130 research outputs found
The method of unitary clothing transformations in the theory of nucleon-nucleon scattering
The clothing procedure, put forward in quantum field theory by Greenberg and
Schweber, is applied for the description of nucleon-nucleon (N-N) scattering.
We consider pseudoscalar, vector and scalar meson fields interacting with 1/2
spin fermion ones via the Yukawa-type couplings to introduce trial interactions
between "bare" particles. The subsequent unitary clothing transformations are
found to express the total Hamiltonian through new interaction operators that
refer to particles with physical (observable) properties, the so-called clothed
particles. In this work, we are focused upon the Hermitian and
energy-independent operators for the clothed nucleons, being built up in the
second order in the coupling constants. The corresponding analytic expressions
in momentum space are compared with the separate meson contributions to the
one-boson-exchange potentials in the meson theory of nuclear forces. In order
to evaluate the T-matrix of the N-N scattering we have used an equivalence
theorem that enables us to operate in the clothed particle representation (CPR)
instead of the bare particle representation with its huge amount of virtual
processes. We have derived the Lippmann-Schwinger-type equation for the CPR
elements of the T-matrix for a given collision energy in the two-nucleon sector
of the Hilbert space of hadronic states and elaborated a code for its numerical
solution in momentum space.Comment: 10 pages, 3 eps figures, proceedings of "19th International IUPAP
Conference on Few-Body Problems in Physics"(FB19), 30Aug-05Sep 2009, Bonn,
German
Reaction Mechanisms of the Proton - Deuteron Breakup Process at GeV Energies
The deuteron fragmentation by fast protons has been studied both near the
kinematics of quasi-free proton - proton scattering and far away from it. We
have concentrated on the interplay between different reaction mechanisms
associated with the antisymmetrization of the initial and final states and
rescattering contributions. A multiple-scattering-expansion technique has been
applied to evaluate the reaction amplitude. An essential element of this
approach in the momentum representation is the use of the effective nucleon-
nucleon interaction constructed by Love and Franey as a two-body t-matrix for
the incident proton scattering on a bound nucleon in the deuteron. Along with
the five-fold cross sections, the proton analyzing power and the deuteron
analyzing powers have been calculated as function of the momentum of the
outgoing fast proton. The results are compared with the data obtained by the
Gatchina-Saclay collaboration.Comment: LaTeX, 27 pages, 11 figures, to be published in Few Body System
The One-Body and Two-Body Density Matrices of Finite Nuclei and Center-of-Mass Correlations
A method is presented for the calculation of the one-body and two-body
density matrices and their Fourier transforms in momentum space, that is
consistent with the requirement for translational invariance, in the case of a
nucleus (a finite self-bound system). We restore translational invariance by
using the so-called fixed center-of-mass approximation for constructing an
intrinsic nuclear ground state wavefunction by starting from a
non-translationally invariant wavefunction and applying a projection
prescription. We discuss results for the one-body and two-body momentum
distributions of the 4He nucleus calculated with the Slater determinant of the
harmonic oscillator orbitals, as the initial non-translationally invariant
wavefunction. Effects of such an inclusion of CM correlations are found to be
quite important in the momentum distributions.Comment: 5 pages, incl. 2 figures; Proc. Int. Conf. on Frontiers in Nuclear
Structure, Astrophysics and Reactions (FINUSTAR), Kos, Greece, Sept.200
The one-body and two-body density matrices of finite nuclei with an appropriate treatment of the center-of-mass motion
The one-body and two-body density matrices in coordinate space and their
Fourier transforms in momentum space are studied for a nucleus (a
nonrelativistic, self-bound finite system). Unlike the usual procedure,
suitable for infinite or externally bound systems, they are determined as
expectation values of appropriate intrinsic operators, dependent on the
relative coordinates and momenta (Jacobi variables) and acting on intrinsic
wavefunctions of nuclear states. Thus, translational invariance (TI) is
respected. When handling such intrinsic quantities, we use an algebraic
technique based upon the Cartesian representation, in which the coordinate and
momentum operators are linear combinations of the creation and annihilation
operators a^+ and a for oscillator quanta. Each of the relevant multiplicative
operators can then be reduced to the form: one exponential of the set {a^+}
times other exponential of the set {a}. In the course of such a normal-ordering
procedure we offer a fresh look at the appearance of "Tassie-Barker" factors,
and point out other model-independent results. The intrinsic wavefunction of
the nucleus in its ground state is constructed from a
nontranslationally-invariant (nTI) one via existing projection techniques. As
an illustration, the one-body and two-body momentum distributions (MDs) for the
4He nucleus are calculated with the Slater determinant of the
harmonic-oscillator model as the trial, nTI wavefunction. We find that the TI
introduces important effects in the MDs.Comment: 13 pages, incl. 3 figures - to appear in Eur. Phys. J.
Modelling a Particle Detector in Field Theory
Particle detector models allow to give an operational definition to the
particle content of a given quantum state of a field theory. The commonly
adopted Unruh-DeWitt type of detector is known to undergo temporary transitions
to excited states even when at rest and in the Minkowski vacuum. We argue that
real detectors do not feature this property, as the configuration "detector in
its ground state + vacuum of the field" is generally a stable bound state of
the underlying fundamental theory (e.g. the ground state-hydrogen atom in a
suitable QED with electrons and protons) in the non-accelerated case. As a
concrete example, we study a local relativistic field theory where a stable
particle can capture a light quantum and form a quasi-stable state. As
expected, to such a stable particle correspond energy eigenstates of the full
theory, as is shown explicitly by using a dressed particle formalism at first
order in perturbation theory. We derive an effective model of detector (at
rest) where the stable particle and the quasi-stable configurations correspond
to the two internal levels, "ground" and "excited", of the detector.Comment: 13 pages, references added, final versio
- …