6,926 research outputs found
The Solar pp and hep Processes in Effective Field Theory
The strategy of modern effective field theory is exploited to pin down
accurately the flux factors for the and processes in the Sun.
The technique used is to combine the high accuracy established in few-nucleon
systems of the "standard nuclear physics approach" (SNPA) and the systematic
power counting of chiral perturbation theory (ChPT) into a consistent effective
field theory framework. Using highly accurate wave functions obtained in the
SNPA and working to \nlo3 in the chiral counting for the current, we make
totally parameter-free and error-controlled predictions for the and
processes in the Sun.Comment: 5 pages, aipproc macros are included. Talk given at International
Nuclear Physics Conference 2001, Berkeley, California, July 30 - August 3,
200
Chiral Dynamics and Heavy-Fermion Formalism in Nuclei: Exchange Axial Currents
URL: http://www-spht.cea.fr/articles/T93/013 http://fr.arxiv.org/abs/hep-ph/9301295International audienceChiral perturbation theory in heavy-fermion formalism is developed for meson-exchange currents in nuclei and applied to nuclear axial-charge transitions. Calculation is performed to the next-to-leading order in chiral expansion which involves graphs up to one loop. The result turns out to be very simple. The previously conjectured notion of \lq\lq chiral filter mechanism\rq\rq\ in the time component of the nuclear axial current on the space component of the nuclear electromagnetic current is verified to that order. As a consequence, the phenomenologically observed soft-pion dominance in the nuclear process is given a simple interpretation in terms of chiral symmetry in nuclei. In this paper we focus on the axial current, relegating the electromagnetic current which can be treated in a similar way to a separate paper. We discuss the implication of our result on the enhanced axial-charge transitions observed in heavy nuclei and clarify the relationship between the phenomenological meson-exchange description and the chiral Lagrangian description.Lorqu'on cherche à appliquer la théorie des perturbations chirales à des processus où entrent en jeu les nucléons et les mésons, on se heurte à une difficulté liée à la masse du nucléon qui est grande par rapport aux petits paramètres de la théorie (impulsions externes, masse du pion, constante de désintégration du pion). Dans ce travail les auteurs appliquent le formalisme des baryons lourds pour lever cette difficulté. Grossièrement parlant ce formalisme travaille avec des champs des nucléons redéfinis de manière à éliminer les effets de la masse du nucléon. Cela permet aux auteurs d'identifier l'ordre en nombre de boucles à l'ordre de la perturbation chirale. Les calculs deviennent plus rigoureux et en sont grandement simplifiés. Les auteurs appliquent leur méthode au calcul, jusqu'au deuxième ordre, de l'interaction du courant électrofaible avec les noyaux
The Solar Proton Burning Process Revisited In Chiral Perturbation Theory
The proton burning process p + p -> d + e(+) + \nu(e), important for the
stellar evolution of main-sequence stars of mass equal to or less than that of
the Sun, is computed in effective field theory using chiral perturbation
expansion to the next-to-next-to leading chiral order. This represents a
model-independent calculation consistent with low-energy effective theory of
QCD comparable in accuracy to the radiative np capture at thermal energy
previously calculated by first using very accurate two-nucleon wavefunctions
backed up by an effective field theory technique with a finite cut-off. The
result obtained thereby is found to support within theoretical uncertainties
the previous calculation of the same process by Bahcall and his co-workers.Comment: 30 pages, 2 eps files, aaspp4.sty needed, slightly modified, to be
published in Ap.
The Solar hep Process in Effective Field Theory
Using effective field theory, we calculate the S-factor for the hep process
in a totally parameter-free formulation. The transition operators are organized
according to chiral counting, and their matrix elements are evaluated using the
realistic nuclear wave functions obtained in the
correlated-hyperspherical-harmonics method. Terms of up to
next-to-next-to-next-to-leading order in heavy-baryon chiral perturbation
theory are considered. Fixing the only parameter in the theory by fitting the
tritium \beta-decay rate, we predict the hep S-factor with accuracy better than
\sim 20 %.Comment: 4 pages, Revtex. Minor revision has been mad
Parameter-Free Calculation of the Solar Proton Fusion Rate in Effective Field Theory
Spurred by the recent complete determination of the weak currents in
two-nucleon systems up to in heavy-baryon chiral perturbation
theory, we carry out a parameter-free calculation of the solar proton fusion
rate in an effective field theory that combines the merits of the standard
nuclear physics method and systematic chiral expansion. Using the tritium
beta-decay rate as an input to fix the only unknown parameter in the effective
Lagrangian, we can evaluate with drastically improved precision the ratio of
the two-body contribution to the well established one-body contribution; the
ratio is determined to be (0.86\pm 0.05) %. This result is essentially
independent of the cutoff parameter for a wide range of its variation (500 MeV
\le \Lambda \le 800 MeV), a feature that substantiates the consistency of the
calculation.Comment: 10 pages. The argument is considerably more sharpened with a reduced
error ba
Raman Scattering Study of the Lattice Dynamics of Superconducting LiFeAs
We report an investigation of the lattice dynamical properties of LiFeAs
using inelastic light scattering. Five out of the six expected phonon modes are
observed. The temperature evolution of their frequencies and linewidths is in
good agreement with an anharmonic-decay model. We find no evidence for
substantial electron-phonon coupling, and no superconductivity-induced phonon
anomalies.Comment: 5 pages, 3 figures, 1 tabl
Electronic structures of doped anatase : (M=Co, Mn, Fe, Ni)
We have investigated electronic structures of a room temperature diluted
magnetic semiconductor : Co-doped anatase . We have obtained the
half-metallic ground state in the local-spin-density approximation(LSDA) but
the insulating ground state in the LSDA++SO incorporating the spin-orbit
interaction. In the stoichiometric case, the low spin state of Co is realized
with the substantially large orbital moment. However, in the presence of oxygen
vacancies near Co, the spin state of Co becomes intermediate. The
ferromagnetisms in the metallic and insulating phases are accounted for by the
double-exchange-like and the superexchange mechanism, respectively. Further,
the magnetic ground states are obtained for Mn and Fe doped ,
while the paramagnetic ground state for Ni-doped .Comment: 5 pages, 4 figure
Radiative Neutron-Proton Capture in Effective Chiral Lagrangians
We calculate the cross-section for the thermal
process in chiral perturbation theory to next-to-next-to-leading order using
heavy-fermion formalism. The exchange current correction is found to be
in amplitude and the chiral perturbation at one-loop order
gives the cross section \sigma_{th}^{np}=(334\pm 2)\ {\mbox mb} which is in
agreement with the experimental value (334.2\pm 0.5)\ {\mbox mb}. Together
with the axial charge transitions, this provides a strong support for the power
of chiral Lagrangians for nuclear physics.Comment: 9 pages, revtex, uses epsfig.sty, 2 uuencoded figure
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