Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Sea quark polarization measurement by W ± → µ± in PHENIX 2012
C. Kim,∗1,∗2 Y. Goto,∗1 T. Iguri,∗1,∗3 Y. Imazu,∗1 M. Kim,∗1,∗4 T. Moon,∗1,∗5 T. Murakami,∗6 J. Murata,∗3T. Nagashima,∗1,∗3 I. Nakagawa,∗1 S. Park,∗1,∗4 W. Saito,∗1,∗3 R. Seidl,∗1 K. Tanida,∗1,∗4 and I. Yoon∗1,∗4The leptonic decayed W ± bosons measurement atRHIC aims to achieve precise constraint of each ﬂvaordecomposed sea quark’s spin contribution to the whole
proton spin 12 .
The W ± → µ± analysis by using the dataset of year
2012 shares the basic strategy set in the 2011 analysis1) , but also has several advantages such as enhanced
statistics and upgraded W trigger for data acquisition.
Table 1 shows a few key features of the recent longitudinally polarized pp collisions.

tially indirect approach by using likelihood to the W .
The procedure for the estimation is as follows. First,
calculate the W likelihood by using the data and NLO
level Monte Carlo sample. Second, construct the overall probability density function (PDF) for signal andbackgrounds by using three types of component PDFs
based on W likelihood. Each component PDF corresponds to a signal, muonic backgrounds, and hadronic
backgrounds, respectively. Finally, estimate the S/BG
ratio via the overall PDF and unbinned max. likelihood ﬁt technique. The estimated S/BG ratio in thepreliminary condition is summarized in Table 2. Also,
AL calculated by applying the above S/BG ratio canbe seen in ﬁgure 1.

Table 1. Results of recent polarized pp collisions inPHENIX Muon Arms.
Year2009201120122013√

s500
500
510
510

L (pb−1 )
8.6
16.7
31.5
146.0

P (%)
39.0
48.0
51.9
55

FoM (LP 2 )
1.3
3.8
8.5
44.2

The main observable in this analysis is the single
spin asymmetry (AL ) calculated by muons decayedfrom desired W bosons.
The major background source is muons from inﬂight decayed low pT (pT ≤ 3 GeV/c) hadrons, whichmimic high pT muons as well as various irreducible
muonic backgrounds. Owing to the dominance of thesebackgrounds in addition to smearing in momentum
and charge determination, observing distinct Jacobianpeak in W ± → µ± measurement in forward rapidity
is not expected, unlike the W ± → e± measurementin mid-rapidity. Therefore, accurate estimation of theS/BG ratio plays an essential role in a reliable signal
extraction process.
Fig. 1. AL in Run 12 (preliminary). Empty squared box
indicates systematic error by dilution factor.

Table 2. S/BG ratio in Run 12 (preliminary).
Channel−South µnsig
88.87+16.97−16.28nµnhad

S/BG

44.42

177.77+19.60−18.600.40+0.12−0.10After the preliminary, various reﬁnements have been
made or are underway, such as inclusion of kinematic
variables from the Forward Silicon Vertex Detector(FVTX), applying overall trigger eﬃciency, update indetector eﬃciency, and further ﬁne tune of hadronic
PDF. Among them, inclusion of FVTX variables andﬁne tune of hadronic PDF are expected to play a signiﬁcant role in improving statistics in the region ofinterest (W likelihood ≥ 0.92) as well as reduced errors, which will enable better estimation of the S/BG
ratio and AL .

+24.38+0.10
South µ+ 92.48+20.55−19.91 44.88 258.74−23.31 0.30−0.08+15.45+0.09
North µ− 38.95+11.90−11.15 42.71 139.78−14.56 0.21−0.07+18.93+0.10
North µ+ 72.37+15.75−15.04 38.67 185.69−17.98 0.32−0.09

To estimate the S/BG ratio properly, we use a par∗1
∗2
∗3
∗4
∗5
∗6RIKEN Nishina Center
Department of Physics, Korea University
Department of Physics, Rikkyo University
Department of Physics, Seoul National University
Department of Physics, Yonsei University
Department of Physics, Kyoto University

References1) H. Oide et al: RIKEN Accel. Prog. Rep. 46 xviii- 89 -