Majorana neutrinos in the triple gauge boson coupling ZZZ*

Confirmed by the measurement of neutrino oscillations, neutrino mass is recognized as a genuine manifestation of physics beyond the Standard Model, while its originating mechanism remains a mystery. Moreover, the proper field-theory description of neutrinos, whether they are Majorana or Dirac type, must be linked to such a mechanism. The present work addresses the calculation, estimation, and analysis of one-loop contributions from virtual Majorana neutrinos, light and heavy as well, to the neutral gauge boson coupling ZZZ, which participates in Z-boson pair production from e(+)e(-) collisions. This task is carried out in the framework defined by a seesaw variant in which light neutrinos remain massless at tree level, then becoming massive radiatively. The ZZZ* coupling, with Z* an off-shell Z boson, is defined by two form factors, namely, f(4), characterizing CP-odd effects, and f(5), which is CP-even. Constraints from the Large Hadron Collider on both of these quantities are currently O(10(-4)). Our calculation yields CP-nonpreserving contributions to ZZZ, which are absent in the framework of the sole Standard Model. Our estimations show that the f(4) contribution might be as large as O(10(-7)) for heavy-neutrino masses similar to 1 TeV. CP-even contributions f(5) are also generated, which are, in general, larger than their CP-odd counterparts. We estimate them to be as large as O(10(-4)) at a center-of-mass energy of 500 GeV, in ethorne- collisions.