Session FH: Proton Helicity Structure

Di-jet Measurements at Mid and Intermediate Rapidities $(-0.8 < \eta < 1.8)$ at the STAR Detector

One of the primary goals of the STAR spin program is to determine the spin-dependent gluon distribution, $\Delta g(x)$, of the proton. To date, the strongest constraints on $\Delta g(x)$ from STAR have come from measurements of the longitudinal double-spin asymmetry, $A_{LL}$, in inclusive jet production. In contrast to inclusive jets, di-jet correlation measurements provide access to partonic kinematics, at leading order, and thus give better constraints on the behavior of $\Delta g(x)$ as a function of partonic momentum fraction $x$. Furthermore, di-jet measurements at larger rapidity sample lower $x$ values where $\Delta g(x)$ is poorly constrained. In addition to $A_{LL}$, the di-jet cross section can be measured and compared to NLO pQCD calculations to confirm that detector and reconstruction effects are well understood. The mid-rapidity di-jet cross section result, utilizing 17 pb$^{-1}$ of 57\% polarized p-p data at $\sqrt{s} = 200$ GeV collected in 2009, will be presented. The status of the cross section and $A_{LL}$ measurements at forward rapidities will also be discussed.   

Dijet Longitudinal Double Spin Asymmetry at $\sqrt s = 510$ GeV in $\vec{p}\vec{p}$ collisions at STAR

The production of jets in $\vec{p}\vec{p}$ collisions at STAR is dominated by quark-gluon and gluon-gluon scattering processes. The dijet longitudinal double spin asymmetry $(A_{LL})$ is sensitive to the polarized parton distributions and may be used to extract information about the intrinsic gluon spin contribution $(\Delta G)$ to the spin of the proton. The measurement of dijet $A_{LL}$ at $\sqrt s = 510$ GeV will extend the current constraints on $\Delta G$ to a lower gluon momentum fraction and allow for reconstruction of the partonic kinematics at leading order. A status report on the present work towards the dijet $A_{LL}$ measurement from $\sim80 pb^{-1}$ of $\sim53\%$ polarized proton data at $\sqrt s = 510$ GeV taken during the 2012 RHIC run will be given.   

The inclusive jet cross-section measurement at $\sqrt{s}=200$ GeV p+p collisions at STAR

Inclusive jet and dijet measurements in polarized p+p collisions at $\sqrt{s} = 200$ and $500$ GeV are an essential part of the STAR program to understand the internal structure of the proton. Inclusive jet production probes the integral of the gluon distribution over a certain range of $x$ with a given jet transverse momentum. The jet cross-section measurement is one of the fundamental observables to test quantum chromodynamics (QCD). In addition to this, the inclusive jet cross-section measurement is a test of understanding the jet energy scale as well as trigger and detector efficiencies which are important to justify the use of inclusive jets in asymmetry measurements. We will present the status of the mid-rapidity ($|\eta|<1$) inclusive jet cross-section analysis of 18.2 pb$^{-1}$ of data sampled by STAR during the $\sqrt{s} = 200$ GeV RHIC run in 2009.   

Neutral Pion Cross Section and Spin Asymmetries at $0.8 < \eta < 2.0$ and $\sqrt{s} = 200$ GeV at STAR

The STAR endcap electromagnetic calorimeter (EEMC) was designed to allow measurement of cross sections and spin observables in an intermediate pseudorapidity range, $0.8 < \eta < 2.0$, and with full azimuth. Using the EEMC to measure double longitudinal spin asymmetries in photonic channels---such as inclusive neutral pions, prompt photon, and prompt photon + jet---allows access to $\Delta G$ covering a lower Bjorken-$x$ region than current mid-rapidity measurements. Transverse spin asymmetries, shown to be near zero at $\eta$ near zero and as large as 10\% at $\eta$ near four, measured with the EEMC occupy a previously unmeasured region in the 3D pseudorapidity, transverse momentum, and Feynman-$x$ phase space. The neutral pion cross section measurement verifies that the signal of interest can be properly reconstructed and isolated from the background and can quantify the applicability of pQCD predictions. The status of the neutral pion cross section and asymmetry measurements will be discussed, and results obtained from up to 8.0 pb$^{-1}$ of data from the 2006 RHIC run will be shown.   

Measurement of Luminosity Using Coincidence Counting and its Effect on Relative Luminosity

$\Delta G(x)$, the spin dependent gluon distribution in the proton is presently not constrained due to the lack of data at low-x. Even a sizable $\Delta G(x)$ will result in small double longitudinal helicity asymmetries, down to $A_{LL}\sim 10^{-4}$, in this region. In order to be sensitive to such small asymmetries, the relative luminosity between same sign and opposite sign helicity proton-proton interactions must be understood at the same level of precision. Luminosity may be estimated from event yields in luminosity monitoring detectors. To reduce noise, the luminosity is often measured by counting coincidences of two detectors, typically placed symmetrically about the nominal proton-proton interaction point. This method requires collision pileup corrections, for which there are standard procedures. However, a collision vertex requirement might also be imposed and many details of the experimental configuration such as the acceptance of the detectors, the vertex resolution and the longitudinal profile of the particle bunches in the beam must be considered. We present results from a Monte Carlo simulation that seeks to quantify these effects. Our goal is to reduce the systematic uncertainty associated with relative luminosity in polarized proton-proton collision experiments.   

Azimuthal Single-Spin Asymmetries of Charged Hadrons in Jets at $\sqrt{s} = 200$ GeV $p^{\uparrow}p$ Collisions at STAR

The transverse spin structure of the proton is accessible through azimuthal asymmetry measurements of charged hadrons in jets produced in polarized proton collisions. The STAR detector allows for full jet reconstruction and charged particle identification in the pseudorapidity range $|\eta| < 1.0$. Transverse single-spin asymmetry calculations allow for extraction of the Collins and Sivers moments. A status report of the current analysis from 20 pb$^{-1}$ of $\sqrt{s}=200$ GeV $p^{\uparrow}p$ data at 63\% polarization collected during the 2012 run will be given.   

Constraining $\Delta G$ at low-x with Double Longitudinal Spin Asymmetries for Forward Hadrons in PHENIX

Recent global analyses that include polarized p+p data from RHIC through 2009 suggest for the first time a positive contribution of the gluon polarization, $\Delta G$, to the overall proton spin. The data sets included in the analysis constrain $\Delta g(x)$ in the range $0.05< x <0.2$, leaving the lower x region nearly unconstrained. This low-x region can be accessed via a double helicity asymmetry in hadron production at large pseudorapidity, with a dominant contribution from collisions between a high-momentum quark and a low-momentum gluon. At PHENIX, we measure cluster $A_{LL}$ at large pseudorapidity ($3.1<\eta<3.9$) using the Muon Piston Calorimeter (MPC). The majority of the clusters ($>80\%$) come from $\pi^{0}$ decay where the photon showers in the calorimeter overlap. Simulations using the event generator PYTHIA have shown that measuring forward $\pi^{0}$'s can access $\Delta g(x)$ for $x \sim$ $10^{-2}$ for inclusive $\pi^{0}$'s or down to $x \sim$ $10^{-3}$ for the dihadron channel. Here, we present the status of $A_{LL}$ measurements in the MPC at $\sqrt{s}=500\: GeV$ from the 2011 through 2013 runs. This data will help to provide stronger constraints on the form of $\Delta g(x)$ in ongoing global analyses.   

Transverse Single-spin Asymmetries from $p^{\uparrow}+p\to jet+X$ and $p^{\uparrow}+p\to jet+\pi^{\pm}+X$ in the Central Pseudorapidity Range and $\sqrt{s}=500$ GeV at STAR

Current extractions of the Sivers and transversity functions come from measurements of transverse single-spin asymmetries (SSA) from SIDIS and fragmentation functions from lepton-lepton scattering. The limited kinematic reach of these measurements leaves poor constraints on the behavior of the Sivers and transversity functions at higher values of Bjorken-$x$. One avenue to enrich our understanding at this higher kinematic range is through jet production from high-energy polarized-proton collisions. The STAR detector at RHIC has seen hints of non-zero SSA's for the Collins effect from $p^{\uparrow}+p\to jet+\pi^{\pm}+X$ at $|\eta|<1$ and $\sqrt{s}=200$ GeV. Extending transverse SSA measurements for the first time to jet production in the central pseudorapidity range at 500 GeV allows one to examine the Sivers and Collins effects for a different mix of partonic subprocesses than those found at 200 GeV. Moreover, the increased gluon participation allows a more favorable examination of the gluon Collins-like effect. In 2011 STAR integrated 25 pb$^{-1}$ of luminosity from $p^{\uparrow}+p$ at $\sqrt{s}=500$ GeV with 48\% polarization. The status of the analysis of inclusive jet SSA's from these data will be shown.   

Recent results on W A$_L$ in longitudinally polarized $p+p$ collisions at STAR

The production of $W^\pm$ bosons in longitudinally polarized $p+p$ collisions is a powerful tool to study the spin-flavor structure of the proton, because the spin-dependent $W$ roduction cross section $\Delta\sigma=\sigma(\overrightarrow{p} p) - \sigma(\overleftarrow{p} p)$ is directly sensitive to the polarization of the quarks and anti-quarks in the proton. This contribution will report on the recent W measurements performed at the STAR experiment at RHIC. The STAR Electromagnetic Calorimeters are used to trigger on electrons and positrons from the weak decay of the $W$ boson and to provide a measure of the lepton energy, while the STAR Time Projection Chamber allows for reconstruction of the lepton track and its charge sign. During 2012 running period, the STAR experiment had collected an integrated luminosity of $\sim$80 pb$^{-1}$ at $\sqrt{s}=510$ GeV with an average beam polarization of $\sim$55\%. Preliminary results for the single-spin asymmetry, $A_L = \Delta\sigma / \left(\sigma(\overrightarrow{p}p) + \sigma(\overleftarrow{p}p) \right)$, from the 2012 dataset will be presented, as well as projections for the recently completed 2013 running period.   

Inclusive Jet Production in Longitudinally Polarized $pp$ Collisions at STAR

Gluon-gluon and quark-gluon scattering dominate jet production in longitudinally polarized proton-proton collisions at RHIC. The inclusive jet double-helicity asymmetry, $A_{LL}$, measured by STAR places significant constraints on gluon polarization in the proton. The preliminary results of inclusive jet $A_{LL}$ at $\sqrt{s} = 200$ GeV collected during the 2009 RHIC run show the first experimental evidence of non-zero gluon polarization over the Bjorken-$x$ range, $0.05 < x < 0.2$, sampled at RHIC. Furthermore, data collected at $\sqrt{s} = 510$ GeV during the 2012 RHIC run allow access to the gluon polarization at lower $x$. In this talk, I will present the results of the 2009 inclusive jet $A_{LL}$ at 200 GeV and discuss the current status of the analysis of the 2012 inclusive jet $A_{LL}$ at 510 GeV.