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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December 18, 2014
Congratulations to David Tlusty from the Czech Technical University in Prague for the successful defense of his PhD thesis, titled "A Study of Open Charm Production in p+p Collisions at STAR".

December 18, 2014
Congratulations to Qiye Shou from Shanghai Institute of Applied Physics, CAS/China for the successful defense of his PhD thesis, titled "A Study of Charge Separation Effect and Chiral Magnetic Wave in Strong Magnetic Fields at RHIC".

December 8, 2014
Congratulations to Li Yi from Purdue University for the successful defense of her PhD thesis, titled "Study Quark Gluon Plasma by Particle Correlations in Heavy Ion Collisions".

November 14, 2014
Howard H. Wieman wins 2015 APS Bonner Prize "For developing foundational experimental and theoretical tools to enable and guide generations of experiments in relativistic heavy ion physics. The combination of experiment and theory led to the initial discoveries at RHIC, ongoing precision studies of the properties of hot nuclear matter, and to exploration of the nuclear matter phase diagram."

November 3, 2014
L. Chanaka De Silva (Creighton) and Chi Yang (USTC) joined Joey Butterworth (Rice) as junior council representatives.

October 26, 2014
Congratulations to Yi Guo from University of Science and Technology of China for the successful defense of his PhD thesis, titled "Dielectron production in √s = 200 GeV p+p and Au+Au collisions at RHIC"

October 15, 2014
Ms. Sabita Das a PhD student of IOP, Bhubaneswar has won the best poster award at recently concluded ISMD conference in September 8-12, 2014, Italy.

October 13, 2014
Congratulations to Dronika Solanki from University of Rajasthan for the successful defense of her PhD thesis, titled "Limiting fragmentation and azimuthal anisotropy of photons in heavy ion collisions at RHIC energies"

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