154 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.
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
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
Approximate Treatment of Lepton Distortion in Charged-Current Neutrino Scattering from Nuclei
The partial-wave expansion used to treat the distortion of scattered
electrons by the nuclear Coulomb field is simpler and considerably less
time-consuming when applied to the production of muons and electrons by low and
intermediate-energy neutrinos. For angle-integrated cross sections, however, a
modification of the "effective-momentum-transfer" approximation seems to work
so well that for muons the full distorted-wave treatment is usually
unnecessary, even at kinetic energies as low as an MeV and in nuclei as heavy
as lead. The method does not work as well for electron production at low
energies, but there a Fermi function usually proves adequate. Scattering of
electron-neutrinos from muon decay on iodine and of atmospheric neutrinos on
iron are discussed in light of these results.Comment: 11 pages, LaTeX, submitted to Phys. Rev.
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
Soft-photon corrections in multi-body meson decays
The effects due to soft-photon emission (and the related virtual corrections)
in multi-body decays of B, D, and K mesons are analysed. We present analytic
expressions for the universal O(alpha) correction factors which can be applied
to all multi-body decay modes where a tight soft-photon energy cut in the
decaying-particle rest-frame is applied. All-order resummations valid in the
limit of small and large velocities of the final-state particles are also
discussed. The phenomenological implications of these correction factors in the
distortion of Dalitz-plot distributions of K -> 3 pi decays are briefly
analysed.Comment: 8 pages, 2 figures (v2: minor modifications - published version
Z-boson as "the standard candle" for high precision W-boson physics at LHC
In this paper we propose a strategy for measuring the inclusive W-boson
production processes at LHC. This strategy exploits simultaneously the unique
flexibility of the LHC collider in running variable beam particle species at
variable beam energies, and the configuration flexibility of the LHC detectors.
We propose their concrete settings for a precision measurement of the Standard
Model parameters. These dedicated settings optimise the use of the Z boson and
Drell-Yan pair production processes as ``the standard reference candles''. The
presented strategy allows to factorise and to directly measure those of the QCD
effects which affect differently the W and Z production processes. It reduces
to a level of 10^{-4} the impact of uncertainties in the partonic distribution
functions (PDFs) and in the transverse momentum of the quarks on the
measurement precision. Last but not the least, it reduces by a factor of 10 an
impact of systematic measurement errors, such as the energy scale and the
measurement resolution, on the W-boson production observables.Comment: 20 pages, 4 figure
Mesonic decay constants in lattice NRQCD
Lattice NRQCD with leading finite lattice spacing errors removed is used to
calculate decay constants of mesons made up of heavy quarks. Quenched
simulations are done with a tadpole improved gauge action containing plaquette
and six-link rectangular terms. The tadpole factor is estimated using the
Landau link. For each of the three values of the coupling constant considered,
quarkonia are calculated for five masses spanning the range from charmonium
through bottomonium, and one set of quark masses is tuned to the B(c).
"Perturbative" and nonperturbative meson masses are compared. One-loop
perturbative matching of lattice NRQCD with continuum QCD for the heavy-heavy
vector and axial vector currents is performed. The data are consistent with the
vector meson decay constants of quarkonia being proportional to the square root
of their mass and the B(c) decay constant being equal to 420(13) MeV.Comment: 25 pages in REVTe
Microscopic calculation of the inclusive electron scattering structure function in O-16
We calculate the charge form factor and the longitudinal structure function
for O and compare with the available experimental data, up to a momentum
transfer of 4 fm. The ground state correlations are generated using the
coupled cluster [exp(S}] method, together with the realistic v-18 NN
interaction and the Urbana IX three-nucleon interaction. Center-of-mass
corrections are dealt with by adding a center-of-mass Hamiltonian to the usual
internal Hamiltonian, and by means of a many-body expansion for the computation
of the observables measured in the center-of-mass system
- …