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Register a new userTransverse single spin asymmetry for very forward $pi^{0}$ production in polarized proton-proton collisions at $sqrt{s}$ = 510 GeV
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Transverse single spin asymmetry, $A_{N}$, of very forward $pi^{0}$ production from polarized $p + p$ collisions provides new information toward an understanding of its production mechanism. $A_{N}$ of forward $pi^{0}$ in the pseudorapidity region of $3 < eta < 4$ has been described by the partonic structure of the proton in the perturbative QCD framework. However, recent data indicates a potential contribution from not only partonic but also diffractive interactions. In order to provide a new insight on the origin of the $A_{N}$, we measured the very forward $pi^{0}$ production in the pseudorapidity region of $6 < eta$ from $sqrt{s}$ = 510 GeV polarized $p + p$ collisions at RHIC in 2017. We report our measurement of the very forward $pi^{0}$ over the transverse momentum range of $0 < p_{T} < 1$ GeV/$c$ and the preliminary result.
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Transverse single-spin asymmetries of very forward neutral pions generated in polarized $p + p$ collisions allow us to understand the production mechanism in terms of perturbative and non-perturbative strong interactions. During 2017 the RHICf Collaboration installed an electromagnetic calorimeter in the zero-degree region of the STAR detector at the Relativistic Heavy Ion Collider (RHIC) and measured neutral pions produced at pseudorapidity larger than 6 in polarized $p$+$p$ collisions at $sqrt{s}$ = 510 GeV. The large non-zero asymmetries increasing both in longitudinal momentum fraction $x_{F}$ and transverse momentum $p_{T}$ have been observed at low transverse momentum $p_{T} < 1$ GeV/$c$ for the first time at this collision energy. The asymmetries show an approximate $x_{F}$ scaling in the $p_{T}$ region where non-perturbative processes are expected to dominate. A non-negligible contribution from soft processes may be necessary to explain the nonzero neutral pion asymmetries.
In the 2001-2002 running period of the Relativistic Heavy Ion Collider (RHIC), transversely polarized protons were accelerated to 100 GeV for the first time, with collisions at sqrt{s} = 200 GeV. We present results from this run for single transverse spin asymmetries for inclusive production of neutral pions, photons and neutrons of the energy region 20 - 100 GeV for forward and backward production for angles between 0.3 mrad and 2.2 mrad with respect to the polarized proton direction. An asymmetry of A_N = (-0.090 +- 0.006 +- 0.009) x (1.0^{+0.47}_{-0.24}) was observed for forward neutron production, where the errors are statistical and systematic, and the scale error is from the beam polarization uncertainty. The forward photon and pi^0, and backward neutron, photon, and pi^0 asymmetries were consistent with zero. The large neutron asymmetry indicates a strong interference between a spin-flip amplitude, such as one pion exchange which dominates lower energy neutron production, and remaining spin non-flip amplitudes such as Reggeon exchange.
We report high-precision measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for midrapidity inclusive jet and dijet production in polarized $pp$ collisions at a center-of-mass energy of $sqrt{s}=200,mathrm{GeV}$. The new inclusive jet data are sensitive to the gluon helicity distribution, $Delta g(x,Q^2)$, for gluon momentum fractions in the range from $x simeq 0.05$ to $x simeq 0.5$, while the new dijet data provide further constraints on the $x$ dependence of $Delta g(x,Q^2)$. The results are in good agreement with previous measurements at $sqrt{s}=200,mathrm{GeV}$ and with recent theoretical evaluations of prior world data. Our new results have better precision and thus strengthen the evidence that $Delta g(x,Q^2)$ is positive for $x > 0.05$.
The STAR Collaboration reports measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for neutral pions produced at forward directions in polarized proton-proton collisions, at a center-of-mass energy of $510$ GeV. Results are given for transverse momenta in the range $2<p_{T}<10$ GeV/$c$ within two regions of pseudorapidity that span $2.65<eta<3.9$. These results are sensitive to the polarized gluon parton distribution function, $Delta g(x)$, down to the region of Bjorken $x sim 10^{-3}$. The asymmetries observed are less than $pm 5 cdot 10^{-3}$ in magnitude, and will help constrain the contribution to the spin of the proton from polarized gluons at low $x$, when combined with other measurements as part of a global analysis.
PHENIX measurements are presented for the cross section and double-helicity asymmetry ($A_{LL}$) in inclusive $pi^0$ production at midrapidity from $p$$+$$p$ collisions at $sqrt{s}=510$~GeV from data taken in 2012 and 2013 at the Relativistic Heavy Ion Collider. The next-to-leading-order perturbative-quantum-chromodynamics theory calculation is in excellent agreement with the presented cross section results. The calculation utilized parton-to-pion fragmentation functions from the recent DSS14 global analysis, which prefer a smaller gluon-to-pion fragmentation function. The $pi^{0}A_{LL}$ results follow an increasingly positive asymmetry trend with $p_T$ and $sqrt{s}$ with respect to the predictions and are in excellent agreement with the latest global analysis results. This analysis incorporated earlier results on $pi^0$ and jet $A_{LL}$, and suggested a positive contribution of gluon polarization to the spin of the proton $Delta G$ for the gluon momentum fraction range $x>0.05$. The data presented here extend to a currently unexplored region, down to $xsim0.01$, and thus provide additional constraints on the value of $Delta G$. The results confirm the evidence for nonzero $Delta G$ using a different production channel in a complementary kinematic region.
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