arXiv subjects
Publications and source records attributed to Andrea Banfi.
We report on the status of the QCD analysis of dijet azimuthal decorrelations. We emphasise the relevance of resummation of soft and collinear enhancements in describing these observables in the region where the two jets are nearly back-to-back in the transverse plane. We also discuss the sources of theoretical uncertainties and possible research directions aimed at their reduction.
We provide a theoretical study of a novel variable introduced in Ref. \cite{WV} to study the transverse momentum of the Z boson at hadron colliders. The variable we consider has experimental advantages over the standard $p_T$ distribution enabling more accurate measurement at low $p_T$. We provide an all orders perturbative estimate for this variable at the next-to--leading logarithmic accuracy and compare the results to those for the standard $p_T$ distribution. We test our resummation, at the two-loop level, by comparing its expansion to fixed-order perturbative estimates and find agreement with our expectations.
A quantity that promises to reveal important information on perturbative and non-perturbative QCD dynamics is the azimuthal decorrelation between jets in different hard processes. In order to access this information fixed-order NLO predictions need to be supplemented by resummation of logarithmic terms which are large in the region where the jets are nearly back-to-back in azimuth. In the present letter we carry out this resummation to next-to--leading logarithmic accuracy explaining the important role played by the recombination scheme in general resummations for such jet observables.
Three-jet event-shape distributions can be exploited to investigate the dependence of hadronisation effects on the colour and the geometry of the underlying hard event. We present here the first comparison of data in e+e- annihilation and state-of-the-art theoretical predictions, including resummation of large logarithms at next-to-leading logarithmic accuracy matched to exact next-to-leading order and leading non-perturbative power corrections.
Heavy-quark jets are important in many of today's collider studies and searches, yet predictions for them are subject to much larger uncertainties than for light jets. This is because of strong enhancements in higher orders from large logarithms, ln(p_t/m_Q). We propose a new definition of heavy-quark jets, which is free of final-state logarithms to all orders and such that all initial-state collinear logarithms can be resummed into the heavy-quark parton distributions. Heavy-jet spectra can then be calculated in the massless approximation, which is simpler than a massive calculation and reduces the theoretical uncertainties by a factor of three. This provides the first ever accurate predictions for inclusive b- and c-jets, and the latter have significant discriminatory power for the intrinsic charm content of the proton. The techniques introduced here could be used to obtain heavy-flavour jet results from existing massless next-to-leading order calculations for a wide range of processes. We also discuss the experimental applicability of our flavoured jet definition.
We study the mismatch between a full calculation of non-global single-logarithms in the large-N_c limit and an approximation based on free azimuthal averaging, and the consequent angular-ordered pattern of soft gluon radiation in QCD. We compare the results obtained in either case to those obtained from the parton showers in the Monte Carlo event generators HERWIG and PYTHIA, with the aim of assessing the accuracy of the parton showers with regard to such observables where angular ordering is merely an approximation even at leading-logarithmic accuracy and which are commonly employed for the tuning of event generators to data.
We revisit the impact of the jet algorithm on predictions of energy flow into gaps between hard jets, defined using the kt clustering procedure. The resulting prediction has two distinct components: a primary emission piece that is related to independent emission of soft gluons by the hard jets and a correlated emission (non-global) piece known only in the large N_c limit. We analytically compute the dependence of the primary emission term on the jet algorithm, which gives significantly more insight than our previous numerical study of the same. We also point out that the non-global component of the answer is reduced even more significantly by the clustering than suggested previously in the literature. Lastly we provide improved predictions for the latest ZEUS photoproduction data, assessing the impact of our latest findings.
A short overview of recent developments on resummations in QCD is presented.
We review the status of non-perturbative analyses of multi-jet event shape distributions and mean values, highlighting the physical insight on QCD dynamics they can provide.
It is common, in both theoretical and experimental studies, to separately discuss quark and gluon jets. However, even at parton level, widely-used jet algorithms fail to provide an infrared safe way of making this distinction. We examine the origin of the problem, and propose a solution in terms of a new "flavour-kt" algorithm. As well as being of conceptual interest this can be a powerful tool when combining fixed-order calculations with multi-jet resummations and parton showers. It also has applications to studies of heavy-quark jets.
Next-to-leading logarithmic final-state resummed predictions have traditionally been calculated, manually, separately for each observable. In this article we derive NLL resummed results for generic observables. We highlight and discuss the conditions that the observable should satisfy for the approach to be valid, in particular continuous globalness and recursive infrared and collinear safety. The resulting resummation formula is expressed in terms of certain well-defined characteristics of the observable. We have written a computer program, CAESAR, which, given a subroutine for an arbitrary observable, determines those characteristics, enabling full automation of a large class of final-state resummations, in a range of processes.
This article introduces definitions for a number of new event shapes and jet-rates in hadron-hadron dijet production. They are designed so as to be measurable in practice at the Tevatron and the LHC, and to be global so that they can be resummed with currently available techniques. We explain how to vary their sensitivity to beam fragmentation, limiting its impact for purely perturbative studies, or deliberately enhancing it so as to focus on non-perturbative effects. Explicit next-to-leading logarithmic resummed results are presented, as obtained with CAESAR.
We present joint threshold and recoil resummed transverse momentum distributions for heavy quark hadroproduction, at next-to-leading logarithmic accuracy. We study the dependence of these distributions on the production channel, the color configurations and the differences with the pure threshold-resummed distribution.
We show that a resummation of infrared logarithms is needed to obtain a sensible theoretical description of dijet rates when symmetric cuts are applied to the transverse energies of both jets. We also present the next-to-leading logarithmic (NLL) resummation we carried out for DIS production of two jets selected with the cone algorithm.
We consider dijet production in the region where symmetric cuts on the transverse energy, $E_t$, are applied to the jets. In this region next-to--leading order calculations are unreliable and an all-order resummation of soft gluon effects is needed, which we carry out. Although, for illustrative purposes, we choose dijets produced in deep inelastic scattering, our general ideas apply additionally to dijets produced in photoproduction or $γγ$ processes and should be relevant also to the study of prompt di-photon $E_t$ spectra in association with a recoiling jet, in hadron-hadron processes
We build a computer code that fully automates the resummation of jet-observable distributions at next-to-leading logarithmic accuracy. As an application we present results for a jet shape in hadronic dijet production.
We present a numerical program, CAESAR, that allows us to resum large logarithmic contributions to jet observables in a fully automated way. As an application we obtain the first next-to-leading logarithmic distributions for event shapes in hadronic dijet production.
We review theoretical methods employed to study non-perturbative contributions to e^+e^- event-shapes and discuss their phenomenological relevance.