arXiv subjects
Publications and source records attributed to M. H. Seymour.
An overview is given of general-purpose event generators, especially PYTHIA and HERWIG. The current status is summarized, some recent physics improvements are described, and planned future projects are outlined.
We calculate the subjet rates for jets produced in hadron collisions. The kt algorithm is used to define the jets and allows the theoretical calculation to sum both the leading and next-to-leading logarithms in the resolution variable, ycut. We also ensure that our calculation matches exactly the leading order in alpha_s result and has sensible behaviour near thresholds.
Monte Carlo models of DIS e-gamma scattering must describe hard emissions as well as the soft and collinear limits. Comparison with the observed experimental hadronic energy flow has shown that various models underestimate the high-p_T contribution, in particular at low-x. We have attempted to tune the Herwig, Pythia and Ariadne models to improve the agreement with the data, and to understand the physical implications of the changes required. Unless the physics of these processes is understood it will not be possible to unfold the photon structure functions F_2^γfrom the e^+e^- data without large model dependent systematic errors.
An excess of events with a rapidity gap between jets, over what would be expected from non-diffractive processes, has been observed at HERA. A process based on a perturbative QCD calculation of colour singlet exchange has been added to HERWIG. With this addition, HERWIG is able to describe the number of events with a gap between jets over the number without a gap. This gap fraction is predicted to rise at large rapidity intervals between jets which would only be visible if the detector coverage were increased.
HERWIG (Hadron Emission Reactions With Interfering Gluons) is a multipurpose Monte Carlo event generator for the simulation of hard collisions between hadrons, leptons and photons. This note describes the new features of version 5.9. These include adding over 100 new particles, 1800 new decay modes and supporting new decay types. A complete space-time picture, including heavy meson mixing and secondary decays, is available for all event types and includes an optional colour rearrangement model. Initial state radition of photons in e^+e^- is is now included. Many other processes and features have been upgraded.
The high energy photoproduction of jets is being observed at the ep collider, HERA. It may be that the HERA centre-of-mass energy is sufficiently large that the production of more than one pair of jets per ep collision becomes possible, owing to the large number density of the probed gluons. We construct a Monte Carlo model of such multiparton interactions and study their effects on a wide range of physical observables. The conclusion is that multiple interactions could have very significant effects upon the photoproduction final state and that this would for example make extractions of the gluon density in the photon rather difficult. Total rates for the production of many (i.e. > 2) jets could provide direct evidence for the presence of multiple interactions, although parton showering and hadronization significantly affect low transverse energy jets.
This report is an overview of the gamma-gamma physics capabilities of LEP2, and covers the following topics: structure functions, equivalent photon approximation, tagging conditions etc, soft and semihard physics, large-$p_t$ processes, heavy-quark physics, and exclusive channels.
This report is a survey on QCD Event Generator issues of relevance for LEP 2. It contains four main sections: a summary of experience from LEP 1, extrapolations to LEP 2 energies, Monte Carlo descriptions and standardization issues.
We study the effect of infrared renormalons upon shape variables that are commonly used to determine the strong coupling constant in $e^+e^-$ annihilation into hadronic jets. We consider the model of QCD in the limit of large $n_f$. We find a wide variety of different behaviours of shape variables with respect to power suppressed effects induced by infrared renormalons. In particular, we find that oblateness is affected by $1/Q$ non--perturbative effects even away from the two jet region, and the energy--energy correlation is affected by $1/Q$ non--perturbative effects for all values of the angle. On the contrary, variables like thrust, the $c$ parameter, the heavy jet mass, and others, do not develop any $1/Q$ correction away from the two jet region at the leading $n_f$ level. We argue that $1/Q$ corrections will eventually arise at subleading $n_f$ level, but that they could maintain an extra $\as(Q)$ suppression. We conjecture therefore that the leading power correction to shape variables will have in general the form $α^n_{\rm S}(Q)/Q$, and it may therefore be possible to classify shape variables according to the value of $n$.
At $e^+e^-$ and $ep$ colliders, the large fluxes of almost on-shell photons accompanying the lepton beams lead to the photoproduction of jets. As the centre-of-mass energy is increased, regions of smaller $x$ in the parton densities are explored and these are regions of high parton density. As a result, the probability for more than one hard partonic scattering occurring in a single $γγ$ or $γp$ collision can become significant. This effect has been simulated using an eikonal prescription combined with the HERWIG Monte Carlo program. The possible effects of multiple hard interactions on event shapes and jet cross sections have been studied in this framework at a range of energies relevant to HERA and LEPII. The results indicate that the effects could be significant.
The unique nature of the photon can be investigated in hitherto unexplored kinematic regions at LEP2. We discuss the theoretical significance of deep inelastic measurements and present a prescription that allows a theoretically and experimentally sensible separation of the so-called `anomalous' and `hadronic' components of the target photon. We perform preliminary studies regarding the ability to reconstruct the $γ^* γ$ CM energy (and hence $x$) and the usefulness of the easier to measure electron structure function.