Hello I was reading in Griffiths that in the case of electron neutrino scattering, one can apply Casimir's trick to sum over input and output spin states, including the neutrinos. My question is, how come you are allowed to include in the sum the nonphysical right handed helicity state ? And, yes, I understand that the final average takes into account that there is only one neutrino spin. But how does the nonphysical helicity not contribute mathematically, since it is included ? Thank you.