Higgs Searches at Lep and at the Tevatron

After years of efforts to push the LEP performance to, and indeed beyond, the limits of what had been believed possible, hints of a signal of a Higgs boson at 115GeV/c appeared in June 2000, were confirmed in September, and confirmed again in November. Spending an additional six-month period with LEP would have given the unambiguous opportunity of a fundamental discovery. Instead, this possibility was handed over to the Tevatron, for which at least six more years will be needed to confirm the existence of a Higgs boson around 115GeV/c. The upgrades performed at LEP and needed at the Tevatron, together with the physics outcomes, are briefly mentioned in turn. 1 LEP (1989-2000): “2001, A Spoilt Odyssey” As described in P. Renton’s presentation 1), the Luminosity, the Energy and the Precision (L,E,P) of the measurements made at LEP and SLD (available at the time of the conference) allowed an indirect prediction of the Higgs boson mass to be made in the framework of the standard model, mH = 118 +63 −42 GeV/c , (1) as obtained with the as-yet most precise determination of the QED coupling constant evaluated at the Z mass. 2) The prediction of such a light Higgs boson emphasized the interest of the direct search at LEP. All searches carried out during the first phase of LEP through the Higgstrahlung process ee → Hf̄f were unsuccessful, and led to a lower limit of 65.6GeV/c on the standard model Higgs boson mass at 95% C.L. 3) It was time, in 1995, to go to the second phase of LEP. As shown in Fig. 1, a centreof-mass energy of 192GeV (which was foreseen to be reached with the available equipment) allowed a 5σ-sensitivity of 100GeV/c to be achieved on the standard model Higgs boson mass, through the search for the process ee → HZ. Similarly, a centre-of-mass energy of 209GeV (actually reached in 2000) increased this sensitivity to 115GeV/c.