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J. P. Lansberg

Publications and source records attributed to J. P. Lansberg.

At least 91 records · Page 5Linked to original sources
arXiv2008

Two-photon decay of pseudoscalar quarkonia

We report on our recent evaluation of the two-photon width of the pseudoscalar quarkonia, eta_c(nS) and eta_b(nS) in an approach based on Heavy-Quark Spin Symmetry (HQSS). To what concerns the 1S state eta_c, our parameter-free computation agrees with experiments, as well as most of other theoretical works. On the other hand, our computation for the 2S-state looks 2S like a confirmation that there may exist an anomaly related to the decay of eta_c(2S), especially in the light of the new preliminary result of the Belle collaboration. We also point out that the essentially model-independent ratio of eta_b two-photon width to the Upsilon leptonic width and the eta_b two-photon width could be used to extract the strong coupling constant alpha_s.

hep-ph↗
arXiv2008

Possible solution of the J/psi production puzzle

We argue that the s-channel cut contribution to J/psi hadroproduction can be significantly larger than the usual cut contribution of the color-singlet mechanism, which is known to underestimate the experimental measurements. A scenario accounting for intermediate $c\bar(c)$ interactions is proposed that reproduces the data at low- and mid-range transverse momenta P_T from the Fermilab Tevatron and BNL Relativistiv Heavy Ion Collider. The J/psi produced in this manner are polarized predominantly longitudinally.

hep-ph↗
arXiv2007

Hard exclusive electroproduction of a pion in the backward region

We study the scaling regime of pion electroproduction in the backward region, eN->e'N'pi. We compute the leading-twist amplitude in the kinematical region, where it factorises into a short-distance matrix element and long-distance dominated nucleon Distribution Amplitudes and nucleon to pion Transition Distribution Amplitudes. Using the chiral limit of the latter, we obtain a first estimate of the cross section, which may be experimentally studied at JLab or Hermes.

hep-ph↗
arXiv2007

Transition Distribution Amplitudes for gamma* gamma collisions

We study the exclusive production of pi-pi and rho-pi in hard gamma* gamma scattering in the forward kinematical region where the virtuality of one photon provides us with a hard scale in the process. The newly introduced concept of Transition Distribution Amplitudes (TDA) is used to perform a QCD calculation of these reactions thanks to two simple models for TDAs. The sizable cross sections for rho-pi and pi-pi production may be tested at intense electron-positron colliders such as CLEO and B factories (Belle and BaBar).

hep-ph↗
arXiv2007

Production of a pion in association with a high-Q2 dilepton pair in antiproton-proton annihilation at GSI-FAIR

We evaluate the cross section for anti-p p -> l+ l- pi0 in the forward direction and for large lepton pair invariant mass. In this kinematical region, the leading-twist amplitude factorises into a short-distance matrix element, long-distance dominated antiproton Distribution Amplitudes and proton to pion Transition Distribution Amplitudes (TDA). Using a modelling inspired from the chiral limit for these TDAs, we obtain a first estimate of this cross section, thus demonstrating that this process can be measured at GSI-FAIR.

hep-ph↗
arXiv2007

Transition Distribution Amplitudes

We give a brief overview of the theoretical status of the Transition Distribution Amplitudes and discuss their experimental near future.

hep-ph↗
arXiv2007

Hadroproduction of J/psi and Upsilon in association with a heavy-quark pair

We calculate the cross sections for the direct hadroproduction of J/psi and Upsilon associated with a heavy-quark pair of the same flavour at leading order in alpha_S and v in NRQCD. These processes provide an interesting signature that could be studied at the Tevatron and the LHC and also constitute a gauge-invariant subset of the NLO corrections to the inclusive hadroproduction of J/psi and Upsilon. We find that the fragmentation approximation commonly used to evaluate the contribution of these processes to the inclusive quarkonium production sizeably underestimates the exact calculation in the kinematical region accessible at the Tevatron. Both J/psi and Upsilon are predicted to be unpolarised, independently of their transverse momentum.

hep-ph↗
arXiv2006

Exploring backward pion electroproduction in the scaling regime

We use general relations between the Transition Distribution Amplitudes (TDAs), entering the description of the p -> pi0 transition, and the proton Distribution Amplitudes (DAs) in the soft-pion limit to estimate the size of the amplitude for backward electroproduction of pi0 at large Q2.

hep-ph↗
arXiv2006

Off-shell and non-static contributions to heavy-quarkonium production

We have shown that if one relaxes the constraint that the quarks in a heavy quarkonium are at rest and on-shell, new contributions to the discontinuity of the production amplitude appear. These can be seen as a s-cut in the amplitude and are on the same footage as the classical cut of the Colour-Singlet Model (CSM), where the heavy quarks forming the quarkonium are put on-shell by hypothesis. We treat this cut in a gauge-invariant manner by introducing necessary new 4-point vertices, suggestive of the colour-octet mechanism. We have further shown that this cut contributes at least as much as the LO CSM at large P_T. However, the 4-point vertices cannot be totally constrained and an ambiguity remains to what concerns their actual contribution. Theoretical insights from meson photoproduction are discussed in that context.

hep-ph↗
arXiv2006

Two-photon width of eta_b, eta'_b and eta''_b from Heavy-Quark Spin Symmetry

We predict the two-photon width of the pseudoscalar bottomonia, Gamma(eta_b -> gamma gamma), Gamma(eta'_b -> gamma gamma) and Gamma(eta''_b -> gamma gamma) within a Heavy-Quark Spin-Symmetry setting following the same line as our recent work for the corresponding decays for charmonia. Binding-energy effects are included for excited states and are shown to shift up the eta'_b and eta''_b widths by 10 %. We point out that the essentially model independent ratio Gamma(eta_b -> gamma gamma)/Gamma(Upsilon -> ll) and the branching ratio BR(eta_b -> gamma gamma) obtained could be used to extract the coupling constant alpha_s.

hep-ph↗
arXiv2006

J/psi, psi' and Upsilon Production at Hadron Colliders: a review

We give an overview of the present status of knowledge of the production of J/psi, psi' and Upsilon in high-energy hadron collisions. We first present two early models, namely the Colour-Singlet Model (CSM) and the Colour-Evaporation Model (CEM). The first is the natural application of pQCD to quarkonium production and has been shown to fail dramatically to describe data, the second is its phenomenological counterpart and was introduced in the spirit of the quark-hadron duality in the late seventies. Then, we expose the most recent experimental measurements of J/psi, psi' and Upsilon prompt and direct production at nonzero p_T from two high-energy hadron colliders, the Tevatron and RHIC. In a third part, we review six contemporary models describing J/psi, psi' and Upsilon production at nonzero p_T.

hep-ph↗
arXiv2006

Two-photon width of eta_c and eta'_c from Heavy-Quark Spin Symmetry

We evaluate the two-photon width of the pseudoscalar charmonia, Gamma(eta_c -> gamma gamma) and Gamma(eta'_c -> gamma gamma), within a Heavy-Quark Spin-Symmetry setting and show that whereas the former agrees with experiment, the latter is more than twice larger than the recent measurement by CLEO. When binding-energy effects are included in the eta'_c case, the discrepancy is worse, pointing out at a possible anomaly in the eta'_c decay.

hep-ph↗
arXiv2006

Backward DVCS and Proton to Photon Transition Distribution Amplitudes

We analyse deeply-virtual Compton scattering on a proton target, gamma* P -> P' gamma in the backward region and in the scaling regime. We define the transition distribution amplitudes which describe the proton to photon transition. Model-independent predictions are given to test this description, for current or planned experiments at JLab or by Hermes.

hep-ph↗
arXiv2006

Exclusive meson pair production in gamma* gamma scattering at small momentum transfer

We study the exclusive production of pi pi and rho pi in hard gamma* gamma scattering in the forward kinematical region where the virtuality of one photon provides us with a hard scale in the process. The newly introduced concept of Transition Distribution Amplitudes (TDA) is used to perform a QCD calculation of these reactions thanks to two simple models for TDAs. Cross sections for rho pi and pi pi production are evaluated and compared to the possible background from the Bremsstrahlung process. This picture may be tested at intense electron-positron colliders such as CLEO and B factories. The cross section e gamma -> e' pi0 pi0 is finally shown to provide a possible determination of the pi0 axial form factor, FA, at small t, which seems not to be measurable elsewhere.

hep-ph↗
arXiv2005

New contributions to heavy-quarkonium production

We reconsider quarkonium production in a field-theoretical setting and we show that the lowest-order mechanism for heavy-quarkonium production receives in general contributions from two different cuts. The first one corresponds to the usual colour-singlet mechanism. The second one has not been considered so far. We treat it in a gauge-invariant manner, and introduce new 4-point vertices, suggestive of the colour-octet mechanism. These new objects enable us to go beyond the static approximation. We show that the contribution of the new cut can be as large as the usual colour-singlet mechanism at high transverse momentum for J/psi. In the psi' case, theoretical uncertainties are shown to be large and agreement with data is possible.

hep-ph↗
arXiv2005

A toy model for generalised parton distributions

We give the results of a simple model for the diagonal and off-diagonal valence quark distributions of a pion. We show that structure can be implemented in a gauge-invariant manner. This explicit model questions the validityof the momentum sum rule, and gives an explicit counter-example to the Wandzura-Wilczek ansatz for twist-3 GPD's.

hep-ph↗
arXiv2005

Quarkonium Production at High-Energy Hadron Colliders

This thesis can be divided into several parts. The first one consists in a detailed historical review of quarkonium physics, centered on the different production models successively introduced. Then comes a review of the currently available data on high-energy hadronic production of J/psi, psi' and Upsilon's. In a third part, we concentrate on the extension of the Colour-Singlet Model beyond the static approximation. In this extension, we emphasise that a simple generalisation of the static case is not sufficient and new 4-point vertices are to be introduced to restore gauge invariance. Since these new vertices are not fully constrained, we propose a method to study this freedom. Finally, we apply our approach to the production at the Tevatron and RHIC, where we show that a combination of our contributions with fragmentation within the Colour-Octet Mechanism is able to reproduce the measurements both for cross sections and for polarisation.

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