# What is parity violation?

1. Apr 3, 2012

### zheng89120

Can someone explain in easy to understand terms, what parity violation is? specifically:

"Only the left-handed components of particles and right-handed components of antiparticles participate in weak interactions in the standard model."

BONUS: if you're like a particle physicist or something, could you also explain:

"A spin-1 exchange particle can be described by a vector (V) or axial vector (A) operator. Both would conserve parity, but if the operator is mixed, parity can be violated. The observation of maximal parity violation and purely left-handed neutrinos requires equal contributions of vector and axial vector with opposite sign (V-minus-A theory)."

2. Apr 4, 2012

### ofirg

Parity is a symmetry transformation under which the spatial coordinates change sign
x$\rightarrow$-x,y$\rightarrow$-y,z$\rightarrow$-z
Note that this can't be obtained by any rotation.

Vector - changes sign under parity
Axial Vector - doesn't change sign under parity

spin 1/2 particles can be either left handed or right handed, referring to two different representations of the Lorentz group.

for massless fermions, the consequences are:

Right handed fermions have spin aligned with their momentum.
Left handed fermions have spin anti aligned with their momentum.

since monentum is a vector and spin ( angular momentum) an axial vector, parity transforms a Right handed fermion to a left handed one and vice versa.

because only left handed particles and not their right handed counterparts participate in the weak interaction, parity is violated.

If a spin 1 particle interacts with a vector operator or an axial vector particle, its partiy properties can be defined appropriately as to conserve parity.

but if it interacts with both, parity is necessary violated.

hope that helps.