$b$ mesons lifetime 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?
 A: The answer is some kind of a compromise between the fact that the b-quark has a high mass, and the composition of the CKM matrix which governs weak decays of quarks.
As one can quickly notice from the CKM matrix, each quark has the largest probability (order of 1) to evolve into a quark of the same generation (diagonal elements of the matrix): d<->u, s<->c, b<->t. Probabilities of transitions to quarks of other generations are considerably lower: you need to take a given off-diagonal element, which is already small, below 0.25, and take that squared to get the probability of a given transition.
So far, I was talking about transitions, but what determines the lifetimes of hadrons is allowed decays. One can quickly notice that for the strange and bottom quarks, the decay to the quarks of the same generation are actually prohibited by the mass difference: a hadron cannot decay in something heavier than it! Therefore, b- and s-hadrons have no other choice than decay via CKM-suppressed off-diagonal channels.
In the meanwhile, for the charm quark, decays to strange quarks (CKM-friendly diagonal transitions) are allowed, which reduces their lifetime.
Now, adding the mass to the soup. The heavier the particle, the larger is its number of allowed decay channels, and so the small the lifetime. Or, it would be so if not the CKM effects I described just above.
As a consequence, what do we get:

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*strange hadrons (kaons, hyperons) live long, $O(10^{-10}-10^{-8})$ seconds, which in modern particle-physics experiments means having crossed about a meter without a decay. They also are low-mass, which means usually only 1-3 decay channels are allowed, making them easily identifiable (see for example "V0 particle" terminology).

*charm hadrons (D mesons, $\Lambda_c$ baryons, etc) have a lifetime of $O(10^{-13})$ seconds. Which means just a few mm of displacement in the detector. They would then decay to a bunch of strange particles + light hadrons (pions, protons), but their mass is too small to be more imaginative than that. One can indeed do c-tagging at modern experiments based on these features.

*beauty (if you prefer, bottom) hadrons (B mesons, $\Lambda_b$ baryons, etc): although being heavier than their charm colleagues, they actually live longer! This is a consequence of the CKM structure (and, for picky ones, also some QCD effects). This means a typical lifetime of 1 picosecond, so a displacement of ~1cm in a modern detectors. That is exactly the reason why all particle physics experiments want precise vertex detectors: you need to get close enough to the collision point in order to measure this 1 cm displacement and be a happy owner of a beauty hadron.  In addition, B hadrons are heavy, which means hundreds of allowed decay chains. You will get mad trying to reconstruct each one separately, so at high energies they prefer to just treat them as jets. If these jets originate from a point which is not a collision vertex, but rather displaced by about 1 cm from it – congratulations, you are doing a b-tagging (in its most simplistic form)! Once you remember that b-hadrons often decay into something + c-hadrons (so, a short displacement after a longer displacement), you enhance your b-tagging power.

So, a short and over-simplified summary:

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*if you have a long-lived particle which flies a long distance before decaying into 2-3 other particles, this is a strange hadron.

*if you have a particle which flies just a tiny bit (some detectors are probably not precise enough to see this displacement from the collision vertex) and then decays to a "small" jet of strange and light stuff – that's likely a charm.

*if you got a particle which flies a few centimeters (but not meters and not micrometers) and then decays to a "fatty" jet with many final-state particles, that's likely a b-hadron.

All of this is of course true only for weakly-decaying particles. And keep in mind that particle decays are probabilistic, you will always find some particles which decayed in the first picosecond regardless of its mean lifetime.
A: 
The reason is that b mesons are long lived

That is not the reason b-taggin is interesting and is used.  There is also c tagging,and the lifetimes are similarly short, notlong.

Heavy-flavourjet identification exploits the properties of the hadrons originated in the jet to discriminate heavy flavour(b-,c-) initiated jets from those arising from light partons

From wikipedia

b-tagging is important because:



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*The physics of bottom quarks is quite interesting; in particular, it sheds light on CP violation.





*Some important high-mass particles (both recently discovered and hypothetical) decay into bottom quarks. Top quarks very nearly always do so, and the Higgs boson is expected to decay into bottom quarks more than any other particle given its mass has been observed to be about 125 GeV. Identifying bottom quarks helps to identify the decays of these particles.


