arXiv subjects

Carlos Lourenco

Publications and source records attributed to Carlos Lourenco.

16 recordsLinked to original sources

arXiv2019

NRQCD colour-octet expansion vs LHC quarkonium production: signs of a hierarchy puzzle?

The observation of unpolarized quarkonium production in high energy pp collisions, at mid rapidity, implies a significant violation of the non-relativistic QCD (NRQCD) velocity scaling rules. A precise experimental confirmation of this picture could definitely rule out the current formulation of the factorization expansion. This conclusion relies on current perturbative determinations of the short-distance kinematic factors and may be reverted if improved calculations would modify, in a very specific way, their transverse momentum dependences. That solution would result, however, in a full degeneracy in the presently assumed basis of $^{2S+1}L_J$ Fock states. Therefore, whatever the outcome, improved polarization measurements will challenge and improve our fundamental understanding of quarkonium production.

hep-ph↗

arXiv2018

The fate of quarkonia in heavy-ion collisions at LHC energies: a unified description of the sequential suppression patterns

Measurements made at the LHC have shown that the production of the ${\rm J}/ψ$, $ψ$(2S), $Υ$(1S) and $Υ$(2S) quarkonia is suppressed in Pb-Pb collisions, with respect to the extrapolation of the pp production yields. The $ψ$(2S) and $Υ$(2S) states are more strongly suppressed than the ground states and the level of the suppression changes with the centrality of the collision. We show that the measured patterns can be reproduced by a simple model, where all quarkonia are treated in a unified way, starting from the recent realisation that, in pp collisions, the probability of quarkonium formation has a universal dependence on the binding-energy of the bound state. The hot-medium suppression effect is parametrized by a penalty factor in the binding energy, identical for all (S- and P-wave) charmonium and bottomonium states, including those that indirectly contribute to the measured results through feed-down decays. This single parameter, computed through a global fit of all available suppression patterns, fully determines the hierarchy of nuclear effects, for all states and centrality bins. The resulting faithful description of the data provides convincing evidence in favour of the conjecture of sequential quarkonium suppression induced by QGP formation.

hep-ph↗

arXiv2018

Universal kinematic scaling as a probe of factorized long-distance effects in high-energy quarkonium production

Dimensional analysis reveals general kinematic scaling rules for the momentum, mass, and energy dependence of Drell-Yan and quarkonium cross sections. Their application to mid-rapidity LHC data provides strong experimental evidence supporting the validity of the factorization ansatz, a cornerstone of non-relativistic QCD, still lacking theoretical demonstration. Moreover, data-driven patterns emerge for the factorizable long-distance bound-state formation effects, including a remarkable correlation between the S-wave quarkonium cross sections and their binding energies. Assuming that this scaling can be extended to the P-wave case, we obtain precise predictions for the not yet measured feed-down fractions, thereby providing a complete picture of the charmonium and bottomonium feed-down structure. This is crucial information for quantitative interpretations of quarkonium production data, including studies of the suppression patterns measured in nucleus-nucleus collisions.

hep-ph↗

arXiv2018

From identical S- and P-wave pT/M spectra to maximally distinct polarizations: probing NRQCD with chi states

A global analysis of ATLAS and CMS measurements reveals that, at mid-rapidity, the directly-produced $χ_{c1}$, $χ_{c2}$ and J/$ψ$ mesons have differential cross sections of seemingly identical shapes, when presented as a function of the mass-rescaled transverse momentum, $p_{\rm T}/M$. This identity of kinematic behaviours among S- and P-wave quarkonia is certainly not a natural expectation of non-relativistic QCD (NRQCD), where each quarkonium state is supposed to reflect a specific family of elementary production processes, of significantly different $p_{\rm T}$-differential cross sections. Remarkably, accurate kinematic cancellations among the variegated NRQCD terms (colour singlets and octets) of its factorization expansion can lead to a surprisingly good description of the data. This peculiar tuning of the NRQCD mixtures leads to a clear prediction regarding the $χ_{c1}$ and $χ_{c2}$ polarizations, the only observables not yet measured: they should be almost maximally different from one another, and from the J/$ψ$ polarization, a striking exception in the global panorama of quarkonium production. Measurements of the difference between the $χ_{c1}$, $χ_{c2}$ and J/$ψ$ polarizations, complementing the observed identity of momentum dependences, represent a decisive probe of NRQCD.

hep-ph↗

arXiv2014

Quarkonium production in the LHC era: a polarized perspective

Polarization measurements are usually considered as the most difficult challenge for the QCD description of quarkonium production. In fact, global data fits for the determination of the non-perturbative parameters of bound-state formation traditionally exclude polarization observables and use them as a posteriori verifications of the predictions, with perplexing results. With a change of perspective, we move polarization data to the centre of the study, advocating that they actually provide the strongest fundamental indications about the production mechanisms, even before we explicitly consider perturbative calculations. Considering psi(2S) and Y(3S) measurements from LHC experiments and state-of-the-art NLO short-distance calculations in the framework of non-relativistic QCD factorization (NRQCD), we perform a search for a kinematic domain where the polarizations can be correctly reproduced together with the cross sections, by systematically scanning the phase space and accurately treating the experimental uncertainties. This strategy provides a straightforward solution to the "quarkonium polarization puzzle" and reassuring signs that the theoretical framework is reliable. At the same time, the results expose unexpected hierarchies in the non-perturbative NRQCD parameters, that open new paths towards the understanding of bound-state formation in QCD.

hep-ph↗

arXiv2013

Minimal physical constraints on the angular distributions of two-body boson decays

The angular distribution of the two-body decay of a boson of unknown properties is strongly constrained by angular momentum conservation and rotation invariance, as well as by the nature of the detected decay particles and of the colliding ones. Knowing the border between the "physical" and "unphysical" parameter domains defined by these "minimal constraints" (excluding specific hypotheses on what is still subject of measurement) is a useful ingredient in the experimental determinations of angular distributions and can provide model-independent criteria for spin characterizations. In particular, analysing the angular decay distribution with the general parametrization for the J = 2 case can provide a model-independent discrimination between the J = 0 and J = 2 hypotheses for a particle produced by two real gluons and decaying into two real photons.

hep-ph↗

arXiv2011

Determination of chi_c and chi_b polarizations from dilepton angular distributions in radiative decays

The angular distributions of the decay products in the successive decays chi_c (chi_b) to J/psi (Upsilon) gamma and J/psi (Upsilon) to l+l- are calculated as a function of the angular momentum composition of the decaying chi meson and of the multipole structure of the photon radiation, using a formalism independent of production mechanisms and polarization frames. The polarizations of the chi states produced in high energy collisions can be derived from the dilepton decay distributions of the daughter J/psi or Upsilon mesons, with a reduced dependence on the details of the photon reconstruction or simulation. Moreover, this method eliminates the dependence of the polarization measurement on the actual details of the multipole structure of the radiative transition. Problematic points in previous calculations of the chi_c decay angular distributions are identified and clarified.

hep-ph↗

arXiv2011

Model-independent constraints on the shape parameters of dilepton angular distributions

The coefficients determining the dilepton decay angular distribution of vector particles obey certain positivity constraints and a rotation-invariant identity. These relations are a direct consequence of the covariance properties of angular momentum eigenstates and are independent of the production mechanism. The Lam-Tung relation can be derived as a particular case, simply recognizing that the Drell-Yan dilepton is always produced transversely polarized with respect to one or more quantization axes. The dilepton angular distribution continues to be characterized by a frame-independent identity also when the Lam-Tung relation is violated. Moreover, the violation can be easily characterized by measuring a one-dimensional distribution depending on one shape coefficient.

hep-ph↗

arXiv2010

Rotation-invariant observables in parity-violating decays of vector particles to fermion pairs

The di-fermion angular distribution observed in decays of inclusively produced vector particles is characterized by two frame-independent observables, reflecting the average spin-alignment of the produced particle and the magnitude of parity violation in the decay. The existence of these observables derives from the rotational properties of angular momentum eigenstates and is a completely general result, valid for any J=1 state and independent of the production process. Rotation-invariant formulations of polarization and of the decay parity-asymmetry can provide more significant measurements than the commonly used frame-dependent definitions, also improving the quality of the comparisons between the measurements and the theoretical calculations.

hep-ph↗

arXiv2010

Towards the experimental clarification of quarkonium polarization

We highlight issues which are often underestimated in the experimental analyses on quarkonium polarization: the relation between the parameters of the angular distributions and the angular momentum composition of the quarkonium, the importance of the choice of the reference frame, the interplay between observed decay and production kinematics, and the consequent influence of the experimental acceptance on the comparison between experimental measurements and theoretical calculations. Given the puzzles raised by the available experimental results, new measurements must provide more detailed information, such that physical conclusions can be derived without relying on model-dependent assumptions. We describe a frame-invariant formalism which minimizes the dependence of the measurements on the experimental acceptance, facilitates the comparison with theoretical calculations, and probes systematic effects due to experimental biases. This formalism is a direct and generic consequence of the rotational invariance of the dilepton decay distribution and is independent of any assumptions specific to particular models of quarkonium production. The use of this improved approach, which exploits the intrinsic multidimensionality of the problem, will significantly contribute to a faster progress in our understanding of quarkonium production, especially if adopted as a common analysis framework by the LHC experiments, which will soon perform analyses of quarkonium polarization in proton-proton collisions.

hep-ph↗

arXiv2010

A new approach to quarkonium polarization studies

Significant progress in understanding quarkonium production requires improved polarization measurements, fully considering the intrinsic multidimensionality of the problem. We propose a frame-invariant formalism which minimizes the dependence of the measured result on the experimental acceptance, facilitates the comparison with theoretical calculations, and provides a much needed control over systematic effects due to detector limitations and analysis biases. This formalism is a direct and generic consequence of the rotational invariance of the dilepton decay distribution and is independent of any assumptions specific to particular models of quarkonium production.

hep-ph↗

arXiv2010

Rotation-invariant relations in vector meson decays into fermion pairs

The rotational properties of angular momentum eigenstates imply the existence of a frame-independent relation among the parameters of the decay distribution of vector mesons into fermions. This relation is a generalization of the Lam-Tung identity, a result specific to Drell-Yan production in perturbative QCD, here shown to be equivalent to the dynamical condition that the dilepton always originates from a transversely polarized photon.

hep-ph↗

arXiv2009

J/psi polarization from fixed-target to collider energies

The determination of the magnitude and "sign" of the J/psi polarization crucially depends on the reference frame used in the analysis of the data and a full understanding of the polarization phenomenon requires measurements reported in two "orthogonal" frames, such as the Collins-Soper and helicity frames. Moreover, the azimuthal anisotropy can be, in certain frames, as significant as the polar one. The seemingly contradictory J/psi polarization results reported by E866, HERA-B and CDF can be consistently described assuming that the most suitable axis for the measurement is along the direction of the relative motion of the colliding partons, and that directly produced J/psi's are longitudinally polarized at low momentum and transversely polarized at high momentum. We make specific predictions that can be tested on existing CDF data and by LHC measurements, which should show a full transverse polarization for direct J/psi mesons of pT > 25 GeV/c.

hep-ph↗

arXiv2009

Energy dependence of J/psi absorption in proton-nucleus collisions

Charmonium states are expected to be considerably suppressed in the case of quark-gluon plasma formation in high-energy heavy-ion collisions. However, a robust identification of suppression patterns as signatures of a deconfined QCD medium requires a detailed understanding of the "normal nuclear absorption" already present in proton-nucleus collisions, where the charmonium production cross sections increase less than linearly with the number of target nucleons. We analyse the J/psi production cross sections measured in proton-nucleus collisions in fixed target experiments, with proton beam energies from 200 to 920 GeV, and in d-Au collisions at RHIC, at sqrt(s)=200 GeV, in the framework of the Glauber formalism, using several sets of parton distributions with and without nuclear modifications. The results reveal a significant dependence of the "absorption cross section" on the kinematics of the J/psi and on the collision energy. Extrapolating the observed patterns we derive the level of absorption expected at Elab=158 GeV, the energy at which the heavy-ion data sets were collected at the CERN SPS.

hep-ph↗

arXiv2006

Open questions in quarkonium and electromagnetic probes

In my ("not a summary") talk at the Hard Probes 2006 conference, I gave "a personal and surely biased view on only a few of the many open questions on quarkonium and electromagnetic probes". Some of the points reported in that talk are exposed in this paper, having in mind the most important of all the open questions: do we have, today, from experimental data on electromagnetic probes and quarkonium production, convincing evidence that shows, beyond reasonable doubt, the existence of "new physics" in high-energy heavy-ion collisions?

nucl-ex↗

arXiv2001

Searching for Quark Matter at the CERN SPS

This article gives a brief overview of some recent advances in our understanding of the physics of dense strongly interacting matter, from measurements done at the CERN SPS. The presently available results are very interesting, and are likely to reflect the production of a new state of matter in central Pb-Pb collisions, at the highest SPS energies. However, important questions require further work. Particular emphasis is given to developments made since the Quark Matter 1999 conference, and to issues that justify the continuation of the SPS heavy ion physics program beyond year 2000.

hep-ex↗