951 research outputs found
String-derived D4 flavor symmetry and phenomenological implications
In this paper we show how some flavor symmetries may be derived from the
heterotic string, when compactified on a 6D orbifold. In the body of the paper
we focus on the family symmetry, recently obtained in
orbifold constructions. We show how this flavor symmetry constrains fermion
masses, as well as the soft SUSY breaking mass terms. Flavor symmetry breaking
can generate the hierarchy of fermion masses and at the same time the flavor
symmetry suppresses large flavor changing neutral current processes.Comment: 17 pages, no figur
CP asymmetries in penguin-induced B decays in general left-right models
We study CP asymmetries in penguin-induced b -> s\bar{s}s decays in general
left-right models without imposing manifest or pseudomanifest left-right
symmetry. Using the effective Hamiltonian approach, we evaluate CP asymmetries
in B^\pm -> \phi K^{(\ast)\pm} decays as well as mixing induced B meson decays
B -> J/\psi K_s and B -> \phi K_s decays. Based on recent measurements
revealing large CP violation, we show that nonmanifest type model is more
favored than manifest or pseudomanifest type.Comment: 16 pages, 12 eps figure
Finite Hilbert stability of (bi)canonical curves
We prove that a generic canonically or bicanonically embedded smooth curve
has semistable m-th Hilbert points for all m. We also prove that a generic
bicanonically embedded smooth curve has stable m-th Hilbert points for all m
\geq 3. In the canonical case, this is accomplished by proving finite Hilbert
semistability of special singular curves with G_m-action, namely the
canonically embedded balanced ribbon and the canonically embedded balanced
double A_{2k+1}-curve. In the bicanonical case, we prove finite Hilbert
stability of special hyperelliptic curves, namely Wiman curves. Finally, we
give examples of canonically embedded smooth curves whose m-th Hilbert points
are non-semistable for low values of m, but become semistable past a definite
threshold.
(This paper subsumes the previous submission and arXiv:1110.5960).Comment: To appear in Inventiones Mathematicae, 2012. The final publication is
available at http://www.springerlink.co
Theory of biopolymer stretching at high forces
We provide a unified theory for the high force elasticity of biopolymers
solely in terms of the persistence length, , and the monomer spacing,
. When the force f>\fh \sim k_BT\xi_p/a^2 the biopolymers behave as Freely
Jointed Chains (FJCs) while in the range \fl \sim k_BT/\xi_p < f < \fh the
Worm-like Chain (WLC) is a better model. We show that can be estimated
from the force extension curve (FEC) at the extension
(normalized by the contour length of the biopolymer). After validating the
theory using simulations, we provide a quantitative analysis of the FECs for a
diverse set of biopolymers (dsDNA, ssRNA, ssDNA, polysaccharides, and
unstructured PEVK domain of titin) for . The success of a specific
polymer model (FJC or WLC) to describe the FEC of a given biopolymer is
naturally explained by the theory. Only by probing the response of biopolymers
over a wide range of forces can the -dependent elasticity be fully
described.Comment: 20 pages, 4 figure
Size Dependence of Metal-Insulator Transition in Stoichiometric Fe3O4 Nanocrystals
Magnetite (Fe3O4) is one of the most actively studied materials with a famous
metal-insulator transition (MIT), so-called the Verwey transition at around 123
K. Despite the recent progress in synthesis and characterization of Fe3O4
nanocrystals (NCs), it is still an open question how the Verwey transition
changes on a nanometer scale. We herein report the systematic studies on size
dependence of the Verwey transition of stoichiometric Fe3O4 NCs. We have
successfully synthesized stoichiometric and uniform-sized Fe3O4 NCs with sizes
ranging from 5 to 100 nm. These stoichiometric Fe3O4 NCs show the Verwey
transition when they are characterized by conductance, magnetization, cryo-XRD,
and heat capacity measurements. The Verwey transition is weakly size-dependent
and becomes suppressed in NCs smaller than 20 nm before disappearing completely
for less than 6 nm, which is a clear, yet highly interesting indication of a
size effect of this well-known phenomena. Our current work will shed new light
on this ages-old problem of Verwey transition.Comment: 18 pages, 4 figures, Nano Letters (accepted
B --> Phi K_S and Supersymmetry
The rare decay B --> Phi K_S is a well-known probe of physics beyond the
Standard Model because it arises only through loop effects yet has the same
time-dependent CP asymmetry as B --> Psi K_S. Motivated by recent data
suggesting new physics in B --> Phi K_S, we look to supersymmetry for possible
explanations, including contributions mediated by gluino loops and by Higgs
bosons. Chirality-preserving LL and RR gluino contributions are generically
small, unless gluinos and squarks masses are close to the current lower bounds.
Higgs contributions are also too small to explain a large asymmetry if we
impose the current upper limit on B(B_s --> mu mu). On the other hand,
chirality-flipping LR and RL gluino contributions can provide sizable effects
and while remaining consistent with related results in B --> Psi K_S, Delta
M_s, B --> X_s gamma and other processes. We discuss how the LR and RL
insertions can be distinguished using other observables, and we provide a
string-based model and other estimates to show that the needed sizes of mass
insertions are reasonable.Comment: 33 pages, 32 figures, Updated version for PRD. Includes discussions
of other recent works on this topic. Added discussions & plots for gluino
mass dependence and effects of theoretical uncertaintie
Role of quantum coherence in chromophoric energy transport
The role of quantum coherence and the environment in the dynamics of
excitation energy transfer is not fully understood. In this work, we introduce
the concept of dynamical contributions of various physical processes to the
energy transfer efficiency. We develop two complementary approaches, based on a
Green's function method and energy transfer susceptibilities, and quantify the
importance of the Hamiltonian evolution, phonon-induced decoherence, and
spatial relaxation pathways. We investigate the Fenna-Matthews-Olson protein
complex, where we find a contribution of coherent dynamics of about 10% and of
relaxation of 80%.Comment: 5 pages, 3 figures, included static disorder, correlated environmen
On the Origin of Peak-dip-hump Structure in the In-plane Optical Conductivity of the High Cuprates; Role of Antiferromagnetic Spin Fluctuations of Short Range Order
An improved U(1) slave-boson approach is applied to study the optical
conductivity of the two dimensional systems of antiferromagnetically correlated
electrons over a wide range of hole doping and temperature. Interplay between
the spin and charge degrees of freedom is discussed to explain the origin of
the peak-dip-hump structure in the in-plane conductivity of high
cuprates. The role of spin fluctuations of short range order(spin singlet pair)
is investigated. It is shown that the spin fluctuations of the short range
order can cause the mid-infrared hump, by exhibiting a linear increase of the
hump frequency with the antiferromagnetic Heisenberg coupling strength
- …