II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Recent progress of polarized Drell–Yan experiment at Fermilab,SpinQuest (E1039)
K. Nagai,∗1 Y. Goto,∗2 Y. Miyachi,∗3 K. Nakano,∗2,∗4 G. Nukazuka,∗2,∗3 S. Sawada,∗2,∗5 and T. -A. Shibata∗2,∗6
The SpinQuest (E1039) experiment aims to investigate the structure of the proton through the polarized ﬁxed-target Drell–Yan process at Fermilab. In theDrell–Yan process, an antiquark (q̄) in a hadron anda quark (q) in another hadron annihilate and decay
into a lepton pair (ℓ + ℓ̄) via a virtual photon (γ ∗ ):q + q̄ → γ ∗ → ℓ + ℓ̄. The Drell–Yan process is a suitable probe to study antiquarks in the proton because
the antiquark is always involved in this process.
One of the most important unresolved puzzles of theproton is the “proton spin puzzle.” The proton spinhas been considered to be carried by the quark spins.
However, the EMC experiment at CERN showed thatthe contributions of spins of quarks and antiquarks tothe proton spin are much less than 100%.1,2) Further
experiments later conﬁrmed that these contributions
constitute only approximately 30% of the proton spin.
Many possible contributions have been considered tosolve this puzzle. One of them is the contribution of
the orbital angular momenta (OAM) of quarks andantiquarks.
The Sivers function, which is a function of theBjorken x and the transverse momentum of the quarks,
can give some hints about the OAM contributions.
The Sivers function is a kind of so-called transversemomentum-dependent parton distribution function(TMD). It represents the correlation beween the transverse momentum of a quark and the spin of the parent
hadron. If the Sivers functions of antiquarks are nonzero, then the contributions of the OAM of antiquarks
to the proton spin are not zero. The Sivers function(fSivers ) is proportional to the single-spin asymmtry(AN ): fSivers ∝ AN ∝ (NL − NR )/(NL + NR ), where
N is the number of Drell–Yan dimuons and the subscripts L and R denote the direction of the virtual photon (left and right, respectively). Therefore, the Sivers
functions can be experimentally accessed by measuring
the single-spin asymmetry.
The semi-inclusive deep inelastic scattering (SIDIS)
experiments have found the non-zero value of singlespin asymmetries of quarks. The contributions ofquarks and antiquarks are not separated by SIDIS.
The SpinQuest experiment will measure the single-spin
asymmetry of d¯ and ū in the Drell–Yan process using
a 120 GeV proton beam and polarized hydrogen anddeuterium targets.
∗1
∗2
∗3
∗4
∗5∗6Fig. 1. The E906 spectrometer.3) The target system has
been renewed, and dark-photon hodoscopes are installed for SpinQuest.

The SpinQuest spectrometer is basically the same as
the SeaQuest (E906) spectrometer3) (Fig. 1). We are
now updating it for the SpinQuest experiment. The
major update is the installation of the polarized targets, as the E906 experiment used only unpolarizedtargets. The details have already been reported in theprevious progress report.4) We installed another setof hodoscope planes to detect potential dark photon
events. Its optimization is underway.
The detectors are being repaired and updated. All
the drift chambers for SpinQuest were used in the E906
experiment, but one of the drift chambers was damaged after the E906 experiment was completed. We
have determined the cause of damage and have repaired the drift chamber. Some of the less eﬃcient
proportional tubes and hodoscopes have also been repaired. The optimization of their operation is now inprogress.
The beam time is expected to begin in the middleof 2021. After two years of data aquisition, we expect
to ﬁnd an important piece in the proton spin puzzle.
References

1) J. Ashman et al. (EMC Collaboration), Phys. Lett. B
206, 364 (1988).
2) J. Ashman et al. (EMC Collaboration), Nucl. Phys. B
328, 1 (1989).
3) C. A. Aidala et al. (SeaQuest Collaboration), Nucl. Instrum. Methods Phys. Res. A 930, 49 (2019).
4) Y. Miyachi et al., RIKEN Accel. Prog. Rep. 53, 81(2020).

High Energy Nuclear Physics, LANLRIKEN Nishina Center
Faculty of Science, Yamagata University
School of Science, Tokyo Institute of Technology
Institute of Particle and Nuclear Studies, KEK
College of Science and Technology, Nihon University- 59 -1