arXiv subjects
Publications and source records attributed to J. R. Forshaw.
We perform a global fit to high energy precision electroweak data in a Higgs model containing the usual isospin doublet plus a real isospin triplet. The analysis is performed in terms of the oblique parameters S, T and U and we show that the mass of the lightest Higgs boson can be as large as 2 TeV.
We have modified the HERWIG event generator to incorporate diffractive interactions. All standard HERWIG hard subprocesses are available.
The diffractive production of vector bosons at the Tevatron is studied. We take a hard pomeron flux and use the H1 parton density functions extracted from their measurement of F_2^{D(3)}. To this we add a reggeon exchange contribution. We find that the ratio of diffractive to non-diffractive W boson production is in good agreement with the CDF data. We note that the poorly understood reggeon contribution might be as large as the pomeron contribution in the kinematic range of the data. We also note that our pomeron exchange contribution is much smaller than earlier predictions due to our use of a hard pomeron flux. All our results have been obtained using the lowest order matrix elements and we have compared to the results obtained using a modified version of Herwig which is capable of generating a wide variety of diffractive scattering processes. Gap survival is estimated using Pythia and is found to be around 60%. We make predictions which could be compared to future measurements of diffractive Drell-Yan production.
We describe a formalism for solving the BFKL equation with a coupling that runs for momenta above a certain infrared cutoff. By suitably choosing matching conditions proper account is taken of the fact that the BFKL diffusion implies that the solution in the infrared (fixed coupling) regime depends upon the solution in the ultraviolet (running coupling) regime and vice versa. Expanding the BFKL kernel to a given order in the ratio of the transverse momenta allows arbitrary accuracy to be achieved.
We have performed a detailed phenomenological investigation of the hard colour singlet exchange process observed at the Tevatron in events that have a large rapidity gap between outgoing jets. We include the effects of multiple interactions to obtain a prediction for the gap survival factor. Comparing the data on the fraction of gap events with the prediction from BFKL pomeron exchange we find agreement provided that a constant value of alpha_s is used in the BFKL calculation. Moreover, the value of alpha_s is in line with that extracted from measurements made at HERA.
We employ a parameterisation of the proton dipole cross section previously extracted from electroproduction and photoproduction data to predict the diffractive structure function F2D(3)(Q^2, beta, xpom). Comparison with HERA H1 data yields good agreement.
We calculate the subjet rates for jets produced in hadron collisions. The kt algorithm is used to define the jets and allows the theoretical calculation to sum both the leading and next-to-leading logarithms in the resolution variable, ycut. We also ensure that our calculation matches exactly the leading order in alpha_s result and has sensible behaviour near thresholds.
We report on a successful attempt to extract the cross-section for the high-energy scattering of colour dipoles of fixed transverse size off protons using electroproduction and photoproduction total cross-section data, subject to the constraint provided by the ratio of the overall photon dissociation cross-section to the total cross-section.
The study of those rapidity gap processes where a large momentum is transferred across the rapidity gap provides an ideal opportunity to understand the gap producing mechanism wholly within the framework of QCD perturbation theory. The current theoretical and experimental status of this `high-t diffraction' is reviewed.
We examine the effect of suppressing the emission of gluons which are close by in rapidity in the BFKL framework. We show that, after removing the unphysical collinear logarithms which typically arise in formally higher orders of the perturbative expansion, the effect of the rapidity veto is greatly reduced. This is an important result, since it supports the use of multi-Regge and quasi-multi-Regge kinematics which are implemented in the leading and next-to-leading order BFKL formalism.
We study the diffractive production of photons in gamma-p and gamma-gamma collisions. We specifically compute the rates for gamma*-p -> gamma-X and for gamma*-gamma* -> gamma-gamma, where X denotes the proton dissociation. We focus on the rates at large momentum transfers, -t >> Lambda^2, where we are most confident in the use of QCD perturbation theory. However, our calculations do allow us to study the -t -> 0 behaviour of the gamma*-gamma*-> gamma-gamma process in the region where the incoming photons are sufficiently virtual.
We argue that the massive Yang-Mills model of Kunimasa and Goto, Slavnov, and Cornwall, in which massive gauge vector bosons are introduced in a gauge-invariant way without resorting to the Higgs mechanism, may be useful for studying diffractive scattering of strongly interacting particles. We perform in this model explicit calculations of S-matrix elements between quark states, at tree level, one loop, and two loops, and discuss issues of renormalisability and unitarity. In particular, it is shown that the S-matrix element for quark scattering is renormalisable at one-loop order and is only logarithmically non-renormalisable at two loops. The discrepancies in the ultraviolet regime between the one-loop predictions of this model and those of massless QCD are discussed in detail. Also, some of the similarities and differences between the massive Yang-Mills model and theories with a Higgs mechanism are analysed at the level of the S-matrix. Finally, we briefly discuss the high-energy behaviour of the leading-order amplitude for quark-quark elastic scattering in the diffractive region. The above analysis sets up the stage for carrying out a systematic computation of the higher order corrections to the two-gluon exchange model of the Pomeron using massive gluons inside quantum loops.
This talk summarises our work on the calculation of small-$x$ jet rates within the BFKL and CCFM approaches. The two approaches are proven to yield the same results at the leading logarithm level to order ${\bar α}_{S}^{3}$. The proof is then extended to all orders.
This is a summary of the contributions on the next-to-leading order corrections to the BFKL equation which were presented to the `Small-x and Diffraction' working group at the 1998 Durham Workshop on HERA Physics.
The contributions to the deep inelastic scattering structure function which arise from emission of zero, one, two or three resolvable gluons and any number of unresolvable ones are computed to order ${\bar α}_{S}^{3}$. Coherence effects are taken into account via angular ordering and are demonstrated to yield (at the leading logarithm level) the identical results to those obtained assuming the multi-Regge kinematics of BFKL.
Diffractive scattering in the presence of a large momentum transfer is an ideal place to study the short distance rapidity gap producing mechanism. Previous studies (experimental and theoretical) in this area have focussed on gaps between jets and on high-$t$ vector meson production. We propose the measurement of double dissociation at high-$t$. We examine the numerous advantages to studying this more inclusive process and conclude that it is an ideal place to study short distance diffraction.
Some recent developments in the physics of photon induced reactions are discussed. My presentation is biased towards HERA physics with the other summary talk (of David Miller) being biased towards the gamma-gamma topics. Within the context of the data which were presented, I concentrate upon the the following topics: diffraction; jets; prompt photons; open charm and charmonium.
Some selected topics in photoproduction are reviewed. In particular, the focus is on quasi-real photon-proton scattering as measured at HERA. Single & dijet rates and jet profiles are discussed, as are open charm production and inelastic charmonium production. The many intriguing theoretical issues which can be addressed through study of these processes are highlighted.