RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-4. Hadron Physics

Measurement of anti-quark ﬂavor asymmetry in the proton at
SeaQuest
K. Nakano,∗1,∗2 Y. Goto,∗2 Y. Kunisada∗1 Y. Miyachi,∗3 S. Miyasaka,∗1 K. Nagai,∗1,∗2 S. Sawada,∗2,∗4
T.-A. Shibata,∗1,∗2 S. Tamamushi∗1 for the E906/SeaQuest Collaboration¯ inThe distributions of light anti-quarks (ū and d)
the proton had been assumed to be ﬂavor symmetric,¯i.e., d(x)= ū(x), because they should be created viagluon splitting (g → q q̄) with the same probability
because of their almost equal mass and ﬂavor independence. However, a large asymmetry has been observedby the NMC, NA51, and E866/NuSea experiments1) .
Various theoretical models are being examined to understand the origin of this ﬂavor asymmetry.2) The
SeaQuest experiment at Fermilab aims to measure the¯ at high x, up to 0.45, to reveal the x
asymmetry d/ū¯
dependence of d/ū.
SeaQuest utilizes a 120-GeV
proton beam from the√Fermilab Main Injector ( s = 15 GeV). It employs
liquid-hydrogen (LH2 ) and liquid-deuterium (LD2 )targets to measure the Drell-Yan process in p + p andp + d reactions. In the Drell-Yan process, a quark in
one hadron and an anti-quark in the other hadron annihilate into a virtual photon and then decay into a
lepton pair: q + q̄ → γ ∗ → l+ + l− . At the forward
rapidity (xF  0) and under p-n isospin symmetry,the ratio of p + p and p + d cross-sections relates to theanti-quark ﬂavor asymmetry as¯ 2) σp+d (x2 )d(x11+,(1)≈2σp+p (x2 )2ū(x2 )where x2 denotes the Bjorken x of partons in thetarget-side hadron. Most systematic errors in the measurement cancel out in this ratio.
The SeaQuest spectrometer detects the ﬁnal-statemuon pair of the Drell-Yan process. Details and therecent status of SeaQuest were reported last year.
3)
SeaQuest has been acquiring physics data sinceNovember 2013 and had recorded 1.1 × 1018 beam protons on targets as of summer 2016. About 50% of therecorded data were analyzed.
The cross-section ratio σpd /2σpp was derived fromdimuon yields measured on the LH2 and LD2 targets. We selected dimuons with an invariant masslarger than 4.2 GeV, where the Drell-Yan events aredominant. The yields have been corrected for trackreconstruction eﬃciency and background dimuons.
Figure 1 shows a preliminary result for σpd /2σpp measured by SeaQuest. The systematic uncertainty arises
from the impurity of the LD2 target and the inaccuracyof the dimuon-yield corrections.
∗1
∗2
∗3
∗4

School of Science, Tokyo Institute of Technology
RIKEN Nishina CenterFaculty of Science, Yamagata University
Institute of Particle and Nuclear Studies, KEK- 118 -¯ was extracted
The anti-quark ﬂavor asymmetry d/ūfrom σpd /2σpp . The cross section was computed at
the leading order in αs without nuclear correction for
deuterium. Figure 2 shows a preliminary result for¯ with the¯ obtained by SeaQuest. It reveals d/ū
d/ūbest ever precision at 0.20 < x2 < 0.58. The systematic uncertainty includes the uncertainty of the parton distribution function (PDF) used in this extraction¯ > 1(CT10LO4) ). Very interestingly, it suggests d/ū1)
at high x, despite the E866 result. Further analyses
are being made to improve the measurement accuracy
and investigate the diﬀerence with respect to E866.
Physics data are being acquired in ﬁscal year 2017.
We expect to record about 4 × 1017 beam protons on
targets. We have upgraded the spectrometer, namelyachieving a wider acceptance and a faster DAQ. The
statistics with the full dataset will be four times betterat high x than the preliminary result.

Fig. 1. Ratio of p + p and p + d Drell-Yan cross sections
versus Bjorken x of target-side partons (x2 ).

¯Fig. 2. Anti-quark ﬂavor asymmetry (d/ū)versus x.

References
1) R. S. Towell et al., Phys. Rev. D 64, 052002 (2001).
2) W. C. Chang et al., Prog. Part. Nucl. Phys. 79, 95(2014).
3) K. Nagai et al., RIKEN Accel. Prog. Rep., 49, 114(2016).
4) H. L. Lai et al., Phys. Rev. D 82, 074024 (2010).