Inclusive cross section and single transverse-spin asymmetry of very forward neutron production at PHENIX
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nclusive Cross Section and Single TransverseSpin Asymmetry of Very Forward Neutron Production at PHENIX1 Yuji Goto for the PHENIX Collaboration RIKEN Nishina Center for AcceleratorBased Science, Wako, Saitama 3510198, Japan RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 119735000, USA Abstract—The cross section and xF dependence of AN of very forward neutron production in polarized p + p collisions at s = 200 GeV were measured in the PHENIX experiment at RHIC. The measured cross sec tions were consistent with the xF scaling claimed by the ISR experiment. Significant negative AN was observed in the forward region, and no significant backward AN was observed. We also measured s and pT dependence of AN of very forward neutron production at
s from 62.4 to 500 GeV.
DOI: 10.1134/S1063779614010390 1
1. INTRODUCTION
cles from collisions are swept out. As a beam luminos ity monitor, Beam Beam Counters (BBCs) are used. They are mounted around the beam pipe located ±144 cm away from the collision point and cover ±(3.0–3.9) and 2π in pseudorapidity and azimuth spaces, respectively. The data was collected by two sets of triggers for the neutron measurement. One was the ZDC trigger for neutron inclusive measurements by requiring energy deposit in either side of the ZDC (the north side or the south side) above 5 GeV. The other was the ZDC ⊗ BBC trigger, a coincidence trig ger of the ZDC trigger with BBC hits which were defined as one or more charged particles in both sides of BBCs.
The origin of the nucleon spin 1/2 has been inves tigated with polarized p + p collisions in the PHENIX experiment at RHIC. With the first polarized p + p collisions at s = 200 GeV at RHIC, a large single transversespin asymmetry (AN) for neutron produc tion in very forward kinematics was discovered by a polarimeter development experiment [1]. The discov ery of the large AN for neutron production is also new important information to understand the production mechanism of the very forward neutron. The cross section of very forward neutron production was mea sured at ISR and Fermilab [2]. They measured a for ward peak in the xF distribution around xF = 0.8 and found only a small s dependence. The cross section was also measured at HERA in e + p collisions [3]. They found a suppression of the forward peak. CERNNA49 measured very forward neutron cross section in p + p reaction [4]. Their cross section was twice large as those measured at ISR and Fermilab, and consistent with that at HERA. In order to under stand the production mechanism of the forward neu tron, more data are necessary, and the asymmetry measurement gives new information.
3. RESULTS The differential cross section, dσ/dxF, in the inte grated pT region, 0 < pT < 0.11xF GeV/c for forward neutron production in p + p collisions at s = 200 GeV was determined using two pT distributions: a Gaussian form, as used in HERA analysis [3], and an exponential form, used for ISR analysis [2]. The results are plotted in Fig. 1. The measured cross sec tion wa
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