Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

PHENIX W→ µ measurements from the 2013 data-taking period
R. Seidl,∗1 Y. Goto,∗1 T. Iguri,∗1,∗3 Y. Imazu,∗1 M. Kim,∗1,∗4 C. Kim,∗1,∗5 T. Moon,∗1,∗6 T. Murakami,∗2J. Murata,∗3 T. Nagashima,∗1,∗3 I. Nakagawa,∗1 S. Park,∗1,∗4 ,W. Saito,∗1,∗3 K. Tanida,∗1,∗4 and I. Yoon∗1,∗4The parity violation of the weak interaction accesses only left-handed particles and right-handed antiparticles. In longitudinally polarized proton-proton
collisions one therefore can acess fixed helicities of thequarks and antiquarks involved in the production ofreal W bosons. Furthermore the charge of the produced W predominantly selects the quark and antiquark flavors involved. W+ are mostly generated bya u and anti-d quark while W− are mostly generated
by a d and an anti-u quark. The PHENIX experimenthas the capabilities to detect Ws inclusively throughtheir electron and muon decays at central and forward
rapidities respectively. In the 2013 data taking period
RHIC was entirely run√ with polarized protons at centerof mass energies of s = 510 GeV to finish the RHIC
W program1) . In PHENIX all major muon trigger upgrades installed and commissioned over the last several
years as well as the forward vertex tracker FVTX were
fully operational for this run. Furthermore PHENIX
implemented various improvements in the operationof the detector to increase the data taking efficiency
without sacrificing quality. In a limited vertex region( more can be used for the forward W analysis ) a
total luminosity of about 146 pb−1 were accumulatedwith average longitudinal beam polarizations of 54%,which is close to five times the data accumulated in
the previous year with comparable polarizations. The
accumulated figure of merit for single spin W asymmetries are displayed in Fig. 1 for the three most recent
500 or 510 GeV data taking periods.
Having developed the W → µ analysis already inthe previous two years, most of the offline quality assurance is finished and the analysis to the single spin
asymmetries relevant to access the sea quark polarizations in the nucleon is close to be available to the public. One aspect relevant in this analysis and in particular the extraction of W production cross sections is theevaluation of the overall trigger efficiency for candidate
events found to be likely W signal events in a MC anddata based W likelihood calculation. As various muon
triggers cover only certain rapidity ranges these trigger efficiencies need to be obatained independently forvarious rapidity bins, detector arms and muon charges.
Fig. 2 shows the total trigger efficiencies after weighting each according to their relative contribution to thefinal W candidate event sample.
∗1
∗2
∗3
∗4
∗5∗6Fig. 1. Figure of merit P 2 L accumulated in PHENIX as afunction of the day in the run relevant to the forward
W analysis. The different colors correspond to the 2011
data taking (red), 2012 (blue) and 2013 (dark green).

South µ + Trigger Efficiency
Trigger efficiency

Trigger efficiency

South µ - Trigger Efficiency
0.8
0.7
0.6
0.5

0.8
0.7
0.6
0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0

1.2

1.4

1.6

1.8

2

2.2

2.4

02.6
η

0.8
0.7
0.6
0.5

1.4

1.6

1.8

22.22.4

2.6
η

2.4

2.6
η

0.8
0.7
0.6
0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1
0

1.2

North µ + Trigger Efficiency
Trigger efficiency

Trigger efficiency

North µ - Trigger Efficiency

0.1
1.2

1.4

1.6

1.8

2

2.2

2.4

2.6
η

0

1.2

1.4

1.6

1.8

2

2.2

Fig. 2. Overall trigger efficiencies separated by muon arm
and charge as a function of rapidity. Various individual
trigger contributions are shown as histogram stack.

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

References1) E. C. Aschenauer et al., arXiv:1304.0079 [nucl-ex].

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