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Matteo Cacciari

Publications and source records attributed to Matteo Cacciari.

At least 55 records · Page 3Linked to original sources
arXiv2007

Power corrections for jets at hadron colliders

We discuss non-perturbative QCD corrections to jet distributions in hadron collisions, focussing on hadronisation and underlying event contributions. Using soft gluon resummation and Monte-Carlo modelling we show that hadronisation dominates at small values of the jet radius R, behaving as 1/R, while underlying event corrections grow with the jet area. This provides a handle to disentangle them and parametrize them in terms of measurable QCD parameters, which might enjoy a degree of universality.

hep-ph↗
arXiv2007

Jet Areas, and What They are Good For

We introduce the concept of the area of a jet, and show how it can be used to perform the subtraction of even a large amount of diffuse noise from hard jets.

hep-ph↗
arXiv2006

Dispelling the N^3 myth for the Kt jet-finder

At high-energy colliders, jets of hadrons are the observable counterparts of the perturbative concepts of quarks and gluons. Good procedures for identifying jets are central to experimental analyses and comparisons with theory. The Kt family of successive recombination jet finders has been widely advocated because of its conceptual simplicity and flexibility and its unique ability to approximately reconstruct the partonic branching sequence in an event. Until now however, it had been believed that for an ensemble of N particles the algorithmic complexity of the Kt jet finder scaled as N^3, a severe issue in the high multiplicity environments of LHC and heavy-ion colliders. We here show that the computationally complex part of Kt jet-clustering can be reduced to two-dimensional nearest neighbour location for a dynamic set of points. Borrowing techniques developed for this extensively studied problem in computational geometry, Kt jet-finding can then be performed in N ln N time. Code based on these ideas is found to run faster than all other jet finders in current use.

hep-ph↗
arXiv2006

FastJet: a code for fast k_t clustering, and more

Two main classes of jet clustering algorithms, cone and k_t, are briefly discussed. It is argued that the former can be often cumbersome to define and implement, and difficult to analyze in terms of its behaviour with respect to soft and collinear emissions. The latter, on the other hand, enjoys a very simple definition, and can be easily shown to be infrared and collinear safe. Its single potential shortcoming, a computational complexity believed to scale like the number of particles to the cube (N^3), is overcome by introducing a new geometrical algorithm that reduces it to N ln N. A practical implementation of this approach to k_t-clustering, FastJet, is shown to be orders of magnitude faster than all other present codes, opening the way to the use of k_t-clustering even in highly populated heavy ion events.

hep-ph↗
arXiv2006

Power-suppressed effects in heavy quark fragmentation functions

This talk summarizes the results of a phenomenological analysis of heavy quark fragmentation data published by the CLEO and BELLE collaborations at \sqrt{s} = 10.6 GeV and by the LEP collaborations at \sqrt{s} = 91.2 GeV. Several theoretical ingredients are employed: next-to-leading order initial conditions, evolution and coefficient functions; soft-gluon resummation to next-to-leading-log accuracy; a next-to-leading order matching condition for the crossing of the bottom threshold in the evolution. Important initial-state electromagnetic radiation effects in the CLEO and BELLE data are also accounted for. We find that with reasonably simple choices of a non-perturbative correction to the fixed-order initial condition for the evolution, the data from CLEO and BELLE can be fitted with remarkable accuracy. The fitted fragmentation function, when evolved to LEP energies, does not however represent fairly the D* fragmentation spectrum measured by ALEPH. Large non-perturbative corrections to the coefficient functions of the meson spectrum are needed in order to reconcile CLEO/BELLE and ALEPH results.

hep-ph↗
arXiv2005

QCD Predictions for Charm and Bottom Production at RHIC

We present up-to-date QCD predictions for open charm and bottom production at RHIC in nucleon-nucleon collisions at \sqrt{S} = 200 GeV. The electron spectrum resulting from heavy flavor decays is also evaluated for direct comparison to the PHENIX and STAR data. These predictions seek to establish a rigorous benchmark, including the theoretical uncertainties, against which nuclear collision data can be compared to obtain evidence for nuclear effects.

hep-ph↗
arXiv2005

A Study of Heavy Flavoured Meson Fragmentation Functions in e+ e- annihilation

We compare QCD theoretical predictions for heavy flavoured mesons fragmentation spectra in e+ e- annihilation with data from CLEO, BELLE and LEP. We include several effects in our calculation: next-to-leading order initial conditions, evolution and coefficient functions. Soft-gluon effects are resummed at next-to-leading-log accuracy. A matching condition for the crossing of the bottom threshold in evolution is also implemented at next-to-leading order accuracy. Important initial-state electromagnetic radiation effects in the CLEO and BELLE data are accounted for. We find that, with reasonably simple choices of a non-perturbative correction to the fixed-order initial condition for the evolution, the data from CLEO and BELLE can be fitted with remarkable accuracy. The fitted fragmentation function, when evolved to LEP energies, does not however represent fairly the D* fragmentation spectrum measured by ALEPH. Large non-perturbative corrections to the coefficient functions of the meson spectrum are needed in order to reconcile CLEO/BELLE and ALEPH results. Non-perturbative parameters extracted from the fits to e+ e- fragmentation data for D/D* and B mesons are tabulated. They can be employed in the theoretical predictions for the production of charmed and bottomed mesons in hadron-hadron, photon-hadron and photon-photon collisions.

hep-ph↗
arXiv2005

Crossing Heavy-Flavour Thresholds in Fragmentation Functions

In analogy with parton distribution functions, also parton fragmentation functions obey matching conditions when crossing heavy-flavour thresholds. We compute these matching conditions at next-to-leading order in the strong coupling constant alpha_s in the MSbar scheme. Our results can be used for the dynamical generation of the heavy-flavour component in next-to-leading order fits to light-hadrons fragmentation functions. Furthermore, when computing perturbatively the charm fragmentation function from first principles and evolving it to higher scales, our matching conditions should be used for consistency when crossing the bottom threshold.

hep-ph↗
arXiv2005

QCD Predictions for Charm and Bottom Production at RHIC

We make up-to-date QCD predictions for open charm and bottom production at RHIC in nucleon-nucleon collisions at \sqrt{S} = 200 GeV. We also calculate the electron spectrum resulting from heavy flavor decays to allow direct comparison to the data. A rigorous benchmark, including the theoretical uncertainties, is established against which nuclear collision data can be compared to obtain evidence for nuclear effects.

hep-ph↗
arXiv2004

Rise and Fall of the Bottom Quark Production Excess

We review the history of comparisons between bottom production measurements and QCD predictions. We challenge the existence of a `significant discrepancy', and argue that standard approaches to QCD calculations do a good job in describing the experimental findings.

hep-ph↗
arXiv2003

Heavy Quark Production: Theory vs. Experiment

The current status of the comparisons between some experimental results and theoretical predictions for heavy quark production is reviewed. It is shown that the combination of new theoretical tools and better experimental input allows for a good description of charm, bottom and top hadroproduction, with no significant discrepancies between theory and experiment. Theoretical progress in the resummation of large logarithms and inclusion of power corrections for the heavy quark fragmentation function is also discussed.

hep-ph↗
arXiv2003

Perturbative and non-perturbative aspects of heavy-quark fragmentation

We describe a new approach to heavy-quark fragmentation which is based on a resummed perturbative calculation and parametrization of power corrections, concentrating on the z -> 1 limit, where the heavy meson carries a large fraction of the momentum of the initial quark. It is shown that the leading power corrections in this region are controlled by the scale m(1-z). Renormalon analysis is then used to extend the perturbative treatment of soft and collinear radiation to the non-perturbative regime. Theoretical predictions are confronted with data on B-meson production in e+e- annihilation.

hep-ph↗
arXiv2003

Charm Cross Sections for the Tevatron Run II

We present a calculation of the D^{*+}, D^+ and D^0 meson single inclusive production cross section for the Tevatron Run II. We use the FONLL approach in perturbative QCD, which, besides including the known next-to-leading order corrections, also provides for the resummation at the next-to-leading logarithmic level of terms enhanced at large p_T by powers of log(p_T/m), where m is the charm mass and p_T is its transverse momentum. Non-perturbative effects in charm hadronization are extracted, in moment space, from recent ALEPH data for D^* fragmentation in e^+e^- collisions.

hep-ph↗
arXiv2003

Soft-Gluon Resummation for Bottom Fragmentation in Top Quark Decay

We study soft-gluon radiation in top quark decay within the framework of perturbative fragmentation functions. We present results for the b-quark energy distribution, accounting for soft-gluon resummation in both the MSbar coefficient function and in the initial condition of the perturbative fragmentation function. The results show remarkable improvement and the b-quark energy spectrum in top quark decay exhibits very little dependence on factorization and renormalization scales. We present some hadron-level results in both x_B and moment space by including non-perturbative information determined from e+e- data.

hep-ph↗
arXiv2003

Heavy-Quark Fragmentation

We study perturbative and non-perturbative aspects of heavy-quark fragmentation into hadrons, emphasizing the large-x region, where x is the energy fraction of the detected hadron. We first prove that when the moment index N and the quark mass m get large simultaneously with the ratio (N Lambda/m) fixed, the fragmentation function depends on this ratio alone. This opens up the way to formulate the non-perturbative contribution to the fragmentation function at large N as a shape function of m(1-x) which is convoluted with the Sudakov-resummed perturbative result. We implement this resummation and the parametrization of the corresponding shape function using Dressed Gluon Exponentiation. The Sudakov exponent is calculated in a process independent way from a generalized splitting function which describes the emission probability of an off-shell gluon off a heavy quark. Non-perturbative corrections are parametrized based on the renormalon structure of the Sudakov exponent. They appear in moment space as an exponential factor, with a leading contribution scaling as (N Lambda/m) and corrections of order (N Lambda/m)^3 and higher. Finally, we analyze in detail the case of B-meson production in e+e- collisions, confronting the theoretical predictions with LEP experimental data by fitting them in moment space.

hep-ph↗
arXiv2002

Perturbative and Non-Perturbative Issues in Heavy Quark Fragmentation

We review the state-of-the-art of our understanding of heavy quark fragmentation. Recent e^+e^- data for B mesons are compared to the most up-to-date theoretical predictions, and the need for inclusion of a non-perturbative component is discussed. Experimental analyses in moments space are suggested, and it is pointed out how perturbative and non-perturbative contributions are to be properly matched. Failure to do so can result in large phenomenological discrepancies. An example is given for B^+ hadroproduction at the Tevatron.

hep-ph↗
arXiv2001

The LEP Trail to Non-Perturbative QCD

This talk summarizes the presentations given in the Hadronic Physics session at the LEPTRE meeting, emphasizing the importance of LEP data in our quest for a successful approach to non-perturbative QCD.

hep-ph↗