24 research outputs found
Muon capture in nuclei: an ab initio approach based on quantum Monte Carlo methods
An ab initio quantum Monte Carlo method is introduced for calculating total
rates of muon weak capture in light nuclei with mass number . As a
first application of the method, we perform a calculation of the rate in He
in a dynamical framework based on realistic two- and three-nucleon interactions
and realistic nuclear charge-changing weak currents. The currents include one-
and two-body terms induced by - and -meson exchange, and
-to- excitation, and are constrained to reproduce the empirical
value of the Gamow-Teller matrix element in tritium. We investigate the
sensitivity of theoretical predictions to current parametrizations of the
nucleon axial and induced pseudoscalar form factors as well as to two-body
contributions in the weak currents. The large uncertainties in the measured
values obtained from bubble-chamber experiments (carried out over 50 years ago)
prevent us from drawing any definite conclusions.Comment: 6 pages, 1 figur
Muon Capture in Nuclei: An \u3ci\u3eab initio\u3c/i\u3e Approach Based on Green\u27s Function Monte Carlo Methods
An ab initio Greenâs function Monte Carlo (GFMC) method is introduced for calculating total rates of muon weak capture in light nuclei with mass number A †12. As a first application of the method, we perform a calculation of the rate in 3H and 4He in a dynamical framework based on realistic two- and three-nucleon interactions and realistic nuclear charge-changing weak currents. The currents include one- and two-body terms induced by Ï-and Ï-meson exchange, and N-to-Î excitation, and are constrained to reproduce the empirical value of the Gamow-Teller matrix element in tritium. We investigate the sensitivity of theoretical predictions to current parametrizations of the nucleon axial and induced pseudoscalar form factors as well as to two-body contributions in the weak currents. The large uncertainties in the measured 4He rates obtained from bubble-chamber experiments (carried out over 50 years ago) prevent us from drawing any definite conclusions. No data exist for 3H, but results are compared to those of a recent Faddeev calculation as a validation of the present GFMC method
Magnetic structure of few-nucleon systems at high momentum transfers in a EFT approach
The five low-energy constants (LECs) in the electromagnetic current derived
in chiral effective field theory (EFT) up to one loop are determined by a
simultaneous fit to the =--3 nuclei magnetic moments and to the
deuteron magnetic form factor and threshold electrodisintegration at backward
angles over a wide range of momentum transfers. The resulting parametrization
then yields predictions for the He/H magnetic form factors in excellent
accord with the experimental values for momentum transfers ranging up to
GeV/c, beyond the expected regime of validity of the EFT
approach. The calculations are based on last-generation two-nucleon
interactions including high orders in the chiral expansion and derived by
Entem, Macheleidt, and Nosyk [Phys.\ Rev.\ C {\bf 96}, 024004 (2017)] and by
Piarulli {\it et al.} [Phys.\ Rev.\ C {\bf 94}, 054007 (2016)], using different
EFT formulations. In the = calculations, (chiral) three-nucleon
interactions are also accounted for. The model dependence resulting from these
different formulations of the interactions is found to be mild for momentum
transfer below GeV/c. An analysis of the convergence of the chiral
expansion is also provided.Comment: 12 pages, 8 figure
Coulomb Sum Rule for \u3csup\u3e4\u3c/sup\u3eHE
We determine the Coulomb sum for 4He using the world data on 4He(e,eâČ) and compare the results to calculations based on realistic interactions and including two-body components in the nuclear charge operator. We find good agreement between theory and experiment when using free-nucleon form factors. The apparent reduction of the in-medium GEp implied by IA-interpretation of the L/T-ratios measured in 4He(e,eâČp) and 4He(eâ,eâČpâ) is not confirmed
Ab initio calculation of the electromagnetic and neutral-weak response functions of 4He and 12C
Precise measurement of neutrino oscillations, and hence the determination of
their masses demands a quantitative understanding of neutrino-nucleus
interactions. To this aim, two-body meson-exchange currents have to be
accounted for along within realistic models of nuclear dynamics. We summarize
our progresses towards the construction of a consistent framework, based on
quantum Monte Carlo methods and on the spectral function approach, that can be
exploited to accurately describe neutrino interactions with atomic nuclei over
the broad kinematical region covered by neutrino experiments.Comment: 8 pages, 4 figure, Proceedings of the 21st International Conference
on Few-Body Problems in Physics, Chicago, Illinois, US
Partial Muon Capture Rates in A = 3 and A = 6 Nuclei with Chiral Effective Field Theory
Searches for neutrinoless double-ÎČ decay rates are crucial in addressing questions within fundamental symmetries and neutrino physics. The rates of these decays depend not only on unknown parameters associated with neutrinos, but also on nuclear properties. In order to reliably extract information about the neutrino, one needs an accurate treatment of the complex many-body dynamics of the nucleus. Neutrinoless double-ÎČ decays take place at momentum transfers on the order of 100MeV /c and require both nuclear electroweak vector and axial current matrix elements. Muon capture, a process in the same momentum transfer regime, has readily available experimental data to validate these currents. In this Letter, we present results of ab initio calculations of partial muon capture rates for 3He and 6Li nuclei using variational and Green\u27s function Monte Carlo computational methods. We estimate the impact of the three-nucleon interactions, the cutoffs used to regularize two-nucleon (2N) interactions, and the energy range of 2N scattering data used to fit these interactions
Comparative Study of \u3csup\u3e6\u3c/sup\u3eHe ÎČ-Decay Based on Different Similarity-Renormalization-Group Evolved Chiral Interactions
We report on a study of the Gamow-Teller matrix element contributing to â¶HeÎČ decay with similarity renormalization group (SRG) versions of momentum- and configuration-space two-nucleon interactions. These interactions are derived from two different formulations of chiral effective field theory (ÏEFT)âwithout and with the explicit inclusion of Î isobars. We consider evolution parameters ÎSRG in the range between 1.2 and 2.0 fmâ1 and, for the Î-less case, also the unevolved (bare) interaction. The axial current contains one- and two-body terms, consistently derived at tree level (no loops) in the two distinct ÏEFT formulations we have adopted here. The â¶He and â¶Li ground-state wave functions are obtained from hyperspherical-harmonics (HH) solutions of the nuclear many-body problem. In A = 6 systems, the HH method is limited at present to treat only two-body interactions and non-SRG evolved currents. Our results exhibit a significant dependence on ÎSRG of the contributions associated with two-body currents, suggesting that a consistent SRG-evolution of these is needed in order to obtain reliable estimates. We also show that the contributions from one-pion-exchange currents depend strongly on the model (chiral) interactions and on the momentum- or configuration-space cutoffs used to regularize them. These results might prove helpful in clarifying the origin of the sign difference recently found in no-core-shell-model and quantum Monte Carlo calculations of the â¶He Gamow-Teller matrix element
Magnetic Structure and Radiative Captures of Few-Nucleon Systems: Status and Prospects
We review the main ingredients for an ab-initio study of few-nucleon reactions of astrophysical interest within the chiral effective field theory approach, with particular attention to radiative captures relevant for Big Bang Nucleosynthesis and stellar evolution. We conclude with an outlook for ongoing and future work
Local chiral potentials and the structure of light nuclei
We present fully local versions of the minimally non-local nucleon-nucleon
potentials constructed in a previous paper [M.\ Piarulli {\it et al.}, Phys.\
Rev.\ C {\bf 91}, 024003 (2015)], and use them in hypersperical-harmonics and
quantum Monte Carlo calculations of ground and excited states of H, He,
He, He, and Li nuclei. The long-range part of these local
potentials includes one- and two-pion exchange contributions without and with
-isobars in the intermediate states up to order ( denotes
generically the low momentum scale) in the chiral expansion, while the
short-range part consists of contact interactions up to order . The
low-energy constants multiplying these contact interactions are fitted to the
2013 Granada database in two different ranges of laboratory energies, either
0--125 MeV or 0--200 MeV, and to the deuteron binding energy and singlet
scattering length. Fits to these data are performed for three models
characterized by long- and short-range cutoffs, and
respectively, ranging from fm down to
fm. The long-range (short-range) cutoff regularizes the one- and
two-pion exchange (contact) part of the potential.Comment: 29 pages, 3 figure
Reply to Comment on \u27Quasielastic Lepton Scattering and b=Back-to-Back Nucleons in the Short-Time Approximation\u27
We briefly review the concept of scaling and how it occurs in quasielastic electron and neutrino scattering from nuclei, and then the particular approach to scaling in the short-time approximation. We show that, whereas two-nucleon currents do significantly enhance the transverse electromagnetic response, they do not spoil scaling, but, in fact, enhance it. We provide scaling results obtained in the short-time approximation that verify this claim. The enhanced scaling, although obtained empirically, is not âaccidentalââas claimed in [O. Benhar, Phys. Rev. C 105, 049801 (2022)]âbut rather reflects quasielastic kinematics and the dominant role played by pion-exchange interactions and currents in the quasielastic regime