I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Asymmetry of antimatter in the proton†
Y. Goto,∗1 S. Miyasaka,∗2 K. Nagai,∗2 F. Sanftl,∗2 Y. Kudo,∗3 Y. Miyachi,∗3 K. Nakano,∗1,∗2 S. Nara,∗3S. Sawada,∗4 and T. -A. Shibata∗1,∗2 for the SeaQuest Collaboration

†∗1
∗2
∗3
∗4

Condensed from the article in Nature 590, 561 (2021)
RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology
Department of Physics, Yamagata University
Institute of Particle and Nuclear Studies, KEK- S16 -2.5

2

SeaQuest/E906
Syst. uncert.
NuSea/E866
NA51

1.5d/ uThe structure of the proton is a prototypical example of a strongly coupled and correlated systemwith quarks and gluons interacting according to quantum chromodynamics (QCD). An essential feature of
QCD is its ability to create matter-antimatter quark
pairs inside the proton that exist only for a very shorttime. Their ﬂeeting existence makes the antimatter
quarks within protons diﬃcult to study, but their existence is discernible in reactions in which a matterantimatter quark pair annihilates. In this picture ofquark-antiquark creation by the strong force, the probability distributions as a function of momentum forthe presence of up and down antimatter quarks shouldbe nearly identical, given that their masses are very
similar and small compared to the mass of the proton.1) Here, we provide evidence from muon-pair production measurements that these distributions are considerably diﬀerent, with down antimatter quarks moreabundant than up antimatter quarks over a wide rangeof momenta.
In the Drell-Yan process in hadron-hadron collisions,a quark and an antiquark annihilate into a virtual photon, which decays into a lepton-antilepton pair.2) The
ratio of the Drell-Yan cross-section on a deuterium target to that on a hydrogen target has a direct sensitivity¯¯ are the distributions
to d(x)/ū(x),where ū(x) and d(x)of up and down antiquarks in the proton, respectively,
as a function of the fractional momentum (x) of the¯ from the Drell-Yan process wasproton. The ratio d/ūﬁrst reported by NA51 at CERN.3) This result is consistent with the down antiquark dominance reported
in deep inelastic scattering by NMC at CERN.4) The
Fermilab NuSea experiment5) was able to measure thex dependence of the down antiquark dominance withan 800-GeV proton beam in the kinematic range of
0.015 < x < 0.35.
The Fermilab SeaQuest experiment was designed toinvestigate the ﬂavour asymmetry at higher x values
than NuSea with the newly constructed experimental apparatus. With a proton beam at an energy of
120 GeV, liquid hydrogen and deuterium targets, anda focusing magnet of 10 Tm after the target region, theexperiment was optimized for the study of target antiquarks in the intermediate region, with x around 0.3,
by detecting muon (μ+ μ− ) pairs from decays of thevirtual photons produced in the Drell-Yan process.
¯Figure 1 shows the ratios d(x)/ū(x)in the proton

1Alberg and Miller
0.5
Basso, Bourrely, Pasechnik and Soffer
000.1

0.2

0.3

0.4x¯
Fig. 1. Ratios d(x)/ū(x).

(red ﬁlled circles) with their statistical (vertical bars)
and systematic (yellow boxes) uncertainties extractedfrom the present data based on next-to-leading-order
calculations of the Drell-Yan cross-sections. Also
shown are the results obtained by the NuSea experiment (open black squares) with statistical and systematic uncertainties added in quadrature. The trends
between the two experiments at higher x are quite different. No explanation has been found yet for these
diﬀerences. The horizontal bars on the data points
indicate the width of the bins.
The present data are reasonably described by thepredictions of the statistical parton distributions ofBasso et al.6) (green band) and by the chiral eﬀective perturbation theory of Alberg & Miller7) (cyanband), which are also shown in Fig. 1. These two calculations emphasize rather diﬀerent non-perturbative¯mechanisms that lead to the diﬀerences in d(x)andū(x). The present data show that d¯ is greater thanū for the entire x range measured in this experiment.
This provides important support for these and othernon-perturbative mechanisms of the QCD structure of
the proton that were disfavoured by the NuSea results.
References1) D. A. Ross, C. T. Sachrajda, Nucl. Phys. B 149, 497(1979).
2) S. D. Drell, T. M. Yan, Phys. Rev. Lett. 25, 316 (1970);
erratum 25, 902 (1970).
3) A. Baldit et al., Phys. Lett. B 332, 244 (1994).
4) P. Amaudruz et al., Phys. Rev. Lett. 66, 2712 (1991);
M. Arneodo et al., Phys. Rev. D 50, R1 (1994).
5) R. S. Towell et al., Phys. Rev. D 64, 052002 (2001).
6) E. Basso et al., Nucl. Phys. A 948, 63 (2016).
7) M. Alberg, G. A. Miller, Phys. Rev. C 100, 035205(2019).

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