170 research outputs found
Spin structure of the nucleon at low energies
The spin structure of the nucleon is analyzed in the framework of a
Lorentz-invariant formulation of baryon chiral perturbation theory. The
structure functions of doubly virtual Compton scattering are calculated to
one-loop accuracy (fourth order in the chiral expansion). We discuss the
generalization of the Gerasimov-Drell-Hearn sum rule, the Burkhardt-Cottingham
sum rule and moments of these. We give predictions for the forward and the
longitudinal-transverse spin polarizabilities of the proton and the neutron at
zero and finite photon virtuality. A detailed comparison to results obtained in
heavy baryon chiral perturbation theory is also given.Comment: 29 pp, 14 fig
A new window on Strange Quark Matter as the ground state of strongly interacting matter
If strange quark matter is the true ground state of matter, it must have
lower energy than nuclear matter. Simultaneously, two-flavour quark matter must
have higher energy than nuclear matter, for otherwise the latter would convert
to the former. We show, using an effective chiral lagrangian, that the
existence of a new lower energy ground state for two-flavour quark matter, the
pion condensate, shrinks the window for strange quark matter to be the ground
state of matter and sets new limits on the current strange quark mass
Compton scattering on the proton, neutron, and deuteron in chiral perturbation theory to O(Q^4)
We study Compton scattering in systems with A=1 and 2 using chiral
perturbation theory up to fourth order. For the proton we fit the two
undetermined parameters in the O(Q^4) p amplitude of McGovern to
experimental data in the region MeV, obtaining a
chi^2/d.o.f. of 133/113. This yields a model-independent extraction of proton
polarizabilities based solely on low-energy data: alpha_p=12.1 +/- 1.1 (stat.)
+/- 0.5 (theory) and beta_p=3.4 +/- 1.1 (stat.) +/- 0.1 (theory), both in units
of 10^{-4} fm^3. We also compute Compton scattering on deuterium to O(Q^4). The
d amplitude is a sum of one- and two-nucleon mechanisms, and contains
two undetermined parameters, which are related to the isoscalar nucleon
polarizabilities. We fit data points from three recent d scattering
experiments with a chi^2/d.o.f.=26.6/20, and find alpha_N=13.0 +/- 1.9 (stat.)
+3.9/-1.5 (theory) and a beta_N that is consistent with zero within sizeable
error bars.Comment: 57 pages, 16 figures. Substantial changes. Correction of errors in
deuteron calculation results in different values for isoscalar
polarizabilities. Results for the proton are unaffected. Text modified to
reflect this change, and also to clarify various point
Hybrid stars with the color dielectric and the MIT bag models
We study the hadron-quark phase transition in the interior of neutron stars
(NS). For the hadronic sector, we use a microscopic equation of state (EOS)
involving nucleons and hyperons derived within the Brueckner-Bethe-Goldstone
many-body theory, with realistic two-body and three-body forces. For the
description of quark matter, we employ both the MIT bag model with a density
dependent bag constant, and the color dielectric model. We calculate the
structure of NS interiors with the EOS comprising both phases, and we find that
the NS maximum masses are never larger than 1.7 solar masses, no matter the
model chosen for describing the pure quark phase.Comment: 11 pages, 5 figures, submitted to Phys. Rev.
Nucleon mass, sigma term and lattice QCD
We investigate the quark mass dependence of the nucleon mass M_N. An
interpolation of this observable, between a selected set of fully dynamical
two-flavor lattice QCD data and its physical value, is studied using
relativistic baryon chiral perturbation theory up to order p^4. In order to
minimize uncertainties due to lattice discretization and finite volume effects
our numerical analysis takes into account only simulations performed with
lattice spacings a5. We have also restricted ourselves to
data with m_pi<600 MeV and m_sea=m_val. A good interpolation function is found
already at one-loop level and chiral order p^3. We show that the
next-to-leading one-loop corrections are small. From the p^4 numerical analysis
we deduce the nucleon mass in the chiral limit, M_0 approx 0.88 GeV, and the
pion-nucleon sigma term sigma_N= (49 +/- 3) MeV at the physical value of the
pion mass.Comment: 12 pages, 4 figures, revised journal versio
Two-loop corrections to the decay rate of parapositronium
Order corrections to the decay rate of parapositronium are
calculated. A QED scattering calculation of the amplitude for electron-positron
annihilation into two photons at threshold is combined with the technique of
effective field theory to determine an NRQED Hamiltonian, which is then used in
a bound state calculation to determine the decay rate. Our result for the
two-loop correction is in units of times the
lowest order rate. This is consistent with but more precise than the result
of a previous calculation.Comment: 26 pages, 7 figure
Effective theory of the Delta(1232) in Compton scattering off the nucleon
We formulate a new power-counting scheme for a chiral effective field theory
of nucleons, pions, and Deltas. This extends chiral perturbation theory into
the Delta-resonance region. We calculate nucleon Compton scattering up to
next-to-leading order in this theory. The resultant description of existing
p cross section data is very good for photon energies up to about 300
MeV. We also find reasonable numbers for the spin-independent polarizabilities
and .Comment: 29 pp, 9 figs. Minor revisions. To be published in PR
Predictive powers of chiral perturbation theory in Compton scattering off protons
We study low-energy nucleon Compton scattering in the framework of baryon
chiral perturbation theory (BPT) with pion, nucleon, and (1232)
degrees of freedom, up to and including the next-to-next-to-leading order
(NNLO). We include the effects of order , and , with
MeV the -resonance excitation energy. These are
all "predictive" powers in the sense that no unknown low-energy constants enter
until at least one order higher (i.e, ). Estimating the theoretical
uncertainty on the basis of natural size for effects, we find that
uncertainty of such a NNLO result is comparable to the uncertainty of the
present experimental data for low-energy Compton scattering. We find an
excellent agreement with the experimental cross section data up to at least the
pion-production threshold. Nevertheless, for the proton's magnetic
polarizability we obtain a value of fm, in
significant disagreement with the current PDG value. Unlike the previous
PT studies of Compton scattering, we perform the calculations in a
manifestly Lorentz-covariant fashion, refraining from the heavy-baryon (HB)
expansion. The difference between the lowest order HBPT and BPT
results for polarizabilities is found to be appreciable. We discuss the chiral
behavior of proton polarizabilities in both HBPT and BPT with the
hope to confront it with lattice QCD calculations in a near future. In studying
some of the polarized observables, we identify the regime where their naive
low-energy expansion begins to break down, thus addressing the forthcoming
precision measurements at the HIGS facility.Comment: 24 pages, 9 figures, RevTeX4, revised version published in EPJ
Chiral effective field theories of the strong interactions
Effective field theories of the strong interactions based on the approximate
chiral symmetry of QCD provide a model-independent approach to low-energy
hadron physics. We give a brief introduction to mesonic and baryonic chiral
perturbation theory and discuss a number of applications. We also consider the
effective field theory including vector and axial-vector mesons.Comment: 22 pages, 9 figures, proceedings of "Many-Body Structure of Strongly
Interacting Systems", Mainz, Germany, Feb. 23-25 201
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