Joint ECFA-NuPECC-APPEC Workshop “Synergies between the EIC and the LHC”

I review recent theory developments in QCD, largely stimulated by recent LHC data and by the needs of the forthcoming LHC high luminosity upgrade, emphasizing the synergies between the physics programs of the EIC and the HL-LHC.

Overview of EIC physics

What has COMPASS taught us on spin of hadrons?

The HERAPDF2.0 parton densities represent the current state of the art in determining the longitudinal structure of the proton using data from Deep Inelastic Scattering (DIS) experiments alone. Their precision is at the few percent level at intermediate Bjorken-x, but deteriorates fast for x–>1 and also below x 10−3. The high x region in particular can also be constrained using LHC data, though at the price of increased theoretical complexity and currently with tensions between data sets. In this study we investigate how the picture may evolve with time in the future by using only DIS ingredients in proton PDF fits. We start by including simulated data from the Electron Ion Collider (EIC), which are known to improve matters at large x. We then additionally add simulated data from the Large Hadron electron Collider (LHeC), the Future Circular Collider (FCC) in ep mode, or both. The LHeC is shown to potentially revolutionise the precision at both large and small x, even with EIC data are included. In the longer term the FCC-eh may allow further progress at low x, extending the sensitivity toward x=10-7. We also study the results of leaving the strong coupling as a free parameter in the fits. Once again, the LHeC and/or FCC-eh yield significant improvements even beyond the currently world-leading potential of an EIC determination.

There are many opportunities to study gluon distributions at both the EIC and LHC. Most of these studies are complementary and the combined information will allow us to get a more complete picture of the gluonic properties of protons and nuclei. This applies to collinear gluon PDFs, but also to the gluon Transverse Momentum Distributions (TMDs), Generalized Parton Distributions (GPDs), Generalized TMDs (GTMDs), Double distributions and diffractive distributions, as will be addressed in this talk.

The Electron-Proton/Ion Collider experiment (ePIC) will be the first detector at the future Electron-Ion Collider (EIC), which will be built at Brookhaven National Laboratory. The central region of ePIC follows a standard collider detector layout optimized for electron-ion Deep Inelastic Scattering (DIS) with low-mass tracking, high precision electromagnetic calorimetry, extensive particle identification (PID) capability and hadron calorimetry covering a phase space of 2π in azimuth and |η| < 4 (~3.5) for calorimetry (tracking and PID). In addition to the central detector, ePIC has extensive beamline instrumentation in both the outgoing electron and ion directions

Many ePIC subsystems utilize novel technologies which may be relevant for future nuclear and high-energy experiments. This presentation will provide an overview of the ePIC detector while highlighting several of these synergistic technologies.

The future Electron-Ion Collider (EIC), with its large range of center-of-mass energies in combination with high luminosity and polarization of both the electron and the proton/light-ion beams will transform our understanding of Quantum Chromo-Dynamics (QCD).
Its new state-of-the-art detector, ePIC, will open a unique opportunity for high precision measurements of both cross sections and spin-asymmetries in e+p(A) collisions.

The ePIC Collaboration has been established in July 2022 to build a general purpose detector designed to investigate the whole EIC core science program.
Studies of key measurements are ongoing in order to demonstrate that ePIC is capable of delivering on its mission.
Processes taken into consideration are chosen for both their relevance to the core science and the specific challenges that they pose to the detector.
This talk will highlight current and planned activities by the Physics Working Groups of ePIC in the context for the Technical Design Report and the Early Science Report, which will explore the impact of EIC early science during its first 5 years of running.

The future Electron-Ion Collider (EIC) will enable a broad scientific program spanning topics in nuclear physics not yet fully understood at past or present facilities. Among them is understanding the origin of mass and spin of the proton and being able to study the 3D structure of the proton (partonic imaging). Additionally, there are goals in electron + heavy-ion collisions aimed at understanding nuclear parton distribution functions and studying saturation, among other topics. In order to meet these and other physics goals, specialized detectors integrated with the outgoing hadron beamline are required. While these far-forward detectors are not new in an of themselves, the technology to be used at the EIC will be employed for the first time and has many synergies with other high-energy experiments. This presentation will primarily focus on these technologies and their application to the EIC physics program, and beyond.

The dual-radiator RICH (dRICH) detector of the ePIC experiment at the Electron-Ion Collider (EIC) will employ Silicon Photomultipliers (SiPMs) for single-photon Cherenkov light detection. Covering an area of $\sim$ 3 m$^{2}$ with 3$\times$3 mm$^{2}$ pixels and more than 300k readout channels, this will be the first collider experiment to utilize SiPMs at such a scale for single-photon applications in high-energy physics. SiPMs are chosen for their cost-effectiveness, high photon detection efficiency, and robust performance in magnetic fields ($\sim$ 1 T at the detector location). The dRICH will provide crucial particle identification over a broad momentum range (1$-$50 GeV/c) in the hadronic endcap region.

Despite their advantages, SiPMs are not radiation-hard, requiring comprehensive R&D efforts to ensure the preservation of their single-photon counting capabilities and control of dark count rates (DCR) over the experiment’s lifetime. Strategies to mitigate performance degradation include operating the sensors at low temperatures, exploiting precise timing with fast time-to-digital conversion (TDC) electronics, and recovering radiation damage through high-temperature annealing cycles.

In this talk, we present an overview of the ePIC-dRICH photodetector system with highlights from the R&D performed for the operation of the SiPM optical readout in the ePIC experiment, where a large number of SiPMs were tested for usability in single-photon applications in a moderate radiation environment. Irradiated SiPMs underwent various annealing procedures to test their recovery capability from radiation damage. Particular attention was given to an annealing procedure exploiting the Joule effect, where high temperatures were achieved via self-heating of the sensor.

We will also present recent beam test results of a large-area, 2048-channel detector prototype that was successfully tested with particle beams at CERN-PS in October 2023 and May 2024. The photodetector surface is modular and consists of novel photodetection units (PDUs) developed by INFN, each comprising 256 SiPM sensors, cooling infrastructure, and TDC electronics within a compact volume. The results demonstrate promising performance for the first SiPM-based Cherenkov detector in a frontier QCD experiment, paving the way for its operation at the EIC.

The energy momentum tensor (EMT) and its form factors have been one of the major theme of hadron physics in the past decades. In this talk I will discuss how one can get an experimental access to them through deeply virtual Compton scattering, and why this kind of process, measured at the JLab and the EIC is not enough to provide constraints on the gluon sector.

Understanding the internal structure of hadrons requires a detailed study of parton distributions. In particular, distributions in the transverse directions—Transverse Momentum Dependent distributions (TMDs) and Generalized Parton Distributions (GPDs)—provide essential insights. Lattice QCD and Large Momentum Effective Theory (LaMET) offer ab initio calculations of TMDs and GPDs. In our research, we have carried out a comprehensive study of key TMD ingredients, including Collins–Soper kernel and the soft function. As a continuation and extension of existing work on GPDs, we present the first structured results for skewness-dependent GPDs based on Lattice QCD and LaMET. In this talk, I will highlight both the key results and novel contributions of our research on TMDs and GPDs, and provide an introductory overview of the Lattice QCD and LaMET methodologies.

In recent years, there has been a growing interest in the possibility of extracting parton distributions of QCD from Lattice simulations. Since it is not possible to extract the light-cone distributions via Lattice QCD, they are extracted in the Euclidean region $z_E^2 = - z^2 > 0$, and then matched to the light-cone one, through the use of perturbative math kernels. In the case of heavy-quark distributions, e.g., the charm PDF, the presence of an additional scale gives rise to power corrections of the type $z^2 m_Q^2$. Since the condition $z^2 m_Q^2 \rightarrow 0$ is difficult to achieve at the current lattice spacing, these mass corrections must be included. We propose a method to include this effect at the level of the perturbative matching kernels, needed to connect the off-light-cone distributions (extracted from Lattice) to the light-cone ones that we probe in actual experiments. We discuss the quantitative impact of these corrections at one-loop.

The future electron-hadron collider (EIC) at Brookhaven National Laboratory and the High-Luminosity Large Hadron Collider (HL-LHC) at CERN represent the next frontiers for high precision quantum chromodynamics (QCD) measurements in nuclear physics. The staggered timelines, together with the synergy between the physics programs of the two accelerators, offer unique opportunities to advance the understanding of QCD in both the hot and cold temperature regimes. Thanks to its outstanding detection capabilities, the ATLAS experiment at the LHC can reconstruct proton-proton (p+p), proton-nucleus (p+A), and nucleus-nucleus (A+A) collisions with high precision. In this talk, I will review recent heavy-ion results from ATLAS that are directly related to the physics landscape of the LHC. I will present cold QCD measurements carried out in p+A and ultra-peripheral A+A collisions, demonstrating their direct connection to the EIC program. I will also discuss how the unprecedented precision of the EIC in mapping the nucleon and the nuclear structure will benefit the (HL-) LHC heavy-ion program and possible future synergies between the EIC and the HL-LHC.

After the ongoing Run 3 of the CERN LHC, the CMS detector will undergo a set of upgrades in order to meet the challenges of the upcoming High Luminosity era of the LHC. Among the more ambitious aspects of the Phase-2 upgrades of CMS are the replacement of the entire silicon tracking system, a new high-granularity combined electromagnetic calorimeter system for the endcap region of the detector, the addition of timing layers for pile-up rejection and particle ID, and the extension of the muon system with a new RPC and GEM chambers. The level-1 trigger and data acquisition will also be upgraded to handle the larger interaction rate. This talk will summarize the upgrades of CMS and their impact on physics performance with an eye towards synergies with the EIC program.

The ATLAS experiment at the LHC is preparing for significant Phase-II upgrades in anticipation of Run 4 and the High-Luminosity LHC era. These upgrades are crucial not only for the pp physics program but also for advancing heavy ion physics.
In this talk, I will give an overview of the current status of the ATLAS Phase-II upgrades, emphasizing their importance for the ATLAS Heavy Ion program in Run 4 and beyond. Particular attention will be paid to the Zero Degree Calorimeter (HL-ZDC) and its pivotal role in event characterization and centrality measurements. I will also discuss key synergies between the ATLAS Heavy Ion program and the future Electron-Ion Collider at Brookhaven National Laboratory.

Precise measurements of hard scattering processes are a corner stone of the LHC physics programme. On theory side, the large energy transfer in these processes allows for higher-order perturbative computations in QCD to achieve remarkable precision. Combined these measurements and predictions are able to put stringent constraints on PDFs, coupling constants and masses. In this talk I will give a review the necessary ingredients, state-of-the-art and challenges of perturbative computations beyond NLO QCD, both for hadron-hadron as well as lepton-hadron collisions. On the way I will highlight some recent results and applications.

Forward physics provides invaluable information for the study of
saturation in QCD. While the color-glass-condensate constitutes the
theoretical framework to study saturation within QCD, there is also a
consistent approach that allows for a perturbative factorization of
cross sections for forward production in terms of PDFs and parton-level
scattering, called Improved TMD factorization. It is valid both for LHC
and EIC physics. Furthermore, it can be formulated in momentum space and
is ideal for Monte Carlo approaches. We report on progress on the
precision of such calculations.

The scale dependence of parton distributions, such as PDFs and GPDs, is set by the anomalous dimensions of composite operators and can be computed perturbatively. While in principle straightforward, in practice such calculations are often complicated due to operator mixing. In this talk, we show that powerful consistency relations for the anomalous dimensions, based on conjugations, can be set up by analyzing symmetries of and relations between the operators at hand. We explicitly discuss such relations for two types of mixing, namely mixing with gauge-variant (alien) operators and mixing with total-derivative operators in non-forward kinematics.

The role of the LHC data in constraining nuclear parton distributions (nPDFs) is of fundamental significance. In this talk I present the evolution of our knowledge about nPDFs from pre-LHC era to the current time. I focus on recent results especially from the nCTEQ group and provide the current status before the time of EIC.

We will present an overview of phenomenological studies of nuclear effects in photon-nucleus scattering using heavy ion ultraperipheral collisions (UPCs) at the LHC. In particular, we will discuss how exclusive charmonium and inclusive dijet production in Pb-Pb UPCs at the LHC can help to better constrain nuclear parton distribution (PDFs) in the small-x shadowing region. We will also discuss implications of these results for the studies of small-x partons and QCD dynamics at the Electron-Ion Collider (EIC).

Double parton scattering (DPS) is the process in which two hard scatters occur in a single proton-proton collision. This process involves a pair of partons from each proton and thus allows us to learn about the correlations between them. I will discuss the basics of DPS as well as the various sources of these correlations such as spin effects, number and momentum rules and the perturbative splitting contribution. Finally I will show how these effects can be captured in double parton distribution functions.

Since 1980s, TMD factorization provides a stringent mathematical formalism to treat 3D hadron structure. However, the baseline MC generators still rely on 1D collinear factorization. The TMD Parton Branching (PB) method was developed to include elements of TMD physics in MC generators.

In this talk, I discuss the Sudakov form factor of the TMD PB evolution equation and its relation to that of Collins-Soper-Sterman. I focus especially on the non-perturbative input and present the recent results on intrinsic-kt vs center-of-mass (in)dependence in different approaches.

Transverse momentum dependent parton distribution functions (TMDs) encode essential information on the transverse motion of partons inside nucleons, as well as their spin-orbit correlations. In this talk I will show how, by looking at the azimuthal asymmetries in the inclusive production of pairs of vector quarkonia ($J/\psi$, $\psi(2S)$, $\Upsilon$ mesons), one can probe different TMDs. In particular, quark and antiquark TMDs could be accessed in pion-proton scattering at the COMPASS and AMBER experiments at CERN, where the contributions of gluons is negligible. The impact of the present and future fixed-target experiments at the LHC (SMOG, LHCspin) on this kind of studies will be discussed as well.

We present a phenomenological analysis for hadron-in-jet production in (un)polarized $pp$ collisions within a TMD approach, based on recent extractions of TMD fragmentation functions (TMD FFs) from SIDIS and $e^+e^-$ processes. The general agreement of our estimates with the available data corroborates the hypothesis of the TMD factorization for such processes as well as of the universality of TMD FFs. Open issues and future improvements, on both the experimental and the theoretical side, are also discussed.

The search for gluon saturation using diffractive vector meson production is discussed reviewing experimental results from HERA and UPC at the LHC.
A few lessons potentially useful for EIC are mentioned.

In this talk we review CMS results on photoproduction (coherent and incoherent) of vector mesons and open charm in ultra-peripheral collisions, on double pomeron scattering, as well as on exclusive photon fusion in pp. At the end of the talk, we also discuss femtoscopy and its relevance at EIC.

Ultra-peripheral collisions are collisions between two nuclei (or protons) without overlap - the impact parameter is larger than the sum of the nuclear radii. Strong interactions are thus heavily suppressed but electromagnetic interactions are allowed. The electromagnetic fields can be treated as an equivalent flux of photons (Fermi-Weizsacker-Williams), and these photons may interact with the other, target nucleus in a variety of ways.

Ultra-peripheral collisions have attracted an increased interest in recent years, both at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC). Initially, the focus was on exclusive and coherent Vector Meson production. These interactions are valuable as probes of the nuclear gluon content. In recent years, more general inclusive photonuclear interactions (gamma+A --> X) have also gained attention.

Owing to the upgrade of the ALICE readout system before LHC Run 3, ALICE has been able to study the latter type of photonuclear interactions starting with the heavy-ion run in 2023. Photoproduction of several particle species have been investigated. These include charged pions, kaons, protons; strange particles K0, Lambda; and open charm D0. In this talk, the ALICE results on ultra-peripheral collisions will be reviewed with a focus on the most recent inclusive photoproduction results. These results will provide an important baseline for the EIC, for example by constraining theoretical calculations and Monte Carlo models before the first results arrive.

The ultrarelativistic collisions of the heavy ions provide rich spectrum of possibilities of discussing the nucleus response to photons.
Newly published neutron and proton multiplicities measured in the ALICE experiment in ultraperipheral conditions allow investigating the influence of the electromagnetic fields on colliding nuclei for the $^{208}$Pb+$^{208}$Pb at $\sqrt{s_{NN}}$ = 5.02 TeV. The theoretical predictions are done within hybrid model(s) including equivalent photon approximations (EPA), GiBUU or INCL modeling preequilibrium processes and generating the ensemble of excited nuclei which decay with statistical afterburners: GEM2 and GEMINI++.
The cross sections of the mass-charge distributions of nuclear remnants as well as the neutrons, protons and other charged particle multiplicities are estimated. We concentrate on production of protons and isotopes coming from the electromagnetic dissociation. Special attention is devoted to emission of a single proton. The cross section for 1p emission is very close to the maximal available one based on reactions of photon with individual nucleons. Our pre-equilibrium processes explain simultaneusly the tail of neutron energy distributions in the nuclear rest frame observed in $\gamma + A$ collisions in the past.
A possibility to study such processes in $e+A$ collisions at EIC will be presented.

We discuss the central exclusive photoproduction of $\pi^{+}\pi^{-}$ pairs in diffractive photon-proton and in proton-proton collisions at high energies. We consider the resonant ($\rho^{0}$, $\omega$, $f_{2}(1270)$) and non-resonant (Drell-S\"oding) contributions. Our calculation is based on the tensor-pomeron approach. For the $pp\to pp\pi^{+}\pi^{-}$ reaction, we calculate differential cross sections as a function of the two-pion invariant mass. We discuss the important role of the Drell-S\"oding mechanism in shaping the $\rho(770)$ resonance line. Our research is relevant in the context of ALICE, ATLAS, CMS, and LHCb measurements in $pp$ collisions at the LHC, even when the leading protons are not detected and instead only rapidity-gap conditions are checked experimentally. Our results can also serve as basis for the description of coherent $\pi^{+}\pi^{-}$ production in ultra-peripheral $p$A and AA collisions. This approach can be directly applied to the analysis of photoproduction and small-$Q^{2}$ electroproduction in $ep$ collisions at high energies. Such data exist from the HERA experiments and will be obtained in the future at the EIC.

The presentation is based on arXiv:2508.06334 [hep-ph].

We investigate the exclusive production of the pseudoscalar meson $\eta_c$ in electron-ion collisions through $\gamma^* \gamma$ interactions. At high energies, this process is dominated by photon-photon fusion, enhanced by the strong nuclear photon flux. We present cross-section predictions for future facilities (EIC, EicC, LHeC), focusing on rapidity, transverse momentum, and photon virtuality distributions. The analysis explores different models for the $\eta_c$ wave function, showing how such measurements can constrain the meson's transition form factor and offer new insights into its internal structure.

In recent years, the description of lepton-pair production in ultra-peripheral collisions (UPCs) of heavy nuclei has attracted significant attention, particularly with the release of new STAR data. When the dilepton pair is produced almost back-to-back, hence at small $q_T$, the observable starts to be sensitive to the photon Wigner distribution. They are related via Fourier transform to the generalised transverse momentum dependent distributions (GTMDs), which depend on two internal transverse momenta: $k_T$ and $\Delta_T$. The former is the usual intrinsic transverse momentum also found in the transverse momentum-dependent distributions, whereas the latter corresponds to the (transverse) momentum transfer.
Thus, the Wigner distribution provides a priori the most theoretical robust description of such an observable. Moreover, if one is interested in exploring the gluonic content of nuclear matter (e.g., by considering heavy quarkonium production), the QED channel corresponds to the background of the process. Therefore, understanding the photon GTMDs is crucial to single out the QCD channel and probe the gluon GTMDs.
In this talk I will present the theoretical framework that describes UPCs of heavy ions in terms of photon GTMDs. In particular, I will address the counter-intuitive features that the presence of the two aforementioned transverse momenta generates. I will also discuss how the mass of the produced system modifies both the cross section and some of the various asymmetries that can be measured in this process, based on RHIC and LHC kinematics.

In this contribution, we suggest to consider photon-photon scattering as a useful source of information on transverse-momentum-dependent fragmentation functions (TMD FFs), complementing SIDIS and e+e- annihilation processes, which provide most of the present phenomenological information on TMD FFs. As a first illustrative example, we study two-hadron azimuthal asymmetries around the jet thrust axis in processes where, in a circular lepton collider one tagged, deeply virtual photon scatters off an untagged quasireal photon, both originating from the initial lepton beams, producing inclusively an almost back-to-back light-hadron pair with large transverse momentum, in the gamma-gamma center-of-mass frame. Similar processes, in a more complicated environment due to the presence of initial hadronic states, can also be studied in ultraperipheral collisions at the LHC and the planned future hadron colliders.

Reference paper:
S. Anedda, F. Murgia, C. Pisano, Phys. Rev. D 112, 014013 (2025)

Exclusive diffractive meson production represents a promising channel for investigating gluonic saturation inside nucleons and nuclei. I’ll discuss a systematic framework to deal with beyond leading power corrections at small-x, including the saturation regime, and obtain the γ∗ → M(ρ, φ, ω) impact factor with both incoming photon and outgoing meson carrying arbitrary polarizations. This is of particular interest, as the saturation scale at modern colliders, although entering a perturbative regime, is not large. As a result, higher-twist terms can lead to significant effects.

Based on:
R. Boussarie, M. Fucilla, L. Szymanowski and S. Wallon, Phys. Rev. Lett. 134 (2025)
no.4, 041901 and Phys. Rev. D 111 (2025) no.1, 014032

In recent years, numerous studies have aimed at improving the precision of the theoretical description of the nonlinear QCD regime of gluon saturation in high-energy collisions, in order to match the precision of the data coming from the LHC and the future EIC. In particular, NLO QCD corrections have been calculated for many high-energy processes sensitive to gluon saturation.

In most of such NLO calculations, the chosen regularization procedure is dimensional regularization for the transverse integrals complemented by a naive cut-off for the light-cone momentum k^+ integrals. Although convenient in that context, this regularization procedure has disadvantages. On the one hand, it does not allows us to disentangle soft and rapidity divergences, which are then both regulated by the cut-off. On the other hand, it complicates the comparison with results obtained in other regimes of QCD, for example in collinear or TMD factorizations, using other regularization schemes.

As an alternative, we discuss how to implement in these NLO gluon saturation calculations various new rapidity regulators proposed by the TMD or SCET communities. As a first application and consistency check, we revisit the calculation of the NLO corrections to the F_L DIS structure function at low x in the dipole factorization approach, now with these rapidity regulators together with dimensional regularization. When combining the results from all diagrams, we find as expected that the UV divergences are canceling each other, as well as the soft divergences. The only surviving divergence is the rapidity divergence associated with the Balitsky-Kovchegov evolution of the dipole operator. Among the finite NLO corrections to F_L, the only ones which are found to depend on the rapidity regularization scheme are associated with the scheme dependence concerning the choice of evolution variable for the BK equation.

Derivation of finite energy corrections to high energy eikonal scattering processes within the Color Glass Condensate (CGC) effective theory has been a prominent topic over the last decade. In this talk, we will give a brief overview of next-to-eikonal corrections to parton propagators and their applications to various observables.

Instabilities in the non-linear evolution equation for gluon distributions in the saturation regime when approaching collinear kinematics were noticed a decade ago. Since then, several ad hoc solutions to the issue have been proposed with some success in improving stability in arbitrary ways. Here, we will discuss a top-down approach which fully encompasses both full collinear kinematics and saturation effects by changing the base asumptions in the effective field theoretical description of gluon saturation. In the spirit of JHEP 07 (2022) 080 and JHEP 10 (2024) 056 where observables were discussed, we will detail the evolution equation for the relevant gluon distributions and the so-called Improved BK limit of this equation.

We present a determination of the gluon density $xg$ from simulated measurements of the longitudinal structure function of the proton at the Electron-Ion Collider (EIC). Using the Rosenbluth separation method, we show that the EIC can achieve high precision in the extraction of $F_L$, even with modest luminosities. We further show the complementarity between the EIC, HERA, and fixed-target results.

Deeply virtual Compton scattering (DVCS) provides one of the most promising avenues to access generalized parton distributions (GPDs), which encode the three-dimensional structure and mechanical properties of hadrons. However, the extraction of GPDs from experimental observables requires solving an ill-posed inverse problem, passing through the intermediate stage of calculating Compton Form Factors (CFFs). In this work, I present recent developments in applying machine learning techniques for a global analysis of DVCS data, which allow for model-independent extraction of CFFs with controlled uncertainties. Applications to JLab data highlight both the successes and limitations of current analyses, while simulated EIC data demonstrate the potential for precision studies of hadron structure at future facilities. I will discuss feasibility tests, systematic uncertainties, and open challenges in bridging machine learning methods with QCD phenomenology. This work paves the way toward a more robust and data-driven tomography of the nucleon.