On leptonic decay of a heavy quarkonium with a Higgs-boson emission

A leptonic (l̄l) decay of a heavy quark-antiquark bound state T (Q̄Q) with a Higgs-boson H emission is investigated. The applying of the well-known low-energy theorem to meson-Higgs coupling allows one to estimate the probability of the decay T (Q̄Q) → l̄lH. The only a simple version of the Standard Model extension containing two-Higgs doublet is considered. It is well-known that some extensions of the Standard Model (SM) admit the existence of new physical bound states (hadrons), composed of heavy quarks and antiquarks including the 4th generation quarks (Q4) [1]. The question of the existence of 4th generation fermions (f4) is among the most important, intriguing and not solved yet one in the modern elementary particle physics. We know that, e.g., the heterotic string phenomenology in E6 model leads to the 4th generation of leptons (l4) and Q4 with a relatively stable massive neutrino of 4th generation (ν4) [2]. A possible virtual contributions of 4th generation particles have been advocated by recent analysis [3] of precision data on the SM parameters. The following question arises: what about the recent limits on the masses of f4 ? It turns out that l4 and Q4 are not excluded under the condition that the Dirac ν4 is a (quasi)stable particle and it has a mass around 50 GeV [3], and the rest of a spectrum of f4-particles satisfies their direct experimental constraints on the masses m4