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We calculate the cross section and single-spin azimuthal asymmetry, A_n(t) for inclusive neutron production in pp collisions at forward rapidities relative to the polarized proton. Absorptive corrections to the pion pole generate a relative phase between the spin-flip and non-flip amplitudes, which leads to an appreciable spin asymmetry. However, the asymmetry observed recently in the PHENIX experiment at RHIC at very small |t|~0.01GeV^2 cannot be explained by this mechanism.
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Leading neutron production on protons is known to be subject to strong absorptive corrections, which have been under debate for a long time. On nuclear targets these corrections are significantly enhanced and push the partial cross sections of neutron production to the very periphery of the nucleus. As a result, the A-dependences of inclusive and diffractive neutron production turn out to be similar. The mechanism of pi-a_1 interference, which successfully explained the observed single-spin asymmetry of neutrons in polarized pp interactions, is extended here to polarized pA collisions. Corrected for nuclear effects it explains well the magnitude and sign of the asymmetry A_N observed in inelastic events, resulting in a violent break up of the nucleus. However the excessive magnitude of A_N observed in the diffractive sample, remains a challenge.
We calculate absorptive corrections to single pion exchange in the production of leading neutrons in pp collisions. Contrary to the usual procedure of convolving the survival probability with the cross section, we apply corrections to the spin amplitudes. The non-flip amplitude turns out to be much more suppressed by absorption than the spin-flip one. We identify the projectile proton Fock state responsible for the absorptive corrections as a color octet-octet 5-quarks configuration. Calculations within two very different models, color-dipole light-cone description, and in hadronic representation, lead to rather similar absorptive corrections. We found a much stronger damping of leading neutrons than in some of previous estimates. Correspondingly, the cross section is considerably smaller than was measured at ISR. However, comparison with recent measurements by the ZEUS collaboration of neutron production in deep-inelastic scattering provides a strong motivation for challenging the normalization of the ISR data. This conjecture is also supported by preliminary data from the NA49 experiment for neutron production in pp collisions at SPS.
We study the longitudinal polarization of the Sigma_bar and Xi_bar anti-hyperons in polarized high energy pp collisions at large transverse momenta, extending a recent study for the Lambda_bar anti-hyperon. We make predictions by using different parametrizations of the polarized parton densities and models for the polarized fragmentation functions. Similar to the Lambda_bar polarization, the Xi_bar0 and Xi_bar+ polarizations are found to be sensitive to the polarized anti-strange sea in the nucleon. The Sigma_bar- and Sigma_bar+ polarizations show sensitivity to the light sea quark polarizations, Delta bar u(x) and Delta bar d(x), and their asymmetry.
We study the longitudinal polarization of hyperons and anti-hyperons at forward pseudorapidity, $2.5<eta<4$, in singly polarized $pp$ collisions at RHIC energies by using different parameterizations of the polarized parton densities and different models for the polarized fragmentation functions. The results show that the $Sigma^+$ polarization is able to distinguish different pictures on spin transfer in high energy fragmentation processes; and the polarization of $Lambda$ and $barLambda$ hyperons can provide sensitivity to the helicity distribution of strange sea quarks. The influence from beam remnant to hyperon polarization in the forward region is also discussed.
This paper is a write-up of the ideas that were presented, developed and discussed at the third International Workshop on QCD Challenges from pp to A-A, which took place in August 2019 in Lund, Sweden. The goal of the workshop was to focus on some of the open questions in the field and try to come up with concrete suggestions for how to make progress on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.
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