STAR focus: Electroweak bosons provide new probe of proton spin structure

The proton is composed of two up and one down flavor quarks that are bound together by the strong force. The strong force is mediated by gluons which in turn can spawn virtual quark and antiquark pairs known as sea quarks. Since the masses of the up and down (anti)quarks are small and nearly identical, it was expected that their number densities would also be very similar. However, experiments in the 1990s observed a significant asymmetry in the number of down and up antiquarks in the proton. Theories that successfully explained this asymmetry inspired predictions of even larger asymmetries in the polarized sector, generating renewed interest in determining the spin orientation of the proton’s antiquarks. The data from the STAR experiment, reported in this paper, Phys. Rev. Lett. 113, 072301 (2014), provide new constraints on these antiquark polarizations and shed light on the origin of the proton’s sea quarks.

Left Fig. The asymmetry in the production cross section for W± bosons in proton collisions with a positive (negative) proton beam helicity, σ+-), defined as AL=(σ+ - σ-)/(σ+ + σ-) is shown as a function of the W boson’s decay lepton pseudorapidity. The filled(open) points correspond to the W+(W-) asymmetries, in comparison to predictions from different polarized parton distribution functions.

In this measurement, the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) studied longitudinally polarized proton collisions at a center of mass energy of 0.5 TeV. W bosons are produced in these collisions by the annihilation of a quark and an antiquark. The quark flavors in the interaction can be determined by the charge of the W boson produced. The analysis by the STAR collaboration reports a significant asymmetry in the number of W bosons produced with a positive vs. negative helicity proton beam (shown in the figure above), and the data prefer a sizable, positive up flavor antiquark polarization in the kinematic range covered at RHIC.

Posted Aug. 13, 2014

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