https://doi.org/10.1140/epjc/s10052-020-08474-x
Regular Article – Theoretical Physics
The cross section of 
as a litmus test of isospin violation in the decays of vector charmonia into 
1
Laboratori Nazionali di Frascati dell’INFN, Frascati, Italy
2
INFN Sezione di Perugia, 06100, Perugia, Italy
3
Università di Perugia, 06100, Perugia, Italy
Received:
24
July
2020
Accepted:
12
September
2020
Published online:
29
September
2020
Under the aegis of isospin conservation, the amplitudes in Born approximation, i.e., considering the only one-photon-exchange mechanism, of the decay 
, where 
is a vector charmonium, and of the reaction at the mass, are parametrized by the same electromagnetic coupling. It follows that, the modulus of such a coupling can be extracted from the data on the two observables: the decay branching fraction and the annihilation cross section. By considering the first two vector charmonia, 
and 
, it is found that, especially in the case of , there is a substantial discrepancy between the values of the modulus of the same electromagnetic coupling extracted from the branching ratio and the cross section. We propose, as a possible explanation for such a disagreement, the presence in the decay amplitude of isospin-violating contributions driven by two different mechanisms, that, however, appear to be more favored in the than in the decays.
© The Author(s) 2020
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