Large-distance ultraperipheral collisions of two relativistic ions are considered. The clouds of photons surrounding the ions are responsible for their distant electromagnetic interaction. The perturbative approach and the method of equivalent photons are described. It is shown that the total cross section of these collisions increases rapidly with increasing energy and is especially large for heavy ions. Some experimental data and their comparison with theoretical approaches are described. Further proposals are discussed.
It is argued that the cross sections of ultraperipheral interactions of heavy nuclei can become comparable in value to those of their ordinary hadronic interactions at high energies. Simple estimates of corresponding "preasymptotic energy thresholds" are provided.The~method of equivalent photons is compared with the perturbative approach. The~situation at NICA/FAIR energies is discussed.
The comparison of photoproduction cross sections for $c\bar{c}$ and b-b(bar) in PbPb and pPb collisions can give sensitivity to nuclear shadowing effects. The photoproduction of vector mesons is even more sensitive to the underlying gluon distributions. In this study we present the cross sections and rapidity dependence of the photoproduction of heavy quarks and exclusive production of vector mesons in ultraperipheral pPb and PbPb collisions at the Large Hadron Collider at sqrt(s_NN)=5 TeV and sqrt(s_NN)=2.76$ TeV, respectively. The potentials of using these processes for constraining nuclear …
It is advocated that geometry of the interaction region of two heavy nuclei colliding at large impact parameters is important for the relative role of light-by-light scattering and QCD-initiated processes. Exclusive production of resonances is possible by dense electromagnetic fields in the interior space between the nuclei. The cross section of these processes is evaluated and some examples are considered. It is speculated that the exclusive production of $ρ^0$-mesons by two-photon processes forbidden by the Landau-Yang rule may become allowed within strong magnetic fields.
Using the Gribov-Glauber model for photon-nucleus scattering and a generalization of the vector meson dominance model for the hadronic structure of the photon, we make predictions for the cross section of incoherent $ρ$ photoproduction in Pb-Pb ultraperipheral collisions (UPCs) in the Large Hadron Collider kinematics. We find that the effect of the inelastic nuclear shadowing is significant and leads to an additional 25% suppression of the incoherent cross section. Comparing our predictions to those of the STARlight Monte Carlo framework, we observe very significant differences.
We make predictions for cross sections of $ρ$ and $ϕ$ vector meson photoproduction in ultraperipheral Xe-Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV. Analyzing the momentum transfer distribution of $ρ$ mesons in this process, we explore the feasibility of extracting the nuclear density of $^{129}$Xe, which is needed in searches for dark matter with Xenon-based detectors.
The automated generation of arbitrary exclusive final states produced via photon fusion in ultraperipheral high-energy collisions of protons and/or nuclei is implemented in the MadGraph5_aMC@NLO and HelacOnia Monte Carlo codes. Cross sections are calculated in the equivalent photon approximation using $γ$ fluxes derived from electric dipole and charge form factors, and incorporating hadronic survival probabilities. Multiple examples of $γγ$ cross sections computed with this setup, named gamma-UPC, are presented for proton-proton, proton-nucleus, and nucleus-nucleus ultraperipheral collisions (…
We show that the recent ALICE measurements of exclusive J/ψ production in ultraperipheral PbPb collisions at 2.76 TeV provide the first direct experimental evidence for the strong nuclear gluon shadowing in lead at $x \sim 10^{-3}$. The evidence is based on the comparison of the nuclear suppression factor S(x\approx 0.001)=0.61^{+0.05}_{-0.04} found in the analysis of the coherent J/ψ photoproduction cross sections measured by ALICE with the nuclear gluon shadowing predicted by the global fits of nuclear parton distributions and by the leading twist theory of nuclear shadowing.
Photoproduction of heavy quarks in ultraperipheral collisions can help elucidate important features of the physics of heavy quarks in Quantum Chromodynamics (QCD). Due to the dependence on parton distributions it can also potentially offer some constraining ability in the determination of nuclear parton distributions. In the present study we consider next-to-leading order (NLO) photoproduction of heavy quarks in ultraperipheral proton-proton (pp), proton-nucleus (pA), and nucleus-nucleus (AA) collisions at the CERN Large Hadron Collider (LHC). Total cross sections and rapidity distributions ar…
Photoproduction of heavy quarks and exclusive production of vector mesons in ultraperipheral proton-nucleus and nucleus-nucleus collisions depend significantly on nuclear gluon distributions. In the present study we investigate quantitatively the extent of the applicability of these processes at the Large Hadron Collider (LHC) in constraining the shadowing component of nuclear gluon modifications.
We study exclusive $π^+ π^-$ pair production near $ρ^0$ resonance peak in ultraperipheral heavy ion collisions. Pion pair can either be produced via two photon fusion process or from the decay of $ρ^0$ in photon-nuclear reaction. At very low pair transverse momentum, the electromagnetic and nuclear amplitudes become comparable. We show that the Coulomb nuclear interference amplitude gives rise to sizable $\cos ϕ$ and $\cos 3ϕ$ azimuthal asymmetries, which can be used to constrain the phase of the dipole-nucleus scattering amplitude.
Exclusive photoproduction of vector mesons in the perturbative two-gluon exchange formalism depends significantly on nucleon and nuclear gluon distributions. In the present study we calculate total cross sections and rapidity distributions of $J/ψ(1s)$, $ψ(2s)$, $Υ(1s)$, $Υ(2s)$, and $Υ(3s)$ in ultraperipheral proton-lead (pPb) and lead-lead (PbPb) collisions at the CERN Large Hadron Collider (LHC) at $\sqrt{s_{_{NN}}}=5$ TeV and $\sqrt{s_{_{NN}}}=2.76$ TeV respectively. Effects of gluon shadowing are investigated and potentials for constraining nuclear gluon modifications are discussed.
Within the leading logarithmic approximation of perturbative QCD and the leading twist approach to nuclear shadowing, we compare the cross sections of photoproduction of $J/ψ$ and $ψ(2S)$ mesons on nuclei in Pb-Pb UPCs in the LHC kinematics. We find that the nuclear suppression of these cross sections due to the nuclear gluon shadowing is large and rather similar in the $J/ψ$ and $ψ(2S)$ cases.
We argue that with an increase of the collision energy, elastic photoproduction of $ρ$ mesons on nuclei becomes affected by the significant cross section of photon inelastic diffraction into large masses, which results in the sizable inelastic nuclear shadowing correction to $σ_{γA \to ρA}$ and the reduced effective $ρ$-nucleon cross section. We take these effects into account by combining the vector meson dominance model, which we upgrade to include the contribution of high-mass fluctuations of the photon according to QCD constraints, and the Gribov-Glauber approximation for nuclear shadowing…
We compute the cross section of inclusive dijet photoproduction in ultraperipheral Pb-Pb collisions at the LHC using next-to-leading order perturbative QCD. We demonstrate that our theoretical calculations provide a good description of various kinematic distributions measured by the ATLAS collaboration. We find that the calculated dijet photoproduction cross section is sensitive to nuclear modifications of parton distribution functions (PDFs) at the level of 10 to 20%. Hence, this process can be used to reduce uncertainties in the determination of these nuclear PDFs, whose current magnitude is…
The $D$-wave admixture in quarkonium wave functions is acquired from the photonlike structure of $V\to Q\bar Q$ transition in the light-front frame widely used in the literature. Such a $D$-wave ballast is not justified by any nonrelativistic model for $Q-\bar Q$ interaction potential and leads to falsified predictions for the cross sections in heavy quarkonium production in ultra-peripheral nuclear collisions. We analyze this negative role of $D$-wave contribution by comparing with our previous studies based on a simple non-photon-like "$S$-wave-only" $V\to Q\bar Q$ transition in the $Q\bar Q…
Threshold behavior of the cross sections of ultraperipheral nuclear interactions is studied. Production of $e^+e^-$ and $μ^+μ^-$ pairs as well as $π^0$ and parapositronium is treated. The values of corresponding energy thresholds are presented and the total cross sections of these processes at the newly constructed NICA and FAIR facilities are estimated.
We point out a significance of ultraperipheral photoproduction of vector mesons in the Coulomb field of nuclei as a means of measuring the radius of the neutral vector meson. This new contribution to the production amplitude is very small compared to the conventional diffractive amplitude, but because of large impact parameters inherent to the ultraperipheral Coulomb mechanism its impact on the diffraction slope is substantial. We predict appreciable and strongly energy dependent increase of the diffraction slope towards very small momentum transfer.The magnitude of the effect is proportional …
It is shown that production of low-mass $e^+e^-$-pairs in ultraperipheral nuclear collisions is enhanced due to the Sommerfeld-Gamow-Sakharov (SGS) factor. This effect is especially strong near the threshold of creation of unbound $e^+e^-$-pairs with low masses in the two-photon fusion. Coulomb attraction of the non-relativistic components of such pairs may lead to the increased intensity of 511 keV photons. It can be recorded at the NICA collider and has some astrophysical implications. The analogous effect can be observed at LHC in dilepton production.
We discuss production of two $μ^+μ^-$ pairs in ultraperipheral ultrarelativistic heavy ion collisions at the LHC. We take into account electromagnetic (two-photon) double-scattering production and for a first time direct $γγ$ production of four muons in one scattering. We study the unexplored process $γγ\to μ^+μ^-μ^+μ^-$. We present predictions for total and differential cross sections. Measurable nuclear cross sections are obtained and corresponding differential distributions and counting rates are presented.
By using the theoretical approaches describing well the available data on $t$-integrated coherent photoproduction of light and heavy vector mesons in Pb-Pb ultraperipheral collisions (UPCs) at the Large Hadron Collider (LHC) in Run 1, we calculate the momentum transfer distributions for this process for $ρ$ and $J/ψ$ vector mesons in the kinematics of Run 2 at the LHC. We demonstrate that nuclear shadowing not only suppresses the absolute value of the cross sections, but also shifts the momentum transfer distributions toward smaller values of the momentum transfer $|t|$. This result can be int…
In this paper we investigate the production of a true muonium state, which is an atom consisting of a $μ^+ μ^-$ bound state, by $γγ$ interactions in ultraperipheral $PbPb$ collisions considering an accurate treatment of the absorptive corrections and for the nuclear form factor. The rapidity distributions and cross sections are estimated considering the RHIC, LHC and FCC energies. Our results indicate that the experimental analysis can be useful to observe, for the first time, the true muonium state.
We argue that the STAR data on $J/ψ$ photoproduction in $d+Au$ ultraperipheral collisions (UPCs) at the momentum transfer $t \neq 0$ primarily probes elastic and possibly nucleon-dissociative $J/ψ$ photoproduction on quasi-free nucleons, while being essentially insensitive to expectedly weak nuclear modifications of the gluon distribution in the deuteron. Analyzing the STAR detector capabilities for selection of UPC events, we explore the possibility that the contribution of nucleon-dissociative $J/ψ$ photoproduction is negligibly small and obtain a very good description of the $t$ dependence …
In this paper we perform a systematic study of the exclusive dilepton production by $γγ$ interactions in $PbPb$ collisions at the LHC Run 2 energies considering different levels of precision for the treatment of the absorptive corrections and for the nuclear form factor. The rapidity and invariant mass distributions are estimated taking into account the experimental cutoffs and a comparison with the recent ALICE and ATLAS data for the $e^+ e^-$ and $μ^+ μ^-$ production is presented.
We calculate the cross section of inclusive dijet photoproduction in ultraperipheral collisions (UPCs) of heavy ions at the CERN Large Hadron Collider using next-to-leading order perturbative QCD and demonstrate that it provides a good description of the ATLAS data. We study the role of this data in constraining nuclear parton distribution functions (nPDFs) using the Bayesian reweighting technique and find that it can reduce current uncertainties of nPDFs at small $x$ by a factor of 2. We also make predictions for diffractive dijet photoproduction in UPCs and examine its potential to shed ligh…
We make predictions for the cross sections of coherent photoproduction of $ρ$, $ϕ$, $J/ψ$, $ψ(2S)$, and $Υ(1S)$ mesons in Pb-Pb ultraperipheral collisions (UPCs) at $\sqrt{s_{NN}}=5.02$ TeV in the kinematics of run 2 at the Large Hadron Collider extending the approaches successfully describing the available Pb-Pb UPC data at $\sqrt{s_{NN}}=2.76$ TeV. Our results illustrate the important roles of hadronic fluctuations of the photon and inelastic nuclear shadowing in photoproduction of light vector mesons on nuclei and the large leading twist nuclear gluon shadowing in photoproduction of quarkon…
Perturbative and non-perturbative terms of the cross sections of ultraperipheral production of lepton pairs in ion collisions are taken into account. It is shown that production of low-mass $e^+e^-$ pairs is strongly enhanced (compared to perturbative estimates) due to the non-perturbative Sommerfeld-Gamow-Sakharov (SGS) factor. Coulomb attraction of the non-relativistic components of those pairs leads to the finite value of their mass distribution at lowest masses. Their annihilation can result in the increased intensity of 511 keV photons. It can be recorded at the NICA collider and is espec…
In this letter we propose the search of dark photons in the decay of pions produced by $γγ$ interactions in ultraperipheral $PbPb$ collisions. The cross section is estimated considering an accurate treatment for the absorptive corrections and for the nuclear form factor. Predictions for the event rates are presented considering the expected luminosities for the LHC, High -- Luminosity LHC and High -- Energy LHC as well as for the Future Circular Collider. Our results indicate that a future experimental analysis of the pion production in ultraperipheral heavy ion collisions can be useful to pro…
The double meson production in ultraperipheral heavy ions collisions is addressed, focusing on the particular case of rho-J/Psi from two-photon reactions. The cross section at photon level is obtained using distinct parameterizations for the gluon distribution on the light meson. The resulting estimates for the nuclear case are presented and discussed. As a by product, we estimate the double rho production cross section using the Pomeron-exchange factorization relations.
We discuss hadron production in the forward rapidity region in ultraperipheral proton-lead collisions at the LHC and proton-gold collisions at RHIC. Our discussion is based on the Monte Carlo simulations of the interactions of virtual photons emitted by a fast moving nucleus with a proton beam. We simulate the virtual photon flux with the STARLIGHT event generator and then particle production with the SOPHIA, DPMJET, and PYTHIA event generators. We show the rapidity distributions of charged and neutral particles, and the momentum distributions of neutral pions and neutrons at forward rapiditie…
The heavy quark production in ultraperipheral heavy ion collisions is investigated, in particular we focus on the results from the coherent interactions given by the two-photon process. One addresses the heavy quark total cross sections at photon level considering the saturation model and the BFKL dynamics in the color dipole picture. The corresponding cross sections at nuclear level are presented. It is verified that the QCD dynamics implies an enhancement of the cross section in comparison with previous calculations.
We make predictions for the cross sections of diffractive dijet photoproduction in $pp$, $pA$ and $AA$ ultraperipheral collisions (UPCs) at the LHC during Runs 1 and 2 using next-to-leading perturbative QCD. We find that the resulting cross sections are sufficiently large and, compared to lepton-proton scattering at HERA, have an enhanced sensitivity to small observed momentum fractions in the diffractive exchange, commonly denoted $z_{P}^{\rm jets}$, and an unprecedented reach in the invariant mass of the photon-nucleon system $W$. We examine two competing schemes of diffractive QCD factoriza…
The exclusive $Υ$ photoproduction in ultraperipheral proton-nucleus and nucleus-nucleus collisions at LHC energies is being investigated using pQCD framework which constrain the gluon distribution in the proton and nuclei at low $x$. Rapidity distributions of $Υ$ photoproduction with different gluon distribution parameterization and gluon shadowing for nuclear PDFs including photon flux suppression for strong interaction, are being presented at LHC energies. Predictions done for ultraperipheral collisions at $\sqrt s$=$8.16$ TeV in pPb, and at $\sqrt s$=$5.02$ TeV in PbPb collisions which are …
We discuss nuclear cross sections for $AA \to AAV$ and $AA \to AAVV$ reactions with one or two vector mesons in the final state. Our analysis is done in the impact parameter space equivalent photon approximation. This approach allows to consider the above processes taking into account distance between colliding nuclei. We consider both ultraperipheral and semi-central collisions. We are a first group which undertook a study of single $J/ψ$ photoproduction for different centrality bins. We show that one can describe new ALICE experimental data by including geometrical effects of collisions in t…
Exclusive process $γγ\rightarrow J/ψ+γ$ production in ultraperipheral proton and nuclear collisions at the HL-LHC and FCC
School of Physics, Liaoning University（辽宁大学物理学院）；Department of Physics and Electronics, School of Mathematics and Physics, Beijing University of Chemical Technology（北京化工大学数学与科学学院物理电子系）；School of Physics and Electronic Technology, Liaoning Normal University（辽宁师范大学物理与电子技术学院）
Diffractive vector meson photo-production in oxygen-oxygen and neon-neon ultraperipheral collisions at energies available at the CERN Large Hadron Collider
In this paper, we discuss the exclusive photoproduction of ground and excited states of $ψ(1S,2S)$ and $Υ(1S,2S)$ in ultraperipheral collisions (UPCs). Using the potential model 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/ψ(1S,2S) $ and $Υ(1S)$ in $γp$ collisions. We extend the calculations to the nuclear target case applying them to $AA$ UPCs with the use of the shadowing and finite coherence length effects fitted to the data. Our results are compared to the recent LHC data, in both incoherent…
A brief introduction to the physics of ultraperipheral collisions at collider energies is given. Photon-hadron (proton/ nucleus) and photon-photon interactions can be studied in a hitherto unexplored energy regime.
Using the general notion of cross section fluctuations in hadron--nucleus scattering at high energies, we derive an expression for the cross section of incoherent $J/ψ$ photoproduction on heavy nuclei $dσ_{γA \to J/ψY}/dt$, which includes both elastic $dσ_{γp \to J/ψp}/dt$ and proton-dissociation $dσ_{γp \to J/ψY}/dt$ photoproduction on target nucleons. We find that with good accuracy, $dσ_{γA \to J/ψY}/dt$ can be expressed as a product of the sum of the $dσ_{γp \to J/ψp}/dt$ and $dσ_{γp \to J/ψY}/dt$ cross sections, which have been measured at HERA, and the common nuclear shadowing factor, wh…
We consider $J/ψ$ photoproduction in ion--ion ultraperipheral collisions (UPCs) at the LHC and RHIC in the coherent and incoherent quasielastic channels with and without accompanying forward neutron emission and analyze the role of nuclear gluon shadowing at small $x$, $x=10^{-4}-10^{-2}$, in these processes. We find that despite the good agreement between large nuclear gluon shadowing and the ALICE data in the coherent channel, in the incoherent channel, the leading twist approximation predicts the amount of nuclear suppression which is by approximately a factor of $1.5$ exceeds that seen in …
We present the first NLO pQCD study of coherent exclusive $J/ψ$ photoproduction in ultraperipheral heavy-ion collisions (UPCs) at the LHC. Taking the generalized parton distributions (GPDs) in their forward limit, as parton distribution functions (PDFs), we quantify the NLO contributions in the rapidity-differential cross section, show that the real part of the amplitude must not be neglected, study the gluon and quark contributions, chart the scale-choice and PDF uncertainties, and compare the NLO results with LHC and HERA data. We show that the scale dependence is significant but a scale cho…
We present a new approach for calculating electromagnetic excitation of nuclei as well as probabilities of emission and distributions of neutrons from decays of excited nuclear systems for ultrarelativistic, ultraperipheral heavy ion collisions. Excitation functions for $γ$ + Pb $\to$ Pb$^*$ are parametrized using physics-motivated components: excitation of giant resonances, quasi-deuteron absorption mechanism, excitation of nucleon resonances as well as high-energy dissociation of protons and neutrons. Neutron emission (up to 10 neutrons) from low-energy excitations of $^{208}$Pb is modelled …
We discuss the physics of large impact parameter interactions at the LHC: ultraperipheral collisions (UPCs). The dominant processes in UPCs are photon-nucleon (nucleus) interactions. The current LHC detector configurations can explore small $x$ hard phenomena with nuclei and nucleons at photon-nucleon center-of-mass energies above 1 TeV, extending the $x$ range of HERA by a factor of ten. In particular, it will be possible to probe diffractive and inclusive parton densities in nuclei using several processes. The interaction of small dipoles with protons and nuclei can be investigated in elasti…
We argue that already the first year run at the LHC with 3.5 TeV proton beams will provide a quick and effective way to test the dynamics of the color dipole-gluon interactions and the small x behavior of the gluon density in proton from study of the vector meson photoproduction in ultraperipheral proton-proton collisions.
Ultraperipheral collisions at collider energies are a useful tool to study photon-hadron (proton/nucleus) and photon-photon interactions in a hitherto unexplored energy regime. Theoretical tools to study these processes are briefly described. Some current results and problems are discussed
We present predictions for the rapidity-differential cross sections of exclusive $Υ$ photoproduction in ultraperipheral collisions (UPCs) of lead ions at the Large Hadron Collider (LHC). We work in the framework of collinear factorization at next-to-leading order (NLO) in perturbative QCD, modeling the generalized parton distributions (GPDs) through the Shuvaev transform of nuclear parton distribution functions (nPDFs). While the effects due to the GPD modeling turn out to be small, the direct NLO predictions still carry significant nPDF-originating uncertainties and depend strongly on the cho…
We present calculations of coherent photoproduction of vector mesons ($J/Ψ$ and $Υ$) with leading-order parton distribution functions to check new kinds of corrections of the DGLAP equations and nuclear modifications. The input gluon distribution of the proton is the dynamical parton model from the DGLAP equations with GLR-MQ-ZRS (Gribov-Levin-Ryskin, Mueller-Qiu, Zhu-Ruan-Shen) modifications. From comparison between several other gluon distribution models, we find that the dynamical gluon distribution fits with the results of meson photoproduction experiments in the high energy region. The ca…
With the attempts of extending the hydrodynamic framework of heavy-ion collision to proton-proton and other small and low energy systems, we are confronted with the question of how small the system can get and still be safely modelled as a fluid. One of the transport coefficients required in the $2^{nd}$ order relativistic viscous hydrodynamics is the shear relaxation time, inclusion of which solves the causality violation problem in the Navier-Stokes equation. In phenomenological studies this coefficient has been taken as a constant and much attention has gone into finding and fixing the shea…
