909 research outputs found
Neutron skin uncertainties of Skyrme energy density functionals
Background: Neutron-skin thickness is an excellent indicator of isovector
properties of atomic nuclei. As such, it correlates strongly with observables
in finite nuclei that depend on neutron-to-proton imbalance and the nuclear
symmetry energy that characterizes the equation of state of neutron-rich
matter. A rich worldwide experimental program involving studies with rare
isotopes, parity violating electron scattering, and astronomical observations
is devoted to pinning down the isovector sector of nuclear models. Purpose: We
assess the theoretical systematic and statistical uncertainties of neutron-skin
thickness and relate them to the equation of state of nuclear matter, and in
particular to nuclear symmetry energy parameters. Methods: We use the nuclear
superfluid Density Functional Theory with several Skyrme energy density
functionals and density dependent pairing. To evaluate statistical errors and
their budget, we employ the statistical covariance technique. Results: We find
that the errors on neutron skin increase with neutron excess. Statistical
errors due to uncertain coupling constants of the density functional are found
to be larger than systematic errors, the latter not exceeding 0.06 fm in most
neutron-rich nuclei across the nuclear landscape. The single major source of
uncertainty is the poorly determined slope L of the symmetry energy that
parametrizes its density dependence. Conclusions: To provide essential
constraints on the symmetry energy of the nuclear energy density functional,
next-generation measurements of neutron skins are required to deliver precision
better than 0.06 fm.Comment: 5 pages, 4 figure
Recent Developments in Precision Electroweak Physics
Developments in precision electroweak physics in the two years since the
symposium are briefly summarized.Comment: Update on recent developments, prepared for the publication of the
Proceedings of Alberto Sirlin Symposium, New York University, October 2000.
10 pages, 1 figur
Masses of a Fourth Generation with Two Higgs Doublets
We use sampling techniques to find robust constraints on the masses of a
possible fourth sequential fermion generation from electroweak oblique
variables. We find that in the case of a light (115 GeV) Higgs from a single
electroweak symmetry breaking doublet, inverted mass hierarchies are possible
for both quarks and leptons, but a mass splitting more than M(W) in the quark
sector is unlikely. We also find constraints in the case of a heavy (600 GeV)
Higgs in a single doublet model. As recent data from the Large Hadron Collider
hints at the existence of a resonance at 124.5 GeV and a single Higgs doublet
at that mass is inconsistent with a fourth fermion generation, we examine a
type II two Higgs doublet model. In this model, there are ranges of parameter
space where the Higgs sector can potentially counteract the effects of the
fourth generation. Even so, we find that such scenarios produce qualitatively
similar fermion mass distributions.Comment: v2: 9 pages, 7 figures, improved analysis of Higgs decay constraints,
typos corrected and reference adde
Weak mixing angle at low energies
We determine the weak mixing angle in the MS-bar scheme at energy scales
relevant for present and future low energy electroweak measurements. We relate
the renormalization group evolution of the weak mixing angle to the
corresponding evolution of the QED coupling and include higher-order terms in
alpha_s and alpha that had not been treated in previous analyses. We also
up-date the analysis of non-perturbative hadronic contributions and argue that
the associated uncertainty is small compared to anticipated experimental
errors. The resulting value of the low-energy weak mixing angle is sin^2
theta_W (0) = 0.23867 +- 0.00016.Comment: 21 pages; 1 figure and some references added, some changes in text;
final version as publishe
A Simple Analytic Solution for Tachyon Condensation
In this paper we present a new and simple analytic solution for tachyon
condensation in open bosonic string field theory. Unlike the B_0 gauge
solution, which requires a carefully regulated discrete sum of wedge states
subtracted against a mysterious "phantom" counter term, this new solution
involves a continuous integral of wedge states, and no regularization or
phantom term is necessary. Moreover, we can evaluate the action and prove Sen's
conjecture in a mere few lines of calculation.Comment: 44 pages
Fluctuations around the Tachyon Vacuum in Open String Field Theory
We consider quadratic fluctuations around the tachyon vacuum numerically in
open string field theory. We work on a space spanned
by basis string states used in the Schnabl's vacuum solution. We show that the
truncated form of the Schnabl's vacuum solution on is
well-behaved in numerical work. The orthogonal basis for the new BRST operator
on and the quadratic forms of potentials
for independent fields around the vacuum are obtained. Our numerical results
support that the Schnabl's vacuum solution represents the minimum energy
solution for arbitrary fluctuations also in open string field theory.Comment: 16 pages, 2 figures, some comments and one table added, version to
appear in JHE
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