Ultraperipheral Heavy Ion Collisions Research Articles

Exclusive Four-pion Photoproduction in Ultra-peripheral Heavy-ion Collisions at RHIC and LHC Energies

We study the photoproduction of exclusive $2\pi^+2\pi^-$ mesons in ultra-peripheral heavy-ion collisions at RHIC and LHC energies. Predictions in photon-nucleus interactions are calculated for various resonances at central and forward rapidities. The recent H1 preliminary data are utilized to improve the description of the poorly known $\gamma p \to 4\pi^\pm p$ process. We present the comparisons of our results to the available STAR data at RHIC, and made predictions for LHC energies.

Heavy quarkonia production at energies available at the CERN Large Hadron Collider and future electron-ion colliding facilities using basis light-front quantization wave functions

We study exclusive charmonium and bottomonium production in ultra-peripheral heavy-ion collisions and electron-ion collisions within the dipole picture. We employ heavy quarkonium light-front wavefunctions obtained within the basis light-front quantization framework, which has some features of the light-front holographic QCD. We focus on comparison with measurements of exclusive charmonium and bottomonium production in ultraperipheral $pp$, $p$Pb and PbPb collisions at LHC and find reasonable agreement with the cross sections. We also discuss the coherent production cross-section ratios of excited states to the ground state for charmonium and bottomonium, which exhibit insensitivity to the dipole model parameters. We show that electron-ion collisions provide an opportunity for a quantitative study of heavy quarkonium structure.

Probing the linear polarization of photons in ultraperipheral heavy ion collisions

We propose to measure the linear polarization of the external electromagnetic fields of a relativistic heavy ion through azimuthal asymmetries in dilepton production in ultraperipheral collisions. The asymmetries estimated with the equivalent photon approximation are shown to be sizable.

Inclusive dijet photoproduction in ultraperipheral heavy ion collisions at the CERN Large Hadron Collider in next-to-leading order QCD

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 comparable to the size of the calculated nuclear modifications of the dijet photoproduction cross section.

Light-by-light scattering in ultraperipheral heavy-ion collisions at low diphoton masses

We present a study of photon-photon scattering in the mass range $W_{\gamma\gamma} < 5$ GeV. We extend earlier calculations of this cross section for $W_{\gamma\gamma}>$ 5 GeV into the low mass range where photoproduction of the pseudoscalar resonances $\eta$, $\eta^{'}(958)$ contributes to two-photon final states. We present the elementary photon-photon cross section as a function of diphoton mass $M_{\gamma\gamma}$ arising from lepton and quark loop diagrams, and the visible cross section obtained with the gamma-gamma decay branching fractions of the resonances $\eta$, $\eta^{'}(958)$, $\eta_c(1S)$, $\eta_c(2S)$, $\chi_{c0}(1P)$. We derive the corresponding cross sections in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV by folding the elementary cross section with the heavy-ion photon fluxes. We consider the dominating background of the two-photon final state which arises from gamma decays of photoproduced $\pi^{0}$-pairs. Such $\pi^{0}$-pairs contribute to the background when only two of the four decay photons are within the experimental acceptance, while the other two photons escape undetected. We reduce this background by applying cuts on asymmetries of transverse momenta of the two photons and indicate how the background can be further suppressed using a multivariate sideband analysis. We present the cross section for the signal and the background at midrapidity $|\eta| <$ 0.9, and in the forward rapidity range 2.0 $< \eta <$ 4.5.

Coherent photoproduction of J/ψ in nucleus-nucleus collisions in the color dipole approach

We investigate the exclusive photoproduction of $J/\psi$-mesons in ultraperipheral heavy ion collisions in the color dipole approach. We first test a number of dipole cross sections fitted to inclusive $F_2$-data against the total cross section of exclusive $J/\psi$-production on the free nucleon. We then use the color-dipole formulation of Glauber-Gribov theory to calculate the diffractive amplitude on the nuclear target. The real part of the free nucleon amplitude is taken into account consistent with the rules of Glauber theory. We compare our results to recent published and preliminary data on exclusive $J/\psi$ corrections in ultraperipheral lead-lead collisions at $\sqrt{s_{NN}}=2.76 \, \rm{TeV}$ and $\sqrt{s_{NN}} = 5.02 \, \rm{TeV}$. Especially at high $\gamma A$ energies there is room for additional shadowing corrections, corresponding to triple-Pomeron terms or shadowing from large mass diffraction.

Recent Theoretical Results for Electromagnetically-induced Ultraperipheral Reactions of Heavy Ions

We briefly review our works on ultraperipheral heavy ion collisions. We discuss both $\gamma \gamma$ and rescattering of hadronic photon fluctuation induced by one nucleus in the collision partner. Production of one and two leptonic and pionic and $p \bar p$ pairs is discussed as an example of photon-photon processes. The production of single vector mesons ($\rho^0$ or $J/\psi$) is an example of the second category. The double-scattering mechanisms of two $\rho^0$ meson production is discussed in addition.

Importance of mesons in light-by-light scattering in ultraperipheral lead-lead collisions at the LHC

Possibility of PbPb→PbPbγγ measurement at smaller (&lt; 5 GeV) diphoton invariant mass will be presented. Analysis focuses only on ultraperipheral heavy-ion collisions. This estimate shows that the γγ → γγ collisions can be measured at the LHC by ALICE and LHCb experiments for diphoton invariant mass &gt; 2 GeV. Predictions for the γγ → η, γγ → η′ resonance scattering shows that these resonances can be measured with rather good statistics. A possible observation of peaks related to intermediate η, η′(958), ηc(1S), ηc(2S), χc0(1P) mesons will be presented too. Attempts of reduction of background which comes from dipion production will be considered.

Light-by-light Scattering in Lead--Lead Collisions in the ATLAS Experiment

Light-by-light (LbyL) scattering, ! , is a quantum-mechanical process, forbidden by the classical theory of electrodynamics, but possible in Quantum Electrodynamics via a loop diagram. Despite the small cross section, it is theoretically possible to observe this process in ultra-peripheral high-energy heavy-ion collisions. Based on 0.48 nb􀀀1 of 2015 Pb+Pb data, a first direct evidence of LbyL scattering was established by the ATLAS Collaboration in 2017. In total, 13 events were found in the signal region with a background expectation of 2:60:7 events. The excess corresponds to 4.4 significance over the background-only hypothesis. In November 2018, the new dataset of Pb+Pb collisions was collected by the ATLAS experiment with an integrated online luminosity of 1.7 nb􀀀1. This recent dataset has been employed to perform a preliminary study using the control sample from ! e+e􀀀 process.

Recent results for UPC at the LHC

Ultraperipheral heavy ion collisions are a source of photons which can collide with each other producing a pair of particles. This work will be focused on analysis of the light-by-light scattering. Here contribution from fermionic boxes, resonance scattering, VDM-Regge model, two-gluon exchange and pionic background will be compared. Each of these processes dominate at different ranges of two-photon invariant masses. Our calculated nuclear cross section is in good agreement with recently measured ATLAS and CMS data. Predictions including ALICE and LHCb experimental cuts for the next run at the LHC will be shown.

Equivalent photons at the LHC: pp(γγ) → ppl+l-, Pb Pb (γγ) → Pb Pb l+l-

The Large Hadron Collider is considered as a photon-photon collider with the photons produced in ultraperipheral collisions of protons or heavy ions. The equivalent photon approximation is applied to derive analytical formulae for the fiducial cross sections of reactions pp(γγ) → pp μ+μ- and Pb Pb (γγ) → Pb Pb μ+μ-. The results are compared to the measurements reported by the ATLAS collaboration.

Measurements of photo-nuclear jet production in Pb + Pb collisions with ATLAS

Ultra-peripheral heavy ion collisions provide a unique opportunity to study the parton distributions in the colliding nuclei via the measurement of photo-nuclear jet production. An analysis of jet production in ultra-peripheral Pb+Pb collisions at sNN=5.02 TeV performed using data collected with the ATLAS detector in 2015 is described. The data set corresponds to a total Pb+Pb integrated luminosity of 0.38 nb−1. The ultra-peripheral collisions are selected using a combination of forward neutron and rapidity gap requirements. The cross-sections, not unfolded for detector response, are compared to results from Pythia Monte Carlo simulations re-weighted to match a photon spectrum obtained from the STARlight model. Qualitative agreement between data and these simulations is observed over a broad kinematic range suggesting that using these collisions to measure nuclear parton distributions is experimentally realisable.

Probing subnucleon scale fluctuations in ultraperipheral heavy ion collisions

We show that introducing subnucleon scale fluctuations constrained by HERA diffractive J/Ψ production data significantly affects the incoherent diffractive J/Ψ production cross section in ultraperipheral heavy ion collisions. We find that the inclusion of the additional fluctuations increases the ratio of the incoherent to the coherent cross section approximately by a factor of 2, and modifies the transverse momentum spectra of the produced J/Ψ at momenta larger than the scale that corresponds to the distance scale of the subnucleonic fluctuations. We present predictions for J/Ψ production in ultraperipheral heavy ion collisions at sNN=5.02 TeV at the LHC and 200 GeV at RHIC.

Charmonium spectrum and diffractive production in a light-front Hamiltonian approach

We study exclusive charmonium production in diffractive deep inelastic scattering and ultra-peripheral heavy-ion collisions within the dipole picture. The mass spectrum and light-front wavefunctions of charmonium are obtained from the basis light-front quantization approach, using the one-gluon exchange interaction plus a confining potential inspired by light-front holography. We apply these light-front wavefunctions to exclusive charmonium production. The resulting cross sections are in reasonable agreement with electron-proton collision data at HERA and ultra-peripheral nucleus collision measurements at RHIC and LHC. The charmonium cross-section has model dependence on the dipole model. We observe that the cross-section ratio of excited states to the ground state has a weaker dependence than the cross-section itself. We suggest that measurements of excited states of heavy quarkonium production in future electron-ion collision experiments will impose rigorous constraints on heavy quarkonium light-front wave-functions, thus improving our understanding of meson structure, which eventually will help us develop a precise description of the gluon distribution function in the small-x regime.

The role of meson exchanges in light-by-light scattering

We discuss the role of meson exchange mechanisms in γγ→γγ scattering. Several pseudoscalar (π0, η, η′(958), ηc(1S), ηc(2S)), scalar (f0(500), f0(980), a0(980), f0(1370), χc0(1P)) and tensor (f2(1270), a2(1320), f2′(1525), f2(1565), a2(1700)) mesons are taken into account. We consider not only s-channel but also for the first time t- and u-channel meson exchange amplitudes corrected for off-shell effects including vertex form factors. We find that, depending on not well known vertex form factors, the meson exchange amplitudes interfere among themselves and could interfere with fermion-box amplitudes and modify the resulting cross sections. The meson contributions are shown as a function of collision energy as well as angular distributions are presented. Interesting interference effects separately for light pseudoscalar, scalar and tensor meson groups are discussed. The meson exchange contributions may be potentially important in the context of a measurement performed recently in ultraperipheral collisions of heavy ions by the ATLAS collaboration. The light-by-light interactions could be studied in future in electron–positron collisions by the Belle II at SuperKEKB accelerator.

Estimating nonlinear effects in forward dijet production in ultra-peripheral heavy ion collisions at the LHC

Using the framework that interpolates between the leading power limit of the color glass condensate and the high energy (or k_{T}) factorization we calculate the direct component of the forward dijet production in ultra-peripheral mathrm {Pb}–mathrm {Pb} collisions at CM energy 5.1,mathrm {TeV} per nucleon pair. The formalism is applicable when the average transverse momentum of the dijet system P_{T} is much bigger than the saturation scale Q_{s}, P_{T}gg Q_{s}, while the imbalance of the dijet system can be arbitrary. The cross section is uniquely sensitive to the Weizsäcker–Williams (WW) unintegrated gluon distribution, which is far less known from experimental data than the most common dipole gluon distribution appearing in inclusive small-x processes. We have calculated cross sections and nuclear modification ratios using WW gluon distribution obtained from the dipole gluon density through the Gaussian approximation. The dipole gluon distribution used to get WW was fitted to the inclusive HERA data with the nonlinear extension of unified BFKL + DGLAP evolution equation. The saturation effects are visible but rather weak for realistic p_{T} cut on the dijet system, reaching about 20% with the cut as low as 6,mathrm {GeV}. We find that the LO collinear factorization with nuclear leading-twist shadowing predicts quite similar effects.

Searching for Axionlike Particles with Ultraperipheral Heavy-Ion Collisions.

We show that ultraperipheral heavy-ion collisions at the Large Hadron Collider (LHC) can be used to search for axionlike particles with mass below 100GeV. The Z^{4} enhanced photon-photon luminosity from the ions provides a large exclusive production rate, with a signature of a resonant pair of back-to-back photons and no other activity in the detector. In addition, we present both new and updated limits from recasting multiphoton searches at LEP II and the LHC, which are more stringent than those currently in the literature for the mass range 100MeV to 100GeV.

Diffractive charmonium spectrum in high energy collisions in the basis light-front quantization approach

Using the charmonium light-front wavefunctions obtained by diagonalizing an effective Hamiltonian with the one-gluon exchange interaction and a confining potential inspired by light-front holography in the basis light-front quantization formalism, we compute production of charmonium states in diffractive deep inelastic scattering and ultra-peripheral heavy ion collisions within the dipole picture. Our method allows us to predict yields of all vector charmonium states below the open flavor thresholds in high-energy deep inelastic scattering, proton–nucleus and ultra-peripheral heavy ion collisions, without introducing any new parameters in the light-front wavefunctions. The obtained charmonium cross section is in reasonable agreement with experimental data at HERA, RHIC and LHC. We observe that the cross-section ratio σΨ(2s)/σJ/Ψ reveals significant independence of model parameters.

Charmonium production in ultra-peripheral heavy ion collisions with two-photon processes

We calculate the production of large-pT charmonium and narrow resonance state (exotic charmonium) in proton–proton, proton–nucleus, and nucleus–nucleus collisions with the semi-coherent two-photon interactions at Relativistic Heavy Ion Collider (RHIC), Large Hadron Collider (LHC), and Future Circular Collider (FCC) energies. Using the large quasi-real photon fluxes, we present the γγ→H differential cross section for charmonium and narrow resonance state production at large transverse momentum in ultra-peripheral heavy ion collisions. The numerical results demonstrate that the experimental study of ultra-peripheral collisions is feasible at RHIC, LHC, and FCC energies.

Mapping color fluctuations in the photon in ultraperipheral heavy ion collisions at the Large Hadron Collider

We model effects of color fluctuations (CFs) in the light-cone photon wave function and for the first time make predictions for the distribution over the number of wounded nucleons ν in the inelastic photon–nucleus scattering. We show that CFs lead to a dramatic enhancement of this distribution at ν=1 and large ν>10. We also study the implications of different scales and CFs in the photon wave function on the total transverse energy ΣET and other observables in inelastic γA scattering with different triggers. Our predictions can be tested in proton–nucleus and nucleus–nucleus ultraperipheral collisions at the LHC and will help to map CFs, whose first indications have already been observed at the LHC.