220 research outputs found
Chiral power counting of one- and two-body currents in direct detection of dark matter
We present a common chiral power-counting scheme for vector, axial-vector,
scalar, and pseudoscalar WIMP-nucleon interactions, and derive all one- and
two-body currents up to third order in the chiral expansion. Matching our
amplitudes to non-relativistic effective field theory, we find that chiral
symmetry predicts a hierarchy amongst the non-relativistic operators. Moreover,
we identify interaction channels where two-body currents that so far have not
been accounted for become relevant.Comment: 8 pages, 1 table; journal versio
Improved predictions for conversion in nuclei and Higgs-induced lepton flavor violation
Compared to and , the process
conversion in nuclei receives enhanced contributions from Higgs-induced lepton
flavor violation. Upcoming conversion experiments with drastically
increased sensitivity will be able to put extremely stringent bounds on
Higgs-mediated transitions. We point out that the theoretical
uncertainties associated with these Higgs effects, encoded in the couplings of
quark scalar operators to the nucleon, can be accurately assessed using our
recently developed approach based on Chiral Perturbation Theory that
cleanly separates two- and three-flavor observables. We emphasize that with
input from lattice QCD for the coupling to strangeness , hadronic
uncertainties are appreciably reduced compared to the traditional approach
where is determined from the pion--nucleon -term by means of an
relation. We illustrate this point by considering Higgs-mediated lepton
flavor violation in the Standard Model supplemented with higher-dimensional
operators, the two-Higgs-doublet model with generic Yukawa couplings, and the
Minimal Supersymmetric Standard Model. Furthermore, we compare bounds from
present and future conversion and experiments.Comment: 9 pages, 5 figures, journal versio
Extracting the chiral anomaly from gamma pi --> pi pi
We derive dispersive representations for the anomalous process gamma pi -->
pi pi with the pi pi P-wave phase shift as input. We investigate how in this
framework the chiral anomaly can be extracted from a cross-section measurement
using all data up to 1 GeV, and discuss the importance of a precise
representation of the gamma pi --> pi pi amplitude for the hadronic
light-by-light contribution to the anomalous magnetic moment of the muon.Comment: 10 pages, 3 figures; version published in Phys. Rev.
Dispersive determination of the HVP contribution to the muon g − 2
The determination of hadronic vacuum polarization (HVP) from e+e−→ hadrons cross-section data, in the energy region relevant for the anomalous magnetic moment of the muon, has recently been challenged by lattice-QCD calculations, especially for the intermediate window in Euclidean time. In these proceedings we review some frequently-asked questions on the comparison between data-driven and lattice-QCD evaluations of the HVP contribution
Two-pion contribution to hadronic vacuum polarization
We present a detailed analysis of data up to
in the framework of dispersion relations. Starting
from a family of -wave phase shifts, as derived from a previous
Roy-equation analysis of scattering, we write down an extended Omn\`es
representation of the pion vector form factor in terms of a few free parameters
and study to which extent the modern high-statistics data sets can be described
by the resulting fit function that follows from general principles of QCD. We
find that statistically acceptable fits do become possible as soon as potential
uncertainties in the energy calibration are taken into account, providing a
strong cross check on the internal consistency of the data sets, but preferring
a mass of the meson significantly lower than the current PDG average.
In addition to a complete treatment of statistical and systematic errors
propagated from the data, we perform a comprehensive analysis of the systematic
errors in the dispersive representation and derive the consequences for the
two-pion contribution to hadronic vacuum polarization. In a global fit to both
time- and space-like data sets we find and . While the constraints are
thus most stringent for low energies, we obtain uncertainty estimates
throughout the whole energy range that should prove valuable in corroborating
the corresponding contribution to the anomalous magnetic moment of the muon. As
side products, we obtain improved constraints on the -wave,
valuable input for future global analyses of low-energy scattering, as
well as a determination of the pion charge radius, .Comment: 40 pages, 16 figures, 13 tables; version published in JHE
Accurate evaluation of hadronic uncertainties in spin-independent WIMP-nucleon scattering: Disentangling two- and three-flavor effects
We show how to avoid unnecessary and uncontrolled assumptions usually made in
the literature about soft SU(3) flavor symmetry breaking in determining the
two-flavor nucleon matrix elements relevant for direct detection of WIMPs.
Based on SU(2) Chiral Perturbation Theory, we provide expressions for the
proton and neutron scalar couplings and with the
pion-nucleon sigma-term as the only free parameter, which should be used in the
analysis of direct detection experiments. This approach for the first time
allows for an accurate assessment of hadronic uncertainties in spin-independent
WIMP-nucleon scattering and for a reliable calculation of isospin-violating
effects. We find that the traditional determinations of and
are off by a factor of 2.Comment: 6 pages, 2 figures; improved numerical analysis, journal versio
Isospin violation in low-energy pion-nucleon scattering revisited
We calculate isospin breaking in pion-nucleon scattering in the threshold
region in the framework of covariant baryon chiral perturbation theory. All
effects due to quark mass differences as well as real and virtual photons are
consistently included. As an application, we discuss the energy dependence of
the triangle relation that connects elastic scattering on the proton pi+- p -->
pi+- p with the charge exchange reaction pi- p --> pi0 n.Comment: 82 pages, 22 figures, version published in Nucl. Phys.
- …