arXiv subjects
Publications and source records attributed to W. J. Stirling.
We review recent results within the Durham model of central exclusive production. We discuss the theoretical aspects of this approach and consider the phenomenological implications in a variety of processes, comparing to existing collider data and addressing the possibilities for the future.
We present the results of a recent novel application of the `hard exclusive' perturbative formalism to the process gg --> MMbar at large angles, where M (Mbar) is a light meson (anti-meson). As well as discussing the important theoretical features of the relevant leading-order gg --> qqbar(gg) qqbar(gg) 6-parton amplitudes, we also comment on their phenomenological implications. In particular, we consider the central exclusive production of meson pairs at comparatively large transverse momentum k_t, which is expected to proceed via this mechanism. We show that this leads to various non-trivial predictions for a range of exclusive processes, and that the cross sections for the eta' and eta mesons display significant sensitivity to any valence gg component of the meson wavefunctions.
Currently, the long-standing issue concerning the size of the gluonic content of the eta' and eta mesons remains unsettled. With this in mind we consider the central exclusive production (CEP) of eta', eta meson pairs in the perturbative regime, applying the Durham pQCD-based model of CEP and the `hard exclusive' formalism to evaluate the meson production subprocess. We calculate for the first time the relevant parton-level processes gg --> qqbar gg and gg --> gggg, where the final-state gg and qqbar pairs form a pseudoscalar flavour-singlet state. We observe that these amplitudes display some non-trivial and interesting theoretical properties, and we comment on how this can be understood in a MHV framework. Finally, we present a phenomenological study, and show that the cross sections for the CEP of eta', eta meson pairs are strongly sensitive to the size of the gluon content of these mesons. The observation of these processes could therefore provide important and novel insight into this problem.
Selected new results in central exclusive production (CEP) processes within the pQCD-based Durham model are discussed. Topics covered include the CEP of SM and BSM Higgs-like particles, meson pair CEP and the gap survival probability.
We investigate the effect of extending the standard MSTW parameterisation of input parton distribution functions (PDFs) using Chebyshev polynomials. We find evidence that four powers in the polynomial are sufficient for extremely high precision. Applying this to valence and sea quarks we find an improvement in the global fit, but a significant change only in the small-$x$ valence up-quark PDF, $u_V$. We investigate the effect of also extending, and making more flexible, the `nuclear' correction to deuteron structure functions. The extended `Chebyshev' parameterisation results in an improved stability in the deuteron corrections that are required for the best fit to the `global' data. The resulting PDFs have a significantly, but not dramatically, altered valence down-quark distribution, $d_V$. For the extended set of MSTW PDFs, their uncertainties can be obtained using 23, rather than the usual 20, orthogonal `uncertainty' eigenvectors. Since the dominant effect is on the valence quarks, we present a detailed study of the dependence of the valence--sea separation on the predictions for the decay lepton charge asymmetry which results from $W^\pm$ production at the LHC, illustrating the PDFs and the $x$ range probed for different experimental scenarios. We show that the modified MSTW PDFs make significantly improved predictions for these data at the LHC, particularly for high values of the $p_T$ cut of the decay lepton. However, this is a special case, since the asymmetry is extremely sensitive to valence--sea details, and in particular to the combination $u_V-d_V$ of valence PDFs for $x \sim M_W/\sqrt{s}$ at low lepton rapidities. We show that the predictions for a wide variety of total cross sections are very similar to those obtained using the MSTW2008 PDFs, with changes being much smaller than the PDF uncertainties.
We consider the effect of next-to-leading order (NLO) electroweak corrections to Standard Model 2-to-2 processes, taking into account the potentially large double logarithms originating from both real and virtual corrections. A study of the leading Sudakov logarithms is presented and Bloch-Nordsieck (BN) violations are discussed for processes at the CERN Large Hadron Collider. In particular, we focus on the processes Z/photon+jet and also the ratio of Z to photon production. This ratio is known to be insensitive to NLO QCD corrections but this is not expected to be the case for the electroweak corrections. We also comment on the effect of electroweak corrections and the presence of BN violation for QCD processes, in particular dijet production, and also for purely electroweak processes such as W + H and W + Z associated production.
Central exclusive production (CEP) processes in high-energy hadron-hadron collisions provide an especially clean environment in which to measure the nature and quantum numbers (in particular, the spin and parity) of new resonance states. Encouraged by the broad agreement between experimental measurements and theoretical predictions based on the Durham approach, we perform a detailed phenomenological analysis of diphoton and meson pair CEP final states, paying particular attention to the theoretical uncertainties in the predictions, including those from parton distribution functions, higher-order perturbative corrections, and non-perturbative and proton dissociation contributions. We present quantitative cross-section predictions for these CEP final states at the RHIC, Tevatron and LHC colliders.
We study the polarisation of gauge bosons produced at the LHC. Polarisation effects for W bosons manifest themselves in the angular distributions of the lepton and in the distributions of lepton transverse momentum and missing transverse energy. The distributions also depend on the selection cuts, with kinematic effects competing with polarisation effects. The polarisation is discussed for a range of different processes producing W bosons: W+jets, W from top (single and pair) production, W pair production and W production in association with a Z or Higgs boson. The relative contributions of the different polarisation states varies from process to process, reflecting the dynamics of the underlying hard-scattering process. We also present results for the polarisation of the Z boson produced in association with QCD jets at the LHC, and comment on the differences between W and Z production.
The production of charm quark jets in association with electroweak gauge bosons at the LHC can be used as a tool to constrain quark parton distribution functions (PDFs). Motivated by recent measurements at the Tevatron and LHC, we calculate cross sections for $W/Z+c$, comparing these to $W/Z+\textrm{jet}$, for various PDF sets. The cross-section differences can be understood in terms of the different underlying PDFs, with the strange quark distribution being particularly important for $W+c$ production. We suggest measurements of appropriately defined ratios and comment on how these measurements at the LHC can be used to extract information on the strange and charm content of the proton at high $Q^2$ scales.
Motivated by evidence for the existence of dark matter, many new physics models predict the pair production of new particles, followed by the decays into two invisible particles, leading to a momentum imbalance in the visible system. For the cases where all four components of the vector sum of the two `missing' momenta are measured from the momentum imbalance, we present analytic solutions of the final state system in terms of measureable momenta, with the mass shell constraints taken into account. We then introduce new variables which allow the masses involved in the new physics process, including that of the dark matter particles, to be extracted. These are compared with a selection of variables in the literature, and possible applications at lepton and hadron colliders are discussed.
This Report summarises the results of the second year's activities of the LHC Higgs Cross Section Working Group. The main goal of the working group was to present the state of the art of Higgs Physics at the LHC, integrating all new results that have appeared in the last few years. The first working group report Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables (CERN-2011-002) focuses on predictions (central values and errors) for total Higgs production cross sections and Higgs branching ratios in the Standard Model and its minimal supersymmetric extension, covering also related issues such as Monte Carlo generators, parton distribution functions, and pseudo-observables. This second Report represents the next natural step towards realistic predictions upon providing results on cross sections with benchmark cuts, differential distributions, details of specific decay channels, and further recent developments.
We review the recent progress in the theoretical description and experimental observation of multiple parton interactions. Subjects covered include experimental measurements of minimum bias interactions and of the underlying event, models of soft physics implemented in Monte Carlo generators, developments in the theoretical description of multiple parton interactions and phenomenological studies of double parton scattering. This article stems from contributions presented at the Helmholtz Alliance workshop on "Multi-Parton Interactions at the LHC", DESY Hamburg, 13-15 September 2010.
We discuss how the mass of new physics particles involved in a pair of short decay chains leading to two invisible particles, for example slepton pair production, followed by the decay into two leptons and two neutralinos, may be measured in central exclusive production (CEP) with forward proton tagging. We show how the existing mass measurement strategies in CEP may be improved by making full use of the mass-shell constraints, and demonstrate that, with around 30 signal events, the masses of the slepton and neutralino can be measured with an accuracy of a few GeV.
A spin-3/2 top quark excitation originating in string realisations of the RS scenario has an effect on the production of top-antitop pairs in hadron-hadron collisions. We study the additional contribution of this state to the cross section for the LHC. We comment on the prospect of discovery or exclusion for a range of masses and couplings at both 7 and 14 TeV and show that the reach extends significantly with increasing centre-of-mass energy. Results are compared with the effect of a hypothetical spin-1/2 excited top as well as other BSM scenarios which predict the presence of higher dimension effective operators. We also calculate distributions for several observables which can be used to distinguish between the different scenarios in the presence of a deviation from the SM prediction for the ttbar cross section.
We discuss the prospects of observing double parton scattering (DPS) processes with purely leptonic final states at the LHC. We first study same-sign W pair production, which is particularly suited for studying momentum and valence number conservation effects, followed by discussions on double Drell-Yan and production of J/psi pairs. The effects of initial state and intrinsic transverse momentum smearing on pair-wise transverse momentum balance characteristic to DPS are studied quantitatively. We also present a new technique, based on rapidity differences, to extract the DPS component from a double J/psi sample recently studied at the LHCb.
The irreducible background from Z(nunu)+jets, to beyond the Standard Model searches at the LHC, can be calibrated using gamma+jets data. The method utilises the fact that at high vector boson pT, the event kinematics are the same for the two processes and the cross sections differ mainly due to the boson-quark couplings. The method relies on a precise prediction from theory of the Z/gamma cross section ratio at high pT, which should be insensitive to effects from full event simulation. We study the Z/gamma ratio for final states involving 1, 2 and 3 hadronic jets, using both the leading-order parton shower Monte Carlo program Pythia8 and a leading-order matrix element program Gambos. This enables us both to understand the underlying parton dynamics in both processes, and to quantify the theoretical systematic uncertainties in the ratio predictions. Using a typical set of experimental cuts, we estimate the net theoretical uncertainty in the ratio to be of order 7%, when obtained from a Monte Carlo program using multiparton matrix-elements for the hard process. Uncertainties associated with full event simulation are found to be small. The results indicate that an overall accuracy of the method, excluding statistical errors, of order 10% should be possible.
We argue that the recent LHCb observation of J/psi-pair production indicates a significant contribution from double parton scattering, in addition to the standard single parton scattering component. We propose a method to measure the double parton scattering at LHCb using leptonic final states from the decay of two prompt J/psi mesons.
The central exclusive production (CEP) of heavy resonance states that subsequently decay into meson pairs, MMbar, is an important signature for such processes at hadron colliders. However there is a potentially important background from the direct QCD production of meson pairs, as mediated for example by the exclusive gg --> MMbar hard scattering subprocess. This is in fact an interesting process in its own right, testing novel aspects of perturbative QCD technology. We explicitly calculate the gg --> MMbar helicity amplitudes for different meson states within the hard exclusive formalism, and comment on the application of MHV techniques to the calculation. Using these results, we describe how meson pair CEP can be calculated in the perturbative regime, and present some sample numerical predictions for a variety of final states. We also briefly consider the dominant non-perturbative contributions, which are expected to be important when the meson transverse momentum is small.