Charmonium Hadroproduction Research Articles

Relativistic corrections to prompt double charmonium hadroproduction near threshold

We calculate the relativistic corrections to prompt J/ψ pair and J/ψ+ψ(2S) hadroproduction through the color singlet channel within the framework of nonrelativistic-QCD (NRQCD) factorization. The short-distance coefficients are obtained by matching full-QCD and NRQCD calculations at the partonic level, in which both squared amplitude and phase space are expanded in v2. We find that such an expansion of the phase space spoils the convergence of NRQCD factorization near the production threshold. To fix this problem, we propose to modify the matching between full QCD and NRQCD by adopting the physical phase space. In this modified approach, the theoretical uncertainties due to the choice of charm quark mass mc are largely reduced and the overall agreement of our predictions with LHC data is significantly improved, both for total and differential cross sections. Published by the American Physical Society 2024

Fixed-target charmonium production and pion parton distributions

We investigate how charmonium hadroproduction at fixed-target energies can be used to constrain the gluon distribution in pions. Using nonrelativistic QCD (NRQCD) formulation, the $J/\psi$ and $\psi(2S)$ cross sections as a function of longitudinal momentum fraction $x_F$ from pions and protons colliding with light targets, as well as the $\psi(2S)$ to $J/\psi$ cross section ratios, are included in the analysis. The color-octet long-distance matrix elements are found to have a pronounced dependence on the pion parton distribution functions (PDFs). This study shows that the $x_F$ differential cross sections of pion-induced charmonium production impose strong constraints on the pion's quark and gluon PDFs. In particular, the pion PDFs with larger gluon densities provide a significantly better description of the data. It is also found that the production of the $\psi(2S)$ state is associated with a larger quark-antiquark contribution, compared with $J/\psi$.

Curing the unphysical behaviour of NLO quarkonium production at the LHC and its relevance to constrain the gluon PDF at low scales

We address the unphysical energy dependence of quarkonium-hadroproduction cross sections at Next-to-Leading Order (NLO) in alpha _s which we attribute to an over-subtraction in the factorisation of the collinear singularities inside the PDFs in the overline{text {MS}} scheme. Such over- or under-subtractions have a limited phenomenological relevance in most of the scattering processes in particle physics. On the contrary, it is particularly harmful for P_T-integrated charmonium hadroproduction which renders a wide class of NLO results essentially unusable. Indeed, in such processes, alpha _s is not so small, the PDFs are not evolved much and can be rather flat for the corresponding momentum fractions and, finally, some process-dependent NLO pieces are either too small or too large. We propose a scale-fixing criterion which avoids such an over-subtraction. We demonstrate its efficiency for eta _{c,b} but also for a fictitious light elementary scalar boson. Having provided stable NLO predictions for eta _{c,b}P_T-integrated cross sections, sigma ^{mathrm{NLO}}_{eta _Q}, and discussed the options to study eta _{b} hadroproduction, we argue that their measurement at the LHC can help better determine the gluon PDF at low scales and tell whether the local minimum in conventional NLO gluon PDFs around x=0.001 at scales below 2 GeV is physical or not.

Heavy Flavor production with ATLAS

ATLAS has a rich heavy flavor and quarkonia program with measurements and searches that can contribute to a better understanding of Quantum Chromodynamics. Using data from proton–proton collisions in the LHC at √s = 7 TeV the γ(1S, 2S, 3S) production cross sections and the charged B meson production cross section measurements are presented, along with comparisons with theoretical expectations. The observation of J /ψ produced in association with W bosons, accessible for the first time, can provide new insights into the hadro-production of charmonium.

Complete next-to-leading order calculation of theJ/ψandψ′production at hadron colliders

A complete next-to-leading order (NLO) calculation in $\alpha_s$ for the $\jpsi$ and $\psip$ prompt production at the Tevatron, LHC, and RHIC in nonrelativistic QCD is presented. We argue that the next-to-next-to-leading order (NNLO) color-singlet contribution may not be so important as to resolve the large discrepancy between theory and experiment in $\jpsi$ large $p_T$ production cross sections. Therefore, a complete NLO calculation, including both color-singlet and color-octet contribution, is necessary and essential to give a good description for $\jpsi$ and $\psip$ production. We also study the methods to fit the long-distance matrix elements using either two linear combined matrix elements or three matrix elements, and find these two methods can give consistent results. Compared with the measurements at the LHC and RHIC for prompt $\jpsi$ and $\psip$ production, our predictions are found to agree with all data. In particular, the recently released large $p_T$ data (up to 60-70 GeV) at the LHC are in good agreement with our predictions. Our results imply that the universality of color-octet matrix elements holds approximately in charmonium hadroproduction, when one uses fixed order perturbative calculation to describe data (the data in small $p_T$ region are not included). Our work may provide a new test for the universality of color-octet matrix elements, and the color-octet mechanism in general.

Charmonium hadroproduction studies via [formula omitted] at the LHC

In this paper, the potential to study hadroproduction of charmonium via p p ¯ decay at the LHC is discussed. The branching ratio of h c → p p ¯ is estimated to be about 0.6 × 10 − 3 . The studies of charmonium hadroproduction using c c ¯ → p p ¯ and c c ¯ → l + l − are compared. The yield of J / ψ → p p ¯ is estimated to be about 60 events/pb−1 at s = 7 TeV .

Study of ψ′ and χcdecays as feed-down sources ofJ/ψ hadro-production

The interpretation of the J/psi suppression patterns observed in nuclear collisions, at CERN and RHIC, as a signature of the formation of a deconfined phase of QCD matter, requires knowing which fractions of the measured J/psi yields, in pp collisions, are due to decays of heavier charmonium states. From a detailed analysis of the available mid-rapidity charmonium hadro-production cross sections, or their ratios, we determine that the J/psi feed-down contributions from psi' and chi_c decays are, respectively, (8.1 +/- 0.3) % and (25 +/- 5) %. These proton-proton values are derived from global averages of the proton-nucleus measurements, assuming that the charmonium states are exponentially absorbed with the length of matter they traverse in the nuclear targets.

Analysis of charmonium production at fixed-target experiments in the NRQCD approach

We present an analysis of the existing data on charmonium hadro-production based on non-relativistic QCD (NRQCD) calculations at the next-to-leading order (NLO). All the data on J/psi and psi' production in fixed-target experiments and on pp collisions at low energy are included. We find that the amount of color octet contribution needed to describe the data is about 1/10 of that found at the Tevatron.

Soft and hard QCD dynamics in hadroproduction of charmonium

Both hard and soft QCD dynamics are important in charmonium production, as presented here through a next-to-leading order QCD matrix element calculation combined with the colour evaporation model. Observed $x_F$ and $p_\perp$ distributions of $J/\psi$ in hadroproduction at fixed target and $p\bar{p}$ collider energies are reproduced. Quite similar results can also be obtained in a more phenomenologically useful Monte Carlo event generator where the perturbative production of \ccbar pairs is instead obtained through leading order matrix elements and the parton shower approximation of the higher order processes. The soft dynamics may alternatively be described by the soft colour interaction model, originally introduced in connection with rapidity gaps. We also discuss the relative rates of different charmonium states and introduce an improved model for mapping the continuous \ccbar mass spectrum on the physical charmonium resonances.

New extraction of color-octet NRQCD matrix elements from charmonium hadroproduction

We re-analyze Tavatron data on charmonium hadroproduction in the framework of the color-octet model implemented in the event generator PYTHIA taking into account initial-state radiation of gluons and Altarelli-Parisi evolution of final-stage gluons fragmenting into c c pairs. We obtain new values for the color-octet matrix elements relevant to this production process. We discuss the sensitivity of our results to the transverse momentum lower cut-off employed in the generation to avois the problematic p t →0 region, arguing to the transverse momentum our previous extraction of the NRQCD matrix elements for the 3S 1 (8) and 1S 0 (8) + 3P J (8) contributions. Finally we extrapolate to LHC energies to get predictions on the J ψ direct production rate.

Massive-evolution effects on charmonium hadroproduction

The fragmentation functions D a→ H ( x, μ 2) of a heavy hadron H, with mass m H , satisfy the phase-space constraint D a→ H ( x, μ 2)=0 for x< m H 2/ μ 2, which is violated by the naive μ 2 evolution equations. Using appropriately generalized μ 2 evolution equations, we reconsider the inclusive hadroproduction of prompt J/ ψ mesons with high transverse momenta in the framework of the factorization formalism of nonrelativistic quantum chromodynamics, and determine the resulting shifts in the values of the leading colour-octet matrix elements, which are fitted to data from the Fermilab Tevatron.

InclusiveJ/ψandψ(2S)production fromBdecay inpp¯collisions

Using information on B-meson fragmentation functions from CERN LEP 1 and adopting the nonrelativistic QCD factorization formalism proposed by Bodwin, Braaten, and Lepage, we predict the transverse-momentum distribution of J/psi mesons originating from the inclusive decays of b hadrons produced in p p-bar collisions at the Fermilab Tevatron. We determine the relevant colour-octet charmonium matrix elements from fits to CDF data on prompt charmonium hadroproduction and to CLEO data on charmonium production from B-meson decay. Our predictions are found to agree well with recent CDF and D0 data.

Double spin asymmetries in P-wave charmonium hadroproduction

We discuss the double spin asymmetries in P-wave charmonium hadroproduction with non-zero transverse momenta at fixed target energies, s ≃40 GeV, within the framework of the factorization approach. The size of the asymmetries and the projected statistical errors in a future option of HERA with longitudinally polarized protons scattering off a polarized target (HERA- N ) are calculated. Measurements of the χ c1 and χ c2 decays into dilepton plus photon should allow to distinguish between different parametrizations for the polarized gluon distribution in the proton. At higher energies ( s =200 GeV) the situation appears less favourable with the presently envisaged integrated luminosities of the polarized RHIC collider.

Polarised and unpolarised charmonium production at higher orders in ν

We study the unpolarised and polarised hadro-production of charmonium in non-relativistic QCD (NRQCD) at low transverse momentum, including sufficiently higher orders in the relative velocity, ν, so as to study the ratio of χ c1 and χ c2 production rates.

Prompt charmonium production in Z dexays

The color-evaporation model quantitatively describes all data on photoproduction and hadroproduction of charmonium. Although the model is in part nonperturbative, the associated parameters can for instance be determined from the charmonium photoproduction data. At this point its predictions for the prompt production of ψ's at the Z pole are made with no free parameters. We show here that this approach successfully describes all available data on the production of prompt ψ's in Z decays. We also comment on the prompt Z decays into other charmonium states.

The interpretation of rapidity gaps at HERA

In leading twist deep inelastic ep scattering, the virtual photon interaction is fast compared to the time scale of soft color rearrangement. We compare the Pomeron exchange model, in which a neutral cluster is preformed, with a gluon exchange model, in which color is exchanged after the hard interaction. We find several features of the DIS data and of data on exclusive hard processes that favor a gluon exchange scenario. If correct, the postulate of soft color interactions between the produced (̄qq) system and the target has important implications for other processes. In particular, this may explain the puzzles of charmonium hadroproduction.

Hadroproduction of charmonium at large xF.

We present a higher-twist production mechanism for $\eta_c$ and $\chi_J$ charmonium at large momentum fraction $x_F$ in pion-nucleon collisions. The higher-twist contribution is essentially independent of $x_F$ and therefore becomes dominant at large $x_F$, where the leading-twist contribution falls off as $(1-x_F)^3$. We show that the higher-twist mechanism produces longitudinally polarized $\chi_1$ and $\chi_2$. For the $\chi_2$, this is clearly different from the leading-twist prediction of transverse polarization. For the $\chi_1$, the polarization of the leading- and higher-twist contributions is qualitatively similar.

Hadroproduction and polarization of charmonium.

In the limit of heavy-quark mass, the production cross section and polarization of quarkonia can be calculated in perturbative QCD. We study the ${\mathit{p}}_{\mathrm{\ensuremath{\perp}}}$-averaged production of charmonium states in \ensuremath{\pi}N collisions at fixed target energies. The data on the relative production rates of J/\ensuremath{\psi} and ${\mathrm{\ensuremath{\chi}}}_{\mathit{J}}$ are found to disagree with leading twist QCD. The polarization of the J/\ensuremath{\psi} indicates that the discrepancy is not due to poorly known parton distributions nor to the size of higher order effects (K factors). Rather, the disagreement suggests important higher twist corrections, as has been surmised earlier from the nuclear target A dependence of the production cross section.

On theA-dependence in charmonium hadroproduction on nuclei

The processes of hadroproduction ofJ/ψ on nuclei are considered in the approach of multiple scattering of intermediate leading states. It is shown that the description of experimental data available is possible when neglecting inelastic scattering and absorption cross sections of\(c\bar c\) pair in nuclear matter.