Monday, 23 November 2015

dirac equation - Components of the Weyl spinor field


In the Weyl basis we can separate the spinor field into 2 components: the right-chiral spinor and the left-chiral spinor. Each of these fields has again 2 components which are coupled. What is the physical interpretation of these 2 components that make up the left-chiral (or right-chiral) field?


In the Dirac basis the interpretation of the 4 components is:
1. Electron spin-up
2. Electron spin-down
3. Positron spin-up
4. Positron spin-down



So my question is what is the corresponding interpretation in the Weyl basis (in the massless case). Is it like this?
1. Left-chiral electron ψ4
2. Left-chiral positron ψ3
3. Right-chiral electron ψ2
4. Righ-chiral positron ψ1


If this is the case than I don't understand why the left-chiral electron ψ4 couples to left-chiral positron ψ3 as can be seen in the equations:


tψ4+xψ4iyψ4+zψ3=0

tψ3+xψ3+iyψ3zψ4=0
tψ2xψ2+iyψ2zψ1=0
tψ1xψ1iyψ1+zψ2=0



Answer



The meaning of different components in the chiral representation are




  1. Left-handed spin up,

  2. Left-handed spin down,

  3. Right-handed spin up,

  4. Right-handed spin down.


Spin up and down are with respect to some arbitrary axis, which we often set to the z-axis .


Electron and positron(or negative-energy electron) states are identified from the solutions of the Dirac equation. It turns out that in the massless limit (for which the chiral representation is most convenient), the left- and right-handed sectors decouple, and electron(positron) states in the left-handed sector are left(right)-handed, viz. the spin is anti-parallel(parallel) to the momentum, and similarly for the right-handed sector.


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