Bulletin of the American Physical Society
APS April Meeting 2017
Volume 62, Number 1
Saturday–Tuesday, January 28–31, 2017; Washington, DC
Session J16: Investigating Parton Dynamics with Drell-Yan-like ProcessesInvited
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Sponsoring Units: GHP DNP Chair: Paul Reimer, Argonne National Laboratory Room: Washington 3 |
Sunday, January 29, 2017 10:45AM - 11:21AM |
J16.00001: The 3D structure of the nucleon in momentum space: status and perspectives Invited Speaker: Alessandro Bacchetta The distribution of partons inside the nucleon can be described in terms of partonic Transverse Momentum Distributions (TMDs), which extend the concept of standard parton distribution functions (PDFs). We aim at obtaining a precise determination of these quantities based on global fits. A concise overview of this active field of research will be presented, illustrating a selection of recent achievements, current open issues, and future perspectives. [Preview Abstract] |
Sunday, January 29, 2017 11:21AM - 11:57AM |
J16.00002: Transverse single-spin asymmetries of weak bosons and Drell-Yan production at STAR Invited Speaker: Salvatore Fazio The three-dimensional structure of the proton in momentum space can be described via Transverse Momentum Dependent (TMDs) parton distribution functions. One of these TMDs, known as the Sivers function $f^{\perp}_{1T}$, describes the correlation of parton transverse momentum with the transverse spin of the nucleon. In Semi-Inclusive Deep Inelastic Scattering (SIDIS) processes in e+p collisions, the quark Sivers function is associated with a final state effect from the gluon exchange between the struck quark and the target nucleon remnants. On the other hand, in p+p collisions processes, the Sivers asymmetry appears as an initial state interaction effect. As a consequence, the quark Sivers functions are of opposite sign in SIDIS and in p+p and this non-universality is a fundamental prediction from the gauge invariance of QCD. The experimental test of this sign change is one of the open questions in hadronic physics, and can provide a direct verification of color interactions of partons (quarks and gluons) inside the hadrons. Accessing the Sivers TMD function in proton+proton collisions through the measurement of transverse single spin asymmetries (TSSAs) in weak boson production is an effective path to test the fundamental QCD prediction of the non-universality of the Sivers function. Furthermore, it provides data to study the spin-flavor structure of valence and sea quarks inside the proton and to test the evolution of parton distributions. RHIC is the world's only facility that can run transversely polarized p+p collisions at a center-of-mass energy large enough to produce weak bosons. The TSSA amplitude, $A_{N}$, has been measured at STAR in p+p collisions at $\sqrt{s} = 500$~GeV, with a recorded integrated luminosity of $25~\text{pb}^{-1}$. Within relatively large statistical uncertainties, the current data favor theoretical models that include change of sign for the Sivers function relative to observations in SIDIS measurements, if TMD evolution effects are small. RHIC plans to run proton+proton collisions of transversely polarized beams at $\sqrt{s} = 510$~GeV in 2017, delivering an integrated luminosity of $400~\text{pb}^{-1}$. This will allow STAR to perform a precise measurement of TSSAs in both weak boson and Drell-Yan production as well as other observables sensitive to the non-universality of the Sivers function via the Twist-3 formalism, e.g. the TSSA of direct photons. [Preview Abstract] |
Sunday, January 29, 2017 11:57AM - 12:33PM |
J16.00003: Drell-Yan measurement at COMPASS: a place to test the TMD PDFs universality Invited Speaker: Vincent Andrieux For the first time ever, the COMPASS experiment (CERN, SPS) collected in 2015 Drell-Yan (DY) data using a 190 GeV/$c$ pion beam on a transversely polarized NH$_3$ target. The azimuthal modulations of the DY cross-section give access to the set of transverse momentum dependent (TMD) parton distribution functions (PDFs), which describe the spin structure of the nucleon. Those PDFs were already measured in semi-inclusive deep inelastic scattering (SIDIS) by several experiments and especially COMPASS, which dedicated several campaigns between 2002 and 2010 to measure spin (in)dependent azimuthal asymmetries using a 160 GeV/$c$ polarized muon beam on a transversely polarized $^6$LiD or NH$_3$ target. A key interest of extracting those TMD PDFs from different processes is to check the universality and the process-dependent features of TMD PDFs. In this aim, COMPASS is a unique place to test the predicted sign-change of the TMD PDFs using a similar experimental setup and comparable kinematic domain. The main focus of this talk will be set on the physics aspects of the COMPASS polarized Drell-Yan program and related SIDIS results. [Preview Abstract] |
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