Why don't pions explode?

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Offline DoctorBeaver

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Why don't pions explode?
« on: 18/07/2008 14:34:45 »
A pion consists of a quark and an antiquark. So why don't they annihilate each other?
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Offline LeeE

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Why don't pions explode?
« Reply #1 on: 18/07/2008 15:48:41 »
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.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline DoctorBeaver

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Why don't pions explode?
« Reply #2 on: 18/07/2008 19:29:15 »
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?
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Offline Soul Surfer

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Why don't pions explode?
« Reply #3 on: 18/07/2008 19:29:42 »
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
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Offline DoctorBeaver

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Why don't pions explode?
« Reply #4 on: 18/07/2008 19:31:29 »
Ian - that's not the same thing, is it? Surely, that's just decay.
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Offline Soul Surfer

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Why don't pions explode?
« Reply #5 on: 18/07/2008 19:40:08 »
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
« Last Edit: 18/07/2008 19:48:08 by Soul Surfer »
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Offline DoctorBeaver

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Why don't pions explode?
« Reply #6 on: 18/07/2008 19:55:54 »
Thank you, Ian. You're the canine's testes!
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Offline LeeE

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Why don't pions explode?
« Reply #7 on: 19/07/2008 12:42:28 »
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.
« Last Edit: 19/07/2008 12:44:54 by LeeE »
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Offline DoctorBeaver

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Why don't pions explode?
« Reply #8 on: 19/07/2008 12:53:59 »
So could an up annihilate with 2 anti-downs?
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Offline LeeE

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Why don't pions explode?
« Reply #9 on: 19/07/2008 13:52:10 »
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.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline DoctorBeaver

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Why don't pions explode?
« Reply #10 on: 19/07/2008 15:15:42 »
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?
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Offline Soul Surfer

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Why don't pions explode?
« Reply #11 on: 19/07/2008 23:22:47 »
Mesons have zero spin meson decay other than straight of compatible pairs anihilation is via the weak reaction see the reference I quoted earlier.
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Offline DoctorBeaver

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Why don't pions explode?
« Reply #12 on: 19/07/2008 23:59:02 »
Ian - I read the article you referred to. Im going to read it again, though, to try to understand it better.
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Offline chrisdsn

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Why don't pions explode?
« Reply #13 on: 21/07/2008 06:00:56 »
> 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).


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Offline DoctorBeaver

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Why don't pions explode?
« Reply #14 on: 21/07/2008 08:07:19 »
chrisdsn - thank you for your informed reply.

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

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Why don't pions explode?
« Reply #15 on: 23/07/2008 23:59:04 »
> 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.
 

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Offline DoctorBeaver

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Why don't pions explode?
« Reply #16 on: 24/07/2008 07:49:21 »
Thanks, chrisdsn; I think I understand that. Sort of. Ish. Kinda  [;D]

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