1. J Carter,Proc. joint lepton-photon and Int. europhysics Conf. high energy physics, Geneva, edited by S Hegartyet al, (World Scientific, Singapore, 1992)
2. See for example, the MSM is extended by allowing Majorana mass terms for the neutrinos then the four family model with two heavy Majorana neutrinos belonging to the fourth generation has been shown to be perfectly natural[3]
3. C T Hill and E A Paschos,Phys. Lett. B241, 96 (1990)
4. C H Llewelyn Smith,Philos. Trans. R. Soc. London A336, 307 (1991)
5. C Jarlskog,Proc. of the 25th Int. Conf. high energy physics, Singapore (edited by K K Phua and Y Yamaguchi (World Scientific, Singapore, 1990) p. 61