But for massive particles like an electron, the chirality is not conserved in time i.e. if an electron is produced in the state $e_L$ at time t=0, at a later time it becomes a mixture of left-handed and right-handed components.
How can the evolution of a purely left-chiral or right-chiral state of a massive fermion (like the electron) be quantified? In other words, given that the electron was produced in a state of definite chirality at $t=0$, what will it become at a later time $t>0$?
I previously asked a question here but I did not get an answer that I was looking for. Let me explain the answer I'm looking for. We know that a neutrino created in a define flavour state becomes a mixture of flavours at a later instant of time. In the same manner, I want to understand how a state of definite chirality become a state which is a mixture of both chiralities.