Résumé du preprint DAPNIA-07-208

Generalised parton distributions at HERA and prospects for COMPASS.
L. Schoeffel
 We present a model of generalised parton distributions based on a forward ansatz 
in the DGLAP region. We discuss some aspects of the parametrisations, as the dependence 
in t, with factorised and non-factorised approaches, where t is the square of the 
four-momentum exchanged at the hadron vertex. The predictions of this model are 
then compared with DVCS cross sections from H1 and ZEUS, and a related observable, 
the skewing factor, defined as the following ratio imaginary amplitudes : 
$R \\equiv {Im {\\cal A} (\\gamma^*+p \\to \\gamma +p)\\lfrestriction{t=0}} / 
{Im {\\cal A} (\\gamma^*+p \\to \\gamma^* +p)\\lfrestriction{t=0}}$. 
It is an interesting quantity including both the non-forward kinematics and 
the non-diagonal effects. Finally, we discuss the beam charge asymmetry, which is 
certainly the most sensitive observable to the different hypothesis needed in the 
GPDs parametrisations. We show that the approximations done for the t dependence 
lead to significant differences for the predictions in the HERMES kinematic domain and 
prospects are given for COMPASS. 


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