3 ms·

> This subatomic particle is normally made up of a charm quark and an up antiquark, while its antimatter equivalent consists of a charm antiquark and an up quar

by mdoms 5y ago

> This subatomic particle is normally made up of a charm quark and an up antiquark, while its antimatter equivalent consists of a charm antiquark and an up quark. Q: if these subatomic particles consist of a quark and an anti-quark, why don't they self-annihilate?

mkaic 5y ago
as someone who isn't well versed in particle physics i'm also wondering this. can anyone with more experience weigh in?
dukwon 5y ago
Mesons have 'annihilation' decay modes, certainly, but they can also decay in other ways.
ganzuul 5y ago
Charm, Up and Top are all different. They share some properties but not all and therefore annihilation can not take place.
- dukwon 5y ago
  Sure it can, but it has to proceed via the weak interaction, and it's suppressed by the GIM mechanism. For example, D⁰→γγ (c̅u or cu̅ annihilation) is rare and not yet observed, but it's allowed.
  - deleted 5y ago
    [deleted]
gus_massa 5y ago
There are six quarks: down, up, strange, charm, bottom, top. There are six anti-quarks: down, up, strange, charm, bottom, top. Using the strong force, a quark can annihilate only an antiquark of the same type, so the number of charm quark minus the number of charm antiquarks is "almost" conserved. If you start with a system with one charm quark like the particle here, you "allays" will have a system with one charm quark, or 2 charm quarks and on charm antiquark, or ... but "never" a system with no charm quarks like a few photons after annihilation. It's more complicated, because the weak force can annihilate one quark with a different antiquark. (The rules are somewhat complicated, and some annihilations are more usual than others.) So after some time, you can be lucky and a weak interaction can destroy your charm quark and a up antiquark, so the number of charm quark minus the number of charm antiquarks changes "slowly". Here "slowly" means that to see an effect of the weak interaction you must wait like 10E-12 seconds instead of the 10E-24 seconds you must wait until you see an effect of the strong interaction. The numbers change for each decay and are difficult to calculate and may be much longer. So everything in scare quotes means it's just an aproximation. For example the neutron->proton+electron+antineutrino decay use the weak force and you must wait like 10 minutes that is a lot for an unstable particle. The particle discussed in this article has a life of almost a millisecond, that is very long for a particle but short for us. (Electromagnetism can annihilate only an antiquark of the same type, so it's not important here and it's much fainter.) --- On the other hand, you are right. This article is about the decay of a particle D0 into a Ks0 and pions. After some time (probably a few milliseconds after the experiment finalized) the Ks0 decays in more pions. And the new pions and the other pions after some time decay into photons and muons and neutrinos, and after some time the muons decay into electrons and neutrinos. The number of electrons and antielectrons (positrons) are equal, so they annihilate each other and you get more photons. Also, the number of neutrinos is equal to the number of antineutrinos, so they theoretically can annihilate each other, but in practice they just escape. So the global reaction is an annhihilation: real: D0 ---long-story---> photons + neutrinos + antineutrinos optimistic theoretical possibility: D0 ---longer-difficult-story---> photons

by mdoms 5y ago

mkaic 5y ago

as someone who isn't well versed in particle physics i'm also wondering this. can anyone with more experience weigh in?

dukwon 5y ago

Mesons have 'annihilation' decay modes, certainly, but they can also decay in other ways.

ganzuul 5y ago

Charm, Up and Top are all different. They share some properties but not all and therefore annihilation can not take place.

dukwon 5y ago
Sure it can, but it has to proceed via the weak interaction, and it's suppressed by the GIM mechanism. For example, D⁰→γγ (c̅u or cu̅ annihilation) is rare and not yet observed, but it's allowed.
- deleted 5y ago
  [deleted]

gus_massa 5y ago

There are six quarks: down, up, strange, charm, bottom, top. There are six anti-quarks: down, up, strange, charm, bottom, top. Using the strong force, a quark can annihilate only an antiquark of the same type, so the number of charm quark minus the number of charm antiquarks is "almost" conserved. If you start with a system with one charm quark like the particle here, you "allays" will have a system with one charm quark, or 2 charm quarks and on charm antiquark, or ... but "never" a system with no charm quarks like a few photons after annihilation. It's more complicated, because the weak force can annihilate one quark with a different antiquark. (The rules are somewhat complicated, and some annihilations are more usual than others.) So after some time, you can be lucky and a weak interaction can destroy your charm quark and a up antiquark, so the number of charm quark minus the number of charm antiquarks changes "slowly". Here "slowly" means that to see an effect of the weak interaction you must wait like 10E-12 seconds instead of the 10E-24 seconds you must wait until you see an effect of the strong interaction. The numbers change for each decay and are difficult to calculate and may be much longer. So everything in scare quotes means it's just an aproximation. For example the neutron->proton+electron+antineutrino decay use the weak force and you must wait like 10 minutes that is a lot for an unstable particle. The particle discussed in this article has a life of almost a millisecond, that is very long for a particle but short for us. (Electromagnetism can annihilate only an antiquark of the same type, so it's not important here and it's much fainter.) --- On the other hand, you are right. This article is about the decay of a particle D0 into a Ks0 and pions. After some time (probably a few milliseconds after the experiment finalized) the Ks0 decays in more pions. And the new pions and the other pions after some time decay into photons and muons and neutrinos, and after some time the muons decay into electrons and neutrinos. The number of electrons and antielectrons (positrons) are equal, so they annihilate each other and you get more photons. Also, the number of neutrinos is equal to the number of antineutrinos, so they theoretically can annihilate each other, but in practice they just escape. So the global reaction is an annhihilation: real: D0 ---long-story---> photons + neutrinos + antineutrinos optimistic theoretical possibility: D0 ---longer-difficult-story---> photons