arXiv subjects

C. R. Allton

Publications and source records attributed to C. R. Allton.

At least 19 recordsLinked to original sources

arXiv2013

Charmonium Potentials at Finite Temperature

The charmonium states at non-zero temperature are studied on anisotropic lattices with 2 dynamical quark flavours. Non-local operators are used to determine the Nambu-Bethe-Salpeter (NBS) wavefunctions via both conventional fitting methods and the Maximum Entropy Method. The interquark potential is determined from the solution of the Schrodinger equation, given the NBS wavefunction as input following the HAL QCD method. We observe a temperature dependent potential which becomes steeper as the temperature decreases.

hep-lat↗

arXiv2013

Ab Initio Calculation of Finite Temperature Charmonium Potentials

The interquark potential in charmonium states is calculated for the first time in both the zero and non-zero temperature phases from a first-principles lattice QCD calculation. Simulations with two dynamical quark flavours were used with temperatures T in the range 0.4 T_c \lesssim T \lesssim 1.7 T_c, where T_c is the deconfining temperature. The correlators of point-split operators were analysed to gain spatial information about the charmonium states. A method, introduced by the HAL QCD collaboration and based on the Schroedinger equation, was applied to obtain the interquark potential. We find a clear temperature dependence, with the central potential becoming flatter (more screened) as the temperature increases.

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arXiv2010

Polarised correlators at finite temperature

QCD undergoes a deconfining transition at high temperature to a "quark-gluon plasma" phase where hadrons may become unbound. In this work, meson correlation functions at non-zero momentum are studied both in the confined and plasma phases using the Maximum Entropy Method. In particular, both the longitudinal and transverse modes of the vector correlation functions are considered. Only in the case of light quarks in the plasma phase, we find that both longitudinal and transverse spectral functions have a non-zero intercept at zero energy.

hep-lat↗

arXiv2010

An analysis of the nucleon spectrum from lattice partially-quenched QCD

The chiral extrapolation of the nucleon mass, M_n, is investigated using data coming from 2-flavour partially-quenched lattice simulations. A large sample of lattice results from the CP-PACS Collaboration is analysed using the leading one-loop corrections, with explicit corrections for finite lattice spacing artifacts. The extrapolation is studied using finite range regularised chiral perturbation theory. The analysis also provides a quantitative estimate of the leading finite volume corrections. It is found that the discretisation, finite-volume and partial quenching effects can all be very well described in this framework, producing an extrapolated value of M_n in agreement with experiment. Furthermore, determinations of the low energy constants of the nucleon mass's chiral expansion are in agreement with previous methods, but with significantly reduced errors. This procedure is also compared with extrapolations based on polynomial forms, where the results are less encouraging.

hep-lat↗

arXiv2007

Charmonium properties in the quark-gluon plasma

We present results for charmonium correlators and spectral functions in 2-flavour CD on anisotropic lattices. Our results indicate that the S-waves (J/psi and eta_c) survive up to temperatures close to 2T_c, while the P-waves (chi_c0 and chi_c1) melt away below 1.2T_c.

hep-lat↗

arXiv2006

Charmonium spectral functions in N_f=2 QCD at high temperature

Charmonium systems in two-flavour QCD at non-zero temperature are studied at, and above the deconfining transition. Using anisotropic lattices the MEM approach is used to extract spectral functions for these channels. By carefully varying some of the irrelevant parameters in the MEM procedure, we confirm that our systematic effects are under control. The eta_c and J/psi states are found to persist at temperatures >~ 1.3 T_c in agreement with our previous dynamical studies.

hep-lat↗

arXiv2006

Charmonium spectral functions in two-flavour QCD

We compute charmonium spectral functions in 2-flavour QCD using the maximum entropy method and anisotropic lattices. We find that the S-waves (J/psi and eta_c) survive up to temperatures close to 2T_c, while the P-waves (chi_c0 and chi_c1) melt away below 1.3T_c.

hep-lat↗

arXiv2005

Asymptotic Scaling and Monte Carlo Data

It is a generally known problem that the behaviour predicted from perturbation theory for asymptotically free theories like QCD, i.e. asymptotic scaling, has not been observed in Monte Carlo simulations when the series is expressed in terms of the bare coupling g_0. This discrepancy has been explained in the past with the poor convergence properties of the perturbative series in the g_0. An alternative point of view, called Lattice-Distorted Perturbation Theory proposes that lattice artifacts due to the finiteness of the lattice spacing, a, cause the disagreement between Monte Carlo data and perturbative scaling. Following this alternative scenario, we fit recent quenched data from different observables to fitting functions that include these cut-off effects, confirming that the lattice data are well reproduced by g_0-PT with the simple addition of terms O(a^n).

hep-lat↗

arXiv2005

Chiral and Continuum Extrapolation of Partially-Quenched Hadron Masses

Using the finite-range regularisation (FRR) of chiral effective field theory, the chiral extrapolation formula for the vector meson mass is derived for the case of partially-quenched QCD. We re-analyse the dynamical fermion QCD data for the vector meson mass from the CP-PACS collaboration. A global fit, including finite lattice spacing effects, of all 16 of their ensembles is performed. We study the FRR method together with a naive polynomial approach and find excellent agreement ~1% with the experimental value of M_rho from the former approach. These results are extended to the case of the nucleon mass.

hep-lat↗

arXiv2005

Unified chiral analysis of the vector meson spectrum from lattice QCD

The chiral extrapolation of the vector meson mass calculated in partially-quenched lattice simulations is investigated. The leading one-loop corrections to the vector meson mass are derived for partially-quenched QCD. A large sample of lattice results from the CP-PACS Collaboration is analysed, with explicit corrections for finite lattice spacing artifacts. To incorporate the effect of the opening decay channel as the chiral limit is approached, the extrapolation is studied using a necessary phenomenological extension of chiral effective field theory. This chiral analysis also provides a quantitative estimate of the leading finite volume corrections. It is found that the discretisation, finite-volume and partial quenching effects can all be very well described in this framework, producing an extrapolated value of M_ρin excellent agreement with experiment. This procedure is also compared with extrapolations based on polynomial forms, where the results are much less enlightening.

hep-lat↗

arXiv2005

Chiral and Continuum Extrapolation of Partially-Quenched Lattice Results

The vector meson mass is extracted from a large sample of partially quenched, two-flavor lattice QCD simulations. For the first time, discretisation, finite-volume and partial quenching artefacts are treated in a unified framework which is consistent with the low-energy behaviour of QCD. This analysis incorporates the leading infrared behaviour dictated by chiral effective field theory. As the two-pion decay channel cannot be described by a low-energy expansion alone, a highly-constrained model for the decay channel of the rho-meson is introduced. The latter is essential for extrapolating lattice results from the quark-mass regime where the rho is observed to be a physical bound state.

hep-lat↗

arXiv2005

The QCD equation of state for two flavours at non-zero chemical potential

We present results of a simulation of 2 flavour QCD on a $16^3\times4$ lattice using p4-improved staggered fermions with bare quark mass $m/T=0.4$. Derivatives of the thermodynamic grand canonical partition function $Z(V,T,μ_u,μ_d)$ with respect to chemical potentials $μ_{u,d}$ for different quark flavours are calculated up to sixth order, enabling estimates of the pressure and the quark number density as well as the chiral condensate and various susceptibilities as functions of $μ_{u,d}$ via Taylor series expansion. Results are compared to high temperature perturbation theory as well as a hadron resonance gas model. We also analyze baryon as well as isospin fluctuations and discuss the relation to the chiral critical point in the QCD phase diagram. We moreover discuss the dependence of the heavy quark free energy on the chemical potential.

hep-ph↗

arXiv2005

Charmonium spectral functions in Nf=2 QCD

We report on a study of charmonium at high temperature in 2-flavour QCD. This is the first such study with dynamical fermions. Using an improved anisotropic lattice action, spectral functions are extracted from correlators in the vector and pseudoscalar channels. No signs of medium-induced suppression of the ground states are seen for temperatures up to 1.5T_c, while at T~2T_c there are clear signs of modifications. The current systematic and statistical uncertainties in our data, in particular the relatively coarse lattice and small volume, do not allow us to draw a firm conclusion at this stage.

hep-lat↗

arXiv2005

QCD at non-zero temperature and density from the lattice

The study of systems as diverse as the cores of neutron stars and heavy-ion collision experiments requires the understanding of the phase structure of QCD at non-zero temperature, T, and chemical potential, mu_q. We review some of the difficulties of performing lattice simulations of QCD with non-zero mu_q, and outline the re-weighting method used to overcome this problem. This method is used to determine the critical endpoint of QCD in the (mu_q,T) plane. We study the pressure and quark number susceptibility at small mu_q.

hep-lat↗

arXiv2005

Thermodynamics of Two Flavor QCD to Sixth Order in Quark Chemical Potential

We present results of a simulation of 2-flavor QCD on a 4x16^3 lattice using p4-improved staggered fermions with bare quark mass m/T=0.4. Derivatives of the thermodynamic grand canonical partition function Z(V,T,mu_u,mu_d) with respect to chemical potentials mu_(u,d) for different quark flavors are calculated up to sixth order, enabling estimates of the pressure and the quark number density as well as the chiral condensate and various susceptibilities as functions of mu_q = (mu_u + mu_d)/2 via Taylor series expansion. Furthermore, we analyze baryon as well as isospin fluctuations and discuss the relation between the radius of convergence of the Taylor series and the chiral critical point in the QCD phase diagram. We argue that bulk thermodynamic observables do not, at present, provide direct evidence for the existence of a chiral critical point in the QCD phase diagram. Results are compared to high temperature perturbation theory as well as a hadron resonance gas model.

hep-lat↗

arXiv2004

Fluctuations in the vicinity of the phase transition line for two flavor QCD

We study the susceptibilities of quark number, isospin number and electric charge in numerical simulations of lattice QCD at high temperature and density. We discuss the equation of state for 2 flavor QCD at non-zero temperature and density. Derivatives of $\ln Z$ with respect to quark chemical potential $(μ_q)$ are calculated up to sixth order. From this Taylor series, the susceptibilities are estimated as functions of temperature and $μ_q$. Moreover, we comment on the hadron resonance gas model, which explains well our simulation results below $T_c$.

hep-lat↗

arXiv2004

Numerical study of the equation of state for two flavor QCD at non-zero baryon density

We discuss the equation of state (EoS) for two flavor QCD at non-zero temperature and density. Derivatives of $\ln Z$ with respect to quark chemical potential $μ_q$ are calculated up to sixth order. From this Taylor series, the pressure, quark number density and associated susceptibilities are estimated as functions of temperature and $μ_q$. It is found that the fluctuations in the quark number density increase in the vicinity of the phase transition temperature and the susceptibilities start to develop a pronounced peak as $μ_q$ is increased. This suggests the presence of a critical endpoint in the $(μ_q, T)$ plane. Moreover, we comment on the hadron resonance gas model, which explains well our simulation results below $T_c$.

hep-lat↗

arXiv2004

Quenching Effects in the Hadron Spectrum

Lattice QCD has generated a wealth of data in hadronic physics over the last two decades. Until relatively recently, most of this information has been within the "quenched approximation" where virtual quark--anti-quark pairs are neglected. This review presents a descriptive discussion of the effects of removing this approximation in the calculation of hadronic masses.

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