21 research outputs found
Effective Field Theory calculations of
In this review we present the recent advances for calculations of the
reactions using chiral effective field theory. Discussed are the
next-to-next-to leading order loop contributions with nucleon and Delta-isobar
for near threshold s-wave pion production. Results of recent experimental
pion-production data for energies close to the threshold are analyzed. Several
particular applications are discussed: (i) it is shown how the measured charge
symmetry violating pion-production reaction can be used to extract the
strong-interaction contribution to the proton-neutron mass difference; (ii) the
role of for the extraction of the pion-nucleon scattering lengths
from pionic atoms data is illuminated.Comment: 68 pages, 23 figures, to be published in Int. J. Mod. Phys.
Low-Energy Lepton-Proton Bremsstrahlung via Effective Field Theory
We present a systematic calculation of the cross section for the
lepton-proton bremsstrahlung process l + p --> l' + p + gamma in chiral
perturbation theory at next-to-leading order. This process corresponds to an
undetected background signal for the proposed MUSE experiment at PSI. MUSE is
designed to measure elastic scattering of low-energy electrons and muons off a
proton target in order to extract a precise value of the proton's r.m.s.
radius. We show that the commonly used peaking approximation, which is used to
evaluate the radiative tail for the elastic cross section, is not applicable
for muon-proton scattering at the low-energy MUSE kinematics. Furthermore, we
point out a certain pathology with the standard chiral power counting scheme
associated with electron scattering, whereby the next-to-next-to-leading order
contribution from the pion loop diagrams is kinematically enhanced and
numerically of the same magnitude as the next-to-leading order corrections. We
correct a misprint in a commonly cited review article.Comment: Accepted for Publication in European Physics Journal A (EPJA). arXiv
admin note: substantial text overlap with arXiv:1712.0996
Analytical Evaluation of Elastic Lepton-Proton Two-Photon Exchange in Chiral Perturbation Theory
We present an exact evaluation of the two-photon exchange contribution to the
elastic lepton-proton scattering process at low-energies using heavy baryon
chiral perturbation theory. The evaluation is performed including
next-to-leading order accuracy. This exact analytical evaluation contains all
soft and hard two-photon exchanges and we identify the contributions missing in
a soft-photon approximation approach. We evaluate the infrared divergent
four-point box diagrams analytically using dimensional regularization. We also
emphasize the differences between muon-proton and electron-proton scatterings
relevant to the MUSE kinematics due to lepton mass differences.Comment: 5 figures. Accepted for publication in Eur. Phys. J. A , A short
version was presented at the workshop in Trento July 202
Quark-quark correlations and baryon electroweak observables
The simple independent quark models have difficulties explaining
simultaneously the totality of the known hyperon magnetic moments and hyperon
semi-leptonic decay rates. We show that both the Goldstone boson loop
contributions and the two-quark effective exchange currents are essential in
explaining these observables.Comment: 7 pages, 1 figur
Fixed-Point Analysis of the Low-Energy Constants in the Pion-Nucleon Chiral Lagrangian
In the framework of heavy-baryon chiral perturbation theory, we investigate
the fixed point structure of renormalization group equations (RGE) for the
ratios of the renormalized low energy constants (LECs) that feature in the
pion-nucleon chiral Lagrangian. The ratios of the LECs deduced from our RGE
analysis are found to be in semi-quantitative agreement with those obtained
from direct fit to the experimental data. The naturalness of this agreement is
discussed using a simple dimensional analysis combined with Wilsonian RGEs.Comment: 10 page
Capture rate and neutron helicity asymmetry for ordinary muon capture on hydrogen
Applying heavy-baryon chiral perturbation theory to ordinary muon capture
(OMC) on a proton, we calculate the capture rate and neutron helicity asymmetry
up to next-to-next-to-leading order. For the singlet hyperfine state, we obtain
the capture rate Gamma_0 = 695 sec^{-1} while, for the triplet hyperfine state,
we obtain the capture rate Gamma_1 = 11.9 sec^{-1} and the neutron asymmetry
alpha_1 = 0.93. If the existing formalism is used to relate these atomic
capture rates to Gamma_{liq}, the OMC rate in liquid hydrogen, then Gamma_{liq}
corresponding to our improved values of Gamma_0 and Gamma_1 is found to be
significantly larger than the experimental value, primarily due to the updated
larger value of g_A. We argue that this apparent difficulity may be correlated
to the specious anomaly recently reported for mu^- + p to n + nu_mu + gamma,
and we suggest a possibility to remove these two "problems" simply and
simultaneously by reexamining the molecular physics input that underlies the
conventional analysis of Gamma_{liq}.Comment: 14 pages, 1 figur
The effect of Kaon Condensation on Quark-Antiquark Condensate in Dense Matter
Assuming that at sufficiently high densities the constituent quarks become
relevant degrees of freedom, we study within the framework of a chiral quark
model the influence of s-wave condensation on the quark-antiquark
condensates. We find that, in linear density approximation, the presence of a
condensate quenches the condensate, but that the
condensate remains unaffected up to the chiral order under consideration. We
discuss the implication of the suppressed condensate for
flavor-dependent chiral symmetry restoration in dense matterComment: 17 pages, 3 figures, two subsections are removed. To appear in Nucl.
Phys.