Quark Mass Threshold Research Articles

A QCD Debye mass in a broad temperature range

The Debye mass sets a scale for the screening of static charges and the scattering of fast charges within a gauge plasma. Inspired by its potential cosmological applications, we determine a QCD Debye mass at 2-loop order in a broad temperature range (1 GeV ... 10 TeV), demonstrating how quark mass thresholds get smoothly crossed. Along the way, integration-by-parts identities pertinent to massive loops at finite temperature are illuminated.

Quark mass thresholds in QCD thermodynamics

We discuss radiative corrections to how quark mass thresholds are crossed, as a function of the temperature, in basic thermodynamic observables such as the pressure, the energy and entropy densities, and the heat capacity of high temperature QCD. The indication from leading order that the charm quark plays a visible role at surprisingly low temperatures, is confirmed. We also sketch a way to obtain phenomenological estimates relevant for generic expansion rate computations at temperatures between the QCD and electroweak scales, pointing out where improvements over the current knowledge are particularly welcome.

Anomalous pion decay in effective QCD at finite temperature

Abstract The width of the process π 0 → γγ is calculated within effective QCD at finite temperature. We use two different effective models which describe chiral symmetry breaking and its restoration at finite temperature in SU(2) ƒ : (i) an effective lagrangian with a nonlocal separable interaction kernel, (ii) A- and B-scaledNJL lagrangians which implement the scale anomaly of QCD. We calculate the temperature dependences of the quark and pion masses as well as of the pion-park-antiquark coupling strength below and above the double quark mass threshold. At zero temperature we obtain a fairly good agreement with the experimental value for the decay width. The temperature dependence of the decay width is related to that of the pion mass. Γ π 0 → γγ is enhanced in the vicinity of the Mott temperature ( M π = 2 m q ) for the nonlocal as well as for the A-scaled model and suppressed for the B-scaled model. We compare our results with the SU(2) Nambu-Jona-Lasinio model which is contained as a limiting case in our approach.

On the role of the quark mass thresholds in extrapolations of the running αs

The accuracy of the conventional treatment of quark mass thresholds in the QCD running coupling constant α s , based on the step approximation to the β-function, is investigated. The errors of extrapolating α s ( μ) from low energies to μ = M z , implied by this approximation, are shown to be of the same magnitude as the typical next-to-next-to-leading order contributions to α s and tend to increase the resulting α s . The importance of the proper choice of matching points is emphasized.

Next-to-next-to-leading order QCD analysis of the Gross-Llewellyn Smith sum rule and the higher twist effects

We present the next-to-next-to-leading order QCD analysis of the Gross-Llewellyn Smith (GLS) sum rule in deep inelastic lepton-nucleon scattering, taking into account dimension-two, twist-four power correction. We discuss in detail the renormalization scheme dependence of the perturbative QCD approximations, propose a procedure for an approximate treatment of the quark mass threshold effects and compare the results of our analysis to the recent experimental data of the CCFR Collaboration. From this comparison we extract the value of the strong coupling constant α s nnl (M z, MS ) = 0.115±0.001 (stat.) ±0.005 (syst.)±0.003 (twist)±0.0005 (scheme) . We stress the importance of an accurate measurement of the GLS sum rule and in particular of its Q 2 dependence.

Quark Mass Thresholds in Quarkonium Systems

We consider the role of quark mass thresholds in quarkonium systems, which are formed by the binding of heavy quark-antiquark pairs and are manifest in the IfJ and Y families of mesons. Such systems provide a probe of the interactions of quarks in a region accessible to the predictions of perturbative quantum chromodynamics (QCD), as well as in the long range region where the interaction is expected to show confining behaviour consistent with our knowledge of light quark systems. A prescription for deriving an interaction linking these two limits has been developed by Buchmiiller, Grunberg and Tye (1980, 1981) and has been successfully applied to the IfJ and Y systems, leading them to suggest a value for the QCD scale parameter AMS" of 500 MeV. In this work we extend this formalism by incorporating the effects of allowing the virtual quarks which contribute to the interaction to have finite masses. These masses must be considered energy dependent, and it is found that while the properties of IfJ and Y states are insensitive to the detailed way in which the masses vary, the presence of mass thresholds has the effect of weakening the ability of this model to set the value of AMS ' with 500 MeV becoming an approximate lower bound.