Soft and Hard Pomeron in the Structure Function of the Proton at Low x and Low Q^2

We study inclusive electroproduction on the proton at low $x$ and low $Q^2$ using a soft and a hard Pomeron. The contribution of the soft Pomeron is based on the Stochastic Vacuum Model, in which a nonperturbative dipole-dipole cross section can be calculated by means of a gauge invariant gluon field strength correlator. To model the hard Pomeron exchange we phenomenologically extend the leading order evolution of a power-behaved structure function, $F_2 \propto x^{- \lambda}$, proposed by L\'opez and Yndur\'ain. This extension allows to consider both the case $Q^2 = 0$ and the region of higher $Q^2$ on the basis of the same parametrization. A good simultaneous fit to the data on $F_2$ and on the cross section $\sigma_{\gamma p}$ of real photoproduction is obtained for $\lambda=0.37$. With four parameters we achieve a $\chi^2/\textrm{d.o.f.} = 0.98$ for 222 data points. In addition, we use our model of the inclusive $\gamma^{\ast} p$ interaction to compute the longitudinal structure function $F_L$.Comment: 18 pages, Latex, 6 PS-figures, Regge-exchange neglected, more details concerning the soft Pomeron included, section on the longitudinal structure function added, all conclusions unchanged, final version to appear in Eur. Phys. J.

Role of transverse momentum dependence of unpolarised parton distribution and fragmentation functions in the analysis of azimuthal spin asymmetries

Information on the Sivers distribution and the Collins fragmentation functions and their transverse momentum dependence is mainly based on fitting single spin asymmetry data from semi-inclusive deep inelastic scattering (SIDIS). Independent information, respectively on the Sivers distribution and the Collins fragmentation, can be obtained from Drell-Yan and $e^+e^-$ annihilation processes. In the SIDIS case, the transverse momentum of the final observed hadron, which is the quantity measured, is generated both by the average transverse momentum in the distribution and in the fragmentation functions. As a consequence, these are strongly correlated and a separate extraction is made difficult. In this paper we investigate, in a simple kinematical Gaussian configuration, this correlation, its role on the transverse single spin asymmetries in SIDIS and the consequences for predictions of the Sivers asymmetry in Drell-Yan processes and for the Collins asymmetry in $e^+e^-$ annihilation. We find that, in some cases, these effects can be relevant and must be carefully taken into account.Comment: 21 pages, 9 figures (18 eps plots

Study of the sign change of the Sivers function from STAR Collaboration W/Z production data

Recent data on the transverse single spin asymmetry $A_N$ measured by the STAR Collaboration for $p^\uparrow \, p \to W^\pm/Z^0 \, X$ reactions at RHIC allow the first investigation of the Sivers function in Drell-Yan processes and of its expected sign change with respect to SIDIS processes. A new extraction of the Sivers functions from the latest SIDIS data is performed and a critical assessment of the significance of the STAR data is attempted.Comment: 17 pages, 16 pdf figures, uses jheppub.st

Extracting the Sivers function from polarized SIDIS data and making predictions

The most recent data on the weighted transverse single spin asymmetry $A_{UT}^{\sin(\phi_h-\phi_S)}$ from HERMES and COMPASS collaborations are analysed within LO parton model with unintegrated parton distribution and fragmentation functions; all transverse motions are taken into account, with exact kinematics, in the elementary interactions. The overall quality of the data is such that, for the first time, a rather well constrained extraction of the Sivers function for $u$ and $d$ quarks is possible and is performed. Comparisons with models are made. Based on the extracted Sivers functions, predictions for $A_{UT}^{\sin(\phi_h-\phi_S)}$ asymmetries at JLab are given; suggestions for further measurements at COMPASS, with a transversely polarized hydrogen target and selecting favourable kinematical ranges, are discussed. Predictions are also presented for Single Spin Asymmetries (SSA) in Drell-Yan processes at RHIC and GSI.Comment: 18 pages, 10 figures, version published in PRD, one figure and comments adde

Sivers effect in Drell-Yan processes

The Sivers distributions recently extracted from semi-inclusive deep inelastic scattering data [1] are used to compute estimates for Sivers asymmetries in Drell-Yan processes which are being planned at several facilities (RHIC, COMPASS, J-PARC, PAX, PANDA, NICA (JINR) and SPASCHARM (IHEP)). Most of these asymmetries turn out to be large and could allow a clear test of the predicted sign change of the Sivers distributions when active in SIDIS and Drell-Yan processes. This is regarded as a fundamental test of our understanding, within QCD and the factorization scheme, of single spin asymmetries.Comment: RevTeX4, 12 pages, 23 eps figure

Sivers distribution functions and the latest SIDIS data

We present an extraction of the Sivers distribution functions from the most recent experimental data of the HERMES and COMPASS experiments, assuming a negligible contribution of sea quark Sivers functions.Comment: To appear in the Proceedings of the XIX International Workshop on Deep-Inelastic Scattering and Related Subjects (DIS 2011), April 11-15, 2011, Newport News, VA USA. Typo corrected in Table