Polarized triplet production by circularly polarized photons

A process of pair production by a circularly polarized photon in the field of an unpolarized atomic electron was considered in the Weizaecker-Williams approximation. The degree of longitudinal polarization of a positron and an electron was calculated. An exclusive cross section, as well as a spectral distribution, were obtained. We estimate the accuracy of our calculations at the level of a few percent. We show the identity of the positron polarization for the considered process and for the process of pair production in the screened Coulomb field of a nucleus.     

Resonance effects with the photoproduction of electron-positron pairs in the field of a plane electromagnetic wave

The differential cross section is obtained for the formation of electron-positron pairs by a photon at a Coulomb center in the field of a plane electromagnetic wave whose intensity η = eA′/m? 1 and frequency Ω′/m? 1. Expressions are given for the resonance frequencies of the photon for which the emission of a virtual electron (positron) at a mass shell occurs. It is shown that resonance occurs only for electron or positron energies E > m²/Ω′. The resonance differential cross section under interference conditions is obtained. It is shown that the resonance cross section can exceed by several orders of magnitude the ordinary cross section for the photoproduction of pairs without a field.     

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). Received: 19 December 1997 / Published online: 10 March 1998     

Resonant electron-positron pair photoproduction on a nucleus in a pulsed light field

The resonant photoproduction of an electron-positron pair on a nucleus in the field of a pulsed light wave is studied theoretically. The approximation where the electromagnetic pulse duration is much longer than the characteristic time of wave oscillations is considered. The interaction of the electron and positron with the Coulomb potential of the nucleus is considered in the Born approximation. An analytical expression for the resonant differential cross section is derived for the range of moderately strong external fields. This cross section contains a resonant peak whose height and width are determined by the external pulsed wave characteristics. The resonant cross section for pair photoproduction on a nucleus in a pulsed laser field can exceed the corresponding cross section for pair photoproduction on a nucleus in the absence of an external field by an order of magnitude.     

Quantum Processes beyond the Aharonov-Bohm Effect

We consider QED processes in the presence of an infinitely thin and infinitely long straight string with a magnetic flux inside it. The bremsstrahlung from an electron passing by the magnetic string and the electron-positron pair production by a single photon are reviewed. Based on the exact electron and positron solutions of the Dirac equation in the external Aharonov-Bohm potential we present matrix elements for these processes. The dependence of the resulting cross sections on energies, directions, and polarizations of the involved particles is discussed for low energies.     

Polarization operator of a photon in a magnetic field

The polarization operator of a photon in a static uniform magnetic field has been studied at photon energies both above and below the threshold of electron–positron pair production by a photon. In the first order of the fine-structure constant α, expressions for the refractive index of a photon with a certain polarization in both low and high fields as compared to the critical field H₀ = 4.41 × 10¹³ G have been obtained. Both the purely quantum range of photon energies, where the particles of a pair are produced at the lowest Landau levels, and the region of applicability of the semiclassical approximation in the case of the population of high energy levels have been considered. A general spectral integral formula has been obtained with divergent threshold terms separated in an explicit form.     

Nonresonant photocreation of electron-positron pair on a nucleus in the field of a pulsed light wave

The nonresonant Coulomb photocreation of electron-positron pair on a nucleus in the field of a pulsed light wave is theoretically investigated. The approximation is examined when pulsewidth is considerably greater than the characteristic time of wave oscillations. The interaction of an electron and a positron with a Coulomb potential of a nucleus is considered in the first order of perturbation theory (the Born approximation). An analytic expression for the nonresonant differential cross section was obtained for the range of moderately strong fields in the case relativistic and nonrelativistic energies. It is shown, that the nonresonant cross section of Coulomb photocreation of nonrelativistic pair in the field of a pulsed light wave is two times larger than the cross section of Coulomb photocreation in the absence of an external field.     

Concept of formation length in radiation theory

The features of electromagnetic processes are considered which connected with finite size of space region in which final particles (photon, electron–positron pair) are formed. The longitudinal dimension of the region is known as the formation length. If some external agent is acting on an electron while traveling this distance the emission process can be disrupted. There are different agents: multiple scattering of projectile, polarization of a medium, action of external fields, etc. The theory of radiation under influence of the multiple scattering, the Landau–Pomeranchuk–Migdal (LPM) effect, is presented. The probability of radiation is calculated with an accuracy up to “next to leading logarithm” and with the Coulomb corrections taken into account. The integral characteristics of bremsstrahlung are given, it is shown that the effective radiation length increases due to the LPM effect at high energy. The LPM effect for pair creation is also presented. The multiple scattering influences also on radiative corrections in a medium (and an external field too) including the anomalous magnetic moment of an electron and the polarization tensor as well as coherent scattering of a photon in a Coulomb field. The polarization of a medium alters the radiation probability in soft part of spectrum. Specific features of radiation from a target of finite thickness include: the boundary photon emission, interference effects for thin target, multi-photon radiation. The theory predictions are compared with experimental data obtained at SLAC and CERN SPS. For electron–positron colliding beams following items are discussed: the separation of coherent and incoherent mechanisms of radiation, the beam-size effect in bremsstrahlung, coherent radiation and mechanisms of electron–positron creation.     

Integrated Cross Sections of High-Energy <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>–</Superscript> Pair Electroproduction by an Electron in an Atomic Field

The integrated cross sections of e⁺e^– pair electroproduction by an ultrarelativistic electron in an atomic field are studied. The importance of various contributions to the integrated cross sections is discussed. The Coulomb corrections have turned out to make a large contribution to both differential and integrated cross sections even at moderate values of the nuclear charge number. The interaction of the electron emitting a virtual photon with the atomic field is shown to contribute significantly to the cross sections differential with respect to the electron transverse momentum. However, this interaction affects only slightly the cross section differential only with respect to the positron transverse momentum.     

Results from the CMS collaboration on the search of heavy dilepton and diphoton resonances

A survey of the results on the search of new heavy resonances decaying into muon, electron, or photon pairs in the CMS experiment at proton beams of the LHC in the run of 2015 for the total center of mass energy √s = 13 TeV is presented. The limits for heavy resonance production cross sections are set. Excess of diphoton events in the region of mass spectra near 750 GeV with a local statistical significance of 3.4 σ and a global statistical significance of 1.6 σ is observed.     

Electroproduction of electron-positron pair in oriented crystal at high energy

The process of electroproduction of the electron-positron pair by high energy electron in an oriented single crystal is investigated. Two contributions are considered: the direct (one-step) process via the virtual intermediate photon and the cascade (two-step) process when the electron emits the real photon moving in the field of axis and afterwards the photon converts into the pair. The spectrum of created positron (electron) is found. It is shown that the probability of the process is strongly enhanced comparing with the corresponding amorphous medium.     

Interference effect in the photoproduction of electron-positron pairs on a nucleus in the field of two light waves

In the general relativistic case, the nonresonance photoproduction of electron-positron pairs on a nucleus in the field of two circularly polarized light waves propagating in one direction is theoretically investigated. It is shown that there are two essentially different kinematical regions: the noninterference region, where the Bunkin-Fedorov quantum parameters play the role of multiphoton parameters, and the interference region, where interference effects are important and where quantum interference parameters are treated as multiphoton parameters. The interference effect is found in the photoproduction of an electron-positron pair on a nucleus in the field of two light waves. This effect occurs in the interference region and is due to the production of an electron-positron pair in the plane spanned by the light wave vector and the incident-photon momentum and to stimulated correlated emission and absorption of photons of the two waves. The cross sections for pair photoproduction on a nucleus in the above kinematical regions are determined for moderately strong fields. It is shown that, in the interference region, the partial cross sections for the case where there is a correlated emission (absorption) of the same number of photons of the two waves can significantly exceed the corresponding cross sections in any other geometry.     

Perturbative Study of Bremsstrahlung and Pair-Production by Spin-½ Particles in the Aharonov–Bohm Potential

In the presence of an external Aharonov–Bohm potential, we investigate the two QED processes of the emission of a bremsstrahlung photon by an electron, and the production of an electron–positron pair by a single photon. Calculations are carried out using the Born approximation within the framework of covariant perturbation theory to lowest non-vanishing order in α. The matrix element for each process is derived, and the corresponding differential cross-section is calculated. In the non-relativistic limit, the resulting angular and spectral distributions and some polarization properties are considered, and compared to results of previous works.     

Photoproduction of top quarks in peripheral heavy ion collisions

In relativistic heavy ion collisions, top quarks can be produced by photon-gluon fusion when a photon from the Weizs?cker-Williams virtual photon field of one nucleus interacts with a gluon in the other nucleus. Photoproduction with heavy ions at the Large Hadron Collider (LHC) will be the first accessible non-hadronic top production channel. We calculate the photoproduction cross sections, pair mass and top quark rapidity distributions in peripheral lead-lead and oxygen-oxygen collisions. The cross sections are sensitive to the top quark charge and the large- gluon distribution in the nucleus. We find a cross section of 15 pb in oxygen-oxygen collisions, leading to 210 pairs in a one month ( s) LHC run. In pA collisions, the rate is higher, 1100 pairs per month for pO. A comparison of the AA and pA data might allow for a study of gluon shadowing at high Q . Received: 18 May 2001 / Revised version: 13 June 2001 / Published online: 24 August 2001     

Formation of a pair and an electron by a photon in the field of a nucleus

A general expression is calculated for the differential cross section of the formation of an electron-positron pair and a photon by a photon in the Coulomb field of an atomic nucleus. Particular kinematic cases of the process which have not previously been investigated are considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 91–96, October, 1978.     

Use of Furry-Sommerfeld-Maue wave functions in pair production at medium and high energies

The analytic expression for the differential pair production cross section, obtained using full Furry-Sommerfeld-Maue wave functions for the electron and positron, is integrated numerically over angles without further approximation. The resulting Coulomb corrections to the total pair production cross section are compared with existing interpolations in the energy range 10 MeV to 50 MeV, and their experimental relevance is discussed.     

Photon gluon fusion cross sections at HERA energy

We present cross sections for heavy flavour production through photon gluon fusion in electron proton collisions at HERA energy. The electron photon vertex is taken into account explicitly, and theQ ² of the exchanged photon ranges from nearly zero (almost real photon) to the kinematically allowed maximum. In our approach the scale is set by the mass of the produced quarks. Our formalism is also applicable to the production of light quarks as long as the invariant mass of the pair is sufficiently high, so we also give cross sections for \(u\bar u,d\bar d\) and \(s\bar s\) production.     

Total nuclear photon absorption cross sections for some light elements

Absolute nuclear photon absorption cross sections have been measured for the elements Li, Be, C, O, Al, Si and Ca from 10 MeV up to photon energies beyond the meson production threshold. Magnetic Compton spectrometers and a bremsstrahlung spectrum with fixed end-point energy were used. The cross sections show structure in the region of the giant resonance and fall off smoothly towards higher energies. In the giant resonance region recent 1p-1h calculations are in poor agreement with these measurements except for one calculation for carbon, which included low lying excited states of the residual mass-11 system. The cross section in the intermediate region (40 to 140 MeV) can be described by the quasideuteron model with the density of deuteron-like structures taken as 8 NZ/A. The moments of the measured cross sections are compared with sum rule predictions. The integrated cross sections from 10 MeV up to the meson production threshold (140 MeV) exceed the classical dipole sum by a factor of 1.4 to 2.     

Resonance bremsstrahlung of an electron by a nucleus in the field of a plane electromagnetic wave

The resonance behavior of the brems Strahlung of an electron by a nucleus in the field of a plane electromagnetic wave is investigated. The differential cross section of the process in the resonance region is calculated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 12–17, March, 1980.The authors express their gratitude to A. A. Sokolov for his attention to this research.