Flavour Independent Search for Hadronically Decaying Neutral Higgs Bosons at LEP

TL;DR

This paper addresses flavour-independent Higgs decays to hadrons to reduce model dependence in Higgs searches by combining LEP-2 results for the $e^+e^- \rightarrow hZ$ channel across four experiments and three hadronic topologies. By omitting $b$-tagging and employing mass-dependent selections (notably in the four-jet channel) and a likelihood-ratio CLs framework, it provides the first combined limits on hadronically decaying neutral Higgs bosons. The combined result yields a 95% CL lower limit of $m_h = 112.9$ GeV/$c^2$ (with an expected limit of 113.0 GeV/$c^2$ under SM-like production and BR$(h\rightarrow hadrons)=1$), and cross-section exclusions span roughly 10–60% of the SM value for $60$–$100$ GeV/$c^2$ Higgs, improving to a few–30% with the full data set. These findings offer model-independent constraints that complement standard SM Higgs searches and illustrate the robustness of LEP-2 in testing non-standard Higgs decays, albeit with no significant signal observed.

Abstract

Hadronic decays of Higgs bosons, not necessarily into b-quarks, have been searched for using data collected at LEP-2. Such searches are complementary to the usual Standard Model Higgs searches and lead to more model-independence. Preliminary results obtained by the four LEP collaborations are presented for the hZ production mechanism (using the q-qbarq-qbar, q-qbarnu-nubar and q-qbarl+l- channels) and combined for the first time.

Figures (2)

• Figure 1: Combined LEP confidence levels as a function of the Higgs boson mass in the flavour independent hypothesis, computed with the likelihood ratio technique O-stat, assuming production cross-sections equal to those in the Standard Model and BR(h$\rightarrow$ hadrons) = 1.0. The confidence levels for the signal and background-only hypotheses are shown in the upper (a) and lower (b) plots, respectively. The curves are the observed (solid) and expected median (dashed) confidences from background-only experiments, and the bands are the corresponding 68.3 % and 95 % confidence intervals. In the lower plot, the dot-dashed line shows the expected median confidence from experiments including an expected signal of mass given in abscissa.
• Figure 2: Combined flavour independent LEP observed and expected 95 % CL upper limits on the production cross-section as a function of the Higgs boson mass, normalised to the expected Standard Model values, computed assuming BR(h$\rightarrow$ hadrons) = 1.0. The computation was done with the likelihood ratio technique O-stat. The curves are the observed (solid) and expected median (dashed) excluded ratios, and the bands correspond to 68.3 % and 95 % confidence intervals from the background-only experiments.