Charged-pion cross sections and double-helicity asymmetries in polarizedp+pcollisions ats=200  GeV

Abstract

We present midrapidity charged-pion invariant cross sections, the ratio of the π − to π þ cross sections and the charge-separated double-spin asymmetries in polarized p þ p collisions at ffiffi ffi s p ¼ 200 GeV. While the cross section measurements are consistent within the errors of next-to-leading-order (NLO) perturbative quantum chromodynamics predictions (pQCD), the same calculations overestimate the ratio of the charged-pion cross sections. This discrepancy arises from the cancellation of the substantial systematic errors associated with the NLO-pQCD predictions in the ratio and highlights the constraints these data will place on flavor-dependent pion fragmentation functions. The charge-separated pion asymmetries presented here sample an x range of ∼0.03-0.16 and provide unique information on the sign of the gluon-helicity distribution.

Measurement of charged pion double spin asymmetries at midrapidity in longitudinally polarized p+p collisions at s=510 GeV

“…The 2013 detector configuration was similar to the published results at ffiffi ffi s p ¼ 200 GeV [13] in 2009, except that the hadron-blind detector was no longer installed. Due to the higher collision energy and collision rates in 2013, the energy thresholds of the EMCal triggers were increased by a factor of ≈2-3 compared to in 2009 and events were triggered by particles leaving at least 2.2, 3.7, 4.7 or 5.6 GeV energy deposits in the EMCal for the various trigger types.…”

“…A positive gluon spin, coupled with the positive up quark polarization and negative down quark polarization would result in π þ asymmetries to be the largest, followed by π 0 and, then, π − . The charge-separated pion asymmetry results at ffiffi ffi s p ¼ 200 GeV have already been published [13].…”

Transverse-single-spin asymmetries of charged pions at midrapidity in transversely polarized $p{+}p$ collisions at $\sqrt{s}=200$ GeV

“…Given that pion candidates are triggered using an electromagnetic calorimeter, the overall reconstruction efficiencies are much lower than for neutral pions or electrons, see for example [22] for more details on pion cross section measurements using PHENIX.…”

Measurement of double helicity asymmetries ($A_{LL}$) at mid- and forward rapidities in longitudinally polarized $p+p$ collisions at $\sqrt{s}$ = 510 GeV with PHENIX experiment

“…Therefore, in principle the ordering of the three asymmetries directly shows the sign of the gluon helicity probed. PHENIX has measured all charges at 200 GeV [1,2] but due to trigger limitations the precision is not sufficient so far. At 510 GeV the charged pion analysis is ongoing while the neutral pion analysis will be discussed below.…”