8,882 research outputs found
Effects of Relativistic Dynamics in near Threshold
The cross-section for threshold production in proton-proton
collisions is evaluated in the framework of the covariant spectator
description. The negative energy intermediate states are included
non-perturbatively and seen to yield a considerably smaller contribution, when
compared to perturbative treatments. A family of OBE-models with different
off-shell scalar coupling is considered.Comment: 10 pages, 3 figures, 1 tabl
On the number of classes of conjugate Hall subgroups in finite simple groups
In this paper we find the number of conjugate -Hall subgroups in all
finite almost simple groups. We also complete the classification of -Hall
subgroups in finite simple groups and correct some mistakes from our previous
paper.Comment: article in press in "Journal of algebra
On first-order phase transition in microcanonical and canonical non-extensive systems
Two examples of Microcanonical Potts models, 2-dimensional nearest neighbor
and mean field, are considered via exact enumeration of states and analytical
asymptotic methods. In the interval of energies corresponding to a first order
phase transition, both of these models exhibit a convex dip in the entropy vs
energy plot and a region with negative specific heat within the dip. It is
observed that in the nearest neighbor model the dip flattens and disappears as
the lattice size grows, while in the mean field model the dip persists even in
the limit of an infinite system. If formal transitions from microcanonical to
canonical ensembles and back are performed for an infinite but non-extensive
system, the convex dip in the microcanonical entropy plot disappears.Comment: 10 pages, 8 figure
New Numerical Method for Fermion Field Theory
A new deterministic, numerical method to solve fermion field theories is
presented. This approach is based on finding solutions to the lattice
functional equations for field theories in the presence of an external source
. Using Grassmann polynomial expansions for the generating functional ,
we calculate propagators for systems of interacting fermions. These
calculations are straightforward to perform and are executed rapidly compared
to Monte Carlo. The bulk of the computation involves a single matrix inversion.
Because it is not based on a statistical technique, it does not have many of
the difficulties often encountered when simulating fermions. Since no
determinant is ever calculated, solutions to problems with dynamical fermions
are handled more easily. This approach is very flexible, and can be taylored to
specific problems based on convenience and computational constraints. We
present simple examples to illustrate the method; more general schemes are
desirable for more complicated systems.Comment: 24 pages, latex, figures separat
D-Instantons in Non-Critical Open String Theory
We show that the strength of the leading non-perturbative effects in
non-critical string theory is of the order . We show
how this restricts the space of consistent theories. We also identify
non-critical one dimensional D-instantons as dynamical objects which exchange
closed string states and calculate the order of their size.Comment: 11 pages, latex, no figures, revised version accepted for publication
in Physics Letters
Conference Discussion of the Nuclear Force
Discussion of the nuclear force, lead by a round table consisting of T.
Cohen, E. Epelbaum, R. Machleidt, and F. Gross (chair). After an invited talk
by Machleidt, published elsewhere in these proceedings, brief remarks are made
by Epelbaum, Cohen, and Gross, followed by discussion from the floor moderated
by the chair. The chair asked the round table and the participants to focus on
the following issues: (i) What does each approach (chiral effective field
theory, large Nc, and relativistic phenomenology) contribute to our knowledge
of the nuclear force? Do we need them all? Is any one transcendent? (ii) How
important for applications (few body, nuclear structure, EMC effect, for
example) are precise fits to the NN data below 350 MeV? How precise do these
fits have to be? (iii) Can we learn anything about nonperturbative QCD from
these studies of the nuclear force? The discussion presented here is based on a
video recording made at the conference and transcribed afterward.Comment: Discussion at the 21st European Conference on Few Body Problems
(EFP21) held at Salamanca, Spain, 30 Aug - 3 Sept 201
Barrier Paradox in the Klein Zone
We study the solutions for a one-dimensional electrostatic potential in the
Dirac equation when the incoming wave packet exhibits the Klein paradox (pair
production). With a barrier potential we demonstrate the existence of multiple
reflections (and transmissions). The antiparticle solutions which are
necessarily localized within the barrier region create new pairs with each
reflection at the potential walls. Consequently we encounter a new paradox for
the barrier because successive outgoing wave amplitudes grow geometrically.Comment: 10 page
A New Thermodynamics, From Nuclei to Stars
Equilibrium statistics of Hamiltonian systems is correctly described by the
microcanonical ensemble. Classically this is the manifold of all points in the
body phase space with the given total energy. Due to Boltzmann's principle,
, its geometrical size is related to the entropy
. This definition does not invoke any information theory, no
thermodynamic limit, no extensivity, and no homogeneity assumption, as are
needed in conventional (canonical) thermo-statistics. Therefore, it describes
the equilibrium statistics of extensive as well of non-extensive systems. Due
to this fact it is the {\em fundamental} definition of any classical
equilibrium statistics. It can address nuclei and astrophysical objects as
well. All kind of phase transitions can be distinguished sharply and uniquely
for even small systems. For transitions in nuclear physics the scaling to an
hypothetical uncharged nuclear matter with an ratio like realistic
nuclei is not needed.Comment: 17 pages, 7 figures, Latex file, presented at XLI International
Winter Meeting on Nuclear Physics, Bormio, It. Figure captions now in more
detai
Nuclear matter in the chiral limit and the in-medium chiral condensate
We investigate nuclear matter, i.e. the nuclear equation-of-state (EOS) as
well as the relativistic mean fields in the chiral limit. The investigations
are based on a chiral nucleon-nucleon EFT interaction where the explicit and
implicit pion mass dependence is known up to next-to-leading order. The nuclear
bulk properties are found to remain fairly stable in the chiral limit. Based on
the same interaction the in-medium scalar condensate is derived, both in
Hartree-Fock approximation as well as from the Brueckner G-matrix, making
thereby use of the Hellman-Feynman theorem. Short distance physics which
determines the reduction of the in-medium nucleon mass is found to play only a
minor role for the reduction of the chiral condensate.Comment: 30 pages, 5 figs. To appear in Nuclear Physics
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