232 research outputs found
Nuclear matter in the chiral limit and the in-medium chiral condensate
We investigate nuclear matter, i.e. the nuclear equation-of-state (EOS) as
well as the relativistic mean fields in the chiral limit. The investigations
are based on a chiral nucleon-nucleon EFT interaction where the explicit and
implicit pion mass dependence is known up to next-to-leading order. The nuclear
bulk properties are found to remain fairly stable in the chiral limit. Based on
the same interaction the in-medium scalar condensate is derived, both in
Hartree-Fock approximation as well as from the Brueckner G-matrix, making
thereby use of the Hellman-Feynman theorem. Short distance physics which
determines the reduction of the in-medium nucleon mass is found to play only a
minor role for the reduction of the chiral condensate.Comment: 30 pages, 5 figs. To appear in Nuclear Physics
Three-Nucleon Forces from Chiral Effective Field Theory
We perform the first complete analysis of nd scattering at
next-to-next-to-leading order in chiral effective field theory including the
corresponding three-nucleon force and extending our previous work, where only
the two-nucleon interaction has been taken into account. The three-nucleon
force appears first at this order in the chiral expansion and depends on two
unknown parameters. These two parameters are determined from the triton binding
energy and the nd doublet scattering length. We find an improved description of
various scattering observables in relation to the next-to-leading order results
especially at moderate energies (E_lab = 65 MeV). It is demonstrated that the
long-standing A_y-problem in nd elastic scattering is still not solved by the
leading 3NF, although some visible improvement is observed. We discuss
possibilities of solving this puzzle. The predicted binding energy for the
alpha-particle agrees with the empirical value.Comment: 36 pp, 20 figure
The spin-dependent nd scattering length - a proposed high-accuracy measurement
The understanding of few-nucleon systems at low energies is essential, e.g.
for accurate predictions of element abundances in big-bang and stellar fusion.
Novel effective field theories, taking only nucleons, or nucleons and pions as
explicit degrees of freedom, provide a systematic approach, permitting an
estimate of theoretical uncertainties. Basic constants parameterising the short
range physics are derived from only a handful of experimental values. The
doublet neutron scattering length a_2 of the deuteron is particularly sensitive
to a three-nucleon contact interaction, but experimentally known with only 6%
accuracy. It can be deduced from the two experimentally accessible parameters
of the nd scattering length. We plan to measure the poorly known "incoherent"
nd scattering length a_{i,d} with 10^{-3} accuracy, using a Ramsey apparatus
for pseudomagnetic precession with a cold polarised neutron beam at PSI. A
polarised target containing both deuterons and protons will permit a
measurement relative to the incoherent np scattering length, which is know
experimentally with an accuracy of 2.4\times 10^{-4}.Comment: 5 pages LaTeX2e, 1 .eps figure. To be published in Nucl. Inst.
Methods A as part of the Proceedings of the 9th International Workshop on
Polarized Solid Targets and Techniques in Bad Honnef (Germany), 27th - 29th
October 200
Correlated two-pion exchange and large-N(C) behavior of nuclear forces
The effect of correlated scalar-isoscalar two-pion exchange (CrTPE) modes is
considered in connection with central and spin-orbit parts of the NN force. The
two-pion correlation function is coupled directly to the scalar form factor of
the nucleon which we calculate in the large-N(C) limit where the nucleon can be
described as a soliton of an effective chiral theory. The results for the
central NN force show a strong repulsive core at short internucleon distances
supplemented by a moderate attraction beyond 1 fm. The long-range tail of the
central NN potential is driven by the the pion-nucleon sigma term and
consistent with the effective meson exchange. The spin-orbit part of
the NN potential is repulsive. The large-N(C) scaling behavior of the
scalar-isoscalar NN interaction is addressed. We show that the spin-orbit part
is O(1/N^2(C)) in strength relative to the central force resulting in the ratio
suggested by the 1/N(C) expansion for N(C)=3. The latter is in
agreement with our numerical analysis and with the Kaplan-Manohar large-N(C)
power counting. Unitarization of the scattering amplitude plays here
an important role and improves the tree level results. Analytical
representations of the CrTPE NN potential in terms of elementary functions are
derived and their chiral content is discussed.Comment: 29 pages, 7 figure
Signatures of three-nucleon interactions in few-nucleon systems
Recent experimental results in three-body systems have unambiguously shown
that calculations based only on nucleon-nucleon forces fail to accurately
describe many experimental observables and one needs to include effects which
are beyond the realm of the two-body potentials. This conclusion owes its
significance to the fact that experiments and calculations can both be
performed with a high accuracy. In this review, both theoretical and
experimental achievements of the past decade will be underlined. Selected
results will be presented. The discussion on the effects of the three-nucleon
forces is, however, limited to the hadronic sector. It will be shown that
despite the major successes in describing these seemingly simple systems, there
are still clear discrepancies between data and the state-of-the-art
calculations.Comment: accepted for publication in Rep. Prog. Phy
The Axial-Vector Current in Nuclear Many-Body Physics
Weak-interaction currents are studied in a recently proposed effective field
theory of the nuclear many-body problem. The Lorentz-invariant effective field
theory contains nucleons, pions, isoscalar scalar () and vector
() fields, and isovector vector () fields. The theory exhibits a
nonlinear realization of chiral symmetry and has three
desirable features: it uses the same degrees of freedom to describe the
axial-vector current and the strong-interaction dynamics, it satisfies the
symmetries of the underlying theory of quantum chromodynamics, and its
parameters can be calibrated using strong-interaction phenomena, like hadron
scattering or the empirical properties of finite nuclei. Moreover, it has
recently been verified that for normal nuclear systems, it is possible to
systematically expand the effective lagrangian in powers of the meson fields
(and their derivatives) and to reliably truncate the expansion after the first
few orders. Here it is shown that the expressions for the axial-vector current,
evaluated through the first few orders in the field expansion, satisfy both
PCAC and the Goldberger--Treiman relation, and it is verified that the
corresponding vector and axial-vector charges satisfy the familiar chiral
charge algebra. Explicit results are derived for the Lorentz-covariant,
axial-vector, two-nucleon amplitudes, from which axial-vector meson-exchange
currents can be deduced.Comment: 32 pages, REVTeX 4.0 with 12pt.rtx, aps.rtx, revsymb.sty,
revtex4.cls, plus 14 figures; two sentences added in Summary; two references
adde
The alpha-particle based on modern nuclear forces
The Faddeev-Yakubovsky equations for the alpha-particle are solved. Accurate
results are obtained for several modern NN interaction models, which include
charge-symmetry breaking effects in the NN force, nucleon mass dependences as
well as the Coulomb interaction. These models are augmented by three-nucleon
forces of different types and adjusted to the 3N binding energy. Our results
are close to the experimental binding energy with a slight overbinding. Thus
there is only little room left for the contribution of possible 4N interactions
to the alpha-particle binding energy. We also discuss model dependences of the
binding energies and the wave functions.Comment: 22 pages REVTeX 4, 12 figures, table with TM parameters added, typos
corrected, version as published in PR
Universal Correlations in Pion-less EFT with the Resonating Group Model: Three and Four Nucleons
The Effective Field Theory "without pions" at next-to-leading order is used
to analyze universal bound state and scattering properties of the 3- and
4-nucleon system. Results of a variety of phase shift equivalent nuclear
potentials are presented for bound state properties of 3H and 4He, and for the
singlet S-wave 3He-neutron scattering length a_0(3He-n). The calculations are
performed with the Refined Resonating Group Method and include a full treatment
of the Coulomb interaction and the leading-order 3-nucleon interaction. The
results compare favorably with data and values from AV18(+UIX) model
calculations. A new correlation between a_0(3He-n) and the 3H binding energy is
found. Furthermore, we confirm at next-to-leading order the correlations,
already found at leading-order, between the 3H binding energy and the 3H charge
radius, and the Tjon line. With the 3H binding energy as input, we get
predictions of the Effective Field Theory "without pions" at next-to-leading
order for the root mean square charge radius of 3H of (1.6\pm 0.2) fm, for the
4He binding energy of (28\pm 2.5) MeV, and for Re(a_0(3He-n)) of (7.5\pm
0.6)fm. Including the Coulomb interaction, the splitting in binding energy
between 3H and 3He is found to be (0.66\pm 0.03) MeV. The discrepancy to data
of (0.10\mp 0.03) MeV is model independently attributed to higher order charge
independence breaking interactions. We also demonstrate that different results
for the same observable stem from higher order effects, and carefully assess
that numerical uncertainties are negligible. Our results demonstrate the
convergence and usefulness of the pion-less theory at next-to-leading order in
the 4He channel. We conclude that no 4-nucleon interaction is needed to
renormalize the theory at next-to-leading order in the 4-nucleon sector.Comment: 24 pages revtex4, including 8 figures as .eps files embedded with
includegraphicx, leading-order results added, calculations include the LO
three-nucleon interaction explicitly, comment on Wigner bound added, minor
modification
Modelling nucleon-nucleon scattering above 1 GeV
Motivated by the recent measurement of proton-proton spin-correlation
parameters up to 2.5 GeV laboratory energy, we investigate models for
nucleon-nucleon (NN) scattering above 1 GeV. Signatures for a gradual failure
of the traditional meson model with increasing energy can be clearly
identified. Since spin effects are large up to tens of GeV, perturbative QCD
cannot be invoked to fix the problems. We discuss various theoretical scenarios
and come to the conclusion that we do not have a clear phenomenological
understanding of the spin-dependence of the NN interaction above 1 GeV.Comment: 36 pages, 8 figure
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