# I Does a particle and its anti-particle always annihilate?

#### Frank Castle

When a particle and its corresponding anti-particle interact do they always annihilate or are there other possible interactions that can occur, such as them scattering off of one another?

If the former is true, why do they always annihilate? If the latter is true, is it the case that the most probable interaction is that they annihilate (hence why in pop-sci it is always said that matter and anti-matter annihilate if they come into contact)?

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#### Arman777

Gold Member
When a particle and its corresponding anti-particle interact do they always annihilate or are there other possible interactions that can occur, such as them scattering off of one another?

If the former is true, why do they always annihilate? If the latter is true, is it the case that the most probable interaction is that they annihilate (hence why in pop-sci it is always said that matter and anti-matter annihilate if they come into contact)?
Well I think yes they should annihilate.
When they interect, energy,charge and angular momentum should conserved.Thats why I think the only possible solution is annihilation.

#### Frank Castle

When they interect, energy,charge and angular momentum should conserved.Thats why I think the only possible solution is annihilation.
These quantities can also be conserved if they don't annihilate though, for example, if they simply scatter off one another.

#### mfb

Mentor
Scattering is possible, and it is the most likely outcome in most cases. Various other reactions are also possible if there is enough energy (e. g. proton plus antiproton -> hyperon plus kaon plus antiproton - changing the proton but not the antiproton).

If you bring macroscopic amounts of antimatter into contact with macroscopic amounts of matter, the scattering is irrelevant - the particles will annihilate and make a big explosion. If you shoot antimatter particles into matter, scattering is important to follow how the particle loses energy - but in the end the antimatter will annihilate and energy is released.

#### Frank Castle

If you bring macroscopic amounts of antimatter into contact with macroscopic amounts of matter, the scattering is irrelevant - the particles will annihilate and make a big explosion. If you shoot antimatter particles into matter, scattering is important to follow how the particle loses energy - but in the end the antimatter will annihilate and energy is released.
Why is this the case? Is it because on a macroscopic time-scale the probability that matter and antimatter (that initially scattered) to annihilate is large enough that it dominates over any probability for scattering?

#### mfb

Mentor
Scattering process do happen, but you don't have to know about them to understand the outcome. It doesn't matter if the particles scatter x times before they annihilate.

#### Frank Castle

Scattering process do happen, but you don't have to know about them to understand the outcome. It doesn't matter if the particles scatter x times before they annihilate.
But why is it that macroscopically the matter and antimatter will completely annihilate? Couldn't there be small amounts of each left over that have scattered and then propagated away freely? Or is it that when there are macroscopic amounts of matter and antimatter in contact, particle number is so dense that it is impossible for any of the matter and antimatter not to be involved in a collision in which they annihilate?!

#### mfb

Mentor
On Earth, matter is everywhere. Every antimatter particle you have on Earth will hit a matter particle quickly unless you keep the antimatter levitated in an extremely good vacuum.

#### Frank Castle

On Earth, matter is everywhere. Every antimatter particle you have on Earth will hit a matter particle quickly unless you keep the antimatter levitated in an extremely good vacuum.
So is my point in the previous post correct then? Even if there were only one antimatter particle, as there is so much matter (on Earth), although the antimatter particle my initially scatter with a few matter particles (of the same species) it will very quickly be involved in an interaction in which it annihilates with the corresponding matter particle.

Mentor
Sure.

#### Frank Castle

Ok cool, thanks for your help.

I'm assuming this is why it is so difficult to even maintain a single antimatter particle in a laboratory on Earth, since although it may initially scatter it will very quickly be involved in an interaction in which it is annihilated.

#### Orodruin

Staff Emeritus
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Note that, as mfb said, the typical antimatter particle will scatter a lot before annihilating. In fact, a positron will typically lose all its kinetic energy before annihilating. This is why you get two 511 keV photons from the annihilation. If the positron did not stop first, the photons would have a higher total energy (and generally the photon energies would be different).

#### Frank Castle

Note that, as mfb said, the typical antimatter particle will scatter a lot before annihilating. In fact, a positron will typically lose all its kinetic energy before annihilating. This is why you get two 511 keV photons from the annihilation. If the positron did not stop first, the photons would have a higher total energy (and generally the photon energies would be different).
So is the point that a typical antimatter particle will scatter, but if there is a macroscopic amount of matter present in the region that it is propagating then it will very quickly annihilate?

#### Orodruin

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So is the point that a typical antimatter particle will scatter, but if there is a macroscopic amount of matter present in the region that it is propagating then it will very quickly annihilate?
No, it will eventually annihilate after scattering multiple times and losing most of its kinetic energy. On a human time scale, this eventually is a rather short time but the scatter time scale is even shorter.

#### Frank Castle

No, it will eventually annihilate after scattering multiple times and losing most of its kinetic energy. On a human time scale, this eventually is a rather short time but the scatter time scale is even shorter.
Ah ok. So when it is said that matter and antimatter annihilate one another this happens over some finite time scale with many scattering events before complete annihilation, although (as you said) this will be a relatively short amount of time on human timescales.

#### mfb

Mentor
Right.
"Short amount of time" is here something shorter than a nanosecond.

#### snorkack

What can a neutrino and an antineutrino annihilate to, even if they meet each other?

#### Frank Castle

Short amount of time" is here something shorter than a nanosecond.
Is something that one can compute in principle?

#### nikkkom

When a particle and its corresponding anti-particle interact do they always annihilate
"Annihilate" is actually not a precise scientific term. They simply can react, and for heavy particles there are many possible end states, not at all limited to gammas. There could be pions, kaons, muons, electrons and positrons, etc.
Although in practice, daughter particles would themselves quickly decay or react with more matter, and almost all energy _eventually_ is converted to light and heat.

#### mfb

Mentor
What can a neutrino and an antineutrino annihilate to, even if they meet each other?
Two other neutrinos (similar to scattering then)
Two photons, although that is an extremely unlikely process.
Other particle pairs, if their energy is sufficient
Single Z or Higgs, if their energy is sufficient
Is something that one can compute in principle?
Sure, it is routinely done in simulating particle physics experiments.

#### Frank Castle

Sure, it is routinely done in simulating particle physics experiments.
Is it simply the inverse of the decay rate for the process $e^{+}e^{-}\rightarrow\gamma\gamma$?

#### mfb

Mentor
The scattering processes? No.

#### Frank Castle

The scattering processes? No.
No, I was meaning calculating the lifetime of an electron positron pair before annihilating.

#### mfb

Mentor
If positronium forms, that is the inverse of the decay rate $e^{+}e^{-}\rightarrow\gamma\gamma$, sure. Otherwise we have to consider all possible processes with electrons bound to atoms.

"Does a particle and its anti-particle always annihilate?"

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