Search for Yukawa Production of a Light Neutral Higgs Boson at LEP

TL;DR

This paper reports a search for a light neutral Higgs boson in the 4-12 GeV range at LEP, setting upper limits on production cross-sections within a 2HDM framework, and constraining related theoretical models.

Contribution

First search for light Higgs bosons in the specified mass range at LEP within a 2HDM, providing new upper limits on Yukawa production cross-sections and constraining related models.

Findings

No significant excess observed over background.

Set 95% CL upper limits on Yukawa production parameters.

Constraints imposed on models explaining muon g-2 anomaly.

Abstract

Within a Two-Higgs-Doublet Model (2HDM) a search for a light Higgs boson in the mass range of 4-12 GeV has been performed in the Yukawa process e+e- -> b bbar A/h -> b bbar tau+tau-, using the data collected by the OPAL detector at LEP between 1992 and 1995 in e+e- collisions at about 91 GeV centre-of-mass energy. A likelihood selection is applied to separate background and signal. The number of observed events is in good agreement with the expected background. Within a CP-conserving 2HDM type II model the cross-section for Yukawa production depends on xiAd = |tan beta| and xihd = |sin alpha/cos beta| for the production of the CP-odd A and the CP-even h, respectively, where tan beta is the ratio of the vacuum expectation values of the Higgs doublets and alpha is the mixing angle between the neutral CP-even Higgs bosons. From our data 95% C.L. upper limits are derived for xiAd within the range of 8.5 to 13.6 and for xihd between 8.2 to 13.7, depending on the mass of the Higgs boson, assuming a branching fraction into tau+tau- of 100%. An interpretation of the limits within a 2HDM type II model with Standard Model particle content is given. These results impose constraints on several models that have been proposed to explain the recent BNL measurement of the muon anomalous magnetic moment.