Sourendu Gupta

The acceptance probability in the hybrid Monte Carlo method

Abstract We study the acceptance probability in the hybrid Monte Carlo method applied to lattice QCD with dynamical fermions We investigate in detail its dependence upon the step-size and the parameters β, m q and V , and suggest a simple phenomenological formula for this dependence.

Glueball-like screening masses in pure SU(3) at finite temperatures

We investigate the finite-temperature excitation spectrum in the gluon sector of SU (3) pure gauge theory through measurements of screening masses in correlations of loop operators. We develop the classification of such operators under the symmetry group of the “z-slice”. In the confined phase of the theory, we find that the spectrum dynamically realizes the zero-temperature symmetries. We observe a large thermal shift of the 0++ glueball mass. In the deconfined phase, the spectrum distinguishes between operators coupling to electrically and magnetically polarised gluon fields. The former yields a screening mass equal to the Wilson-line screening mass; the latter a method for the measurement of the magnetic mass in the high-temperature limit.

Final state J / psi suppression in nuclear collisions

The hadroproduction of J /ψ Ψ, Ψ, and γ on nuclear targets shows two initial state effects: a modification of the structure functions in nuclei leads a reduced production relative to p-p interactions, and parton scattering in nuclear matter broadens the transverse momentum spectrum in comparison to that in p-p collisions. If charmonium or bottonium states are to be used as probes for the nature of dense matter formed in nucleus-nucleus collisions, both these effects must first be taken into account. We do this and discuss the resulting final state features of J / ψ Ψ and Ψ production in high energy heavy ion experiments.

The quark propagator at finite temperature

Abstract We study the quark propagator at finite temperature on euclidean lattices in the Landau gauge, and compare the results to an O(g2) lattice weak coupling calculation. The screening mass obtained from spatial correlation functions in the chiral symmetric phase is close to the Matsubara frequency. The temporal correlation functions yield a much smaller screening mass, which approaches the perturbative result, meff2=g2T2/6, for T≳1.75 Tc. Deviations from the perturbative behaviour are seen for Tc⩽T⩽1.75 Tc. For T⩽Tc, the screening masses from both spatial and temporal correlation functions are large and close to half the mass of the ϱ-meson. Dispersion relations do not show any significant deviations from free particle behaviour.

Double parton scattering contribution toW+jets

We investigate the double parton scattering production ofW±+jets in proton colliders. At high energies an observable signal is expected from double parton collisions when events with small transverse momentumW together with two jets balancing inpT are selected. Double parton scattering is also a potentially serious background to several rare processes whose signal is aW++W−(→ev+jets) final state. A simplepTW cut seems to be sufficient to separate the genuineW-pair events from the double scattering background.

The physical degrees of freedom in hot quenched QCD

Abstract We demonstrate the absence of a low-lying bosonic pole in the spectral function for the pseudoscalar meson channel for T > T c . We also extract a set of effective four-Fermi couplings and show that this is small inthe vector channel for all T > T c , but not in the pseudovector channel. A perturbative estimate is compatible with this result. The computations have been performed in quenched QCD on lattices with N τ = 4.

Hadron Properties just before Deconfinement

Abstract We have investigated hadron screening masses, the chiral condensate, and the pion decay constant close to the deconfinement phase transition in the confined phase of QCD. The simulations were done in the quenched approximation, on a lattice of size 32 3 × 8. We examined temperatures ranging from 0.75 T c up to 0.92 T c . We see no sign of a temperature dependence in the chiral condensate or the meson properties, but some temperature dependence for the nucleon screening mass is not excluded.

Dynamics near a first-order phase transition with the Metropolis and Swendsen-Wang algorithms

We have simulated the 10-state 2d Potts model with the Metropolis and Swendsen-Wang algorithms, in order to compare their performances near a first-order phase transition. We show that a simple Markov model relates the frequencies of the flips between coexisting phases and the auto-correlation times. It leads to a coherent description of the dynamics. The ratio of the algorithm efficiencies has little temperature dependence. A slight increase of the cluster algorithm efficiency with volume, as compared to the Metropolis method, is suggested.

Spatial and temporal hadron correlators below and above the chiral phase transition

Hadronic correlation functions at finite temperature in QCD, with four flavours of dynamical quarks, have been analyzed both above and below the chiral symmetry restoration temperature. We have used both point and extended sources for spatial as well as temporal correlators. The effect of periodic temporal boundary conditions for the valence quarks on the spatial meson correlators has also been investigated. All our results are consistent with the existence of individual quarks at high temperatures. A measurement of the residual interaction between the quarks is presented.

Gluon asymmetries in the leptoproduction of J/ψ

We study J/ψ production, in deep inelastic scattering experiments with polarised beams and polarised targets. The spin asymmetries are seen to depend strongly on the particular form of the spin dependent gluon distributions used. Therefore, it should be possible in these experiments, to discriminate between different parametrizations of polarised gluon distributions, and hence between the distinctly different physical pictures of the proton spin underlying these parametrizations.