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Register a new userConstraints on Gluon Sivers Distribution from RHIC Results
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M. Anselmino
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We consider the recent RHIC data on the transverse single spin asymmetry (SSA) A_N, measured in p(transv. polarized) p --> pion X processes at mid-rapidity by the PHENIX Collaboration. We analyze this experimental information within a hard scattering approach based on a generalized QCD factorization scheme, with unintegrated, transverse momentum dependent (TMD), parton distribution and fragmentation functions. In this kinematical region, only the gluon Sivers effect could give a large contribution to A_N; its vanishing value is thus used to give approximate upper limits on the gluon Sivers function (GSF). Additional constraints from the Burkardt sum rule for the Sivers distributions are also discussed.
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We study the transverse single-spin asymmetries for $p^uparrow pto pi, X$ and $p^uparrow pto gamma, X$ within the so-called color gauge invariant generalized parton model (CGI-GPM) which, in addition to spin and transverse momentum effects, includes initial and final state interactions with the polarized proton remnants. We compute all relevant contributions, focusing in particular on the process dependence of the gluon Sivers function, which, for these processes, can always be expressed as a linear combination of two independent, universal terms. This study extends and completes a previous one, where only quark initiated partonic processes were considered. We then perform a combined phenomenological analysis of RHIC data on transverse single-spin asymmetries in $p^uparrow pto pi, X$ and $p^uparrow pto D, X$, putting the first preliminary constraints on these two gluon Sivers functions. We show how their size can be estimated by means of these data, and use our results to provide predictions for the process $p^uparrow pto J/psi,X$, comparing them with data, and $p^uparrow pto gamma, X$, for which experimental information will soon become available. Corresponding estimates within the simpler GPM approach, without initial and final state interactions and with a single universal gluon Sivers function, are also given, showing that a clear discrimination between these two models is, for the moment, not possible.
We study the production of prompt-photons at RHIC in the context of a generalised parton model framework, with a view to obtain information on the gluon Sivers function (GSF). At RHIC energy ($sqrt{s}=200$ GeV), the Compton process, $gqtogamma q$ contributes significantly to the production of direct-photons at midrapidity and dominates it in the negative (backward) rapdity region. We find that for direct photons, asymmetries of upto 10% are allowed by a maximal gluon Sivers function. However, the asymmetry obtained using existing fits of the GSF available is literature is negligible. We also estimate the impact that photons produced via fragmentation can have on the signal and find that their inclusion can dilute the asymmetry by between 10-50% of the direct-photon value. Finally, using the Colour-Gauge Invariant generalised parton model (CGI-GPM) approach, we consider the effects of initial state and final state interactions which can affect the universality of the Sivers functions in different processes. We find that the inclusion of these effects leads to the size of the gluon contributions being roughly halved. However, in the backward region which we are interested in, the sizes of the quark contributions are suppressed even further, leading to increased dominance of the gluon contributions.
Within a generalized parton model approach, with inclusion of spin and intrinsic transverse momentum effects, we show how the latest, highly precise, midrapidity data on the transverse single spin asymmetry measured in $pptopi^0, X$ by the PHENIX Collaboration at RHIC [1], can be used to get a first estimate on the still poorly known gluon Sivers distribution. To this end we also adopt the present information on the quark Sivers functions, as extracted from semi-inclusive deeply inelastic scattering data. This analysis updates a previous study by some of us where a first bound on this distribution was obtained [2].
Using momentum sum rule for evolution equations for Double Parton Distribution Functions (DPDFs) in the leading logarithmic approximation, we find that the double gluon distribution function can be uniquely constrained via the single gluon distribution function. We also study numerically its evolution with a hard scale and show that an approximately factorized ansatz into the product of two single gluon distributions performs quite well at small values of $x$ but is always violated for larger values, as expected.
The prediction of single spin asymmetry (SSA) in double $rm J/psi$ production in proton-proton collision is given out within the framework of non-relativistic QCD, using the recently obtained best fit parameters for the gluon Sivers function extracted from PHENIX data in $rm p+p^uparrowtopi^0+X$. The color singlet state $rm ^3 S_1^{(1)}$ and color octet state $rm ^3 S_1^{(8)}$ are considered to the SSA contribution in the double $rm J/psi$ hadroproduction. Our result shows that a sizable asymmetry can be estimated as functions of different kinematic variables.
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