Fourth Generation Lepton Searches

The current mass limits from LEP are 80.5 GeV (90.3 GeV) for a Majorana (Dirac) ν′ decaying to τW and 101.9 GeV (or 100.8 GeV) for τ′ decaying to ν′W (or νW). We shall be able to use LHC data to exclude a range of mass combinations above these limits. This is in lieu of a dedicated search for the simplest leptonic extension of the standard model, which surprisingly still remains to be done. An even simpler though less motivated search is possible if one assumes that ν′ → W ℓ (ℓ = µ, e) is the dominant decay. . . . a τ′ mass up to at least 250 GeV would be excluded very early with 1 fb⁻¹ of data. We estimate that the corresponding limit with present data would be at least 600 GeV. Here again no dedicated search has been reported.

From here.

Three observed excesses in the signal region of multi-lepton searches reported to date at the LHC prevent much more strict limits on fourth generation charged lepton masses and fourth generation neutrinos from being revised in the linked paper. This probably reflects unduly broad event selection criteria for published multi-lepton event searches designed for other purposes such as the search for a Higgs boson, rather dedicated searches for fourth generation leptons which the criteria could be be rigorous. It probably does not reflect a sub-discovery threshold signal of fourth generation leptons, although this can't be ruled out from available published data.

Of course, it would also be very exciting if all or any of the three observed excesses were a signal of beyond the Standard Model (BSM) physics.

The benefits in terms of ruling out BSM theories or BSM parameter spaces of a dedicated fourth generation lepton event search at the LHC that could increase exclusions from 80.5 GeV for a fourth generation Majorana neutrino, 90.3 GeV for the fourth generation Dirac neutrino, and 100.8-101.9 GeV for a fourth generation charged lepton set in experiments at the LEP which ceased operations in the year 2000, to about 600 GeV for each of these form the LHC, would be substantial. Few BSM experimental limits are so stale.

An increased mass limitation on a fourth generation "fertile" neutrino (aka active neutrino) to about 600 GeV would be particular relevant as it would significantly impact the parameter space of dark matter theories and other aspects of neutrino physics. The heaviest of the three current fertile neutrino species is realistically probably not more than about 0.1 eV, so it would be very surprising given the absence of a fertile neutrino species between 0.1 eV and 80.5 GeV to find a fertile neutrino species between 80.5 GeV and 600 GeV. But, the extra experimental confirmation would be nice to have anyway.

The searches may not have been done already, mostly because a variety of factors already strongly disfavor the SM4 extension of the Standard Model that simply adds a fourth generation of Standard Model fermions to the existing three.