arXiv subjects
Publications and source records attributed to A. D. Martin.
We compare predictions based on small x (QCD) dynamics with recent data for deep inelastic events containing forward jets or forward pi0 mesons. We quantify the effect of imposing the (higher order) consistency condition on the BFKL equation and study uncertainties inherent in the QCD predictions. We also estimate the cross-section for the forward production of two jets.
We study the properties of dijet production in deep inelastic scattering using a unified BFKL/DGLAP framework, which includes important subleading ln (1/x) contributions. We calculate the azimuthal decorrelation between the jets. We compute the cross section for dijet production as a function of Q^2 and the jet transverse momentum, as well as calculate the total dijet rate. We compare the predictions with HERA data.
We show that a knowledge of diagonal partons at a low scale is sufficient to determine the off-diagonal (or skewed) distributions at a higher scale, to a good degree of accuracy. We quantify this observation by presenting results for the evolution of off-diagonal distributions from a variety of different inputs.
We show that the off-diagonal (or skewed) parton distributions are completely determined at small $x$ and $ξ$ by the (conventional) diagonal partons. We present predictions which can be used to estimate the off-diagonal distributions at small $x$ and $ξ$ at any scale.
We estimate the cross section for gamma p -> Upsilon p by two independent methods. First, by studying the corrections to the naive leading-order QCD formula and, second, by using parton-hadron duality. The estimates are in good agreement with each other and with the recent measurements of the cross section at HERA.
We analyse the data for the proton structure function $F_2$ over the entire $Q^2$ domain, including especially low $Q^2$, in terms of perturbative and non-perturbative QCD contributions. The small distance configurations are given by perturbative QCD, while the large distance contributions are given by the vector dominance model and, for the higher mass $q \overline{q}$ states, by the additive quark approach.
We summarize the discussions on the value of the survival probabilities of large rapidity gap (LRG) processes at Durham WS'98.
We calculate the cross sections for neutrino interactions with (isoscalar) nuclear targets in the energy domain all the way up to 10^12 GeV. Small x QCD effects are included by using a unified BFKL/DGLAP formalism which embodies non-leading log(1/x) contributions. The few free parameters which specify the input parton distributions are determined by fitting to HERA deep inelastic data. The attenuation of neutrinos transversing the Earth at different nadir angles is calculated for a variety of energy spectra for neutrinos originating from different sources (from Active Galactic Nuclei, Gamma ray bursts, top-down models), as well as for atmospheric neutrinos. For this purpose we solve the transport equation which includes regeneration due to neutral current neutrino interactions, besides attenuation.
We extend a recent global analysis of nucleon parton distributions carried out at next-to-leading order (NLO) in the MSbar scheme to provide distributions in the so-called DIS scheme. We pay particular attention to the translation of the heavy quark distributions in going from the MSbar to the DIS scheme. We repeat the global analysis at leading order (LO) and comment on the major effects produced when going from LO to NLO. Finally we include in the global analysis a freely parameterised form of possible higher twist contributions to make an exploratory study of the size of these effects as a function of x.
We construct off-diagonal parton distributions defined on the interval 0 < X < 1 starting from the off-forward distributions defined by Ji. We emphasize the particular role played by the symmetry relations in the "ERBL-like" region. We find the evolution equations for the off-diagonal distributions which conserve these symmetries. We present numerical results of the evolution, and verify that the analytic asymptotic forms of the parton distributions are reproduced. We also compare the constructed off-diagonal distributions with the non-forward distributions defined by Radyushkin and comment on the singularity structure of the basic amplitude written in terms of the off-diagonal distributions.
We analyse the data for the proton structure function $F_2$ over the entire $Q^2$ domain, including especially low $Q^2$, in terms of perturbative and non-perturbative QCD contributions. The small distance configurations are given by perturbative QCD, while the large distance contributions are given by the vector dominance model and, for the higher mass $q \bar{q}$ states, by the additive quark approach. The interference between states of different $q\bar{q}$ mass (in the perturbative contribution) is found to play a crucial role in obtaining an excellent description of the data throughout the whole $Q^2$ region, including photoproduction.
We study the effects of intrinsic transverse momentum smearing on J/psi production both at the Tevatron and at HERA. For the case of large-p_T J/psi production at the Tevatron, the effects due to k_T smearing are mild. On the other hand, inelastic J/psi photoproduction at HERA is very sensitive to the k_T smearing and, in fact, with a reasonable value of it is possible to resolve the large-z discrepancy seen by comparing non-relativistic QCD (NRQCD) predictions with the HERA data. We conclude that, with the present kinematic cuts, photoproduction at HERA is not a good test of NRQCD.
Diffractive dijet photoproduction is proposed as a probe of the off-diagonal gluon distribution and its evolution. Predictions for the transverse momentum distribution of the jets are given. Differences with DGLAP evolution are highlighted.
We present a new analysis of parton distributions of the proton. This incorporates a wide range of new data, an improved treatment of heavy flavours and a re-examination of prompt photon production. The new set (MRST) shows systematic differences from previous sets of partons which can be identified with particular features of the new data and with improvements in the analysis. We also investigate the sensitivities of the results to (i) the uncertainty in the determination of the gluon at large x, (ii) the value of alpha_S(M_Z^2) and (iii) the minimum Q^2 cut on the data that are included in the global fit.
We propose exclusive diffractive dijet photoproduction as an ideal measure of the off-diagonal gluon distribution at high scales. We solve the off-diagonal evolution equations for the gluon and quark singlet over the full kinematic domain. We discuss the nature of the solutions of these equations, which embody both DGLAP and ERBL evolution. We give predictions for the transverse momentum distribution of the jets. In particular we quantify the enhancement arising from the evolution of the off-diagonal parton distributions.
We perform an exploratory study of higher twist contributions to deep inelastic scattering. We estimate the size of two major sources of higher twist, namely absorptive corrections and the vector meson dominance (VMD) contribution. We find that they give a sizeable higher twist component of F_2. For example at x = 0.01 it is about 8% (17%) at Q^2 = 10 GeV^2 (4 GeV^2), reaching up to 27% at x = 10^{-4} and Q^2 = 4 GeV^2. At the smaller x value the largest contribution comes from absorptive corrections, while at the larger values of x the VMD term dominates.
We present a simple physical description of the off-diagonal gluon distribution g (x,x^\prime,Q^2) and splitting function P_{gg}(x,x^\prime) which enter the amplitude for diffractive vector meson production. We study the off-diagonal effects both on the evolution in Q^2 and on the input distribution. We predict the ratio $R$ of the off-diagonal to the diagonal gluon density, x^\prime g(x,x^\prime,Q^2)/xg(x, Q^2), as a function of the kinematic variables.
We outline the existing descriptions of the charm component of the deep inelastic proton structure function F2. We discuss recent approaches to include charm mass effects in the parton evolution equations and the coefficient functions.