arXiv subjects

Sourendu Gupta

Publications and source records attributed to Sourendu Gupta.

125 records · Page 7Linked to original sources

arXiv1997

Higher Orders in the Colour-Octet Model of J/ψProduction

We study the hadro- and photo-production of $\bar cc$ and $\bar bb$ mesons at low transverse momentum to high orders in the relative velocity of the pair, v, in non-relativistic QCD. We evaluate cross sections to order v^7 for $η_c$, sufficient for studies of photo-production in the almost-elastic region. For all other charmonium states we find the cross section to order v^9, sufficient for studies of the ratio of $χ_{c1}$ and $χ_{c2}$ production rates. We find recurrence formulae for generating terms at even higher orders, should they be needed.

hep-ph↗

arXiv1997

Thermal Effects on Two-photon Decays of Pseudo-scalars

We study the effect of finite temperatures and Fermion density on the effective pseudo-scalar-photon vertex induced by the triangle diagram. The manifestly covariant calculations show that when the pseudo-scalar mass is much less than the temperature, then there is a large enhancement of the decay rate. Alternatively, when the temperature is much higher than the Fermion mass, or the Fermion chemical potential is large, the lifetime is enhanced. Other related processes, and applications of these results to cosmology and astrophysics, are discussed.

hep-ph↗

arXiv1996

J/psi Spin Asymmetries in the Colour-Octet Model

We investigate the pattern of asymmetries in the colour-octet model for inclusive production of charmonium with polarised initial state hadrons and photons. We find that singly polarised asymmetries vanish. The double polarised asymmetry for J/psi involves a new combination of long-distance matrix element. We also investigate the asymmetries for eta_c, chi_c and Upsilon production. The asymmetries distinguish well between different models of charmonium production.

hep-ph↗

arXiv1996

Collinear Divergences at One-loop Order for External Particles in a Heat-bath

In hard interactions between external particles incident on a heat-bath, we show that large logarithms are generated when a radiated or absorbed gauge boson is collinear with the initial fermion momentum. These logarithms can be absorbed into process independent splitting/absorption probabilities. Unlike the zero-temperature case, however, they depend explicitly on the temperature and the scale of the interaction.

hep-ph↗

arXiv1996

Colour-octet Contributions to $x_F$ Distributions in $J/ψ$ Hadro-production at Fixed Target Energies

We study J/ψproduction at fixed-target energies, including the colour-octet production mechanisms predicted by NRQCD. In an earlier paper, we found that the octet components were crucial in understanding p_T-integrated forward (x_F > 0) cross-sections. Here we make a detailed comparison of the theoretical predictions with measured x_F distributions from fixed-target experiments. We find that the model predictions agree well with data. Taking into account higher orders in NRQCD, we argue that sigma(chi_1)/sigma(chi_2)=0.6, in excellent agreement with data. Similiar arguments also predict that sigma(chi)/sigma(J/psi) is roughly equal in photo- and hadro-production.

hep-ph↗

arXiv1996

Colour-octet Contributions to J/psi Hadroproduction at Fixed Target Energies

We investigate the integrated cross section for forward $\jpsi$ production in proton-proton and pion-proton collisions at centre of mass energies upto 60 GeV. We find that colour-octet contributions to the cross section are crucially important, and their inclusion produces a very good description of the data. Since the values of non-perturbative matrix elements are fixed by other experiments, these predictions are parameter-free.

hep-ph↗

arXiv1995

Bloch-Nordsieck Thermometers: One-loop Exponentiation in Finite Temperature QED

We study the scattering of hard external particles in a heat bath in a real-time formalism for finite temperature QED. We investigate the distribution of the 4-momentum difference of initial and final hard particles in a fully covariant manner when the scale of the process, $Q$, is much larger than the temperature, $T$. Our computations are valid for all $T$ subject to this constraint. We exponentiate the leading infra-red term at one-loop order through a resummation of soft (thermal) photon emissions and absorptions. For $T>0$, we find that tensor structures arise which are not present at $T=0$. These carry thermal signatures. As a result, external particles can serve as thermometers introduced into the heat bath. We investigate the phase space origin of $\log(Q/m)$ and $\log(Q/T)$ terms.

hep-ph↗

arXiv1995

Hadron Properties just before Deconfinement

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 \mbox{$32^{3}\times 8$}. We examined temperatures ranging from $0.75\tc$ up to $0.92\tc$. 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.

hep-lat↗

arXiv1995

Jet Missing PT for Quark-Gluon Plasma Thermometry

At the LHC, where hard multi-jet events are common, large imbalances in the total jet-\pt{} can arise due to the interaction of jets with the plasma formed in heavy-ion collisions. We find that this missing-\pt{} spectrum yields a measurement of the initial temperature of the plasma. Using commonly accepted guesses for this temperature, and ISAJET estimates for jet cross sections, we predict an order of thousand events at LHC with unbalanced $\pt>50$ GeV.

hep-ph↗

arXiv1994

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.

hep-lat↗

arXiv1993

Interface Tensions and Perfect Wetting in the Two-Dimensional Seven-State Potts Model

We present a numerical determination of the order-disorder interface tension, \sod, for the two-dimensional seven-state Potts model. We find $\sod=0.0114\pm0.0012$, in good agreement with expectations based on the conjecture of perfect wetting. We take into account systematic effects on the technique of our choice: the histogram method. Our measurements are performed on rectangular lattices, so that the histograms contain identifiable plateaus. The lattice sizes are chosen to be large compared to the physical correlation length. Capillary wave corrections are applied to our measurements on finite systems.

hep-lat↗

arXiv1993

Instantons and Surface Tension at a First-Order Transition

We study the dynamics of the first order phase transition in the two dimensional 15-state Potts model, both at and off equilibrium. We find that phase changes take place through nucleation in both cases, and finite volume effects are described well through an instanton computation. Thus a dynamical measurement of the surface tension is possible. We find that the order-disorder surface tension is compatible with perfect wetting. An accurate treatment of fluctuations about the instanton solution is seen to be of great importance.

hep-lat↗

arXiv1993

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 realises 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.

hep-lat↗

arXiv1993

Electromagnetic Signals and Backgrounds in Heavy-Ion Collisions:

Aspects of the dilepton spectrum in heavy-ion collisions are discussed, with special emphasis on using lattice computations to guide the phenomenology of finite temperature hadronic matter. The background rates for continuum dileptons expected in forthcoming experiments are summarised. Properly augmented by data from ongoing measurements at HERA, these rates will serve as a calibrating background for QGP searches. Recent results on the temperature dependence of the hadronic spectrum obtained in lattice computations below the deconfinement transition are summarised. Light vector meson masses are strongly temperature dependent. Accurate measurements of a resolved $ρ$-peak in dimuon spectra in present experiments are thus of fundamental importance. [file-length=200k characters; instructions for processing given; some Latex versions give error messages: ignore them completely]

hep-ph↗

arXiv1993

Finite-Size Scaling at Phase Coexistence

{}From a finite-size scaling (FSS) theory of cumulants of the order parameter at phase coexistence points, we reconstruct the scaling of the moments. Assuming that the cumulants allow a reconstruction of the free energy density no better than as an asymptotic expansion, we find that FSS for moments of low order is still complete. We suggest ways of using this theory for the analysis of numerical simulations. We test these methods numerically through the scaling of cumulants and moments of the magnetization in the low-temperature phase of the two-dimensional Ising model. (LaTeX file; ps figures included as shar file)

hep-lat↗

arXiv1993

Critical Hysteresis

Hysteresis is observed at second order phase transitions. Universal scaling formulæ for the areas of hysteresis loops are written down. Critical exponents are defined, and related to other exponents for static and dynamic critical phenomena. These relations are verified with Langevin dynamics in both the critical and tricritical mean-field models. A finite-size scaling relation is tested in the two-dimensional Ising model with heat-bath dynamics. (Shar file; 2 figures created through dvips directives from 4 PS files included)

cond-mat↗

arXiv1992

The Degrees of Freedom in Hot Quenched QCD

In quenched QCD, on lattices with $\nt=4$, the absence of a pole in the pseudoscalar meson channel for $T>T_c$ is demonstrated. A set of effective 4-fermi couplings is extracted. It is observed that this coupling is small in the vector channel for all $T>T_c$, but not in the pseudoscalar channel. The temperature dependence of hadronic parameters below \tc{} is found to be small. NOTE: Latex source, no figures, needs espcrc2.sty from hep-lat macros.

hep-lat↗