2,734 research outputs found
CORE and the Haldane Conjecture
The Contractor Renormalization group formalism (CORE) is a real-space
renormalization group method which is the Hamiltonian analogue of the Wilson
exact renormalization group equations. In an earlier paper\cite{QGAF} I showed
that the Contractor Renormalization group (CORE) method could be used to map a
theory of free quarks, and quarks interacting with gluons, into a generalized
frustrated Heisenberg antiferromagnet (HAF) and proposed using CORE methods to
study these theories. Since generalizations of HAF's exhibit all sorts of
subtle behavior which, from a continuum point of view, are related to
topological properties of the theory, it is important to know that CORE can be
used to extract this physics. In this paper I show that despite the folklore
which asserts that all real-space renormalization group schemes are necessarily
inaccurate, simple Contractor Renormalization group (CORE) computations can
give highly accurate results even if one only keeps a small number of states
per block and a few terms in the cluster expansion. In addition I argue that
even very simple CORE computations give a much better qualitative understanding
of the physics than naive renormalization group methods. In particular I show
that the simplest CORE computation yields a first principles understanding of
how the famous Haldane conjecture works for the case of the spin-1/2 and spin-1
HAF.Comment: 36 pages, 4 figures, 5 tables, latex; extensive additions to conten
Extrapolation of K to \pi\pi decay amplitude
We examine the uncertainties involved in the off-mass-shell extrapolation of
the decay amplitude with emphasis on those aspects that
have so far been overlooked or ignored. Among them are initial-state
interactions, choice of the extrapolated kaon field, and the relation between
the asymptotic behavior and the zeros of the decay amplitude. In the inelastic
region the phase of the decay amplitude cannot be determined by strong
interaction alone and even its asymptotic value cannot be deduced from
experiment. More a fundamental issue is intrinsic nonuniqueness of off-shell
values of hadronic matrix elements in general. Though we are hampered with
complexity of intermediate-energy meson interactions, we attempt to obtain a
quantitative idea of the uncertainties due to the inelastic region and find
that they can be much larger than more optimistic views portray.Comment: 16 pages with 5 eps figures in REVTE
Probing New Physics From CP Violation in Radiative B Decays
When new CP-violating interactions are dominated by flavor changing neutral
particle exchanges, that may occur in many extensions of the standard model. We
examine a type 3 two Higgs doublet model and find that direct CP asymmetries
can be as large as about 25% . Time-dependent and time-integrated
mixing-induced CP asymmetries up to 85 and 40 %, respectively, are possible
without conflict with other constraints. It mainly requirs an enhanced
chromo-magnetic dipole decay to be close to the present experimental
bound.Comment: 7 pages, latex, no figure
Is the ground state of Yang-Mills theory Coulombic?
We study trial states modelling the heavy quark-antiquark ground state in
SU(2) Yang-Mills theory. A state describing the flux tube between quarks as a
thin string of glue is found to be a poor description of the continuum ground
state; the infinitesimal thickness of the string leads to UV artifacts which
suppress the overlap with the ground state. Contrastingly, a state which
surrounds the quarks with non-abelian Coulomb fields is found to have a good
overlap with the ground state for all charge separations. In fact, the overlap
increases as the lattice regulator is removed. This opens up the possibility
that the Coulomb state is the true ground state in the continuum limit.Comment: 10 pages, 9 .eps figure
Model independent properties of two-photon exchange in elastic electron proton scattering
We derive from first principles, as the C-invariance of the electromagnetic
interaction and the crossing symmetry, the general properties of two-photon
exchange in electron-proton elastic scattering. We show that the presence of
this mechanism destroys the linearity of the Rosenbluth separation.Comment: 12 pages, no figures- Corrected misprints, changes in P. 7. No
changes in conclusion
Higher Twist Effects in the Drell-Yan Angular Distribution
We study the Drell-Yan process at large
using perturbative QCD. A higher-twist mechanism suggested by Berger and
Brodsky is known to qualitatively explain the observed dependence of the
muon angular distribution, but the predicted large behavior differs
quantitatively from observations. We have repeated the model calculation taking
into account the effects of nonasymptotic kinematics. At fixed-target energies
we find important corrections which improve the agreement with data. The
asymptotic result of Berger and Brodsky is recovered only at much higher
energies. We discuss the generic reasons for the large corrections at high
. A proper understanding of the data would give important
information on the pion distribution amplitude and exclusive form factor.Comment: 8 pages in Latex with 3 figures appended as Postscript files,
HU-TFT-94-12, LBL-35430. (The introductory part has been slightly altered and
three references have been added
Flavor Changing Scalar Interactions
The smallness of fermion masses and mixing angles has recently been been
attributed to approximate global symmetries, one for each fermion type.
The parameters associated with these symmetry breakings are estimated here
directly from observed masses and mixing angles. It turns out that although
flavor changing reaction rates may be acceptably small in electroweak theories
with several scalar doublets without imposing any special symmetries on the
scalars themselves, such theories generically yield too much CP violation in
the neutral kaon mass matrix. Hence in these theories CP must also be a good
approximate symmetry. Such models provide an alternative mechanism for CP
violation and have various interesting phenomenological features.Comment: 18 pages. UTTG-22-92; LBL 33016; UCB 92/3
On the Gerasimov-Drell-Hearn sum rule for the deuteron
The Gerasimov-Drell-Hearn sum rule is evaluated for the deuteron by explicit
integration up to 550 MeV including contributions from the photodisintegration
channel and from coherent and incoherent single pion production as well. The
photodisintegration channel converges fast enough in this energy range and
gives a large negative contribution, essentially from the resonant
state near threshold. Its absolute value is about the same size as the sum of
proton and neutron GDH values. It is only partially cancelled by the single
pion production contribution. But the incoherent channel has not reached
convergence at 550 MeV.Comment: 6 pages latex including 3 postscript figures, talk at the 15th Int.
Conf. on Few-Body Problems in Physics, Groningen, Netherlands, 22-26 July
1997. To be published in Nucl. Phys.
A Perturbative/Variational Approach to Quantum Lattice Hamiltonians
We propose a method to construct the ground state of local
lattice hamiltonians with the generic form , where
is a coupling constant and is a hamiltonian with a non degenerate ground
state . The method is based on the choice of an exponential ansatz
, which is a sort of generalized
lattice version of a Jastrow wave function. We combine perturbative and
variational techniques to get succesive approximations of the operator
. Perturbation theory is used to set up a variational method which
in turn produces non perturbative results. The computation with this kind of
ansatzs leads to associate to the original quantum mechanical problem a
statistical mechanical system defined in the same spatial dimension. In some
cases these statistical mechanical systems turn out to be integrable, which
allow us to obtain exact upper bounds to the energy. The general ideas of our
method are illustrated in the example of the Ising model in a transverse field.Comment: 27 pages, three .ps figures appended, DFTUZ 94-2
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