Nuclear parton density functions from dijet photoproduction at the EIC

We study the potential of dijet photoproduction measurements at a future electron-ion collider (EIC) to better constrain our present knowledge of the nuclear parton distribution functions. Based on theoretical calculations at next-to-leading order and approximate next-to-next-to-leading order of perturbative QCD, we establish the kinematic reaches for three different EIC designs, the size of the parton density function modifications for four different light and heavy nuclei from He-4 over C-12 and Fe-56 to Pb-208 with respect to the free proton, and the improvement of EIC measurements with respect to current determinations from deep-inelastic scattering and Drell-Yan data alone as well as when also considering data from existing hadron colliders.

Figure 1

Dijet photoproduction in electron-lead ion collisions at eRHIC and MEIC with electron beam energies of 12 to 21 GeV and ion beam energies per nucleon of 40 to 100 GeV. Shown are differential cross sections in the average jet transverse momentum (top left), average rapidity (top right), and observed parton momentum fractions in the probed nucleon (bottom left) and photon (bottom right).

Figure 2

Dijet photoproduction in electron-ion collisions with beam energies of 16 and 100 GeV, respectively, at eRHIC for different nuclei: Pb-208 (full black lines), Fe-56 (dashed blue lines), C-12 (dotted green lines), and He-4 (dot-dashed red lines). Shown are the ratios of electron-ion over electron-proton cross sections as a function of the average jet transverse momentum (top left), average rapidity (top right), and observed parton momentum fractions in the probed nucleon (bottom left) and photon (bottom right). Error bars indicate the expected experimental precision.

Figure 3

Dijet photoproduction in electron-lead ion collisions with beam energies of 16 and 100 GeV, respectively, at eRHIC. Shown is the ratio of electron-lead ion over electron-proton cross sections (full black lines) including the current nuclear PDF uncertainty from the nCTEQ15 fit to DIS and DY data only (red-shaded bands) as well as the relative gluon contribution in the lead ion (dashed blue lines) and the direct photon contribution (dot-dashed orange) to the total cross section as a function of the average jet transverse momentum (top left), average rapidity (top right), and observed parton momentum fractions in the probed nucleon (bottom left) and photon (bottom right). Error bars indicate the expected experimental precision.

Figure 4

Same as Fig. 3 for the nCTEQ15 fit including also inclusive pion data from D-Au collisions at BNL RHIC, and for the central EPPS16 fit (dotted green lines) to, in particular, dijet data from the CERN LHC as well as the corresponding (green-shaded) error bands.

Figure 5

Ratios of nuclear PDFs for Pb-208 over C-12 for up quarks (left) and gluons (right) at ${\mu }_A=6\mathrm{GeV}$ using the nCTEQ15 (full black line) and EPPS16 (dotted green line) fits and their associated nuclear PDF uncertainties (red-shaded and green-shaded bands).