Measurement of double helicity asymmetries ($A_{LL}$) in $\pi^{0}$ and $\pi^{\pm}$ production at mid-rapidity in longitudinally polarized $p+p$ collisions at $\sqrt{s}$ = 510 GeV with PHENIX experiment

Apr 13, 2016, 11:40 AM
SR4a (DESY Hamburg)


DESY Hamburg

Spin Physics WG6 Spin Physics


Ralf Seidl (RIKEN)


One of the main goals of the RHIC spin program is the determination of the gluon helicity contribution to the proton spin. This can be accessed by measuring double spin asymmetries ($A_{LL}$) of pion production at mid-rapidity in longitudinally polarized proton collisions with the PHENIX experiment. The ordering of the asymmetries with the charge of the final state pions can in addition directly infer the sign of the gluon spin contribution. Charged and neutral pions are reconstructed in the central PHENIX tracking system and electromagnetic calorimeters, respectively. The asymmetries are evaluated between collisions of bunches with the same and opposite helicity after correcting for differences in luminosity and for beam polarizations. The $A_{LL}$ measurements of pion production at $\sqrt{s}$ = 200 GeV have been published previously. To extend our understanding of the gluon polarization to a lower gluon momentum fraction ($x$), high statistics data was collected at a higher $\sqrt{s}$ = 510 GeV in 2012-2013. We present the recently published $\pi^{0}$ production results and the status of the $\pi^{\pm}$ $A_{LL}$ measurements at mid-rapidity at $\sqrt{s}$ = 510 GeV. The neutral pion result confirms a positive contribution of the gluon spins to the proton spin for $x >$ 0.05 and provides additional constraints on the gluon polarization down to $x \sim$ 0.01.

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