Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Status of measurements of Drell–Yan process at FNAL SeaQuest
K. Nakano,∗1,∗2 Y. Goto,∗2 Y. Miyachi,∗3 K. Nagai,∗1,∗4 W. Saito,∗1 S. Sawada,∗2,∗5 T.-A. Shibata,∗1,∗2 andH. Yamada∗1 for the E906/SeaQuest Collaboration
The partonic structure of the proton is one of
the most vital topics in the present study of hadronphysics. The SeaQuest experiment is being carried outat Fermi National Accelerator Lab (FNAL) in USA
to investigate the partonic structure with the Drell–Yan process. In the Drell–Yan process, a quark in one
hadron and an anti-quark in another hadron annihilate
into a virtual photon and then decay into a lepton pairas q + q̄ → γ ∗ → l+ + l− , as shown in Fig. 1. The crosssection at the leading order of αS is expressed asd2 σ4πα2 1 ∑ 2= B Tei ·B
Tdx dx
9x x s i}{ B B T T
qi (x )q̄i (x ) + q̄iB (xB )qiT (xT ) ,(1)where x is the Bjorken scaling variable, s is thesquare of the center-of-mass energy of two interacting hadrons, qi (x) is the parton distribution functionof a ﬂavor i, and the superscripts “B” and “T” denotepartons in the beam hadron and target hadron, respectively. When the process is measured at( forward rapid-)
ity as SeaQuest does, the second term q̄iB (xB )qiT (xT )of Eq. (1) becomes negligible since xB ≫ xT andq̄(xB ) ∼ 0 at large xB . This implies that a quark
mostly originates from the beam with xB and an antiquark from the target with xT . Therefore, the Drell–
Yan process at forward rapidity is directly proportional
to the anti-quark distributions, q̄(x).
SeaQuest utilizes the proton beam extracted fromthe FNAL Main Injector with E = 120 GeV as well as
several types of targets to cover wide physics topics.
Liquid hydrogen (LH2 ) and deuterium (LD2 ) targets
are used for the measurement of the Drell–Yan processes in p + p and p + d reactions. Carbon, iron,and tungsten targets are also used for the p + A reaction. The SeaQuest spectrometer detects the ﬁnalstate muon pair of the Drell–Yan process, the detailsof which were reported in the past.2) SeaQuest acquired physics data from November 2013 to July 2017with a summer accelerator shutdown each year. It hasrecorded 1.4 × 1018 beam protons on targets, and isactively analyzing approximately 40% of the recordeddata.
The primary purpose of SeaQuest is to measure the¯ inﬂavor asymmetry of light anti-quarks (ū and d)
the proton. Since a large asymmetry has been observed by the NMC, NA51, and E866/NuSea experi∗1
∗2
∗3
∗4
∗5

School of Science, Tokyo Institute of Technology
RIKEN Nishina Center
Faculty of Science, Yamagata University
Academia Sinica, Taiwan
Institute of Particle and Nuclear Studies, KEK- 117 -ments, various theoretical models are being examined
to understand the origin of this ﬂavor asymmetry.1)
SeaQuest analyzed high-mass Drell–Yan events in p+pand p+d reactions and obtained a preliminary result of¯d(x)/ū(x)at 0.10 < x < 0.58, as reported last year.3)
Further analyses are being performed to improve thestatistics and measurement accuracy.
Another purpose of SeaQuest with p + p and p + dreactions is to measure the angular distribution ofﬁnal-state leptons in the Drell–Yan process. The angular distribution is theoretically predicted to satisfy
the Lam-Tung relation but was found by several experiments to violate it. This violation can originate
from, for example, QCD higher-order radiation and theBoer-Mulders eﬀect.4) It can be investigated further
with high-precision data from SeaQuest. The angulardependence of the detection eﬃciency is being studied
in detail by tuning detector responses and background
distributions in simulation.5)
SeaQuest also measures the nuclear eﬀect via theDrell–Yan process in p + A reactions. Approximately
1/5 of the beam protons were applied to the carbon,
iron and tungsten targets. The nuclear eﬀect is deﬁned
as the diﬀerence in per-nucleon cross sections between
σ/Ap+A and p+p, namely p+A
σp+p ̸= 1. It involves various
physics mechanisms and is widely examined by measurements and theories. SeaQuest is ﬁrst focusing on
the Drell–Yan process in the large-xT region (≳ 0.2) toextract the partonic energy loss in cold nuclear matter.6) Background events in p + A reactions are being
identiﬁed in detail, where the rates of all backgroundtypes are diﬀerent from those in p + p.7)
hBeam

hTargetXlq*ql

X

Fig. 1. Feynman diagram of the Drell–Yan process.

References
1) W. C. Chang et al., Prog. Part. Nucl. Phys. 79, 95(2014).
2) K. Nagai et al., RIKEN Accel. Prog. Rep. 49, 114(2016).
3) K. Nakano et al., RIKEN Accel. Prog. Rep. 50, 118(2017).
4) M. Lambertsen et al., Phys. Rev. D 93, 114013 (2016).
5) B. Ramson, PoS DIS 2016, 021 (2016).
6) H. Xing et al., Nucl. Phys. A 879, 77 (2012).
7) P. J. Lin, PoS DIS 2017, 174 (2017).