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Register a new userTesting the universality of the Collins function in pion-jet production at RHIC
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By adopting a generalised parton model approach at leading order in QCD, including spin and intrinsic parton motion effects, we study the Collins azimuthal asymmetries for pions within a large-$p_T$ jet produced at mid-rapidity in polarised hadronic collisions. Using available information on the quark transversity distributions and the pion Collins functions, as extracted from semi-inclusive deeply inelastic scattering and $e^+e^-to h_1 h_2, X$ processes, we compute estimates for the Collins asymmetries in kinematical configurations presently investigated at RHIC by the STAR Collaboration. Collins-like asymmetries, involving linearly polarised gluons, are also considered. Our predictions, compared against available preliminary data, show a very good agreement, even if some discrepancies, to be further scrutinized both theoretically and experimentally, appear in the transverse momentum dependence of the Collins asymmetry. These results are in favour of the predicted universality of the Collins function and of a mild, if any, evolution with the hard scale of the asymmetries.
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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.
The latest data released by the BaBar Collaboration on azimuthal correlations measured for pion-kaon and kaon-kaon pairs produced in $e^+e^-$ annihilations allow, for the first time, a direct extraction of the kaon Collins functions. These functions are then used to compute the kaon Collins asymmetries in Semi Inclusive Deep Inelastic Scattering processes, which result in good agreement with the measurements performed by the HERMES and COMPASS Collaborations.
Background: A resonance has been observed by the ANDY Collaboration at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory in Cu+Au collisions at center-of-mass energy $sqrt{s} = 200$ GeV and at forward rapidity with an average mass of 18.15 GeV. The Collaboration suggests that it is a $b overline b b overline b$ tetraquark state decaying to two $Upsilon$(1S) states, each measured through the $Upsilon rightarrow ggg$ channel. Purpose: Their suggestion is investigated assuming that the two $Upsilon$ states are produced through the materialization of a $|uud boverline b b overline b rangle$ Fock state in the projectile. Methods: The $Upsilon$ pair mass and rapidity distributions arising from such a state are calculated. The production of an $X_b(b overline b b overline b)$ tetraquark state from the same Fock configuration is also investigated. The dependence on bottom quark mass and their transverse momentum range is also studied. Results: It is found that double $Upsilon$ production from these $|uud b overline b b overline b rangle$ states peak in the rapidity range of the ANDY detector. The $Upsilon$ pair and $X_b$ masses are, however, higher than the mass reported by the ANDY Collaboration. Conclusions: The results obtained from these calculations are incompatible with the ANDY result. They are, however, compatible with previous predictions of $b overline b b overline b$ tetraquark masses.
The production of phi mesons in Au+Au collisions at RHIC and their propagation in a hot and dense nuclear medium is studied within the microscopic quark-gluon string model. The inverse slope parameter of the transverse mass distribution agrees well with that extracted from the STAR data, while the absolute yield of phi is underestimated by a factor 2. It appears that the fusion of strings alone cannot increase the phi yield either. Less than 30% of detectable phis experience elastic scattering, this rate is insufficient for the full thermalization of phi. The directed flow of phi at |y|<2 demonstrates strong antiflow behavior, whereas its elliptic flow rises up to about 3.5% in the same rapidity interval. As a function of transverse momentum it rises linearly with increasing p_t, in agreement with the STAR data, and saturates at p_t > 2 GeV/c.
Transverse single spin asymmetries in pp --> pion X processes, while on a quite firm ground experimentally, are still a much debated phenomenological issue. We consider them in a transverse momentum dependent factorization scheme. After revisiting a previous result, we give new estimates of the Collins contribution by adopting the latest information on the Collins and transversity functions, as extracted from SIDIS and e+e- data.
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