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STAR focus: Precision Measurement of the Longitudinal Double-Spin Asymmetry for Inclusive Jet Production in Polarized Proton Collisions at √s = 200 GeV

A long-standing puzzle in quantum chromodynamics asks how the intrinsic spins and orbital angular momenta of the quarks, antiquarks, and gluons sum to give the proton spin of ħ/2. Polarized deep-inelastic lepton scattering (DIS) experiments have shown that the quark and antiquark spins account for less than one-third of the total. The DIS measurements provide only very weak constraints on the gluon spin contribution to the proton spin, and essentially nothing is known experimentally about the orbital angular momentum contribution. The ability of polarized proton collisions to probe gluon polarization was a primary motivation for establishing the RHIC spin program.

In a recent publication, Phys. Rev. Lett. 115, 092002 (2015), the STAR Collaboration presents the first experimental evidence for positive gluon polarization in the Bjorken-x region x > 0.05. The paper, which is highlighted as a PRL Editors’ Suggestion, presents a measurement of the longitudinal double-spin asymmetry, ALL, for midrapidity inclusive jet production in 200 GeV pp collisions. The data, which were recorded in 2009, provide nearly a 20-fold increase in the event statistics compared to previous STAR inclusive jet ALL measurements, and the analysis features improved jet reconstruction and correction techniques to reduce systematic uncertainties.

The measured inclusive jet ALL vs. parton jet pT is compared to predictions from several recent NLO global analyses of the polarized parton distributions. The error bars are statistical. The gray boxes show the size of the systematic uncertainties. The uncertainty bands associated with the NLO global analyses, which are of the order of or larger than the spread among the model calculations, are omitted for clarity.

The figure shows the results compared to predictions from several recent next-to-leading-order (NLO) global analyses of DIS, semi-inclusive DIS, and previous RHIC pp data. The results draw a narrow road through the uncertainty bands associated with those previous NLO global analyses, leading to substantially tighter constraints on gluon polarization than were provided by the previous world data. Very recently, these results have been included in new NLO global analyses by the DSSV and NNPDF groups. Both groups find that the gluon polarization in the Bjorken-x region x > 0.05 differs from zero by over 3σ, and contributes approximately 40% of the total proton spin.

Posted Aug. 27, 2015

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June 21, 2015
Congratulations to Dr. Evan Sangaline for receiving the RHIC/AGS Users’ Thesis Award. Evan’s thesis is titled: Ups and Downs with a Bit of Strange: A STAR Analysis of π/K/p Spectra at High pT in Au+Au Collisions at √sNN = 7.7, 11.5, 19.6, 27.0, 39.0, and 62.4 GeV and Its Physics Implications.

June 18, 2015
Congratulations to Olga Rusnakova from Czech Technical University in Prague on successfully defending her Ph.D. thesis titled: Heavy flavor physics in the STAR experiment.


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