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We have six leptons and six quarks. Yet most of what we see around us is made of only neutrons, protons and electrons. For $\mu^-$ and $\tau^-$, I think the reason is that these are unstable particles and quickly decays into $e^-$. But I have the following doubts.

  1. Why do neutrinos, $\nu_e,\nu_\mu$ and $\nu_\tau$ do not form ordinary matter?

  2. We do know about certain baryons and mesons containing $s$ quark in addition to $u$ and $d$. Are they all unstable? Why?

  3. Is any bound state containing $c,b$ or $t$ quark unstable?

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  1. They do! All three types of neutrinos are around us in great abundance and they are absolutely part of "ordinary matter" (as opposed for example to dark matter). The reason neutrinos do not combine with other particles to form something like atoms is because they do not have any electric charge.

  2. Yes. The standard reference website with the most up-to-date information is http://pdglive.lbl.gov/. You can look there at the lifetimes of the $\Lambda$ and $\Sigma$ baryons and check for yourself that they are very small.

  3. Yes.

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    $\begingroup$ Solid answer. To the OP's trailing question in 2., I'd point out that the mass difference of the s quark from that of the u is over 90MeV, so it can not be offset/fudged by nuclear binding energies to prevent the inevitable weak decay of one to the other. $\endgroup$ – Cosmas Zachos Jul 28 at 20:37
  • $\begingroup$ I would like to add to this correct answer that all the particles you list, in contrast to atoms and molecules , are only seen by the fit of mathematical formula's to measurements. These formulas are very successful in fitting data in the microcosm, and predicting new data. We treat the particles they hypothesize as we treat the ones we can find in chemical reactions. $\endgroup$ – anna v Jul 29 at 3:48
  • $\begingroup$ "because they do not have any electric charge." And also strong interactions? $\endgroup$ – mithusengupta123 Jul 29 at 13:46
  • $\begingroup$ @mithusengupta123 Yes, correct. They do not have strong interactions either, so they do not combine to form particles such as mesons and baryons either. $\endgroup$ – Heterotic Jul 29 at 17:43
  • $\begingroup$ @Heterotic, "All three types of neutrinos are around us in great abundance and they are absolutely part of "ordinary matter" (as opposed for example to dark matter)." If there are right-handed (sterile) neutrinos, they can have humongous Majorana masses. They are thus "clumpable" by sheer gravity instead of being relativistic/hot. Hence they are possible cold dark matter candidates. $\endgroup$ – MadMax Jul 29 at 18:33

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