Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

PHENIX local polarimetry analysis status
M. Kim,∗4,∗1 Y. Goto,∗1 T. Iguri,∗3,∗1 Y. Imazu,∗1 C. Kim,∗5,∗1 T. Moon,∗6,∗1 T. Murakami,∗2 J. Murata,∗3T. Nagashima,∗3,∗1 I. Nakagawa,∗1 S. Park,∗4,∗1 R. Seidl,∗1 W. Saito,∗3,∗1 K. Tanida,∗4,∗1 and I. Yoon∗4,∗1

∗1
∗2
∗3
∗4
∗5
∗6eN(φ)One of the main goals of the PHENIX experimentat the Relativistic Heavy Ion Collider (RHIC) is tostudy the proton spin structure through spin asymmetry measurements of production cross sections using various probes from polarized p + p collisions. In
2013,
we ran longitudinally polarized p + p collisions at√
s = 510 GeV. Since the stable polarization directionof proton beams circulating in the ring is perpendicular to the ground, the polarization direction is kept
perpendicular except in the experimental hall duringlongitudinally polarized beam collision runs.
Measuring proton beam polarization along the longitudinal direction at the collision is important in double longitudinal spin asymmetry ALL measurements.
The polarization in RHIC is measured by RHIC polarimeters at the position where the spin vector is vertical; however, once the polarization direction is changed
to be longitudinal, some transverse component of thebeam polarization can remain. This should be accounted as a systematic error in the ALL calculation.
The PHENIX local polarimeter measures the transverse component of proton beam polarization at thecollision point with single transverse spin asymmetry
AN in forward neutron production. A large AN for theneutron production was measured in the PHENIX experiment.1) For forward neutron production, which has
a small transverse momentum pT , the large AN cannot be explained by perturbative QCD, but the One
Pion Exchange (OPE) models explain it well. In theOPE model, finite AN is accompanied by the interference of the spin-flip amplitude of pion-exchange andthe spin-nonflip amplitude of other Reggeon exchange.
A detailed theoretical study is still ongoing.
The PHENIX local polarimeter consists of Zero Degree Calorimeters (ZDCs) and Shower Max Detectors (SMDs) and is located downstream of the beam
dipole-bending-magnet outside of the interaction region. ZDC is a hadron calorimeter, and SMD is ahodoscope composed of plastic scintilator strips. The
neutron’s position is calculated by centroid method using energy deposited in the SMD.
For obtaining the local polarimeter data, a transversely polarized commissioning fill, a longitudinallypolarized commissioning fill, and physics fills are used.
81M events were taken for the transverse commision-

χ2 / ndf

0.03
0.02
0.01

27.97 / 6

p00.03737 ± 0.00048

p10.09854 ± 0.01273

1.5

20-0.01
-0.02
-0.03
-0.04

00.5

12.53φFig. 1. eN fitting

ing fill, and 280M events were taken for the longitudinal commissioning fill. During physics runs, the local
polarimeter trigger was used with a prescale, and thescaled trigger rate was about 100-200 Hz.
Events with 70-300 GeV energy are considered in order to avoid background photons; beam scraping backgrounds from the beam pipe, which deposit low energy
in ZDC; and events that directly hit the optical fibers
for ZDC readout. Further, only the events with atleast two hits in the SMD scintillator strips in both x,y coordinates are considered in order to reject the photon background. A fiducial acceptance cut with radius= 0.5-4.0 cm from the SMD center is applied in order
to avoid shower leakage or the smearing effect at thecenter caused by the SMD with about 1 cm position
resolution. After the event selection, 19M events areused for the transverse run analysis.
The measured analyzing power AN measured is defined aseN (φ)
1
AN measured = sin(φ−φp .
0)eN is the raw asymmetry calculated with the squareroot formula,1) and its values for the south detector(counter-clockwise direction from the collision point)in the transverse run are plotted in Fig.1. Only thestatistical error is calculated for the error bars. p is theproton beam polarization, and it is 0.369±0.061 for theyellow beam (beam runs counter-clockwise direction)
during the transverse run. The calculated AN measured
is 0.101 ± 0.018. For the transverse component calculation, we calculate AN measured of longitudinal runs inthe same manner, then divide that by AN measured of
the transverse run. The transverse component is about
1% or less during the commissioning run.
Currently, modification of the pedestal and gain parameters for ZDC and SMD is ongoing. After these arefinialized, the codes with new parameters will be rerun.
Further, the systematic errors will be estimated.

RIKEN Nishina Center
Department of Physics, Kyoto University
Department of Physics, Rikkyo University
Department of Physics and Astronomy, Seoul National University
Department of Physics, Korea University
Department of Physics, Yonsei University

References
1) A. Adare et al. : Phys. Rev. D 88, 032006 (2013).

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