In this talk, there will be a historical introduction to the physics of UPCs from both theory and experimental perspectives.
Relativistic heavy ions are sources of strong electromagnetic fields which produce photon-induced interactions. These interactions are usually studied in ultra-peripheral collisions (UPCs) of the relativistic heavy ions. The UPCs can produce di-lepton or di-hadron pairs via the γ-γ interactions or produce vector mesons via the γ-nuclear interactions. Both the photo-produced vector mesons and...
In this talk, we will present the recent experimental results on the physics of ultra-peripheral heavy ion collisions using the ALICE detector, including the measurements of both heavy and light vector mesons in both ultra-peripheral p-Pb and Pb-Pb collisions.
Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This talk presents a series of measurements of such processes performed by the ATLAS Collaboration. Measurements of exclusive dilepton production (electron, muon, and tau pairs) are discussed. These processes provide strong constraints on the nuclear...
Overview of latest UPC results from CMS.
Ultra peripheral collisions has a large range of observables which are kinematically hard to be measured. These includes light vector mesons from coherent effects and other probes requiring soft particle tracking. The LHCb experiment has unique capabilities to study multiple UPC observables, thanks to its low transverse momentum tracking and particle identification. This presentation will...
I will review calculations of vector meson production in ultra peripheral collisions. I will focus on exclusive J/Psi and rho production at RHIC and LHC and address their potential to elucidate nuclear effects including gluon saturation, as well as nuclear structure, sub-nucleonic fluctuations, and entanglement driven interference effects. I will focus on dipole models and compare to...
In this talk a solution of the target-rapidity Balitsky-Kovchegov (BK) equation will be presented considering the complete impact-parameter dependence, including the orientation of the dipole with respect to the impact-parameter vector. The target-rapidity formulation of the BK equation introduces non-locality in rapidity. Three different prescriptions are considered to take into account the...
We study the coherent photoproduction of light vector mesons in $AA$ collisions using the color dipole approach. We use the Glauber--Gribov formalism, however, it has to be supplemented by the gluon shadowing, since the coherence length of higher dipole Fock states is smaller than the nucleus radius. We fit this gluon shadowing to the deep inelastic structure function $F_2$ (E665) and $\rho$...
In ultra-peripheral collisions (UPCs), the photon fluctuates to a quark-antiquark dipole which then elastically scatters off the nucleus, emerging as vector meson and opposite-charge pseudoscalar meson pair. The excellent particle identification capabilities of ALICE enable the study of photoproduced pi+pi- and K+K- pairs at midrapidity in Pb--Pb collisions at √sNN = 5.02 TeV. We will present...
In this talk, we discuss the exclusive photoproduction of ground and excited states of $\psi(1S,2S)$ and $\Upsilon(1S,2S)$ in ultraperipheral collisions (UPCs). Using the potential approach in order to obtain the vector meson wave function, we find a good agreement of our calculations with data from the LHC and HERA colliders for $J/\psi (1S,2S) $ and $\Upsilon(1S)$ in $\gamma p$ collisions....
Vector meson production in high-energy UPCs is an excellent tool to study the dynamics of small-x gluons inside nuclei. Employing the Color Glass Condensate (CGC) effective field theory, I will describe the impact that gluon saturation has in exclusive and semi-inclusive J/\psi production in UPC reactions.
In the first part of the talk, I will focus on exclusive J/\psi production and show...
Coherent photoproduction has long been studied in UPCs. Although interference between the two nuclear targets has been studied, other quantum mechanical problems have received much less attention. In this talk, I will discuss some open problems, including studying the limits of coherence, and quantum mechanical aspects of multiple vector meson production in a single UPC.
Ultra-strong electromagnetic field in relativistic heavy-ion collisions can generate a large flux of linearly polarized quasi-real photons. Photons emitted by one nucleus can interact with the other whole nucleus or individual nucleons to produce vector meson ($\gamma + A \rightarrow V + A$). On the other hand, interactions between photons emitted by two nuclei can produce lepton pairs...
Exclusive vector meson photoproduction in ultraperipheral heavy-ion collisions is a well-established tool to probe the gluon structure of the colliding nuclei. This talk will focus on the observation of entanglement-enabled spin interference in the ρ0 meson photoproduction, in the form of angular anisotropy. Such an anisotropy appears due to two different factors: the first is that the photons...
Exclusive vector meson photoproduction in ultra-peripheral collisions (UPC)
is a unique tool to study quantum chromodynamics in nucleus-nucleus
collisions. In a UPC, photon induced interaction takes place on one
of the nuclei, while the other is the source of virtual photon. Neutrons
may be emitted from the target nucleus and eventually detected
in a very forward calorimeter. Here we...
Gluons are found to become increasingly dominant constituents of nuclear matter when being probed at higher energies or smaller Bjorken-x values. This has led to the question of the ultimate fate of nuclear gluonic structure at extreme density. In ultraperipheral collisions (UPCs) of relativistic heavy ions, the coherent heavy flavor vector meson production via photon-nuclear interactions is...
In ultra-peripheral collisions (UPCs), exclusive vector meson photoproduction, e.g., $\rho^{0}$ and $J/\psi$, has been considered one of the most sensitive probes to the gluon structure in heavy nuclei. Recently, it was discovered that the linear polarization of the photons involved in these processes can enable measurements of the nuclear geometry through the so-called entanglement-enabled...
Double Parton Scattering (DPS) is a important way for which we can investigate the parton distributions of the proton and the nucleus. Although, we know that such scatterings should occur in high energy collisions, the formalism to describe it lack of answers to questions like --- is there a universal effective cross section? In direction to explore such questions, we investigate DPS in...
We revisit the Wigner function approach to the impact parameter dependent
dilepton pair production developed in [M. Klusek-Gawenda, WS, A. Szczurek Phys.Lett.B 814 (2021) 136114]. We study the distribution of the angle between difference and sum of lepton transverse momenta, and show how it relates to the orbital angular momentum of leptons. The dependence on impact paramter is discussed,...
We will discuss photon photon physics at the LHC when protons are intact in the final state and can be detected in PPS (CMS) or AFP (ATLAS). We will review the sensitivities to quartic anomalous couplings and the search for axion-like particles in pp collisions. In addition, we will present the complementary gamma gamma physics at lower masses in heavy ion collisions.
Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This talk presents a series of measurements of dilepton production from photon fusion performed by the ATLAS Collaboration. Recent measurements of exclusive dielectron production in ultra-peripheral collisions (UPC) are presented. These processes provide...
Relativistic ultra-peripheral heavy-ion collisions (UPCs) generate an extremely intense electromagnetic field, offering an ideal setting for investigating the electromagnetic excitation of the vacuum. The lowest-order QED excitation involves the creation of lepton pairs through two-photon fusion, commonly referred to as the Breit-Wheeler (BW). In this presentation, we will report a...
The Lorentz-boosted electromagnetic field, arising from colliding nuclei, can be treated as a flux of quasi-real photons. Consequent photonuclear ($\propto$ $Z^{2}$) and photon-photon ($\propto$ $Z^{4}$) processes could reasonably explain the observed enhancements of $J/\psi$ and $e^{+}e^{-}$ pair production at very low transverse momenta ($p_{T}$) in peripheral heavy-ion collisions. The STAR...
Ultraperipheral nucleus-nucleus collisions produce very large photon fluxes such that fundamental quantum-mechanical processes can be observed and studied in a novel way. In this presentation, an observation of the $\tau$ lepton photoproduction at LHC is reported, using ultraperipheral lead-lead collision data collected by CMS. This measurement paves the way for a precise determination of the...
We study the production of $2\pi^+2\pi^-$ in ultraperipheral heavy-ion collisions at RHIC and LHC energies. The recent H1 preliminary data [1] are utilized to improve the description of the poorly known $\gamma p \to 4\pi^\pm p$ process. Predictions for photon-nucleus interactions were calculated for various excited states of $\rho$ meson. We will present an agreement of theory with the...
The gluon density has been observed to increase rapidly with energy, which would eventually violate unitarity. At high energies, however, nonlinear effects start to become important, slowing down the evolution of the gluon density and hence giving rise to gluon saturation. While there have already been strong hints of saturation in the currently available data, a definite measurement of...
We argue that diffractive photo-production of jets in coherent nucleus-nucleus ultra-peripheral collisions (UPCs) at high energy is a golden channel to study gluon saturation. By ``coherent'' we mean elastic processes in which both nuclei emerge unbroken after the collision and the final state exhibits large rapidity gaps. We study such processes within the colour glass condensate effective...
We calculate the Next to Leading Corrections to single and double inclusive hadron production in Deep Inelastic Scattering (DIS) and in Ultra-Peripheral Collisions (UPC) using the Color Glass Condensate effective theory of QCD at small x. We argue that single inclusive hadron production where momentum of the produced hadron is of the order of Saturation scale provides a sensitive probe of...
We study the proposal that the ratio of Psi(2s) and J/Psi exclusive photoproduction cross-sections might serve as an indicator of the presence of non-linear QCD evolution, related to the presence of high and potentially saturated gluon densities in both the proton and a lead nucleus. Our study employs recent fits of the GBW and BGK dipole model and provides predictions for both exclusive...
We compute the cross section for the inclusive photo-production of a pair of jets at next-to-leading order accuracy in the Color Glass Condensate framework. We study the emergence of the large Sudakov logarithms in the back-to-back limit, and show that they cannot appear consistently unless the low-x resummation is kinematically constrained.
We consider processes with production of a leading heavy vector meson, dijet followed by rapidity gap in UPC. We argue that the knock out mechanism of the elastic scattering of hard Pomeron off constituents of nucleon/nucleus dominates in a wide range of momentum transfer due to the structure of the resummed Pomeron. The suit of rapidity gap processes in which neutrons are detected in the...
Photoproduction of dijets is an important complementary probe of the partonic structure of protons, nuclei and real photons in QCD. We will review applications of the next-to-leading order (NLO) perturbative QCD in the framework of collinear factorization to inclusive and diffractive dijet photoproduction in heavy-ion ultraperipheral collisions (UPCs) in the kinematics of the Large Hadron...
Inclusive dijet photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs) has been promoted as a probe of nuclear parton distribution functions (nPDFs). However, due to the requirement of no nuclear overlap in such events, the impact parameter space is restricted. This becomes important in dijet production, where the requirement of having high-$p_{\rm T}$ jets means that one has to...
In relativistic heavy ion collisions, the charged ions produce an intense flux of equivalent photons. Thus, photon-induced processes are the dominant interaction mechanism when the colliding nuclei have a transverse separation larger than the nuclear diameter. In these ultra-peripheral collisions (UPCs), the photon provides a clean, energetic probe of the partonic structure of the nucleus,...
The light-by-light scattering (LbyL), $\gamma\gamma\to\gamma\gamma$, is a rare process in the Standard Model (SM) in which two photons interact and produce another pair of photons in the final state. The first direct evidence of this process at the LHC was established by the ATLAS experiment in 2017 [1] and subsequently confirmed by the CMS experiment in 2019 [2], both using data collected in...
We explore a new ratio of observables in ultra-peripheral A + A and A + p collisions as a potential signal of saturation physics. We consider the ratio R 1 of elastic vector meson photo-production cross section to the inclusive hadron or jet photo-production cross section. The ratio can be measured in the γ + A and γ + p collisions taking place in the UPCs. We label the ratios R 1 (A) and R 1...
Photon-photon reactions and the production of the J/ψ meson through photonuclear reactions have been extensively studied in ultra-peripheral heavy-ion collisions, in which the impact parameter is larger than twice the nuclear radius. In recent years, coherently photoproduced J/ψ and dilepton production via photon-photon interactions have also been observed in nucleus-nucleus (A–A) collisions...
The strong electromagnetic field generated by the colliding nuclei in heavy-ion collisions can be represented by a spectrum of photons leading to photon-induced interactions. While such interactions are traditionally studied in ultra-peripheral collisions (UPC) without any nuclear overlap, significant enhancements of dilepton pairs and J/$\psi$ production at very low transverse momentum...
Ultraperipheral collisions of relativistic heavy ion beams lead to a diverse set of photon-nucleus (photonuclear) interactions. The measurements of particle production in photonuclear reactions can shed light on the QCD dynamics of the novel, extremely asymmetric colliding systems, with energies between those available at RHIC and the LHC. Previous studies by ATLAS indicate significant...
The automated generation of arbitrary exclusive final states produced via photon fusion in ultraperipheral high-energy collisions of protons and/or nuclei, A B \to A X B, is implemented in the MadGraph5_aMC@NLO and HELAC-Onia Monte Carlo codes. Cross sections are calculated in the equivalent photon approximation using γ fluxes derived from electric dipole and charge form factors, and...
Ultraperipheral heavy-ion collisions provide the first opportunity to study collisions between photons and heavy ions in high-energy colliders. The quasi-real photons emitted by the nuclei may fluctuate into a hadronic state and in fact this hadronic contribution will provide the bulk of the photon-hadron cross section. In case of a heavy-ion target the partonic structure of these resolved...
In this talk I will address two topics. First I will discuss the production
of pions at very large rapidities (y > 7) in p-Pb and Pb-Pb collisions at the
LHC. These pions are produced through the magnetic excitation of nucleons in
the target due to the strong magnetic field generated by the projectile, i.e.,
N -> Delta -> N' pion. This process has a very large cross section and can
be...
The STARlight Monte Carlo calculates cross sections and generates events for a variety of ultra-peripheral collisions. In particular two-photon production of dilepton pairs and single mesons, and photonuclear production of vector mesons. The collision geometry plays a major role in these interactions, and the photon spectrum is thus calculated in impact parameter space. The photonuclear vector...
The coherent photons induced by relativistic heavy ions are highly linearly polarized, in close analogy to the linear polarization of gluons in a large nucleus. We proposed to measure the photon polarization through azimuthal asymmetries in dilepton production in ultraperipheral collisions. Our prediction for the asymmetries were soon confirmed by the STAR experiment with high precision. We...
The Forward Calorimeter (FoCal) is a high-granularity forward calorimeter to be installed as an ALICE upgrade subsystem during the LHC Long Shutdown 3 and take data during the LHC Run 4. It consists of a compact silicon-tungsten sampling electromagnetic calorimeter (FoCal-E) with pad and pixel readout layers to achieve high spatial and energy resolutions and a hadron calorimeter based on...
We discuss possible future studies of photon-photon (light-by-light)
scattering using planned FoCal and ALICE 3 detectors.
We include different mechanisms of $\gamma\gamma\to\gamma\gamma$
scattering such as double-hadronic photon fluctuations,
$t/u$-channel neutral pion exchange or resonance excitations
($\gamma \gamma \to R$) and deexcitation ($R \to \gamma \gamma$).
The broad range...
The ALICE experiment has undergone a major detector upgrade for Run 3, expanding its detection capabilities for a wide variety of studies. The new continuous readout, “trigger-less mode”, will significantly enhance the physics potential for ultra-peripheral collision analyses. In this talk, we will discuss some of the physics analyses that can be carried out in ultra-peripheral collisions...
One of the main goals of future electron-ion colliders is to improve our understanding of the structure of hadrons. We study the exclusive $\eta_c$ production by $\gamma^* \gamma$ nteractions in eA collisions and demonstrate that future experimental analysis of this process can be used to improve the description of the $\eta_c$ transition form factor. The rapidity, transverse momentum and...
Abstract to be provided
The study of photon-induced reactions in collisions of heavy nuclei at RHIC and the LHC has become an important direction of the research program of these facilities in recent years. In particular, the production of vector mesons in ultra-peripheral collisions (UPC) has been intensively studied. Owing to the intense photon fluxes, the two nuclei participating in such processes undergo...
The experimental study of photon – induced interactions in hadronic collisions became a reality in the last years, motivated by the possibility of improving our understanding of the Standard Model as well as by the opportunity to use these processes as an alternative tool to search for New Physics. Recent results provided the first observation of the light – by – light (LbL) scattering,...
Relativistic heavy-ion collisions generate extremely strong electromagnetic fields, providing an ideal environment to study the electromagnetic excitation of the vacuum. The Breit-Wheeler process, the lowest-order decay mode of the QED vacuum excitation into electron-positron pairs, has been experimentally verified by the STAR collaboration, stimulating further investigations of higher-order...
We study the lepton pair photoproduction in peripheral heavy-ion collisions based on the formalism in our previous work [Phys. Rev. D 104, 056011 (2021)]. We present the numerical results for the distributions of the transverse momentum, azimuthal angle and invariant mass for e+e− and μ+μ− pairs as functions of the impact parameter and other kinematic variables in Au+Au collisions. Our...
In ultraperipheral heavy-ion collisions, a photon emitted by one nucleus can interact with the other nucleus via photon-gluon fusion and lead to heavy quark production. The resulting yields of open charm or beauty depend on the photon flux and the gluon distribution within nuclei, enabling this process to probe the nuclear gluon distribution function. We extend the previous calculations for...
In 2017, the ATLAS collaboration published an article that confirmed the experimental phenomenon of light-by-light scattering [1]. This process entails the creation of photon pairs as a consequence of the interaction between strong electromagnetic fields. The effect was successfully recorded through the intense photon flux generated during ultra-peripheral collisions of heavy ions. Two...
We study the $J/\psi$ photoproduction in ultrarelativistic lead-lead collisions at the LHC energy. In the present approach, as an example, we use a simple model based on a vector dominance picture and multiple scattering of the hadronic (cc̄) state in a cold nucleus. Equivalent photon approximation in the impact parameter space, which successfully describes data for ultraperipheral processes,...
In the high energy limit, a boson has enough energy to fluctuate into a quark-antiquark pair, i.e., a color dipole, before interacting with the target. Therefore, the interaction between two bosons, such as $\gamma, W^\pm, Z$ or $g$, can be studied through the dipole-dipole interaction. In this work, we investigate these interactions in ultraperipheral collisions and estimate the double vector...
