Session CC: 3D Imaging of Hadrons

Probing the simplest mesons to study the transverse spatial structure of hadrons

According to the constituent quark model, mesons are the hadrons with the simplest structures since they are built of two valence quarks only. This relative simple structure gives the mesons one of the best place in our quest to understand the structure and interaction of hadrons on the basis of QCD. The pion, the lightest meson, has been studied in details over the last decades. Measurements of the pion form factor are bringing important information about the pion structure and the meson electroproduction mechanism. Pion form factor data also allow one to extract the pion transverse charge density, an important quantity related to the GPDs that can also provide interesting information on the spatial structure of the nucleon's meson cloud. In this talk, I will present the transverse pion charge density extracted from world data of the pion form factor and their comparison to proton transverse charge densities. The kaon, the next simplest system available for studies, is relatively unexploited to date due to the lack of the necessary experimental facilities. I will also discuss the current status and opportunities for kaon studies in the 12 GeV era at Jefferson Laboratory and future facilities like the Electron-Ion Collider, as well as instrumentation required for such studies.   

Studies of Chiral-Odd GPDs in Hard Exclusive Pseudoscalar Meson Production

Deeply virtual exclusive electroproduction of pseudoscalar mesons in hard scattering processes provides a unique avenue to access chiral-odd GPDs. The latter enter the soft matrix elements in transverse virtual photon-proton scattering. Transverse photon polarization dominates $\pi^0$, $\eta$, and $K$ production in the multi GeV region owing to the fact that $t$-channel meson pole contribution either does not occur ($\pi^0$ and $\eta$), or is predicted to be relatively small ($K^+, K^o$). We will present ongoing studies of spin and azimuthal asymmetries in hard exclusive production of pseudoscalar mesons at Jefferson Lab, and proposed future measurements to study chiral-odd GPDs at JLab and Electron Ion Collider. Combination of measurements of cross sections, spin and azimuthal asymmetries with a longitudinally polarized beam and both unpolarized and longitudinally polarized proton targets for $\pi^o$, $\eta$ and $K^+, K^o$ electroproduction will provide a detailed test of the mechanism for pseudoscalar meson electroproduction including strangeness, and allow to perform flavor decomposition of underlying chiral-odd GPDs.   

Deeply Virtual Meson Production at Jefferson Lab

A longstanding goal in nuclear and particle physics has been to describe the three dimensional structure of the nucleon in terms of the quarks and gluon fields. Exclusive electron scattering reactions at high momentum transfers directly related to Generalized Parton Distributions (GPDs). Most reactions studied, such as DVCS or vector meson electroproduction, are primarily sensitive to the chiral-even GPDs. Very little is known about the chiral-odd GPDs, except that $H_T$ becomes the transversity function $h_1$ in the forward limit. It turns out that pseudoscalar meson electroproduction, and especially $\pi^0$ and $\eta$ production, were identified as especially sensitive to the parton helicity-flip subprocesses. Dedicated experiments to study Deeply Virtual Meson Production have been carried out in Hall B at Jefferson Lab. The cross sections and asymmetries of the exclusive pseudoscalar meson electroproduction processes in a very wide kinematic range of $Q^2$, $x_B$ and t have been measured with CLAS. The comparison of these data with the theoretical models will help us to better understand the connection of these processes with the longitudinal and transverse GPDs.   

Transverse target single-spin asymmetry in inclusive electroproduction of charged pions and kaons by HERMES

HERMES is a polarized deep inelastic scattering experiment at DESY. It uses 27.6 GeV electron/positron beam at DESY-HERA and internal gas targets. In this talk, recent results from the analyses on azimuthal asymmetry of $\pi^\pm$ and $K^\pm$ production with unpolarized beam on transverselly polarized hydrogen target[1] will be presented. $\pi^+$, $\pi^-$, $K^+$ and $K^-$ are detected in high-statistics inclusive measurements as functions of $P_T$ and $X_F$. $A_{UT}$ is measured and the sin$\psi$ amplitude is extracted for these particles where $\psi$ is the azimuthal angle determined by the beam axis, hadron momentum vector and the target spin direction. The results show clear positive amplitudes for $\pi^+$ and $K^+$ and a slightly negative amplitude for $\pi^-$. The results are interpreted based on the three relative scales, $\Lambda_{QCD}$, $P_T$ and $Q$. The Sivers and Collins mechanisms and twist-three quark-gluon correlation function are relevant in different kinematic regions. Correlations between $P_T$ and $X_F$ can be studied with these data.\\[4pt] [1] A. Airapetian et al. HERMES, Transverse target single-spin asymmetry in inclusive electroproduction of charged pions and kaons, Phys. Lett. B 728 (2014) 183.   

Recnt COMPASS results on transverse spin asymmetries in SIDIS

The nucleon spin structure has been studied in COMPASS at CERN by measurements of spin asymmeties in the semi inclusive deep inelastic scattering(SIDIS) with a muon at 160 GeV/c off a transversely polarized nucleon target. These allow investigation of the Collins and Sivers asymmetries and two-hadron production asymmetries. The Collins and the two-hadron production asymmetries have a connection to the transverse spin distribution function called transversity. The Sivers asymmetry gives Sivers function which is one of the transverse momentum dependenct parton distribution functions of the nucleon. Recent results of the analysis with the SIDIS data obtained in 2008 and 2010 for proton will be shown.   

Measurement of the Transverse Single-Spin Asymmetries for Forward $\pi^{0}$ and Electromagnetic Jets in Correlation with Midrapidity Jet-like Event\ s at STAR in $p+p$ Collisions at $\sqrt{s}$ = 500 GeV

Large transverse single-spin asymmetries ($A_N$) have been observed for forward inclusive hadron production in $p+p$ collisions at various experiments. In the collinear perturbative scattering picture, twist-3 multi-parton correlations can give rise to such an asymmetry. A transversely polarized quark can also give rise to a spin-dependent distribution of its hadron fragments via the twist-3 fragmentation function contribution. The observed $A_N$ may involve contributions from both processes. These can be disentangled by studying asymmetries for jets, direct photons and jet-fragments. The STAR Forward Meson Spectrometer (FMS), a Pb-glass electromagnetic calorimeter covering the pseudo-rapidity ($\eta$) range 2.6-4.2 and full azimuth, can detect photons, neutral pions and $\eta$ mesons. We are measuring $A_N$ for $\pi^{0}$ and electromagnetic(EM) jets reconstructed from photons in the FMS in $p+p$ collisions at $\sqrt{s}$ = 500 GeV that were recorded during the 2011 RHIC run. We report, here, $A_N$ for forward EM-jets and its sensitivity to the event topology of having a coincedence EM-jet at midrapidity.   

Analysis of gluon polarisation for single hadron production with high-pT hadrons in the low Q2

We present recent COMPASS result on gluon polarisation. We recently analysed longitudinal double spin asymmetry $A_{LL}(p_T)$ for single hadron production with high-$p_T$ hadrons in the low $Q^2$ regime using all COMPASS data. LiD and NH3 were used as polarized deuteron and proton targets. The asymmetries, which are almost consistent with zero, are compared to theoretical predictions with various $\Delta G$ hypotheses.   

Electroproduction in the Target Fragmentation Region

The Semi-Inclusive DIS process in the Target Fragmentation Region (TFR), when the hadron is produced in the fragmentation process of the target remnants, can be described through the so-called Fracture Functions (FFs). They represent the joint probability of producing the final hadron from the target remnants, when a parton of the target nucleon is struck by the virtual photon in a hard scattering process. Like the ordinary parton distribution functions, the FFs are universal objects, thus they can be measured in one experiment at a given hard scale and then used to make predictions for other experiments, at another hard scale. Measurements of the Lambda multiplicities and polarization asymmetries in TFR, in particular, will provide information on corresponding Fracture Functions. The study of its $Q^2$ dependence at JLab and EIC also will test the perturbative framework implied by Fracture Functions, simultaneously encoding the information on the interacting parton and on the fragmentation of the spectator system. We will present ongoing studies of electroproduction in TFR at Jefferson Lab, and proposed future measurement at upgraded JLab and Electron Ion Collider.   

Nucleon Spin Flavor Decomposition through Semi-Inclusive DIS Measurements on a Polarized 3He target

Semi-Inclusive Deep-Inelastic Scattering reactions can provide quark flavor tagging through the detection of the leading-hadron. Precise measurements of double-spin asymmetry $A_{1N}^h$ ($h=\pi^+, \pi^-, K^+, K^-$) can provide strong constrains on valence quark polarization, as well as sea quark polarization, and to access the possible polarized sea flavor asymmetry $\Delta \bar{u} - \Delta \bar{d}$. We emphasize that there are three independent methods to access polarized sea flavor asymmetry: 1. Next-Leading-Order QCD global fit method, as in DSSV2008, LSS2010 etc., 2. Leading-Order ``purity method,'' as used by HERMES and COMPASS experiments, 3. Leading-Order ``Christova-Leader'' method in which helicity asymmetry of the yield difference $\pi^+ - \pi^-$ is used to extract valence quark polarization $\Delta u_v$ and $\Delta d_v$, and polarized sea flavor asymmetry can be extracted when combining with inclusive spin structure $g_{1p}$ and $g_{1n}$. Taking a JLab-12GeV SIDIS experiment (PR12-14-008) as an example we show the sensitivity to polarized parton distributions and the polarized sea's flavor asymmetry.   

Measurement of longitudinal single-spin asymmetries for $W^{\pm}$ boson production in polarized $p+p$ collisions at $\sqrt{s}=510$ GeV at RHIC

$W^\pm$ boson production in longitudinally polarized $p+p$ collisions provides unique and clean access to the individual helicity polarizations of quarks and antiquarks in the colliding protons. Due to maximal violation of parity in the production, $W^\pm$ bosons couple to left-handed quarks and right-handed anti quarks and hence offer direct probes of their respective helicity distributions in the nucleon. These can be observed in measured large parity-violating longitudinal single-spin asymmetries $A_L$, as a function of decay lepton pseudo-rapidity $\eta$, for $W^\pm$ boson production at the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The final results of W $A_L$ measured between $|\eta|<1.1$ at $\sqrt{s}=510$ GeV from the dataset collected in 2011 and 2012 running periods with an integrated luminosity of 86 pb$^{-1}$ and an average beam polarization of $\sim$55\% will be presented. The status for the analysis of W $A_L$ from the larger dataset collected during 2013 (more than 3 times of 2012) at $\sqrt{s}=510$ GeV with an average beam polarization of $\sim$54\% will be reported.