Apr 16 – 20, 2018
Kobe University Convention Centre / Kobe International Conference Center
Asia/Tokyo timezone

Recent STAR Results on the Measurement of the Unpolarized and Polarized Light Quark Flavor Structure of W-Boson Production at RHIC at BNL

Apr 18, 2018, 9:25 AM


WG6: Spin and 3D structure WG1-WG6 Joint Session


Bernd Surrow (Temple University)


The STAR experiment at the Relativistic Heavy-Ion Collider at Brookhaven National
Laboratory is carrying out a spin physics program in high-energy polarized proton collisions
to gain a deeper insight into the spin structure and
dynamics of the proton.

The collision of polarized protons at $\sqrt{s}=510\,$GeV opens a new era of spin-flavor
structure measurements from $W^{+}/W^{-}$ boson production through a measurement of the longitudinal single-spin asymmetry, $A_{L}$. In addition, the measurement of the cross-section ratio of $W^{+}/W^{-}$ boson production provides an independent way to probe the ratio of unpolarized $\bar{d}$ and $\bar{u}$ quark distribution functions. $W^{-(+)}$ bosons are produced in $\bar{u}+d\,(\bar{d}+u)$ collisions and can be detected through their leptonic decays, $e^{-}+\bar{\nu}_{e}\;(e^{+}+\nu_{e})$, where only the respective charged lepton is measured. The discrimination of $\bar{u}+d\;(\bar{d}+u)$ quark combinations requires distinguishing between high $p_{T}$ $e^{-(+)}$ through their opposite charge sign, which in turn requires precise tracking information.

The main STAR detector sub-systems used in this measurement are the Time Projection Chamber (TPC) and the Barrel and Endcap Electromagnetic Calorimeters (BEMC, EEMC). The TPC provides tracking inside an axial magnetic field of $0.5\,$T for a pseudorapidity range of $|\eta|<1.3$. The BEMC and EEMC provide full azimuthal coverage for $|\eta|<1$ and $1.09<\eta<2$, respectively. This detector acceptance range measuring scattered $e^{-(+)}$ from $W^{-(+)}$ boson decays in polarized proton collisions at $\sqrt{s}=510\,$GeV allows to probe a kinematic range of roughly $0.05

The status and recent STAR results will be presented on the measurement of $W^{+}/W^{-}$ longitudinal single-spin asymmetry and the cross-section ratio based on data samples taken in 2011, 2012 and 2013. These results are expected to provide important constraints through a global analysis on the understanding of $\bar{d}$ and $\bar{u}$ quark distribution functions.

Primary author

Bernd Surrow (Temple University)


Jana Bielcikova (Acad. of Sciences of the Czech Rep. (CZ))

Presentation materials