100 research outputs found
Current constraints on the EFT for the \Lambda N --> N N transition
The relation between the low energy constants appearing in the effective
field theory description of the \Lambda N --> N N transition potential and the
parameters of the one-meson-exchange model previously developed are obtained.
We extract the relative importance of the different exchange mechanisms
included in the meson picture by means of a comparison to the corresponding
operational structures appearing in the effective approach. The ability of this
procedure to obtain the weak baryon-baryon-meson couplings for a possible
scalar exchange is also discussed.Comment: 10 pages, 6 figure
Octet baryon magnetic moments from lattice QCD: Approaching experiment from a three-flavor symmetric point
Lattice QCD calculations with background magnetic fields are used to determine the magnetic moments of the octet baryons. Computations are performed at the physical value of the strange quark mass, and two values of the light quark mass, one corresponding to the SU(3)(F)-symmetric point, where the pion mass is m(pi) similar to 800 MeV, and the other corresponding to a pion mass of m(pi) similar to 450 MeV. The moments are found to exhibit only mild pion-mass dependence when expressed in terms of appropriately chosen magneton units-the natural baryon magneton. A curious pattern is revealed among the anomalous baryon magnetic moments which is linked to the constituent quark model, however, careful scrutiny exposes additional features. Relations expected to hold in the large-N-c limit of QCD are studied; and, in one case, a clear preference for the quark model over the large-Nc prediction is found. The magnetically coupled Lambda-Sigma(0) system is treated in detail at the SU(3)(F) point, with the lattice QCD results comparing favorably with predictions based on SU(3)(F) symmetry. This analysis enables the first extraction of the isovector transition magnetic polarizability. The possibility that large magnetic fields stabilize strange matter is explored, but such a scenario is found to be unlikely
Unitary Limit of Two-Nucleon Interactions in Strong Magnetic Fields
Two-nucleon systems are shown to exhibit large scattering lengths in strong
magnetic fields at unphysical quark masses, and the trends toward the physical
values indicate that such features may exist in nature. Lattice QCD
calculations of the energies of one and two nucleons systems are performed at
pion masses of and 806 MeV in uniform, time-independent
magnetic fields of strength {\bf B}| \sim 10^{19}10^{20}$ Gauss to determine
the response of these hadronic systems to large magnetic fields. Fields of this
strength may exist inside magnetars and in peripheral relativistic heavy ion
collisions, and the unitary behavior at large scattering lengths may have
important consequences for these systems.Comment: Accepted journal versio
The Magnetic Structure of Light Nuclei from Lattice QCD
Lattice QCD with background magnetic fields is used to calculate the magnetic
moments and magnetic polarizabilities of the nucleons and of light nuclei with
, along with the cross-section for the transition , at the flavor SU(3)-symmetric point where the pion mass is MeV. These magnetic properties are extracted from nucleon and nuclear
energies in six uniform magnetic fields of varying strengths. The magnetic
moments are presented in a recent Letter. For the charged states, the
extraction of the polarizability requires careful treatment of Landau levels,
which enter non-trivially in the method that is employed. The nucleon
polarizabilities are found to be of similar magnitude to their physical values,
with fm and
fm, exhibiting a
significant isovector component. The dineutron is bound at these heavy quark
masses and its magnetic polarizability, fm differs significantly from twice that of the neutron. A
linear combination of deuteron scalar and tensor polarizabilities is determined
by the energies of the deuteron states, and is found to be
fm. The magnetic
polarizabilities of the three-nucleon and four-nucleon systems are found to be
positive and similar in size to those of the proton, fm, fm, fm. Mixing between the
deuteron state and the spin-singlet state induced by the background
magnetic field is used to extract the short-distance two-nucleon counterterm,
, of the pionless effective theory for systems (equivalent to
the meson-exchange current contribution in nuclear potential models), that
dictates the cross-section for the process near threshold.
Combined with previous determinations of NN scattering parameters, this enables
an ab initio determination of the threshold cross-section at these unphysical
masses.Comment: 49 pages, 24 figure
Ab initio calculation of the radiative capture process
Lattice QCD calculations of two-nucleon systems are used to isolate the
short-distance two-body electromagnetic contributions to the radiative capture
process , and the photo-disintegration processes
. In nuclear potential models, such contributions are
described by phenomenological meson-exchange currents, while in the present
work, they are determined directly from the quark and gluon interactions of
QCD. Calculations of neutron-proton energy levels in multiple background
magnetic fields are performed at two values of the quark masses, corresponding
to pion masses of and 806 MeV, and are combined with pionless
nuclear effective field theory to determine these low-energy inelastic
processes. Extrapolating to the physical pion mass, a cross section of
is obtained at an incident neutron speed of $v=2,200\
m/s\sigma^{expt}(np \to d\gamma)
= 334.2(0.5)\ mb$
Microscopic approach to the proton asymmetry in the nonmesonic weak decay of Λ hypernuclei
The nonmesonic weak decay of polarized Λ hypernuclei is studied with a microscopic diagrammatic formalism in which one- and two-nucleon-induced decay mechanisms, Λ→N → NN and Λ→NN → NNN, are considered together with (and on the same ground of) nucleon final state interactions. We adopt a nuclear matter formalism extended to finite nuclei via the local density approximation. Our approach adopts different one-meson-exchange weak transition potentials, while the strong interaction effects are accounted for by a Bonn nucleon-nucleon interaction. We also consider the two-pion-exchange effect in the weak transition potential. Both the twonucleon-induced decay mechanism and the final state interactions reduce the magnitude of the asymmetry. The quantum interference terms considered in the present microscopic approach give rise to an opposite behavior of the asymmetry with increasing energy cuts to that observed in models describing the nucleon final state interactions semiclassically via the intranuclear cascade code. Our results for the asymmetry parameter in Λ¹²C obtained with different potential models are consistent with the asymmetry measured at KEK.Facultad de Ciencias ExactasInstituto de FÃsica La Plat
Pi-K Scattering in Full QCD with Domain-Wall Valence Quarks
We calculate the pi+ K+ scattering length in fully-dynamical lattice QCD with
domain-wall valence quarks on MILC lattices with rooted staggered sea-quarks at
a lattice spacing of b=0.125 fm, lattice spatial size of L =2.5 fm and at pion
masses of m_pi=290, 350, 490 and 600 MeV. The lattice data, analyzed at
next-to-leading order in chiral perturbation theory, allows an extraction of
the full pi K scattering amplitude at threshold. Extrapolating to the physical
point gives m_pi a_3/2 = -0.0574 (+- 0.0016)(+0.0024 -0.0058) and m_pi a_1/2 =
0.1725 (+- 0.0017)(+0.0023 -0.0156) for the I=3/2 and I=1/2 scattering lengths,
respectively, where the first error is statistical and the second error is an
estimate of the systematic due to truncation of the chiral expansion.Comment: 14 pages, 9 figure
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