Constraining extra-fermion(s) from the Higgs boson data

TL;DR

The paper develops a general effective-field-theory framework to constrain extra-fermions (EF) that modify Higgs couplings via mixing and loop effects, using a global fit to Higgs-rate data. It expresses Higgs observables in terms of $c_t$, $c_b$, $c_ au$, $c_{gg}$, and $c_{ m abla ext{ } abla}$, and analyzes how EF scenarios map onto these parameters. The results show EF-induced corrections can improve the fit relative to the SM, identify best-fit points (e.g., $c_b\approx 2.08$, $c_{gg}\approx 0.66$, $c_{ m abla ext{ } abla}\approx -1.09$ for fixed $c_t=c_ au=1$) and reveal symmetries in parameter space, as well as predictive power for single EF models (notably a color-charged EF with $Q_{q'}=-7/3$). Indirect Higgs-data constraints become powerful enough to bound EF masses up to ~200 TeV and motivate targeted channels to measure bottom and tau Yukawas more precisely, thereby sharpening EF searches.

Abstract

First, we study the fit of the Higgs boson rates, based on all the latest collider data, in the effective framework for any Extra-Fermion(s) [EF]. The best-fit results are presented in a generic formalism allowing to apply those for the test of any EF scenario. The variations of the fit with each one of the five fundamental parameters are described, and, the obtained fits can be better than in the Standard Model (SM). We show how the determination of the EF loop-contributions to the Higgs couplings with photons and gluons is relying on the knowledge of the top and bottom Yukawa couplings (affected by EF mixings); for determining the latter coupling, the relevance of the investigation of the Higgs production in association with bottom quarks is emphasized. In the instructive approximation of a single EF, we find that the constraints from the fit already turn out to be quite predictive, in both cases of an EF mixed or not with SM fermions, and especially when combined with the extra-quark (-lepton) mass bounds from direct EF searches at the LHC (LEP) collider. In the case of an unmixed extra-quark, non-trivial fit constraints are pointed out on the Yukawa couplings for masses up to ~200 TeV. In particular, we define the extra-dysfermiophilia, which is predicted at 68.27% C.L. for any single extra-quark (independently of its electric charge). Another result is that, among any components of SM multiplet extensions, the extra-quark with a -7/3 electric charge is the one preferred by the present Higgs fit.

Figures (10)

• Figure 1: Best-fit regions at $68.27\%{\rm C.L.}$ (in green), $95.45\%{\rm C.L.}$ (yellow) and $99.73\%{\rm C.L.}$ (grey) in the plane $c_{\gamma\gamma}$ versus $c_{gg}$, for $c_\tau=1$. Each one of the four figures [a,b,c,d] is associated to a certain $c_b$ value written (in blue) on the figure itself. In each figure, the regions are drawn for three $c_t$ values, the corresponding value being indicated nearby the relevant region; the regions for the lowest, intermediate, highest $c_t$ values are respectively shown by the plain contours, colored filled domains, dotted contours. The SM (black) point, at $c_t=c_b=c_\tau=1$, $c_{\gamma\gamma}=c_{gg}=0$, is shown on the plot [b]. Finally, the four best-fit point locations are indicated by crosses in the plot [c]. The theoretically predicted lines for extra-quarks of type $b'$ (plot [c,d]) and $t'$ (plot [b]) are also represented (in red).
• Figure 2: Best-fit regions at $68.27\%{\rm C.L.}$ (green), $95.45\%{\rm C.L.}$ (yellow) and $99.73\%{\rm C.L.}$ (grey) in the plane $c_{\gamma\gamma}$ versus $c_{gg}$, for $c_b=2.08$. Each one of the three figures is obtained for a $c_\tau$ value which is indicated (in blue). In each figure, the regions are drawn for three $c_t$ values [same conventions as in Fig.(\ref{['Fig:cbVAR']})]. The predicted (red) line for an extra-quark of type $b'$ is also represented in the plot [b].
• Figure 3: Best-fit regions at $68.27\%{\rm C.L.}$, $95.45\%{\rm C.L.}$ and $99.73\%{\rm C.L.}$ in the plane $c_{\gamma\gamma}$ versus $c_\tau$, for the case of an extra-lepton with electric charge, $Q_{\ell'}=-1$, corresponding to $c_t=c_b=1$, $c_{gg}=0$. The two best-fit points are indicated (in black).
• Figure 4: Best-fit regions at $68.27\%{\rm C.L.}$, $95.45\%{\rm C.L.}$ and $99.73\%{\rm C.L.}$ in the plane $c_{\gamma\gamma}$ versus $c_{gg}$, for $c_t=c_b=c_\tau=1$. Also represented are the predicted (red plain) lines for extra-quarks with the several electric charges, $Q_{q'}= -1/3$, $2/3$, $-4/3$, $5/3$, $-7/3$ and $8/3$. The extreme (red dashed) lines for, $Q_{q'}=0$, and, $\vert Q_{q'} \vert =\vert Q_{q'} \vert_{\rm pert.} = \sqrt{4\pi /\alpha}$, are shown as well. The four best-fit points are indicated (in black).
• Figure 5: Best-fit regions at $68.27\%{\rm C.L.}$, $95.45\%{\rm C.L.}$ and $99.73\%{\rm C.L.}$ in the plane of the $\vert m_{f'} \vert$ absolute mass (in GeV) versus the $\tilde{Y}_{f'}$ coupling, with $c_t=c_b=c_\tau=1$, for the cases $Q_{q'}= - 1/3$, $5/3$, $8/3$ and $Q_{\ell'} =-1$. Also represented as dashed lines are the lower perturbative limit on Yukawa couplings, $-4\pi$ [black horizontal lines], and the direct LHC bounds, $m_{b'}>611$ GeV, $m_{q_{5/3}}>611$ GeV, or LEP constraint, $m_{\ell'}>63.5$ GeV ($>101.9$ GeV) for $m_{\ell'}-m_{\nu'}>7$ GeV ($>15$ GeV) [purple vertical lines]. The mass ranges start at the Higgs mass, $m_h$ (see Footnote [\ref{['FootMh']}]), except for the $\ell'$-lepton case (see text) where the $m_h$ value is indicated by a [black dashed vertical] line.