Physics Opportunities with a Fixed-Target Program at the Electron-Ion Collider

Abstract

A fixed-target program at the Electron-Ion Collider (EIC) would broaden the facility's scientific reach by providing key measurements for studies of cold nuclear matter (CNM), the QCD phase diagram, and nuclear reactions relevant for space radiation. Constraining CNM effects is essential for interpreting observables in proton-nucleus ($p+A$) and nucleus-nucleus ($A+A$) collisions, yet these effects are poorly understood at low center-of-mass energies. In particular, the range $\sqrt{s}\approx10$--20 GeV, where several CNM effects may compete at similar scales, has not been explored with high statistical precision. Mapping the QCD phase diagram similarly requires high-statistics $A+A$ data with broad rapidity and transverse-momentum coverage to probe the onset of deconfinement and the possible location of the QCD critical point (CP). Currently, such data are limited at 4.5 GeV $< \sqrt{s_{NN}} < 7.7$ GeV A fixed-target program at the EIC would fill these gaps, providing CNM baselines and complementary data for QCD CP studies. By delivering high luminosity and systematic measurements across a broad range of nuclear targets, the program would link $p+A$ and $A+A$ systems at the same center-of-mass energies, enabling a unified, quantitative description of cold QCD matter and clarifying the interpretation of QGP signatures in $A+A$ collisions at low energies where comparable $p+A$ data are lacking. Finally, the program would offer a unique opportunity to measure nuclear cross sections critical for improving cosmic-ray models, including studies of space-radiation protection for both autonomous spacecraft and long-duration human spaceflight.

Figures (7)

• Figure 1: The $\sqrt{s_{NN}}$ coverage of $p+A$ collisions in fixed-target experiments: NA3 Badier:1983dg, NA38 NA38:1998lyg, NA50 NA50:2006rdpNA50:2003fvu, NA60 NA60:2010wey, E444 Anderson:1979tt, E772 Alde:1990waAlde:1991sw, E789 Kowitt:1993nsLeitch:1995yc, E866 Leitch:1999ea, E906 (SeaQuest) Lin:2017eoc, HERA-B HERA-B:2006bhyHERA-B:2008ymp, SMOG (LHCb) LHCb:2022sxsLHCb:2018jry, NA60+ NA60:2022sze. The projected energy coverage of the proposed EIC fixed-target measurements is shown in red.
• Figure 2: The $\sqrt{s_{NN}}$ coverage in fixed-target $A+A$ collisions: HADES HADES:2020ver, BM@N Baranov:2018cdk, AGS Odyniec:2013aaa, STAR FXT STAR:2020davSTAR:2021yiuSTAR:2024zvjSTAR:2024zncSTAR:2022etbSTAR:2021fge, STAR BES II STAR:2017salSTAR:2013gus, NA49 NA49:2006gaj, NA61/SHINE, NA60 NA60:2009una, NA60+ NA60:2022sze, CBM Agarwal:2023otg, J-PARC heavy-ion program Koch:1986udOzawa:2022sam, SMOG LHCb:2022qvjLHCb:2025ixz. The projected energy coverage of the proposed EIC fixed-target measurements is shown in red.
• Figure 3: Nucleon–nucleon center-of-mass energy $\sqrt{s_{NN}}$ and estimated instantaneous fixed-target luminosities for representative $p+\mathrm{Au}$ and $\mathrm{Au}+\mathrm{Au}$ collisions at the EIC, assuming a 1 mm thick gold foil and an effective beam current of $I_{\mathrm{eff}} = 10$ mA.
• Figure 4: Schematic longitudinal view of a possible ePIC fixed-target configuration based on the ePIC geometry ePIC:software, including the high-performance Detection of Internally Reflected Cherenkov light (hpDIRC) detector, as well as the central and forward tracking detectors. The point-like target is located on the hadron beam side, in the backward region at $z = -3290$ mm with respect to the nominal interaction point (IP6). Fixed-target measurements primarily exploit the forward detector systems.
• Figure 5: The dimuon transverse mass $m_T^{\mu\mu}$ kinematic phase space as a function of the parton momentum fraction $x_2$ for $J/\psi \rightarrow \mu^{+}\mu^{-}$ decays in fixed-target $p+p$ collisions at ePIC at $\sqrt{s}=19.4$ GeV. The transverse mass is defined as $m_T^{\mu \mu}=\sqrt{M_{\mu \mu}^2+p_T^2}$, where $M_{\mu \mu}$ and $p_T$ denote the invariant mass and transverse momentum of the dimuon, respectively. The simulation uses the nominal magnetic field strength of 1.7 T and the target located at $z = -3290$ mm, as illustrated in Fig. \ref{['fig:ePIC_FXT_setup']}. Results are obtained using truth-level muon tracks requiring at least four hits in the tracking detectors.