Measurements of leading neutron production in polarized pp collisions at RHIC-PHENIX

In 2001-2002, we performed an experiment to search for non-zero single transverse spin asymmetry in very forward angle, 0-2.8 mrad, with first polarized pp collision in √(s) = 200 GeV at RHIC, and an unexpected large asymmetry for leading neutron production was discovered. For further investigation, we have measured the cross section with the PHENIX detector in the similar kinematics region. The observed asymmetries are consistent with the previous results measured in an experiment at the 12 o'clock interaction point at RHIC. The measured cross section are consistent with the Feynman-x scaling claimed by the ISR experiments which measured leading neutrons in pp collisions at √(s) = 30.6-62.7 GeV. These cross sections in large Feynman-x, as well as those in ep collisions at HERA experiments, are well described by a pion exchange model. Therefore, the observed large asymmetry for leading neutrons is considered to come from an interference between spin-flip amplitude with the pion exchange and non-flip with Reggeon exchanges.