qft notes 4
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8/14/2019 QFT notes 4
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QFT
Unit 4: The Spin-Statistics Theorem
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The Spin-Statistics Theorem
! Anti-commutation relations (fermions) may be usedonly on half-integer spin particles.
! Commutation relations (bosons) may be used onlyon integer spin particles.
! We saw a hint of this last time, because we got atrivial L from spin-0 anti-commutators." But not yet clear why this is the case, or if the problem
can be fixed.
! The rest of this section is to prove the theorem forspin = 0.
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Outline
1. Prove that !+ and !- yield local, Lorentz-
Invariant interactions.
2. Require the transition amplitude to be
constant in different frames.
3. Show that this requirement is impossible
(for spin 0) for anti-commutators, and for
any field operator other than !.
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Local, Lorentz-Invariant Interactions
! Let’s consider the Hamiltonian for free,
non-interacting spin-0 fields, with "0 = #0.
! Now let’s define !+, !-
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Local, Lorentz-Invariant Interactions
! Time-evolving these:
! Notice two things:" The sum of these is just !
" !+ and !- are Lorentz scalars, ie when sandwichedbetween the unitary Lorentz matrices, their argumentis Lorentz-transformed.
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Schrodinger & Heisenberg Pictures
! When systems are time-evolved, there are
two ways of thinking about it:
" Heisenberg: The initial and final states areconstant, but the Hamiltonian in the evolutionoperator is time-dependent.
" Schrodinger: The operator is constant, but thestates evolve in time.
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Transition Amplitudes
! Let’s choose the Heisenberg picture. The
transition amplitude is:
! Let’s chop this integral up into many little
pieces. How can we interpret the time-
ordering symbol in light of relativity?
" Time-ordering must be frame independent.
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Transition Amplitudes
! If separation is time-like, no problem. Ifseparation is space-like, we must require:
! But in fact:
(see problem 4.1)
This is never zero.
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Resolving this Problem
! There must be some linear combination of !+ and !- that will work – otherwise we can’t add
even a simple interaction term to the
Lagrangian.
! Let’s try the most general linear combination.
We find:
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Resolving the Problem, cntd.
! This is zero if and only if:" Choose anti-commutators
" Choose |$| = 1
! But this second requirement gives us (up to aphase shift) ! again!
! We tried to start with a, a†, but it seems that thefundamental object is instead !.
! Further, we must choose the anti-commutation
operators to avoid a trivial L.
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