Gluon fragmentation into quarkonium at next-to-leading order using FKS subtraction

We present the calculation at next-to-leading order (NLO) in alpha_s of the fragmentation function of a gluon into heavy quarkonium in the color-octet spin-singlet S-wave channel. To calculate the real NLO corrections, we adapt a subtraction scheme introduced by Frixione, Kunszt, and Signer. Ultraviolet and infrared divergences in the real NLO corrections are calculated analytically by evaluating the phase-space integrals of the subtraction terms using dimensional regularization. The subtracted phase-space integrals are then evaluated in 4 space-time dimensions. The divergences in the virtual NLO corrections are also calculated analytically. After renormalization, all the divergences cancel. The NLO corrections significantly increase the fragmentation probability for a gluon into the spin-singlet quarkonium states eta_c and eta_b.Comment: 52 pages, 6 figure

Parton Model Calculation of Inclusive Charm Production by a Low-energy Antiproton Beam

The cross section for inclusive charm production by a low-energy antiproton beam is calculated using the parton model and next-to-leading order perturbative QCD. For an antiproton beam with a momentum of 15 GeV, the charm cross section at next-to-leading order in the QCD coupling constant changes by more than an order of magnitude as the charm quark mass is varied from 1.3 to 1.7 GeV. The variations can be reduced by demanding that the same value of the charm quark mass give the measured charm cross sections for fixed-target experiments with a proton beam. The resulting estimate for the charm cross section from a low-energy antiproton beam is large enough to allow the study of charm meson mixing.Comment: 9 pages, 4 figure

Charm-pair Rescattering Mechanism for Charmonium Production in High-energy Collisions

A new mechanism for heavy quarkonium production in high-energy collisions called the "s-channel cut" was proposed in 2005 by Lansberg, Cudell, and Kalinovsky. We identify this mechanism physically as the production of a heavy quark and anti-quark that are on-shell followed by their rescattering to produce heavy quarkonium. We point out that in the NRQCD factorization formalism this rescattering mechanism is a contribution to the color-singlet model term at next-to-next-to-leading order in perturbation theory. Its leading contribution to the production rate can be calculated without introducing any additional phenomenological parameters. We calculate the charm-pair rescattering (or s-channel cut) contribution to the production of J/psi at the Tevatron and compare it to estimates by Lansberg et al. using phenomenological models. This contribution competes with the leading-order term in the color-singlet model at large transverse momentum but is significantly smaller than the next-to-leading-order term. We conclude that charm-pair rescattering is not a dominant mechanism for charmonium production in high-energy collisions.Comment: 12 pages, 10 figure

Using Line Shapes to Discriminate between Binding Mechanisms for the X(3872)

We construct line shapes for the X(3872) that generalize the Flatte and zero-range line shapes that have been considered previously. These line shapes are associated with scattering amplitudes that are exactly unitary for real values of the interaction parameters and can be derived from a renormalizable quantum field theory. The new line shapes can be used to discriminate between the alternative binding mechanisms in which the X(3872) is generated either dynamically by charm meson interactions or by a resonance near the D^{*0} Dbar^0 threshold. If the resonance is identified with the P-wave charmonium state chi'_{c1}, the interaction parameters can be constrained by using charmonium phenomenology. We analyze data on the X(3872) and also data from the Belle and Babar Collaborations on the invariant mass distribution of the charm mesons from the decay B -> K + D^{*0} Dbar^0 up to 4000 MeV. Our analysis is compatible with the mechanism for the X(3872) being either a fine-tuning of charm meson interactions or the fine-tuning of the chi'_{c1} to the D^{*0} Dbar^0 threshold. In particular, the data do not exclude a separate chi'_{c1} resonance between the D^{*+} D^- threshold and 4000 MeV.Comment: 50 pages, 5 figures, 2 tables. Version published in Physical Review

Production of the X(3872) at the Tevatron and the LHC

We predict the differential cross sections for production of the X(3872) at the Tevatron and the Large Hadron Collider from both prompt QCD mechanisms and from decays of b hadrons. The prompt cross section is calculated using the NRQCD factorization formula. Simplifying assumptions are used to reduce the nonperturbative parameters to a single NRQCD matrix element that is determined from an estimate of the prompt cross section at the Tevatron. For X(3872) with transverse momenta greater than about 4 GeV, the predicted cross section is insensitive to the simplifying assumptions. We also discuss critically a recent analysis that concluded that the prompt production rate at the Tevatron is too large by orders of magnitude for the X(3872) to be a weakly-bound charm-meson molecule. We point out that if charm-meson rescattering is properly taken into account, the upper bound is increased by orders of magnitude and is compatible with the observed production rate at the Tevatron.Comment: 29 pages, 5 figure

Automation of the matrix element reweighting method

Matrix element reweighting is a powerful experimental technique widely employed to maximize the amount of information that can be extracted from a collider data set. We present a procedure that allows to automatically evaluate the weights for any process of interest in the standard model and beyond. Given the initial, intermediate and final state particles, and the transfer functions for the final physics objects, such as leptons, jets, missing transverse energy, our algorithm creates a phase-space mapping designed to efficiently perform the integration of the squared matrix element and the transfer functions. The implementation builds up on MadGraph, it is completely automatized and publicly available. A few sample applications are presented that show the capabilities of the code and illustrate the possibilities for new studies that such an approach opens up.Comment: 41 pages, 21 figure

Automatic spin-entangled decays of heavy resonances in Monte Carlo simulations

We present a general method that allows one to decay narrow resonances in Les Houches Monte Carlo events in an efficient and accurate way. The procedure preserves both spin correlation and finite width effects to a very good accuracy, and is therefore particularly suited for the decay of resonances in production events generated at next-to-leading-order accuracy. The method is implemented as a generic tool in the MadGraph framework, giving access to a very large set of possible applications. We illustrate the validity of the method and the code by applying it to the case of single top and top quark pair production, and show its capabilities on the case of top quark pair production in association with a Higgs boson.Comment: 17 pages, 6 figures, published versio

Unravelling ttˉht\bar{t}h via the matrix element method

Associated production of the Higgs boson with a top-antitop pair is a key channel to gather further information on the nature of the newly discovered boson at the LHC. Experimentally, however, its observation is very challenging due to the combination of small rates, difficult multi-jet final states and overwhelming backgrounds. In the Standard Model the largest number of events is expected when h->bb, giving rise to a WWbbbb signature, deluged in tt+jets. A promising strategy to improve the sensitivity is to maximally exploit the theoretical information on the signal and background processes by means of the matrix element method. We show how, despite the complexity of the final state, the method can be efficiently applied to discriminate the signal against combinatorial and tt+jets backgrounds. Remarkably, we find that a moderate integrated luminosity in the next LHC run will be enough to make the signature involving both W's decaying leptonically as sensitive as the single-lepton one.Comment: 4 pages, 2 figure

J/psi production at sqrt(s)=1.96 and 7 TeV: Color-Singlet Model, NNLO* and polarisation

We study J/psi production in pp collisions at sqrt(s)=1.96 and 7 TeV using the Colour-Singlet Model (CSM), including next-to-leading order (NLO) corrections and dominant alphaS^5 contributions (NNLO*). We find that the CSM reproduces the existing data if the upper range of the NNLO* is near the actual --but presently unknown-- NNLO. The direct yield polarisation for the NLO and NNLO* is increasingly longitudinal in the helicity frame when P_T gets larger. Contrary to what is sometimes claimed in the literature, the prompt J/psi yield polarisation in the CSM is compatible with the experimental data from the CDF collaboration, when one combines the direct yield with a data-driven range for the polarisation of J/psi from chi(c).Comment: Contributed to the 22nd International Conference On Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2011), Annecy, France, May 23 - 28, 2011. 4 pages, 4 figures, uses iopams.sty, iopart12.clo, iopart.cls (included