Publications (19)73.57 Total impact
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ABSTRACT: Using lattice simulations, we study the infrared behavior of a particularly interesting SU(2) gauge theory, with six massless Dirac fermions in the fundamental representation. We compute the running gauge coupling derived nonperturbatively from the Schrödinger functional of the theory, finding no evidence for an infrared fixed point up through gauge couplings g¯2 of order 20. This implies that the theory either is governed in the infrared by a fixed point of considerable strength, unseen so far in nonsupersymmetric gauge theories, or breaks its global chiral symmetries producing a large number of composite NambuGoldstone bosons relative to the number of underlying degrees of freedom. Thus either of these phases exhibits novel behavior.  [Show abstract] [Hide abstract]
ABSTRACT: We present a novel technique for the determination of the topological susceptibility (related to the variance of the distribution of global topological charge) from lattice gauge theory simulations, based on maximumlikelihood analysis of the Markovchain Monte Carlo time series. This technique is expected to be particularly useful in situations where relatively few tunneling events are observed. Restriction to a lattice subvolume on which topological charge is not quantized is explored, and may lead to further improvement when the global topology is poorly sampled. We test our proposed method on a set of lattice data, and compare it to traditional methods.  [Show abstract] [Hide abstract]
ABSTRACT: Composite dark matter candidates, which can arise from new stronglycoupled sectors, are wellmotivated and phenomenologically interesting, particularly in the context of asymmetric generation of the relic density. In this work, we employ lattice calculations to study the electromagnetic form factors of electroweakneutral darkmatter baryons for a threecolor, QCDlike theory with Nf = 2 and 6 degenerate fermions in the fundamental representation. We calculate the (connected) charge radius and anomalous magnetic moment, both of which can play a significant role for direct detection of composite dark matter. We find minimal Nf dependence in these quantities. We generate massdependent crosssections for dark matternucleon interactions and use them in conjunction with experimental results from XENON100, excluding dark matter candidates of this type with masses below 10 TeV.  [Show abstract] [Hide abstract]
ABSTRACT: A new framework for evaluating hydrodynamic models of relativistic heavy ion collisions has been developed. This framework, a Comprehesive Heavy Ion Model Evaluation and Reporting Algorithm (CHIMERA) has been implemented by augmenting UVH 2+1D viscous hydrodynamic model with eccentricity fluctuations, preequilibrium flow, and the Ultrarelativistic Quantum Molecular Dynamic (UrQMD) hadronic cascade. A range of initial temperatures and shear viscosity to entropy ratios were evaluated for four initial profiles, $N_{part}$ and $N_{coll}$ scaling with and without preequilibrium flow. The model results were compared to pion spectra, elliptic flow, and femtoscopic radii from 200 GeV Au+Au collisions for the 020% centrality range.Two sets of initial density profiles, $N_{part}$ scaling with preequilibrium flow and $N_{coll}$ scaling without were shown to provide a consistent description of all three measurements. 
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ABSTRACT: We present results on the chiral and deconfinement properties of the QCD transition at finite temperature. Calculations are performed with 2+1 flavors of quarks using the p4, asqtad and HISQ/tree actions. Lattices with temporal extent N_tau=6, 8 and 12 are used to understand and control discretization errors and to reliably extrapolate estimates obtained at finite lattice spacings to the continuum limit. The chiral transition temperature is defined in terms of the phase transition in a theory with two massless flavors and analyzed using O(N) scaling fits to the chiral condensate and susceptibility. We find consistent estimates from the HISQ/tree and asqtad actions and our main result is T_c=154 +/ 9 MeV.  [Show abstract] [Hide abstract]
ABSTRACT: We present results for screening masses of mesons built from light and strange quarks in the temperature range of approximately between 140MeV to 800MeV. The lattice computations were performed with 2+1 dynamical light and strange flavors of improved (p4) staggered fermions along a line of constant physics defined by a pion mass of about 220MeV and a kaon mass of 500MeV. The lattices had temporal extents N τ =4, 6 and 8 and aspect ratios of N s /N τ ≥4. At least up to a temperature of 140MeV the pseudoscalar screening mass remains almost equal to the corresponding zero temperature pseudoscalar (pole) mass. At temperatures around 3T c (T c being the transition temperature) the continuum extrapolated pseudoscalar screening mass approaches very close to the free continuum result of 2πT from below. On the other hand, at high temperatures the vector screening mass turns out to be larger than the free continuum value of 2πT. The pseudoscalar and the vector screening masses do not become degenerate even for a temperature as high as 4T c . Using these mesonic spatial correlation functions we have also investigated the restoration of chiral symmetry and the effective restoration of the axial symmetry. We have found that the vector and the axialvector screening correlators become degenerate, indicating chiral symmetry restoration, at a temperature which is consistent with the QCD transition temperature obtained in previous studies. On the other hand, the pseudoscalar and the scalar screening correlators become degenerate only at temperatures larger than 1.3T c , indicating that the effective restoration of the axial symmetry takes place at a temperature larger than the QCD transition temperature.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the QCD equation of state for temperatures corresponding to the transition region with physical mass values for two degenerate light quark flavors and a strange quark using an improved staggered fermion action (p4action) on lattices with temporal extent N_tau=8. We compare our results with previous calculations performed at twice larger values of the light quark masses as well as with results obtained from a resonance gas model calculation. We also discuss the deconfining and chiral aspects of the QCD transition in terms of renormalized Polyakov loop, strangeness fluctuations and subtracted chiral condensate. We show that compared to the calculations performed at twice larger value of the light quark mass the transition region shifts by about 5 MeV toward smaller temperatures Comment: 7 pages, LaTeX, 6 figures; minor corrections, typos corrected, references added 
Article: The HotQCD Collaboration
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ABSTRACT: We calculate the equation of state in 2+1 flavor QCD at finite temperature with physical strange quark mass and almost physical light quark masses using lattices with temporal extent Nτ=8. Calculations have been performed with two different improved staggered fermion actions, the asqtad and p4 actions. Overall, we find good agreement between results obtained with these two O(a2) improved staggered fermion discretization schemes. A comparison with earlier calculations on coarser lattices is performed to quantify systematic errors in current studies of the equation of state. We also present results for observables that are sensitive to deconfining and chiral aspects of the QCD transition on Nτ=6 and 8 lattices. We find that deconfinement and chiral symmetry restoration happen in the same narrow temperature interval. In an appendix we present a simple parametrization of the equation of state that can easily be used in hydrodynamic model calculations. In this parametrization we include an estimate of current uncertainties in the lattice calculations which arise from cutoff and quark mass effects.  [Show abstract] [Hide abstract]
ABSTRACT: We analyze baryon number, strangeness and electric charge fluctuations as well as their correlations in QCD at high temperature. We present results obtained from lattice calculations performed with an improved staggered fermion action (p4action) at two values of the lattice cutoff with almost physical up and down quark masses and a physical value for the strange quark mass. We compare these results, with an ideal quark gas at high temperature and a hadron resonance gas model at low temperature. We find that fluctuations and correlations are well described by the former already for temperatures about 1.5 times the transition temperature. At low temperature qualitative features of the lattice results are quite well described by a hadron resonance gas model. Higher order cumulants, which become increasingly sensitive to the light pions, however show deviations from a resonance gas in the vicinity of the transition temperature.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the spatial string tension in (2+1) flavor QCD with physical strange quark mass and almost physical light quark masses using lattices with temporal extent N_tau=4,6 and 8. We compare our results on the spatial string tension with predictions of dimensionally reduced QCD. This suggests that also in the presence of light dynamical quarks dimensional reduction works well down to temperatures 1.5T_c.  [Show abstract] [Hide abstract]
ABSTRACT: We present results on the equation of state in QCD with two light quark flavors and a heavier strange quark. Calculations with improved staggered fermions have been performed on lattices with temporal extent Nt =4 and 6 on a line of constant physics with almost physical quark mass values; the pion mass is about 220 MeV, and the strange quark mass is adjusted to its physical value. High statistics results on large lattices are obtained for bulk thermodynamic observables, i.e. pressure, energy and entropy density, at vanishing quark chemical potential for a wide range of temperatures, 140 MeV < T < 800 MeV. We present a detailed discussion of finite cutoff effects which become particularly significant for temperatures larger than about twice the transition temperature. At these high temperatures we also performed calculations of the trace anomaly on lattices with temporal extent Nt=8. Furthermore, we have performed an extensive analysis of zero temperature observables including the light and strange quark condensates and the static quark potential at zero temperature. These are used to set the temperature scale for thermodynamic observables and to calculate renormalized observables that are sensitive to deconfinement and chiral symmetry restoration and become order parameters in the infinite and zero quark mass limits, respectively.  [Show abstract] [Hide abstract]
ABSTRACT: We study the finite temperature transition in QCD with three flavors of equal masses using the R and RHMC algorithms on lattices with temporal extent Nτ=4 and 6. For the transition temperature in the continuum limit we find r0Tc=0.429(8) for the light pseudoscalar mass corresponding to the endpoint of the 1st order transition region. When comparing the results obtained with the R and RHMC algorithms for p4fat3 action we see no significant stepsize errors down to a lightest pseudoscalar mass of mpsr0=0.4.  [Show abstract] [Hide abstract]
ABSTRACT: We present a study of the flavor symmetry breaking in the pion spectrum for various improved staggered fermion actions. To study the effects of link fattening and tadpole improvement, we use three different variants of the p4 action  p4fat3, p4fat7, and p4fat7tad. These are compared to Asqtad and also to naive staggered. To study the pattern of symmetry breaking, we measure all 15 meson masses in the 4flavor staggered theory. The measurements are done on a quenched gauge background, generated using a oneloop improved Symanzik action with $\beta=10/g^2 = 7.40, 7.75,$ and 8.00, corresponding to lattice spacings of approximately a = .31 fm., .21 fm., and .14 fm. We also study how the lattice scale set by the $\rho$ mass on each of these ensembles compares to one set by the static quark potential. 
Article: Transition temperature in QCD
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ABSTRACT: We present a detailed calculation of the transition temperature in QCD with two light and one heavier (strange) quark mass on lattices with temporal extent Nτ=4 and 6. Calculations with improved staggered fermions have been performed for various light to strange quark mass ratios in the range, 0.05≤m̂l/m̂s≤0.5, and with a strange quark mass fixed close to its physical value. From a combined extrapolation to the chiral (m̂l→0) and continuum (aT≡1/Nτ→0) limits we find for the transition temperature at the physical point Tcr0=0.457(7) where the scale is set by the Sommerscale parameter r0 defined as the distance in the static quark potential at which the slope takes on the value, (dVq̅ q(r)/dr)r=r0=1.65/r02. Using the currently best known value for r0 this translates to a transition temperature Tc=192(7)(4) MeV. The transition temperature in the chiral limit is about 3% smaller. We discuss current ambiguities in the determination of Tc in physical units and also comment on the universal scaling behavior of thermodynamic quantities in the chiral limit.  [Show abstract] [Hide abstract]
ABSTRACT: We discuss recent algorithmic improvements in simulating finite temperature QCD on a lattice. In particular, the Rational Hybrid Monte Carlo(RHMC) algorithm is employed to generate lattice configurations for 2+1 flavor QCD. Unlike the Hybrid R Algorithm, RHMC is reversible, admitting a Metropolis accept/reject step that eliminates the $\mathcal{O}(\delta t^2)$ errors inherent in the R Algorithm. We also employ several algorithmic speedups, including multiple time scales, the use of a more efficient numerical integrator, and Hasenbusch preconditioning of the fermion force. 
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ABSTRACT: We summarize the latest results from performing Lattice QCD calculations with two different discretization schemes, asqtad and p4fat3, on the LLNL BG/L supercomputer, and present a plan for continued running to complete the calculation of the QCD transition temperature and equation of state.
Publication Stats
2k  Citations  
73.57  Total Impact Points  
Top Journals
Institutions

20122014

Boston University
 Center for Computational Science
Boston, Massachusetts, United States


2013

University of Washington Seattle
 Department of Physics
Seattle, Washington, United States


20092011

Lawrence Livermore National Laboratory
 Physics Division
Livermore, CA, United States


20062008

Columbia University
 Department of Physics
New York, New York, United States
