# Quark-antiquark annihilation

1. Jul 22, 2012

### TrickyDicky

I was wondering, for instance in a neutral pion or the j/ψ meson, what prevents quark-antiquark annihilation during its short lifetime? I mean what allows them to be particles, shouldn't annihilation be inmediate? is it the time it takes quark and antiquark to collide inside the particle?

2. Jul 22, 2012

### fzero

Yes, you can think of the lifetime as a measure of the average amount of time that it takes for the antiparticle to collide with the particle. It turns out that this is inversely proportional to the probability of finding the antiparticle at the same location of the particle, $|\psi(0)|^2$. Here $\psi(x)$ is the wavefunction of the antiparticle in coordinates where the particle is located at $x=0$.

Note that the annihilation process is statistical. In a large sample of particle-antiparticle bound states, some bound states will annihilate faster, while others slower. The lifetime is the average amount of time we have to wait for around 63% of an initial sample to decay.

3. Jul 22, 2012

### Staff: Mentor

All those particles decay extremely quickly, about 10-16s for the neutral pion (which cannot decay via the strong interaction), about 10-20s for the J/psi, less for most (all?) other states.

Last edited: Jul 22, 2012
4. Jul 22, 2012

### TrickyDicky

Ok, thanks.

5. Jul 23, 2012

### tom.stoer

The pions (or in general the light pseudo-scalar mesons) are protected by the special fact that they are (nearly) Goldstone bosons of the spontaneously broken chiral symmetry of QCD. With massless quarks (i.e. exact chiral symmetry) and QCD only the pseudo-scalar mesons would be exactly massless and stable. Mechanisms for their decay are related to weak interactions and axial anomaly.