Higgs-flavon mixing and LHC phenomenology in a simplified model of broken flavor symmetry

The LHC phenomenology of a low-scale gauged flavor symmetry model with inverted hierarchy is studied, through introduction of a simplified model of broken flavor symmetry. A new scalar (a flavon) and a new neutral top-philic massive gauge boson emerge with mass in the TeV range, along with a new heavy fermion associated with the standard model top quark. After checking constraints from electroweak precision observables, we investigate the influence of the model on Higgs boson physics, notably on its production cross section and decay branching fractions. Limits on the flavon $\phi$ from heavy Higgs boson searches at the LHC at 7 and 8 TeV are presented. The branching fractions of the flavon are computed as a function of the flavon mass and the Higgs-flavon mixing angle. We also explore possible discovery of the flavon at 14 TeV, particularly via the $\phi \to Z^0{Z}^0$ decay channel in the $2\ell 2{\ell }^{\prime }$ final state, and through standard model Higgs boson pair production $\phi \to hh$ in the $b\overline{b}\gamma \gamma$ final state. We conclude that the flavon mass range up to 500 GeV could be probed down to quite small values of the Higgs-flavon mixing angle with $100{\mathrm{fb}}^{-1}$ of integrated luminosity at 14 TeV.

Figure 1

Contribution to the one-loop effective potential from Eq. (3). Each black dot means an insertion of the vertex and a zero-momentum external scalar.

Figure 2

The constraint on the mass of the heavy fermion $m_T$ and the mixing angle of the left-handed fermions $s_L$ from the electroweak oblique parameters. The flavon mass $m_{\phi }$ is chosen to be 300 GeV. In the upper panel, the scalar mixing angle is $s_H^2=0.01$. In the lower panel, $s_H^2=0.20$.

Figure 3

Feynman diagrams for the additional contributions to $h\to Z^0\gamma$. Both $t$ and $T$ appear in the fermion loops.

Figure 4

The ratio between $\mathrm{Br}(h\to Z^0\gamma )$ in the NP model and the SM is shown as a function of the mass of the heavy fermion $m_T$ and the mixing angle of the left-handed fermions $s_L$. We show the result in the region where the model can fit the EWPO at 3 $\sigma$ C.L. We choose $m_{\phi }=300\mathrm{GeV}$ and ${\lambda }_{\mathrm{\Phi }}=1.0$. In the upper panel, we choose $s_H^2=0.01$. In the lower panel, $s_H^2=0.20$.

Figure 5

The Feynman diagram by which the $(H^{†}H){\overline{Q}}_L\tilde{H}{t}_R$ operator can be generated. The ${\mathrm{\Psi }}_{u,c}$ are the heavy fermion eigenstates of the first and the second generations in the UV completion of the flavor symmetry model with inverted hierarchy.

Figure 6

The SM-like Higgs pair production cross section in the NP model (without the flavon resonance effect). We choose ${\lambda }_{\mathrm{\Phi }}/(4\pi )=0.01$ in the calculation.

Figure 7

The deviation of $|c_{\phi gg}/c_{hgg}^{SM}{|}^2$ from the heavy fermion limit $s_H^2$ is plotted as a function of ${\lambda }_{\mathrm{\Phi }}$ and the flavon mass $m_{\phi }$.

Figure 8

The decay branching ratios of the most important $\phi$ decay channels. Only the $W^{+}{W}^{-},Z^0{Z}^0,hh$, and $t\overline{t}$ channels are significant in $\phi$ decay. In this calculation, we choose $M=2000\mathrm{GeV}$, and $\lambda =1.1$.

Figure 9

The lower bound of the $2\sigma$ exclusion region of $s_H^2$ from the $Z^0{Z}^0$ channel heavy SM Higgs boson searches at 7 and 8 TeV is shown as a function of ${\lambda }_{\mathrm{\Phi }}$ and $m_{\phi }$. We choose $M=2000\mathrm{GeV}$ in this calculation. In the red region, the constraint from fitting the Higgs boson inclusive cross section is stronger. The peaks and valleys in the figure reflect the structure of the experimental exclusion bound.

Figure 10

The expected lower bound of the $2\sigma$ exclusion region of $s_H^2$ from searches for heavy scalar decays to $Z^0{Z}^0$ at the LHC at 14 TeV with $100{\mathrm{fb}}^{-1}$ integrated luminosity is shown as a function of ${\lambda }_{\mathrm{\Phi }}$ and $m_{\phi }$. We choose $M=2000\mathrm{GeV}$ in this calculation.

Figure 11

The reconstructed diphoton, $b\overline{b}$, and $hh$ mass distributions are shown at 14 TeV with $100{\mathrm{fb}}^{-1}$ integrated luminosity. We choose $m_{\phi }=400\mathrm{GeV}$. The total cross section is rescaled to the value of the SM-like Higgs boson with the same mass. The decay branching ratio $\mathrm{Br}(\phi \to hh)$ is set to be 100%. In the left and middle panels, the events passed all of the cuts except the $\gamma \gamma$ and $bb$ invariant mass cuts. In the right panel, the events passed all of the cuts. Note that the horizontal scale differs in the three distributions.

Figure 12

The $2\sigma$ C.L. exclusion bound and the $5\sigma$ C.L. observation bound of the flavon production strength $\mu$ from the $pp\to \phi \to hhX\to b\overline{b}\gamma \gamma X$ and the $pp\to \phi \to ZZX\to 2\ell 2{\ell }^{\prime }$ channels. $\mu \equiv \sigma (pp\to \phi \to ZZ)/\sigma (pp\to H)$ for the $ZZ$ channel and $\mu \equiv \sigma (pp\to \phi \to hh)/\sigma (pp\to H)$ for the $hh$ channel. The region above the curves will be excluded (discovered) at $2\sigma$ ($5\sigma$) C.L.

Figure 13

Combination of the $2\sigma$ exclusion region of $s_H^2$ results for the $\phi \to Z^0{Z}^0\to 2\ell 2{\ell }^{\prime }$ search and the $\phi \to hh\to b\overline{b}\gamma \gamma$ search. In the upper part of the figure, in the irregularly shaped region above the broad-dashed line, the $\phi \to hh\to b\overline{b}\gamma \gamma$ search yields a stronger constraint.

Figure 14

The $5\sigma$ discovery significance of the required value of $s_H^2$ from a combination of the $\phi \to Z^0{Z}^0\to 2\ell 2{\ell }^{\prime }$ search and the $\phi \to hh\to b\overline{b}\gamma \gamma$ search. In the upper part of the figure, in the irregularly shaped region above the broad-dashed line, the $\phi \to hh\to b\overline{b}\gamma \gamma$ process is more sensitive to the NP model.