How Does Antimatter React with Matter and What Influences Particle Stability?

[jonatron5]

Well my question is two fold. Everyone knows that Antimatter anihlates matter on contact. Why? What causes this?

Also as elements get heavier and heavier they are less and less stable.

Would Antimatter become increasingly stable as you added more antiparticles?

 

[DaveC426913]

https://en.wikipedia.org/wiki/Annihilation

Would Antimatter become increasingly stable as you added more antiparticles?

No. Antimatter behaves pretty much exactly like matter, just opposite charges. It behaves the same as matter when interacting with gravity, EMR, etc.

 

[jtbell]

Antimatter in isolation is just as stable (or unstable) as matter in isolation. A positron sitting all by itself will last forever, just like an electron sitting all by itself. An anti-muon sitting all by itself will eventually decay, with a half-life of 2.2 microseconds, same as a muon sitting all by itself.

 

[ChrisVer]

Antimatter in isolation is just as stable (or unstable) as matter in isolation.

Well that is approximately true - there is CP violation... and once upon a period of our universe, it had to be even larger.

 

[mfb]

Well that is approximately true - there is CP violation

It is true for every known particle where an assignment as "matter" or "antimatter" is meaningful, this is guaranteed by CPT symmetry. CP violation allows that some mesons decay faster than their antiparticles, but which one is matter and which one is antimatter?

 

[ChrisVer]

It is true for every known particle where an assignment as "matter" or "antimatter" is meaningful, this is guaranteed by CPT symmetry. CP violation allows that some mesons decay faster than their antiparticles, but which one is matter and which one is antimatter?

Well what about neutrinos?
And in fact when I wrote it I was thinking 1 of Sakharov's condition for the matter-antimatter asymmetry.

 

[mfb]

Neutrinos have no known decay mode.

 

[vanhees71]

It is true for every known particle where an assignment as "matter" or "antimatter" is meaningful, this is guaranteed by CPT symmetry. CP violation allows that some mesons decay faster than their antiparticles, but which one is matter and which one is antimatter?

That's a question of convention. You start with the elementary particles in the standard model. You have quarks and leptons and their antiparticles (concerning the neutrinos it's not yet clear whether they are their own antiparticles or not; in the usual standard model they are not, being represented by Dirac fields).

Now mesons in the parton model are bound states of quarks and antiquarks. So it's not a priori clear how to label them as particle or antiparticle. Here's a nice table, where you can look it up:

https://de.wikipedia.org/wiki/Liste_der_Mesonen#Anmerkungen_zu_den_neutralen_Kaonen

An explanation for the systematics of this convention, see the article on mesons:

https://de.wikipedia.org/wiki/Meson

 

[ChrisVer]

@mfb they don't, but they can cause a particle to decay...
An example is the \mu \rightarrow e \gamma.
If the CP-V phase in the PMNS matrix is non-zero, then there would be a difference in such a decay mode between \mu^+, \mu^-...or so I guess.

 

[mfb]

Now mesons in the parton model are bound states of quarks and antiquarks. So it's not a priori clear how to label them as particle or antiparticle.

That's why I excluded particles where this assignment is not meaningful. Are $B_0$ and $\bar B_0$ matter/antimatter pairs? Their CP eigenstates? Their mass eigenstates? If yes, which one is matter and which one is antimatter? How do we even define the lifetime for particles that are not eigenstates of their decay interaction? That just doesn't make sense.

@mfb they don't, but they can cause a particle to decay...
An example is the \mu \rightarrow e \gamma.
If the CP-V phase in the PMNS matrix is non-zero, then there would be a difference in such a decay mode between \mu^+, \mu^-...or so I guess.

The muon and antimuon lifetime should be exactly the same, and I don't see how neutrino mixing would be relevant here.

 

[ChrisVer]

The muon and antimuon lifetime should be exactly the same, and I don't see how neutrino mixing would be relevant here.

Because
\mu^+ \rightarrow W \nu_\mu \rightarrow W \nu_e \rightarrow \gamma e^+
vs
\mu^- \rightarrow W \bar{\nu}_\mu \rightarrow W \bar{\nu}_e \rightarrow \gamma e^-
Shouldn't be the same if there is CPV for the neutrinos (i.e. \bar{\nu} behave differently than \nu) ?

 

[mfb]

I still think CPT covers that. Even if not, it would be at most a 10^-50 effect.

 

[vanhees71]

That's why I excluded particles where this assignment is not meaningful. Are $B_0$ and $\bar B_0$ matter/antimatter pairs? Their CP eigenstates? Their mass eigenstates? If yes, which one is matter and which one is antimatter? How do we even define the lifetime for particles that are not eigenstates of their decay interaction? That just doesn't make sense.
The muon and antimuon lifetime should be exactly the same, and I don't see how neutrino mixing would be relevant here.

The $B_0$ and $\overline{B_0}$ are charge eigenstates and thus one is the charge-conjugated eigenstate of the other. They are not weak-isospin eigenstates and thus mix (CP violation), i.e. For the neutral kaons you even name the flavor eigenstates $K_s$ and $K_l$ (K short and K long) due to their very different life times. Originally they were thought as two different particles called $\theta$ and $\tau$ (not to be mixed up with the modern $\tau$, which is a charged lepton rather than a meson).