LEP shut down at the end of 2000 with a Higgs lower-mass limit of 114.4 GeV, after an unconfirmed ALEPH excess near 115 GeV — and the Higgs, discovered in 2012 near 125 GeV, was never reachable by LEP

physicshistory-of-sciencehiggs-bosonLEPCERNALEPHcollider-history

In its final run, CERN's Large Electron-Positron collider (LEP) was pushed to a centre-of-mass energy of 206 GeV, at which the ALEPH Collaboration observed a handful of Higgs-like events corresponding to a mass around 115 GeV. The other three LEP experiments — DELPHI, L3, and OPAL — did not confirm ALEPH's excess, and the collider could not be pushed to higher energy. CERN management decided to shut LEP down at the end of 2000, publishing a lower limit of mH > 114.4 GeV at 95% confidence.

John Ellis explicitly addresses — and rejects — the speculation that this shutdown decision caused LEP to miss the Higgs discovery: the particle was ultimately found at the LHC in 2012 near 125 GeV, a mass LEP could not have reached without major extra investment in accelerating cavities that would have risked jeopardizing the subsequent LHC programme. The counterfactual ("LEP missed it") and the actual mass gap (125 GeV vs. a maximum reach well below that) are both load-bearing parts of the same finding — the shutdown was not the missed opportunity it is sometimes framed as, because per Ellis reaching the real particle's mass would have required that substantial cavity investment to have been made back in the 1990s, a commitment there had been concern would jeopardise the LHC programme — the year-2000 decision itself was not what put 125 GeV out of reach.