312,696 research outputs found
A simple Proof of Stolarsky's Invariance Principle
Stolarsky [Proc. Amer. Math. Soc. 41 (1973), 575--582] showed a beautiful
relation that balances the sums of distances of points on the unit sphere and
their spherical cap -discrepancy to give the distance integral of
the uniform measure on the sphere a potential-theoretical quantity (Bj{\"o}rck
[Ark. Mat. 3 (1956), 255--269]). Read differently it expresses the worst-case
numerical integration error for functions from the unit ball in a certain
Hilbert space setting in terms of the -discrepancy and vice versa
(first author and Womersley [Preprint]). In this note we give a simple proof of
the invariance principle using reproducing kernel Hilbert spaces
Metastable dark matter mechanisms for INTEGRAL 511 keV rays and DAMA/CoGeNT events
We explore dark matter mechanisms that can simultaneously explain the
galactic 511 keV gamma rays observed by INTEGRAL/SPI, the DAMA/LIBRA annual
modulation, and the excess of low-recoil dark matter candidates observed by
CoGeNT. It requires three nearly degenerate states of dark matter in the 4-7
GeV mass range, with splittings respectively of order an MeV and a few keV. The
top two states have the small mass gap and transitions between them, either
exothermic or endothermic, can account for direct detections. Decays from one
of the top states to the ground state produce low-energy positrons in the
galaxy whose associated 511 keV gamma rays are seen by INTEGRAL. This decay can
happen spontaneously, if the excited state is metastable (longer-lived than the
age of the universe), or it can be triggered by inelastic scattering of the
metastable states into the shorter-lived ones. We focus on a simple model where
the DM is a triplet of an SU(2) hidden sector gauge symmetry, broken at the
scale of a few GeV, giving masses of order \lsim 1 GeV to the dark gauge
bosons, which mix kinetically with the standard model hypercharge. The purely
decaying scenario can give the observed angular dependence of the 511 keV
signal with no positron diffusion, while the inelastic scattering mechanism
requires transport of the positrons over distances \sim 1 kpc before
annihilating. We note that an x-ray line of several keV in energy, due to
single-photon decays involving the top DM states, could provide an additional
component to the diffuse x-ray background. The model is testable by proposed
low-energy fixed target experiments.Comment: 27 pp, 19 figures; v2. minor clarification, added refs; v3. corrected
observed rate of positron production, added new section responding to
criticisms of arXiv:0904.1025; v4. corrected typos in eqs. (6) and (40
Pair Correlation Function of Wilson Loops
We give a path integral prescription for the pair correlation function of
Wilson loops lying in the worldvolume of Dbranes in the bosonic open and closed
string theory. The results can be applied both in ordinary flat spacetime in
the critical dimension d or in the presence of a generic background for the
Liouville field. We compute the potential between heavy nonrelativistic sources
in an abelian gauge theory in relative collinear motion with velocity v =
tanh(u), probing length scales down to r_min^2 = 2 \pi \alpha' u. We predict a
universal -(d-2)/r static interaction at short distances. We show that the
velocity dependent corrections to the short distance potential in the bosonic
string take the form of an infinite power series in the dimensionless variables
z = r_min^2/r^2, uz/\pi, and u^2.Comment: 16 pages, 1 figure, Revtex. Corrected factor of two in potential.
Some changes in discussio
Chiral dynamics and peripheral transverse densities
In the partonic (or light-front) description of relativistic systems the
electromagnetic form factors are expressed in terms of frame-independent charge
and magnetization densities in transverse space. This formulation allows one to
identify the chiral components of nucleon structure as the peripheral densities
at transverse distances b = O(M_pi^{-1}) and compute them in a parametrically
controlled manner. A dispersion relation connects the large-distance behavior
of the transverse charge and magnetization densities to the spectral functions
of the Dirac and Pauli form factors near the two-pion threshold at timelike t =
4 M_pi^2. Using relativistic chiral effective field theory in the leading-order
approximation, we (a) derive the asymptotic behavior (Yukawa tail) of the
isovector transverse densities in the "chiral" region b = O(M_pi^{-1}) and the
"molecular" region b = O(M_N^2/M_pi^3); (b) perform the heavy-baryon expansion;
(c) explain the relative magnitude of the peripheral charge and magnetization
densities in a simple mechanical picture; (d) include Delta intermediate states
and study the densities in the large-N_c limit of QCD; (e) quantify the spatial
region where the chiral components are numerically dominant; (f) calculate the
chiral divergences of the b^2-weighted moments of the transverse densities
(charge and magnetic radii) and determine their spatial support. Our approach
provides a concise formulation of the spatial structure of the nucleon's chiral
component and offers new insights into basic properties of the chiral
expansion. It relates the information extracted from low-t elastic form factors
to the generalized parton distributions probed in peripheral high-energy
scattering processes.Comment: 52 pages, 13 figure
Non-linear screening of external charge by doped graphene
We solve a nonlinear integral equation for the electrostatic potential in
doped graphene due to an external charge, arising from a Thomas-Fermi (TF)
model for screening by graphene's electron bands. In particular, we study
the effects of a finite equilibrium charge carrier density in graphene,
non-zero temperature, non-zero gap between graphene and a dielectric substrate,
as well as the nonlinearity in the band density of states. Effects of the
exchange and correlation interactions are also briefly discussed for undoped
graphene at zero temperature. Nonlinear results are compared with both the
linearized TF model and the dielectric screening model within random phase
approximation (RPA). In addition, image potential of the external charge is
evaluated from the solution of the nonlinear integral equation and compared to
the results of linear models. We have found generally good agreement between
the results of the nonlinear TF model and the RPA model in doped graphene,
apart from Friedel oscillations in the latter model. However, relatively strong
nonlinear effects are found in the TF model to persist even at high doping
densities and large distances of the external charge.Comment: 12 pages including 6 figure
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