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Register a new userConstraining Sea Quark Distributions Through $W^{pm}$ Cross Section Ratios Measured at STAR
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M. Posik
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Over the past several years the STAR experiment at RHIC has been contributing to our understanding of the proton structure. Through its instrumentation, STAR is well equipped to measure $W rightarrow u + e$ in $sqrt{s}$ = 500/510 GeV proton-proton collisions at mid-rapidity (-1.1 $le eta le $ 1.1) . The $W$ cross section ratio ($W^+/W^-$) is sensitive to unpolarized $u$, $d$, $bar{u}$, and $bar{d}$ quark distributions. At these kinematics, STAR is able to measure the quark distributions near Bjorken-$x$ values of 0.1. The RHIC runs in 2011, 2012 and 2013 at $sqrt{s}$ = 500/510 GeV saw a significant increase in delivered luminosity from previous years. This resulted in a total data sample being collected of about 352 pb$^{-1}$ of integrated luminosity. The increased statistics will lead to a higher precision measurement of the $W^+/W^-$ cross section ratio than was previously measured by STARs 2009 run, as well as allow for a measurement of its $eta$ dependence at mid-rapidity. Presented here is an update of the $W$ cross section ratio analysis from the STAR 2011, 2012 and 2013 runs.
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Over the past several years, parton distribution functions (PDFs) have become more precise. However there are still kinematic regions where more data are needed to help constrain global PDF extractions, such as the ratio of the sea quark distributions $bar{d}$/$bar{u}$ near the valence region. Furthermore, current measurements appear to suggest different high-$x$ behaviors of this ratio. The $W$ cross section ratio ($W^+$/$W^-$) is sensitive to the unpolarized quark distributions at large $Q^2$ set by the $W$ mass. Such a measurement can be used to help constrain the $bar{d}$/$bar{u}$ ratio. The STAR experiment at RHIC is well equipped to measure the leptonic decays of $W$ bosons, in the mid-pseudorapdity range $left(|eta| leq 1 right)$, produced in proton-proton collisions at $sqrt{s}$ = 500/510 GeV. At these kinematics STAR is sensitive to quark distributions near $x$ of 0.16. STAR can also measure $W^+$/$W^-$ in a more forward region ranging from 1.0 $< eta <$1.5, which extends the sea quark sensitivity to higher $x$. RHIC runs from 2011 through 2013 have collected about 350 pb$^{-1}$ of integrated luminosity, and an additional 350 pb$^{-1}$ from the 2017 run. These proceedings will present preliminary results of the 2011-2013 $W^+$/$W^-$ cross section ratio measurements. Additionally, the $W/Z$ cross section ratio, differential and total $W$ and $Z$ cross sections are presented.
Over the past several years, parton distribution functions (PDFs) have become more precise. However there are still kinematic regions where more data are needed to help constrain global PDF extractions, such as the sea quark distributions $bar{d}$/$bar{u}$ near the valence region (Bjorken-x $approx$ 0.1 - 0.3).~Current measurements appear to suggest different high-x behaviors of these distributions, leading to large uncertainties in global fits. The charged W cross section ratio (W$^+$/W$^-$) is sensitive to the unpolarized $u,;d,;bar{u},$ and $bar{d}$ quark distributions at large $Q^2$ set by the $W$ mass and could help shed light on this discrepancy. The STAR experiment at RHIC is well equipped to measure the leptonic decays of W bosons, in the mid-rapidity range $left(|eta| leq 1 right)$, produced in proton+proton collisions at $sqrt{s}$ = 500/510 GeV. At these kinematics STAR is sensitive to quark distributions near Bjorken-x of 0.16. STAR can also measure the W cross section ratio in a more forward bin ranging from 1.1 $< eta <$ 2.0, which extends the sea quark sensitivity to higher x. RHIC runs from 2011 through 2013 have collected about 350 pb$^{-1}$ of integrated luminosity, and a 2017 run is expected to provide an additional 400 pb$^{-1}$. Presented here are preliminary results for the 2011-2012 charged W cross section ratios ($sim$100pb$^{-1}$) and an update on the 2013 charged W cross section analysis ($sim$250 pb$^{-1}$).
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 $0.05 < x < 0.25$.
One of the primary goals of the spin program at the Relativistic Heavy Ion Collider (RHIC) is to determine the polarization of the sea quarks and gluons in the proton. The polarization of the sea quarks is probed through the production of $W^{-(+)}$ bosons via the annihilation of $bar{u}+d,(bar{d}+u)$, at leading order. In this proceedings we report measurements of the single-spin asymmetry, $A_{L}$, for $W$ boson production at $sqrt{s} = 510$ GeV, and the new constraints these results place on the antiquark helicity distributions. Recent results on the longitudinal double-spin asymmetry, $A_{LL}$, for inclusive and di-jet production at $sqrt{s} = 200$ GeV are also presented. The inclusive jet results provide the first experimental indication of non-zero gluon polarization in the $x$ range probed at RHIC.
We use the meson cloud model to calculate $bar{d}(x) - bar{u}(x)$ and $ bar{d}(x)/bar{u}(x)$ in the proton. We show that a modification of the symmetric, perturbative part of the light quark sea provides better agreement with the ratio $ bar{d}(x)/bar{u}(x).
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