II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Transverse single spin asymmetries of forward neutrons in p + p,
√p + Al and p + Au collisions at sN N = 200 GeV as a function oftransverse and longitudinal momenta†
R. Seidl∗1 and the PHENIX Collaboration
In 2015 the PHENIX collaboration took data withtransversely polarized proton beams in proton-proton,
proton-Al and proton-Au collisions at center-of-massenergies of 200 GeV per nucleon. One of the mesurements that were performed with this dataset are veryforward neutron single-spin asymmetries that were detected with the zero-degree calorimeter in the direction of the polarized proton beam. It is located ca
18m downstream of the beam collision point. As reported, nonzero single spin asymmetries for protonproton collisions were already known,1) but the factthat sign and magnitude of such asymmetries werediﬀerent in proton-nucleus collisions2) initially cameas a surprise. Since then it has been realized, thatapart from meson interchange,3) in p-A collisions increasingly also ulatraperipheral collisions (UPC) cancreate such asymmetries.4) In UPC, a nucleus emits
virtual photons that can collide with the proton andcreate ﬁnal states with forward neutrons, most notably
via the production of a Delta resonance.
Previously, these results were extracted integrated
over the transverse momentum pT of the neutron andthe longitudional momentum fraction xF relative tothe proton beam momentum and its direction. Using Bayesian unfolding as implemented in root thePHENIX experiment has now extracted also the explicit xF and pT dependence of these asymmetries forthe ﬁrst time. Additionally, the asymmetries were calculated in (anti) correlation with activity in other forward detectors (beam-beam counters, BBC) that are
predominantly sensitive to hard collisions and thus allow to enhance or reduce the contribution of the mostly
very soft UPC events.
It was found that the meson-based calculations
describe the proton-proton data very well, showing
roughly a linear transverse momentum dependencewith very little xF dependence. A comparison between
inclusive and hard-collision enahanced (BBC tagged)
asymmetries shows that some UPC contributions maybe present. In p+Al collisions the inclusive asymmetries are comparable with zero over most of the phasespace while the hard-collision enhanced sample is comparable to the p + p case. In contrast, the BBC-vetoedsample (softer processes such as UPC enhanced) shows
already positive asymmetries as shown in Fig. 1. The
asymmetries in this case are initially rising but do show
an indication of a decrease at higher transverse mo†∗1

Fig. 1. Neutron single spin asymmetries for p+Al collisions
either measured inclusively (blue), enhancing hard collisions (green, labelled BBC tagged) or enhancing UPC
events (red, labeled BBC veto) as a function of transverse momentum pT in bins of xF .

menta. Theoretical calculations4) that include both effects qualitatively describe the behavior but do not reproduce the details very well. However, the UPC based
calculations only take decays of resonances with onepion in addition to the neutron into account while unpolarized MCs show that other contributions are alsopresent. This likely explains the very sharp kinematic
dependence of the theory calculations as the phasespace for neutrons from Delta decays is very limited.
In p+Au collisions the contributions from UPC areeven larger as they increase quadratically with thecharge of the nucleus. Here, even some UPC events
seem to remain in the hard-collision enhanced data
sample where the asymmetries are nearly vanishing
while both inclusive and UPC enhanced asymmetries
are positive. They reach up to 40% and show a similar
behavior as in the UPC enhanced p+Al case.
These results will help further understand the mechanisms that create very forward neutron single spin
asymmetries.
References
1) Y. Fukao et al., Phys. Lett. B 650, 325 (2007).
2) C. Aidala et al., Phys. Rev. Lett. 120, 022001 (2018).
3) B. Z. Kopeliovich et al., Phys. Rev. D 84, 114012 (2011).
4) G. Mitsuka, Phys. Rev. C 95, 044908 (2017).

Condensed from the article in Phys. Rev. D 105, 032004(2022)RIKEN Nishina Center- 50 -1