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At least 1,783 records · Page 99
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DOE OSTI2014

Study of hadronic event-shape variables in multijet final states in pp collisions at √s = 7 TeV

Event-shape variables, which are sensitive to perturbative and nonperturbative aspects of quantum chromodynamic (QCD) interactions, are studied in multijet events recorded in proton-proton collisions at √s = 7 TeV. Events are selected with at least one jet with transverse momentum p T > 110 GeV and pseudorapidity |η| < 2.4, in a data sample corresponding to integrated luminosities of up to 5 fb –1 . As a result, the distributions of five event-shape variables in various leading jet p T ranges are compared to predictions from different QCD Monte Carlo event generators.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Search for top quark decays t → qH , with H → γγ, in $ \sqrt{s}=13 $ TeV pp collisions using the ATLAS detector

This study presents a search for flavour-changing neutral currents in the decay of a top quark into an up-type (q = c, u) quark and a Higgs boson, where the Higgs boson decays into two photons. The proton-proton collision data set analysed amounts to 36.1 fb –1 at √s=13 TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for, where one top quark decays into qH and the other decays into bW . Both the hadronic and leptonic decay modes of the W boson are used. No significant excess is observed and an upper limit is set on the t → cH branching ratio of 2.2 × 10 –3 at the 95% confidence level, while the expected limit in the absence of signal is 1.6 × 10 –3 . The corresponding limit on the tcH coupling is 0.090 at the 95% confidence level. The observed upper limit on the t → uH branching ratio is 2.4 × 10 –3 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

IRIS-HEP Strategic Plan for the Next Phase of Software Upgrades for HL-LHC Physics

The quest to understand the fundamental building blocks of nature and their interactions is one of the oldest and most ambitious of human scientific endeavors. CERN's Large Hadron Collider (LHC) represents a huge step forward in this quest. The discovery of the Higgs boson, the observation of exceedingly rare decays of $B$ mesons, and stringent constraints on many viable theories of physics beyond the Standard Model (SM) demonstrate the great scientific value of the LHC physics program. The next phase of this global scientific project will be the High-Luminosity LHC (HL-LHC) which will collect data starting circa 2029 and continue through the 2030s. The primary science goal is to search for physics beyond the SM and, should it be discovered, to study its implications. In the HL-LHC era, the ATLAS and CMS experiments will record around 100 times as many collisions as were used to discover the Higgs boson (and at twice the energy). Both NSF and DOE are making large detector upgrade investments so the HL-LHC can operate in this high-rate environment. Similar investment in software R&D for acquiring, managing, processing and analyzing HL-LHC data is critical to maximize the return-on-investment in the upgraded accelerator and detectors. This report presents a strategic plan for a possible second 5-year funded phase (2023 through 2028) for the Institute for Research and Innovation in Software for High Energy Physics (IRIS-HEP) which will close remaining software and computing gaps to deliver HL-LHC science.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

Search for pair and single production of new heavy quarks that decay to a Z boson and a third-generation quark in pp collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

A search is presented for the production of new heavy quarks that decay to a Z boson and a third-generation Standard Model quark. In the case of a new charge +2/3 quark (T ), the decay targeted is T → Zt, while the decay targeted for a new charge -1/3 quark (B) is B → Zb. The search is performed with a dataset corresponding to 20.3 fb -1 of pp collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum Z boson candidate reconstructed from a pair of oppositely charged same-flavor leptons (electrons or muons), and are analyzed in two channels defined by the absence or presence of a third lepton. Hadronic jets, specifically those with properties consistent with the decay of a b -hadron, are also required to be present in selected events. Different requirements are made on the jet activity in the event in order to enhance the sensitivity to either heavy quark pair production mediated by the strong interaction, or single production mediated by the electroweak interaction. No significant excess of events above the Standard Model expectation is observed, and lower limits are derived on the mass of vector-like T and B quarks under various branching ratio hypotheses, as well as upper limits on the magnitude of electroweak coupling parameters.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Cosmic-Ray Feedback Heating of the Intracluster Medium

Active galactic nuclei (AGNs) play a central role in solving the decades-old cooling-flow problem. Although there is consensus that AGNs provide the energy to prevent catastrophically large star formation, one major problem remains: How is the AGN energy thermalized in the intracluster medium (ICM)? We perform a suite of three-dimensional magnetohydrodynamical adaptive mesh refinement simulations of AGN feedback in a cool core cluster including cosmic rays (CRs). CRs are supplied to the ICM via collimated AGN jets and subsequently disperse in the magnetized ICM via streaming, and interact with the ICM via hadronic, Coulomb, and streaming instability heating. We find that CR transport is an essential model ingredient at least within the context of the physical model considered here. When streaming is included, (i) CRs come into contact with the ambient ICM and efficiently heat it, (ii) streaming instability heating dominates over Coulomb and hadronic heating, (iii) the AGN is variable and the atmosphere goes through low-/high-velocity dispersion cycles, and, importantly, (iv) CR pressure support in the cool core is very low and does not demonstrably violate observational constraints. However, when streaming is ignored, CR energy is not efficiently spent on the ICM heating and CR pressure builds up to a significant level, creating tension with the observations. Overall, we demonstrate that CR heating is a viable channel for the AGN energy thermalization in clusters and likely also in ellipticals, and that CRs play an important role in determining AGN intermittency and the dynamical state of cool cores.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Combination of searches for heavy resonances decaying into bosonic and leptonic final states using 36 fb - 1 of proton-proton collision data at s = 13 TeV with the ATLAS detector

Searches for new heavy resonances decaying into different pairings of W, Z, or Higgs bosons, as well as directly into leptons, are presented using a data sample corresponding to 36.1 fb -1 of pp collisions at $ \sqrt{s}=13 $ TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting bosonic decay modes in the qqqq, ννqq, ℓνqq, ℓℓqq, ℓνℓν, ℓℓνν, ℓνℓℓ, ℓℓℓℓ, qqbb, ννbb, ℓνbb, and ℓℓbb final states are combined, searching for a narrow-width resonance. Likewise, analyses selecting the leptonic ℓν and ℓℓ final states are also combined. These two sets of analyses are then further combined. No significant deviation from the Standard Model predictions is observed. Three benchmark models are tested: a model predicting the existence of a new heavy scalar singlet, a simplified model predicting a heavy vector-boson triplet, and a bulk Randall-Sundrum model with a heavy spin-2 Kaluza-Klein excitation of the graviton. Cross section limits are set at the 95% confidence level using an asymptotic approximation and are compared with predictions for the benchmark models. These limits are also expressed in terms of constraints on couplings of the heavy vector-boson triplet to quarks, leptons, and the Higgs boson. The data exclude a heavy vector-boson triplet with mass below 5.5 TeV in a weakly coupled scenario and 4.5 TeV in a strongly coupled scenario, as well as a Kaluza-Klein graviton with mass below 2.3 TeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

The P2 experiment: A future high-precision measurement of the weak mixing angle at low momentum transfer

Here we describe the research and development work for the P 2 experiment which aims for a high precision determination of the weak mixing angle sin 2 θ W to a precision of 0.15% at a four-momentum transfer of 4.5×10 –3 GeV 2 . This accuracy, comparable to existing measurements at the Z pole, allows for a sensitive test of the Standard Model up to a mass scale of 50 TeV, extendable to 60 TeV. The weak mixing angle is connected to the weak charge of the proton which will be extracted from a measurement of the parity violating cross section asymmetry –39.94×10 –9 in elastic electron-proton scattering. A total accuracy of 0.57×10 –9 is achievable in a measurement time of 11000 h using a 150μA polarized electron beam impinging on a 60 cm liquid hydrogen target. The P 2 asymmetry is smaller than any asymmetry measured so far in electron scattering with an unprecedented goal for the accuracy. The use of a solenoid spectrometer with 100% Φ-acceptance as well as an atomic hydrogen trap polarimeter are new features, which have never before been used in parity-violation experiments. In order to collect the enormous statistics required for this measurement, the new Mainz Energy-Recovering Superconducting Accelerator (MESA) is under construction. Plans for the associated beam control system and the polarimetry are described in this article as well. A liquid hydrogen high-power target with an extremely low noise level of 10 ppm needs to be designed and constructed. We report in addition on the conceptual design of the P 2 spectrometer, its Cherenkov detectors, the integrating read-out electronics as well as the ultra-thin, fast tracking detectors. The physics program of the MESA facility comprises indirect, high precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclear physics, single-spin asymmetries, and a possible future extension to the measurement of hadronic parity violation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Gas Filled RF Resonator Hadron Beam Monitor for Intense Neutrino Beam Experiments

MW-class beam facilities are being considered all over the world to produce an intense neutrino beam for fundamental particle physics experiments. A radiation-robust beam monitor system is required to diagnose the primary and secondary beam qualities in high-radiation environments. We have proposed a novel gas-filled RF-resonator hadron beam monitor in which charged particles passing through the resonator produce ionized plasma that changes the permittivity of the gas. The sensitivity of the monitor has been evaluated in numerical simulation. A signal manipulation algorithm has been designed. A prototype system will be constructed and tested by using a proton beam at the MuCool Test Area at Fermilab.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Comparative study of hadron- and γ-triggered azimuthal correlations in relativistic heavy-ion collisions

In the framework of a multi-phase transport model, initial fluctuations in the transverse parton density lead to all orders of harmonic flows. Hadron-triggered azimuthal correlations include all contributions from harmonic flows, hot spots, and jet-medium excitations, which are isolated by using different initial conditions. We found that different physical components dominate different pseudorapidity ranges of dihadron correlations. Because γ-triggered azimuthal correlations can only be caused by jet-medium interactions, a comparative study of hadron- and γ -triggered azimuthal correlations can reveal more dynamics about jet-medium interactions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Searches for third-generation squark production in fully hadronic final states in proton-proton collisions at $ \sqrt{s} = 8$ TeV

We searched for third-generation squarks in fully hadronic final states and presented them using data samples corresponding to integrated luminosities of 19.4 or 19.7 fb -1 , collected at a centre-of-mass energy of 8 TeV with the CMS detector at the LHC. Three mutually exclusive searches are presented, each optimized for a different decay topology. They include a multijet search requiring one fully reconstructed top quark, a dijet search requiring one or two jets originating from b quarks, and a monojet search. Furthermore, no excesses above the standard model expectations are seen, and limits are set on top and bottom squark production in the context of simplified models of supersymmetry.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Yields of weakly bound light nuclei as a probe of the statistical hadronization model

The statistical hadronization model successfully describes the yields of hadrons and light nuclei from central heavy-ion collisions over a wide range of energies. It is a simple and efficient phenomenological framework in which the relative yields for very high energy collisions are essentially determined by a single model parameter—the chemical freeze-out temperature. Recent measurements of yields of hadrons and light nuclei covering over nine orders of magnitudes from the ALICE collaboration at the Large Hadron Collider were described by the model with remarkable accuracy with a chemical freeze-out temperature of 156.5 ± 1.5 MeV. A key physical question is whether (at least to a good approximation) the freeze-out temperature can be understood, literally, as the temperature at which the various species of an equilibrated gas of hadrons (including resonances) and nuclei chemically freeze out, as the model assumes, or whether it successfully parametrizes the yield data for a different reason. This paper analyzes the yields of weakly bound light nuclei—the deuteron and the hypertriton—to probe this issue. Such nuclei are particularly sensitive to assumptions of the model because their binding energies are at a scale far below both typical hadronic scales and the freeze-out temperature. Here, the analysis depends only on outputs of the statistical hadronization model, known hadronic properties and standard assumptions of kinetic theory while making no additional dynamical assumptions about the dynamics of heavy-ion collisions. The analysis indicates that a key assumption underlying the model—that hadrons (and nuclei), just prior to chemical freeze-out temperature, are in thermal equilibrium and are sufficiently dilute as to have particle distributions accurately described statistically by a nearly ideal gas of hadrons and nuclei with masses given by their free space values—appears to be inconsistent with the chemical freeze-out temperature output by the model, at least for these weakly bound nuclei. Implications of this analysis for the interpretation of parameters extracted from the model are discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

PHENIX: Beyond 15 years of discovery

The PHENIX experiment at BNL’s Relativistic Heavy Ion Collider (RHIC) was designed to uncover properties of the quark–gluon plasma (QGP) via rare penetrating probes. Over the past 15 years, the collaboration has delivered on its promised measurements, often with exciting results beyond those originally foreseen. That the QGP behaves as a nearly perfect fluid and that non-photonic electrons are substantially suppressed has led to the use of heavy quarks as probes of the medium. The PHENIX silicon vertex detectors are opening a new arena for QGP studies, and the MPC-EX, a novel forward calorimeter with silicon readout, accesses low-x physics via direct photons with unprecedented precision. PHENIX has proposed sPHENIX, a major upgrade using the recently acquired BaBar solenoid and full calorimetric coverage and high rate capabilities. sPHENIX will reconstruct jets and extend observables to higher transverse momentum, where comparisons to results from the Large Hadron Collider (LHC) heavy-ion program will provide the most insightful. Following the RHIC program, the nuclear physics community has identified an electron ion collider (EIC) as crucial to the next generation of QCD investigations. The BaBar magnet and sPHENIX calorimetry will be an excellent foundation for a new collaborative pursuit of discovery.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Hadron Analyses Data Repository Online Project

The study of excited states of the nucleon is a major effort of present day nuclear physics. The spectrum reflects the internal dynamics of QCD. The past decade has seen a dramatic increase in the precision and range of experimental measurements from MAMI@Mainz and CEBAF@JLab. The analyses of these data, both in Europe and the US, have reached a level of robustness and sophistication that the results on the production of N*'s demand to be shared openly in the nuclear reaction community and beyond. Indeed, there is a demand for such information in fields beyond hadron spectroscopy, such as Heavy Ion collisions. Reaction calculations demand amplitudes and not just simple representations in terms of resonances. The aim of this project is to set up a publicly accessible database to archive (a) the full range of data on cross-sections and polarization asymmetries measured in hadro and photo-production of baryon resonances, (b) for each excited baryon its mass, width, couplings and transition formfactors, (c) for each analysis, the partial wave amplitudes with a detailed exposition of the methods used, (e) where appropriate these will be compared with the detailed predictions of QCD.

Pennington, Michael↗
DOE OSTI2015

Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

Results of a search for H → ττ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb –1 and 20.3 fb –1 at centre-of-mass energies of √s=7 TeV and √s=8 TeV respectively. All combinations of leptonic (τ → ℓνν¯ with ℓ = e, μ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalized to the Standard Model expectation, of μ = 1.43 –0.37 +0.43 is consistent with the predicted Yukawa coupling strength in the Standard Model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗