[DoctorBeaver (OP)]

A pion consists of a quark and an antiquark. So why don't they annihilate each other?

[LeeE]

I believe that the anti-quark isn't of the same type as the quark i.e. up + anti-down, as opposed to up + anti-up.

[DoctorBeaver (OP)]

That's interseting. I shall have to investigate further. Thank you.

So is it only particle/anti-particle pairs of the same type that will annihilate?

[Soul Surfer]

In effect they do!  Because they are not stable.  A pi meson mostly turns into a mu meson and a neutrino The mu meson eventually turns into an electron and two neutrinos the electron is stable. energy is also released with each transition

[DoctorBeaver (OP)]

Ian - that's not the same thing, is it? Surely, that's just decay.

[Soul Surfer]

You are forgetting that as well as the pi plus and pi minus mesons there is a pi 0 meson which decays much faster. This is because it is a compatible particle and antiparticle and decays via electromagnetic annihilation.  it does last long enough to be considered as a separate entity.

see   

http://en.wikipedia.org/wiki/Pion 

This should answer most of your questions

[DoctorBeaver (OP)]

Thank you, Ian. You're the canine's testes!

[LeeE]

So is it only particle/anti-particle pairs of the same type that will annihilate?

Annihilation seems to depend upon cancellation of equal but opposite charge - the up and anti-up quarks have 2/3 charge but the down and anti-down only have 1/3 charge, so there's a charge imbalance between an up quark and an anti-down quark, which implies that they cannot annihilate.

Up and down quarks also have unequal mass, so if annihilation could be initiated by somehow removing all charges from the quark/anti-quark combination there'd be some mass left over, roughly in the range of up/anti-up quarks.  However, because you'd have no charge left over (because you had to neutralise it to get the annihilation) you wouldn't be able to build a new up/anti-up quark from the remaining mass.

Actually, I'm not sure what models for annihilation have been postulated but it seems to me that if the charge of a particle is one of it's fundamental properties then removal or cancellation of the charge makes the particle impossible, so it must self-destruct, releasing it's mass as energy.  If this is so, then the imbalance in mass wouldn't matter because the particles self-annihilate.

[DoctorBeaver (OP)]

So could an up annihilate with 2 anti-downs?

[LeeE]

Well, anti-downs have +ve charge, as does the up, so I'd say no.  However, the neutron is an up + two downs, with a net charge of zero, but then none of them are anti-particles.  Perhaps a quark can only take part in a single interaction at any one time (because it can't do two mutually exclusive things at the same time) so simultaneous three-way interactions aren't allowed.  It would be interesting to see what would happen in an interaction between say, an up quark and an anti-top quark (which is much more massive than the up quark) though, as they both have 2/3 charges, but of opposite polarity.  If they didn't annihilate, would they have to form a meson, because both have +1/2 spin?  I'm not sure if spins can simply be added.

[DoctorBeaver (OP)]

If they didn't annihilate, would they have to form a meson, because both have +1/2 spin?  I'm not sure if spins can simply be added.

If you added the spins together to give spin-1, wouldn't that change them into bosons? Is that allowed?

[Soul Surfer]

Mesons have zero spin meson decay other than straight of compatible pairs anihilation is via the weak reaction see the reference I quoted earlier.

[DoctorBeaver (OP)]

Ian - I read the article you referred to. Im going to read it again, though, to try to understand it better.

[chrisdsn]

> A pion consists of a quark and an antiquark. So why don't they annihilate each other?

They do. Pions aren't stable particles. The pi-zero (up and anti-up) has
a mean life-time of ~10^{-16} seconds (zero point ... fifteen more zeros ... some
number seconds). Pretty much all of the time they annihilate to produce two
photons.

The charged pions (pi-plus and pi-minus) are made up of one-up and one-down
quark and so need a flavour changing process to decay. These exist, but are
weaker than the electromagnetic processes at low energies (Hence the name
of the force that provides such interactions: the Weak force). Their mean
lifetime are ~ 10^{-8} seconds (most of the time they decays into a muon
and a neutrino).

[DoctorBeaver (OP)]

chrisdsn - thank you for your informed reply.

Can't particles only change flavour if symmetry is unbroken?

[chrisdsn]

> Can't particles only change flavour if symmetry is unbroken?

The breaking of the electro-weak symmetry via the Higgs mechanism
doesn't change the fact that flavour changing interactions exist,
however it does lead to the gauge-bosons - the W bosons - that
mediate these interactions having a (heavy) mass; for an unbroken
symmetry the gauge boson mass would be zero (as it is for photons
and gluons). This mass is ~80GeV (GeV == Giga Electron Volts) which
is very large compared to the mass of the pions, ~140 MeV (Mega
Electron Volts), or even a proton (~1GeV). This disparity of
scales means that if you're looking at pion-scale energies the
probability of producing a virtual W-boson is quite small and hence
the force is weak; if you were looking at energies ~80GeV this
suppression mechanism wouldn't apply and the weak force wouldn't
actually be very weak.
 

[DoctorBeaver (OP)]

Thanks, chrisdsn; I think I understand that. Sort of. Ish. Kinda  []

I know symmetry-breaking theories that allow too much flavour changing have to be discarded, which is why I asked.