Experimental overview on UPC results at STAR.
The ALICE Collaboration has extensively studied coherent and incoherent J/$\psi$ photoproduction, as well as coherent $\psi$' in ultra-peripheral Pb-Pb collisions using the Run 2 data set, including results from exclusive and dissociative J/$\psi$ mesons in ultra-peripheral p-Pb interactions, together also with the |t|-dependence of (in)coherent J/$\psi$. These measurements, which studied...
Exclusive processes in high energy collisions can be described in terms of either a collinear factorization or a dipole approach. While these approaches have a common limit, they also differ in many ways. For example one can consider it more important to resum large logarithms of either transverse or longitudinal momenta. There can also be different views about whether the dominant transverse...
I will present an overview on the topic of Monte Carlo event generators for UPCs and photon-mediated processes
We will present an overview of theoretical and phenomenological studies on the partonic structure of nuclei and small-x dynamics of QCD using photon-nucleus scattering in UPCs. In particular, we review implications of exclusive charmonium and inclusive dijet production in Pb-Pb UPCs at the LHC as well as discuss the potential of new measurements of diffractive dijet and inclusive heavy quark...
Over the last years, several authors have demonstrated that the study of exclusive processes in hadronic collisions provides new possibilities for the investigation of the Quantum Electrodynamics and Quantum Chromodynamics at high energies, as well as to searching for Beyond the Standard Model (BSM) Physics. In this talk, I will review the recent predictions for the production of new particles...
I will review the status of our understanding of collectivity in ultra peripheral heavy ion and p+A collisions.
Ultraperipheral collisions (UPCs) of relativistic heavy ions provide a unique laboratory for studying photon-induced reactions due to their intense photon flux. Vector mesons, such as $J/\Psi$ and $\Psi(2S)$, can be produced via $\gamma$ + A interactions, offering a powerful tool for probing the gluonic properties of the target nucleus. The $J/\Psi$ and $\Psi(2S)$ mesons introduce distinct...
The fully stripped ions used in heavy ion collisions at the LHC are an excellent source of high-energy quasi-real photons. These can interact with photons emitted by the oncoming nucleus, or with the nucleus itself, either directly in inelastic processes or diffractively via pomeron exchange. Diffractive photonuclear processes can produce exclusive vector mesons that are uniquely sensitive to...
A new phenomenon, gluon saturation, is expected to appear at high energies in QCD, where the splitting and production of gluons achieve a dynamic balance. Gluon saturation is expected to appear at lower energies in large nuclei. Due to its high sensitivity to the gluon distribution, the diffractive photoproduction of J/Ѱ is an ideal tool to search for the onset of gluon saturation.
ALICE...
The exclusive photoproduction of multi-hadron final states in ultraperipheral
collisions (UPCs) provides a unique avenue to explore quantum chromodynamics (QCD) and the nature of resonances emerging from gluonic interactions.
The ALICE Collaboration has recently performed measurements of exclusive
four-pion photoproduction, revealing data that suggest the presence of two resonances....
Diffractive vector meson production is a golden channel in the search for gluon saturation. The color glass condensate (CGC) framework has been successfully applied to describe diffractive vector meson production in $e+p$ collisions at HERA. Predictions for $\gamma+Pb$ collisions realized in ultraperipheral collisions at the LHC are however overestimating the experimental data at large center...
We study exclusive $J/\Psi$, $\Upsilon$, $b \bar{b}$ and $c \bar{c}$ photoproduction for proton and Pb targets in the high-energy limit, with the energy dependence computed using the linear Balitsky-Fadin-Kuraev-Lipatov and the nonlinear Balitsky-Kovchegov evolution equations. The difference between these two evolution equations can be directly attributed to gluon saturation physics. We find...
This talk will discuss the experimental results of ALICE, LHCb and STAR collaborations.
The photoproduction of J/Ѱ is investigated in the coherent collisions at LHC energy (√s = 5.02 TeV), for distinct transverse momentum and centrality classes: ultra-peripheral collisions (UPC) and peripheral collisions across different event geometries. An application of the convolution between the photon flux and the photonuclear cross section is explored for UPCs. In the the peripheral...
The gluon density in protons and nuclei has been observed to increase very rapidly with energy. At high energies, however, nonlinear effects of QCD start to become important, slowing down the evolution of the gluon density and hence giving rise to gluon saturation. This gluon saturation can be naturally described using the color-glass condensate (CGC) effective field theory.
In practice,...
We consider the differential cross-section for single-inclusive jet production with transverse momentum P⊥ in Deep Inelastic Scattering (DIS) at small Bjorken xBj, mediated by a virtual photon with virtuality Q2. We focus on the kinematic regime where the jet is produced in the target fragmentation hemisphere of the Breit frame, and with P⊥ ≪ Q. For a longitudinally polarised photon, we...
We predict the possibility of detecting signatures of the long-sought saturation in ultraperipheral heavy-ion collisions at currently accessible energies of the LHC. Saturation, as predicted by QCD at high energies, occurs when gluon densities in hadrons become so large that gluons begin to recombine with each other, stabilizing their population. To probe these effects, we study incoherent...
The Large Hadron Collider (LHC) has paved the way for an increasingly deeper understanding of hadronic structure. Particularly exciting is the potential to exploit the high center-of-mass energies to investigate gluon saturation in nuclei and nucleons through Ultra Peripheral Collisions (UPC).
We consider the exclusive photoproduction of a photon-pion pair, $\gamma + P \rightarrow \gamma +...
Momentum transfer distributions $d\sigma/dt$ in diffractive electroproduction of heavy quarkonia on nuclear targets are studied within a rigorous quantum-mechanical approach based on the light-front Green function formalism [1]. Such a formalism allows to include naturally the higher-twist and leading-twist shadowing corrections, as well as
the color transparency effects. Model calculations...
We present our recent results regarding the numerical solution of the
BK equation. We discuss the solution of the NLO BK equation
including the dependence on the impact parameter. We present the
obtained solutions and discuss their properties.
We also discuss solutions to the BK equation formulated in the target
rapidity and including the full impact parameter dependence.We utilise the...
Deeply Virtual Meson Production (DVMP) represents a key channel for investigating BFKL dynamics and gluon saturation within nucleons and nuclei. Describing many DVMP observables requires a treatment beyond leading power. We present a systematic framework to address next-to-leading power corrections at small-$x$, including the saturation regime, and apply it to calculate the $\gamma^{*}...
In this talk, I will focus on an interesting subclass of 2 to 3 exclusive processes, in which a pair of light mesons of large invariant mass is produced through photoproduction. These processes provide access, at leading power, to both chiral-even and chiral-odd quark generalised parton distributions (GPDs), depending on the nature and on the polarisation of the mesons. For the final-state...
I will report on our recent complete one-loop study of exclusive photoproduction of vector quarkonia off protons in Collinear Factorisation (CF) including the scale evolution of the Generalised Parton Distributions (GPDs). We have confirmed the perturbative instability of the cross section at high photon-proton-collision energies ($W_{\gamma𝑝}$) at Next-to-Leading Order (NLO) in $\alpha_s$ and...
In this talk, I will discuss a class of 2 to 3 exclusive processes which can be used to probe generalised parton distributions (GPDs). One of the main advantages of such exclusive processes is that they have an enhanced sensitivity to the x-dependence of GPDs, compared to standard 2 to 2 exclusive processes such as Deeply Virtual Compton Scattering (DVCS), which only give moment-type...
Diffractive vector meson photoproduction has long been recognized as an unparalleled probe of the gluon distribution within nuclei, potentially key to elucidating non-linear QCD effects that lead to universal states of dense gluonic matter. $\phi$ meson photoproduction is particularly useful for studying small-$x$ dynamics, especially gluon saturation, due to its sensitivity to larger dipole...
Coherent J/$\psi$ photoproduction in heavy-ion collisions has emerged as a sensitive probe of both the initial-state gluonic structure of nuclei and the dynamical electromagnetic fields generated by relativistic ions. While traditionally studied in ultra-peripheral collisions (UPCs), recent experimental observations by ALICE and STAR collaborations reveal surprising enhancements in low-$p_T$ ...
The new forward calorimeter, FoCal, extends the ALICE physics programme with the capability, unique at the LHC, of investigating gluon parton distribution functions (PDFs) down to Bjorken-x of ~10^-6. In this kinematic range, the gluon distributions are expected to behave non-linearly.
FoCal is a high-granularity forward calorimeter to be installed as an ALICE upgrade subsystem during the...
The LHC is not only the energy-frontier collider for parton-parton collisions, but it has proven to work as a powerful photon collider providing photon-photon collisions at center-of-mass energies never reached before. I will present results of the gamma-UPC Monte Carlo event generator [1] that can calculate any arbitrary exclusive final states produced via photon fusion in ultraperipheral...
We present Upcgen, a Monte Carlo event generator designed for simulating dilepton pair production in ultra-peripheral collisions (UPCs) [1]. In Upcgen the dilepton pair production cross section A+A -> A+A + ll is calculated by folding the elementary gamma gamma -> ll cross section with the two-photon luminosity produced by the colliding nuclei. The calculation of the two-photon luminosity is...
The STARlight Monte Carlo event generator was developed more than 20 years ago and has been used extensively for the study of ultra-peripheral relativistic heavy ion collisions ever since.
STARlight can calculate cross sections for the two-photon production of dilepton pairs and single mesons and for both coherent and incoherent photonuclear production of vector mesons. The photon spectrum...
Photoproduction processes have gained a renewed interest following the approval of the EIC, making their implementation in Monte Carlo event generators highly desirable. I will present recent efforts to develop a POWHEG-BOX extension simulating dijet production from direct and resolved photons at NLO, leveraging the Weizsäcker-Williams Approximation. Its wide-ranging applicability will enable...
We discuss recent developments in the Sherpa event generator with regard to UPCs. Different form factors for the photon flux for both protons and heavy nuclei are compared and finite size effects of the emitters are taken into account. We present preliminary results for jet prodcution at NLO matched to the parton shower using MC@NLO, as well as NLO QED calculations for exclusive muon-pair...
We present the recent developments in simulations of ultra-peripheral collisions with the Pythia 8 Monte Carlo event generator. We first review the status of recent comparisons and tuning efforts to HERA photoproduction data which provide the baseline of photon-nucleon interactions. Then we present the vector-meson-dominance (VMD) based approach to model interactions between a real photon and...
We present a comprehensive comparison with the Sartre event generator and available exclusive diffraction data from UPC of heavy ions. Sartre now takes all phenomena unique to UPC into account, i.e. interference effects, neutron emissions due to secondary and tertiary photon interactions, as well as a photon pt, paired with a reliable saturated small-x QCD model (IPSat). This enables us to use...
Generalized parton distributions (GPDs), probed through exclusive processes like deeply virtual Compton scattering, serve as indispensable tools in filling in the gaps of the angular momentum sum rules as well as mapping the partonic spatial structure of hadrons. In this work, we extend the utility of GPDs in mapping the proton’s spatial structure beyond the typical one-body partonic picture,...
In ultra-peripheral collisions (UPCs), emitted photons may participate in a hard-scattering process with partons within the struck nucleus, providing a clean, energetic probe of its parton distributions. This talk presents measurements of jet production in UPCs performed with the ATLAS detector using high-statistics 2018 Pb+Pb data. Events are selected using requirements on jet production and...
The STAR experiment collected large-statistics datasets from isobar collisions of $^{96}_{44}\text{Ru}+^{96}_{44}\text{Ru}$ and $^{96}_{40}\text{Zr}+^{96}_{40}\text{Zr}$ ions in 2018, offering a valuable opportunity to study Coulomb dissociation via Ultra-Peripheral Collisions (UPCs). Despite having identical mass numbers, these isobars exhibit distinct proton and neutron distributions,...
High-multiplicity proton–proton and proton–lead collisions at LHC energies feature signatures similar to those observed in Pb--Pb collisions, such as e.g. strangeness enhancement and collective expansion effects, which are traditionally attributed to the formation of the quark–gluon plasma. One way to study them is through the measurement of $p_T$-spectra for identified hadrons such as charged...
Vector, scalars and tensor mesons are accessible in the mass spectrum of di-hadrons measured in UPC events. The LHCb experiment is able to identify pions, kaons, proton, electrons, muons and photons at a very low transverse momentum, enabling the observation of a broad spectrum of mesons produced from different kinds of photon and pomeron interactions. This presentation will discuss the...
To be included
Inelastic photoproduction of charm has been used previously to constrain the proton gluon distribution at low-x, using e-p collisions. Ultra-peripheral heavy-ion collisions provide an opportunity to use the same mechanism to study the less known nuclear gluon distribution. In these collisions, a photon emitted from one nucleus interacts with a gluon in the target nucleus, producing a pair of...
We compute the differential cross section for inclusive $\mathrm{D}^0$ production in ultra-peripheral collisions within the Color Glass Condensate framework. Our predictions are found to be in good agreement with preliminary CMS data. Furthermore, we parametrize saturation effects by a nuclear modification ratio for photoproduction $R_{pA}$ and examine the collinear factorization limit of the...
Abstract: We present theoretical predictions for the inclusive D0 production cross section in ultraperipheral Pb–Pb collisions at the LHC, as a function of transverse momentum and rapidity. The calculations are performed within the Fixed-Order Next-to-Leading Logarithm (FONLL) framework, which is validated using D^* photoproduction cross sections measured in electron–proton collisions at HERA....
We demonstrate that hadron-hadron collisions at the LHC can be used to extract inclusive photoproduction cross sections. We focus on quarkonium cross sections in proton-lead collisions. We illustrate this by constructing a Monte Carlo for both the signal and background and show that, despite the background having a significantly larger cross section, the signal can be experimentally...
The anomalous magnetic moment of the tau lepton ($a_{\tau}$) is a sensitive probe for the search for deviations from the Standard Model predictions and thus for new physics. This study investigates the feasibility of measuring $a_{\tau}$ using ultra-peripheral collisions (UPCs) at the LHC, where photon-photon interactions ($\gamma\gamma\to \tau^+\tau^-$) produce tau lepton pairs. We focus on...
In ultra-relativistic heavy-ion collisions, high rates of $\gamma\gamma$ processes occur through the interaction of the large electromagnetic fields of the heavy nuclei. For large impact parameters between the nuclei, i.e. interaction distances larger than the nuclei's radii, the di-photon interaction can be the only one taking place, leading to very clean signatures in the detector. One of...
To be included
I will discuss the recent NLO calculations for gamma-gamma physics.
The current state of art estimations lead to cross sections
for $AA \to \gamma \gamma AA$ which are somewhat
smaller than the measured ones by the ATLAS and CMS Collaborations.
We calculate inelastic contribution to $\gamma \gamma \to \gamma \gamma$
scattering process in $A A \to \gamma \gamma X Y$ where $X, Y = A, A'$.
We include processes of coupling of photons to individual...
In ultra-relativistic heavy-ion collisions, the Lorentz-contracted electromagnetic fields of the ions generate intense quasi-real photon fluxes. These lead to photon-induced interactions that have been observed in ultra-peripheral collisions (UPCs), such as vector meson and lepton-pair production. The high photon flux also enables the occurrence of multiple photon-induced processes in a single...
Kaon pairs can be produced via photoproduction ($\gamma$-Pomeron) or photon-photon fusion ($\gamma\gamma$). A significant contribution to kaon pair production comes from the decay of scalar, tensor, and vector mesons. The study will provide a comprehensive description of dikaon production at both the elementary ($\gamma\gamma \to m \to K^+ K^-$) and nuclear ($Pb Pb \to Pb Pb K^+ K^-$) levels....
One of the most pressing questions in both hot and cold QCD communities is what the physics mechanism responsible for modified parton densities in heavy nuclei is. One promising channel to address this question is the photoproduction of vector mesons, which is considered a clean probe to the nuclear parton structures.
We present a measurement of $\pi^+\pi^-\pi^+\pi^-$ and $\pi^+\pi^-$...
The EIC is set to start running around the same time as LHC Run 5. What could be the synergies between EIC and LHC UPC measurements at this time. The EIC is a wonderful machine for studying multidimensional correlations between partons. At the same time the LHC will have greatly expanded detector capabilities that should allow a new era of UPC measurements. I will attempt to sketch these...
Relativistic heavy-ion collisions generate extremely strong electromagnetic (EM) fields, providing an ideal environment to study the EM excitation of the vacuum. The Breit-Wheeler process, which involves the electron-position pair production via photon-photon interactions, represents the lowest-order decay mode of the QED vacuum excitation. This process was first observed by the STAR...
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 RHIC data. The description of this process can be achieved by introducing the so-called Wigner distribution of the photons. They are related via Fourier transform to the generalised transverse momentum...
Photon-induced processes in ultra-peripheral collisions have attracted considerable attention in recent years due to their potential to probe various aspects of nuclear and particle physics. A key feature of these processes is the azimuthal modulation of particle production, which provides critical insights into the underlying dynamics of interactions involving quasi-real photons. In this talk...
In ultraperipheral collisions, the invariant mass spectrum of $\pi^+ \pi^-$ pairs is very complex due to the numerous production channels and intermediate states. The quantum ambiguity between production channels, referred to as the Entanglement Enabled Spin Interference (EESI) effect, leads to angular anisotropy in the final state. The most dominant contribution to the invariant mass spectrum...
In ultraperipheral Pb+Pb collisions, intense electromagnetic fields enable the generation of magnetic monopole pairs via the Schwinger mechanism. Due to their high ionization and unique trajectories in a solenoidal magnetic field, monopoles are expected to leave a large number of clusters in the innermost ATLAS pixel detector without associated reconstructed charged-particle tracks or...
Ultraperipheral collisions (UPCs) of relativistic heavy ion beams lead to a diverse set of photon-nucleus (photonuclear) interactions. Measurements of particles produced in photonuclear reactions can shed light on the QCD dynamics of these novel, extremely asymmetric colliding systems, with energies between those available at RHIC and the LHC. Previous studies by ATLAS have characterized...
Understanding collectivity in small systems like proton-proton and proton-ion collisions remains a key challenge in heavy-ion physics. Recent LHC studies show signs of collectivity in high-multiplicity photonuclear collisions, where a quasi-real photon from one nucleus interacts with the other nucleus. In this talk, we present CMS measurements of two- and multi-particle correlations from...
Investigating collective behavior due to the formation of a fluid-like medium in small collision systems has been a significant focus in the field. A tell-tale signature of this would be the medium's response to the initial state in small collision systems, as predicted by fluid-dynamic models.
Recent RHIC studies of small systems have shown a hierarchy of elliptic anisotropy coefficients...
It is not understood how collective flow can occur in low-multiplicity systems such as minimum-bias proton-proton collisions. The problem is that these system appears to be too short lived to form equilibrated matter. To understand flow in these systems and the flow observed by ATLAS for UPC events, one typically need a flow mechanism that is not equilibrated.
In this talk, I discuss...
Ultra-peripheral heavy-ion collisions provide a unique laboratory for studying the structure of both nuclear and nucleon targets. In photonuclear interactions, the high-energy photons provided by the ultra-Lorentz boosted ions serve as a “femtoscope” for probing sub-femtoscale nuclear properties of the target, revealing details of the gluon distributions within nuclear matter. Substantial...
In this study, the production of strange particles is investigated in ultra-peripheral Pb–Pb collisions (UPCs) at $\sqrt{s_{\rm{NN}}}$ = 5.36 TeV, using data from the ALICE experiment at the LHC. The analysis focuses on selecting photo-nuclear reactions, where a quasi-real photon emitted by the fully ionized nucleus either acts as a point-like particle interacting with a parton in the...
In this talk, I will present our investigation of the T-odd nucleon energy correlator at small-x and elucidate its connection with spin-dependent odderons. The probe of this nucleon energy correlator involves measuring a single transverse spin asymmetry for the energy pattern in the deep inelastic scattering. We demonstrate that this asymmetry can be served as a clean and sensitive observable...
In this talk I plan to discuss the next to leading order corrections to light by light scattering process in QCD+QED. Recent experimental observation of this fundamental process in ultra peripheral collision at LHC has revived the interest to precisely predict its cross-section. We discuss
two radically different computational approaches, both exact in the fermion mass dependence,...
The Lorentz-boosted electromagnetic field surrounding relativistic heavy ions can be treated as a large flux of linearly polarized, quasi-real photons. The coherently photoproduced $J/\psi$ mesons inherit the linear polarization of these photons, leading to an azimuthal asymmetry in their decay products relative to the impact parameter. This asymmetry can be estimated using the second-order...
We consider production of very forward neutrons in photon-nucleus and proton-nucleus inelastic collisions, which are detected in the LHC detectors by zero degree calorimeters (ZDC) with efficiency close to one. We argue that such measurements would provide a new probe of nuclear shadowing in soft and hard regimes of QCD. In particular, we calculate the distribution over the number of...
Abstract: Exclusive dijet production in ultraperipheral lead-lead (PbPb) collisions has been proposed as a probe to constrain the elliptically polarized gluon distributions of heavy ions. Specifically, the second Fourier harmonic ($v_2$) of the angle ($\Delta\phi$) between the three-momentum vector sum ($\vec{Q}_T = \vec{p}_{T,1} + \vec{p}_{T,2}$) and vector difference ($\vec{q}_T =...
