Equivalent Photon Approximation Research Articles

Spatial resolution of dijet photoproduction in near-encounter ultraperipheral nuclear collisions

We present next-to-leading order perturbative QCD predictions for inclusive dijet photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs) within the impact-parameter dependent equivalent photon approximation. Taking into account the finite size of both the photon-emitting nucleus and the target nucleus, we show that this process is sensitive to the transverse-plane geometry of the UPC events. We show that this leads to a sizable, 20–40% effect for large values of the zγ variable in the dijet photoproduction cross section in lead-lead UPCs at 5.02 TeV compared to the widely used pointlike approximation where the nuclear radius is accounted for only as a sharp cutoff in the photon flux calculation. This resolution of the spatial degrees of freedom is a result of having high-transverse-momentum jets in the final state, which at the large-zγ kinematics requires a highly energetic photon in the initial state, thus biasing the collisions to small impact-parameter “near-encounter” configurations. We further discuss the role of the forward-neutron event-class selection in isolating the photonuclear cross section in the nucleus-nucleus collisions, and employ the needed electromagnetic breakup survival factor in our predictions. Published by the American Physical Society 2024

Multiturn simulation of radiative Bhabha scattering in the equivalent photon approximation

In this paper, we present a Monte Carlo event generator for radiative Bhabha scattering, based on the method of equivalent photons, and optimized for multiturn tracking simulations in the framework. We demonstrate the accuracy of the event generator with successful benchmarks of the luminosity and event cross section against currently existing alternative tools. Consequently, we use it to investigate existing estimates of the radiative Bhabha scattering beam lifetime at the FCC-ee, as well as the impact of radiative Bhabha scattering on the beam dynamics, using full nonlinear tracking models. Published by the American Physical Society 2024

Resolved photons in Sherpa

We present the first complete simulation framework, in the event generator, for resolved photon interactions at next-to leading order accuracy. It includes photon spectra obtained through the equivalent-photon approximation, parton distribution functions to parametrize the hadronic structure of quasi-real photons, the matching of the parton shower to next-to leading order QCD calculations for resolved photon cross sections, and the modelling of multiple-parton interactions. We validate our framework against a wide range of photo-production data from and experiments, observing good overall agreement. We identify important future steps relevant for high-quality simulations at the planned Electron–Ion Collider.

Light-by-light scattering in ultraperipheral collisions of heavy ions at two future detectors

We discuss possible future studies of photon-photon (light-by-light) scattering using a planned FoCal and ALICE 3 detectors. We include different mechanisms of γγ→γγ scattering such as double-hadronic photon fluctuations, t/u-channel neutral pion exchange or resonance excitations (γγ→R) and deexcitation (R→γγ). The broad range of (pseudo)rapidities and lower cuts on transverse momenta open a necessity to consider not only dominant box contributions but also other subleading contributions. Here we include low mass resonant R=π0, η, η′ contributions. The resonance contributions give intermediate photon transverse momenta. However, these contributions can be eliminated by imposing windows on diphoton invariant mass. We study and quantify individual box contributions (leptonic, quarkish). The electron/positron boxes dominate at low Mγγ<1 GeV diphoton invariant masses. The PbPb→PbPbγγ cross section is calculated within equivalent photon approximation in the impact parameter space. Several differential distributions are presented and discussed. We consider four different kinematic regions. We predict a cross section in the (mb-b) range for typical ALICE 3 cuts, a few orders of magnitude larger than for the current ATLAS or CMS experiments. We also consider the two-π0 background which can, in principle, be eliminated at the new kinematical range for the ALICE 3 measurements by imposing dedicated cuts on diphoton transverse momentum and/or so-called vector asymmetry. Published by the American Physical Society 2024

Ultra-peripheral collisions of charged hadrons in extensive air showers

We discuss the electromagnetic collisions of high energy protons, pions and kaons with atmospheric nuclei. In particular, we use the equivalent photon approximation to estimate (i) the diffractive collisions where the projectile scatters inelastically off a nucleus, and (ii) the usual radiative processes (bremsstrahlung, pair production and photonuclear interactions) of these charged hadrons in the air. We then include the processes in the simulator AIRES and study how they affect the longitudinal development of extensive air showers. For 109-11 GeV proton primaries we find that they introduce a very small reduction (below 1%) in the average value of both X max and ΔX max. At a given shower age (relative slant depth from X max), these electromagnetic processes slightly increase the number of charged particles at the shower maximum and reduce the number of muons when it is old, decreasing by 1% the muon-to-(γ + e) near the ground level.

New constraint on neutrino magnetic moment and neutrino millicharge from LUX-ZEPLIN dark matter search results

Elastic neutrino-electron scattering represents a powerful tool to investigate key neutrino properties. In view of the recent results released by the LUX-ZEPLIN collaboration, we provide a first determination of the limits achievable on the neutrino magnetic moment and neutrino millicharge, whose effect becomes non-negligible in some beyond the Standard Model theories. In this context, we evaluate and discuss the impact of different approximations to describe the neutrino interaction with atomic electrons. The new LUX-ZEPLIN data allows us to set a very competitive limit on the neutrino magnetic moment when compared to the other laboratory bounds, namely ${\ensuremath{\mu}}_{\ensuremath{\nu}}^{\mathrm{eff}}<1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}{\ensuremath{\mu}}_{\mathrm{B}}$ at 90% C.L., which improves by a factor of 2.5 the Borexino collaboration limit and represents the second best world limit after the recent XENONnT result. Moreover, exploiting the so-called equivalent photon approximation, we obtain the most stringent limit on the neutrino millicharge, namely $|{q}_{\ensuremath{\nu}}^{\mathrm{eff}}|<1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{e}_{0}$ at 90% C.L., which represents a great improvement with respect to the previous laboratory bounds.

Production of tensor glueball in ultraperipheral pp, AuAu and PbPb collisions

In this work, we discuss the production of a tensor glueball in the proton–proton and nuclei–nuclei ultraperipheral collisions (UPCs). The cross section is calculated in the equivalent photon approximation (EPA). The total UPCs cross section is related to the matrix elements of gamma gamma rightarrow pi ^0 G_2 process, which can be described in terms of the neutral pion and tensor glueball G_2’s distribution amplitudes. The predictions for the rapidity distributions of UPCs cross sections and total cross sections are presented. The theoretical uncertainty of the whole calculation is considered. We consider the decay modes of the final-state tensor glueball and estimate the corresponding number of events. We estimate that there are about 10 tensor glueball G_2rightarrow phi phi signal events in PbPb collisions at the LHC. We also argue that the lead–lead UPC experiments at future HE-LHC and FCC experiments will provide larger opportunities to measure tensor glueballs.

Azimuthal decorrelation for photon induced dijet production in ultra-peripheral collisions of heavy ions

We study the azimuthal angular decorrelation of the dijet production via photon fusion in ultra-peripheral heavy ion collisions. The impact parameter dependent cross section of quark-antiquark pairs production is derived using the equivalent photon approximation, and the contribution from final-state QCD radiations to the azimuthal angular distribution are calculated within Soft-Collinear Effective Theory. We carry out the QCD resummation of large logarithms of the azimuthal angle as well as the jet radius at next-to-leading logarithmic accuracy. In the end we present the normalized differential cross section for azimuthal decorrelation of the dijet pair and find that our results are consistent with the measurements reported by the ATLAS collaboration.

Gamma-UPC: automated generation of exclusive photon-photon processes in ultraperipheral proton and nuclear collisions with varying form factors

The automated generation of arbitrary exclusive final states produced via photon fusion in ultraperipheral high-energy collisions of protons and/or nuclei, A B overset{upgamma upgamma}{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 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 (UPCs) at the Large Hadron Collider and Future Circular Collider. Total photon-fusion cross sections for the exclusive production of spin-0, 2 resonances (quarkonia, ditauonium, and Higgs boson; as well as axions and gravitons), and for pairs of particles (J/ψJ/ψ, WW, ZZ, Zγ, toverline{t} , HH) are presented. Differential cross sections for exclusive dileptons and light-by-light scattering are compared to LHC data. This development paves the way for the upcoming automatic event generation of any UPC final state with electroweak corrections at next-to-leading-order accuracy and beyond.

Top quark effective couplings from top-pair tagged photoproduction in pe− collisions

We present a detailed study, at the fast detector simulation level, of top-pair photoproduction in semileptonic mode at the Large Hadron-electron Collider (LHeC) and Future Circular Collider (FCC-he). We work in full tree-level QED, not relying on the equivalent photon approximation, taking into account the complete photoproduction kinematics. This allows us to define three photoproduction regions based on the angular acceptance range of the electron tagger. Those regions provide different degrees of sensitivity to top quark effective couplings. We focus on the $t\overline{t}\ensuremath{\gamma}$ dipole couplings and the left-handed vector $tbW$ coupling for which we determine limits at both energies and in different photoproduction regions. We find that the LHeC and FCC-he will yield tight direct bounds on top dipole moments, greatly improving on current direct limits from hadron colliders, and direct limits on the $tbW$ coupling as restrictive as those expected from the HL-LHC. We also consider indirect limits from $b\ensuremath{\rightarrow}s\ensuremath{\gamma}$ branching ratio and $CP$ asymmetry, that are well known to be very sensitive probes of top electromagnetic dipole moments.

Observing true tauonium via two-photon fusion at e+e− and hadron colliders

The feasibility of observing true tauonium, the bound state of two tau leptons, $\mathcal{T}_0\equiv(\tau^+\tau^-)_0$, via photon-photon collisions at $e^+e^-$ colliders and at the LHC, is studied. The production cross sections of the process $\gamma\gamma\to\mathcal{T}_0\to\gamma\gamma$ -- as well as those of all relevant backgrounds: spin-0 and 2 charmonium resonances decaying to diphotons, and light-by-light scattering -- are computed in the equivalent photon approximation for $e^+e^-$ collisions at BES III ($\sqrt{s} = 3.8$ GeV), Belle II ($\sqrt{s} = 10.6$ GeV), and FCC-ee ($\sqrt{s} = 91.2$ GeV), as well as for ultraperipheral p-p, p-Pb, and Pb-Pb collisions at the LHC. Despite small $\mathcal{T}_0$ production cross sections and a final state swamped by decays from overlapping pseudoscalar and tensor charmonium states -- the $\mathrm{\chi_{c2}}$, $\mathrm{\eta_{c}(2S)}$, and $\mathrm{\chi_{c0}}$ states have masses only 2.5, 84, and 139 MeV away, respectively, from the $\mathcal{T}_0$ peak -- evidence and observation of the ground state of the heaviest leptonium appears feasible at Belle II and FCC-ee, respectively, with in-situ high-precision measurements of the irreducible backgrounds.

Compton-like dark photon production in electron–nucleus collisions

The Compton-like production of massive dark photons is investigated in ultrarelativistic electron–ion collisions by considering the kinetic mixing between the dark photon and the standard model photon. The quasi-real photons in the heavy ion are described by the equivalent photon approximation, and the model is employed to calculate the integrated cross section and event rates as a function of the dark photon mass, m γ′, and mixing parameter, ɛ. Predictions are shown for electron–ion colliders (EICs) in the mass range 100 ⩽ m γ′ ⩽ 500 MeV. Numerical results are provided within the kinematic coverage of the planned machines: an EIC in China (EicC), a polarized EIC at Jefferson Lab (JLEIC), an EIC/USA (EIC), a large hadron electron collider (LHeC) and a future circular collider (FCC-eA). It complements existing search strategies for dark photons in the considered mass interval.

Photoproductions of dileptons, photons, and light vector mesons in p−p , p−Pb , and Pb−Pb collisions at Large Hadron Collider energies

The photoproduction of large $p_{T}$ dileptons, photons and light vector mesons in p-p, p-Pb and Pb-Pb collisions at LHC energies is studied, where the fragmentation processes and the ultra-incoherent photon channel are included. An exact treatment is developed for photoproduction processes in heavy ion collisions, which recovers the equivalent photon approximation (EPA) in the limit $Q^{2}\rightarrow0$ and can avoid double counting effectively. The full kinematical relations are also achieved. We present the results as the distributions in $Q^{2}$, $p_{T}$ and $y_{r}$, the total cross sections are also estimated. The numerical results indicate that: the contribution of photoproduction processes is evident in the large $p_{T}$ and $y_{r}$ regions, and starts to play a fundamental role in p-Pb collisions. The ultra-incoherent photon emission is a important channel of photoproduction processes, which can provide the meaningful contributions. EPA is only applicable in small $y$ and $Q^{2}$ domains, and is very sensitive to the values of $y_{\mathrm{max}}$ and $Q^{2}_{\mathrm{max}}$. The EPA errors appear when $y>0.29$ and $Q^{2}>0.1~\mathrm{GeV}^{2}$, and are rather serious in p-Pb and Pb-Pb collisions. When dealing with widely kinematical regions, the exact treatment needs to be adopted.

Lepton pair production in ultraperipheral collisions

The lepton pair production in ultraperipheral collisions is studied in the classical field approximation. We derive a general form of the cross section in terms of photon distributions that depend on the transverse momentum and coordinate based on the wave packet form of nuclear wave functions. Such a general form of the cross section in the classical field approximation contains the results of the generalized equivalent photon approximation (EPA) as well as the corrections beyond EPA in the Born approximation. By rewriting the general form of the cross section in light-cone coordinates, we find a good connection with the transverse momentum dependent distribution factorization formalism in the Born approximation. Our numerical results are consistent with current experimental data.

Relativistic impulse approximation in the atomic ionization process induced by millicharged particles

The millicharged particle has become an attractive topic to probe physics beyond the Standard Model. In direct detection experiments, the parameter space of millicharged particles can be constrained from the atomic ionization process. In this work, we develop the relativistic impulse approximation (RIA) approach, which can duel with atomic many-body effects effectively, in the atomic ionization process induced by millicharged particles. The formulation of RIA in the atomic ionization induced by millicharged particles is derived, and the numerical calculations are obtained and compared with those from free electron approximation and equivalent photon approximation. Concretely, the atomic ionizations induced by mllicharged dark matter particles and millicharged neutrinos in high-purity germanium (HPGe) and liquid xenon (LXe) detectors are carefully studied in this work. The differential cross sections, reaction event rates in HPGe and LXe detectors, and detecting sensitivities on dark matter particle and neutrino millicharge in next-generation HPGe and LXe based experiments are estimated and calculated to give a comprehensive study. Our results suggested that the next-generation experiments would improve 2-3 orders of magnitude on dark matter particle millicharge δχ than the current best experimental bounds in direct detection experiments. Furthermore, the next-generation experiments would also improve 2-3 times on neutrino millicharge δν than the current experimental bounds.

Lepton pair production through two photon process in heavy ion collisions

This paper investigates the electromagnetic production of lepton pairs with low transverse momentum in relativistic heavy ion collisions. We estimate the initial photons' transverse momentum contributions by employing models where the average transverse momentum squared of the incoming photon can be calculated in the equivalent photon approximation. We further derive an all order QED resummation for the soft photon radiation, which gives an excellent description of the ATLAS data in ultra-peripheral collisions at the LHC. For peripheral and central collisions, additional $p_T$-broadening effects from multiple interaction with the medium and the magnetic field contributions from the quark-gluon plasma are also discussed.

Investigating the production of leptoquarks by means of zeros of amplitude at photon electron collider

Leptoquarks belong to the possible candidates for explaining various anomalies in flavor physics. Nonetheless, their existence is yet to be confirmed on the experimental side. In this paper we show how zeros of the single-photon tree-level amplitude can be used to extract information as regards leptoquarks in the case of e–gamma colliders. A small number of standard model backgrounds keep the signal clean in this kind of colliders. Unlike other colliders, the zeros of the single-photon amplitude here depend on sqrt{s} and on the mass of leptoquark along with its electric charge. We perform a PYTHIA based simulation for reconstructing the leptoquark from its decay products of the first generation and estimating the background with luminosity of 100 hbox {fb}^{-1}. Our analysis is done for all the leptoquarks that can be seen at an e–gamma collider with three different masses (70 GeV, 650 GeV and 1 TeV) and three different center of momentum energies (200 GeV, 2 TeV and 3 TeV). The effects of non-monochromatic photons on the zeros of the amplitude under laser backscattering and the equivalent photon approximation have also been addressed.

Light-by-light scattering and spacetime noncommutativity

The aim of this article is to explore a potential usability of a photon-photon self-interaction from the noncommutative quantum electrodynamics (NCQED) in the case of light-by-light scattering ($\gamma\gamma\to\gamma\gamma$) in ultraperipheral Pb+Pb collisions, a reaction measured recently by ATLAS and planned for future experiments in hadron-hadron colliders. We compute the total cross section from both, the full one-loop standard model (SM) and the tree-level NCQED amplitudes, in the equivalent photon approximation with impact parameters, for various noncommutative scales, $\rm\Lambda_{NC}$, and incoming nuclear spin-energy combinations. We find that NCQED contribution to the cross section has considerable increase at diphoton invariant mass range higher than $\rm\Lambda_{NC}$, while the SM contribution is strongly suppressed in such region. Our results show that the current ATLAS $\rm\sqrt{s_{NN}} = 5.02$ TeV experiment can only probe $\rm\Lambda_{NC}<100$ GeV region. On the other hand, future hadron-hadron collider proposals could have the potential to extend to $\rm\Lambda_{NC}\lesssim 300$ GeV region, making the performance of $\rm Pb+Pb(\gamma\gamma)\to Pb+Pb\gamma\gamma$ scattering on testing space-time noncommutativity close to that of the previously proposed photon-photon mode of linear electron-positron collider.

Initial transverse-momentum broadening of Breit-Wheeler process in relativistic heavy-ion collisions

We calculate the cross section and transverse-momentum (P⊥) distribution of the Breit-Wheeler process in relativistic heavy-ion collisions and their dependence on collision impact parameter (b). To accomplish this, the Equivalent Photon Approximation (EPA) was generalized in a more differential way compared to the approach traditionally used for inclusive collisions. In addition, a lowest-order QED calculation with straightline assumption was performed as a standard baseline for comparison. The cross section as a function of b is consistent with previous calculations using the equivalent one-photon distribution function. Most importantly, the P⊥ shape from this model is strongly dependent on impact parameter and can quantitatively explain the P⊥ broadening observed recently by RHIC and LHC experiments. This broadening effect from the initial QED field strength should be considered in studying possible trapped magnetic field and multiple scattering in a Quark-Gluon Plasma (QGP). The impact-parameter sensitive observable also provides a controllable tool for studying extreme electromagnetic fields.

Equivalent photons in proton–proton and ion–ion collisions at the Large Hadron Collider

The equivalent photon approximation is used to calculate fiducial cross sections for dimuon production in ultraperipheral proton–proton and lead–lead collisions. Analytic formulas taking experimental cuts into account are derived. The results are compared with the measurements reported by the ATLAS collaboration.