Highlighting the STAR Collaboration

Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1904
DOI: 10.1126/science.310.5756.1904

The members of the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) were disappointed by the article “Dueling experiments close in on source of proton's spin” (A. Cho, News of the Week, 4 Nov., p. 757). The article provides a description of the recent results from the PHENIX and COMPASS experiments on the contribution that gluons make to the spin of the proton but does not mention that the STAR experiment has yielded important, complementary new results that bear directly on this question as well.

STAR, like PHENIX, measures asymmetries in high-energy polarized proton collisions. The STAR results, like the PHENIX and COMPASS results, argue against the extreme model that the gluons in the proton are fully polarized.

In the language of perturbative quantum chromodynamics (pQCD), the STAR and PHENIX measurements involve the same initial states—a mixture of quark+quark, quark+gluon, and gluon+gluon collisions—but different final states. Interpreting jet asymmetries such as those measured by STAR introduces less theoretical uncertainty regarding final-state effects. Furthermore, the STAR results extend to considerably higher transverse momentum, where the applicability of pQCD for interpreting the observed spin asymmetries is believed to be quite reliable. In contrast, the reported PHENIX and COMPASS measurements that provide the greatest discriminating power among competing models of the proton involve rather low transverse momentum pions and quite low momentum transfer di-hadrons, respectively, where questions still remain regarding the reliability of the corresponding pQCD calculations that have been performed to date.

All three experiments are complementary. Each involves a unique mix of theoretical uncertainties, so the fact that they lead to a consistent conclusion is far more convincing than is the case for any subset of them.

  • *Spokesperson of STAR

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