Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Determination of the detector acceptance correction for thePHENIX W → µ analysis
S. Park∗1 for the PHENIX collaboration
The cross section and single spin asymmetry of parity violating W production in longitudinally polarized
proton collisions have been studied at the Relativistic
Heavy Ion Collider as an important probe for sea quark
polarization in the proton. At PHENIX, W bosons canbe measured via their decays into electrons at midrapidities (|η| < 0.35) and muons at forward/backward
rapidities (1.2 < |η| < 2.4). Since the preliminary result of the W → µ channel was obtained with the highstatistics data sample of 20131) , analysis progress has
been made towards the publication. One importantcomponent is to get the final acceptance correction of
the forward muon spectrometer that is applied on themeasured W candidates to calculate the total crosssection. The total cross section of W → lν in protonproton collisions can be written as:

Data

0.025

MC

0.02

0.015

0.01

0.005

0-3-2-1

01

23

azimuth angle [rad](a)0.02Data (all)0.018

MC0.0160.014
0.012
0.010.008

σ(pp → W +(−) X) × BR(W +(−) → l+(−) ν)
1 (Nobs − Nbg )(1 − fZ )
=
L
<A·>

0.006
0.004
0.002

(1)

0-2-10123azimuth angle [rad](b)where L is total integrated luminosity, Nobs(bg) is thenumber of observed signal (background) events, A · 
is the acceptance and total efficiency correction, andfZ is the fraction of leptonic decays of Z bosons in thesignal.
In order to determine the detector acceptance,
Monte-Carlo (MC) simulation samples were produced
using the PYTHIA event generator and the PHENIX
detector simulation packages based on GEANT3. The
MC samples were passed through the same analysis
software as the real data. Realistic detector responses
such as data driven detector hit efficiency, gain andpedestal calibrations, dead HV modules and readout
channels under different beam and detector condition
are considered. Figure 1a shows an example of azimuthal distributions of single muon candidates in thenorth side of the detector for a representative datasample and MC sample.
In this study, we also found that the detector performance varied throughout the data taking periods. Thisis mainly caused by instability of the readout channels and the number of enabled HV modules of thePHENIX Muon Tracking Chambers2) . The numberof inactive readout channels and disabled HV modules were scanned for different data taking period in2013, and we found variations especially in readout
channels. The possible causes of the variation in readout channels are problems in gain and pedestal calibrations or detector malfuctions while the HV module∗1-3Fig. 1. (a) Azimuthal distributions of a representative data
sample (blue) and MC (red). Each bin covers about
0.05 rad. (b) Azimuthal distributions of averaged data
(black) and the same MC plot (red) as in (a).

status changes typically when the background rate ishigh. When an output current of a module exceeds aset current, the HV channel in the module becomesdisabled to protect the detector. Therefore, we introduced azimuthal angle dependent correction factors
between data and MC to include this fluctuation in thesimulation. Figure 1b shows the averaged data distribution along with the MC plot for the representative
data sample. The averaged data distribution is obtained by summing the entire 2013 data set with aweight of luminosity for different data taking periods.
Then, the ratio between the averaged data and MC istaken as the correction factor for each bin. Using thecorrection factors, A ·  is being estimated including
the trigger, reconstruction and analysis cut efficiencies.
Currently, the estimation of systematic uncertainty for
A ·  is also ongoing and will be finalized in the immediate future.
References
1) R. Seidl: RIKEN Accel. Prog. Rep. 48, 104 (2014).
2) H. Akikawa et al.: Nucl. Instr. and Meth. A 499, 537(2003).

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