191 research outputs found
A remark on the Primakoff effect
The coherent-nuclear reaction a + A -> a* + A is in the small-angle region
dominated by the one-photon-exchange mechanism, often referred to as the
Primakoff effect. In this region information about the electromagnetic decay a*
-> a + gamma can be obtained. Well-known examples are the two-photon decays of
the pi- and eta-mesons. Also decays of charged hadrons can be studied. For
charged hadrons the one-photon-exchange amplitude comes with a Coulomb-phase
factor and a Coulomb-form factor, which depend on the ratio between transverse-
and logitudinal-momentum transfers, the latter being fixed. At the peak of the
cross-section distribution, where the two momentum transfers are equal, the
form factor could cut down the cross-section value by as much as 40 %.
Consequently, a determination of a radiative-decay rate that relies on the peak
value becomes sensitive to a proper treatment of the Coulomb-form factor.Comment: 5 pages, 2 figure
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
Coulomb-nuclear interference in pion-nucleus bremsstrahlung
Pion-nucleus bremsstrahlung offers a possibility of measuring the structure
functions of pion-Compton scattering from a study of the
small-momentum-transfer region where the bremsstrahlung reaction is dominated
by the single-photon-exchange mechanism. The corresponding cross-section
distribution is characterized by a sharp peak at small momentum transfers. But
there is also a hadronic contribution which is smooth and constitutes an
undesired background. In this communication the modification of the
single-photon exchange amplitude by multiple-Coulomb scattering is investigated
as well as the Coulomb-nuclear interference term.Comment: 21 pages, 5 figures. Eqs.(51,52) corrected; some new figure
Comparison of eta and eta' production in the pp -> pp eta(eta') reactions near threshold
The total cross section of the pp -> pp eta' reaction has been measured at
two energies near threshold by detecting the final protons in a magnetic
spectrometer. The values obtained are about a factor of 70 less than for the
corresponding eta production, in good agreement with the predictions of a
one-pion-exchange model.Comment: 10 pages, Latex with 3 eps figure
Cross section and analyzing power of pol{p}p -> pn pi+ near threshold
The cross section and analyzing power of the pol{p}p -> pn pi+ reaction near
threshold are estimated in terms of data obtained from the pol{p}p -> d pi+ and
pp -> pp pi0 reactions. A simple final state interaction theory is developed
which depends weakly upon the form of the pion-production operator and includes
some Coulomb corrections. Within the uncertainties of the model and the input
data, the approach reproduces well the measured energy dependence of the total
cross section and the proton analyzing power at a fixed pion c.m. angle of
90deg, from threshold to T_p = 330 MeV. The variation of the differential cross
section with pion angle is also very encouraging.Comment: 20 pages, Latex including 4 eps figure
Meson production in high-energy electron-nucleus scattering
Pseudoscalar mesons can be produced and studied in high-energy
electron-nucleus scattering. We review and extend our previous analysis of
meson production in the nuclear Coulomb field. The
decay rates are most directly determined for mesons produced in the
double-Coulomb region where both photons are nearly real, and provided the
background-hadronic contribution remains small. The larger the mass of the
meson the higher the electron energy needed to assure such a condition.Comment: 19 pages, 2 figure
Scattering Wave Functions at Bound State Poles
The normalisation relation between the bound and scattering S-state wave
functions, extrapolated to the bound state pole, is derived from the
Schroedinger equation. It is shown that, unlike previous work, the result does
not depend on the details of the potential through the corresponding Jost
function but is given uniquely in terms of the binding energy. The
generalisations to higher partial waves and one-dimensional scattering are
given.Comment: 15 pages Latex. No graph
Eta-Helium Quasi-Bound States
The cross section and tensor analysing power t_20 of the d\vec{d}->eta 4He
reaction have been measured at six c.m. momenta, 10 < p(eta) < 90 MeV/c. The
threshold value of t_20 is consistent with 1/\sqrt{2}, which follows from
parity conservation and Bose symmetry. The much slower momentum variation
observed for the reaction amplitude, as compared to that for the analogous
pd->eta 3He case, suggests strongly the existence of a quasi-bound state in the
eta-4He system and optical model fits indicate that this probably also the case
for eta-3He.Comment: LaTeX, uses elsart.sty, 10 pages, 3 Postscript figures, Submitted to
Physics Letters
Eta physics at threshold
The production of eta and eta-prime mesons in elementary nucleon-nucleon
collisions has been investigated at the synchrotrons CELSIUS, COSY and SATURNE.
The magnitude and energy dependence of the total cross section as well as the
occupation distribution of the phase space serve as observables for
investigating the mechanisms underlying the production processes and the
interaction of mesons with nucleons. The precise data on the eta and eta-prime
creation via the pp --> pp eta(eta-prime) reactions allowed to settle the
general features of the eta and eta-prime meson production and revealed the
sensitivity of the mentioned observables to the nucleon-nucleon-meson final
state interaction. The particular production properties, like for example the
determination of the dominating exchange processes which lead to the excitation
of the S_11 nucleon isobar in the case of eta creation, must be established by
confrontation with other observables. The present status of this investigation
with an emphasis on the results of the COSY-11 collaboration is briefly
presented. The available data are interpreted in view of the production
mechanism and the meson-nucleon interaction.Comment: 3 pages, 3 figures, Presented at Conference on Quarks and Nuclear
Physics (QNP 2002), Julich, Germany, 9-14 Jun 200
Chiral corrections to the isovector double scattering term for the pion-deuteron scattering length
The empirical value of the real part of the pion-deuteron scattering length
can be well understood in terms of the dominant isovector -double
scattering contribution. We calculate in chiral perturbation theory all
one-pion loop corrections to this double scattering term which in the case of
-scattering close the gap between the current-algebra prediction and the
empirical value of the isovector threshold T-matrix . In addition
to closing this gap there is in the -system a loop-induced off-shell
correction for the exchanged virtual pion. Its coordinate space representation
reveals that it is equivalent to -exchange in the deuteron. We evaluate
the chirally corrected double scattering term and the off-shell contribution
with various realistic deuteron wave functions. We find that the off-shell
correction contributes at most -8% and that the isovector double scattering
term explains at least 90% of the empirical value of the real part of the -scattering length.Comment: 4 pages, 2 figures, to be published in The Physical Review
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