = 13 TeV with the ATLAS detector

A search is performed for a long-lived particle decaying into a final state that includes a pair of muons of opposite-sign electric charge, using proton-proton collision data collected at √ s = 13 TeV by the ATLAS detector at the Large Hadron Collider corresponding to an integrated luminosity of 32.9 fb -1 . No significant excess over the Standard Model expectation is observed. Limits at 95% confidence level on the lifetime of the long-lived particle are presented in models of new phenomena including gauge-mediated supersymmetry or decay of the Higgs boson, H, to a pair of dark photons, Z D . Lifetimes in the range c τ = 1 – 2400 cm are excluded, depending on the parameters of the model. In the supersymmetric model, the lightest neutralino is the next-to-lightest supersymmetric particle, with a relatively long lifetime due to its weak coupling to the gravitino, the lightest supersymmetric particle. The lifetime limits are determined for very light gravitino mass and various assumptions for the neutralino mass in the range 300–1000 GeV. In the dark photon model, the lifetime limits are interpreted as exclusion contours in the plane of the coupling between the Z D and the Standard Model Z boson versus the Z D mass (in the range 20–60 GeV), for various assumptions for the H → Z D Z D branching fraction.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Search for a Standard Model Higgs Boson in the H to ZZ to l(+)l(-)v(v)over-bar Decay Channel with the ATLAS Detector

A search for a heavy standard model Higgs boson decaying via H {yields} ZZ {yields} {ell}{sup +}{ell}{sup -} {nu}{bar {nu}}, where {ell} = e, {mu}, is presented. It is based on proton-proton collision data at {radical}s = 7 TeV, collected by the ATLAS experiment at the LHC in the first half of 2011 and corresponding to an integrated luminosity of 1.04 fb{sup -1}. The data are compared to the expected standard model backgrounds. The data and the background expectations are found to be in agreement and upper limits are placed on the Higgs boson production cross section over the entire mass window considered; in particular, the production of a standard model Higgs boson is excluded in the region 340 < m{sub H} < 450 GeV at the 95% confidence level.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Search for New Phenomena in tt¯ Events with Large Missing Transverse Momentum in Proton-Proton Collisions at √s=7 TeV with the ATLAS Detector

A search for new phenomena in tt¯ events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on 1.04 fb⁻¹ of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV and a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, top-partner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV.

Aad, G.↗
DOE OSTI2012

Search for the Higgs Boson in the H→WW*→l⁺νl⁻ν¯ Decay Channel in pp Collisions at √s=7 TeV with the ATLAS Detector

A search for the Higgs boson has been performed in the H→WW*→l⁺νl⁻ν¯ channel (l=e/μ) with an integrated luminosity of 2.05 fb⁻¹ of pp collisions at √s=7 TeV collected with the ATLAS detector at the Large Hadron Collider. No significant excess of events over the expected background is observed and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range 110 GeV<mH<300 GeV. The observations exclude the presence of a standard model Higgs boson with a mass 145<mH<206 GeV at 95% confidence level.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Search for Dark Matter in Events with a Hadronically Decaying W or Z Boson and Missing Transverse Momentum in p p Collisions at s = 8 TeV with the ATLAS Detector

A search is presented for dark matter pair production in association with a W or Z boson in pp collisions representing 20.3 fb -1 of integrated luminosity at $ \sqrt{s} $ = 8 TeV using data recorded with the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet with the jet mass consistent with a W or Z boson, and with large missing transverse momentum are analyzed. The data are consistent with the standard model expectations. Limits are set on the mass scale in effective field theories that describe the interaction of dark matter and standard model particles, and on the cross section of Higgs production and decay to invisible particles. In addition, cross section limits on the anomalous production of W or Z bosons with large missing transverse momentum are set in two fiducial regions.

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 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

Search for a Charged Higgs Boson Produced in the Vector-Boson Fusion Mode with Decay H ± → W ± Z using p p Collisions at s = 8 TeV with the ATLAS Experiment

A search for a charged Higgs boson, H ± , decaying to a W ± boson and a Z boson is presented. The search is based on 20.3 fb⁻¹ of proton-proton collision data at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the LHC. The H ± boson is assumed to be produced via vector-boson fusion and the decays W ± →qq'¯ and Z→e⁺e⁻/μ⁺μ⁻ are considered. The search is performed in a range of charged Higgs boson masses from 200 to 1000 GeV. No evidence for the production of an H ± boson is observed. Upper limits of 31–1020 fb at 95% C.L. are placed on the cross section for vector-boson fusion production of an H ± boson times its branching fraction to W ± Z. The limits are compared with predictions from the Georgi-Machacek Higgs triplet model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗