6,260 research outputs found
On The Expected Photon Spectrum in B -> X_s + gamma and Its Uses
Measuring the photon energy spectrum in radiative B decays provides essential
help for gaining theoretical control over semileptonic B transitions. The
hadronic recoil mass distribution in B -> X_u \ell\nu promises the best
environment for determining |V_ub|. The theoretical uncertainties are largest
in the domain of low values of the lepton pair mass q^2. Universality relations
allow to describe this domain reliably in terms of the photon spectrum in B ->
X_s + \gamma. A method is proposed to incorporate 1/m_b corrections into this
relation. The low-E_\gamma tail in radiative decays is important in the context
of extracting |V_ub|. We argue that CLEO's recent fit to the spectrum
underestimates the fraction of the photon spectrum below 2 GeV. Potentially
significant uncertainties enter in the theoretical evaluation of the integrated
end-point lepton spectrum or the B -> X_u \ell\nu width with a too high value
of the lower cut on q^2 in alternative approaches to |V_ub|.Comment: 24 pages, 6 figures, LaTeX. Revised: Complete version. Numerical
predictions are improved and the estimate for the decay fraction revised. The
theoretical expectations for the decay fraction and the spectrum itself are
given on the plot
On the theory of pseudogap anisotropy in the cuprate superconductors
We show by means of the theory of the order parameter phase fluctuations that
the temperature of "closing" (or "opening") of the gap (and pseudogap) in the
electron spectra of superconductors with anisotropic order parameter takes
place within a finite temperature range. Every Fourier-component of the order
parameter has its own critical temperature
Right-Handed New Physics Remains Strangely Beautiful
Current data on CP violation in B_d -> eta' K_S and B_d -> phi K_S, taken
literally, suggest new physics contributions in b -> s transitions. Despite a
claim to the contrary, we point out that right-handed operators with a single
weak phase can account for both deviations thanks to the two-fold ambiguity in
the extraction of the weak phase from the corresponding CP-asymmetry. This
observation is welcome since large mixing in the right-handed sector is favored
by many GUT models and frameworks which address the flavor puzzle. There are
also interesting correlations with the B_s system which provide a way to test
this scenario in the near future.Comment: 7 pages, 9 figures; published version: added 1 reference and 1
clarificatio
Dynamics and thermalization of the nuclear spin bath in the single-molecule magnet Mn12-ac: test for the theory of spin tunneling
The description of the tunneling of a macroscopic variable in the presence of
a bath of localized spins is a subject of great fundamental and practical
interest, and is relevant for many solid-state qubit designs. Instead of
focusing on the the "central spin" (as is most often done), here we present a
detailed study of the dynamics of the nuclear spin bath in the Mn12-ac
single-molecule magnet, probed by NMR experiments down to very low temperatures
(T = 20 mK). We find that the longitudinal relaxation rate of the 55Mn nuclei
in Mn12-ac becomes roughly T-independent below T = 0.8 K, and can be strongly
suppressed with a longitudinal magnetic field. This is consistent with the
nuclear relaxation being caused by quantum tunneling of the molecular spin, and
we attribute the tunneling fluctuations to the minority of fast-relaxing
molecules present in the sample. The transverse nuclear relaxation is also
T-independent for T < 0.8 K, and can be explained qualitatively and
quantitatively by the dipolar coupling between like nuclei in neighboring
molecules. We also show that the isotopic substitution of 1H by 2H leads to a
slower nuclear longitudinal relaxation, consistent with the decreased tunneling
probability of the molecular spin. Finally, we demonstrate that, even at the
lowest temperatures, the nuclear spins remain in thermal equilibrium with the
lattice phonons, and we investigate the timescale for their thermal
equilibration. After a review of the theory of macroscopic spin tunneling in
the presence of a spin bath, we argue that most of our experimental results are
consistent with that theory, but the thermalization of the nuclear spins is
not.Comment: 24 pages, 18 figures. Experimental study of the spin bath dynamics in
quantum nanomagnets, plus an extensive review and application of the theor
On The Potential of Minimal Flavour Violation
Assuming the Minimal Flavour Violation hypothesis, we derive the general
scalar potential for fields whose background values are the Yukawa couplings.
We analyze the minimum of the potential and discuss the fine-tuning required to
dynamically generate the mass hierarchies and the mixings between different
quark generations. Two main cases are considered, corresponding to Yukawa
interactions being effective operators of dimension five or six (or,
equivalently, resulting from bi-fundamental and fundamental scalar fields,
respectively). At the renormalizable and classical level, no mixing is
naturally induced from dimension five Yukawa operators. On the contrary, from
dimension six Yukawa operators one mixing angle and a strong mass hierarchy
among the generations result.Comment: 33 pages, 6 figures; Note added in proof on the stability of the
minima of the scalar potential; results unchanged; references adde
On the Occurrence of Finite-Time-Singularities in Epidemic Models of Rupture, Earthquakes and Starquakes
We present a new kind of critical stochastic finite-time-singularity, relying
on the interplay between long-memory and extreme fluctuations. We illustrate it
on the well-established epidemic-type aftershock (ETAS) model for aftershocks,
based solely on the most solidly documented stylized facts of seismicity
(clustering in space and in time and power law Gutenberg-Richter distribution
of earthquake energies). This theory accounts for the main observations (power
law acceleration and discrete scale invariant structure) of critical rupture of
heterogeneous materials, of the largest sequence of starquakes ever attributed
to a neutron star as well as of earthquake sequences.Comment: Revtex document of 4 pages including 1 eps figur
BCS pairing in a trapped dipolar Fermi gase
We present a detailed study of the BCS pairing transition in a trapped
polarized dipolar Fermi gas. In the case of a shallow nearly spherical trap, we
find the decrease of the transition temperature as a function of the trap
aspect ratio and predict the existence of the optimal trap geometry. The latter
corresponds to the highest critical temperature of the BCS transition for a
given number of particles. We also derive the phase diagram for an ultracold
trapped dipolar Fermi gases in the situation, where the trap frequencies can be
of the order of the critical temperature of the BCS transition in the
homogeneous case, and find the critical value of the dipole-dipole interaction
energy, below which the BCS transition ceases to exist. The critical dipole
strength is obtained as a function of the trap aspect ratio. Alternatively, for
a given dipole strength there is a critical value of the trap anisotropy for
the BCS state to appear. The order parameter calculated at criticality,
exhibits nover non-monotonic behavior resulted from the combined effect of the
confining potential and anisotropic character of the interparticle
dipole-dipole interation.Comment: 14 pages, 3 figure
Dynamics of two interacting Bose-Einstein condensates
We analize the dynamics of two trapped interacting Bose-Einstein condensates
and indentify two regimes for the evolution: the regime of slow periodic
oscillations and the regime of strong non-linear mixing leading to the damping
of the relative motion of the condensates. We compare our predictions with an
experiment recently performed at JILA.Comment: 4 pages RevTeX, 3 eps figure
Prediction of extreme events in the OFC model on a small world network
We investigate the predictability of extreme events in a dissipative
Olami-Feder-Christensen model on a small world topology. Due to the mechanism
of self-organized criticality, it is impossible to predict the magnitude of the
next event knowing previous ones, if the system has an infinite size. However,
by exploiting the finite size effects, we show that probabilistic predictions
of the occurrence of extreme events in the next time step are possible in a
finite system. In particular, the finiteness of the system unavoidably leads to
repulsive temporal correlations of extreme events. The predictability of those
is higher for larger magnitudes and for larger complex network sizes. Finally,
we show that our prediction analysis is also robust by remarkably reducing the
accessible number of events used to construct the optimal predictor.Comment: 5 pages, 4 figure
Unnatural Origin of Fermion Masses for Technicolor
We explore the scenario in which the breaking of the electroweak symmetry is
due to the simultaneous presence and interplay of a dynamical sector and an
unnatural elementary Higgs. We introduce a low energy effective Lagrangian and
constrain the various couplings via direct search limits and electroweak and
flavor precision tests. We find that the model we study is a viable model of
dynamical breaking of the electroweak symmetry.Comment: 20 pages, 7 eps figure
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