widths of the Z boson at CERN's Large Electron–Positron Collider (LEP). Nonetheless, searches at high-energy colliders for particles from a fourth generation continue, but as yet no evidence has been observed. In such searches, fourth-generation particles are denoted by the same symbols as third-generation ones with an added prime (e.g. b′ and t′). The lower bound for a fourth generation of quark (b′, t′) masses is currently at 1.4 TeV from experiments at the LHC. The lower bound for a fourth generation neutrino () mass is currently at about 60 GeV (millions of times larger than the upper bound for the other 3 neutrino masses). The lower bound for a fourth generation charged lepton () mass is currently 100GeV and proposed upper bound of 1.2 TeV from unitarity considerations. If the Koide formula continues to hold, the masses of the fourth generation charged lepton would be 44 GeV (ruled out) and b′ and t′ should be 3.6 TeV and 84 TeV respectively. (The maximum possible energy for protons in the LHC is about 6 TeV.) Origin The origin of multiple generations of fermions, and the particular count of 3, is an unsolved problem of physics. String theory provides a cause for multiple generations, but the particular number depends on the details of the compactification of the D-brane intersections. Additionally, E grand unified theories in 10 dimensions compactified on certain orbifolds down to 4‑D naturally contain 3 generations of matter. This includes many heterotic string theory models. See also Grand Unified Theory Koide formula Neutrino mass hierarchy References Quarks