II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Semi-inclusive deep inelastic scattering at the Electron Ion Collider
R. Seidl∗1
The electron-ion collider, EIC, will be a newly build
accelerator at Brookhaven National Laboratory to collide polarized electrons with polarized protons andlight ions, as well as unpolarized heavier nuclei. It willbe the perfect place to study the√ strong interaction
with collision energies between s = 29 to 141 GeV
and luminosities that are expected to be three ordersof magnitude higher than achieved at the HERA ringin Germany. In early 2020 the US DOE oﬃce of science acknowledged the EIC oﬃcially as a project thatwill be built in the next 10 years. A large community of more than 1000 members has already formed
with interest in the EIC, organized as the EIC usergroup.1) Within this group a call for a comprehensive
Yellow report was formed that updates earlier publications on the physics goals2,3) and closely looks at
the technological options for EIC detectors to have inorder to fulﬁll these physics goals. The main processat the EIC is deeply inelastic lepton-nucleon (or nucleus) scattering, DIS, in which only the scattered lepton is detected. The kinematic variables x, which is
the momentum fraction of the nucleon a parton has,and the momentum transfer of the process Q2 can beextracted. Typically scales of Q2 > 1 GeV2 are considered as in that case the extracted cross sections andspin asymmetries can be factorized into hard scattering processes between partons that can be describedby perturbative QCD and nonperturbative parton distribution functions, PDFs. PDFs describe the distribution of quarks and gluons within the nucleon as a function of the momentum fraction x at a given scale Q2 .
In semi-inclusive DIS at least one ﬁnal state hadron is
also detected and the type, charge, momentum fraction z and transverse momentum relative to the boson
mediating the lepton-parton scattering informs on theﬂavor, spin and intrinsic transverse momentum of thestruck parton with the help of fragmentation functions.
These semi-inclusive DIS processes cover most of themain physics goals of the EIC. For example, the spin
contribution by individual sea quark ﬂavors to the total
nucleon spin can be probed this way. Also the threedimensional momentum picture of the nucleon can beobtained via SIDIS measurements and the closely related Sivers function and the tensor charges for quarks,
sea-quarks and gluons (gluons only for the Sivers function).
As an example of the work by the SIDIS group of theyellow report, the four-dimensional coverage of SIDIS
hadrons was studied at all envisioned collision energies
taking into account a potential detector conﬁguration
ranging from −3.5 < η < 3.5 with varying particle∗1Fig. 1. Particle identification acceptance ratios for pionsfrom semi-inclusive events at the EIC as a function ofmomentum fraction z and transverse momentum PT , inbins of x and Q2 .

identiﬁcation (PID) ranges depending on suitable technologies. Figure 1 displays the coverage for pions. One
can see that at intermediate x and Q2 the maximum
PID momentum of only 6 GeV limits the expected coverage while elsewhere the coverage is suﬃcient. Thisis caused by the fact that at central rapidities no compact PID detector can cover a larger momentum range.
Using these detector coverages and realistic smearing,
pseudo-data was created resembling actual measurements extrapolated to about 10 fb−1 , which corresponds to about a year of EIC running. This pseudodata was then used in global ﬁts by theorists to extract
the impact on various physics goals of the EIC such as
the tensor charges that potentially relates to physics
beyond the standard model,4) the Sivers function,5)
as well as the expected scale dependence of the functions that relate to the three-dimensional momentumpicture of the nucleon. The Yellow Report is now publicly available6) and provides the basis for the ongoing
review process of the DOE.
References

1) http://www.eicug.org/ .
2) A. Accardi et al., Eur. Phys. J. A 52, 268 (2016)(arXiv:1212.1701 [nucl-ex]).
3) D. Boer et al., arXiv:1108.1713 [nucl-th].
4) L. Gamberg, Z. B. Kang, D. Pitonyak, A. Prokudin,
N. Sato, R. Seidl, Phys. Lett. B 816, 136255 (2021).
5) A. Vladimirov, R. Seidl, in preparation.
6) R. Abdul Khalek et al., arXiv:2103.05419 [physics.insdet].

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