18,714 research outputs found
Yukawa Bound States of a Large Number of Fermions
We consider the bound state problem for a field theory that contains a Dirac
fermion that Yukawa couples to a (light) scalar field . We are
interested in bound states with a large number of particles. A Fermi
gas model is used to numerically determine the dependence of the radius of
these bound states on and also the dependence of the binding energy on .
Since scalar interactions with relativistic 's are suppressed two regimes
emerge. For modest values of the state is composed of non-relativistic
particles. In this regime as increases decreases. Eventually the
core region becomes relativistic and the size of the state starts to increase
as increases. As a result, for fixed Yukawa coupling and mass, there
is a minimum sized state that occurs roughly at the value of where the core
region first becomes relativistic. We also compute an elastic scattering form
factor that can be relevant for direct detection if the dark matter is composed
of such particles.Comment: 14 pages, 7 figure
Structure- and laser-gauges for the semiconductor Bloch equations in high-harmonic generation in solids
The semiconductor Bloch equations (SBEs) are routinely used for simulations
of strong-field laser-matter interactions in condensed matter. In systems
without inversion or time-reversal symmetries, the Berry connections and
transition dipole phases (TDPs) must be included in the SBEs, which in turn
requires the construction of a smooth and periodic structure gauge for the
Bloch states. Here, we illustrate a general approach for such a structure-gauge
construction for topologically trivial systems. Furthermore, we investigate the
SBEs in the length and velocity gauges, and discuss their respective advantages
and shortcomings for the high-harmonic generation (HHG) process. We find that
in cases where we require dephasing or separation of the currents into
interband and intraband contributions, the length gauge SBEs are
computationally more efficient. In calculations without dephasing and where
only the total current is needed, the velocity gauge SBEs are structure-gauge
independent and are computationally more efficient. We employ two systems as
numerical examples to highlight our findings: an 1D model of ZnO and the 2D
monolayer hexagonal boron nitride (h-BN). The omittance of Berry connections or
TDPs in the SBEs for h-BN results in nonphysical HHG spectra. The structure-
and laser-gauge considerations in the current work are not restricted to the
HHG process, and are applicable to all strong-field matter simulations with
SBEs
Effective Theory and Simple Completions for Neutrino Interactions
We consider all the dimension 6 operators as well as some simple extensions
of the standard model that give new contributions to neutrino interactions with
matter. Such interactions are usually parametrized by , where and are neutrino flavor indices taking the
values , and . In the simple models we consider the
's are much more constrained than in the
operator-based model-independent approach. Typically the 's are restricted to be smaller in magnitude than around . In
some of the leptoquark models, a specific pattern for the leptoquark Yukawa
couplings allows the diagonal element to be as large as
, or one of , . We discuss
the interplay between neutrino physics and leptoquark searches at the LHC.Comment: 12 pages, 2 figure
Lepton Flavorful Fifth Force and Depth-dependent Neutrino Matter Interactions
We consider a fifth force to be an interaction that couples to matter with a
strength that grows with the number of atoms. In addition to competing with the
strength of gravity a fifth force can give rise to violations of the
equivalence principle. Current long range constraints on the strength and range
of fifth forces are very impressive. Amongst possible fifth forces are those
that couple to lepton flavorful charges or . They
have the property that their range and strength are also constrained by
neutrino interactions with matter. In this brief note we review the existing
constraints on the allowed parameter space in gauged . We find two regions where neutrino oscillation experiments are at
the frontier of probing such a new force. In particular, there is an allowed
range of parameter space where neutrino matter interactions relevant for long
baseline oscillation experiments depend on the depth of the neutrino beam below
the surface of the earth.Comment: 6 pages, 5 figure
Strong CMB Constraint On P-Wave Annihilating Dark Matter
We consider a dark sector consisting of dark matter that is a Dirac fermion
and a scalar mediator. This model has been extensively studied in the past. If
the scalar couples to the dark matter in a parity conserving manner then dark
matter annihilation to two mediators is dominated by the P-wave channel and
hence is suppressed at very low momentum. The indirect detection constraint
from the anisotropy of the Cosmic Microwave Background is usually thought to be
absent in the model because of this suppression. In this letter we show that
dark matter annihilation to bound states occurs through the S-wave and hence
there is a constraint on the parameter space of the model from the Cosmic
Microwave Background.Comment: 5 pages, 3 figure
Supersymmetry and Goldstino-like Mode in Bose-Fermi Mixtures
Supersymmetry is assumed to be a basic symmetry of the world in many high
energy theories, but none of the super partners of any known elementary
particle has been observed yet. We argue that supersymmetry can also be
realized and studied in ultracold atomic systems with a mixture of bosons and
fermions, with properly tuned interactions and single particle dispersion. We
further show that in such non-releativistic systems supersymmetry is either
spontaneously broken, or explicitly broken by a chemical potential difference
between the bosons and fermions. In both cases the system supports a sharp
fermionic collective mode or the so-called Goldstino, due to supersymmetry. We
also discuss possible ways to detect the Goldstino mode experimentally.Comment: 4 pages. V4: published versio
Nanocrystalline iron at high pressure
X-ray diffraction measurements were performed on nanocrystalline iron up to 46 GPa. For nanocrystalline epsilon-Fe, analysis of lattice parameter data provides a bulk modulus, K, of 179±8 GPa and a pressure derivative of the bulk modulus, K[prime], of 3.6±0.7, similar to the large-grained control sample. The extrapolated zero-pressure unit cell volume of nanocrystalline epsilon-Fe is 22.9±0.2 Å^3, compared to 22.3±0.2 Å^3 for large-grained epsilon-Fe. No significant grain growth was observed to occur under pressure
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