Friday, September 27, 2019

quantum field theory - Matching Dirac/Majorana/Weyl Spinor Degrees of Freedom in Minkowski signature


Question: How do we match the real degrees of freedom (DOF) of Dirac/Majorana/Weyl Spinor in terms of their quantum numbers (spin, momentum, etc) in any dimensions [1+1 to 9+1] in Minkowski signature?


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(p.s. This question is partially inspired by this one, but I am asking something more generally in any dimensions from 1+1 to 9+1 in Minkowski signature)


My question concerns that how to match each component of spinors in physical degrees of freedom (real degrees of freedom of Dirac/Majorana/Weyl Spinor) reflecting into their quantum numbers.


For example in 3+1d, in Weyl basis,




For 3+1d Dirac spinor,



we have 4 component complex spinor thus we have 8 real degrees of freedom. For the massless particles, in the boost limit along the z direction, pzE or pzE, we can match



8=2×2×2,

as 8 real DOF=2 (spin up/down)×2 (momentum up/down)×2 (particle/ anti-particle)
More precisely, the 8 real DOF becomes the following:



ψpar(x)=u(p)eipx=u(p)ei(Etpx)=(pσζspˉσζs)ei(Etpx), s=1,2


ψanti(x)=v(p)eipx=v(p)ei(Etpx)=(pσηspˉσηs)ei(Etpx), s=1,2


particle, p3=pz=E, spin up,


ψpar(x)=2E(0010)eipx



particle, p3=pz=E, spin down,


ψpar(x)=2E(0100)eipx


particle, p3=pz=E, spin up,


ψpar(x)=2E(1000)eipx


particle, p3=pz=E, spin down,


ψpar(x)=2E(0001)eipx


anti-particle, p3=pz=E, spin up,


ψanti(x)=2E(0010)eipx


anti-particle, p3=pz=E, spin down,


ψanti(x)=2E(0100)eipx



anti-particle, p3=pz=E, spin up,


ψanti(x)=2E(1000)eipx


anti-particle, p3=pz=E, spin down,


ψanti(x)=2E(0001)eipx





For 3+1d Weyl spinor,



we separate Dirac into left-hand and right-hand Weyl spinors, thus


We have the particle is the same as the anti-particle, which is the real representation. Thus we have



for left-handed (PL)



4=(2)×2,

as 4 real DOF=(21/2 helicity : spin up/down lock momentum up/down)×2 (particle / anti-particle)



similarly for right-handed (PR)



4=(2)×2,

as 4 real DOF=2(+1/2 helicity : spin up/down lock momentum up/down)×2 (particle / anti-particle)







For 3+1d Majorana spinor,



We have the particle is the same as the anti-particle, which is the real representation. Thus we have



4=2×2×1,

as 4 real DOF=2 (spin up/down)×2 (momentum up/down)×1 (particle = anti-particle)



How about other dimensions?


There will be certainly an even-odd dimensions subtlty. Also the the helicity like degrees of freedom (the spin projected along the boost direction) may NOT be enough for other dimensions higher than 3+1d.




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