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A. D. Martin

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

At least 127 records · Page 7Linked to original sources
arXiv2008

Diffractive dissociation re-visited for predictions at the LHC

We describe the formalism, and present the results, for a triple-Regge analysis of the available pp and p\bar{p} high-energy data which explicitly accounts for absorptive corrections. In particular, we allow for the gap survival probability, S^2, in single proton diffractive dissociation. Since for pp scattering the value of S^2 is rather small, the triple-Pomeron vertex obtained in this analysis is larger than that obtained in the old analyses where the suppression caused by the absorptive corrections was implicitly included in an effective vertex. We show that the bare triple-Pomeron coupling that we extract from the pp and p\bar{p} data is consistent with that obtained in a description of the gamma p -> J/psi + Y HERA data. The analyses of the data prefer a zero slope, corresponding to the small size of the bare vertex, giving the hope of a smooth matching to the perturbative QCD treatment of the triple-Pomeron coupling.

hep-ph↗
arXiv2008

Soft Diffraction at the LHC

We present a triple-Regge analysis of the available pp --> p + X high-energy data accounting for absorptive corrections. We describe a model for high-energy soft interactions which includes the whole set of multi-Pomeron (n --> m) vertices, and give predictions for the LHC.

hep-ph↗
arXiv2008

Rapidity gap survival probability and total cross sections

We discuss recent calculations of the survival probability of the large rapidity gaps in exclusive processes of the type pp --> p+A+p at high energies. Absorptive or screening effects are important, and one consequence is that the total cross section at the LHC is predicted to be only about 90 mb.

hep-ph↗
arXiv2008

Parton Distributions and QCD at LHCb

We consider the impact that can be made on our understanding of parton distributions (PDFs) and QCD from early measurements at the LHCb experiment. The high rapidity values make the experiment uniquely suited to a detailed study of small-x parton distributions and hence will make a significant contribution towards the clarification of both experimental and theoretical uncertainties on PDFs and their applications.

hep-ph↗
arXiv2008

Recent Progress in Global PDF Analysis

We discuss selected topics in the forthcoming MSTW 2008 determination of parton distributions by global analysis. The tolerance parameter controlling the uncertainties on the parton distributions is now determined by a new dynamic procedure for each eigenvector of the covariance matrix. New data sets fitted include Tevatron Run II data on inclusive jet production, the lepton charge asymmetry from W decays and the Z rapidity distribution. Predictions are given for the total W and Z cross sections at the Tevatron and LHC.

hep-ph↗
arXiv2008

One-loop gg->bb(bar) effects in the main irreducible background to exclusive H->bb(bar) production at the LHC

We calculate the amplitude of gg->bb(bar) production for the colour singlet, J_z=0 di-gluon state at O(α_S^2) order. We consider the cancellation, and a realistic cutoff, of the infrared divergent terms. We show that the one-loop radiative QCD contributions effectively reduce the Born level result for the central exclusive bb(bar) cross section at the LHC. This process is essentially the only irreducible QCD background to the exclusive H->bb(bar) signal.

hep-ph↗
arXiv2008

Early LHC measurements to check predictions for central exclusive production

We show how the early data runs of the LHC can provide valuable checks of the different components of the formalism used to predict the cross sections of central exclusive processes. The `soft' rapidity gap survival factor can be studied in electroweak processes, such as W+gaps events, where the bare amplitude is well known. The generalized gluon distribution, in the appropriate kinematic region, can be probed by exclusive Upsilon production. The perturbative QCD effects, especially the Sudakov-like factor, can be probed by exclusive two- and three-jet production. We discuss the possible role of enhanced absorptive corrections which would violate the soft-hard factorization implied in the usual formalism, and suggest ways that the LHC may explore their presence.

hep-ph↗
arXiv2007

Soft diffraction at the LHC: a partonic interpretation

We present a `new generation' model for high energy proton-proton `soft' interactions. It allows for a full set of multi-Pomeron vertices, as well as including multichannel eikonal scattering. It describes the behaviour of the proton-proton total, sigma(total), and elastic dsigma(el)/dt, cross sections together with those for low and high mass proton dissociation. Although the model contains a comprehensive set of multi-Pomeron diagrams, it has a simple partonic interpretation. Including the more complicated multi-Pomeron vertices reduces the absorptive effects as compared to the predictions in which only the triple-Pomeron vertex is considered. Tuning the model to describe the available `soft' data in the CERN ISR - Tevatron energy range, we predict the total, elastic, single- and double-diffractive dissociation cross sections at the LHC energy. An inescapable consequence of including multichannel eikonal and multi-Pomeron effects is that the total cross section is expected to be lower than before: indeed, we find sigma(total) \simeq 90 mb at the LHC energy. We also present differential forms of the cross sections. In addition we calculate soft diffractive central production.

hep-ph↗
arXiv2007

Update of Parton Distributions at NNLO

We present a new set of parton distributions obtained at NNLO. These differ from the previous sets available at NNLO due to improvements in the theoretical treatment. In particular we include a full treatment of heavy flavours in the region near the quark mass. In this way, an essentially complete set of NNLO partons is presented for the first time. The improved treatment leads to a significant change in the gluon and heavy quark distributions, and a larger value of the QCD coupling at NNLO, alpha_S(M_Z^2) = 0.1191 +/- 0.002(expt.) +/- 0.003(theory). Indirectly this also leads to a change in the light partons at small x and modifications of our predictions for W and Z production at the LHC. As well as the best-fit set of partons, we also provide 30 additional sets representing the uncertainties of the partons obtained using the Hessian approach.

hep-ph↗
arXiv2007

Small x gluon from exclusive J/psi production

Exclusive J/psi production, gamma* p -> J/psi p, offers a unique opportunity to determine the gluon density of the proton in the small x domain. We use the available HERA data to determine the gluon distribution in the region 10^{-4} <~ x <~ 10^{-2} and 2 <~ Q^2 <~ 10 GeV^2, where the uncertainty on the gluon extracted from the global parton analyses is large. The gluon density is found to be approximately flat at the lower scale; it is compared with those of recent global analyses.

hep-ph↗
arXiv2007

Uncertainties of predictions from parton distributions. I: experimental errors

We determine the uncertainties on observables arising from the errors on the experimental data that are fitted in the global MRST2001 parton analysis. By diagonalizing the error matrix we produce sets of partons suitable for use within the framework of linear propagation of errors, which is the most convenient method for calculating the uncertainties. Despite the potential limitations of this approach we find that it can be made to work well in practice. This is confirmed by our alternative approach of using the more rigorous Lagrange multiplier method to determine the errors on physical quantities directly. As particular examples we determine the uncertainties on the predictions of the charged-current deep-inelastic structure functions, on the cross-sections for W production and for Higgs boson production via gluon--gluon fusion at the Tevatron and the LHC, on the ratio of W-minus to W-plus production at the LHC and on the moments of the non-singlet quark distributions. We discuss the corresponding uncertainties on the parton distributions in the relevant x,Q^2 domains. Finally, we briefly look at uncertainties related to the fit procedure, stressing their importance and using sigma_W, sigma_H and extractions of alpha_S(M_Z^2) as examples. As a by-product of this last point we present a slightly updated set of parton distributions, MRST2002.

hep-ph↗
arXiv2007

Bright Solitary-Matter-Wave Collisions in a Harmonic Trap: Regimes of Soliton-like Behaviour

Systems of solitary-waves in the 1D Gross-Pitaevskii equation, which models a trapped atomic Bose-Einstein condensate, are investigated theoretically. To analyse the soliton-like nature of these solitary-waves, a particle analogy for the solitary-waves is formulated. Exact soliton solutions exist in the absence of an external trapping potential, which behave in a particle-like manner, and we find the particle analogy we employ to be a good model also when a harmonic trapping potential is present. In the case of two solitons, the particle model is integrable, and the dynamics are completely regular. The extension to three particles supports chaotic regimes. The agreement between the particle model and the wave dynamics remains good even in chaotic regimes. In the case of a system of two solitary waves of equal norm, the solitons are shown to retain their phase difference for repeated collisions. This implies that soliton-like regimes may be found in 3D geometries where solitary waves can be made to repeatedly collide out of phase, stabilising the condensate against collapse.

cond-mat.other↗
arXiv2007

Parton Distributions for the LHC

We present a preliminary set of updated NLO parton distributions. For the first time we have a quantitative extraction of the strange quark and antiquark distributions and their uncertainties determined from CCFR and NuTeV dimuon cross sections. Additional jet data from HERA and the Tevatron improve our gluon extraction. Lepton asymmetry data and neutrino structure functions improve the flavour separation, particularly constraining the down quark valence distribution.

hep-ph↗
arXiv2007

New Physics with Tagged Forward Protons at the LHC

The addition of forward proton detectors to LHC experiments will significantly enlarge the potential for studying New Physics. A topical example is Higgs production by the central exclusive diffractive process, pp -> p+H+p. We discuss the exclusive production of Higgs bosons in both the SM and MSSM. Special attention is paid to the backgrounds to the H -> bbbar signal.

hep-ph↗
arXiv2007

Improved predictions for g-2 of the muon and α_{\rm QED}(M_Z^2)

We update the Standard Model predictions of the anomalous magnetic moment of the muon, a_μ= (g-2)/2, and the value of the QED coupling at the Z-boson mass, incorporating the new e^+e^- \to ππdata obtained by CMD-2 and KLOE, as well as the corrected SND data, and other improvements. The prediction for a_μ=11659180.4 (5.1) \times 10^{-10} is about 3 \times 10^{-10} lower than before, and has a smaller uncertainty, which corresponds to a 3.4σdeviation from the measured value. The prediction for the QED coupling is α(M_Z^2)^{-1}= 128.937 \pm 0.030.

hep-ph↗