Helicity violation in strong interaction?

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Discussion Overview

The discussion revolves around the concept of helicity violation in strong interactions, specifically in the context of proton-antiproton scattering via pion exchange. Participants explore the implications of helicity changes in matrix elements and the conservation of helicity in strong interactions.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a matrix element for proton-antiproton scattering and notes that helicity changes are required, suggesting helicity is not conserved.
  • Another participant questions the assumption that helicity is conserved in strong interactions.
  • A third participant explains that scalar or pseudoscalar interactions, like those involving pions, inherently involve helicity flips due to Lorentz invariance and the Dirac equation.
  • This participant also mentions the breaking of chiral symmetry in QCD and connects it to the role of pions as Goldstone bosons.
  • A later reply acknowledges the previous points and expresses gratitude for the clarification, indicating a realization of a misunderstanding.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the conservation of helicity in strong interactions, as there are differing views on the implications of helicity changes and the nature of the interactions involved.

Contextual Notes

The discussion includes assumptions about the nature of the interactions and the role of helicity in particle physics, which may not be fully resolved. The implications of chiral symmetry breaking and its effects on helicity conservation are also noted but not conclusively addressed.

metter
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I have a proton and an antiproton scattering, via a pion exchange.

The matrix element has the form:
M=g*(\bar{u}_{1}\gamma ^{5}u_{2})\frac {1} {q^2-m^2}( \bar{v}_{1}\gamma ^{5}v_2)
Wher g is my coupling constant, and q the 4-momentum of the pion.

The problem is that when I compute the currents (\bar{u}_{1}\gamma ^{5}u_2) and (\bar{v}_{1}\gamma ^{5}v_2) in the helicity basis this terms are non zero only for a change of the helicity( my righthanded proton should change into a lefthanded proton and the same for my antiproton).

This would imply that the matrix element for a helicity 1 state going to a helicity -1 state is not zero, which implies helicity is not conserved.

Where am I getting it wrong?
 
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Why do you think helicity conserved in strong interactions?
 
A scalar (or pseudoscalar) interaction is always L-R. This follows from Lorentz invariance, or from a more brute-force point of view, from the Dirac equation. The pion is a pseudoscalar, so you expect a helicity flip!

Chiral symmetry is broken in QCD, with pions being the goldstone bosons of the breaking. So this is totally consistent.

This leads to the infamous question that appears on graduate board exams for particle-physics PhD candidates: if there was no Higgs, what would be the mass of the Z boson? Answer: proton mass!
 
Thnak you very much. You are right. I was confusing something, and your answers made me realize that.

Thanks again
 

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