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P V Landshoff

Publications and source records attributed to P V Landshoff.

At least 37 records · Page 2Linked to original sources
arXiv1999

Soft and hard pomerons

Regge theory provides an excellent description of small-$x$ structure-function data from $Q^2=0$ up to the highest available values. The large-$Q^2$ data should also be described by perturbative QCD: the two descriptions must agree in the region where they overlap. However, at present there is a serious lack in our understanding of how to apply perturbative QCD at small $x$. The usual lowest-order or next-to-lowest order expansion is not valid, at least not until $Q^2$ becomes much larger than is usually assumed; a resummation is necessary, but as yet we do not know how to do this resummation.

hep-ph↗
arXiv1999

QCD Pressure and the Trace Anomaly

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.

hep-th↗
arXiv1999

Urgent problems at small x

Regge theory provides an excellent fit to small-x structure-function data from Q^2=0 right up to the highest available values, but it also teaches us that conventional approaches to perturbative evolution are incorrect.

hep-ph↗
arXiv1998

Introduction to Equilibrium Thermal Field Theory

Within the next few years experiments at RHIC and the LHC will seek to create in the laboratory a quark-gluon plasma, the phase of matter through which the Universe passed very early in its life. It is believed that the plasma will survive long enough to reach thermal equilibrium. I give an introduction to the formalism of thermal field theory, the combination of statistical mechanics and quantum field theory needed to describe the plasma in thermal equilibrium, in a way that tries to keep close to the physics it describes.

hep-ph↗
arXiv1998

Renormalisation of the Nonperturbative Thermal Pressure

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.

hep-ph↗
arXiv1998

Small x: Two Pomerons!

Regge theory provides a very simple and economical description of data for (i) the proton structure function with x<0.07 and all available Q^2 values, (ii) the charm structure function, and (iii) gamma p --> J/psi p. The data are all in agreement with the assumption that there is a second pomeron, with intercept about 1.4 . They suggest also that the contribution from the soft pomeron is higher twist. This means that there is an urgent need to make perturbative evolution compatible with Regge theory at small and not-so-small x, and to reassess the magnitude of higher-twist contributions at quite small x.

hep-ph↗
arXiv1997

Final State Interactions in WW Production

It is shown that colour transparency causes nonperturbative colour-singlet final-state interactions to have a negligible effect on the production rate and the dijet mass spectra in $e^+e^-\to WW$. However, the same cannot be said of nonperturbative colour-octet exchange, for which we show that there are indications of observable effects, though we are unable to present precise estimates.

hep-ph↗
arXiv1997

Foam Diagram Summation at Finite Temperature

We show that large-$N$ $ϕ^4$ 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.

hep-ph↗
arXiv1997

Gauge Invariance and Factorisation in Exclusive Meson Production

The structure of the nonperturbative vector meson vertex function complicates the proof of the factorisation theorem for the reaction $γ^*p\to Vp$. It leads to additional contributions but, in a simple model for the vertex function, gauge invariance ensures that they cancel and factorisation is preserved.

hep-ph↗
arXiv1997

Introduction to thermal field theory

Within the next few years experiments at RHIC and the LHC will seek to create in the laboratory a quark-gluon plasma, the phase of matter in which the Universe was initially created. It is believed that the plasma will survive long enough to reach thermal equilibrium. I give an introduction to the formalism of thermal field theory, the combination of statistical mechanics and quantum field theory needed to describe the plasma in thermal equilibrium, in a way that tries to keep close to the physics it describes.

hep-ph↗
arXiv1997

Soft interactions

Existing data on total cross sections, on elastic scattering at small, moderate and large values of $t$, and on diffraction dissociation, reveal a surprisingly simple phenomenology, but they throw up many questions for the LHC to answer.

hep-ph↗
arXiv1996

Simple Physical Approach to Thermal Cutting Rules

A first-principles derivation is given of the imaginary part of a Green's function in real-time thermal field theory. The analysis and its conclusions are simpler than in the usual circled-vertex formalism. The relationship to Cutkosky-like cutting rules is explained.

hep-ph↗
arXiv1996

The Interest of Large-t Elastic Scattering

Existing data for large-$t$ $pp$ elastic-scattering differential cross-sections are energy-independent and behave as $t^{-8}$. This has been explained in terms of triple-gluon exchange, or alternatively through triple-singlet exchange. A discussion is given of the problems raised by each of these explanations, and of the possibility that at RHIC or LHC energies the exchange of three BFKL pomerons might result in a rapid rise with energy.

hep-ph↗
arXiv1996

The Soft Pomeron

The soft pomeron successfully correlates a wide variety of data. Its properties seem rather simple: it couples to single quarks and its coupling factorises.

hep-ph↗
arXiv1996

The BFKL Pomeron: Can it be Detected?

An estimate is derived for the absolute magnitude of BFKL pomeron exchange at $t=0$. The analysis takes account of energy conservation and of the need realistically to model nonperturbative contributions to the BFKL integral from infrared regions. Experiment finds that there is little or no room for a significant BFKL term in soft processes, and this constrains its magnitude in hard and semihard processes, so that it is unlikely to be detectable.

hep-ph↗
arXiv1995

γp \to Vp and γ^* p \to Vp

Exclusive vector meson production from real and virtual photons is a good probe of pomeron physics. In particular, it sheds light on how the soft pomeron is related to nonperturbative QCD, and how it couples to heavy flavours. It also raises the question whether there are two pomerons, a soft one and a hard one.

hep-ph↗
arXiv1995

INTRODUCTION TO THE POMERON STRUCTURE FUNCTION

In many ways the pomeron is like the photon, but there are important differences. Factorisation allows us to define a pomeron structure function, even though the pomeron is not a particle. Although we have a model for the light-quark content of the pomeron, which led to the prediction that a surprisingly large fraction of events at HERA would have an extremely-fast final-state proton, its charm and gluon content will have to be got from experiment. Because the pomeron is not a particle, we cannot derive a momentum sum rule.

hep-ph↗