596 research outputs found
A New Spin on Neutrino Quantum Kinetics
Recent studies have demonstrated that in anisotropic environments a coherent
spin-flip term arises in the Quantum Kinetic Equations (QKEs) which govern the
evolution of neutrino flavor and spin in hot and dense media. This term can
mediate neutrino-antineutrino transformation for Majorana neutrinos and
active-sterile transformation for Dirac neutrinos. We discuss the physical
origin of the coherent spin-flip term and provide explicit expressions for the
QKEs in a two-flavor model with spherical geometry. In this context, we
demonstrate that coherent neutrino spin transformation depends on the absolute
neutrino mass and Majorana phases.Comment: 11 pages, 3 figures, Major changes compared to v1, accepted for
publication in Physics Letters
Neutrino Quantum Kinetics
We present a formulation of the quantum kinetic equations (QKEs) which govern
the evolution of neutrino flavor at high density and temperature. Here, the
QKEs are derived from the ground up, using fundamental neutrino interactions
and quantum field theory. We show that the resulting QKEs describe coherent
flavor evolution with an effective mass when inelastic scattering is
negligible. The QKEs also contain a collision term. This term can reduce to the
collision term in the Boltzmann equation when scattering is dominant and the
neutrino effective masses and density matrices become diagonal in the
interaction basis. We also find that the QKE's include equations of motion for
a new dynamical quantity related to neutrino spin. This quantity decouples from
the equations of motion for the density matrices at low densities or in
isotropic conditions. However, the spin equations of motion allow for the
possibility of coherent transformation between neutrinos and antineutrinos at
high densities and in the presence of anisotropy. Although the requisite
conditions for this exist in the core collapse supernova and compact object
merger environments, it is likely that only a self consistent incorporation of
the QKEs in a sufficiently realistic model could establish whether or not
significant neutrino-antineutrino conversion occurs.Comment: Revised version, published in Physical Review
The pionic beta decay in chiral perturbation theory
Within the framework of chiral perturbation theory with virtual photons and
leptons, we present an updated analysis of the pionic beta decay including all
electromagnetic contributions of order e**2 p**2. We discuss the extraction of
the Cabibbo-Kobayashi-Maskawa matrix element |Vud| from experimental data. The
method employed here is consistent with the analogous treatment of the Kl3
decays and the determination of |Vus|.Comment: 8 pages, 1 figure, latex file, uses EPJC macro
Radiative corrections to K_{l3} decays
We present a complete calculation of the K_{l3} decays K^+ --> pi^0 l^+ nu_l
and K^0 --> pi^- l^+ nu_l to O(p^4, (m_d-m_u) p^2, e^2 p^2) in chiral
perturbation theory with virtual photons and leptons. We introduce the concept
of generalized form factors and kinematical densities in the presence of
electromagnetism, and propose a possible treatment of the real photon emission
in K^+_{l3} decays. We illustrate our results by applying them to the
extraction of the Kobayashi--Maskawa matrix element |V_{us}| from the
experimental K^+_{e3} decay parameters.Comment: 13 page
<VAP> Green Function in the Resonance Region
We analyse the three-point function of vector, axial-vector and
pseudoscalar currents. In the spirit of large N_C, a resonance dominated Green
function is confronted with the leading high-energy behaviour from the operator
product expansion. The matching is shown to be fully compatible with a chiral
resonance Lagrangian and it allows to determine some of the chiral low-energy
constants of O(p^6).Comment: 13 pages, 2 figures. Published version. Results and conclusions
unchange
Shining LUX on Isospin-Violating Dark Matter Beyond Leading Order
Isospin-violating dark matter (IVDM) has been proposed as a viable scenario
to reconcile conflicting positive and null results from direct detection dark
matter experiments. We show that the lowest-order dark matter-nucleus
scattering rate can receive large and nucleus-dependent corrections at
next-to-leading order (NLO) in the chiral expansion. The size of these
corrections depends on the specific couplings of dark matter to quark flavors
and gluons. In general the full NLO dark-matter-nucleus cross-section is not
adequately described by just the zero-energy proton and neutron couplings.
These statements are concretely illustrated in a scenario where the dark matter
couples to quarks through scalar operators. We find the canonical IVDM scenario
can reconcile the null XENON and LUX results and the recent CDMS-Si findings
provided its couplings to second and third generation quarks either lie on a
special line or are suppressed. Equally good fits with new values of the
neutron-to-proton coupling ratio are found in the presence of nonzero heavy
quark couplings. CDMS-Si remains in tension with LUX and XENON10/100 but is not
excluded.Comment: 11 pages, 5 figure
Neutrinoless double beta decay in chiral effective field theory: lepton number violation at dimension seven
We analyze neutrinoless double beta decay () within the
framework of the Standard Model Effective Field Theory. Apart from the
dimension-five Weinberg operator, the first contributions appear at dimension
seven. We classify the operators and evolve them to the electroweak scale,
where we match them to effective dimension-six, -seven, and -nine operators. In
the next step, after renormalization group evolution to the QCD scale, we
construct the chiral Lagrangian arising from these operators. We develop a
power-counting scheme and derive the two-nucleon currents up
to leading order in the power counting for each lepton-number-violating
operator. We argue that the leading-order contribution to the decay rate
depends on a relatively small number of nuclear matrix elements. We test our
power counting by comparing nuclear matrix elements obtained by various methods
and by different groups. We find that the power counting works well for nuclear
matrix elements calculated from a specific method, while, as in the case of
light Majorana neutrino exchange, the overall magnitude of the matrix elements
can differ by factors of two to three between methods. We calculate the
constraints that can be set on dimension-seven lepton-number-violating
operators from experiments and study the interplay between
dimension-five and -seven operators, discussing how dimension-seven
contributions affect the interpretation of in terms of the
effective Majorana mass .Comment: Matches version published in JHE
Neutrinoless Double Beta Decay and Lepton Flavor Violation
We point out that extensions of the Standard Model with low scale (~TeV)
lepton number violation (LNV) generally lead to a pattern of lepton flavor
violation (LFV) experimentally distinguishable from the one implied by models
with GUT scale LNV. As a consequence, muon LFV processes provide a powerful
diagnostic tool to determine whether or not the effective neutrino mass can be
deduced from the rate of neutrinoless double beta decay. We discuss the role of
\mu -> e \gamma and \mu -> e conversion in nuclei, which will be studied with
high sensitivity in forthcoming experiments.Comment: 4 pages, 3 figure
V(us) Determination from Hyperon Semileptonic Decays
We analyze the numerical determination of the quark mixing factor V(us) from
hyperon semileptonic decays. The discrepancies between the results obtained in
two previous studies are clarified. Our fits indicate sizeable SU(3) breaking
corrections, which unfortunately can only be fully determined from the data at
the first order. The lack of a reliable theoretical calculation of second-order
symmetry breaking effects translates into a large systematic uncertainty, which
has not been taken into account previously. Our final result, V(us) = 0.226 +/-
0.005, is not competitive with the existing determinations from K(l3), K(l2)
and \tau decays.Comment: 16 pages, no figures. References added and other minor change
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