Searches for Charged Higgs Bosons at LEP

The four LEP experiments Aleph, Delphi, L3 and Opal updated their searches for pair production of charged Higgs bosons using more than 210/pb luminosity collected per experiment in the year 2000. Combining it with previously collected data, a significant deviation from background (equivalent to 4.4 sigma) is found by the L3 collaboration for low values of the branching ratio H+/- ->tau nu around masses of about 68 GeV. This excess is however not seen by the other LEP collaborations and thus a lower limit on the charged Higgs mass is set at 78.5 GeV at 95% confidence level. All results reported here are still preliminary.


Theoretical Framework
In the Standard Model 1 , the particle masses are generated through the coupling to the Higgs field. The Standard Model is minimal in the sense that it contains only one Higgs doublet which corresponds to four degrees of freedom. Three out of these area 'eaten up' by the longitudinal polarization states of the W and Z bosons, the remaining one is the physical Higgs field. However, there is no a priori reason why there should only be one such doublet. Two Higgs Doublet Models (2HDMs) 2 have eight degrees of freedom coming from the Higgs sector. Five of these correspond to the Higgs particles out of which two are the charged Higgs bosons H + and H − . Their masses are predicted to be equal.
The charged Higgs sector of 2HDMs can be parameterized by the mass of the charged Higgs boson m(H ± ) and tan β which is the ratio of the vacuum expectation values of the two doublets.
Two types of 2HDMs are distinguished: The absence of flavor changing neutral currents puts a lower limit on the charged Higgs mass in e.g. type II models without further particle content 3 , while type I models are much less restricted by experimental data. For low values of tan β, the dominant decay modes are H + → cs and τ + ν τ (and charge conjugates) while for high values values of tan β the three body decay H ± → W ± * A dominates. Of the LEP experiments, only Opal has performed a search for three body decays of charged Higgs bosons 4 . There is a good agreement between the observed data and the expected background.
This document reports on the searches for pair production of H + and H − in e + e − collisions at LEP. The dominant process is e + e − → Z * /γ * → H + H − .

Experimental signatures and event selection
The branching ratio BR(H ± → τ ν) and the charged Higgs mass are not predicted by theory and therefore left as parameters of the search. It is assumed that cs and τ ν τ are only decay modes of the charged Higgs boson, leading to three different final states: τ + ν τ τ −ν τ (leptonic channel), csτ −ν τ a (semileptonic channel) and cscs (hadronic channel). Leptonic Channel: The leptonic channel is characterized by two acoplanar taus (electrons, muons or low multiplicity jets) and missing energy. No mass reconstruction is attempted since at least four neutrinos are present in the final state. The most important backgrounds are e + e − → W + W − → ℓ + νℓ −ν , γγ collisions and e + e − → τ τ (γ) where the γ escapes detection.
For example, Delphi uses a likelihood method to discriminate between signal and background based on τ polarization variables and the polar angle of the τ decay products 5 . The distribution of the likelihood variable is shown in Figure 1. No significant excess is observed.
Semileptonic Channel: In this channel, high multiplicity events consistent with two jets plus an additional electron, muon or low multiplicity jet are selected. The events also must have a high amount of missing energy. The background is dominated by the process e + e − → W + W − → qqτ ν. As an example, Aleph uses linear discriminants in the analysis based on variables like the τ decay angle in the Higgs rest frame, τ polarization probability, Higgs production angle etc 6 . An example of the mass distribution after a cut on the discriminant is shown in Figure 2a. Data favor the background-only hypothesis.
Hadronic Channel: The signature of this channel is a four jet structure and no missing energy. The most important backgrounds are W pair and qq production. For example, L3 uses in their four jet analysis (among other cuts) a neural network to discriminate qq events 7 . The jets are grouped into pairs such that the probability of the pairs having equal mass is maximized. A cut on the Higgs production polar angle is applied to reduce the W + W − background. The Higgs candidate mass distribution is shown in Figure 2b. A significant excess is observed at m(H ± ) ≈ 68 GeV. Table 1 summarizes the observed and expected limits for fully hadronic and fully leptonic branching ratio as well as the observed limits independent of the branching ratio for the individual a The charge conjugate reaction is implied throughout this letter    Figure 3 shows the excluded region in the m(H ± ) vs. BR(H ± → τ ν) plane for the combination of the four LEP experiments 8 .

Summary
Searches for pair production of charged Higgs bosons in e + e − collisions were performed by the four LEP experiments. A significant (4.4 σ deviation from the expected background) excess is observed around m H ± ≈ 68 GeV in the L3 data over the last years, which is however not seen by the other LEP experiments. Work is continuing to better understand the nature of this excess and the difference between the observations of the four LEP experiments. Combining the L3 data with the other three LEP experiments, the deviation from background is significantly smaller than what one expects from a charged Higgs boson of mass 68 GeV, thus a lower limit of 78.5 GeV is set on m(H ± ) at 95% confidence level. All results are still preliminary.