-
[show abstract]
[hide abstract]
ABSTRACT: First quasiclassical correction to the differential cross section of
high-energy electron-positron photoproduction in the electric field of a heavy
atom is obtained with the exact account of the field. This correction is
responsible for the charge asymmetry ${\cal A}$ in this process. When the
transverse momentum of at least one of the produced particles is much larger
than the electron mass $m$, the charge asymmetry can be as large as tens
percent. We also estimate the contribution $\tilde{\cal A}$ to the charge
asymmetry coming from the Compton-type diagram. For heavy nuclei, this
contribution is negligible. For light nuclei, $\tilde{\cal A}$ is noticeable
only when the angle between the momenta of electron and positron is of order of
$m/\omega$ ($\omega$ is the photon energy) while the transverse momenta of both
particles are much larger than $m$.
11/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The process $e^+e^- \rightarrow N\bar N$ is studied nearby a threshold with
account for polarizations of all initial and final particles. The nucleon
polarization $\bm \zeta^N$ reveals a strong energy dependence due to that of
the nucleon electromagnetic form factors $G_E(Q^2)$ and $G_M(Q^2)$ caused by
the final-state interaction of nucleons. It is shown that the modulus of the
ratio of these form factors and their relative phase can be determined by
measuring $\bm \zeta^N$ along with the differential cross section. The
polarization degree is analyzed using Paris $N\bar N$ optical potential for
calculation of the form factors. It turns out that $|\bm \zeta^N|$ is high
enough in a rather wide energy range above the threshold. Being especially high
for longitudinally polarized beams, $|\bm \zeta^N|$ is noticeable even if both
$e^+e^-$ beams are unpolarized.
12/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: The cross section which addresses the spin-flip transitions of a proton (antiproton) interacting with a polarized non-relativistic electron or positron is calculated analytically. In the case of attraction, this cross section is greatly enhanced for sufficiently small relative velocities as compared to the result obtained in the Born approximation. However, it is still very small, so that the beam polarization time turns out to be enormously large for the parameters of $e^{\pm}$ beams available now. This practically rules out a use of such beams to polarize stored antiprotons or protons.
03/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: Low energy $p\bar p$ interaction is considered taking into account the polarization of both particles. The corresponding cross sections are calculated using the Paris nucleon-antinucleon optical potential. Then they are applied to the analysis of the polarization buildup which is due to the interaction of stored antiprotons with polarized protons of a hydrogen target. It is shown that, at realistic parameters of a storage ring and a target, the filtering mechanism provides a noticeable polarization in a time comparable with the beam lifetime.
08/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: The kinetics of the polarization buildup during the interaction of stored protons (antiprotons) with a polarized target is considered. It is demonstrated that for small scattering angles, when a projectile remains in the beam, the polarization buildup is completely due to the spin-flip transitions. The corresponding cross sections turn out to be negligibly small for a hydrogen gas target as well as for a pure electron target. For the latter, the filtering mechanism also does not provide a noticeable beam polarization.
Physical Review E 01/2006; 72(6 Pt 2):066503. · 2.26 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The differential and partially integrated cross sections are considered for bremsstrahlung from high-energy electrons in atomic field with the exact account of this field. The consideration exploits the quasiclassical electron Green's function and wave functions in an external electric field. It is shown that the Coulomb corrections to the differential cross section are very susceptible to screening. Nevertheless, the Coulomb corrections to the cross section summed up over the final-electron states are independent of screening in the leading approximation over a small parameter $1/mr_{scr}$ ($r_{scr}$ is a screening radius, $m$ is the electron mass, $\hbar=c=1$). Bremsstrahlung from an electron beam of the finite size on heavy nucleus is considered as well. Again, the Coulomb corrections to the differential probability are very susceptible to the beam shape, while those to the probability integrated over momentum transfer are independent of it, apart from the trivial factor, which is the electron-beam density at zero impact parameter. For the Coulomb corrections to the bremsstrahlung spectrum, the next-to-leading terms with respect to the parameters $m/\epsilon$ ($\epsilon$ is the electron energy) and $1/mr_{scr}$ are obtained. Comment: 13 pages, 4 figures
04/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: First correction to the high-energy asymptotics of the total $e^+e^-$ photoproduction cross section in the electric field of a heavy atom is derived with the exact account of this field. The consideration is based on the use of the quasiclassical electron Green function in an external electric field. The next-to-leading correction to the cross section is discussed. The influence of screening on the Coulomb corrections is examined in the leading approximation. It turns out that the high-energy asymptotics of the corresponding correction is independent of the photon energy. In the region where both produced particles are relativistic, the corrections to the high-energy asymptotics of the electron (positron) spectrum are derived. Our results for the total cross section are in good agreement with experimental data for photon energies down to a few $MeV$. In addition, the corrections to the bremsstrahlung spectrum are obtained from the corresponding results for pair production.
11/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: First correction to the high-energy asymptotics of the total $e^+e^-$ photoproduction cross section in the electric field of a heavy atom is obtained with the exact account of this field. The consideration is based on the use of the quasiclassical electron Green function in an external electric field. The influence of screening on the Coulomb corrections is examined in the leading approximation. It turns out that the high-energy asymptotics of the corresponding correction is independent of the photon energy. The detailed comparison of our results with experimental data is performed. This comparison has justified the analytical result and allowed us to elaborate a simple ansatz for the next-to-leading correction. Using this ansatz, good agreement with the experimental data is obtained for photon energies above a few $MeV$. In the region where both produced particles are relativistic, the corrections to the high-energy asymptotics of the electron (positron) spectrum are obtained. In addition, analogous corrections to the bremsstrahlung spectrum are derived starting from the corresponding results for pair production.
08/2003;
-
Sh Zh Akhmadaliev,
G Ya Kezerashvili,
S G Klimenko,
R N Lee,
V M Malyshev,
A L Maslennikov,
A M Milov,
A I Milstein,
N Yu Muchnoi,
A I Naumenkov,
V S Panin,
S V Peleganchuk,
G E Pospelov,
I Ya Protopopov,
L V Romanov,
A G Shamov,
D N Shatilov,
E A Simonov, V M Strakhovenko,
Yu A Tikhonov
[show abstract]
[hide abstract]
ABSTRACT: Data analysis of an experiment in which photon splitting in atomic fields was observed is presented. The experiment was performed at the tagged photon beam of the ROKK-1M facility at the VEPP-4M collider. In the energy region of 120-450 MeV, statistics of 1.6x10(9) photons incident on the BGO target was collected. About 400 candidate photon-splitting events were reconstructed. Within the attained experimental accuracy, the experimental results are consistent with the calculated exact atomic-field cross section. The predictions obtained in the Born approximation differ significantly from the experimental results.
Physical Review Letters 09/2002; 89(6):061802. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Photon splitting due to vacuum polarization in the electric field of an atom is considered. We survey different theoretical approaches to the description of this nonlinear QED process and several attempts of its experimental observation. We present the results of the lowest-order perturbation theory as well as those obtained within the quasiclassical approximation being exact in the external field strength. The experiment where photon splitting was really observed for the first time is discussed in details. The results of this experiment are compared with recent theoretical estimations. Comment: 45 pages, 24 figures
11/2001;
-
[show abstract]
[hide abstract]
ABSTRACT: A representation is obtained for the quasiclassical Green functions of the Dirac and Klein-Gordon equations allowing for the
first nonvanishing correction in an arbitrary localized potential which generally possesses no spherical symmetry. This is
used to obtain a solution of these equations in an approximation similar to the Furry-Sommerfeld-Maue approximation. It is
shown that the quasiclassical Green function does not reduce to the Green function obtained in the eikonal approximation and
has a wider range of validity. This is illustrated by calculating the amplitude of small-angle scattering of a charged particle
and the amplitude of Delbrück forward scattering. A correction proportional to the scattering angle was obtained for the amplitude
of charged particle scattering in a potential possessing no spherical symmetry. The real part of the Delbrück forward scattering
amplitude was calculated in a screened Coulomb potential.
Journal of Experimental and Theoretical Physics 12/1999; 90(1):66-78. · 1.03 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The quasiclassical Green functions of the Dirac and Klein-Gordon equations in the external electric field are obtained with the first correction taken into account. The relevant potential is assumed to be localized, while its spherical symmetry is not required. Using these Green functions, the corresponding wave functions are found in the approximation similar to the Furry-Sommerfeld-Maue approximation. It is shown that the quasiclassical Green function does not coincide with the Green function obtained in the eikonal approximation and has a wider region of applicability. It is illustrated by the calculation of the small-angle scattering amplitude for a charged particle and the forward photon scattering amplitude. For charged particles, the first correction to the scattering amplitude in the non-spherically symmetric potential is found. This correction is proportional to the scattering angle. The real part of the amplitude of forward photon scattering in a screened Coulomb potential is obtained. Comment: 20 pages, latex, 1 figure
07/1999;
-
[show abstract]
[hide abstract]
ABSTRACT: Using a new representation for the quasiclassical Green function of the Dirac equation in a Coulomb field, analytical expressions for the high-energy small-angle Delbr\"uck scattering amplitudes are obtained exactly in the parameter Z\alpha. Magnitudes of the amplitudes coincide with the previous results. However, the structure of the expressions obtained is much more simple, which considerably facilitates numerical calculations.
03/1999;
-
[show abstract]
[hide abstract]
ABSTRACT: Various differential cross sections of high-energy photon splitting in the electric fields of heavy atoms are calculated exactly in the parameter Zff. The consideration is based on the quasiclassical approach applicable for small angles between all photon momenta. The expressions obtained are valid for arbitrary transverse momenta of final photons . The detailed investigation of the process is performed taking into account the effect of screening . The exact cross section turns out to be noticeably smaller than the result obtained in the Born approximation. 1 Introduction The first successful observation of high-energy photon splitting in the electric fields of atoms has been recently performed in the Budker Institute of Nuclear Physics. A crystal of Bi 4 Ge 3 O 12 has been used as a target. At the present time, the data processing is almost completed and preliminary results are published in [1]. Theoretical and experimental investigation of this nonlinear QED process is important as...
01/1999;
-
[show abstract]
[hide abstract]
ABSTRACT: Various differential cross sections of high-energy photon splitting in the electric fields of heavy atoms are calculated exactly in the parameter \al. The consideration is based on the quasiclassical approach applicable for small angles between all photon momenta. The expressions obtained are valid for arbitrary transverse momenta of final photons . The detailed investigation of the process is performed taking into account the effect of screening . The exact cross section turns out to be noticeably smaller than the result obtained in the Born approximation.
05/1998;
-
[show abstract]
[hide abstract]
ABSTRACT: The Coulomb corrections to the helicity amplitudes of high-energy photon splitting are examined. The consideration is based on the amplitudes obtained exactly in the parameter Z\alpha within the quasiclassical approach valid for small angles between all photon momenta. We consider the case when the transverse momenta of both final photons are much larger than the electron mass. It is shown that at Z\alpha \sim 1 the Coulomb corrections essentially change the result for the cross section as compared to the Born approximation. The effect of screening is also taken into account.
11/1997;
-
[show abstract]
[hide abstract]
ABSTRACT: The helicity amplitudes of the high-energy photon splitting in the external Coulomb field are obtained exactly in the parameter $\al$. The cases of screened and unscreened potentials are investigated. The consideration is based on the quasiclassical approach, valid for small angles between all photon momenta. New representation of the quasiclassical electron Green function is exploited. General expressions obtained are analyzed in detail for the case of large transverse momenta of both final photons compared to the electron mass.
05/1997;
-
[show abstract]
[hide abstract]
ABSTRACT: Photon splitting due to vacuum polarization in the electric field of an atom is considered. We survey different theoretical approaches to the description of this nonlinear QED process and several attempts of its experimental observation. We present the results of the lowest-order perturbation theory as well as those obtained within the quasiclassical approximation being exact in the external field strength. The experiment where photon splitting was really observed for the first time is discussed in details. The results of this experiment are compared with recent theoretical estimations.
Physics Reports.
-
[show abstract]
[hide abstract]
ABSTRACT: The Coulomb corrections (CC) to the processes of bremsstrahlung and pair production are investigated. The next-to-leading term in the high-energy asymptotics is found. This term becomes very essential in the region of intermediate energies. The influence of screening for CC is small for differential cross section, spectrum, and the total cross section of pair production. The same is true for the spectrum of bremsstrahlung, but not for the differential cross section, where the influence of screening can be very large. The corresponding screening corrections as well as the modification of the differential cross section of bremsstrahlung are found. A comparison of our results for the total cross section of pair production with the experimental data available is performed. This comparison has justified our analytical results and allowed to elaborate a simple ansatz for the next-to-leading correction. The influence of the electron beam shape on CC for bremsstrahlung is investigated. It turns out that the differential cross section is very sensitive to this shape.
Radiation Physics and Chemistry. 75(8):868-873.
-
[show abstract]
[hide abstract]
ABSTRACT: The helicity amplitudes of high-energy photon splitting in an external Coulomb field are obtained exactly in the parameter Zα. The consideration is based on the quasiclassical approach, valid for small angles between all photon momenta. A new representation of the quasiclassical electron Green function is exploited. General expressions obtained are analyzed in detail when the transverse momenta of both final photons are much larger than the electron mass. It is shown that, at Zα∼1, Coulomb corrections essentially change the cross section as compared to the Born approximation. Screened and unscreened potentials are considered.
Phys. Rev. A. 57(4).
