253 research outputs found
Dispersive and absorptive corrections to the pion-deuteron scattering length
We present a parameter--free calculation of the dispersive and absorptive
contributions to the pion--deuteron scattering length based on chiral
perturbation theory. We show that once all diagrams contributing to leading
order to this process are included, their net effect provides a small
correction to the real part of the pion--deuteron scattering length.
At the same time the sizable imaginary part of the pion--deuteron scattering
length is reproduced accurately.Comment: Numerical error corrected. Results for dispersive corrections changed
- conclusions unchanged. Version as accepted by Phys. Lett.
Heavy-quark spin symmetry partners of the revisited
We revisit the consequences of the heavy-quark spin symmetry for the possible
spin partners of the . We confirm that, if the were a
molecular state with the quantum numbers , then in
the strict heavy-quark limit there should exist three more hadronic molecules
degenerate with the , with the quantum numbers , , and
in line with previous results reported in the literature. We
demonstrate that this result is robust with respect to the inclusion of the
one-pion exchange interaction between the mesons. However, this is true
only if all relevant partial waves as well as particle channels which are
coupled via the pion-exchange potential are taken into account. Otherwise, the
heavy-quark symmetry is destroyed even in the heavy-quark limit. Finally, we
solve the coupled-channel problem in the channel with nonperturbative
pions beyond the heavy-quark limit and, contrary to the findings of previous
calculations with perturbative pions, find for the spin-2 partner of the
a significant shift of the mass as well as a width of the order of 50
MeV.Comment: 17 pages, 3 figures, 1 table, version published in Phys.Lett.
Quark mass dependence of the X(3872) binding energy
We explore the quark-mass dependence of the pole position of the X(3872)
state within the molecular picture. The calculations are performed within the
framework of a nonrelativistic Faddeev-type three-body equation for the
system in the channel. The interaction is
parametrised via a pole, and a three-body force is included to render the
equations well defined. Its strength is adjusted such that the X(3872) appears
as a bound state 0.5 MeV below the neutral threshold. We find that
the trajectory of the X(3872) depends strongly on the assumed quark-mass
dependence of the short-range interactions which can be determined in future
lattice QCD calculations. At the same time we are able to provide nontrivial
information on the chiral extrapolation in the channel.Comment: LaTeX2e, 14 pages, 5 figures, references updated and extended, to
appear in Phys.Lett.
Remarks on the study of the X(3872) from Effective Field Theory with Pion-Exchange Interaction
In a recent paper Phys.Rev.Lett. 111, 042002 (2013) (arXiv:1304.0846), the
charmonium state X(3872) is studied in the framework of an effective field
theory. In that work it is claimed that (i) the one-pion exchange (OPE) alone
provides sufficient binding to produce the X as a shallow bound state at the
threshold, (ii) short-range dynamics (described by a contact
interaction) provides only moderate corrections to the OPE, and (iii) the
X-pole disappears as the pion mass is increased slightly and therefore the X
should not be seen on the lattice, away from the pion physical mass point, if
it were a molecular state. In this paper we demonstrate that the results of
Phys.Rev.Lett. 111, 042002 (2013) (arXiv:1304.0846) suffer from technical as
well as conceptual problems and therefore do not support the conclusions drawn
by the authors.Comment: LaTeX2e, 7 pages, 2 figures, to appear in Phys.Rev.
Precision calculation of within chiral perturbation theory
The reaction is calculated up to order in
chiral perturbation theory, where denotes the ratio of the pion to the
nucleon mass. Special emphasis is put on the role of nucleon--recoil
corrections that are the source of contributions with fractional power in
. Using the known near threshold production amplitude for as the only input, the total cross section for
is described very well. A conservative estimate suggests that the theoretical
uncertainty for the transition operator amounts to 3 % for the computed
amplitude near threshold.Comment: 28 page
Towards a high precision calculation for the pion-nucleus scattering lengths
We calculate the leading isospin conserving few-nucleon contributions to pion
scattering on H, He, and He. We demonstrate that the strong
contributions to the pion-nucleus scattering lengths can be controlled
theoretically to an accuracy of a few percent for isoscalar nuclei and of 10%
for isovector nuclei. In particular, we find the -He scattering length
to be where the uncertainties are
due to ambiguities in the -N scattering lengths and few-nucleon effects,
respectively. To establish this accuracy we need to identify a suitable power
counting for pion-nucleus scattering. For this purpose we study the dependence
of the two-nucleon contributions to the scattering length on the binding energy
of H. Furthermore, we investigate the relative size of the leading two-,
three-, and four-nucleon contributions. For the numerical evaluation of the
pertinent integrals, aMonte Carlo method suitable for momentum space is
devised. Our results show that in general the power counting suggested by
Weinberg is capable to properly predict the relative importance of -nucleon
operators, however, it fails to capture the relative strength of - and
-nucleon operators, where we find a suppression by a factor of 5
compared to the predicted factor of 50. The relevance for the extraction of the
isoscalar -N scattering length from pionic H and He is discussed.
As a side result, we show that beyond the calculation of the -H
scattering length is already beyond the range of applicability of heavy pion
effective field theory.Comment: 24 pages, 14 figures, 10 table
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