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.     

Coherent and incoherent radiation from high-energy electron and the LPM effect in oriented single crystal

The process of radiation from high-energy electron in oriented single crystal is considered using the method which permits inseparable consideration of both coherent and incoherent mechanisms of photon emission. The total intensity of radiation is calculated. The theory, where the energy loss of projectile has to be taken into account, agrees quite satisfactory with available CERN data. It is shown that the influence of multiple scattering on radiation process is suppressed due to action of crystal field.     

Electroproduction of an electron-positron pair in a medium

The process of the electron-positron pair creation by a high-energy electron in a medium is analyzed. The spectral distribution over energies of created particles is calculated for the direct and cascade mechanisms of the process. The Coulomb corrections are included. The new formulation of the equivalent photon method is developed, which takes into account the influence of the multiple scattering. It is shown that the effects of multiple scattering can be quite effectively studied in the process under consideration. The text was submitted by the authors in English.     

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.     

Hadron production in 200 GeV μ-copper and μ-carbon deep inelastic interactions

The measurements of the z and p_T² distribution of hadrons produced in the interactions of 200 GeV muons with copper and carbon nuclei are shown in different x_Bj and virtual photon energy intervals. Effects of the jet scattering are seen at the lowest virtual photon energies while for energies above 70 GeV there is no evidence of these effects. Comparison with a theoretical model indicates that at high jet energies the parton fragmentation distance is greater than the nuclear radius and that the parton absorption cross section is less than 10 mb.     

Incoherent bremsstrahlung in nuclei

We have studied the processes A(e, e′γ)X in nuclei, or incoherent bremsstrahlung, and determined expressions for the cross section in terms of the same nuclear response functions R_L, R_T, which appear in inclusive electron scattering (e, e′) in nuclei. Calculations of the cross sections are carried out using a Fermi gas model, complemented by the local-density approximation, to evaluate the response functions. We have carried out a study which shows that the reaction can be used to determine reliably the response functions from experimental data. On the other hand we have compared the incoherent bremsstrahlung with the coherent one in order to see the limits to the tagging technique, which produces monochromatic photons based on the assumption of the dominance of the coherent process. We observe that at energies E_γ < 1 GeV the dominance of the coherent process extends to relatively large scattering angles, making the present technique completely safe. However, as the energy of the electron increases, the region of dominance of the coherent process is reduced to smaller scattering angles. These results should be of use when extending the tagging technique to planned or future electron facilities.     

Photon multiplicity in the hard radiation of 150 GeV electrons in an aligned germanium crystal

Mean values m of photon multiplicity in the radiation of 150 GeV electrons directed at and near the 〈110〉 axis of a 0.185 mm thick Ge crystal cooled to 100 K have been deduced from the measurements of pair conversion probabilities. Depending on the distribution of multiplicity assumed, values of m ranging from 3.8 to 4.3 are obtained for the previously reported anomalous radiation peak.     

Photon emission from bare quark stars

We investigate the photon emission from the electrosphere of a quark star. We show that at temperatures T ≈ 0.1–1 MeV, the dominating mechanism is the bremsstrahlung due to bending of electron trajectories in the mean Coulomb field of the electrosphere. The radiated energy for this mechanism is much larger than that for the Bethe-Heitler bremsstrahlung. The energy flux from the mean field bremsstrahlung also exceeds the one from the tunnel e ⁺ e ^? pair creation. We demonstrate that the LPM suppression of the photon emission is negligible.     

Neutron scattering by spin fluctuations in dilute Al-Mn alloy

Neutron scattering experiments were carried out on Al(0.5%Mn) alloy at T = 80°K and 300°K. Effects of spin fluctuation and conduction electron polarization on the incoherent magnetic scattering cross section were observed.     

The role of incoherent scattering in laser-induced and laser-assisted processes

We use the S-matrix formalism in order to investigate the possible role of incoherent scattering in different processes in a strong laser field. In the cases of above-threshold ionization (ATI) of atoms and above-threshold detachment (ATD) of negative ions, we assume that the ionized (detached) electron may scatter off a target other than its parent ion (atom). We call such a process incoherent in order to distinguish it from the coherent rescattering of an ionized (detached) electron off its parent ion (atom) which occurs in high-order above-threshold ionization (HATI) and high-order above-threshold detachment (HATD) processes. The same line of reasoning can be applied to the laser-assisted processes, such as the laser-assisted electron-ion radiative recombination (LAR) and the laser-assisted electron-atom scattering (LAS). If the density of the ionic beam used in a LAR experiment is high, it is possible that the electron scatters off an ionic target and subsequently recombines with another ion. Analogously, the electron that scatters on an atom in the LAS process may scatter once more off another atom. We show that the contribution of these incoherent processes to the energy spectra can be higher than that of the corresponding coherent processes if the density of atomic (ionic) targets is high enough. The spectra of the incoherent processes have a cutoff-like behavior and the cutoff energies are higher than those of the corresponding coherent processes. These results are explained by semiclassical analysis.     

Experiment studies on the polarized gamma-rays generation at KEK-ATF

Polarized positrons can be created through electron-positron pair creation from circularly polarized gamma-rays. Laser-Compton scattering is an efficient method to generate circularly polarized gamma-rays. A high finesse 2-mirror optical stacking cavity had been installed on the straight section of the electron storage ring at KEK-ATF. A 1064 nm circularly polarized pulsed laser beam was stacked in the cavity. Polarized gamma-rays with a maximum energy of 28.3 MeV were produced via inverse Compton scattering of the enhanced laser pulse off an electron beam of 1.28 GeV. The number of generated gamma photons per collision was estimated by a photon detector. It was found that the experimental result was in agreement with the simulated value.     

Pair creation by a photon in an electric field

The process of pair creation by a photon in a constant and homogeneous electric field is investigated basing on the polarization operator in the field. The total probability of the process is found in a relatively simple form. At high energy the quasiclassical approximation is valid. The corrections to the standard quasiclassical approximation (SQA) are calculated. In the region of relatively low photon energies, where SQA is unapplicable, the new approximation is used. It is shown that in this energy interval the probability of pair creation by a photon in electric field exceeds essentially the corresponding probability in a magnetic field. This approach is valid at the photon energy much larger than the “vacuum” energy in electric field: ω?eE/m. For smaller photon energies the low energy approximation is developed. At ω?eE/m the found probability describes the absorption of soft photon by the particles created by an electric field.     

Reduced forms of the Wigner distribution function for the numerical analysis of rotationally symmetric synchrotron radiation

In an effort to provide a computationally convenient approach to the characterization of partially coherent synchrotron radiation in phase space, a thorough discussion of the minimum dimensionality of the Wigner distribution function for rotationally symmetric sources of arbitrary degrees of coherence is presented. It is found that perfectly coherent, perfectly incoherent and partially coherent sources may all be characterized by a three‐dimensional reduced Wigner distribution function, and some special cases are discussed in which a two‐dimensional reduced Wigner distribution function suffices. An application of the dimension‐reducing formalism to the case of partially coherent radiation from a planar undulator and a circularly symmetric electron beam as can be found in linear accelerators is demonstrated. The photon distribution is convolved over a realistic electron bunch, and how the beta function, emittance and energy spread of the bunch affect the total degree of coherence of the radiation is inspected. Finally the cross spectral density is diagonalized and the eigenmodes of the partially coherent radiation are recovered.     

Induced photon emission from quark jets in ultrarelativistic heavy-ion collisions

We study the induced photon bremsstrahlung from a fast quark produced in AA collisions due to multiple scattering in quark-gluon plasma. For RHIC and LHC conditions, the induced photon spectrum is sharply peaked at a photon energy close to the initial quark energy. In this region, the contribution of the induced radiation to the photon fragmentation function exceeds the ordinary vacuum radiation. Contrary to previous analyses [4–7], our results show that, at RHIC and LHC energies, the final-state interaction effects in quark-gluon plasma do not suppress the direct photon production and may even enhance it at p _T ～5–15 GeV.     

Coherent photo- and lepto-production of vector mesons from deuterium

We discuss the coherent photo- and lepto-production of vector mesons from deuterium at intermediate (virtual) photon energies, 3 GeV ≲ ν ≲ 30 GeV. These processes provide several options to explore the space-time evolution of small size quark-gluon configurations. Furthermore, we study the dependence of the production cross section on the energy and momentum transfer t due to variations of the finite longitudinal interaction length. Kinematic regions are determined where the production cross section is most sensitive to the final state interaction of the initially produced hadronic wave packet. For unpolarized deuteron targets this double scattering contribution can be investigated mainly at large values of the momentum transfer t. For polarized targets kinematic windows sensitive to double scattering are available also at moderate t. We suggest several methods for an investigation of color coherence effects at intermediate energies.     

Search for massive photon pair production at the CERN intersecting storage rings

A search for massive photon pair production at \(\sqrt s = 63 GeV\) has been carried out on the data sample previously employed for the electron pair production study. Positive evidence is reported form _γγ>6GeV, with a production cross-section similar to Drell-Yan electron pairs. The ratio γγ/π⁰π⁰ was measured to be ～10^?3 for ap _T of each γ or γ or π⁰ above 3 GeV/c.     

Theory of the hot free carrier intraband-absorption coefficient of n-inversion layers

The absorption coefficient K of a quasi two dimensional (2D) hot free electron gas is calculated for the first time as a function of the lattice temperature T, the photon angular frequency w, the carrier density N_s as well as the electron temperature T_e when the carriers are scattered by ionized impurities, acoustic phonons and polar optical phonons. Analytical expressions are derived in the limiting cases of non-degeneracy and degeneracy of the electron system. In the quantum limit ħw/k_BT_e ≳ 1 where the interaction between the electron and the photon is inelastic K sensitively depends on the limiting scattering mechanism showing that the electron motion is completely controlled by the photon field. In the classical limit ħw/k_BT_e ⪡ 1 the absorption decreases proportional to w¹ independent of the limiting scattering mechanism in agreement with the experimental data deduced from far-infrared absorptivity measurements on GaAs heterolayers.     

Self-amplifacation of channeling radiation of ultrarelativistic electrons due to loss of transverse energy

When an axially channeled electron emits one photon, its transverse kinetic energy decreases by roughly the same ratio as the longitudinal energy. If this ratio is not small compared to 1, the electron after emission spends a significantly larger fraction of its time in the strong field region than before emission, and the probability of emitting new phonons increases accordingly. This makes the cascade of emissions self-accelerating, and can explain the anomalously large energy radiated by 150 GeV electrons channeled in germanium in a recent experiment. Multiple scattering is important in moderating this mechanism.     

Physical characteristics of X-ray scattering in fat and blood

A Monte Carlo simulation program is constructed in order to trace the histories of photon interactions inside fat and blood under tissue-characterization conditions (energies from 7 to 60 keV and samples from 0.5 to 20 cm). The effect of incident photon energy on the number of coherent and incoherent interactions up to the 3rd order scattering is studied for three different sample sizes. The ratio of scattering to total interactions (including photoelectric absorption) is also studied for fat and blood under the same scattering conditions. Results show considerable differences in the scattering properties of fat and blood over a wide range of energies. This may explain the experimental differences reported by Evans et al. and Kosanetzky et al. The percentage of photons making three incoherent interactions recorded a maximum of 10% which implies the need to correct for multiple scattering in incoherent scattering measurements intended for tissue characterization.     

Conformal approach to fluctuations in hadroproduction

We study two-photon final-states atep-colliders. In particular, we investigate the process γ+p →γ+γ+X at HERA and LEP/LHC energies with the intention of determining its measurability at these accelerators. We find that the transverse momentum distribution will be measured top _T=25 GeV/c at HERA andp _T=50 GeV/c at LEP/LHC. We find that the cross section of this process is too small for it to be a sensitive, direct probe of the gluon content of the proton and the photon. However, we find that it will supplement deep inelastic structure function investigations of the quark distributions within the proton and photon. Further, we predict that at very low transverse momenta, this process will allow the measurement of the box diagramg+g→γ+γ.