arXiv subjects
Publications and source records attributed to S. M. Ryan.
We discuss some selected recent developments in the field of lattice QCD at nonzero temperature and density, describing in particular the transition from the hadronic gas to the quark-gluon plasma, as seen in simulations using Wilson fermions.
We study properties of the thermal transition in QCD, using anisotropic, fixed-scale lattice simulations with $N_f = 2+1$ flavours of Wilson fermion. Observables are compared for two values of the pion mass, focusing on chiral properties. Results are presented for the Polyakov loop, various susceptibilities, the chiral condensate and its susceptibility, and the onset of parity doubling in the light and strange baryonic sector.
The quark-line disconnected diagram is a potentially important ingredient in lattice QCD calculations of the hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon. It is also a notoriously difficult one to evaluate. Here, for the first time, we give an estimate of this contribution based on lattice QCD results that have a statistically significant signal, albeit at one value of the lattice spacing and an unphysically heavy value of the $u/d$ quark mass. We use HPQCD's method of determining the anomalous magnetic moment by reconstructing the Adler function from time-moments of the current-current correlator at zero spatial momentum. Our results lead to a total (including $u$, $d$ and $s$ quarks) quark-line disconnected contribution to $a_μ$ of $-0.15\%$ of the $u/d$ hadronic vacuum polarization contribution with an uncertainty which is 1\% of that contribution.
Theoretical studies of quarkonia can elucidate some of the important properties of the quark--gluon plasma, the state of matter realised when the temperature exceeds 150 MeV, currently probed by heavy-ion collisions experiments at BNL and the LHC. We report on our results of lattice studies of bottomonia for temperatures in the range 100 MeV < T < 450 MeV, introducing and discussing the methodologies we have applied. Of particular interest is the analysis of the spectral functions, where Bayesian methods borrowed and adapted from nuclear and condensed matter physics have proven very successful.
We study the fate of P wave bottomonium states in the quark-gluon plasma, using a spectral function analysis of euclidean lattice correlators. The correlators are obtained from lattice QCD simulations with two light quark flavours on highly anisotropic lattices, treating the bottom quark nonrelativistically. We find clear indications of melting immediately after the deconfinement transition.
We study the temperature dependence of bottomonium for temperatures in the range 0.4 Tc < T < 2.1 Tc, using non-relativistic dynamics for the bottom quark and full relativistic lattice QCD simulations for Nf=2 light flavors. We consider the behaviour of the correlators in Euclidean space, we analyze the associated spectral functions and we study the dependence on the momentum. Our results are amenable to a successful comparison with effective field theories. They help build a coherent picture of the behaviour of bottomonium in the plasma, consistent which the current LHC results.
We study the temperature dependence of bottomonium for temperatures in the range $0.4 T_c < T < 2.1 T_c$, using nonrelativistic dynamics for the bottom quark and full relativistic lattice QCD simulations for $N_f=2$ light flavors on a highly anisotropic lattice. We find that the $Υ$ is insensitive to the temperature in this range, while the $χ_b$ propagators show a crossover from the exponential decay characterizing the hadronic phase to a power-law behaviour consistent with nearly-free dynamics at $T \simeq 2 T_c$.
We present our preliminary analysis for the chamonium and D$_s$ spectra obtained from N$_f=2$ dynamical anisotropic lattices. We use 12$^3\times 80$ lattices with lattice spacing $a_t=7.35$ GeV$^{-1}$ and anisotropy of six. Meson correlators are computed using all-to-all propagators together with variational analysis.
We present results for the charmonium spectrum from $N_f=2$ dynamical QCD simulations on $12^3\times 80$ anisotropic lattices. Using all-to-all propagators we determine the ground and excited states of S, P and D waves and hybrids. We also evaluate the disconnected (OZI suppressed) contribution to the $η_c$ and $J/Ψ$
We present a first study of the charmonium spectrum on N_f=2 dynamical, anisotropic lattices. We take advantage of all-to-all quark propagators to build spatially extended interpolating operators to increase the overlap with states not easily accessible with point propagators such as radially excited states of eta_c, psi, and chi_c, D-waves and hybrid states.
We present methods and results for the tuning of quark and gluon anisotropies for improved actions in N_f = 2 QCD.
We discuss the implementation of QCD on 2+2 anisotropic lattices. Technical details regarding the choice of the action as well as perturbative and non-perturbative improvement are analyzed. The physical applications of the program are presented.
We summarize our lattice QCD study of the form factor at zero recoil in the decay anti-B --> D* lepton anti-neutrino. After careful consideration of all sources of systematic uncertainty, we find, h_A1(1) = 0.913(+0.024-0.017)(+0.017-0.030), where the first uncertainty is from statistics and fitting while the second combined uncertainty is from all other systematic effects.
Multi-dimensional meta-analysis (MDMA) is an innovative technique for investigating complex scientific problems influenced by "external" factors, such as social, medical, economic, political or climatic trends. MDMA extends traditional meta-analysis by identifying significant data from diverse and independent disciplines ("orthogonal dimensions") and incorporating truth tables and non-parametric analysis methods in the interpretation protocol. In this paper, we outline the methodology of MDMA. We then demonstrates how to apply the method to a specific problem: the relationship between asthma and air particulates. The conclusions from the example show that the further reduction of atmospheric particulate levels is not necessarily the answer to the increasing asthma incidence. This example also demonstrates the strength of this method of analysis for complex problems.
We present a comprehensive study of the masses of pseudoscalar and vector mesons, as well as octet and decuplet baryons computed in O(a) improved quenched lattice QCD. Results have been obtained using the non-perturbative definition of the improvement coefficient c_sw, and also its estimate in tadpole improved perturbation theory. We investigate effects of improvement on the incidence of exceptional configurations, mass splittings and the parameter J. By combining the results obtained using non-perturbative and tadpole improvement in a simultaneous continuum extrapolation we can compare our spectral data to experiment. We confirm earlier findings by the CP-PACS Collaboration that the quenched light hadron spectrum agrees with experiment at the 10% level.
We describe a model independent lattice QCD method for determining the deviation from unity for h_{A1}(1), the B -> D* l nu form factor at zero recoil. We extend the double ratio method previously used to determine the B -> D l nu form factor. The bulk of statistical and systematic errors cancel in the double ratios we consider, yielding form factors which promise to reduce present theoretical uncertainties in the determination of V_{cb}. We present results from a prototype calculation at a single lattice spacing corresponding to beta=5.7.
The zero recoil limit of the B -> D l nu form factors is calculated on the lattice, which provides a model-independent determination of |Vcb|. Considering a ratio of form factors, in which the bulk of statistical and systematic errors cancel, we obtain a precise result both for h_+(1) and for h_-(1).
The increasing accuracy of experimental results for the exclusive, semileptonic decay B -> pi l nu requires a similarly accurate calculation of the hadronic matrix elements, to determine |Vub|. We present preliminary results for the form factors of the B to light meson decay mode. Using results from three lattices in the range 5.7 <= beta <= 6.1 we study the dependence on the lattice spacing.