arXiv subjects

A. D. Martin

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

At least 199 records · Page 11Linked to original sources

arXiv2001

The non-forward BFKL amplitude and rapidity gap physics

We discuss the BFKL approach to processes with large momentum transferred through a rapidity gap. The Mueller and Tang scheme to the BFKL non-forward parton-parton elastic scattering amplitude at large $t$, is extended to include higher conformal spins. The new contributions are found to decrease with increasing energy, as follows from the gluon reggeisation phenomenon, and to vanish for asymptotically high energies. However, at moderate energies and high $|t|$, the higher conformal spins dominate the amplitude. We illustrate the effects by studying the production of two high $E_T$ jets separated by a rapidity gap at HERA energies. In a simplified framework, we find excellent agreement with the HERA photoproduction data once we incorporate the rapidity gap survival probability against soft rescattering effects. We emphasize that measurements of the analogous process in electroproduction may probe different summations over conformal spins.

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arXiv2001

MRST2001: partons and alpha_S from precise deep inelastic scattering and Tevatron jet data

We use all the available new precise data for deep inelastic and related hard scattering processes to perform NLO global parton analyses. These new data allow an improved determination of partons and, in particular, the inclusion of the recent measurements of the structure functions at HERA and of the inclusive jets at the Tevatron help to determine the gluon distribution and alpha_S better than ever before. We find a somewhat smaller gluon at low x than previous determinations and that alpha_S(M_Z^2) = 0.119 +/- 0.002 (expt.) +/- 0.003 (theory).

hep-ph↗

arXiv2001

Unintegrated generalised parton distributions

We show how the generalised (or skewed) parton distributions of the proton, H(x, xi; k_t^2, mu^2), unintegrated over the partonic transverse momenta, can be calculated from the known conventional parton distributions, q(x, mu^2) and g(x, mu^2), for small values of the skewedness parameter xi. We demonstrate the procedure by numerically evaluating the skewed unintegrated gluon. We also provide a simple approximate phenomenological form of the distribution, which may be used to make more rapid predictions of observables.

hep-ph↗

arXiv2001

Status of Structure Functions and Partons

We briefly review some of the developments in the study of parton distributions which have occurred since DIS2000, including discussion of uncertainties, shadowing, unintegrated and generalized distributions.

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arXiv2001

Azimuthal decorrelation of forward and backward jets at the Tevatron

We analyse the azimuthal decorrelation of Mueller-Navelet dijets produced in the collisions at Tevatron energies using a BFKL framework which incorporates dominant subleading effects. We show that these effects significantly reduce the decorrelation yet they are still insufficient to give satisfactory description of experimental data. However a good description of the data is obtained after incorporating within formalism the effective rapidity defined by Del Duca and Schmidt.

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arXiv2001

MRST Global Fit Update

We discuss the impact of the most recent data on the MRST global analysis - in particular the new high-E_T jet data and their implications for the gluon and the new small x structure function data. In the light of these new data we also consider the uncertainty in predictions for physical quantities depending on parton distributions, concentrating on the W cross-section at hadron colliders.

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arXiv2001

Probabilities of rapidity gaps in high energy interactions

We show that high energy hadronic reactions which contain a rapidity gap and a hard subprocess have a specific dependence on the kinematic variables, which results in a characteristic behaviour of the survival probability of the gap. We incorporate this mechanism in a two-channel eikonal model to make an essentially parameter-free estimate of diffractive dijet production at the Tevatron, given the diffractive structure functions measured at HERA. The estimates are in surprising agreement with the measurements of the CDF collaboration. We briefly discuss the application of the model to other hard processes with rapidity gaps.

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arXiv2001

Double-diffractive processes in high-resolution missing-mass experiments at the Tevatron

We evaluate, in a model-independent way, the signal-to-background ratio for Higgs -> b bbar detection in exclusive double-diffractive events at the Tevatron and the LHC. For the missing-mass approach to be able to identify the Higgs boson, it will be necessary to use a central jet detector and to tag b quark jets. The signal is predicted to be very small at the Tevatron, but observable at the LHC. However we note that the background, that is double-diffractive dijet production, may serve as a unique gluon factory. We also give estimates for the double-diffractive production of chi_c and chi_b mesons at the Tevatron. We emphasize that a high-resolution missing-mass measurement, on its own, is insufficient to identify rare processes.

hep-ph↗

arXiv2001

High p_T Higgs signal for the LHC

We show that the broad transverse momentum distribution of the Higgs boson produced by WW fusion can provide a viable way to identify H --> b anti-b decays at the LHC, if particular kinematical configurations with large rapidity gaps are selected. We estimate the event rate of the signal and of the QCD b anti-b background. We also discuss Higgs boson detection via the H --> tau tau and H --> WW* decay modes.

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arXiv2001

Summary Talk: First Workshop on Forward Physics and Luminosity Determination at the LHC

An attempt is made to summarize the discussion at the Workshop, except for the panel discussion on the ability of the LHC detectors to accommodate forward reactions. The Workshop focused on two main topics. The first topic was forward physics at the LHC. Predictions were made for forward reactions, including elastic scattering and soft diffractive processes, intopic was forward physics at the LHC. Predictions were made for forward reactions, including elastic scattering and soft diffractive processes, in terms of (multi) Pomeron exchange, using knowledge gained at lower energies. The survival probability of rapidity gaps accompanying hard subprocesses was studied. The nature of the Pomeron, before and after QCD, was exposed, and some aspects of small x physics at the LHC were considered. The second topic of the Workshop concerned the accuracy of the luminosity measuring processes at the LHC. Attention concentrated on three methods. The classic approach based on the optical theorem, secondly, the observation of the pure QED process of lepton-pair (l^+l^-) production by photon-photon fusion and, finally, the measurement of inclusive W and Z production.

hep-ph↗

arXiv2001

Rates for rapidity gap Higgs production

We present model-independent estimates of the signal-to-background ratio for Higgs --> b anti-b detection in double-diffractive events at the Tevatron and the LHC. For the missing-mass approach to be able to identify the Higgs boson, it will be necessary to tag the b quark jets in the central region. The signal is predicted to be very small at the Tevatron, but observable at the LHC. We note that the double-diffractive dijet production, may serve as a unique gluon factory. This process can be used also as a Pomeron-Pomeron luminosity monitor.

hep-ph↗

arXiv2001

The compatibility of diffractive hard scattering in p pbar and ep collisions

We show that the data for diffractive dijet production at the Tevatron p pbar collider are consistent with diffractive deep inelastic e p data collected at HERA. The breakdown of factorization between the two data sets is naturally explained by a strong tendency to populate the rapidity gap in the p pbar diffractive process by secondaries from soft underlying interactions and by bremsstrahlung associated with the presence of the hard dijet subprocess.

hep-ph↗

arXiv2001

Improving alpha_QED(M_Z^2) and the charm mass by analytic continuation

The standard determination of the QED coupling on the Z pole is performed using the latest available data for R. The direct application of analytic continuation techniques is found not to improve the accuracy of the value of α(M_Z^2). However they help to resolve an ambiguity in the values of R in the energy region \sqrt{s} < 2 GeV, which, in turn, reduces the uncertainty in α(M_Z^2). Moreover, they provide a sensitive determination of the mass of the charm quark. The favoured solution, which uses the inclusive data for R for \sqrt{s} < 2 GeV, has a pole mass m_c = 1.33-1.40 GeV and α^{-1}(M_Z^2) = 128.972 +/- 0.026; whereas if the sum of the exclusive channels is used to determine R in this region, we find α^{-1}(M_Z^2) = 128.941 +/- 0.029.

hep-ph↗

arXiv2001

Gluon shadowing in the low x region probed by the LHC

Starting from an unintegrated gluon distribution which satisfies a `unified' equation which embodies both BFKL and DGLAP behaviour, we compute the shadowing corrections to the integrated gluon in the small x domain that will be accessible at the LHC. The corrections are calculated via the Korchegov equation, which incorporates the leading ln(1/x) triple-Pomeron vertex, and are approximately resummed using a simple Pade technique. We find that the shadowing corrections to xg(x,Q^2) are rather small in the HERA domain, but lead to a factor of 2 suppression in the region x \sim 10^-6, Q^2 \sim 4 GeV^2 accessible to experiments at the LHC.

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arXiv2001

Unintegrated parton distributions

We describe how to calculate the parton distributions $f_a(x, k_t^2, μ^2)$, unintegrated over the parton transverse momentum $k_t$, from auxiliary functions $h_a(x, k_t^2)$, which satisfy single-scale evolution equations. The formalism embodies both DGLAP and BFKL contributions, and accounts for the angular ordering which comes from coherence effects in gluon emission. We check that the unintegrated distributions give the measured values of the deep inelastic structure function $F_2(x, Q^2)$.

hep-ph↗

arXiv2000

Soft diffraction at the LHC and properties of the Pomeron

We briefly describe a model for soft diffraction. The model description of ISR, SppbarS and Tevatron data, and the predictions of the total, elastic, single and double diffractive cross sections at the LHC can be found in the original paper. Here we address issues raised at Diffraction 2000, in particular concerning the nature of the resulting Pomeron trajectory.

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arXiv2000

Luminosity monitors at the LHC

We study the theoretical accuracy of various methods that have been proposed to measure the luminosity of the LHC pp collider, as well as for Run II of the Tevatron p barp collider. In particular we consider methods based on (i) the total and forward elastic data, (ii) lepton-pair production and (iii) W and Z production.

hep-ph↗

arXiv2000

A new determination of the QED coupling alpha(M_Z^2) lets the Higgs off the hook

We use the available data on e+ e- -> hadrons to determine the value of the QED coupling at the Z pole. The inclusion of recent preliminary BES-II data in the analysis is found to reduce the ambiguity in the determination of the coupling, particularly that arising from the use of either exclusive or inclusive data in the energy region \sqrt{s} < 2 GeV, in favour of a larger value of α(M_Z^2)^{-1} = 128.978 +/- 0.027. As a consequence the predicted value of the mass of the (Standard Model) Higgs boson is increased, so that the preferred value is just above the LEP2 bound.

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