The mass of the Higgs boson in the Standard Model from precision tests

The LEP data and the measurement of MW confirm the electroweak symmetry breaking by the Higgs mechanism with (2–3) % accuracy. For the Higgs boson mass, fits to the present data (without the SLD result) give Mh = 121−58+207 GeV and Mh < 800 GeV at 95% C.L. We analyze those fits from the point of view of the impact of different measurements on the final result for Mh. All the relevant observables can be grouped into two categories: those which correlate Mh with mt and those which constrain the range of mt. Using this classification we discuss potential improvements in the limits on Mh, anticipating improvements in the precision of the data. Also the sensitivity of the limits on Mh to the value of αs(MZ) is discussed in a toy model which reproduces all the results of the SM but the Z0 → bb width is increased by hand so that Rb = 0.22 and, in consequence, the fitted value of αs(MZ) ≈ 0.11 (qialitatively similar situation occurs in the Minimal Supersymmetric Standard Model with light chargino and right-handed top squark).