8

When one looks up elementary particles on Wikipedia, as one of their properties their parity is stated. For example the Proton has parity $+1$, while a Pion has parity $-1$. I understand that you have to define the parity of some particles to have a reference to measure the parity of all particles (since one could just redefine $+1\mapsto -1$).

How is the parity of a particle measured in experiment?

Sandejo
  • 5,478
Jannik Pitt
  • 952
  • Some elementary introductions such as the one by Perkins devote entire chapters on the elaborate nets of experimental inferences, involving interactions and decays to particles with known parities, leading up to such determinations. You want a vague platitude summary? – Cosmas Zachos May 19 '20 at 16:19
  • 4
    @CosmasZachos Some general ideas would be nice, since this is not easily found on the internet (see e.g. the Wikipedia article for parity). In my opinion thus this question fits this site quite nicely. – Jannik Pitt May 19 '20 at 16:22
  • So you want a listing of examples like this WP one? – Cosmas Zachos May 19 '20 at 16:31
  • 2
    I strongly recommend against using Wikipedia in any circumstance where you want specific equations. The problem is that things like parity are associated with many arbitrary conventions. Good books pick one convention and stick with it. Wikipedia is cobbled together from many different books, so its conventions are not even consistent with itself. Sometimes you'll even see two adjacent equations with incompatible conventions. – knzhou May 19 '20 at 19:41
  • 1
    The same problem goes for trying to learn a convention-heavy subject from StackExchange posts -- everybody is using different conventions. The last time I wrote an answer that was explicit about all possible conventions, it turned out to be thousands of words long. Just stick with one good source! – knzhou May 19 '20 at 19:42

1 Answers1

2

If you look at the particle data group tables, mesons for example you will see for each resonance listed its parity, in the same line as the name and the mass of the resonance. Like the mass, it is an observation from measuring the resonance in experiments, laboriously in experiments over the years .

for example, the parity of the pi- was measured in a specific experiment:

The reaction π−+d→2n has been observed by detecting the two neutrons in coincidence with slow negative mesons incident on a liquid deuterium target. The observed angular correlation of the two neutrons confirms the identification of the process. The process is therefore not forbidden, and this fact may be used to establish the odd relative parity of the pion and the nucleon.

Received 8 June 1954

So it is angular correlations in combination with known parities that establish the unknown ones.Once there is a list of known parity particles and resonances, the parity of new states can be measured , as is proposed to measure the parity of the Higgs in future experiments.

Community
  • 1
anna v
  • 233,453
  • It sounds like this is based on parity conservation, which seems a little circular (and also violated). – akrasia Sep 25 '22 at 17:09
  • @akrasia have a look at the definition of partity https://physics.stackexchange.com/questions/553245/how-do-you-measure-the-parity-of-a-particle/553286?noredirect=1#comment1631030_553286 . As the OP states, for assigning a parity to particles one has to decide on one having a +1 and then the parity of other particles can be measured as linked in the answer, The measurements in experiments , scattering paraticles , will show of parity is not conserved because the distributions will not fit the predictions – anna v Sep 25 '22 at 17:25