QCD Still Works

An excited state of a baryon (three quark composite particle), in this case made of a bottom quark, a strange quark and an up quark, which has long been predicted by quantum chromodynamics but never previously observed, has been detected at the large hadron collider (LHC).

It has a mass of about 6 GeV compared to about 1 GeV for a proton. In a proton, the vast majority of the composite particle's mass is in the gluons that bind the up and down quarks of a proton together. In this excited Xi b baryon, about three quarters of the mass in in the quarks (mostly the bottom quark), and the mass of the gluons for this excited state is about 50% more than in a proton. While it is heavy compared to a proton, it is much lighter than a W boson (roughly 80 GeV), a Z boson (roughly 90 GeV), a Higgs boson (roughly 125 GeV), or a top quark (about 173 GeV).

But, since top quarks have never been observed to become part of composite particles called hadrons (i.e. mesons with two quarks, or baryons with three quarks) before decaying into other particles (99.8% of the time into bottom quarks, 0.17% of the time into strange quarks and 0.07% of the time into down quarks), particles such as the Xi b that include bottom quarks are the heaviest composite particles, with the heaviest hadron topping out at about a sixth of the mass of a Z boson, give or take.

It is really a bit odd. There are four fundamental particles that are heavier than any of the first order composite particles (obviously, protons and neutrons combined to form atomic nuclei that are much heavier than the hadrons that are their component parts, with the atomic nuclei of many isotypes of the heaviest periodic table elements, and many molecules, being heavier than a top quark).