Publications (15)0 Total impact
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Article: Frozen ghosts in thermal gauge field theory
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ABSTRACT: We review an alternative formulation of gauge field theories at finite temperature where unphysical degrees of freedom of gauge fields and the Faddeev-Popov ghosts are kept at zero temperature.04/2009; -
Article: QCD Pressure and the Trace Anomaly
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ABSTRACT: Exact relations between the QCD thermal pressure and the trace anomaly are derived. These are used, first, to prove the equivalence of the thermodynamic and the hydrodynamic pressure in equilibrium in the presence of the trace anomaly, closing a gap in previous arguments. Second, in the temporal axial gauge a formula is derived which expresses the thermal pressure in terms of a Dyson-resummed two-point function. This overcomes the infrared problems encountered in the conventional perturbation-theory approach. Comment: 9 pages plain tex05/1999; -
Article: Some Remarks on the Pomeron and the Odderon in Theory and Experiment
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ABSTRACT: On March 19-21, 1998, a workshop devoted to questions of the pomeron and the odderon in high energy scattering was held in Heidelberg. This note gives a personal account of some of the issues discussed at this workshop. Of course, misconceptions and misunderstandings are to be blamed on us, not on the other participants of the workshop. A puzzle of odderon physics is identified and a convenient reaction for its experimental study is discussed.09/1998; -
Article: Renormalisation of the Nonperturbative Thermal Pressure
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ABSTRACT: We show how the fully resummed thermal pressure is rendered ultraviolet finite by standard zero-temperature renormalisation. The analysis is developed in a 6-dimensional scalar model that mimics QED and has $N$ flavours. The $N\to\infty$ limit of the model can be calculated completely. At a critical temperature, one of the degrees of freedom has vanishing screening mass like the transverse gauge bosons in four-dimensional finite-temperature perturbation theory. The renormalised nonperturbative interaction pressure of this model is evaluated numerically. Comment: 27 pages, plain tex, with 10 figures embedded using epsf06/1998; -
Article: Foam Diagram Summation at Finite Temperature
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ABSTRACT: We show that large-$N$ $\phi ^4$ theory is not trivial if one accepts the presence of a tachyon with a truly huge mass, and that it allows exact calculation. We use it to illustrate how to calculate the exact resummed pressure at finite temperature and verify that it is infrared and ultraviolet finite even in the zero-mass case. In 3 dimensions a residual effect of the resummed infrared divergences is that at low temperature or strong coupling the leading term in the interaction pressure becomes independent of the coupling and is 4/5 of the free-field pressure. In 4 dimensions the pressure is well-defined provided that the temperature is below the tachyon mass. We examine how rapidly this expansion converges and use our analysis to suggest how one might reorganise perturbation theory to improve the calculation of the pressure for the QCD plasma. Comment: 18 pages plain tex, with 8 figures embedded with epsf. Equation (2.15) has been corrected and the consequent changes made to the figures. A further analytic result has been added to the 3-dimensional calculation08/1997; -
Article: Eliminating Infrared Divergences in the Pressure
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ABSTRACT: The pressure of a system in thermal equilibrium is expressed as a mass integral over a sum of thermal propagators. This allows a Dyson resummation and is used to demonstrate that potential infrared divergences are rendered harmless. Comment: 6 pages plain tex, including figures embedded using epsf10/1996; -
Article: Some tests for the helicity structure of the pomeron in $ep$ collisions
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ABSTRACT: We discuss the reaction $e p \to e \tilde{p} X$ in the one-photon exchange approximation, where it is in essence the reaction $\gamma^{*} p \to \tilde{p} X$. A large rapidity gap is required between the particle or particles of the proton remnant $\tilde{p}$ and those of $X$. We define a suitable azimuthal angle $\varphi$ between a leptonic and a hadronic plane. The dependence of the cross section on $\varphi$ is given explicitly and can be used to extract cross sections and interference terms for the reaction $\gamma^{*} p \to \tilde{p} X$ corresponding to the various helicities of the virtual photon $\gamma^{*}$. The interference terms can be used to test models for the large rapidity gap events in a sensitive way. We discuss in detail models with factorizing Pomeron exchange and in particular the Donnachie-Landshoff Pomeron model. We make some remarks on soft colour exchange models and on possible effects of QCD background vacuum fields. We conclude with a suggestion to look for Odderon exchange in exclusive deep inelastic high energy reactions like $\gamma^{*} p \to \tilde{p} \pi^{0}$ and $\gamma^{*} p \to \tilde{p} \eta$.06/1996; -
Article: Thermalisation of Longitudinal Gluons
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ABSTRACT: In the usual real-time finite-temperature gauge theory both the physical and the unphysical degrees of freedom are thermalised. We discuss the alternative approach where only the physical transverse components of the gauge field have bare thermal propagators, whereas the unphysical degrees of freedom are not heated. We show how pinch singularities are avoided: sometimes this requires resummation. If only the hard thermal loop is included in the resummation, the spatially-longitudinal component of the gauge field, which contains an extra collective plasmon mode, becomes fully thermalised, though the Faddeev-Popov ghost and the remaining unphysical component of the gauge field remain frozen. Comment: 10 pages, 1 figure appended as pictex-file, DAMTP 93-06, TUW-93-0303/1993; -
Article: Covariant gauges at finite temperature
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ABSTRACT: A prescription is presented for real-time finite-temperature perturbation theory in covariant gauges, in which only the two physical degrees of freedom of the gauge-field propagator acquire thermal parts. The propagators for the unphysical degrees of freedom of the gauge field, and for the Faddeev-Popov ghost field, are independent of temperature. This prescription is applied to the calculation of the one-loop gluon self-energy and the two-loop interaction pressure, and is found to be simpler to use than the conventional one. Comment: 11 pages plus pictex figures (the leading-temperature contributions given in sect 3.1 were incomplete) CERN-TH-6491/92, published in Nucl Phys B383 (1992) 60705/1992; -
Article: Vacuum structure and diffraction scattering
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ABSTRACT: A simple model of the vacuum is developed in order to explain the properties of the pomeron observed in experiments. In the model, pomeron exchange corresponds to two-gluon exchange. We explain how it can be that when the pomeron couples to a hadron, the two gluons couple predominantly to the same quark in the hadron, and how together their coupling is similar to that of aC=1 isoscalar photon.Zeitschrift für Physik C 01/1987; 35(3):405-416. -
Article: Exclusive production of heavy mesons in photon-photon collisions: The double scattering mechanism
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ABSTRACT: We present two mechanisms which can compete with constituent interchange in the exclusive production at wide angle of heavy mesons in photon-photon collisions. We show that they have the same energy behaviour as constituent interchange, and that their amplitudes are free from large logarithms. We exploit this to calculate the double-scattering diagrams numerically. We find that while constituent interchange dominates at 90, double scattering becomes dominant for angles less than about 45.Zeitschrift für Physik C 01/1983; 20(1):43-52. -
Article: Covariant gauges at finite temperature
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ABSTRACT: A prescription is presented for real-time finite-temperature perturbation theory in covariant gauges, in which only the two physical degrees of freedom of the gauge-field propagator acquire thermal parts. The propagators for the unphysical degrees of freedom of the gauge field, and for the Faddeev-Popov ghost field, are independent of temperature. This prescription is applied to the calculation of the one-loop gluon self-energy and the two-loop interaction pressure, and is found to be simpler to use than the conventional one.Nuclear Physics B. -
Article: Renormalisation of the non-perturbative thermal pressure
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ABSTRACT: We show how the fully resummed thermal pressure is rendered ultraviolet finite by standard zero-temperature renormalisation. The analysis is developed in a six-dimensional scalar model that mimics QED and has N flavours. The N → ∞ limit of the model can be calculated completely. At a critical temperature, one of the degrees of freedom has vanishing screening mass like the transverse gauge bosons in four-dimensional finite-temperature perturbation theory. The renormalised non-perturbative interaction pressure of this model is evaluated numerically.Nuclear Physics B. -
Article: Foam diagram summation at finite temperature
[show abstract] [hide abstract]
ABSTRACT: We show that large-Nφ4 theory is not trivial if one accepts the presence of a tachyon with a truly huge mass, and that it allows exact calculation. We use it to illustrate how to calculate the exact resummed pressure at finite temperature and verify that it is infrared and ultraviolet finite even in the zero-mass case. In three dimensions a residual effect of the resummed infrared divergences is that at low temperature or strong coupling the leading term in the interaction pressure becomes independent of the coupling and is of the free-field pressure. In four dimensions the pressure is well defined provided that the temperature is below the tachyon mass. We examine how rapidly this expansion converges and use our analysis to suggest how one might reorganise perturbation theory to improve the calculation of the pressure for the QCD plasma.Nuclear Physics B. -
Article: QCD pressure and the trace anomaly
[show abstract] [hide abstract]
ABSTRACT: Exact relations between the QCD thermal pressure and the trace anomaly are derived. These are used, first, to prove the equivalence of the thermodynamic and the hydrodynamic pressure in equilibrium in the presence of the trace anomaly, closing a gap in previous arguments. Second, in the temporal axial gauge a formula is derived which expresses the thermal pressure in terms of a Dyson-resummed two-point function. This overcomes the infrared problems encountered in the conventional perturbation-theory approach.Physics Letters B.
