II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Measurement of J/ψ productions in p + d and p + p at SeaQuest
K. Nakano,∗1,∗2 Y. Goto,∗2 Y. Miyachi,∗3 K. Nagai,∗4 S. Sawada,∗2,∗5 and T. -A. Shibata∗2,∗6for the E906/SeaQuest Collaboration
The partonic structure of the proton is one of the mostvital topics in hadron physics. The SeaQuest (E906) experiment at the Fermi National Accelerator Lab (FNAL)
in USA is aimed at measuring the ﬂavor asymmetry¯of light antiquarks in the proton, d(x)/ū(x),at large
Bjorken x (≳ 0.3). It utilizes the 120-GeV proton beamfrom the FNAL Main Injector and targets of liquid hydrogen and liquid deuterium. The preliminary result¯of d(x)/ū(x)using the Drell–Yan process has been reported.1)
The data recorded by SeaQuest include J/ψ productions. The p + d/p + p ratio of J/ψ cross sections issensitive to distributions of both antiquarks and gluons
through the q q̄ annihilation (q q̄ → J/ψ) and gluon fusion (gg → J/ψ), as shown in Fig. 1. The q q̄ annihilation dominates at large Feynman x (xF ≳ 0.4) where
SeaQuest can measure. Therefore, this measurement isexpected to provide additional constraints on parton distribution functions (PDFs), particularly of antiquarks at
the middle Bjorken x. The systematic uncertainties ofthe measurement are largely reduced by taking the ratioof the cross sections.
Muon pairs from J/ψ decays were detected by the
SeaQuest spectrometer.2)
SeaQuest acquired physics data from 2013 to 2017 torecord 1.4 × 1018 beam protons on targets. The ﬁrst halfof the recorded data were analyzed. Figure 2 shows thedistributions of the invariant mass of muon pairs. The
yield of J/ψ was evaluated based on the fraction of theJ/ψ component in this ﬁt. The detection eﬃciency ofJ/ψ was corrected by simulation. The beam intensity
was measured with a secondary-electron emission monitor (SEM) for normalizing the p + d and p + p crosssections.

Fig. 1. Leading diagrams of J/ψ productions.
∗1
∗2
∗3
∗4
∗5
∗6

School of Science, Tokyo Institute of Technology
RIKEN Nishina Center
Faculty of Science, Yamagata University
High Energy Nuclear Physics, Los Alamos National Lab
Institute of Particle and Nuclear Studies, KEK
College of Science and Technology, Nihon University

Figure 3 shows the p + d/p + p ratio of the J/ψ crosssections as a function of xF . The systematic uncertainty of the SeaQuest result arises from the modelingof the combinatorial background and the relative luminosity normalization between targets. The experimental
result is consistent with the two predictions as shown inthe ﬁgure. The analysis including the latter half of therecorded data is underway.

Fig. 2. Distributions of invariant mass of muon pairs. Thepoints are experimental data obtained by SeaQuest. They
were ﬁtted by the components of the four processes,
namely the Drell-Yan process, J/ψ production, ψ ′ production, and combinatorial background. The sum of thefour components is represented by the thick line. :Fig. 3. Cross-section ratio vs. xF . The points are experimental data obtained by SeaQuest. The two lines are predictions3) by the color evapolation model with diﬀerent PDF
sets.

References

1) K. Nagai et al., RIKEN Accel. Prog. Rep. 52, 86 (2019).
2) C. A. Aidala et al., SeaQuest, Nucl. Instrum. Methods
Phys. Res. A 930, 49 (2019).
3) M. L. Mangano et al., Nucl. Phys. B 405, 507 (1993).
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