Rare top decay $t\to c\gamma$ with flavor changing neutral scalar interactions in two Higgs doublet model

Models beyond the standard model with extra scalars have been highly motivated by the recent discovery of the Higgs boson. The two Higgs doublet model type III considers the most general case for the scalar potential, allowing mixing between neutral $CP$-even and $CP$-odd scalar fields. This work presents the results of the study on the $t\to c\gamma$ decay at one loop level if neutral flavor changing is generated by top-charm-Higgs coupling given by the Yukawa matrix. For instance, a value for the branching ratio $\mathrm{Br}(t\to c\gamma )\sim {10}^{-6}$ for $\tan \beta =2.5$ and general neutral Higgs mixing parameters, $1.16\le {\alpha }_1\le 1.5$, $-0.48\le {\alpha }_2\le -0.1$. The number of events for the $t\to c\gamma$ decay with an integrated luminosity of $300{\mathrm{fb}}^{-1}$ is estimated as $10\lesssim N_{\mathrm{Eff}}\lesssim 100$ for the parameters of the model constrained by experimental data.

Figure 1

One loop Feynman diagram with a Higgs boson in the internal line, (a) flavor changing neutral scalar contribution, (b) and (c) charged contributions.

Figure 2

Allowed values for the Yukawa couplings in a scatter plot with points compatible with the experimental value of the $\mathrm{BR}(B\to X_s\gamma )$ and $300\mathrm{GeV}\le m_H^{\pm }\le 600\mathrm{GeV}$ and $\tan \beta =1$, 2.5, 5, 10, and 15.

Figure 3

Allowed values for the Yukawa couplings $Y_{bb}$ in a scatter plot with compatible points with the experimental value of the $\mathrm{BR}(H\to b\overline{b})$ and $\pi /2\le {\alpha }_{1,2}\le \pi /2$.

Figure 4

Allowed values for the Yukawa couplings in a scatter plot with compatible points with the experimental values for $\mathrm{Br}(H\to b\overline{b})$ and $\mathrm{Br}(B\to X_s\gamma )$.

Figure 5

Allowed values for the mixing parameter ${\alpha }_{1,2}$ in a scatter plot with points compatible with the experimental values for $\mathrm{Br}(H\to b\overline{b})$, $\mathrm{Br}(B\to X_s\gamma )$, and $\mathrm{Br}(t\to c\gamma )<5.9\times {10}^{-3}$, for fixed values of $\tan \beta =1$, 2.5, 5, 10, and $Y_{tc}=0.01$.

Figure 6

Allowed values for the mixing parameters ${\alpha }_{1,2}$ in a scatter plot with points compatible with the experimental values for $\mathrm{Br}(H\to b\overline{b})$, $\mathrm{Br}(B\to X_s\gamma )$, and $\mathrm{Br}(t\to c\gamma )<5.9\times {10}^{-3}$, for fixed values of $\tan \beta =1$, 2.5, 5, 10, and $Y_{tc}=-0.04$.

Figure 7

The model III branching ratio for $t\to c\gamma$ as a function of ${\alpha }_1–{\alpha }_2$ in regions $R_1$ and $R_2$.

Figure 8

Effective number of events for $t\to c\gamma$ as a function of $Y_{tc}$ for $\tan \beta =1.56$, 2.5, 5, 10, 15 expected in LHC Run 3.

Figure 9

Allowed regions for ${\alpha }_1$ and ${\alpha }_2$ when $\mathrm{Br}(t\to c\gamma )\sim {10}^{-5}$ is assumed with $-0.385\le Y_{tc}\le -0.307$ for $\tan \beta =1.56$, $-0.267\le Y_{tc}\le -0.135$ for $\tan \beta =2.5$, $-0.173\le Y_{tc}\le -0.035$ for $\tan \beta =5$, $-0.105\le Y_{t}c\le -0.02$ for $\tan \beta =10$ and $-0.08\le Y_{tc}\le -0.02$ for $\tan \beta =15$. In LHC Run 3.

Figure 10

Effective number of events for $t\to c\gamma$ as a function of $\tan \beta$ expected in LHC Run 3.

Figure 11

$Y_{tc}$ coupling as a function of $\tan \beta$ for different FCNC top quark decays. Current experimental limits are taken into account.