RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-4. Hadron Physics

PHENIX Run13 Local Polarimetry
M. Kim∗1,∗2 for the PHENIX collaboration∗1∗2−0.01−χ2 / ndf
χp0132.9 / 11−0.03851 ±± 0.00055−

Fill 17186
17186 BlueBlue Beam
Beam eeLR
Fill
LR

eeLR
LR

Fill 17169 Blue Beam eLReLR

One of the most important goals of the PHENIX
experiment at the Relativistic Heavy Ion Collider(RHIC) is to study the proton spin structure by measuring spin asymmetries during particle production.
The Run13 PHENIX experimentcollided longitudi√
nally polarized protons at s = 510 GeV. Helicity
asymmetry measurements of various probes from theexperiment are ongoing.
RHIC is capable of running polarized proton beams,
and each beam polarization P is measured by RHIC
polarimeters using left-right asymmetry in elastic scattering in the Coulomb Nuclear Interference (CNI) region. Since the stable beam polarization direction in
the RHIC rings is vertical, the beam polarization direction is kept vertical in RHIC except in experimental
halls. For each ring, Spin Rotators, which consist of superconducting helical dipole magnets, rotate the beampolarization direction the 90 degrees at the entrance
and the exit of the PHENIX experimental hall.
Since beam polarization may not purely be longitudinal at the experimental hall, we require measurements of transverse and longitudinal component of P(P⊥ , P� ) at the experimental hall, which is local polarimetry, as they introduce systematic error. Local
polarimetry is especially important in ALL measurements for studying gluon polarization in a proton since
ALL is very small, and P⊥ has potential to introducesignificant error.
PHENIX Local Polarimetry uses large transverse
single spin asymmetry (left-right asymmetry) AN offorward neutron production. Left-right asymmetry
originates from the interference between spin flip andnon-flip amplitudes. One Pion Exchange (OPE) models describe the production cross section of forward
neutron reasonably well, but the pion exchange amplitude is fully spin flip.1) Theoretical study is still ongoing.
PHENIX Local Polarimeter measures energies andtrajectories of forward neutrons. The local polarimeter consists of Zero Degree Calorimeters (ZDCs), andShower Max Detectors (SMDs). ZDC consists of W-Cu
alloys and PMMA (polymethyl methacrylate) based
optical fibers, and collects Cherenkov photons produced by charged secondary particles from neutronshowers. SMD is an X-Y plastic scintillator strip hodoscope. It is located between the 1st and 2nd ZDC
modules, where the approximate hadronic shower ismaximum. X-Y position of a neutron is reconstructedby using shower profile on SMD. Those detectors arelocated 1800 cm away from the collision point, and0.003
0.003

p0p00.002
0.002

−0.02
−0.0010.001

−0.03
−

0.001
− 0.0012 ndf
χ 2 // ndf

7.569 // 11117.569

0.0003416 ± 0.0002509
0.0002509− 0.0003416

000.002− 0.0020.003− 0.003−0.04−0.004− 0.004
−0.05
−

0.005
− 0.005
50100

150200250300ZDC energy (GeV)

0.006− 0.006

5050

100
100

150
150200250300
200250300ZDC energy
energy (GeV)
(GeV)
ZDC

Fig. 1. Observed asymmetry in transverse fill (left), in longitudinal fill (right). The red lines are constant fittings.

cover 10 cm x 10 cm of a transverse plane at each zero
degree area.
The observed left-right asymmetry eN of forward
neutron production is defined as eN = P⊥ AEff
N , where=fA.
fis
thesmearing
effect,correAEffsmear Nsmear
N
lated to the neutron energy-dependent position resolution1) . fsmear is assumed to be constant during theRun13 experiment.We measure AEff
N at transversely
polarized proton beam collisions, where P⊥ = P sothat it is measured by RHIC polarimeters. After that,
we measure eN with the local polarimeter to obtainP⊥ of longitudinally polarized beams.
Fig.1 shows eN plots. The left (right) plot shows eNof transversely (longitudinally) polarized proton beamcollisions.
PY PB
PY PB
As a final result, P⊥Y P⊥B is ∼ O(10−3 ) and P�Y P�Bis ≈ 0.999 in the Run13 experiment. The measured
double helicity asymmetry becomes≈
AMeasured
LL

1 1(P Y P B ALL + P⊥Y P⊥B AT T ) (1)PY PB � �

≈ 0.999ALL + O(10−3 )AT T(2)where Y and B represent two colliding beams, andAT T is the double transverse spin asymmetry. Thisanalysis suppressed the systematic error of AT T termto O(10−3 ). ALL measurements have been done up toO(10−4 ) so far2) , and O(10−3 )AT T is orders of magnitude smaller than 10−4 .3) Therefore, P⊥ does not significantly contribute to systematic error in ALL measurements in Run13.
References
1) A. Adare et al.: Phys. Rev. D 88, 032006 (2013)
2) A. Adare et al.:Phys. Rev. D 90, 012007 (2014)3) K. Boyle: AIP Conf. Proc. 915,335 (2007)

RIKEN Nishina Center
Department of Physics and Astronomy, Seoul National University- 102 -完全版2014_本文.indd 102

15/10/16 17:46