arXiv subjects

J. P. Lansberg

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

99 records · Page 6Linked to original sources

arXiv2005

Heavy Quarkonium Production: Extending CSM and COM

By questioning the applicability of the static approximation of the Colour-Singlet Model, we have seen that the production amplitude receives contributions from two different cuts. The first one in its static limit gives the colour-singlet mechanism. The second one has not been considered so far. We treat it in a gauge-invariant manner by introducing necessary new 4-point vertices, suggestive of the colour-octet mechanism. This new contribution can be as large as the colour-singlet mechanism at high pT, however these vertices are not totally constrained and when the freedom in their determination is fully exploited, we are able to reproduce the production cross-sections at the Tevatron for the J/Psi, Psi' and Upsilon(1S) and at RHIC for the J/Psi.

hep-ph↗

arXiv2004

A Model for the Generalized Parton Distribution of the Pion

We calculate the off-forward structure function of the pion within a simple model where the size of the pion is accounted for using a momentum cut-off. Twist-two and twist-three generalized parton distributions are extracted. Relations between twist-three and twist-two contributions are obtained, the origin of which is not kinematical as they differ from those arising from the Wandzura-Wilczek approximation.

hep-ph↗

arXiv2004

A Simple Model for Generalised Parton Distributions of the Pion

We present here an extension of our model for the pion, which we used previously to calculate its diagonal structure function, to the off-forward case. The imaginary part of the off-forward gamma^* pi -> gamma^* pi scattering amplitude is evaluated in the chiral limit (m_pi=0) and related to the twist-two and twist-three generalised parton distributions H, H3, H3tilde. Non-perturbative effects, linked to the size of the pion and still preserving gauge invariance, are included. Remarkable new relations between H, H3 and H3tilde are obtained and discussed.

hep-ph↗

arXiv2004

A Model for the Off-forward Structure Functions of the Pion

We extend our model for the pion, which we used previously to calculate its diagonal structure function, to the off-forward case. The imaginary part of the off-forward gamma* pi -> gamma* pi scattering amplitude is evaluated in the chiral limit (m_pi=0) and related to the twist-two and twist-three generalised parton distributions, H, H3, H3tilde. Non-perturbative effects, linked to the size of the pion and still preserving gauge invariance, are included. Remarkable new relations between H, H3 and H3tilde are obtained and discussed.

hep-ph↗

arXiv2003

Generalized Parton Distributions of the Pion

Off-forward structure functions of the pion are investigated in twist-two and twist-three approximation. A simple model is used for the pion, which allows to introduce finite size effects, while preserving gauge invariance. Results for the imaginary parts of the gamma^* pi -> gamma^* pi off-forward amplitude and of the structure functions are presented. Generalized Callan-Gross relations are obtained.

hep-ph↗

arXiv2002

Pion Structure Function and Violation of the Momentum Sum Rule

We present a method to evaluate the pion structure functions from a box diagram calculation. Pion and constituent quark fields are coupled through the simplest pseudoscalar coupling. The gamma^* pi -> q \bar q cross-section is evaluated and related to the structure functions. We then show that the introduction of non-perturbative effects, related to the pion size and preserving gauge invariance, provides us with a straighforward relation with the quark distribution. It is predicted that higher-twist terms become negligible for Q^2 larger than about 2 GeV^2 and that quarks in the pion have a momentum fraction smaller than in the proton. We enlarge the discussion concerning this violation of the momentum sum rule, emphasizing that the sum rule is recovered in the chiral limit and also when the finite size condition is not imposed.

hep-ph↗

arXiv2002

The Valence Quark Distribution of the Pion

The pion structure function is investigated in a simple model, where the pion and its constituent quark fields are coupled through the simplest pseudoscalar coupling. The imaginary part of the forward gamma* pi -> gamma* pi scattering amplitude is evaluated and related to the structure functions. It is shown that the introduction of non-perturbative effects, linked to the size of the pion, allows a connection with the quark distribution. It is predicted that higher-twist terms become negligible for Q^2 larger than ~2 GeV^2, that quarks in the pion have a momentum fraction smaller than in the proton case, and that the momentum sum rule is violated for the pion.

hep-ph↗

arXiv2002

A Model for the Pion Structure Function

The pion structure function is investigated in a simple model, where pion and constituent quark fields are coupled through the simplest pseudoscalar coupling. The imaginary part of the forward gamma* pi-> gamma* pi scattering amplitude is evaluated and related to the structure functions. It is shown that the introduction of non-perturbative effects, linked to the size of the pion and preserving gauge invariance, allows a connection with the quark distribution. It is predicted that higher-twist terms become negligible for Q2 larger than about 2 GeV2 and that quarks in the pion have a momentum fraction smaller than in the proton case.

hep-ph↗

arXiv2002

Heavy quarkonium production in hadronic collisions

Different theoretical models which attempt to describe hadronic production of heavy quarkonia are reviewed. Firstly, we consider the Color Singlet Model and point out the large discrepancies between the theoretical predictions and the results from the Tevatron detectors. Then some other models are introduced, quickly discussed and confronted with experimental results. Finally, we suggest possible ways to understand the source of the remaining discrepancies.

hep-ph↗