Phys. Rev. Lett. 110, 091801 (2013)

The standard model of particle physics comprises three 'generations': the known leptons and quarks fall naturally into groups of three particles of increasing mass. But might there be a fourth generation, another tier in the hierarchy at higher mass?

Eric Kuflik and colleagues have explored data from the ATLAS and CMS experiments at CERN, and from the former CDF experiment at Fermilab. They've checked the rates at which the Higgs — with a mass of 125 GeV — is produced in various channels: if there were a fourth generation, the rate of production of this Higgs particle by the fusion of gluons could be enhanced almost ten-fold, and, at the same time, the rate of its decay into photon pairs could be suppressed by as much as a factor of 100, compared with the basic standard-model prediction. Neither effect is seen, and Kuflik et al. strongly exclude the existence of a fourth generation, at 99.95% confidence level for a 125-GeV Higgs.