260 research outputs found
Relativistic eikonal description of A(p,pN) reactions
The authors present a relativistic and cross-section factorized framework for
computing quasielastic A(p,pN) observables at intermediate and high energies.
The model is based on the eikonal approximation and can accomodate both optical
potentials and the Glauber method for dealing with the initial- and final-state
interactions (IFSI). At lower nucleon energies, the optical-potential
philosophy is preferred, whereas at higher energies the Glauber method is more
natural. This versatility in dealing with the IFSI allows one to describe
A(p,pN) reactions in a wide energy range. Most results presented here use
optical potentials as this approach is argued to be the optimum choice for the
kinematics of the experiments considered in the present paper. The properties
of the IFSI factor, a function wherein the entire effect of the IFSI is
contained, are studied in detail. The predictions of the presented framework
are compared with two kinematically different experiments. First, differential
cross sections for quasielastic proton scattering at 1 GeV off 12C, 16O, and
40Ca target nuclei are computed and compared to data from PNPI. Second, the
formalism is applied to the analysis of a 4He(p,2p) experiment at 250 MeV. The
optical-potential calculations are found to be in good agreement with the data
from both experiments, showing the reliability of the adopted model in a wide
energy range.Comment: 34 pages, 14 figures, accepted for publication in Phys. Rev.
Meson production in high-energy electron-nucleus scattering
Pseudoscalar mesons can be produced and studied in high-energy
electron-nucleus scattering. We review and extend our previous analysis of
meson production in the nuclear Coulomb field. The
decay rates are most directly determined for mesons produced in the
double-Coulomb region where both photons are nearly real, and provided the
background-hadronic contribution remains small. The larger the mass of the
meson the higher the electron energy needed to assure such a condition.Comment: 19 pages, 2 figure
Bremsstrahlung Radiation as Coherent State in Thermal QED
Based on fully finite temperature field theory we investigate the radiation
probability in the bremsstrahlung process in thermal QED. It turns out that the
infrared divergences resulting from the emission and absorption of the real
photons are canceled by the virtual photon exchange processes at finite
temperature. The full quantum calculation results for soft photons radiation
coincide completely with that obtained in the semi-classical approximation. In
the framework of Thermofield Dynamics it is shown that the bremsstrahlung
radiation in thermal QED is a coherent state, the quasiclassical behavior of
the coherent state leads to above coincidence.Comment: 8 pages, 2 figure
Color Screening and the Suppression of the Charmonium State Yield in Nuclear Reactions
We discuss the new data for the production of the meson in pA
collisions at 450 GeV at CERN-SPS (of the NA50-collaboration) [1]. We extract
from the CERN data mb under the assumption that the
is produced as a result of the space-time evolution of a point-like
pair which expands with time to the full size of the charmonium
state. In the analysis we assume the existence of a relationship between the
distribution of color in a hadron and the cross section of its interaction with
a nucleon. However, our result is rather sensitive to the pattern of the
expansion of the wave packet and significantly larger values of are not ruled out by the data. We show that recent CERN data confirm the
suggestion of [2] that color fluctuations of the strengths in
charmonium-nucleon interaction are the major source of suppression of the
yield as observed at CERN in both pA and AA collisions.Comment: 10 pages, 5 figures (one with color
Effects of the Neutron Spin-Orbit Density on Nuclear Charge Density in Relativistic Models
The neutron spin-orbit density contributes to the nuclear charge density as a
relativistic effect. The contribution is enhanced by the effective mass
stemming from the Lorentz-scalar potential in relativistic models. This
enhancement explains well the difference between the cross sections of elastic
electron scattering off Ca and Ca which was not reproduced in
non-relativistic models. The spin-orbit density will be examined in more detail
in electron scattering off unstable nuclei which would be available in the
future.Comment: 4 pages with 3 eps figures, revte
The hbar Expansion in Quantum Field Theory
We show how expansions in powers of Planck's constant hbar = h/2\pi can give
new insights into perturbative and nonperturbative properties of quantum field
theories. Since hbar is a fundamental parameter, exact Lorentz invariance and
gauge invariance are maintained at each order of the expansion. The physics of
the hbar expansion depends on the scheme; i.e., different expansions are
obtained depending on which quantities (momenta, couplings and masses) are
assumed to be independent of hbar. We show that if the coupling and mass
parameters appearing in the Lagrangian density are taken to be independent of
hbar, then each loop in perturbation theory brings a factor of hbar. In the
case of quantum electrodynamics, this scheme implies that the classical charge
e, as well as the fine structure constant are linear in hbar. The connection
between the number of loops and factors of hbar is more subtle for bound states
since the binding energies and bound-state momenta themselves scale with hbar.
The hbar expansion allows one to identify equal-time relativistic bound states
in QED and QCD which are of lowest order in hbar and transform dynamically
under Lorentz boosts. The possibility to use retarded propagators at the Born
level gives valence-like wave-functions which implicitly describe the sea
constituents of the bound states normally present in its Fock state
representation.Comment: 8 pages, 1 figure. Version to be published in Phys. Rev.
Suppression of Bremsstrahlung at Non-Zero Temperature
The first-order bremsstrahlung emission spectrum is
at zero temperature. If the radiation is emitted into a region that contains a
thermal distribution of photons, then the rate is increased by a factor
where is the Bose-Einstein function. The stimulated
emission changes the spectrum to for . If this were correct, an infinite amount of energy would be radiated in the
low frequency modes. This unphysical result indicates a breakdown of
perturbation theory. The paper computes the bremsstrahlung rate to all orders
of perturbation theory, neglecting the recoil of the charged particle. When the
perturbation series is summed, it has a different low-energy behavior. For
, the spectrum is independent of and has a value
proportional to .Comment: 16 pages (plain TeX), figures available on reques
Theoretical Expectations For High Mass Photon Pairs in L+ L- Gamma Gamma Events at LEP/SLC
Recently, the L3 collaboration has reported the observation of four events in
the reactions e+ e- --> L+ L- + (2 Photons), L = e, mu, tau, with the invariant
photon pair mass near 60 GeV in a data sample collected in the L3 detector
corresponding to 950,000 produced Z0's. More recently, more data from the other
LEP collaborations have become available. In this paper, we use the Monte Carlo
genrator YFS3 and our recent exact results on e+ e- --> L+ L- + (2 Photons) to
assess the QED expectations for such L3-type high mass photon pair events in e+
e- --> L+ L- + (n Photons) near the Z0 resonance.Comment: 9 pages (LaTeX + 6 uu-encoded figures), UTHEP-93-1002 (version with
corrected preprint number
Bloch-Nordsieck Thermometers: One-loop Exponentiation in Finite Temperature QED
We study the scattering of hard external particles in a heat bath in a
real-time formalism for finite temperature QED. We investigate the distribution
of the 4-momentum difference of initial and final hard particles in a fully
covariant manner when the scale of the process, , is much larger than the
temperature, . Our computations are valid for all subject to this
constraint. We exponentiate the leading infra-red term at one-loop order
through a resummation of soft (thermal) photon emissions and absorptions. For
, we find that tensor structures arise which are not present at .
These carry thermal signatures. As a result, external particles can serve as
thermometers introduced into the heat bath. We investigate the phase space
origin of and terms.Comment: LaTeX file, 29 pages including 3 figure
Radiative corrections to e+e- --> WW --> 4f with RACOONWW
RACOONWW is the first Monte Carlo generator for e+e- --> WW --> 4f(+gamma)
that includes the electroweak O(alpha) radiative corrections in the double-pole
approximation completely. Some numerical results for LEP2 energies are
discussed, and the predictions for the total W-pair cross section are
confronted with LEP2 data.Comment: 7 pages, latex, 5 postscript files, to appear in the Proceedings of
the 5th Zeuthen Workshop on Elementary Particle Theory ``Loops and Legs in
Quantum Field Theory'', Bastei/Koenigstein, Germany, April 9-14, 200
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