# Photon had a non-zero rest mass, based on measured helicity values?

Hi, got an exam tomorrow for particle physics and need a question answered concerning helicity.

How would it be possible to determine if a photon had a non-zero rest mass, based on measured helicity values?

Thanks
Ray

rayveldkamp said:
Hi, got an exam tomorrow for particle physics and need a question answered concerning helicity.

How would it be possible to determine if a photon had a non-zero rest mass, based on measured helicity values?

Thanks
Ray

Well, restmass (which is always referred to as just mass, so don't bother to make the distinction ) mixes up the two different possible chiralities. This is a result of the famous "tau-theta-puzzle" in QFT. To make a long story short : in QFT all elementary particles are massless because the two chiralities each correspond to a different kind of fundamental "particle-property". I mean, in QFT particles are classified based upon their chirality because a left handed chiral particle does not behave in the same way as a right hand chiral particle. So mass has to be zero (before the spontaneous breakdown of symmetry) because otherwise one particle possesses the TWO chiralities at once (because of the mass-term in the Lagrangian) and thus it is NOT fundamental. So basically photons are always massless because they do not interact with the Higgs-field. They contain a distinct chirality-value and thus not different values for ONE photon. Just look at the V-A-current in the elektroweak-theory : only left handed fermions (or right handed anti-fermions) are involved in weak charged currents.

regards
marlon

OK, i understand the your point, but im only in second year undergraduate physics at university so we havent covered a lot of QFT. Basically due to parity violation for the Weak Interaction, participating fermions are emitted and absorbed predominantly in negative helicity states (and positive for anti-fermions), the probability a fermion's in a negative helicity state can be found by

P=0.5(1+v/c) where v is the particle's speed
and for a fermion in a positive helicity
P=0.5(1-v/c)

So from here i see that a massless particle has zero probability of being in a positive helicity. I am just wondering if this same argument can be used to deduce a photon's helicity.
Thanks
Ray

You are correct...it is gonna be something like that...

marlon

Thanks for that, feeling a bit relieved for tomorrow's exam now!

Regards
Ray

good luck man...

marlon