The γγ→ π 0 π 0 and η→ π 0 γγ Transitions in the Extended NJL Model
Abstract
We calculate within the Extended Nambu--Jona-Lasinio model the leading in
1/
N
c
contribution to
γγ→
π
0
π
0
to all orders in the external momenta and quark masses. This result is then combined with the known two-loop Chiral Perturbation Theory and compared with the data and other calculations. A technical difficulty in the same calculation beyond order
p
6
for
η→
π
0
γγ
is identified and for this decay results up to order
p
6
are presented.