W.J. Stirling

New jet cluster algorithms: Next-to-leading order QCD and hadronization corrections

Abstract New algorithms have been proposed as alternatives to the standard JADE-type algorithms for quantifying jet production in e+e− annihilation. We study in detail the “Durham” algorithm, based on clustering according to relative transverse momentum, and introduce a new “Geneva” variation which is expected to exhibit similar theoretical properties of exponentiation at small jet resolution. In particular, we calculate the next-to-leading order perturbative corrections to the three-jet rate for the new algorithms, and we study non-perturbative corrections using the JETSET parton-shower Monte Carlo. In each case we compare to the results obtained with the usual JADE-type algorithms. We conclude that the new algorithms exhibit somewhat smaller perturbative corrections compared to the standard algorithms, and that the non-perturbative corrections are of comparable size. Thus the new algorithms appear to be at least as well suited to precision measurements of the strong coupling.

Spinor techniques for calculating pp → W±/Z0 + jets

Abstract We present techniques which enable one to calculate quickly the amplitudes for many scattering processes in the high-energy limit. As an illustration of the method, these are applied to the diagrams for p p → V + 0, 1 or 2 jets , where V = W ± or Z 0 . The form of the results lends itself to immediate numerical evaluation.

Non-leading corrections to the Drell-Yan cross section at small transverse momentum

Abstract The production of Drell-Yan lepton pairs at small transverse momentum is analyzed to second order in QCD perturbation theory. By calculating exactly all terms involving powers of the large logarithm ln ( Q 2 q T 2 ), we are able to confront the predictions of “soft gluon” cross sections in which these logarithms are summed to all orders. Our results confirm the factorization properties of these cross sections, and yield new information on previously undetermined coefficients.

Drell-Yan cross sections at small transverse momentum

Abstract We present QCD predictions for the production of vector bosons at small transverse momentum in hadron-hadron collisions by the Drell-Yan mechanism. By using Fermilab and ISR data to fix the parametrisation of non-perturbative effects we are able to define clearly the region of transverse momentum in which a leading logarithm analysis applies for the production of W- and Z- bosons. We also investigate the sensitivity of the cross section to recently calculated coefficients in this analysis.

W's, Z's and Jets

Abstract The process p+ p →W±, Z0 plus 2 jets is discussed in the context of perturbative QCD. The magnitude of the expected rate for this process and the correlations anticipated between the jets are presented.

Cross sections for the production of an arbitrary number of photons in electron-positron annihilation

Abstract Spinor techniques are used to derive an expression for the cross section for the process e + e − → N γ, where N is arbitrary. The result is convenient for numerical evaluation. Results are presented for typical cross sections for the production of up to seven photons.

Tagging the Higgs

Abstract We study the effects of jet tagging at small angles on signal and background cross sections in the production of a heavy Higgs boson in pp collisions at LHC/SSC energies, in which the Higgs decay, via a W + W − pair into a charged lepton, a neutrino and a pair of jets. The use of the tagging conditions is shown to improve the signal-to-background ratio by about two orders of magnitude. A more detailed analysis of the decay products can substantially enhance this ratio. We point out that the energy asymmetry of the jets may be very helpful in this respect. Our conclusion is that a 1:1 signal-to-background ratio is feasible at both the LHC and the SSC.

W+W- pair production in high energy hadronic collissions: Signal versus background

Abstract We assess the prospects for measuring the cross sections for the production of W+W- pairs in p p collisions at very high energy, including those pairs resulting from possible Higgs boson production and subsequent decay. We find that if one of the Ws decays hadronically the signal is completely dominated by a background from standard W + two-jet production where the jet-jet mass is order MW. It would appear therefore that a double leptonic decay is required, with a subsequent loss in rate.

Anomalous high-energy behaviour in boson fusion

Abstract The magnitude is discussed of the off-shell correction terms to the equivalent-boson approximation used in SSC phenomenology, with emphasis on Higgs production. Although formally subleading, these terms are NOT small and give rise to cross sections that increase unacceptably at high invariant mass of the boson pair. This is traced to the fact that for off-shell bosons the set of diagrams which has been considered so far is not gauge-invariant. It is concluded that only a much more complete set of Feynman diagrams can yield a reliable prediction for the production of a heavy Higgs boson.

FOUR JET PRODUCTION AT HADRON COLLIDERS

Abstract Some of the characteristic features are examined of four-jet production at hadron colliders given by QCD (2 → 4) subprocesses. It is found that the universal behaviour of parton scattering subprocesses - which is a good approximation for (2 → 2) subprocesses and to a lesser extent for (2 → 3) subprocesses - is violated more significantly in the case of the (2 → 4) parton scattering processes. Methods are suggested for testing if the four-jet data sample receives additional contributions from double hard parton scattering.