Why Are B Mesons Long-Lived and How Does B-Tagging Work?

[Malamala]

Hello! I read about a technique called b-tagging in which one can check if a b meson was produced in a collision by looking for a jet which has a secondary vertex within it. The reason is that b mesons are long lived, so that meson would travel a bit before decaying, hence the secondary vertex. Why are b meson long lived? And why can't we do this with other kind of quarks, like strange or charm? Thank you!

 

[snorkack]

Beautiful quark is noted for having a combination of long lifetime and large decay energy.
Strange quark is even longer lived but has a small decay energy.
Charmed quark has larger decay energy than strange, but shorter lifetime than beauty.
Strangeness:
lowest mesons are charged K, lifetime 12380 ps, energy 493 MeV, and long neutral K, lifetime 51660 ps, energy 497 MeV, lowest hyperon Λ, lifetime 263 ps, energy 1115 MeV
Charm:
lowest meson charged D, lifetime 1,04 ps, energy 1869 MeV, lowest hyperon charmed Λ, lifetime 0,2 ps, energy 2286 MeV
Beauty:
lowest meson charged B, lifetime 1,64 ps, energy, 5279 MeV, lowest hyperon beautiful Λ, lifetime 1,41 ps, energy 5620 MeV.

 

[Malamala]

Beautiful quark is noted for having a combination of long lifetime and large decay energy.
Strange quark is even longer lived but has a small decay energy.
Charmed quark has larger decay energy than strange, but shorter lifetime than beauty.
Strangeness:
lowest mesons are charged K, lifetime 12380 ps, energy 493 MeV, and long neutral K, lifetime 51660 ps, energy 497 MeV, lowest hyperon Λ, lifetime 263 ps, energy 1115 MeV
Charm:
lowest meson charged D, lifetime 1,04 ps, energy 1869 MeV, lowest hyperon charmed Λ, lifetime 0,2 ps, energy 2286 MeV
Beauty:
lowest meson charged B, lifetime 1,64 ps, energy, 5279 MeV, lowest hyperon beautiful Λ, lifetime 1,41 ps, energy 5620 MeV.

Thank you for the reply! My confusion is, why there is a 4 order of magnitude difference between the b mesons lifetime and the charm mesons lifetime. They both decay by strong interaction and I would imagine a more massive particle to decay faster

 

[Vanadium 50]

why there is a 4 order of magnitude difference between the b mesons lifetime and the charm mesons lifetime.

No there isn't. They are comparable.

They both decay by strong interaction

No they don't. They decay by the weak interaction.

 

[mfb]

B-tagging is more general, it's including all methods to detect that there was a B meson (and often it's also detecting the type, i.e. bottom or anti-bottom quark).
Collisions often produce pairs of b quarks which then fly away individually. Isolated b quarks can only decay via the weak interaction, that gives the lightest B hadrons a relatively long lifetime.

You can do the same with charm. It can decay within its generation (to strange), so the lifetime is a bit shorter but still comparable.
Particles with a strange quark live much longer, that leads to different approaches to study them.

 

[Malamala]

No there isn't. They are comparable.
No they don't. They decay by the weak interaction.

Sorry I meant why are they 4 orders of magnitude between strange and bottom mesons lifetime? Also, if charm and bottom have the same lifetime, why is there no such a thing as charm tagging, too? Thank you!

 

[Malamala]

B-tagging is more general, it's including all methods to detect that there was a B meson (and often it's also detecting the type, i.e. bottom or anti-bottom quark).
Collisions often produce pairs of b quarks which then fly away individually. Isolated b quarks can only decay via the weak interaction, that gives the lightest B hadrons a relatively long lifetime.

You can do the same with charm. It can decay within its generation (to strange), so the lifetime is a bit shorter but still comparable.
Particles with a strange quark live much longer, that leads to different approaches to study them.

Thanks a lot for this! I haven't actually heard of charm tagging anywhere before. Is that really done?

 

[Vanadium 50]

I meant why are they 4 orders of magnitude between strange and bottom mesons lifetime

Do you understand the lifetime calculation? The rate goes as Q⁵ which is much larger for b than s. So much larger that other factors have to come into play to keep the lifetime only four orders of magnitude less.

If you don't understand the lifetime calculation, I don't see how I can say much beyond "that's what comes out of the calculation".

I haven't actually heard of charm tagging anywhere before. Is that really done?

Sure. It doesn't work well since you need to reject light quarks and b-quarks so are squeezed at both ends. Also, charm decays have fewer charged particles than b decays, so your tagger has less to work with.

 

[mfb]

Random charm tagging submission on arXiv
For electron-proton collisions (in particular: Belle II) people prefer full event interpretation now: Try to understand the full collision process and associate every particle to your event. Doesn't always work, but where it does you know clearly what happened.