8,630 research outputs found
QCD Radiative Corrections to the Leptonic Decay Rate of the B_c Meson
The QCD radiative corrections to the leptonic decay rate of the meson
are calculated using the formalism of nonrelativistic QCD (NRQCD) to separate
short-distance and long-distance effects. The decay constant is factored
into a sum of NRQCD matrix elements each multiplied by a short-distance
coefficient. The short-distance coefficient for the leading matrix element is
calculated to order by matching a perturbative calculation in full
QCD with the corresponding perturbative calculation in NRQCD. This
short-distance correction decreases the leptonic decay rate by approximately
.Comment: Changed Eq. 2 to read 1/(8 \pi), put in a missing i M_{B_c} in Eq.
18, and put in a normalisation factor of 2 M_{B_c} in Eq. 19
Neutrino masses and flavor symmetries
The problem of neutrino masses and mixing angles is analysed in a class of
supersymmetric grand unified models, with SO(10) gauge symmetry and global U(2)
flavour symmetry. Adopting the seesaw mechanism for the generation of the
neutrino masses, one obtains a mass matrix for the left-handed neutrinos which
is directly related to the parameters of the charged sector, while the unknown
parameters of the right-handed Majorana mass matrix are inglobed in a single
factor.Comment: 17 pages, 1 eps figure, uses graphicx.sty, LaTeX 2e, to be published
on "Il Nuovo Cimento
The Hosotani Mechanism in Bulk Gauge Theories with an Orbifold Extra Space S^1/Z_2
We pursue the possibility of the scenario in which the Higgs field is
identified with the extra-space component of a bulk gauge field. The space-time
we take is M S/Z. We show that a non-trivial
Z-parity assignment allows some of the extra-space component to have
radiatively induced VEV, which strongly modifies the mass spectrum and gauge
symmetry of the theory, realized by oribifolding. In particular we investigate
the dynamical mass generation of zero-mode fermion and spontaneous gauge
symmetry breaking due to the VEV. The gauge theories we adopt are a prototype
model SU(2) and SU(3) model, of special interest as the realistic minimal
scheme to incorporate the standard model SU(2) U(1).Comment: 16 pages 3 figure
Less Minimal Flavour Violation
We consider the approximate U(2)^3 flavour symmetry exhibited by the quark
sector of the Standard Model and all its possible breaking terms appearing in
the quark Yukawa couplings. Taking an Effective Field Theory point of view, we
determine the current bounds on these parameters, assumed to control the
breaking of flavour in a generic extension of the Standard Model at a reference
scale Lambda. In particular, a significant bound from epsilon'/epsilon is
derived, which is relevant to Minimal Flavour Violation as well. In the
up-quark sector, the recently observed CP violation in D -> pi+ pi-, K+ K-
decays might be accounted for in this generic framework, consistently with any
other constraint.Comment: 15 pages, 1 figur
Calculation of the One- and Two-Loop Lamb Shift for Arbitrary Excited Hydrogenic States
General expressions for quantum electrodynamic corrections to the one-loop
self-energy [of order alpha(Zalpha)^6] and for the two-loop Lamb shift [of
order alpha^2(Z\alpha)^] are derived. The latter includes all diagrams with
closed fermion loops. The general results are valid for arbitrary excited non-S
hydrogenic states and for the normalized Lamb shift difference of S states,
defined as Delta_n = n^3 DeltaE(nS) - DeltaE(1S). We present numerical results
for one-loop and two-loop corrections for excited S, P and D states. In
particular, the normalized Lamb shift difference of S states is calculated with
an uncertainty of order 0.1 kHz.Comment: 4 pages, RevTe
U(2) and Maximal Mixing of nu_{mu}
A U(2) flavor symmetry can successfully describe the charged fermion masses
and mixings, and supress SUSY FCNC processes, making it a viable candidate for
a theory of flavor. We show that a direct application of this U(2) flavor
symmetry automatically predicts a mixing of 45 degrees for nu_mu to nu_s, where
nu_s is a light, right-handed state. The introduction of an additional flavor
symmetry acting on the right-handed neutrinos makes the model
phenomenologically viable, explaining the solar neutrino deficit as well as the
atmospheric neutrino anomaly, while giving a potential hot dark matter
candidate and retaining the theory's predictivity in the quark sector.Comment: 20 pages, 1 figur
Knockout of proton-neutron pairs from O with electromagnetic probes
After recent improvements to the Pavia model of two-nucleon knockout from
O with electromagnetic probes the calculated cross sections are compared
to experimental data from such reactions. Comparison with data from a
measurement of the O(e,epn) reaction show much better agreement
between experiment and theory than was previously observed. In a comparison
with recent data from a measurement of the O(,pn) reaction the
model over-predicts the measured cross section at low missing momentum.Comment: 6 pages, 5 figure
Logarithmic SUSY electroweak effects on four-fermion processes at TeV energies
We compute the MSSM one-loop contributions to the asymptotic energy behaviour
of fermion-antifermion pair production at future lepton-antilepton colliders.
Besides the conventional logarithms of Renormalization Group origin, extra SUSY
linear logarithmic terms appear of "Sudakov-type". In the TeV range their
overall effect on a variety of observables can be quite relevant and
drastically different from that obtained in the SM case.Comment: 19 pages and 14 figures, corrected version. e-mail:
[email protected]
Effects of frequency correlation in linear optical entangling gate operated with independent photons
Bose-Einstein coalescence of independent photons at the surface of a beam
splitter is the physical process that allows linear optical quantum gates to be
built. When distinct parametric down-conversion events are used as an
independent photon source, distinguishability arises form the energy
correlation of each photon with its twin. We find that increasing the pump
bandwidth may help in improving the visibility of non-classical interference
and reaching a level of near perfect indistinguishability. PACS: 03.67.Mn,
42.65.Lm, 42.50.St.Comment: Replaced with published versio
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