Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 52 (2019)

Progress in analysis technique for extracting light-antiquark flavor
asymmetry by SeaQuest at Fermilab
K. Nagai,∗1 Y. Goto,∗2 Y. Miyachi,∗3 K. Nakano,∗2,∗4 S. Sawada,∗2,∗5 T. -A. Shibata,∗2,∗6 for theE906/SeaQuest Collaboration
The amounts of d¯ and ū in a proton were expected tobe the same based on the ﬂavor independence of gluon
splitting. However, the NMC experiment at CERN
showed that the total amount of d¯ exceeds that of ū.1,2)
The NA51 experiment at CERN and the E866 experiment at Fermilab measured the Bjorken x dependence¯ 3,4) whereof the light-antiquark ﬂavor asymmetry d/ū,
Bjorken x is the momentum fraction of a parton to a
¯ > 1.0 at x < 0.25 andproton. They found that d/ū¯d/ū < 1.0 at x ∼ 0.3, although the results at x ∼ 0.3
is consistent with 1.0 due to the large statistical un¯ in thiscertainty. It is very important to measure d/ū
unclear region in order to understand the proton structure.
The SeaQuest experiment is a Drell–Yan experiment
performed at the Fermi National Accelerator Laboratory (Fermilab) in Illinois, US. The main purpose ofthis experiment is to measure the light-antiquark ﬂavor asymmetry in a proton in 0.1 < x < 0.45, which includes the unclear region in the previous experiments.
As shown in Fig. 1, the Drell–Yan process is a reaction in which an antiquark in a hadron and a quark inanother hadron annihilate and decay into a lepton pair
via a virtual photon: q + q̄ → γ ∗ → µ+ + µ− .5) The
cross section of the proton-proton Drell–Yan process
at the leading order is expressed asd2 σ
4πα2=
dx1 dx2
9x1 x21s∑

Fig. 1. Diagram of the Drell–Yan process.

took the physics data from 2013 through 2017. The
recorded number of beam protons on targets is 1.4 ×
1018 . At this moment, about 40% of the data are being
analyzed.
The preliminary result of the light-quark ﬂavor
asymmetry was reported.7) The analysis toward theﬁnal result is currently in progress.
There are two diﬃculties that need to be resolvedin the current analysis: the estimation of the beamintensity dependence of the reconstruction eﬃciency
and that of the random background. We are now testing a new method for resolving them based on real data
instead of simulation, namely the so-called extrapolation method.
In this method, we evaluate the cross section ratio
ﬁrst as a function of the beam intensity. Here, we do
not apply any corrections. Then it is ﬁtted with afunction and extrapolated to the zero beam intensity.
The extrapolated value is considered to be the “correct” cross section ratio. Through the simulation, we
conﬁrmed that the extrapolation method removes thebeam-intensity dependence of the reconstruction eﬃciency correctly. The investigation to conﬁrm whether
this method can handle the random background is inprogress.

e2i

i=u,d,s,...

[qi (x1 )q¯i (x2 ) + q¯i (x1 )qi (x2 )] , (1)where x is the Bjorken x, the subscripts (1, 2) denotethe beam and the target, respectively, and q(x) is theparton distribution function. In the SeaQuest acceptance (x1 ≫ x2 ), the last term of Eq. (1) vanishes
because the PDFs of antiquarks are small enough at
the high x region. Therefore, an antiquark of the target proton is always involved in the process and is thus
accessible easily.
The SeaQuest experiment measures the muon pairs
in the ﬁnal state of the Drell–Yan process, which is produced with the 120 GeV proton beam provided by theFermilab Main Injector and the liquid hydrogen anddeuterium targets. The details of the SeaQuest spectrometer have been reported in the past.6) SeaQuest∗1∗2∗3∗4∗5∗6References1) P. Amaudruz et al., Phys. Rev. Lett. 66, 2712 (1991).
2) M. Arneodo et al., Phys. Rev. D 50, R1 (1994).
3) A. Baldit et al., Phys. Lett. B 332, 244 (1994).
4) E. A. Hawker et al., Phys. Rev. Lett. 80, 3715 (1998).
5) S. D. Drell, T. M. Yan, Phys. Rev. Lett. 25, 316 (1970).
6) K. Nagai et al., RIKEN Accel. Prog. Rep. 49, 114(2016).
7) K. Nakano et al., RIKEN Accel. Prog. Rep. 50, 118(2017).

Institute of Physics, Academia Sinica, Taiwan
RIKEN Nishina Center
Department of Physics, Yamagata University
Department of Physics, Tokyo Institute of Technology
Institute of Particle and Nuclear Studies, KEK
College of Science and Technology, Nihon University- 86 -