Editing Meson (section)

=== {{math|P}}-parity ===
{{Main|Parity (physics)}}
{{math|P}}-parity is left-right parity, or spatial parity, and was the first of several "parities" discovered, and so is often called just [[Parity (physics)|"parity"]]. If the universe were reflected in a mirror, most laws of physics would be identical—things would behave the same way regardless of what we call "left" and what we call "right". This concept of mirror reflection is called [[parity (physics)|parity]] ({{math|P}}). [[Gravity]], the [[electromagnetic force]], and the [[strong interaction]] all behave in the same way regardless of whether or not the universe is reflected in a mirror, and thus are said to [[P-symmetry|conserve parity]] ({{math|P}}-symmetry). However, the [[weak interaction]] does'' ''distinguish "left" from "right", a phenomenon called [[parity violation]] ({{math|P}}-violation).

Based on this, one might think that, if the [[wavefunction]] for each particle (more precisely, the [[quantum field]] for each particle type) were simultaneously mirror-reversed, then the new set of wavefunctions would perfectly satisfy the laws of physics (apart from the weak interaction). It turns out that this is not quite true: In order for the equations to be satisfied, the wavefunctions of certain types of particles have to be multiplied by −1, in addition to being mirror-reversed. Such particle types are said to have ''negative'' or ''odd'' parity ({{math|P}}&nbsp;=&nbsp;−1, or alternatively {{math|P}}&nbsp;=&nbsp;&minus;), whereas the other particles are said to have ''positive'' or ''even'' parity ({{math|P}}&nbsp;=&nbsp;+1, or alternatively {{math|P}}&nbsp;=&nbsp;+).

For mesons, parity is related to the orbital angular momentum by the relation:<ref name=PDGQuarkmodel>
{{cite web
 |first1=C. |last1=Amsler
 |display-authors=etal
 |collaboration=[[Particle Data Group]]
 |year=2008
 |title=Quark Model
 |series=Reviews
 |publisher=[[Lawrence Berkeley Laboratory]]
 |url=http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf
}}
</ref><ref name=Amsler-etal-PDG-2008/>

: <math>P = \left( -1 \right)^{L + 1}</math>
where the {{mvar|L}} is a result of the parity of the corresponding [[spherical harmonic]] of the [[wavefunction]]. The "+1" comes from the fact that, according to the [[Dirac equation]], a quark and an antiquark have opposite intrinsic parities. Therefore, the intrinsic parity of a meson is the product of the intrinsic parities of the quark (+1) and antiquark (−1). As these are different, their product is −1, and so it contributes the "+1" that appears in the exponent.

As a consequence, all mesons with no orbital angular momentum ({{mvar|L}}&nbsp;=&nbsp;0) have odd parity ({{math|P}}&nbsp;=&nbsp;−1).