Abstract
We consider a simple extension of the electroweak theory, incorporating one SU(2)L doublet of colour-octet scalars with Yukawa couplings satisfying the principle of minimal flavour violation. Using the HEPfit package, we perform a global fit to the available data, including all relevant theoretical constraints, and extract the current bounds on the model parameters. Coloured scalars with masses below 1.05 TeV are already excluded, provided they are not fermiophobic. The mass splittings among the different (charged and CP-even and CP-odd neutral) scalars are restricted to be smaller than 20 GeV. Moreover, for scalar masses smaller than 1.5 TeV, the Yukawa coupling of the coloured scalar multiplet to the top quark cannot exceed the one of the SM Higgs doublet by more than 80%. These conclusions are quite generic and apply in more general frameworks (without fine tunings). The theoretical requirements of perturbative unitarity and vacuum stability enforce relevant constraints on the quartic scalar potential parameters that are not yet experimentally tested.
Highlights
Model was first proposed by Manohar and Wise [8], we will refer to it as the ManoharWise (MW) model in the following
The standard bounds on S, T and U [75] include in the global fit the ratio Rb = Γ(Z → bb)/Γ(Z → hadrons), which incorporates quantum corrections to Γ(Z → bb) that are enhanced by the large value of the top quark mass [80, 81]
The theoretical constraints can be translated to physical observables like the mass differences, which depend on these parameters
Summary
Since it has colour charge, the new scalar multiplet does not mix with the SM Higgs doublet. With Gμ = GAμ T A the octet gluon field These interactions are determined by the gauge symmetry and do not introduce additional free parameters. In order to guarantee the suppression of unwanted flavour-changing neutral currents, which are extremely suppressed experimentally, we will assume the principle of Minimal Flavour Violation (MFV) [31, 32], which is based on the hypothesis that all Yukawa matrices are proportional to the same flavour structures that break the SU(3)QL ⊗ SU(3)uR ⊗ SU(3)dR symmetry in the SM This is one of the main motivations of the MW model because the only scalar representations that can couple to quarks and be compatible with this principle are the colour octet or singlet electroweak doublets [8]. This assumption removes the imaginary parts of ν4, ν5, ηU and ηD and we end up with only 14 new free parameters
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