17,132 research outputs found

### Polarization effects for pair creation by photon in oriented crystals at high energy

Pair creation by a photon in an oriented crystal is considered in a frame of
the quasiclassical operator method, which includes processes with polarized
particles. Under some quite generic assumptions the general expression is
derived for the probability of pair creation of longitudinally polarized
electron (positron) by circularly polarized photon in oriented crystal. In
particular cases $\vartheta_0 \ll V_0/m$ and $\vartheta_0 \gg V_0/m$
($\vartheta_0$ is the angle of incidence, angle between the momentum of initial
photon and axis (plane) of crystal, $V_0$ is the scale of a potential of axis
or a plane relative to which the angle $\vartheta_0$ is defined) one has
constant field approximation and the coherent pair production theory
correspondingly. Side by side with coherent process the probability of
incoherent pair creation is calculated, which differs essentially from
amorphous one. At high energy the pair creation in oriented crystals is
strongly enhanced comparing with amorphous medium. In appendixes the integral
polarization of positron is found in external field and for coherent and
incoherent mechanisms.Comment: 24 pages, 4 figure

### On propagation of high-energy photon in a medium in presence of an external field

The polarization tensor is calculated which originates from interaction of a
photon with the electron-positron field. The effects of multiple scattering of
electrons and positrons in a medium side by side with an external field are
included. The corresponding general representation of polarization tensor is
found in the form of two-dimentional integral. The both effects may be
essential for propagation of high-energy photons in oriented single crystals.Comment: 12 pages, 1 figur

### Spectrum and polarization of coherent and incoherent radiation and the LPM effect in oriented single crystal

The spectrum and the circular polarization of radiation from longitudinally
polarized high-energy electrons in oriented single crystal are considered using
the method which permits inseparable consideration of both the coherent and the
incoherent mechanisms of photon emission. The spectral and polarization
properties of radiation are obtained and analyzed. It is found that in some
part of spectral distribution the influence of multiple scattering (the
Landau-Pomeranchuk-Migdal (LPM) effect) attains the order of 7 percent. The
same is true for the influence of multiple scattering on the polarization part
of the radiation intensity. The degree of circular polarization of total
intensity of radiation is found. It is shown that the influence of multiple
scattering on the photon polarization is similar to the influence of the LPM
effect on the total intensity of radiation: it appears only for relatively low
energies of radiating electron and has the order of 1 percent, while at higher
energies the crystal field action excludes the LPM effect.Comment: 13 pages, 7 figure

### Coherent and incoherent pair creation by a photon in oriented single crystal

The new approach is developed for study of electron-positron pair production
by a photon in oriented single crystal. It permits indivisible consideration of
both coherent and incoherent mechanisms of pair creation and includes the
action of field of axis (or plane) as well as the multiple scattering of
particles of the created pair (the Landau-Pomeranchuk-Migdal (LPM) effect).
From obtained integral probability of pair creation, it follows that multiple
scattering appears only for relatively low energy of photon, while at higher
photon energy the field action excludes the LPM effect. The found results agree
quite satisfactory with recent CERN experiment.Comment: 10 pages, 3 figure

### 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 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 "vacuum" energy in electric field: $\omega\gg
eE/m$. For smaller photon energies the low energy approximation is developed.
At $\omega\ll eE/m$ the found probability describes the absorption of soft
photon by the particles created by an electric field.Comment: 15 pages, 6 figure

### Quantum theory of transition radiation and transition pair creation

Theory of the transition radiation and the transition pair creation is
developed in the frame of QED. The spectral-angular distributions of
probability of the transition radiation and of the transition pair creation are
found. The total energy losses of and the total probability of pair creation
are calculated and analyzed. Features of radiation and pair creation processes
in a superdence medium (typical for white dwarfs) are discussed.Comment: LaTeX, 12 pages, 3 eps figure

### On the string-inspired approach to QED in external field

Strassler's formulation of the string-derived Bern-Kosower formalism is
extended to consider QED processes in homogeneous constant external field. A
compact expression for the contribution of the one-loop with arbitrary number
of external photon lines is given for scalar QED. Extension to spinor QED is
shortly discussed.Comment: 14 pages,Revtex,no figure

### Pair creation by a photon in a strong magnetic field

The process of pair creation by a photon in a strong magnetic field is
investigated basing on the polarization operator in the field. The total
probability of the process is found in a relatively simple form. The
probability exhibits a "saw-tooth" pattern because of divergences arising when
the electron and positron are created at threshold of the Landau energy levels.
The pattern will be washed out at averaging over any smooth photon energy
distribution. The new results are obtained in the scope of the quasiclassical
approach: 1) in the case when the magnetic field $B \ll B_0, (B_0$ is the
critical field) the new formulation extends the photon energy interval to the
case when the created particles are not ultrarelativistic; 2) the correction to
the standard quasiclassical approximation is found showing the range of
applicability of the approach at high photon energy as well. The very important
conclusion is that for both cases $B \ll B_0$ and $B \geq B_0$ the results of
the quasiclassical calculation are very close to averaged probabilities of
exact theory in a very wide range of photon energies. The quasiclassical
approximation is valid also for the energy distribution if the electron and
positron are created on enough high levels.Comment: 21 pages, 6 figure

### Gluon Radiation off Hard Quarks in a Nuclear Environment: Opacity Expansion

We study the relation between the Baier-Dokshitzer-Mueller-Peigne-Schiff
(BDMPS) and Zakharov formalisms for medium-induced gluon radiation off hard
quarks, and the radiation off very few scattering centers. Based on the
non-abelian Furry approximation for the motion of hard partons in a spatially
extended colour field, we derive a compact diagrammatic and explicitly colour
trivial expression for the N-th order term of the kt-differential gluon
radiation cross section in an expansion in the opacity of the medium. Resumming
this quantity to all orders in opacity, we obtain Zakharov's path-integral
expression (supplemented with a regularization prescription). This provides a
new proof of the equivalence of the BDMPS and Zakharov formalisms which extends
previous arguments to the kt-differential cross section. We give explicit
analytical results up to third order in opacity for both the gluon radiation
cross section of free incoming and of in-medium produced quarks. The N-th order
term in the opacity expansion of the radiation cross section is found to be a
convolution of the radiation associated to N-fold rescattering and a
readjustment of the probabilities that rescattering occurs with less than N
scattering centers. Both informations can be disentangled by factorizing out of
the radiation cross section a term which depends only on the mean free path of
the projectile. This allows to infer analytical expressions for the totally
coherent and totally incoherent limits of the radiation cross section to
arbitrary orders in opacity.Comment: 22 pages, LaTeX, eps-figures included, typos corrected, Comment on
(5.13) added, to appear in NP

### How does transverse (hydrodynamic) flow affect jet-broadening and jet-quenching ?

We give the modification of formulas for $p_{\perp}$-broadening and energy
loss which are necessary to calculate parton interactions in a medium with
flow. Arguments are presented leading to the conclusion that for large
$p_{\perp}$-spectra observed in heavy ion collisions at RHIC, the influence of
transverse flow on the determination of the "quenching power" of the produced
medium is small. This leaves open the question of the interpretation of data in
a consistent perturbative framework.Comment: 8 pages and 4 figures; version v2 includes more consistent notations
and correction of fig.2; version v3 contains a misprint correction, an added
sentence in section 3C and in the Introduction as well as in section 4 a
rephrasing of comments about references [31,32,35

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