Azimuthal transverse single-spin asymmetries of inclusive jets and charged pions within jets from polarized-proton collisions at s=500  GeV

Abstract

We report the first measurements of transverse single-spin asymmetries for inclusive jet and jet þ π AE production at midrapidity from transversely polarized proton-proton collisions at ffiffi ffi s p ¼ 500 GeV. The data were collected in 2011 with the STAR detector sampled from 23 pb −1 integrated luminosity with an average beam polarization of 53%. Asymmetries are reported for jets with transverse momenta 6 < p T < 55 GeV=c and pseudorapidity jηj < 1. Presented are measurements of the inclusive-jet azimuthal transverse single-spin asymmetry, sensitive to twist-3 initial-state quark-gluon correlators; the Collins asymmetry, sensitive to quark transversity coupled to the polarized Collins fragmentation function; and the first measurement of the "Collins-like" asymmetry, sensitive to linearly polarized gluons. Within the present statistical precision, inclusive-jet and Collins-like asymmetries are small, with the latter allowing the first experimental constraints on gluon linear polarization in a polarized proton. At higher values of jet transverse momenta, we observe the first nonzero Collins asymmetries in polarized-proton collisions, with a statistical significance of greater than 5σ. The results span a range of x similar to results from semi-inclusive deep-inelastic scattering but at much higher Q 2 . The Collins results enable tests of universality and factorization breaking in the transverse momentum-dependent formulation of perturbative quantum chromodynamics.

Nonperturbative transverse-momentum-dependent effects in dihadron and direct photon-hadron angular correlations in p+p collisions at s=200 GeV

“…The data show that, at the hard scales and energies probed by RHIC, the nonperturbative away-side widths are consistent within uncertainties at two different center-of-mass energies. Similar conclusions have been drawn by the STAR Collaboration, where polarized TMD observables are consistent within uncertainties between ffiffi ffi s p ¼ 200 and 500 GeV [11,12].…”

“…Additionally, e þ e − annihilation and SIDIS measurements have shown nonzero spin-momentum correlations in the final-state hadronization process [6][7][8]. With the development of robust and theoretically interpretable jet finding algorithms, there have also been several measurements studying nonperturbative hadronization in unpolarized p þ p collisions [9,10] as well as spin-momentum correlations in polarized p þ p collisions [11,12]. Transverse single spin asymmetries of inclusive hadron production in p þ p collisions of up to ∼40% also indicate large spin-momentum correlations within the nucleon and/or the process of hadronization [13]; however, these measurements cannot probe functions within the TMD framework directly because there is not a simultaneous measurement of a small and hard transversemomentum scale.…”

Illuminating Nucleon Structure Through Polarized Proton-Proton Collisions at STAR

“…The STAR Collins asymmetries [15,16] have been compared with model calculations [24,25] based upon SIDIS and e + e data. The comparisons for the p s = 500 GeV data [15] are shown in the right-hand panel of Fig.…”

“…The full kinematics of the boson are reconstructed by summing the hadronic recoil momenta and employing an event-by-event Monte Carlo correction to compensate for recoil momenta that fall outside the STAR acceptance. Charged-pion IFF [17,18] and Collins [15,16] asymmetries from STAR's 2011 and 2012 datasets are presented in Fig. 1.…”

“…Illuminating Nucleon Structure at STAR J. L. Drachenberg [20] (left and center) and charged-pion Collins [15] (right) asymmetries at p s = 500 GeV in comparison to model calculations based on SIDIS [23,24,25]. Weak boson asymmetries show a strong preference for models that assume the sign-change [7], provided suppression due to TMD evolution is small.…”