37 research outputs found
Low-energy QCD: Chiral coefficients and the quark-quark interaction
A detailed investigation of the low-energy chiral expansion is presented
within a model truncation of QCD. The truncation allows for a phenomenological
description of the quark-quark interaction in a framework which maintains the
global symmetries of QCD and permits a expansion. The model dependence
of the chiral coefficients is tested for several forms of the quark-quark
interaction by varying the form of the running coupling, , in the
infrared region. The pattern in the coefficients that arises at tree level is
consistent with large QCD, and is related to the model truncation.Comment: 28 pages, Latex, 6 postscript figures available on request to
[email protected]
Model of supersymmetric quantum field theory with broken parity symmetry
Recently, it was observed that self-interacting scalar quantum field theories
having a non-Hermitian interaction term of the form ,
where is a real positive parameter, are physically acceptable in the
sense that the energy spectrum is real and bounded below. Such theories possess
PT invariance, but they are not symmetric under parity reflection or time
reversal separately. This broken parity symmetry is manifested in a nonzero
value for , even if is an even integer. This paper extends
this idea to a two-dimensional supersymmetric quantum field theory whose
superpotential is . The resulting quantum
field theory exhibits a broken parity symmetry for all . However,
supersymmetry remains unbroken, which is verified by showing that the
ground-state energy density vanishes and that the fermion-boson mass ratio is
unity.Comment: 20 pages, REVTeX, 11 postscript figure
Gluon mass generation in the PT-BFM scheme
In this article we study the general structure and special properties of the
Schwinger-Dyson equation for the gluon propagator constructed with the pinch
technique, together with the question of how to obtain infrared finite
solutions, associated with the generation of an effective gluon mass.
Exploiting the known all-order correspondence between the pinch technique and
the background field method, we demonstrate that, contrary to the standard
formulation, the non-perturbative gluon self-energy is transverse
order-by-order in the dressed loop expansion, and separately for gluonic and
ghost contributions. We next present a comprehensive review of several subtle
issues relevant to the search of infrared finite solutions, paying particular
attention to the role of the seagull graph in enforcing transversality, the
necessity of introducing massless poles in the three-gluon vertex, and the
incorporation of the correct renormalization group properties. In addition, we
present a method for regulating the seagull-type contributions based on
dimensional regularization; its applicability depends crucially on the
asymptotic behavior of the solutions in the deep ultraviolet, and in particular
on the anomalous dimension of the dynamically generated gluon mass. A
linearized version of the truncated Schwinger-Dyson equation is derived, using
a vertex that satisfies the required Ward identity and contains massless poles
belonging to different Lorentz structures. The resulting integral equation is
then solved numerically, the infrared and ultraviolet properties of the
obtained solutions are examined in detail, and the allowed range for the
effective gluon mass is determined. Various open questions and possible
connections with different approaches in the literature are discussed.Comment: 54 pages, 24 figure
Mesons as qbar-q Bound States from Euclidean 2-Point Correlators in the Bethe-Salpeter Approach
We investigate the 2-point correlation function for the vector current. The
gluons provide dressings for both the quark self energy as well as the vector
vertex function, which are described consistently by the rainbow
Dyson-Schwinger equation and the inhomogeneous ladder Bethe-Salpeter equation.
The form of the gluon propagator at low momenta is modeled by a 2-parameter
ansatz fitting the weak pion decay constant. The quarks are confined in the
sense that the quark propagator does not have a pole at timelike momenta. We
determine the ground state mass in the vector channel from the Euclidean time
Fourier transform of the correlator, which has an exponential falloff at large
times. The ground state mass lies around 590 MeV and is almost independent of
the model form for the gluon propagator. This method allows us to stay in
Euclidean space and to avoid analytic continuation of the quark or gluon
propagators into the timelike region.Comment: 21 pages (REVTEX), 8 Postscript figure
Nonlocal symmetries of a class of scalar and coupled nonlinear ordinary differential equations of any order
In this paper we devise a systematic procedure to obtain nonlocal symmetries
of a class of scalar nonlinear ordinary differential equations (ODEs) of
arbitrary order related to linear ODEs through nonlocal relations. The
procedure makes use of the Lie point symmetries of the linear ODEs and the
nonlocal connection to deduce the nonlocal symmetries of the corresponding
nonlinear ODEs. Using these nonlocal symmetries we obtain reduction
transformations and reduced equations to specific examples. We find the reduced
equations can be explicitly integrated to deduce the general solutions for
these cases. We also extend this procedure to coupled higher order nonlinear
ODEs with specific reference to second order nonlinear ODEs.Comment: Accepted for publication in J. Phys. A Math. Theor. 201
The Massive Yang-Mills Model and Diffractive Scattering
We argue that the massive Yang-Mills model of Kunimasa and Goto, Slavnov, and
Cornwall, in which massive gauge vector bosons are introduced in a
gauge-invariant way without resorting to the Higgs mechanism, may be useful for
studying diffractive scattering of strongly interacting particles. We perform
in this model explicit calculations of S-matrix elements between quark states,
at tree level, one loop, and two loops, and discuss issues of renormalisability
and unitarity. In particular, it is shown that the S-matrix element for quark
scattering is renormalisable at one-loop order and is only logarithmically
non-renormalisable at two loops. The discrepancies in the ultraviolet regime
between the one-loop predictions of this model and those of massless QCD are
discussed in detail. Also, some of the similarities and differences between the
massive Yang-Mills model and theories with a Higgs mechanism are analysed at
the level of the S-matrix. Finally, we briefly discuss the high-energy
behaviour of the leading-order amplitude for quark-quark elastic scattering in
the diffractive region. The above analysis sets up the stage for carrying out a
systematic computation of the higher order corrections to the two-gluon
exchange model of the Pomeron using massive gluons inside quantum loops.Comment: 38 pages, Latex, with 7 PostScript pictures. Introduction and Section
6 further enlarged, minor typos correcte
The LOFAR pilot surveys for pulsars and fast radio transients
We have conducted two pilot surveys for radio pulsars and fast transients
with the Low-Frequency Array (LOFAR) around 140 MHz and here report on the
first low-frequency fast-radio burst limit and the discovery of two new
pulsars. The first survey, the LOFAR Pilot Pulsar Survey (LPPS), observed a
large fraction of the northern sky, ~1.4 x 10^4 sq. deg, with 1-hr dwell times.
Each observation covered ~75 sq. deg using 7 independent fields formed by
incoherently summing the high-band antenna fields. The second pilot survey, the
LOFAR Tied-Array Survey (LOTAS), spanned ~600 sq. deg, with roughly a 5-fold
increase in sensitivity compared with LPPS. Using a coherent sum of the 6 LOFAR
"Superterp" stations, we formed 19 tied-array beams, together covering 4 sq.
deg per pointing. From LPPS we derive a limit on the occurrence, at 142 MHz, of
dispersed radio bursts of 107 Jy
for the narrowest searched burst duration of 0.66 ms. In LPPS, we re-detected
65 previously known pulsars. LOTAS discovered two pulsars, the first with LOFAR
or any digital aperture array. LOTAS also re-detected 27 previously known
pulsars. These pilot studies show that LOFAR can efficiently carry out all-sky
surveys for pulsars and fast transients, and they set the stage for further
surveying efforts using LOFAR and the planned low-frequency component of the
Square Kilometer Array.Comment: 18 pages, 10 figures, accepted for A&