Abstract
The single spin asymmetries in very forward neutron production had been first observed about a decade ago at RHIC in transversely polarized proton + proton collision at √s = 200 GeV. Although neutron production near zero degrees is well described by the one-pion exchange (OPE) framework, the OPE appeared to be not satisfactory to describe the observed analyzing power A N . The absorptive correction to the OPE generates the asymmetry as a consequence of a phase shift between the spin flip and non-spin flip amplitudes. However the amplitude predicted by the OPE is too small to explain the large observed asymmetries. Only the model which introduces interference between major pion and small a 1 -Reggeon exchange amplitudes has been successful in reproducing the experimental data. During RHIC Run-15, RHIC delivered polarized proton collisions with Au and Al for the first time, enabling the exploration of the mechanism of transverse single-spin asymmetries with nuclear collisions. A very striking A-dependence was discovered in very forward neutron production at PHENIX in transversely polarized proton + nucleus collision at √s = 200 GeV. Such a dependence has not been predicted from the existing framework which has been succesful in proton + proton collision. In this report, experimental and theoretical efforts are discussed to disentangle the mysterious A-dependence in the very forward neutron asymmetry.
Highlights
The single spin asymmetry of very forward was first discovered[1] at RHIC in transversely polarized proton+proton collision at s = 200 GeV
The production mechanism of the such a neutron is driven by soft process and of which cross section is well described by one pion exchange (OPE) model
The shower-maximum detector (SMD) consists of x-y scintillator strip hodoscopes and they are inserted between the first and second zero-degree calorimeter (ZDC) modules at the position of maximum hadronic shower approximately
Summary
The magnitude of observed asymmetry of the neutron which has the energy fraction of incident proton xF = En/Ep ≥ 0.4 was several percent. The data demonstrate the absolute amplitude of the asymmetries grow as a function of pT. These data are well reproduced by the model calculations[3]
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