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A. D. Martin

Publications and source records attributed to A. D. Martin.

At least 217 records · Page 12Linked to original sources
arXiv2000

Soft diffraction and the elastic slope at Tevatron and LHC energies: a multi-Pomeron approach

We present a formalism for high energy soft processes, mediated by Pomerons, which embodies pion-loop insertions in the Pomeron trajectory, rescattering effects via a two-channel eikonal and high-mass diffractive dissociation. It describes all the main features of the data throughout the ISR to Tevatron energy interval. We give predictions for soft diffractive phenomena at the LHC energy, and we calculate the different survival probabilities of rapidity gaps which occur in various diffractive processes.

hep-ph↗
arXiv2000

Estimating the effect of NNLO contributions on global parton analyses

We use the recent estimates of NNLO splitting functions, made by van Neerven and Vogt, to perform exploratory fits to deep inelastic and related hard scattering data. We investigate the hierarchy of parton distributions obtained at LO, NLO and NNLO, and, more important, the stability of the resulting predictions for physical observables. We use the longitudinal structure function $F_L$ and the cross sections $σ_W, σ_Z$ for $W$ and $Z$ hadroproduction as examples. For $F_L$ we find relatively poor convergence, with increasing order, at small $x$; whereas $σ_{W,Z}$ are much more reliably predicted.

hep-ph↗
arXiv2000

The unintegrated gluon distribution from the CCFM equation

The gluon distribution f(x, k_t^2,mu^2), unintegrated over the transverse momentum k_t of the gluon, satisfies the angular-ordered CCFM equation which interlocks the dependence on the scale k_t with the scale μof the probe. We show how, to leading logarithmic accuracy, the equation can be simplified to a single scale problem. In particular we demonstrate how to determine the two-scale unintegrated distribution f(x,k_t^2,mu^2) from knowledge of the integrated gluon obtained from a unified scheme embodying both BFKL and DGLAP evolution.

hep-ph↗
arXiv2000

Higgs or dijet production in double rapidity gap events

We quantify the rate of double diffractive Higgs/dijet production at $pp$ (or $p\bar{p}$) colliders. The suppression due to QCD bremsstrahlung is calculated perturbatively up to single $\log$ accuracy. The survival probability of the rapidity gaps is discussed. A first comparison with experiment is made.

hep-ph↗
arXiv2000

QCD

We discuss issues of QCD at the LHC including parton distributions, Monte Carlo event generators, the available next-to-leading order calculations, resummation, photon production, small x physics, double parton scattering, and backgrounds to Higgs production.

hep-ph↗
arXiv2000

Ultrahigh energy neutrino physics

We discuss a problem concerning the ultrahigh energy neutrino propagation through the Earth. We present calculation of the neutrino-nucleon cross section at high energies, based on the unifed evolution equation at small x. We also show the solution of the transport equation for different neutrino fluxes originating from active galactic nuclei, gamma ray bursts and top-down model.

hep-ph↗
arXiv2000

Can the Higgs be seen in rapidity gap events at the Tevatron or the LHC?

Double diffractive Higgs production at $pp$ (or $\bar{p}p$) colliders continues to attract attention as a potential signal in the search for the boson. We present improved perturbative QCD estimates of the event rates for both the exclusive and inclusive double diffractive Higgs processes, paying particular attention to the survival probability of the rapidity gaps. We find that the major uncertainty is in the prediction for the survival probability associated with soft rescattering. We show that an analogous process, the double diffractive production of a pair of jets with large values of $E_T$, has an event rate which makes it accessible at the Tevatron. Observation of this process can therefore be used as a luminosity monitor for two- gluon exchange processes, such as the production of a Higgs boson with rapidity gaps on either side.

hep-ph↗
arXiv2000

Unintegrated parton distributions and prompt photon hadroproduction

We introduce a general expression which enables the parton distribution, unintegrated over the parton transverse momentum, to be obtained from the conventional parton densities. We use the formalism to study the effects of the transverse momentum q_t of the incoming partonic system on the calculation of the transverse momentum spectra of prompt photons produced in high energy pp and p\bar{p} collisions. For the purposes of illustration, we use the double logarithm approximation. For large q_t we calculate the effect directly from the perturbative formalism, whereas for small q_t we bound the effect using two extreme hypotheses. In both q_t domains we find that the shapes of the prompt photon spectra are not significantly modified, although the cross sections are enhanced.

hep-ph↗
arXiv1999

Q^2 dependence of diffractive vector meson electroproduction

We give a general formula for the cross section for diffractive vector meson electroproduction, gamma^* p -> Vp. We first calculate diffractive qqbar production, and then use parton-hadron duality by projecting out the J^P = 1^- state in the appropriate mass interval. We compare the Q^2 dependence of the cross section for the diffractive production of rho and J/psi mesons with recent HERA data. We include the characteristic Q^2 dependence associated with the use of the skewed gluon distribution. We give predictions for sigma_L/sigma_T for both rho and J/psi production.

hep-ph↗
arXiv1999

The R ratio in e+ e-, the determination of alpha(Mz^2) and a possible non-perturbative gluonic contribution

We review the determination of the QED coupling at the Z pole, which is a crucial parameter for electroweak theory. We include recent e+e- -> hadron data from Novosibirsk and Beijing to re-evaluate alpha (M_Z^2). We find 1/alpha (M_Z^2) = 128.973 +/- 0.035 or 128.934 +/- 0.040 according, respectively, to whether inclusive or exclusive e+e- -> hadron data are used in the interval 1.4 < \sqrt{s} < 2.1 GeV. The error is mainly due to uncertainties in the data in the low energy region, \sqrt{s} < 2.5 GeV. We find that no advantage is obtained by analytic continuation of the dispersion relation into the complex s plane. We show that the hints of structure for \sqrt{s} ~ 2.5 GeV may be evidence of a non-perturbative gluonic contribution to R(s).

hep-ph↗
arXiv1999

Resummation of double logarithms in electroweak high energy processes

At future linear $e^+e^-$ collider experiments in the TeV range, Sudakov double logarithms originating from massive boson exchange can lead to significant corrections to the cross sections of the observable processes. These effects are important for the high precision objectives of the Next Linear Collider. We use the infrared evolution equation, based on a gauge invariant dispersive method, to obtain double logarithmic asymptotics of scattering amplitudes and discuss how it can be applied, in the case of broken gauge symmetry, to the Standard Model of electroweak processes. We discuss the double logarithmic effects to both non-radiative processes and to processes accompanied by soft gauge boson emission. In all cases the Sudakov double logarithms are found to exponentiate. We also discuss double logarithmic effects of a non-Sudakov type which appear in Regge-like processes.

hep-ph↗
arXiv1999

The QCD description of diffractive processes

We review the application of perturbative QCD to diffractive processes. We introduce the two gluon exchange model to describe diffractive qq(bar) and qq(bar)g production in deep inelastic scattering. We study the triple Regge limit and briefly consider multiple gluon exchange. We discuss diffractive vector meson production at HERA both at t = 0 and large |t|. We demonstrate the non-factorization of diffractive processes at hadron colliders.

hep-ph↗
arXiv1999

Parton Distributions and the LHC: W and Z Production

W and Z bosons will be produced copiously at the LHC proton-proton collider. We study the parton distribution dependence of the total production cross sections and rapidity distributions, paying particular attention to the uncertainties arising from uncertainties in the parton distributions themselves. Variations in the gluon, the strong coupling, the sea quarks and the overall normalisation are shown to lead to small but non-negligible variations in the cross section predictions. Ultimately, therefore, the measurement of these cross sections will provide a powerful cross check on our knowledge of parton distributions and their evolution.

hep-ph↗
arXiv1999

Update of MRST partons

We discuss topical issues concerning the MRST parton analysis. We introduce some new parton sets in order to estimate the uncertainty in the cross sections predicted for W and Z production at the Tevatron and the LHC. We compare with CTEQ5 partons.

hep-ph↗
arXiv1999

Penetration of the Earth by ultrahigh energy neutrinos and the parton distributions inside the nucleon

In this talk we would like to present recent calculations of the cross sections for the ultrahigh energy neutrinos interacting with nucleons. We briefly present the framework of unifed BFKL/DGLAP equations which resums all the leading log(1/x) effects as well as the subleading terms. The few free parameters which specify the input parton distributions are determined by fitting to HERA deep inelastic data. We then use these parton distributions to calculate the cross section for the neutrino-nucleon interactions at very high energies, up to 10^12 GeV. We do also investigate the attenuation of neutrinos when traversing through the Earth. We use the transport equation which we solve for different fluxes of high energy neutrinos (Active Galactic Nucleai, Gamma Ray Bursts, and top-down models). We study the effects of the regeneration of the neutrino flux via neutral current interaction.

hep-ph↗
arXiv1999

Infrared safety of impact factors for colourless particle interactions

We demonstrate, to next-to-leading order accuracy, the cancellation of the infrared singularities in the impact factors which arise in the QCD description of high energy processes A + B -> A' + B' of colourless particles. We study the example where A is a virtual photon in detail, but show that the result is true in general.

hep-ph↗