An infrared singularity in the damping rate for longitudinal gluons in hot QCD

We calculate γ_l(0), the damping rate for longitudinal on-shell gluons with zero momentum in hot QCD using the hard-thermal-loop scheme, and find it to be divergent in the infrared. We argue that this result suggests that the scheme may be sensitive in this sector and thus may need to be improved upon. We discuss the validity of our result and emphasize the fact that it depends on the way we perform our calculation. In particular, we do not exclude the possibility that alternative approaches that we briefly comment on may in principle yield infrared-safe results.     

Calculation of disconnected quark loops in lattice QCD

Calculation of disconnected quark loops in lattice QCD is very time consuming. Stochastic noise methods are generally used to estimate these loops. However, stochastic estimation gives large errors in the calculations of disconnected diagrams. We use the symmetric multi-probing source(SMP) method to estimate the disconnected quark loops, and compare the results with the Z(2) noise method and the spin-color explicit(SCE) method on a quenched lattice QCD ensemble with lattice volume 12~3× 24 and lattice spacing a ≈ 0.1 fm.. The results show that the SMP method is very suitable for the calculation of pseudoscalar disconnected quark loops. However, the SMP and SCE methods do not have an obvious advantage over the Z(2) noise method in the evaluation of the scalar disconnected loops.     

Calculation of the quark and gluon form factors to three loops in QCD

We describe the calculation of the three-loop QCD corrections to quark and gluon form factors. The relevant three-loop Feynman diagrams are evaluated and the resulting three-loop Feynman integrals are reduced to a small set of known master integrals by using integration-by-parts relations. Our calculation confirms the recent results by Baikov et al. for the three-loop form factors. In addition, we derive the subleading \( \mathcal{O}\left( \varepsilon \right) \) terms for the fermion-loop type contributions to the three-loop form factors which are required for the extraction of the fermionic contributions to the four-loop quark and gluon collinear anomalous dimensions. The finite parts of the form factors are used to determine the hard matching coefficients for the Drell-Yan process and inclusive Higgs-production in soft-collinear effective theory.     

QCD quark distributions in mesons

The valence-quark distributions in the π meson and in transversely and longitudinally polarized ρ mesons in the region of intermediate x are obtained on the basis of generalized QCD sum rules. Power-law corrections up to d=6 are taken into account. The quark distributions in the pion agree with those found from the data on Drell-Yan processes. A comparison of the results for the π and ρ mesons show that polarization effects are very significant and that the distribution functions do not have SU(6) symmetry.     

Heavy quark pair correlations in QCD

The azimuthal correlations of heavy quarks produced in high energy pp () collisions are calculated in the perturbative QCD without usual assumptions of the parton model. The virtual nature of the interacting gluons as well as their transverse motion and different polarizations are taken into account within the framework of the semihard processes theory describing the parton distributions in the region of the small Bjorken variable x. We give some predictions for the azimuthal correlations of charm and beauty hadrons produced at Tevatron-collider and LHC. Our approach can be of interest also for HERA energy region, since it shows a difference with the conventional parton model in the small x domain. Received: 7 August 1997 / Revised version: 20 Jauary 1998 / Published online: 6 November 1998     

Massive quark polarization in QCD subprocesses

It is well known that single polarization asymmetries vanish in QCD at momenta that are high relative to quark masses, due to chirality conservation. However, the particular form of those asymmetries for non-zero masses has not been explored. We have calculated the fourth order contributions to massive quark polarization in quark-quark and quark-gluon scattering. The dependence on the non-abelian character of the theory is emphasized and a comparison is made with the Mott scatterng limit. Unusual momentum dependence and large polarizations are noted for some regions of parameter space. the relation to polarized hyperon production is explored.     

The quark condensate in the bag-like QCD vacuum

The quark condensate in the bag model of the QCD vacuum is discussed. It is shown that the systems with J^PC=0⁺⁺ quantum numbers produce a much deeper minimum in the energy density than the recently suggested glueball condensate. Some consequences of this observation are pointed out.     

Hadroproduction of heavy quark flavors in QCD

Within the framework of perturbative QCD, we investigate the hadroproduction of charmed particles via two-gluon as well as quark-pair annihilation. The total production cross section is found to be sensitive to the choice of the scale parameter Q² and in general not free from non-perturbative effects. Charmed-particle production in the high-p_T region is also studied.     

Charm quark energy loss in QCD matter

The energy loss of heavy quarks in a quark-gluon plasma of finite size is studied within the light-cone path integral approach. A simple analytical formulation of the radiative energy loss of heavy quarks is derived. This provides a convenient way to quantitatively estimate the quark energy loss. Our results show that if the energy of a heavy quark is much larger than its mass, the radiative energy loss approaches the radiative energy loss of light quarks.     

Heavy quark interactions in finite temperature QCD

We study the free energy of a heavy quark-antiquark pair in a thermal medium. We construct a simple ansatz for the free energy for two quark flavors motivated by the Debye-Hückel theory of screening.Arrival of the final proofs: 4 July 2005PACS: 11.15.Ha, 12.38.Mh, 25.75.Nq     

Non-relativistic QCD for heavy quark systems

We employ a non-relativistic version of QCD (NRQCD) to study heavy bound states in the lowest approximation without fine structure. We use gluon configurations on a 6³×48 lattice at β=6.2 from the UKQCD Collaboration. For a bare quark mass near that of the b-quark (Ma=1.6) we obtain the bound state masses for the S-, P- and both types of D-waves. We also detect signals for two types of hybriges ( states). The results are sufficiently accurate to confirm that the values of the D-wave mass from both D-waves coincide thus indicating that the cubical invariance of the lattice is restored to full rotational invariance to a good approximation. We also study S- and P-wave masses for values of the range of bare quark mass Ma=1.0, 1.3, 1.6 and 1.9. The results confirm the idea that the S/P-splitting is relatively insensitive to the value of the bare quark mass.     

Moments of isovector quark distributions in lattice QCD

We investigate the connection of lattice calculations of moments of isovector parton distributions to the physical regime through extrapolations in the quark mass. We consider the one-pion loop renormalisation of the nucleon matrix elements of the corresponding operators and thereby develop formulae with which to extrapolate the moments of the unpolarised, helicity and transversity distributions. These formulae are consistent with chiral perturbation theory in chiral limit and incorporate the correct heavy-quark limits. In the polarised cases, the inclusion of intermediate states involving the -isobar is found to be very important. The results of our extrapolations are in general agreement with the phenomenological values of these moments where they are known, and for the first time we make reliable predictions for the low moments of the isovector transversity distribution.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.38.Gc Lattice QCD calculations - 11.30.Rd Chiral symmetries