ABSTRACT The longitudinal double spin asymmetry A_LL in the reaction pp -> pi X has been m... more ABSTRACT The longitudinal double spin asymmetry A_LL in the reaction pp -> pi X has been measured at RHIC with extremely interesting consequences. If the gluon polarization in a proton were as big as needed to resolve the famous "spin crisis" then A_LL would be large and positive. Early RHIC results suggested that A_LL might even be negative, which is impossible in the simple collinear parton model. Later results indicate very small positive values. Recently, for the first time, we derived expressions for the general partonic structure for all hadron spin asymmetries with inclusion of all transverse motion of the partons in a hadron and of the hadrons in a fragmenting parton. Besides the standard soft functions that are present in the collinear treatment, several new spin and k_T dependent soft functions appear and contribute to the cross sections and to spin asymmetries, both transverse and longitudinal. We examine the influence of k_T and of the new terms on A_LL, and in particular whether they could alter the conclusion that the gluon polarization is very small. It turns out that the contribution from these effects is essentially negligible.
Diquark contributions to the nucleon structure functions are given in the framework of the parton... more Diquark contributions to the nucleon structure functions are given in the framework of the parton model, considering the general case where spin 1 and spin 0 diquarks are allowed inside nucleons. The properties of their form factors, required in order for the scaling violations to be compatible with the observed ones, are discussed. It is argues that, assuming appropriate form factors, vector diquark contributions to the unpolarized structure F1 and F2 may satisfy, at leading order, the Callan-Gross relation.
ABSTRACT The longitudinal double spin asymmetry A_LL in the reaction pp -> pi X has been m... more ABSTRACT The longitudinal double spin asymmetry A_LL in the reaction pp -> pi X has been measured at RHIC with extremely interesting consequences. If the gluon polarization in a proton were as big as needed to resolve the famous "spin crisis" then A_LL would be large and positive. Early RHIC results suggested that A_LL might even be negative, which is impossible in the simple collinear parton model. Later results indicate very small positive values. Recently, for the first time, we derived expressions for the general partonic structure for all hadron spin asymmetries with inclusion of all transverse motion of the partons in a hadron and of the hadrons in a fragmenting parton. Besides the standard soft functions that are present in the collinear treatment, several new spin and k_T dependent soft functions appear and contribute to the cross sections and to spin asymmetries, both transverse and longitudinal. We examine the influence of k_T and of the new terms on A_LL, and in particular whether they could alter the conclusion that the gluon polarization is very small. It turns out that the contribution from these effects is essentially negligible.
Diquark contributions to the nucleon structure functions are given in the framework of the parton... more Diquark contributions to the nucleon structure functions are given in the framework of the parton model, considering the general case where spin 1 and spin 0 diquarks are allowed inside nucleons. The properties of their form factors, required in order for the scaling violations to be compatible with the observed ones, are discussed. It is argues that, assuming appropriate form factors, vector diquark contributions to the unpolarized structure F1 and F2 may satisfy, at leading order, the Callan-Gross relation.
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