Session NC: Transverse Momentum Distributions

Indications of process dependence and the Sivers effect

We analyze the spin asymmetry for single inclusive jet production in proton-proton collisions collected by AnDY experiment at the Relativistic Heavy Ion Collider and the Sivers asymmetry data from semi-inclusive deep inelastic scattering experiments. In particular, we consider the role color gauge invariance plays in determining the process-dependence of the Sivers effect. We find that after carefully taking into account the initial-state and final-state interactions between the active parton and the remnant of the polarized hadron, the calculated jet spin asymmetry based on the Sivers functions extracted from HERMES and COMPASS experiments is consistent with the AnDY experimental data. This provides a first indication for the process-dependence of the Sivers effect in these processes.\footnote{ Phys.Rev.Lett. 110 (2013) 232301} We also make predictions for both direct photon and Drell-Yan spin asymmetry, to further test the process-dependence of the Sivers effect in future experiments.   

Spin-Dependent Pion-Induced Drell-Yan Measurement at COMPASS

Transverse-momentum dependent parton distribution functions (TMDs) offer a description of nucleon structure in terms of the parton's transverse momentum and its transverse spin. The so-called Sivers TMD, which represents the correlation between the quark's transverse momentum and the nucleon transverse spin, has been measured in semi-inclusive deep-inelastic scattering (SIDIS) at HERMES and COMPASS to be non-zero, a finding that is indicative of quark orbital angular momentum. A crucial test of the TMD formalism remains to be performed: the measurement of the Sivers TMD and other TMDs in spin-dependent Drell-Yan scattering (DY). The Sivers TMD is predicted to be of equal magnitude but of different sign when accessed from DY compared to SIDIS. At COMPASS (CERN), a 190 GeV beam of negatively charged pions will in 2014 scatter off a fixed target of transversely polarized protons, constituting the worldwide first measurement of spin-dependent DY. The event sample will almost entirely consist of reactions in which the anti-u quark from the beam pion annihilates with a u-quark from the polarized-target proton, avoiding cancellation by the d-quark Sivers TMD, which is of opposite sign to the u-quark Sivers TMD.   

Measurement of $W^{\pm}$ boson single-spin asymmetry in transversely polarized pp collisions at STAR

TMeasurement of $W^{\pm}$ boson single-spin asymmetry in transversely polarized pp collisions at STARhe Sivers function $f_{1T}^{\perp}$ describes the correlation of parton transverse momentum with the transverse spin of the nucleon. There is evidence of a quark Sivers effect in semi-inclusive DIS (SIDIS) measurements. Important aspects of the Sivers effect include its process dependence and the color gauge invariance. As a consequence, the quark Sivers functions are of opposite sign in SIDIS and in Drell-Yan 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 QCD factorization. While luminosities required for a meaningful measurement of asymmetries in Drell-Yan production are challenging, $W^{\pm}$ production with a comparable sensitivity to the predicted sign change can be measured at the STAR experiment. At these large $Q^2$ the result can also provide essential input for the evolution effects of the Sivers function. We report on the first attempt to measure the transverse single spin asymmetry, $A_N$, of the $W^{\pm}$ boson at RHIC. The $W$ kinematics is reconstructed by employing MC-based corrections dictated   

Accessing Unpolarized and Polarized Fragmentation Functions at Belle

Fragmentation functions (FFs) describe the formation of final state particles from a partonic initial state and are directly related to the intriguing QCD phenomenon of confinement. Precise knowledge of these functions is a key ingredient in accessing quantities such as the nucleon spin structure in semi-inclusive deep inelastic scattering and proton-proton collisions. However, FFs can currently not be determined from Quantum Chromodynamics first principles and have to be extracted from experimental data. The Belle experiment at KEK, Japan, provides a large data sample for high precision measurements of unpolarized and polarized FFs. Final results of a measurements of spin-independent FFs for identified pions and kaons will be presented. In addition, we will report on the status of an analysis of the kaon transverse-momentum-dependent Collins FF, which describes the correlation between the transverse polarization of the fragmenting quark and the momentum direction of the produced hadron.   

Target and Beam-Target Double Spin Asymmetry for Semi-Inclusive Deep Inelastic Scattering on Proton and Deuteron

Transverse momentum dependent parton distribution functions (TMDs) describe the transverse momentum and spin of the quarks and gluons inside a nucleon. The TMDs open a window on the contribution of the orbital angular momentum of the quarks and gluons to the total spin of the nucleon. The TMDs can be accessed from the target and double spin asymmetries of semi-inclusive deep inelastic scattering (SIDIS) reactions, where the asymmetries $A_{UL}$ and $A_{LL}$ are convolutions of the fragmentation functions and the TMDs. The EG1-DVCS experiment with CLAS at Jefferson Lab measured semi-inclusive pion production on longitudinally polarized proton and deuteron targets with polarized electrons of approximately 6 GeV. The target single spin and beam-target double spin asymmetries for these reactions are presented.   

Beam-spin Asymmetries from Semi-inclusive Pion Electroproduction

The moment of the semi-inclusive deep inelastic scattering (SIDIS) cross section corresponding to a polarized lepton scattering from an unpolarized target is $A_{LU}^{\sin\phi}$. It has been extracted from measurements of beam-spin asymmetries (BSAs) from pion electro-production in SIDIS. Data were taken with the continuous electron beam accelerator facility (CEBAF) Large Acceptance Spectrometer (CLAS) at Jefferson Lab using a 5.498 GeV longitudinally polarized electron beam and an unpolarized liquid hydrogen target. All three pion channels ($\pi^+$, $\pi^0$ and $\pi^-$) were measured simultaneously over a large range of kinematics at the intermediate $Q^2$ range ($Q^2 \approx$ 1 - 4.5 $GeV^2$). The moment is measured as functions of $z$, $P_T$, $x$, and $Q^{2}$. $A_{LU}^{\sin\phi}$ is a twist-3 structure providing information about quark-gluon-quark correlations. This new high statistical data will provide an important means of studying the transverse degrees of freedom in the nucleon.   

Measurement of single spin asymmetries of charged kaons produced in electrons scattering off a transversely polarized $^{3}$He target

We report the measurement of single-spin asymmetries of charged kaons produced in semi-inclusive deep inelastic scattering of electrons off a transversely polarized $^{3}$He target. The experiment, conducted at Jefferson Lab Hall A using a 5.9 GeV electron beam, covers a range of 0.16 \textless\ x \textless\ 0.35 with 1.4 \textless\ Q$^{2}$ \textless\ 2.7GeV$^{2}$. Both the Collins and Sivers moments for K$^{+}$ and K$^{-}$ are extracted simultaneously by using Maximum likelihood fitting, the corresponding preliminary results will be presented.   

Polarized SIDIS Experiments with SoLID Spectrometer at 12 GeV Jefferson Lab

The Jefferson Lab 12 GeV upgrade will provide unique opportunity to perform polarized semi-inclusive deep inelastic scattering (SIDIS) experiments with very high precision. The measured single and double spin asymmetries (SSA/DSA) in these reactions will provide an access to various transverse momentum dependent parton distribution functions (TMDs). In particular, the experimentally measured SSA on proton and neutron targets can help us in extracting the transversity and Sivers distribution functions of u and d-quarks. Similarly, the measured DSA are sensitive to the quark spin-orbit correlations, and provide access to the worm-gear function ($g_{1T}$). In this talk, we will present our plans for performing these precision measurements in Hall-A using a large acceptance SoLID spectrometer and polarized proton and $^3$He targets with 11 GeV beam. The high luminosities from these targets and the large acceptance of the SoLID spectrometer will allow for a precise 4-dimensional ($x,Q^2,z,P_T$) mapping of SSA and DSA. The data will provide comprehensive information on the correlation between quark angular momentum and the nucleon's spin.   

Transverse neutron spin structure using BigBite and Super BigBite spectrometers in Jefferson Lab's Hall A

New insight into nucleon structure has come from the study of Semi-Inclusive Deep Inelastic Scattering (SIDIS). Detection of leading hadrons associated with the fragmentation of the struck quark in lepton-nucleon DIS at moderate transverse momenta provides access to the transverse momentum dependent parton distributions of the nucleon (TMDs). The transverse target single-spin asymmetries (SSAs) in SIDIS are of particular interest due to their sensitivity to the chiral odd transversity distribution and the ``naive T-odd'' Sivers distribution. The Super BigBite Spectrometer (SBS), currently under construction for use in Jefferson Lab's Hall A following the 12 GeV upgrade of CEBAF, is capable of detecting forward-angle hadrons with a large solid-angle coverage at high luminosity, and is well-suited for the study of SIDIS reactions. Recently approved experiment E12-09-018 will use the SBS equipped with half of the RICH detector from the HERMES experiment for hadron detection, the existing BigBite spectrometer in Hall A for electron detection, and an upgraded polarized $^3He$ target to measure the transverse target SSAs in electron-neutron DIS with unprecedented precision. This talk will present the experiment goals, kinematics, apparatus, and projected results.