Inclusive open charm photoproduction in ultraperipheral collisions at the LHC with G$γ$A-FONLL

TL;DR

The study develops a unified framework for inclusive $D^0$ photoproduction in UPCs by merging FONLL-based heavy-quark production with photon-flux modeling (GγA-FONLL) and validating it against HERA data before applying it to LHC UPCs. It shows that nuclear modifications from EPPS21 and nNNPDF3.0 are essential to describe CMS measurements, while proton-only PDFs overpredict, especially at low $p_T$. Fragmentation modeling (BCFY vs PSSZ) and the charm mass affect normalization and the high-$p_T$ tail, with scale uncertainties dominating the theoretical error. Overall, the work provides a precise baseline for charm photoproduction in UPCs and electron–ion collisions, enabling stringent tests of low-$x$ gluon dynamics and saturation scenarios at current and future facilities.

Abstract

We compute the inclusive $D^{0}$ production cross section in ultraperipheral Pb-Pb collisions at the LHC as a function of the $D^{0}$ transverse momentum and rapidity. These calculations are carried out within the new G$γ$A-FONLL (Generalized Photon-Nucleus FONLL) framework, which can predict photonuclear cross sections for charm and beauty hadrons in electron-proton, electron-nucleus, and ultraperipheral heavy-ion collisions. The framework relies on FONLL (Fixed-Order Next-to-Leading Logarithm) to model heavy-quark production in photonuclear collisions and employs a photon-flux reweighting procedure to describe the production cross sections in ultraperipheral heavy-ion collisions. The G$γ$A calculations are first validated against the photoproduction cross sections of $D^{*}$ in electron-proton collisions at HERA. The predictions for the $D^{0}$ production cross section in ultraperipheral Pb-Pb collisions at the LHC are then presented and compared to the first experimental results obtained by CMS at $\sqrt{\rm s_{NN}}=5.36$ TeV. The predictions are benchmarked against different choices of nuclear parton distribution functions, fragmentation functions, and renormalization and factorization scales.

Figures (25)

• Figure 1: Schematic description of the inclusive $D$ meson production in electron-proton collisions (left) and in ultraperipheral heavy-ion collisions (right). Here: $f_{i/p}$ and $f_{i/A}$ denotes PDF in the proton or nucleus, $F$ fragmentation function and $\hat{\sigma}$ hard scattering process.
• Figure 2: $x$ and $Q^2$ coverage charm production in UPCs collisions. In grey, the existing coverage from fixed-target deep-inelastic photonuclear measurements is presented AbdulKhalek:2019mzd.
• Figure 3: Photon flux scaled by $z$ as a function of the fractional photon energy $z$. The electron flux of Eq.\ref{['eq:eleflux']} is shown for $Q_{\text{max}}^{2}=0.01~\text{GeV}^{2}$ (solid black) and $Q_{\text{max}}^{2}=2~\text{GeV}^{2}$ (dashed blue), while the lead–nucleus flux of Eq.\ref{['eq:nucflux']} appears as a red dotted line.
• Figure 4: Sketch of an ultraperipheral heavy-ion collision before (left) and after (right) the hard scattering.
• Figure 5: (Left) No-breakup probability (EMD) as a function of the photon energy fraction $z$. (Right) No-breakup probability (EMD) as a function of the $D^0~p_T$, in different intervals of the $D^0$ y. The interpolated Chebyshev parametrizations of the photon fluxes used to compute these ratios are taken from Eskola:2024fhf.