20 research outputs found
Using chiral perturbation theory to extract the neutron-neutron scattering length from pi- d -> n n gamma
The reaction pi- d -> n n gamma is calculated in chiral perturbation theory
so as to facilitate an extraction of the neutron-neutron scattering length
(a_nn). We include all diagrams up to O(Q^3). This includes loop effects in the
elementary pi- p -> gamma n amplitude and two-body diagrams, both of which were
ignored in previous calculations. We find that the chiral expansion for the
ratio of the quasi-free (QF) to final-state-interaction (FSI) peaks in the
final-state neutron spectrum converges well. Our third-order calculation of the
full spectrum is already accurate to better than 5%. Extracting a_nn from the
shape of the entire pi- d -> n n gamma spectrum using our calculation in its
present stage would thus be possible at the +-0.8 fm level. A fit to the FSI
peak only would allow an extraction of a_nn with a theoretical uncertainty of
+-0.2 fm. The effects that contribute to these error bars are investigated. The
uncertainty in the rescattering wave function dominates. This suggests
that the quoted theoretical error of +-0.3 fm for the most recent pi- d -> n n
gamma measurement may be optimistic. The possibility of constraining the nn
rescattering wave function used in our calculation more tightly--and thus
reducing the error--is briefly discussed.Comment: 35 pages, 14 eps figures, references and figure added, discussions of
errors extended and clarified, improved conclusions, typos corrected, to be
published in PR
The near threshold N N -> d pi reaction in chiral perturbation theory
The near-threshold n p -> d pi0 cross section is calculated in chiral
perturbation theory to next-to-leading order in the expansion parameter sqrt{M
m_pi}/Lambda_chi. At this order irreducible pion loops contribute to the
relevant pion-production operator. While their contribution to this operator is
finite, considering initial-and final-state distortions produces a linear
divergence in its matrix elements. We renormalize this divergence by
introducing a counterterm, whose value we choose in order to reproduce the
threshold n p -> d pi0 cross section measured at TRIUMF. The energy-dependence
of this cross section is then predicted in chiral perturbation theory, being
determined by the production of p-wave pions, and also by energy dependence in
the amplitude for the production of s-wave pions. With an appropriate choice of
the counterterm, the chiral prediction for this energy dependence converges
well.Comment: 25 pages, REVTeX4, 5 eps figures, 1 reference added, 1 removed,
changed cutoffs shown in Table III, some improvements in the text and
figures, conclusions unaltered, accepted by PR
Has charge symmetry breaking been observed in the d d -> alpha pi0 reaction?
Estimates are made of the d d -> alpha gamma gamma production cross sections
in a model where each neutron-proton pair in the beam and target initiates an n
p -> d gamma reaction. This approach, which successfully reproduces observables
in two-pion production at intermediate energies, suggests that direct
two-photon production could provide a very significant background to the
measurement of the charge-symmetry-breaking (CSB) reaction d d -> alpha pi0. A
non-vanishing CSB cross section has been reported which might be confused with
such two-photon production under the given experimental conditions.Comment: 9 pages, ReVTEX, 2 figures, submitted to Phys. Rev. Let
Observation of the Charge Symmetry Breaking d + d -> 4He + pi0 Reaction Near Threshold
We report the first observation of the charge symmetry breaking d + d -> 4He
+ pi0 reaction near threshold at the Indiana University Cyclotron Facility.
Kinematic reconstruction permitted the separation of 4He + pi0 events from
double radiative capture 4He + gamma + gamma events. We measured total cross
sections for neutron pion production of 12.7 +- 2.2 pb at 228.5 MeV and 15.1 +-
3.1 pb at 231.8 MeV. The uncertainty is dominated by statistical errors.Comment: 7 pages, 2 figures, plain Te
Survey of charge symmetry breaking operators for dd -> alpha pi0
The charge-symmetry-breaking amplitudes for the recently observed d d ->
alpha pi0 reaction are investigated. Chiral perturbation theory is used to
classify and identify the leading-order terms. Specific forms of the related
one- and two-body tree level diagrams are derived. As a first step toward a
full calculation, a few tree-level two-body diagrams are evaluated at each
considered order, using a simplified set of d and alpha wave functions and a
plane-wave approximation for the initial dd state. The leading-order
pion-exchange term is shown to be suppressed in this model because of poor
overlap of the initial and final states. The higher-order one-body and
short-range (heavy-meson-exchange) amplitudes provide better matching between
the initial and final states and therefore contribute significantly and
coherently to the cross section. The consequences this might have for a full
calculation, with realistic wave functions and a more complete set of
amplitudes, are discussed.Comment: REVTeX 4, 35 pages, 8 eps figures, submitted to PR
Realistic Few-Body Physics in the Reaction
We use realistic two- and three-nucleon interactions in a hybrid
chiral-perturbation-theory calculation of the charge-symmetry-breaking reaction
to show that a cross section of the experimentally measured
size can be obtained using LO and NNLO pion-production operators. This result
supports the validity of our power counting scheme and demonstrates the
necessity of using an accurate treatment of ISI and FSI.Comment: 11 pages, 1 figur
Differential cross section and analysing power of the quasi-free pn -> {pp}_s pi- reaction at 353 MeV
In order to establish links between p-wave pion production in nucleon-nucleon
collisions and low energy three-nucleon scattering, an extensive programme of
experiments on pion production is currently underway at COSY-ANKE. The final
proton pair is measured at very low excitation energy, leading to an S-wave
diproton, denoted here as {pp}_s. By using a deuterium target we have obtained
data on the differential cross section and analysing power of the quasi-free
pol{p}n -> {pp}_s pi^- reaction at 353 MeV. The spectator proton p_sp was
either measured directly in silicon tracking telescopes or reconstructed using
the momentum of a detected pi^-. Both observables can be described in terms of
s-, p-, and d-wave pion production amplitudes. Taken together with the
analogous data on the pol{p}p -> {pp}_s pi^0 reaction, full partial wave
decompositions of both processes were carried out.Comment: The interested reader should also study the paper on pizero
production by D.Tsirkov et al., which has also been submitted to the arXi
Towards a field theoretic understanding of NN->NNpi
We study the production amplitude for the reaction NN->NNpi up to
next--to--leading order in chiral perturbation theory using a counting scheme
that takes into account the large scale introduced by the initial momentum. In
particular we investigate a subtlety that arises once the leading loop
contributions are convoluted with the NN wavefunctions as demanded by the
non--perturbative nature of the NN interaction. We show how to properly
identify the irreducible contribution of loop diagrams in such type of
reaction. The net effect of the inclusion of all next-to-leading order loops is
to enhance the leading rescattering amplitude by a factor of 4/3, bringing its
contribution to the cross section for pp->dpi+ close to the experimental value.Comment: 15 Pages, 5 Figure
Three-Body Halo States in Effective Field Theory: Renormalization and Three-Body Interactions in the Helium-6 System
In this paper we study the renormalization of Halo effective field theory applied to the Helium-6 halo nucleus seen as an alpha-neutron-neutron three-body state. We include the 0(+) dineutron channel together with both the 3/2(-) and 1/2(-) neutron-alpha channels into the field theory and study all of the six lowest-order three-body interactions that are present. Furthermore, we discuss three different prescriptions to handle the unphysical poles in the P-wave two-body sector. In the simpler field theory without the 1/2(-) channel present we find that the bound-state spectrum of the field theory is renormalized by the inclusion of a single three-body interaction. However, in the field theory with both the 3/2(-) and 1/2(-) included, the system can not be renormalized by only one three-body operator