Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Probing flavor asymmetry of antiquarks of the protonin the E906/SeaQuest experiment
S. Obata,∗1

S. Miyasaka,∗1 Y. Goto,∗2 Y. Miyachi,∗3 K. Nagai,∗1 K. Nakano,∗1
F. Sanftl,∗1 S. Sawada,∗2,∗4 T.-A. Shibata∗1,∗2 for the E906/SeaQuest experimentE906/SeaQuest is an extension of the earlier DrellYan experiments at Fermi National Accelerator Laboratory (Fermilab), such as E772 and E866/NuSea1) ,and it studies the internal structure of the proton atthe parton level using the Drell–Yan process. In theleading order, the Drell–Yan process is described by
the quark–antiquark annihilation process: q + q̄ →γ ∗ → µ+ +µ− . SeaQuest uses a 120-GeV proton beam
extracted from the Fermilab Main Injector.
¯ was measured in the Bjorken x range
The ratio d/ū0.015 < x < 0.35 in the E866 experiment. The statistically precise part of the data in the range 0.015 <x < 0.2 tends to agree with several models, such as themeson cloud model. However, the data appear to deviate from these models in the larger x region. SeaQuest
will determine the ratio up to x = 0.45 more precisely
than the E866. The SeaQuest spectrometer consists offour tracking stations. Each station has drift chambers
or drift tubes for tracking and hodoscopes for trigger.
The Japanese group is in charge of the drift chambersof the third station. Two drift chambers are aligned
at the station to cover the large acceptance. SeaQuest
completed a two-month commissioning run in spring2012. After an upgrade, a two-year physics run began
in fall 2013.
In the commissioning run, we successfully reconstructed the di-muon mass distribution, and a J/ψ
peak was clearly observed (Fig. 1). The mass resolution was ∼ 0.3 GeV, which meets the requirement.
This proves that the detectors and tracking software
work adequately. After the commissioning run, threemain hardware upgrades were performed. The first one
was an upgrade of the beam quality. During the commissioning run, we observed high-multiplicity events
because the beam had high instantaneous intensity.
This made the track reconstruction difficult. An improvement of the duty factor was confirmed in the current run. The beam tuning is now ongoing, and wewill have a full-intensity beam (1012 protons per second) once it is completed. The second main upgrade
is the installation of a Cherenkov detector in the beamline to measure the beam intensity at 53 MHz RF frequency. This allows us to determine the absolute crosssection of the Drell–Yan process and to generate a vetotrigger in order to avoid the high-intensity part of the∗1∗2∗3∗4beam. The last main upgrade is the improvement of
the hodoscopes and drift chambers. For hodoscopes,
we upgraded the existing PMT bases with a new circuit board to achieve higher rate capabilities. For driftchambers, one new chamber was constructed to ensurea larger acceptance at the third station. The chamber has 5,300 wires, and we manually constructed it in2012. The chamber was installed at the bottom partof the third station. The construction of another newdrift chamber is ongoing. It will be also installed at
the first station.
After the current physics run began, we were working on the optimization of every component for stabledata accumulation. Our current focus is to optimize
the trigger system. The system examines the hit pattern of the hodoscopes to identify the pattern characteristics of the high-mass muon pairs produced fromthe Drell–Yan process or J/ψ decay. We are now trying to suppress the background as much as possible
through the hit-pattern study. The preliminary resultof the mass distribution shows a suppression of thebackground compared to the distribution obtained in
the commissioning run. Once the optimization is completed, we will start taking the Drell–Yan data. The
data accumulated in the next two years will have asignificant impact on our understanding of the internal structure of the proton.

Fig. 1. Di-muon mass distribution reconstructed using thedata taken in the commissioning run. The J/ψ peak isclearly observed. The inset shows the mass distributionin which the normalized background is subtracted.

Graduate School of Science and Engineering, Tokyo Institute of Technology
RIKEN Nishina Center
Faculty of Science, Yamagata University
Institute of Particle and Nuclear Studies, KEK

References
1) E.A. Hawker et al., NuSea, Phys. Rev. Lett. 80, (1998)
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