197 research outputs found
Diffusion with rearranging traps
A model for diffusion on a cubic lattice with a random distribution of traps
is developed. The traps are redistributed at certain time intervals. Such
models are useful for describing systems showing dynamic disorder, such as
ion-conducting polymers. In the present model the traps are infinite, unlike an
earlier version with finite traps, this model has a percolation threshold. For
the infinite trap version a simple analytical calculation is possible and the
results agree qualitatively with simulation.Comment: Latex, five figure
Large effects on \BsBs mixing by vector-like quarks
We calculate the contributions of the vector-like quark model to \BsBs
mixing, taking into account the constraints from the decay . In
this model the neutral bosons mediate flavor-changing interactions at the tree
level. However, \BsBs mixing is dominated by contributions from the box
diagrams with the top quark and the extra up-type quark. In sizable ranges of
the model parameters, the mixing parameter is much different from the
standard model prediction.Comment: 11 pages, 4 figures, To be published in Phys. Rev.
B_s --> mu+ mu- decay in the R-parity violating minimal supergravity
We study B_s --> mu+ mu- in the context of the R-parity violating minimal
supergravity in the high tan beta regime. We find that the lowest value of the
branching ratio can go well below the present LHCb sensitivity and hence B_s
--> mu+ mu- can even be invisible to the LHC. We also find that the present
upper bound on Br(B_s --> mu+ mu-) puts strong constraint on the minimal
supergravity parameter space. The constraints become more severe if the upper
bound is close to its standard model prediction.Comment: 18 pages, 10 figures; version to be published in European Physical
Journal
Dynamical properties of the unitary Fermi gas: collective modes and shock waves
We discuss the unitary Fermi gas made of dilute and ultracold atoms with an
infinite s-wave inter-atomic scattering length. First we introduce an efficient
Thomas-Fermi-von Weizsacker density functional which describes accurately
various static properties of the unitary Fermi gas trapped by an external
potential. Then, the sound velocity and the collective frequencies of
oscillations in a harmonic trap are derived from extended superfluid
hydrodynamic equations which are the Euler-Lagrange equations of a
Thomas-Fermi-von Weizsacker action functional. Finally, we show that this
amazing Fermi gas supports supersonic and subsonic shock waves.Comment: 9 pages, 3 figures, invited talk at the International Workshop
"Critical Stability 2011" (Erice, October 2011), to be published in the
journal Few Body System
\eta-superconductivity in the Hubbard chain with pair hopping
The ground state phase diagram of the 1D Hubbard chain with pair-hopping
interaction is studied. The analysis of the model is performed using the
continuum-limit field theory approach and exact diagonalization studies. At
half-filling the phase diagram is shown to consist of two superconducting
states with Cooper pair center-of-mass momentum Q=0 (BCS-\eta_0 phase) and
Q=\pi (\eta_\pi-phase) and four insulating phases corresponding to the Mott
antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW)
insulator as well as an unconventional insulating phase characterized by the
coexistence of a CDW and a bond-located staggered magnetization. Away from
half-filling the phase diagram consists of the superconducting BCS-\eta_0 and
\eta_\pi phases and the metallic Luttinger-liquid phase. The BCS-\eta_0 phase
exhibits smooth crossover from a weak-coupling BCS type to a strong coupling
local-pair regime. The \eta_\pi phase shows properties of the doublon (zero
size Cooper pair) superconductor with Cooper pair center-of-mass momentum
Q=\pi. The transition into the \eta_\pi- paired state corresponds to an abrupt
change in the groundstate structure. After the transition the conduction band
is completely destroyed and a new \eta_\pi-pair band corresponding to the
strongly correlated doublon motion is created.Comment: 15 pages Revtex, 15 embedded eps figure
and in the Two Higgs Doublet Model with Flavor Changing Neutral Currents
A study of and is presented in the context of a Two Higgs Doublet
Model (2HDM) with flavor changing scalar currents (FCSC). Implications of the
model for the -parameter and for are also considered. The
experimental data on places stringent constraints on the model
parameters. The configuration of the model needed to account for is found
to be irreconcilable with constraints from and . In
particular, if R^{\rm exp}_b>R^{\sss{\rm SM}}_b persists then this version of
2HDM will be ruled out or require significant modifications. Noting that
aspects of the experimental analysis for and may be of some
concern, we also disregard and and give
predictions for these using constraints from and
parameter only. We emphasize the theoretical and experimental advantages of the
observable R_{b+c}\equiv \Gamma(Z\to b\bar b\mbox{ or } c\bar
c)/\Gamma(Z\to\mbox{hadrons}). We also stress the role of R_\ell\equiv
\Gamma(Z\to\mbox{hadrons})/\Gamma(Z\to \ell^+\ell^-) in testing the Standard
Model (SM) despite its dependence on QCD corrections. Noting that in models
with FCNC the amplitude for receives a contribution which grows
with , the importance and uniqueness of precision
measurements for constraining flavor changing currents is
underscored.Comment: 35 pages, 5 Postscript figures, 10 Postscript files used in the tex
file, uses epsf.st
Big GABA II: Water-referenced edited MR spectroscopy at 25 research sites
Accurate and reliable quantification of brain metabolites measured in vivo using 1H magnetic resonance spectroscopy (MRS) is a topic of continued interest. Aside from differences in the basic approach to quantification, the quantification of metabolite data acquired at different sites and on different platforms poses an additional methodological challenge. In this study, spectrally edited γ-aminobutyric acid (GABA) MRS data were analyzed and GABA levels were quantified relative to an internal tissue water reference. Data from 284 volunteers scanned across 25 research sites were collected using GABA+ (GABA + co-edited macromolecules (MM)) and MM-suppressed GABA editing. The unsuppressed water signal from the volume of interest was acquired for concentration referencing. Whole-brain T1-weighted structural images were acquired and segmented to determine gray matter, white matter and cerebrospinal fluid voxel tissue fractions. Water-referenced GABA measurements were fully corrected for tissue-dependent signal relaxation and water visibility effects. The cohort-wide coefficient of variation was 17% for the GABA + data and 29% for the MM-suppressed GABA data. The mean within-site coefficient of variation was 10% for the GABA + data and 19% for the MM-suppressed GABA data. Vendor differences contributed 53% to the total variance in the GABA + data, while the remaining variance was attributed to site- (11%) and participant-level (36%) effects. For the MM-suppressed data, 54% of the variance was attributed to site differences, while the remaining 46% was attributed to participant differences. Results from an exploratory analysis suggested that the vendor differences were related to the unsuppressed water signal acquisition. Discounting the observed vendor-specific effects, water-referenced GABA measurements exhibit similar levels of variance to creatine-referenced GABA measurements. It is concluded that quantification using internal tissue water referencing is a viable and reliable method for the quantification of in vivo GABA levels
Structure of the Odd-A, Shell-stabilized nucleus No102253
In-beam γ-ray spectroscopic measurements have been made on No102253. A single rotational band was identified up to a probable spin of 39/2, which is assigned to the 7/2+[624] Nilsson configuration. The bandhead energy and the moment of inertia provide discriminating tests of contemporary models of the heaviest nuclei. Novel methods were required to interprete the sparse data set associated with cross sections of around 50nb. These methods included comparisons of experimental and simulated spectra, as well as testing for evidence of a rotational band in the γγ matrix
Orientation dependence in molecular dynamics simulations of shocked single crystals
The entry distribution in angular momentum and excitation energy for the formation of 254No has been measured after the 208Pb reaction at 215 and 219 MeV. This nucleus is populated up to spin 22h and excitation energy >˜6 MeV above the yrast line, with the half-maximum points of the energy distributions at ˜5 MeV for spins between 12h and 22h. This suggests that the fission barrier is >˜5 MeV and that the shell-correction energy persists to high spin
- …