The spin or helicity structure

When a Dirac particle is moving fast enough so that its energy E m it can be shown that its spinor becomes an eigenstate of 75 (see, for example, Section 7.10 of Bjorken and DreU, 1964). If and v are spinors for particles and antiparticles respectively, and if we denote by R (right-handed) states with hehcity +5 and by L (left-handed) states with helicity — then one has 

These results sure only true to the extent that E m. neutrinos they sure, of course, exactly true. 

To the extent that we can ignore the masses of the quarks, (1.2.23) tells us that only left-handed quarks participate in h (x). 

Very similm manipulations give the following important results, aU valid to the extent that F m for spin particles A, B  

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