Why are heilicty and chirality equivalent for massless particles?

by Superposed_Cat
Tags: chirality, equivalent, heilicty, massless, particles
P: 270
Salutations, question:
 For massless particles—such as the photon, the gluon, and the (hypothetical) graviton—chirality is the same as helicity; a given massless particle appears to spin in the same direction along its axis of motion regardless of point of view of the observer.
~wikipedia. I'm assuming that's because (correct me if i'm wrong) those particles would travel at c and you could not overtake them so their chirality and helicity are equal.But if a photon was coming towards you then zoomed past you surely the chirality would change because you now see it going away from you?
Mentor
P: 11,819
 But if a photon was coming towards you then zoomed past you surely the chirality would change because you now see it going away from you?
It is now going away, but the spin also changed its radial direction: if it was oriented towards you before, it will be oriented away from you afterwards, and vice versa.
Or, in other words, the spin is always in the direction of motion or against, and this does not change.
 P: 270 But then why does it change for massive particles?
 P: 861 Why are heilicty and chirality equivalent for massless particles? does it change for massive particles? The chirality does not coincide with helicity, because chirality symmetry is explicitly broken
 P: 270 helicity sorry. why does for massive particles?
 Mentor P: 16,317 Please don't take this the wrong way, but do you know what either helicity or chirality are? From your questions, it doesn't look like you do.
 P: 270 No worries I take no offence. I just no what i read on the wiki.
 Mentor P: 16,317 Usually people understand one and are confused about the other. (And answering the question hinges on finding which one they understand) But if you don't know what either of them are, it will be difficult to explain the difference.
P: 861
 Quote by Vanadium 50 Usually people understand one and are confused about the other. (And answering the question hinges on finding which one they understand) But if you don't know what either of them are, it will be difficult to explain the difference.
In fact, can't your just write your state as a superposition of helicity eigenstates, and then look at how it transforms under a lorentz transformation to see that?
 P: 270 V-50 I was wondering why they said you can't change your reference frame because you can't overtake it. Surely if it flyies past you it is the same as overtaking it? P.S. thanks chrisver
 Sci Advisor Thanks P: 4,160 Chirality (aka handedness) = ±1 is the eigenvalue of γ5. It's a Lorentz invariant, but for a particle with nonzero m it's not a constant of the motion, i.e. [H, γ5] ≠ 0. Helicity = ±1 is the eigenvalue of σ·p/|p|. For a free particle it's a constant of the motion, but clearly not a Lorentz invariant. The easiest way to see the relation between the two is to follow Peskin-Schroeder and use the Weyl representation, in which γ5 is diagonal, $$\gamma^5 = \left(\begin{array}{cc}-1&0\\0&1\end{array}\right)$$ The eigenfunctions of γ5 corresponding to ±1 are denoted ψR and ψL, respectively. In terms of ψL and ψR the Dirac equation is $$\left(\begin{array}{cc}-m&E + \sigma \cdot p\\E - \sigma \cdot p&m\end{array}\right) \left(\begin{array}{c}\psi_L\\\psi_R\end{array}\right) = 0$$ and so for m = 0 we have that ψR and ψL are also eigenstates of helicity, σ·p = ±E.
Thanks
P: 1,948
 Quote by Superposed_Cat V-50 I was wondering why they said you can't change your reference frame because you can't overtake it. Surely if it flyies past you it is the same as overtaking it? P.S. thanks chrisver
No, when you overtake a particle, you're moving faster than the particle and in your reference frame the particle is seen moving the opposite direction reversing its helicity. When a particle overtakes you you still see the particle moving in the same direction with no helicity reversal.
 P: 270 Thanks all esp. dauto
P: 364
 Quote by dauto No, when you overtake a particle, you're moving faster than the particle and in your reference frame the particle is seen moving the opposite direction reversing its helicity. When a particle overtakes you you still see the particle moving in the same direction with no helicity reversal.
And perhaps to make it even more obvious, there is no preferred "centre" of a reference frame, i.e. nothing explicitly stating where "you" are relative to the particle. x=0 is not special. So nothing you measure about the particle is going to change as it zooms past some arbitrary location, except it's position of course.

Going to a different frame via a Lorentz boost is a different story though (note again that you don't have to "actually" "overtake" the particle, since "you" are not anywhere in the math)

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