354 research outputs found
Chiral order and fluctuations in multi-flavour QCD
Multi-flavour (N_f>=3) Chiral Perturbation Theory (ChPT) may exhibit
instabilities due to vacuum fluctuations of sea q-bar q pairs. Keeping the
fluctuations small would require a very precise fine-tuning of the low-energy
constants L_4 and L_6 to L_4[crit](M_rho) = - 0.51 * 10^(-3), and
L_6[crit](M_rho) = - 0.26 * 10^(-3). A small deviation from these critical
values -- like the one suggested by the phenomenology of OZI-rule violation in
the scalar channel -- is amplified by huge numerical factors inducing large
effects of vacuum fluctuations. This would lead in particular to a strong
N_f-dependence of chiral symmetry breaking and a suppression of multi-flavour
chiral order parameters. A simple resummation is shown to cure the instability
of N_f>=3 ChPT, but it modifies the standard expressions of some O(p^2) and
O(p^4) low-energy parameters in terms of observables. On the other hand, for
r=m_s/m > 15, the two-flavour condensate is not suppressed, due to the
contribution induced by massive vacuum s-bar s pairs. Thanks to the latter, the
standard two-flavour ChPT is protected from multi-flavour instabilities and
could provide a well-defined expansion scheme in powers of non-strange quark
masses.Comment: Published versio
The two-nucleon electromagnetic charge operator in chiral effective field theory (EFT) up to one loop
The electromagnetic charge operator in a two-nucleon system is derived in
chiral effective field theory (EFT) up to order (or N4LO), where
denotes the low-momentum scale and is the electric charge. The specific
form of the N3LO and N4LO corrections from, respectively, one-pion-exchange and
two-pion-exchange depends on the off-the-energy-shell prescriptions adopted for
the non-static terms in the corresponding potentials. We show that different
prescriptions lead to unitarily equivalent potentials and accompanying charge
operators. Thus, provided a consistent set is adopted, predictions for physical
observables will remain unaffected by the non-uniqueness associated with these
off-the-energy-shell effects.Comment: 16 pages, 10 figure
Electromagnetic Structure and Reactions of Few-Nucleon Systems in EFT
We summarize our recent work dealing with the construction of the
nucleon-nucleon potential and associated electromagnetic currents up to one
loop in chiral effective field theory (EFT). The magnetic dipole
operators derived from these currents are then used in hybrid calculations of
static properties and low-energy radiative capture processes in few-body
nuclei. A preliminary set of results are presented for the magnetic moments of
the deuteron and trinucleons and thermal neutron captures on , , and
He.Comment: Invited talk to the 19th International IUPAP Conference on Few-Body
Problems in Physic
Analysis and interpretation of new low-energy Pi-Pi scattering data
The recently published E865 data on charged K_e4 decays and Pi-Pi phases are
reanalyzed to extract values of the two S-wave scattering lengths, of the
subthreshold parameters alpha and beta, of the low-energy constants l3-bar and
l4-bar as well as of the main two-flavour order parameters: and F_pi
in the limit m_u = m_d = 0 taken at the physical value of the strange quark
mass. Our analysis is exclusively based on direct experimental information on
Pi-Pi phases below 800 MeV and on the new solutions of the Roy equations by
Ananthanarayan et al. The result is compared with the theoretical prediction
relating 2 a_0^0 - 5 a_0^2 and the scalar radius of the pion, which was
obtained in two-loop Chiral Perturbation Theory. A discrepancy at the 1-sigma
level is found and commented upon.Comment: Published version, to appear in Eur. Phys. J.
Electromagnetic processes in a EFT framework
Recently, we have derived a two--nucleon potential and consistent nuclear
electromagnetic currents in chiral effective field theory with pions and
nucleons as explicit degrees of freedom. The calculation of the currents has
been carried out to include NLO corrections, consisting of two--pion
exchange and contact contributions. The latter involve unknown low-energy
constants (LECs), some of which have been fixed by fitting the S- and
P-wave phase shifts up to 100 MeV lab energies. The remaining LECs entering the
current operator are determined so as to reproduce the experimental deuteron
and trinucleon magnetic moments, as well as the cross section. This
electromagnetic current operator is utilized to study the and He
radiative captures at thermal neutron energies. Here we discuss our results
stressing on the important role played by the LECs in reproducing the
experimental data.Comment: Invited talk at the 5th International Conference on Quarks and
Nuclear Physics, to appear in Chinese Physics
Proton-3He elastic scattering at low energies and the "A_y Puzzle"
The Kohn variational principle and the hyperspherical harmonic technique are
applied to study p-3He elastic scattering at low energies. Preliminary results
obtained using several interaction models are reported. The calculations are
compared to a recent phase shift analysis performed at the Triangle University
Nuclear Laboratory and to the available experimental data. Using a
three-nucleon interaction derived from chiral perturbation theory at N2LO, we
have found a noticeable reduction of the discrepancy observed for the A_y
observable.Comment: 9 pages, 9 figures, to be published in the Proceedings of the 19th
International IUPAP Conference on Few-Body Problems in Physics, Bonn, 200
Electromagnetic structure of A=2 and 3 nuclei in chiral effective field theory
The objectives of the present work are twofold. The first is to address and
resolve some of the differences present in independent,
chiral-effective-field-theory (\chiEFT) derivations up to one loop, recently
appeared in the literature, of the nuclear charge and current operators. The
second objective is to provide a complete set of \chiEFT predictions for the
structure functions and tensor polarization of the deuteron, for the charge and
magnetic form factors of 3He and 3H, and for the charge and magnetic radii of
these few-nucleon systems. The calculations use wave functions derived from
high-order chiral two- and three-nucleon potentials and Monte Carlo methods to
evaluate the relevant matrix elements. Predictions based on conventional
potentials in combination with \chiEFT charge and current operators are also
presented. There is excellent agreement between theory and experiment for all
these observables for momentum transfers up to q< 2.0-2.5 (1/fm); for a subset
of them, this agreement extends to momentum transfers as high as q~5-6 (1/fm).
A complete analysis of the results is provided.Comment: 34 pages, Revte
Thermal neutron captures on and He
We report on a study of the and n\,^3He radiative captures at thermal
neutron energies, using wave functions obtained from either chiral or
conventional two- and three-nucleon realistic potentials with the
hyperspherical harmonics method, and electromagnetic currents derived in chiral
effective field theory up to one loop. The predicted and n\,^3He cross
sections are in good agreement with data, but exhibit a significant dependence
on the input Hamiltonian. A comparison is also made between these and new
results for the and n\,^3He cross sections obtained in the conventional
framework for both potentials and currents.Comment: 4 pages, 4 eps figures; references added; corrections to text and
abstract as suggested by referee adde
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