155,696 research outputs found
Model independent constraints on leptoquarks from b -> s l+ l- processes
We list all scalar and vector leptoquark states that contribute to the b -> s
l+ l- effective Hamiltonian. There are altogether three scalar and four vector
leptoquarks that are relevant. For contribution of each state we infer the
correlations between effective operators and find that only two baryon
number-violating vector leptoquarks give rise to scalar and pseudoscalar
four-fermion operators, whereas the scalar states can contribute to those
operators only when two states with same charge are present. We bound the
resulting Wilson coefficients by imposing experimental constraints coming from
branching fractions of B -> K l+ l-, B_s -> mu+ mu-, and B -> X_s mu+ mu-
decays.Comment: 10 pages, 3 figures, 1 table; v2: minor corrections, added reference
The demagnetization factor for randomly packed spheroidal particles
We investigate if the demagnetization factor for a randomly packed powder of
magnetic spheroidal particles depend on the shape of the spheroidal particles
and what the internal variation in magnetization is within such a powder. A
spheroid is an ellipsoid of revolution, i.e. an ellipsoid with two semi-major
axis being equal. The demagnetization factor is calculated as function of
particle aspect ratio using two independent numerical models for several
different packings, and assuming a relative permeability of 2. The calculated
demagnetization factor is shown to depend on particle aspect ratio, not because
of direct magnetic interaction but because the particle packing depend on the
aspect ratio of the particles. The relative standard deviation of the
magnetization across the powder was 3\%-8\%, increasing as the particle shape
deviates from spherical, while the relative standard deviation within each
particle was relatively constant around 5\%.Comment: 7 pages, 9 figure
Vortex structures of rotating spin-orbit coupled Bose-Einstein condensates
We consider the quasi-2D two-component Bose-Einstein condensates with Rashba
spin-orbit (SO) coupling in a rotating trap. An external Zeeman term favoring
spin polarization along the radial direction is also considered, which has the
same form as the non-canonical part of the mechanical angular momentum. The
rotating condensate exhibits rich structures as varying the strengths of
trapping potential and interaction. With a strong trapping potential, the
condensate exhibits a half-quantum vortex-lattice configuration. Such a
configuration is driven to the normal one by introducing the external radial
Zeeman field. In the case of a weak trap potential, the condensate exhibits a
multi-domain pattern of plane-wave states under the external radial Zeeman
field.Comment: 8 pages, 7 figures, two figures are adde
Statistical Inference for Time-changed Brownian Motion Credit Risk Models
We consider structural credit modeling in the important special case where
the log-leverage ratio of the firm is a time-changed Brownian motion (TCBM)
with the time-change taken to be an independent increasing process. Following
the approach of Black and Cox, one defines the time of default to be the first
passage time for the log-leverage ratio to cross the level zero. Rather than
adopt the classical notion of first passage, with its associated numerical
challenges, we accept an alternative notion applicable for TCBMs called "first
passage of the second kind". We demonstrate how statistical inference can be
efficiently implemented in this new class of models. This allows us to compare
the performance of two versions of TCBMs, the variance gamma (VG) model and the
exponential jump model (EXP), to the Black-Cox model. When applied to a 4.5
year long data set of weekly credit default swap (CDS) quotes for Ford Motor
Co, the conclusion is that the two TCBM models, with essentially one extra
parameter, can significantly outperform the classic Black-Cox model.Comment: 21 pages, 3 figures, 2 table
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