EMMI Workshop: Mining for gluon saturation at the LHC and the future EIC

Photon-induced processes will be a key production mechanism at the approved EIC. With this
renewed interest, the demand for precise predictions from state-of-the-art Monte Carlo
generators has increased. In this talk, I will present recent efforts to develop a POWHEG-BOX
extension for simulating photoproduced dijets, including both direct and resolved photon
contributions based on the Weizsäcker-Williams Approximation. This implementation will enable
event generation at NLO+PS accuracy for collisions involving leptons, protons, and heavy ions.
It is particularly relevant for EIC studies and is also applicable to ultra-peripheral
collisions (UPCs) at the LHC.

Scaling amplitudes describing pp elastic scattering differential cross-sections in the dip-bump region at the LHC have been recently derived. We check that this scaling is also verified by the $p\bar p$ cross-section at the highest energy of the Tevatron. Using the energy-phase relations for given plus or minus signature coming from the Regge formalism and deriving the scaling amplitude with only positive signature (i.e. the Pomeron), lead phase predictions which are in agreement with the $p\bar p $ data. An evidence for a negative signature component (i.e. the Odderon) of the scaling amplitudes is thus found. Its amplitude and contribution to the cross-sections are extracted and discussed.

Quarkonia -- bound states of heavy quark-antiquark pairs (charm or bottom) -- offer a unique probe of both perturbative and non-perturbative QCD. While the hard-scale production of the quark-antiquark pairs is calculable via perturbation theory, their hadronization into bound states requires non-perturbative models like NRQCD. In hadronic collisions, the underlying event activity, usually quantified in terms of charged-particle multiplicity density, is mainly determined by soft particle production processes. By correlating measurements of quarkonium production and event activity in small systems, we can learn more about the interplay between hard and soft particle production from initial-state effects, such as gluon saturation or Multiple Partonic Interactions, or from final-state effects. In addition, separating prompt and non-prompt J/$\psi$ helps to study separately the charm and beauty sectors. Finally, measuring the multiplicity in different azimuthal regions with respect to the J/$\psi$ emission direction allows to disentangle the effects of the particles emitted in the same production process as the J/$\psi$ from the effects of the underlying event.

In this talk, recent measurements of prompt and non-prompt J/$\psi$ production by the ALICE Collaboration at midrapidity as a function of charged-particle multiplicity in pp collisions at $\sqrt{s}=13$ TeV will be presented.

We study the diffractive production of a massive quark pair in coherent photon-nucleus collisions within the Color Glass Condensate framework. We work in the correlation limit, where we explicitly distinguish between hard and semi-hard scales. In a first study, we consider the emission of an unmeasured semi-hard gluon with transverse momentum of the order of the saturation scale by the massive quark pair. In this regime, the differential cross section factorizes into a mass-dependent hard factor, which describes the formation of the quark-antiquark, and the diffractive gluon transverse momentum dependent (TMD) distribution. In a second study, we consider the production of a semi-hard quark and a hard antiquark and gluon, which have transverse momenta much larger than the saturation scale. We then factorize the differential cross section into a mass-independent hard factor and a mass-dependent quark diffractive TMD distribution. Our results are the base for future phenomenology predictions for quarkonium and open charm production in the saturation regime in ultraperipheral collisions at the LHC.

We will present new developments in understanding of the JIMWLK Hamiltonian beyond NLO. We will focus on DGLAP and running coupling resummations inside the JIMWLK Hamiltonian.

Ultraperipheral collisions (UPCs) at high energies provide an excellent laboratory to study photon-induced interactions and heavy flavor production in a clean experimental environment. In this work, the inclusive $D^0$ and $B^0$ photoproduction in ultraperipheral $pPb$ and $PbPb$ collisions at LHC energies is analyzed within the color dipole $S$-matrix formalism. Differential cross-sections are evaluated in transverse momentum space, and predictions for rapidity and transverse momentum spectra are obtained using different descriptions of the unintegrated gluon distribution, covering both linear and nonlinear QCD dynamics. The sensitivity of the results to the underlying gluon dynamics is discussed, and comparisons with recent CMS and ALICE preliminary data for $D^0$ photoproduction are presented. Our results indicate that a detailed analysis of these collisions can provide valuable insights into the strong interactions at high energies, as well as into nuclear effects in heavy ion collisions.

We present a numerical evaluation of the diffractive structure functions for DIS using Monte Carlo methods.

We report on progress on the precision of such calculations using
Improved TMD Factorization. It is a consistent approach that allows
for a perturbative factorization of cross sections for forward
production in terms of PDFs and parton-level scattering. It is valid
both for LHC and EIC physics. Furthermore, it can be formulated in
momentum space and is ideal for Monte Carlo approaches.

In 2022, two new expriments able to detect neutrinos started operation in the forward direction of the LHC. This allows using the neutrino flux to study forward particle production, and thus proton structure, in a previously unexplored kinematic regime of very low-x. In this presentation I will give an overview over the LHC neutrino program, first results and prospects for the future. In particular, I will outline the prospects to probe saturation.

We calculate the one-loop corrections to the quark TMD in both projectile and target light-cone gauges using the background field formalism, with the Mandelstam-Leibbrandt (ML) prescription for the extra singularity present in the light-cone gauge propagator. We use the pure rapidity regulator for rapidity divergences. The Collins-Soper-Sterman (CSS) evolution equations are obtained from the one loop renormalization of the quark TMD. We comment on the technical issues that arise when making connections between gluon saturation and TMD factorization.

In this talk photon fluxes associated with light and heavy ions that are relevant in ultraperipheral pA collisions at the LHC are discussed, in particular the comparison between wood saxon and hard sphere models within superchic and pythia generators. Flux effect on particle production, rapidity gaps and two-particle correlations in gamma-proton interactions within pA systems is discussed.

An enhancement in the transverse momentum distribution of Drell-Yan pairs was recently observed in $p$-$A$ collisions, in the proton going direction, by the PHENIX experiment at RHIC.
This observation, along with the absence of any enhancement in the nucleus going direction is consistent with the Cronin effect, and with the observed enhancement in a series of fixed target experiments at Fermilab.
Hard partons from the proton undergo pre-scattering in a large nucleus, prior to the hard process where a dilepton pair with mass $Q$ and transverse momentum $q_T$ is formed.
While earlier studies focussed on the angular distribution, the energy loss of low $q_T$ Drell-Yan pairs, as well as the $q_T$ distribution in the collinear limit [1], we specifically focus on large transverse momentum pairs, where $q_T \sim Q$ the mass of the dilepton pair, using factorized Transverse Momentum Dependent Parton Distribution Functions (TMDPDFs).
While the case of arbitrary $q_T$, at Next-to-Leading Order and Next-to-Leading-Twist (NLO-NLT), yields hundreds of Feynman diagrams, we show that in the limit where $q_T \ll \Lambda_{QCD}$, only about 26 diagrams dominate.
None of these diagrams involve off-forward distributions, allowing for straightforward evaluation using known parameterizations of TMDPDFs.
Evaluating these diagrams we illustrate the dependence of the differential spectrum, as a function of $q_T$, on the excess transverse momentum imparted by the pre-scattering.
Comparing numerical results to data we illustrate, for the first time, the possibility of $x$ and transverse momentum $k_T$ dependent nuclear shadowing.

[1] - Phys.Rev.D 58 (1998) 036001, Nucl. Phys. B 582 (2000) 537-570, Nucl. Phys. A 879 (2012) 77-106.

We study diffractive photoproduction of vector mesons in ultra-peripheral heavy-ion collisions as a probe of the partonic structure of nuclei and the onset of gluon saturation at energies accessible at the LHC. In addition, we present predictions for deeply virtual Compton scattering (DVCS) in ep and eA collisions, focusing on the kinematic ranges that will be explored by the future EIC. Our study is based on the energy-dependent Hotspot model, in which subnucleonic regions of high gluonic density, known as hot spots, fluctuate event by event.
    Incoherent processes are particularly sensitive to fluctuations in the target’s color field, providing direct access to its spatial structure at different transverse scales. We show that the energy dependence of the incoherent cross section exhibits a characteristic non-monotonic behavior, rising with energy, reaching a maximum, and then decreasing as saturation effects become significant.