2,156 research outputs found
On neutral pion electroproduction off deuterium
Threshold neutral pion electroproduction on the deuteron is studied in the
framework of baryon chiral perturbation theory at next-to-leading order in the
chiral expansion. To this order in small momenta, the amplitude is finite and a
sum of two- and three-body interactions with no undetermined parameters. We
calculate the S-wave multipoles for threshold production and the deuteron
S-wave cross section as a function of the photon virtuality. We also discuss
the sensitivity to the elementary neutron amplitudes.Comment: 6 pp, revtex, 3 figs, corrected version, to appear in Phys. Rev.
Enhanced chiral logarithms in partially quenched QCD
I discuss the properties of pions in ``partially quenched'' theories, i.e.
those in which the valence and sea quark masses, and , are
different. I point out that for lattice fermions which retain some chiral
symmetry on the lattice, e.g. staggered fermions, the leading order prediction
of the chiral expansion is that the mass of the pion depends only on , and
is independent of . This surprising result is shown to receive corrections
from loop effects which are of relative size , and which thus
diverge when the valence quark mass vanishes. Using partially quenched chiral
perturbation theory, I calculate the full one-loop correction to the mass and
decay constant of pions composed of two non-degenerate quarks, and suggest
various combinations for which the prediction is independent of the unknown
coefficients of the analytic terms in the chiral Lagrangian. These results can
also be tested with Wilson fermions if one uses a non-perturbative definition
of the quark mass.Comment: 14 pages, 3 figures, uses psfig. Typos in eqs (18)-(20) corrected
(alpha_4 is replaced by alpha_4/2
QCD with domain wall quarks
We present lattice calculations in QCD using a variant of Kaplan fermions
which retain the continuum SU(N)xSU(N) chiral symmetry on the lattice in the
limit of an infinite extra dimension. In particular, we show that the pion mass
and the four quark matrix element related to K_0-K_0-bar mixing have the
expected behavior in the chiral limit, even on lattices with modest extent in
the extra dimension, e.g. N_s=10.Comment: Published version. Minor differences from original. LaTeX, 12 pages
including 2 PostScript figure
Neutrino-deuteron reactions at solar neutrino energies
In interpreting the SNO experiments, very accurate estimates of the \nu d
reaction cross sections are of great importance. We improve the previous
estimates of our group by updating some of its inputs and by taking into
account the results of a recent effective-field-theoretical calculation. The
new cross sections are slightly (\sim 1%) larger than the previously reported
values. We present arguments that lead to the conclusion that it is reasonable
to assign 1% uncertainty to the \nu d cross sections reported here.Comment: 13 pages, 1 figur
Recommended from our members
Exome sequencing and in vitro studies identified podocalyxin as a candidate gene for focal and segmental glomerulosclerosis
Our understanding of focal and segmental glomerulosclerosis (FSGS) has advanced significantly from the studies of rare, monogenic forms of the disease. These studies have demonstrated the critical roles of multiple aspects of podocyte function in maintaining glomerular function. A substantial body of research has suggested that the integral membrane protein podocalyxin (PODXL) is required for proper function of podocytes, possibly by preserving the patency of the slit diaphragm by negative charge-based repulsion. Exome sequencing of affected cousins from an autosomal dominant pedigree with FSGS identified a co-segregating private variant, PODXL p.L442R, affecting the transmembrane region of the protein. Of the remaining 11 shared gene variants, two segregated with disease but their gene products were not detected in the glomerulus. In comparison to wild type, this disease-segregating PODXL variant facilitated dimerization. By contrast, this change does not alter protein stability, extracellular domain glycosylation, cell surface expression, global subcellular localization, or interaction with its intracellular binding partner ezrin. Thus, a variant form of PODXL remains the most likely candidate causing FSGS in one family with autosomal dominant inheritance, but its full effect on protein function remains unknown. Our work highlights the challenge faced in the clinical interpretation of whole exome data for small pedigrees with autosomal dominant diseases
Chiral -exchange NN-potentials: Results for diagrams proportional to g_A^4 and g_A^6
We calculate in (two-loop) chiral perturbation theory the local NN-potentials
generated by the three-pion exchange diagrams proportional to g_A^4 and g_A^6.
Surprisingly, we find that the total isoscalar central -exchange
potential vanishes identically. The individually largest -exchange
potentials are of isoscalar spin-spin, isovector central and isoscalar tensor
type. For these potentials simple analytical expressions can be given. The
strength of these dominant -exchange potentials at r=1.0 fm is 4.6 MeV,
2.9 MeV and 1.4 MeV, respectively. Furthermore, we observe that the spin-spin
and tensor potentials due to the diagrams proportional to g_A^6 do not exist in
the infinite nucleon mass limit.Comment: 8 pages, 5 figure
Chiral 3-exchange NN-potentials: Results for dominant next-to-leading order contributions
We calculate in (two-loop) chiral perturbation theory the local NN-potentials
generated by the three-pion exchange diagrams with one insertion from the
second order chiral effective pion-nucleon Lagrangian proportional to the
low-energy constants . The resulting isoscalar central potential
vanishes identically. In most cases these -exchange potentials are larger
than the ones generated by the diagrams involving only leading order vertices
due to the large values of (which mainly represent virtual
-excitation). A similar feature has been observed for the chiral
-exchange. We also give suitable (double-integral) representations for
the spin-spin and tensor potentials generated by the leading-order diagrams
proportional to involving four nucleon propagators. In these cases the
Cutkosky rule cannot be used to calculate the spectral-functions in the
infinite nucleon mass limit since the corresponding mass-spectra start with a
non-vanishing value at the -threshold. Altogether, one finds that chiral
-exchange leads to small corrections in the region fm where
- and chiral -exchange alone provide a very good strong NN-force as
shown in a recent analysis of the low-energy pp-scattering data-base.Comment: 11 pages, 7 figures, to be published in The Physical Review
Chiral -exchange NN-potentials: Two-loop contributions
We calculate in heavy baryon chiral perturbation theory the local
NN-potentials generated by the two-pion exchange diagrams at two-loop order. We
give explicit expressions for the mass-spectra (or imaginary parts) of the
corresponding isoscalar and isovector central, spin-spin and tensor
NN-amplitudes. We find from two-loop two-pion exchange a sizeable isoscalar
central repulsion which amounts to MeV at fm. There is a
similarly strong isovector central attraction which however originates mainly
from the third order low energy constants entering the chiral -scattering amplitude. We also evaluate the one-loop -exchange diagram
with two second order chiral -vertices proportional to the low
energy constants as well as the first relativistic 1/M-correction
to the -exchange diagrams with one such vertex. The diagrammatic results
presented here are relevant components of the chiral NN-potential at
next-to-next-to-next-to-leading order.Comment: 6 pages, 2 figure
A renormalisation group approach to two-body scattering in the presence of long-range forces
We apply renormalisation-group methods to two-body scattering by a
combination of known long-range and unknown short-range potentials. We impose a
cut-off in the basis of distorted waves of the long-range potential and
identify possible fixed points of the short-range potential as this cut-off is
lowered to zero. The expansions around these fixed points define the power
countings for the corresponding effective field theories. Expansions around
nontrivial fixed points are shown to correspond to distorted-wave versions of
the effective-range expansion. These methods are applied to scattering in the
presence of Coulomb, Yukawa and repulsive inverse-square potentials.Comment: 22 pages (RevTeX), 4 figure
Family of Hermitian Low-Momentum Nucleon Interactions with Phase Shift Equivalence
Using a Schmidt orthogonalization transformation, a family of Hermitian
low-momentum NN interactions is derived from the non-Hermitian Lee-Suzuki (LS)
low-momentum NN interaction. As special cases, our transformation reproduces
the Hermitian interactions for Okubo and Andreozzi. Aside from their common
preservation of the deuteron binding energy, these Hermitian interactions are
shown to be phase shift equivalent, all preserving the empirical phase shifts
up to decimation scale Lambda. Employing a solvable matrix model, the Hermitian
interactions given by different orthogonalization transformations are studied;
the interactions can be very different from each other particularly when there
is a strong intruder state influence. However, because the parent LS
low-momentum NN interaction is only slightly non-Hermitian, the Hermitian
low-momentum nucleon interactions given by our transformations, including the
Okubo and Andreozzi ones, are all rather similar to each other. Shell model
matrix elements given by the LS and several Hermitian low-momentum interactions
are compared.Comment: 10 pages, 7 figure
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