Do spinor boosts have a simple form like rotations in QFT?

[mahdiarnt@yahoo.com]

Weinberg shows in his book on QFT that the effect of a rotation on a
massive particle's spin is the same as applying exp(-i \theta J.n) on
the state, regardless of particle's momentum. In other words just
rotate the spinor in its own representation. This makes calculations
very simple. But what about boosts? There is of course a general
approach described in Weinberg's book using little group elements; but
it is too difficult to apply it each time. Is there any general theorem
for boost like that Weinberg presented for rotations? And also what
about massless particles?

 

[EGoldstein]

I´m finding the exact same problem. In fact I´m trying to solve the first problem in Weinberg Vol I. Chpater 2 and while all the other items I managed to find out, I still hhave many problems in getting the Wigner rotation for the Boost.