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Register a new userMeasurement of longitudinal spin asymmetries for weak boson production in polarized proton-proton collisions at RHIC
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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 $0.05<x<0.2$.
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We report new STAR measurements of the single-spin asymmetries $A_L$ for $W^+$ and $W^-$ bosons produced in polarized proton--proton collisions at $sqrt{s}$ = 510 GeV as a function of the decay-positron and decay-electron pseudorapidity. The data were obtained in 2013 and correspond to an integrated luminosity of 250 pb$^{-1}$. The results are combined with previous results obtained with 86 pb$^{-1}$. A comparison with theoretical expectations based on polarized lepton-nucleon deep-inelastic scattering and prior polarized proton--proton data suggests a difference between the $bar{u}$ and $bar{d}$ quark helicity distributions for $0.05 < x < 0.25$. In addition, we report new results for the double-spin asymmetries $A_{LL}$ for $W^pm$, as well as $A_L$ for $Z/gamma^*$ production and subsequent decay into electron--positron pairs.
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 $0.05 < x < 0.25$.
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.
The production of $W^{pm}$ bosons in longitudinally polarized $vec{p}+vec{p}$ collisions at RHIC provides a new means of studying the spin-flavor asymmetries of the proton sea quark distributions. Details of the $W^{pm}$ event selection in the $e^{pm}$ decay channel at mid-rapidity are presented, along with preliminary results for the production cross section and parity-violating single-spin asymmetry, $A_L$, from the STAR Collaborations 2009 data at $sqrt{s}=500$ GeV.
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 $x>0.05$.
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