Equivalent Photon Approximation Research Articles

Beam normal spin asymmetry in the quasireal Compton scattering approximation

he two-photon exchange contribution to the single spin asymmetries with the spin orientation normal to the reaction plane is discussed for elastic electron-proton scattering in the equivalent photon approximation. In this case, the hadronic part of the two-photon exchange amplitude describes real Compton scattering (RCS). We show that in the case of the beam normal spin asymmetry this approximation selects only the photon helicity flip amplitudes of RCS. At low energies, we make use of unitarity and estimate the contribution of the piN multipoles to the photon helicity flip amplitudes. In the Regge regime, the quasi-RCS (QRCS) approximation allows for a contribution from two-pion exchange, and we provide an estimate of such contributions.

Testing the equivalent photon approximation of the proton in the process $ep \rightarrow \nu W X$

The accuracy of the equivalent photon approximation (EPA) of the proton in describing the inelastic process $ep\to \nu W X$ is investigated. In particular, the scale dependence of the corresponding inelastic photon distribution is discussed. Furthermore, an estimate of the total number of events, including the ones coming from the elastic and quasi-elastic channels of the reaction, is given for the HERA collider.

Production ofe+e−pairs accompanied by nuclear dissociation in ultraperipheral heavy-ion collisions

We present data on e+e− pair production accompanied by nuclear breakup in ultraperipheral gold-gold collisions at a center of mass energy of 200GeV per nucleon pair. The nuclear breakup requirement selects events at small impact parameters, where higher-order diagrams for pair production should be enhanced. We compare the data with two calculations: one based on the equivalent photon approximation, and the other using lowest-order quantum electrodynamics (QED). The data distributions agree with both calculations, except that the pair transverse momentum spectrum disagrees with the equivalent photon approach. We set limits on higher-order contributions to the cross section.Received 7 April 2004DOI:https://doi.org/10.1103/PhysRevC.70.031902©2004 American Physical Society

Suppressing the background process to QED Compton scatteringfor delineating the photon content of the proton

We investigate the QED Compton process (QEDCS) in $e p \rightarrow e \gamma p$ and $e p \rightarrow e \gamma X$ , together with the major background coming from virtual Compton scattering (VCS), where the photon is emitted from the hadronic vertex. We suggest new kinematical constraints which suppress the VCS background and are furthermore suitable for the extraction of the equivalent photon content of the proton at the HERA collider. We show that the cross section, commonly expressed in terms of the proton structure functions, is reasonably well described by the equivalent photon approximation of the proton, also in the inelastic channel in the proposed kinematical region.

Energy dependence of Cronin momentum in saturation model for p + A and A + A collisions

We investigate the QED Compton process (QEDCS) in $e p \to e \gamma p$ and $e p \to e \gamma X$, together with the major background coming from the virtual Compton scattering (VCS), where the photon is emitted from the hadronic vertex. We suggest new kinematical constraints which suppress the VCS background and are furthermore suitable for the extraction of the equivalent photon content of the proton at the HERA collider. We show that the cross section, commonly expressed in terms of the proton structure functions, is reasonably well described by the equivalent photon approximation of the proton, also in the inelastic channel in the proposed kinematical region.

Manifestly covariant analysis of the QED Compton processin $ep\rightarrow e \gamma p$ and $e p \rightarrow e \gamma X $

We calculate the unpolarized QED Compton scattering cross section in a manifestly covariant way. Our approach allows a direct implementation of the specific kinematical cuts imposed in the experiments, {\it e. g.} HERA-H1. We compare the 'exact' cross section in terms of the structure functions $F_{1,2} (x_B,Q^2)$, assuming the Callan-Gross relation, with the one obtained using the equivalent photon approximation (EPA) as well as with the experimental results. We find that the agreement with the EPA is better in $x_{\gamma}$ bins, where $x_{\gamma}$ is the fraction of the longitudinal momentum of the proton carried by the virtual photon, compared to the bins in the leptonic variable $x_l$.

Delineating the polarized and unpolarized photon distributions of the nucleon in eN collisions

The production rates of lepton-photon and dimuon pairs at the HERA collider and the HERMES experiment are evaluated in the leading order equivalent photon approximation. It is shown that the production rates are sufficient to facilitate the extraction of the polarized and unpolarized equivalent photon distributions of the proton and neutron in the available kinematical regions. It is pointed out that these results indicate the possibility of additional, independent, tests concerning the unpolarized and polarized structure functions F 1,2 N and g 1,2 N , respectively, of the nucleon.

Two- and three-photon fusion in relativistic heavy-ion collisions

The production of mesons in ultraperipheral collisions of relativistic heavy ions is reanalyzed using a projection technique to calculate the amplitudes for the appropriate Feynman diagrams. The virtuality of the exchanged photons is fully accounted for in this approach. In the case of two-photon fusion, it is explicitly shown that the inclusion of nuclear form factors validates the equivalent photon approximation. However, this does not apply to three-photon fusion cross sections. The cross section of J/ ψ production in ultraperipheral collisions at RHIC and LHC are shown to be much smaller than the cross sections for the production of C-even mesons of similar masses.

Probing the parton densities of polarized photons at a linear e+e− collider

The present theoretical status of spin-dependent parton densities Δf γ ( x, Q 2) of circularly polarized photons is briefly reviewed. It is then demonstrated that measurements of the deep-inelastic spin asymmetry A 1 γ ≅ g 1 γ / F 1 γ and of di-jet rapidity distributions at a future linear e + e − collider appear to be particularly suited for a determination the spin-dependent photonic quark and gluon densities, respectively. Special emphasis is devoted to a comparison of the different sources of polarized photons at a linear collider: the equivalent photon approximation and backscattered laser (Compton) photons. It is shown that backscattered laser photons are highly favorable, even indispensable, for decent measurements of the Δf γ ( x, Q 2).

Semi-analytic calculations for the impact parameter dependence of electromagnetic multi-lepton pair production

We provide a new general semi-analytic derivation of the impact parameter dependence of lowest order electromagnetic lepton-pair production in relativistic heavy-ion collisions. By using this result we have also calculated the related analytic multiple-pair production in the two-photon external-field model. We have compared our results with the equivalent-photon approximation and other calculations.

Production of low mass electron pairs due to the photon-photon mechanism in central collisions

We calculate the probability for dilepton production in central relativistic heavy ion collisions due to the gamma-gamma mechanism. This is a potential background to more interesting mechanisms. We find that this mechanism is negligible in the CERES experiments. Generally, the contribution due to this mechanism is small in the central region, while it can be large for small invariant masses and forward or backward rapidities. A simple formula based on the equivalent photon approximation and applications to a possible scenario at RHIC are also given.

Multi-pair production cross sections at ultra-relativistic energies

We provide a new semi-analytic calculations for the electromagnetic multi-pair productions in ultra-relativistic heavy-ion collisions. We have compared our results with the equivalent-photon approximation and path integral method.

Bremsstrahlung from electrons and positrons in peripheral relativistic heavy-ion collisions

We study the spectrum of the bremsstrahlung photons coming from the electrons and positrons, which are produced in the strong electromagnetic fields present in peripheral relativistic heavy ion collisions. We compare different approaches, making use of the exact pair production cross section in heavy ion collisions as well as the double equivalent photon approximation.

Bethe-Heitler cross-section for very high photon energies and large muon scattering angles

The cross-section for the process $\gamma + A \to \mu^+ + \mu^- + X$ is studied where the photon energy is of the order of several hundreds of GeV and where one of the leptons is scattered to large angles. This is of practical importance for muon shielding calculations at future linear electron colliders. In addition to the photon pole contribution which was previously considered especially by Y.S.Tsai, we identify another component due to the muon pole (equivalent photon and equivalent muon approximation). This is discussed following the usual Bethe-Heitler formula. Then we give practically useful formulae for inclusive lepton (muon) production along with some numerical examples.

Electromagnetic Lepton-Pair Production in Relativistic Heavy-Ion Collisions

In relativistic heavy-ion collisions, the strong Lorentz-contracted electromagnetic fields are capable of producing copious numbers of lepton pairs through the two-photon mechanism. We provide a new and more general derivation of the Poisson distribution for multiple-pair production and develop a hybrid Monte Carlo technique for calculating the impact parameter dependence and the related multiple-pair production in the two-photon external-field model. Computations for 200 GeV/nucleon sulfur incident on a gold target are compared with recent data. There is good agreement for all but the most energetic pairs, where the theory underpredicts the data. We investigate the ability of the equivalent-photon approximation to predict the impact-parameter dependence of the process, as well as multipair production. We find that the approximation is limited in its ability to address these phenomena.

Production of boundμ+μ−systems in relativistic heavy ion collisions

Dimuonium (the bound system of two muons, $\mu^+\mu^-$-atom) has not been observed yet. In this paper we discuss the electromagnetic production of dimuonium at RHIC and LHC in relativistic heavy ion collisions. The production of parastates is analyzed in the equivalent photon approximation. For the treatment of orthostates, we develop a three photon formalism. We determine the production rates at RHIC and LHC with an accuracy of a few percent and discuss problems related to the observation of dimuonium.

Single pseudoscalar meson production in diffractive $ep$ scattering

Exclusive pseudoscalar meson production in \(ep\) scattering at high energies is a direct probe for a possible “odderon” exchange in soft hadronic processes. Using a simple phenomenological ansatz for the odderon, we demonstrate how it can be separated from the contribution due to photon-photon fusion, and the relevant parameters be measured. Total cross sections and differential distributions are presented for \(\pi^0,\eta,\eta'\), and \(\eta_c\) production. Results are given from both a full calculation and one using the equivalent photon approximation. The accuracy of the latter is discussed.

Relativistic antihydrogen production

Antihydrogen has recently been produced in collisions of antiprotons with ions. While passing through the Coulomb field of a nucleus an antiproton will create an electron-positron pair. In rare cases the positron is bound by the antiproton and an antihydrogen atom produced. We calculate the production of relativistic antihydrogen atoms by bound-free pair production. The cross section is calculated in the semiclassical approximation (SCA), or equivalently in the plane wave Born approximation (PWBA) using exact Dirac-Coulomb wave functions. We compare our calculations to the equivalent photon approximation (EPA).

Antihydrogen production and accuracy of the equivalent photon approximation

The production of antihydrogen in flight in antiproton-nucleus collisions is calculated theoretically in the Plane Wave Born Approximation (which is equivalent to the straight line semiclassical approximation). Antihydrogen has been produced in this way at LEAR/CERN and is presently studied at FERMILAB at various antiproton energies. Dirac wave functions for the leptons are used, taking first order (Z.alpha) corrections into account. Analytical results are obtained for differential cross-sections. Total cross sections are obtained by numerical integration. The dependence on the transverse momentum transfer is studied and the accuracy of the equivalent photon approximation and a recent variant by Munger, Brodsky,and Schmidt is discussed as a function of beam energy.

Single pseudoscalar meson production in diffractive

Exclusive pseudoscalar meson production in ep scattering at high energies is a direct probe for a possible "odderon" exchange in soft hadronic processes. Using a simple phenomenological ansatz for the odderon, we demonstrate how it can be separated from the contribution due to photon-photon fusion, and the relevant parameters be measured. Total cross sections and differential distributions are presented for pi0, eta, eta', and eta_c production. Results are given from both a full calculation and one using the equivalent photon approximation. The accuracy of the latter is discussed.