5,368 research outputs found
Exact solutions to Pauli-Villars-regulated field theories
We present a new class of quantum field theories which are exactly solvable.
The theories are generated by introducing Pauli-Villars fermionic and bosonic
fields with masses degenerate with the physical positive metric fields. An
algorithm is given to compute the spectrum and corresponding eigensolutions. We
also give the operator solution for a particular case and use it to illustrate
some of the tenets of light-cone quantization. Since the solutions of the
solvable theory contain ghost quanta, these theories are unphysical. However,
we also discuss how perturbation theory in the difference between the masses of
the physical and Pauli-Villars particles could be developed, thus generating
physical theories. The existence of explicit solutions of the solvable theory
also allows one to study the relationship between the equal-time and light-cone
vacua and eigensolutions.Comment: 20 pages, REVTeX; minor corrections to normalization
Two-boson truncation of Pauli-Villars-regulated Yukawa theory
We apply light-front quantization, Pauli-Villars regularization, and
numerical techniques to the nonperturbative solution of the dressed-fermion
problem in Yukawa theory in 3+1 dimensions. The solution is developed as a
Fock-state expansion truncated to include at most one fermion and two bosons.
The basis includes a negative-metric heavy boson and a negative-metric heavy
fermion in order to provide the necessary cancellations of ultraviolet
divergences. The integral equations for the Fock-state wave functions are
solved by reducing them to effective one-boson--one-fermion equations for
eigenstates with J_z=1/2. The equations are converted to a matrix equation with
a specially tuned quadrature scheme, and the lowest mass state is obtained by
diagonalization. Various properties of the dressed-fermion state are then
computed from the nonperturbative light-front wave functions. This work is a
major step in our development of Pauli-Villars regularization for the
nonperturbative solution of four-dimensional field theories and represents a
significant advance in the numerical accuracy of such solutions.Comment: 32 pages, 17 figures; requires elsart.cl
Intrinsic Charm Contribution to Double Quarkonium Hadroproduction
Double production has been observed by the NA3 collaboration in and collisions with a cross section of the order of 20-30 pb. The
pairs measured in nucleus interactions at 150 and 280
GeV are observed to carry an anomalously large fraction of the projectile
momentum in the laboratory frame, at 150 GeV and
at 280 GeV. We postulate that these forward pairs
are created by the materialization of Fock states in the projectile containing
two pairs of intrinsic quarks. We calculate the overlap of the
charmonium states with the
Fock state as described by the intrinsic charm model and find that the longitudinal momentum and invariant mass distributions
are both well reproduced. We also discuss double production in
interactions and the implications for other heavy quarkonium production
channels in QCD.Comment: Revtex, APS style, 7 pages, 3 figures in uuencoded fil
Dilepton production in proton-proton and quasifree proton-neutron reactions at 1.25 GeV
We investigate the pp --> pp e+e- and quasifree pn --> pn e+e- reactions
within an effective Lagrangian model at the laboratory kinetic energy of 1.25
GeV for which experimental data have recently been reported by the HADES
Collaboration. The model uses a meson-exchange approximation to describe the
initial nucleon-nucleon (NN) scattering. Contributions to the reaction
amplitudes are included from the NN bremsstrahlung as well as from the
excitation, propagation and radiative decay of the Delta(1230) isobar state. It
is found that the HADES data on the e+e- invariant mass distribution in the pp
--> pp e+e- reaction are reproduced excellently by our model where the Delta
isobar term dominates the spectrum. However, the quasifree pn --> pn e+e- cross
sections are underpredicted in the invariant mass region of 0.40 - 0.55 GeV/c^2
even after including contributions of the eta Dalitz decay and the subthreshold
production and decay of the rho^0 meson via the baryonic resonance N*(1520). In
the case of the quasifree pn --> pn e+e- reaction, a strong sensitivity to the
pion electromagnetic form factor is observed which helps to bring the
calculated cross sections closer to the data in the higher dilepton mass
region.Comment: 12 pages 3, figures, version to appear in Phys. Rev. C (Rapid
Communications
Nonperturbative Renormalization of QED in Light-Cone Quantization
As a precursor to work on QCD, we study the dressed electron in QED
nonperturbatively. The calculational scheme uses an invariant mass cutoff,
discretized light-cone quantization, a Tamm--Dancoff truncation of the Fock
space, and a small photon mass. Nonperturbative renormalization of the coupling
and electron mass is developed.Comment: Latex, 4 pages, 1 figur
On the nonperturbative solution of Pauli--Villars-regulated light-front QED: A comparison of the sector-dependent and standard parameterizations
We consider quantum electrodynamics quantized on the light front in Feynman
gauge and regulated in the ultraviolet by the inclusion of massive,
negative-metric Pauli--Villars (PV) particles in the Lagrangian. The eigenstate
of the electron is approximated by a Fock-state expansion truncated to include
one photon. The Fock-state wave functions are computed from the fundamental
Hamiltonian eigenvalue problem and used to calculate the anomalous magnetic
moment, as a point of comparison. Two approaches are considered: a
sector-dependent parameterization, where the bare parameters of the Lagrangian
are allowed to depend on the Fock sectors between which the particular
Hamiltonian term acts, and the standard choice, where the bare parameters are
the same for all sectors. Both methods are shown to require some care with
respect to ultraviolet divergences; neither method can allow all PV masses to
be taken to infinity. In addition, the sector-dependent approach suffers from
an infrared divergence that requires a nonzero photon mass; due to
complications associated with this divergence, the standard parameterization is
to be preferred. We also show that the self-energy effects obtained from a
two-photon truncation are enough to bring the standard-parameterization result
for the anomalous moment into agreement with experiment within numerical
errors. This continues the development of a method for the nonperturbative
solution of strongly coupled theories, in particular quantum chromodynamics.Comment: 28 pages, 2 figures, LaTeX with elsarticle.cl
Perturbative QCD and factorization of coherent pion photoproduction on the deuteron
We analyze the predictions of perturbative QCD for pion photoproduction on
the deuteron, gamma D -> pi^0 D, at large momentum transfer using the reduced
amplitude formalism. The cluster decomposition of the deuteron wave function at
small binding only allows the nuclear coherent process to proceed if each
nucleon absorbs an equal fraction of the overall momentum transfer.
Furthermore, each nucleon must scatter while remaining close to its mass shell.
Thus the nuclear photoproduction amplitude, M_{gamma D -> pi^0 D}(u,t),
factorizes as a product of three factors: (1) the nucleon photoproduction
amplitude, M_{gamma N_1 -> pi^0 N_1}(u/4,t/4), at half of the overall momentum
transfer, (2) a nucleon form factor, F_{N_2}(t/4), at half the overall momentum
transfer, and (3) the reduced deuteron form factor, f_d(t), which according to
perturbative QCD, has the same monopole falloff as a meson form factor. A
comparison with the recent JLAB data for gamma D -> pi^0 D of Meekins et al.
[Phys. Rev. C 60, 052201 (1999)] and the available gamma p -> pi^0 p data shows
good agreement between the perturbative QCD prediction and experiment over a
large range of momentum transfers and center of mass angles. The reduced
amplitude prediction is consistent with the constituent counting rule, p^11_T
M_{gamma D -> pi^0 D} -> F(theta_cm), at large momentum transfer. This is found
to be consistent with measurements for photon lab energies E_gamma > 3 GeV at
theta_cm=90 degrees and \elab > 10 GeV at 136 degrees.Comment: RevTeX 3.1, 17 pages, 6 figures; v2: incorporates minor changes as
version accepted by Phys Rev
A dynamical model for longitudinal wave functions in light-front holographic QCD
We construct a Schrodinger-like equation for the longitudinal wave function
of a meson in the valence qq-bar sector, based on the 't Hooft model for
large-N two-dimensional QCD, and combine this with the usual transverse
equation from light-front holographic QCD, to obtain a model for mesons with
massive quarks. The computed wave functions are compared with the wave function
ansatz of Brodsky and De Teramond and used to compute decay constants and
parton distribution functions. The basis functions used to solve the
longitudinal equation may be useful for more general calculations of meson
states in QCD.Comment: 12 pages, 2 figures, RevTeX 4.1; expanded discussion, with
calculation details moved to appendice
Classical sum rules and spin correlations in photoabsorption and photoproduction processes
In this paper we study the possibility of generalizing the classical
photoabsorption () sum rules, to processes
and crossed helicity amplitudes. In the first case, using detailed balance, the
sum rule is written as where is a kinematical constant which depends only
on the mass of the particles and the center of mass energy. For other crossed
helicity amplitudes, we show that there is a range of values of and for
which the differential cross section for the process or in which the helicities of the photon and particle have
specific values, is equal to the differential cross section for the process in
which one of these two helicities is reversed (parallel-antiparallel spin
correlation).Comment: 9 pages, 2 figure
Deuteron Electromagnetic Form Factors in the Intermediate Energy Region
Based on a Perturbative QCD analysis of the deuteron form factor, a model for
the reduced form factor is suggested. The numerical result is consistent with
the data in the intermediate energy region.Comment: 9 pages, to appear in Phys.Rev.
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