Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Charged background study for AN of forward neutron production in√s=200 GeV polarized proton-proton collisions in PHENIX
experiment
M. Kim∗1,∗2 for the PHENIX collaboration↑(↓)

Single transverse spin asymmetry AN is deﬁned as
AN =σ↑ − σ↓σ↑ + σ↓(1)where ↑ and ↓ indicate the spin up and down of theincident beam, respectively. In 2015, AN of forward(6.8 < η < 8.8) neutron production in p+p, p+Al
√and p+Au collisions at sN N = 200 GeV was measured at PHENIX using the Zero-Degree Calorimeter(ZDC) and a position-sensitive Shower Max Detector(SMD). Unexpectedly, a strong A dependence in AN
was found, which is not explained by the current theoretical framework for p + p collisions1) . In order to obtain information about the associated mechanisms, theBeam-Beam counter (BBC) correlated AN was measured for “BBC tag” samples and “BBC veto” samples.
A strong correlation with the BBC hit was observed2) .
In the previous result, one of the main systematic
uncertainties of p + p data comes from the proton
backgrounds. In 2008, in order to reduce the charged
backgrounds in the detector acceptance, a charge vetocounter (CV), a plastic pad scintillator in front of theZDC, was operated. To reduce the background uncertainty, the data taken in 2008 were analyzed. Figure
1 shows the charge distribution in the CV versus theparticles x position measured by the SMD.
53X position (cm)10210010-50200

400

600

800

1Charge veto (ADC-pedestal)Fig. 1. The charge distribution in the CV vs the x position
measured by the SMD for p + p from 2008 data. OneMIP peak around the 100 ADC value in the positivex region, where the proton beam bending direction, isshown.

Experimentally the asymmetry is measured by using the square-root formula as shown by Eq. (2) where∗1∗2

RIKEN Nishina Center
Department of Physics and Astronomy, Seoul National University- 107 -Nφ(φ+π) is the number of neutrons detected at azimuthal angle φ (φ + π) when the beam polarization
direction is up (down).
Aﬁt
N =1P

↓Nφ↑ Nφ+π−↑Nφ+πNφ↓↓Nφ↑ Nφ+π+↑Nφ+πNφ↓= (1 − r)ASN + rAbg
N(2)
(3)

The signal asymmetry ASN and the background asymmetry AB
N follow Eq. (3) where r is the fraction ofbackgrounds.
The main backgrounds are protons, generated byelastic, diﬀractive and hard processes. Protons fromelastic and diﬀractive processes nearly follow the beamline, and are swept by the beam bending dipole magnet
to the right side of the detectors, and a small fractionof those protons hit the right side (x > 0 in Fig. 1) ofthe detector. This background fraction is sensitive tothe detector alignment, hence both the 2008 and 2015
data were measured by using the left-right asymmetryof the x distribution of particles.
Protons from hard processes are estimated by thePYTHIA event generator and GEANT Monte Carlosimulations. These protons result in even x distributions in the detector, and the same values are applied
for both for the 2008 and 2015 data.
To solve Eq. (3) for two unknowns, ASN and AB
N,ﬁt
AN and r are measured for two 2008 data samples:
one from the “charge vetoed” sample, which requiresa charge veto counter energy lower than half of theMIP peak, and another from the “charge not vetoed”
sample, which does not require a charge veto cut. ABNof all BBC correlation samples is zero within statistical uncertainties. With AB
N obtained from the 2008
data, ASN of 2015 data is calculated and found to beconsistent with that of the 2008 data.
The resulting uncertainties on the asymmetries fromthe charged background are 13% for the ZDC inclusive
sample, 14% for the “BBC tag” sample, and 55% for
the “BBC veto” sample. The comparably large uncertainty for the “BBC veto” sample comes from thestatistics and the small Aﬁt
N . With this study, we canreduce uncertainties for ZDC inclusive and “BBC tag”
samples.
References
1) B. Z. Kopeliovich, I. K. Potashnikova, I. Schmidt, J.
Soﬀer : Phys. Rev. D 84, 114012 (2011).
2) M. Kim et al., Accel. Prog. Rep. 49, 10 (2017).