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Register a new userMeasurement of Longitudinal Single-Spin Asymmetry for W$^pm$ Production in Polarized Proton+Proton Collisions at STAR
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Qing-Hua Xu
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The sea quark contribution to the nucleon spin is an important piece for a complete understanding of the nucleon spin structure. The production of $W$ bosons in longitudinally polarized $p + p$ collisions at RHIC provides an unique probe for the sea quark polarization, through the parity-violating single-spin asymmetry, $A_L$. At the STAR experiment, $W$ bosons can be effectively detected through the leptonic decay channel $W to e u $ with the Electromagnetic Calorimeters and Time Projection Chamber at mid-rapidity. The previous STAR measurements of $A_L$ for $W$ boson production from datasets taken in 2011 and 2012 have provided significant constraints on the helicity distribution functions of $bar u$ and $bar d$ quarks. In 2013 the STAR experiment collected $p+p$ data with an integrated luminosity of about 250 pb$^{-1}$ at $sqrt s$ = 510 GeV with an average beam polarization of about $56%$, which is about three times the total integrated luminosity of previous years. The final $A_L$ results from the STAR 2013 data sample are presented and are also combined with previous 2011+2012 results. The comparison with theoretical expectations suggests a flavor asymmetry with $Delta bar{u}(x)$ $>$$Delta bar{d}(x)$ for sea quark helicity distributions with $0.05 < x < 0.25$.
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The sea quark contribution to the nucleon spin is an important piece for a complete understanding of the nucleon spin structure. The production of W bosons in longitudinally polarized $p + p$ collisions at RHIC provides an unique probe for the sea quark polarization, through the parity-violating single-spin asymmetry, $A_L$. At the STAR experiment, W bosons through the leptonic decay channel $W to e u $ can be effectively determined with the Electromagnetic Calorimeters and Time Projection Chamber at mid-rapidity. The previous STAR measurements of $A_L$ for W boson production from datasets taken in 2011 and 2012, have provided significant constraints on the helicity distribution functions of $bar u$ and $bar d$ quarks. In 2013 the STAR experiment collected $p+p$ data with an integrated luminosity of about 300 pb$^{-1}$ at $sqrt s$ = 510 GeV with an average beam polarization of about $56%$, which is more than three times larger than the total integrated luminosity of previous years. The preliminary results of W-boson $A_L$ from 2013 data sample will be presented.
We report a new high-precision measurement of the mid-rapidity inclusive jet longitudinal double-spin asymmetry, $A_{LL}$, in polarized $pp$ collisions at center-of-mass energy $sqrt{s}=200$ GeV. The STAR data place stringent constraints on polarized parton distribution functions extracted at next-to-leading order from global analyses of inclusive deep inelastic scattering (DIS), semi-inclusive DIS, and RHIC $pp$ data. The measured asymmetries provide evidence for positive gluon polarization in the Bjorken-$x$ region $x>0.05$.
Transverse single spin asymmetry, $A_{N}$, of very forward $pi^{0}$ production from polarized $p + p$ collisions provides new information toward an understanding of its production mechanism. $A_{N}$ of forward $pi^{0}$ in the pseudorapidity region of $3 < eta < 4$ has been described by the partonic structure of the proton in the perturbative QCD framework. However, recent data indicates a potential contribution from not only partonic but also diffractive interactions. In order to provide a new insight on the origin of the $A_{N}$, we measured the very forward $pi^{0}$ production in the pseudorapidity region of $6 < eta$ from $sqrt{s}$ = 510 GeV polarized $p + p$ collisions at RHIC in 2017. We report our measurement of the very forward $pi^{0}$ over the transverse momentum range of $0 < p_{T} < 1$ GeV/$c$ and the preliminary result.
We report measurements of single- and double- spin asymmetries for $W^{pm}$ and $Z/gamma^*$ boson production in longitudinally polarized $p+p$ collisions at $sqrt{s} = 510$ GeV by the STAR experiment at RHIC. The asymmetries for $W^{pm}$ were measured as a function of the decay lepton pseudorapidity, which provides a theoretically clean probe of the protons polarized quark distributions at the scale of the $W$ mass. The results are compared to theoretical predictions, constrained by recent polarized deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range $0.05<x<0.2$.
We report high-precision measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for midrapidity inclusive jet and dijet production in polarized $pp$ collisions at a center-of-mass energy of $sqrt{s}=200,mathrm{GeV}$. The new inclusive jet data are sensitive to the gluon helicity distribution, $Delta g(x,Q^2)$, for gluon momentum fractions in the range from $x simeq 0.05$ to $x simeq 0.5$, while the new dijet data provide further constraints on the $x$ dependence of $Delta g(x,Q^2)$. The results are in good agreement with previous measurements at $sqrt{s}=200,mathrm{GeV}$ and with recent theoretical evaluations of prior world data. Our new results have better precision and thus strengthen the evidence that $Delta g(x,Q^2)$ is positive for $x > 0.05$.
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