LHC phenomenology of the type II seesaw mechanism: Observability of neutral scalars in the nondegenerate case

This is a sequel to our previous work on LHC phenomenology of the type II seesaw model in the nondegenerate case. In this work, we further study the pair and associated production of the neutral scalars $H^0/A^0$. We restrict ourselves to the so-called negative scenario characterized by the mass order $M_{H^{\pm \pm }}>M_{H^{\pm }}>M_{H^0/A^0}$, in which the $H^0/A^0$ production receives significant enhancement from cascade decays of the charged scalars $H^{\pm \pm },H^{\pm }$. We consider three important signal channels—$b\overline{b}\gamma \gamma$, $b\overline{b}{\tau }^{+}{\tau }^{-}$, $b\overline{b}{\ell }^{+}{\ell }^{-}{\cancel{E}}_T$—and perform detailed simulations. We find that at the 14 TeV LHC with an integrated luminosity of $3000{\mathrm{fb}}^{-1}$, a $5\sigma$ mass reach of 151, 150, and 180 GeV, respectively, is possible in the three channels from the pure Drell-Yan $H^0{A}^0$ production, while the cascade-decay-enhanced $H^0/A^0$ production can push the mass limit further to 164, 177, and 200 GeV. The neutral scalars in the negative scenario are thus accessible at LHC run II.

Figure 1

Branching ratios of $H^{\pm }$ and $H^{\pm \pm }$ versus $M_{\mathrm{\Delta }}$ at some benchmark points of $\mathrm{\Delta }M$ and $v_{\mathrm{\Delta }}$: $(\mathrm{\Delta }M,v_{\mathrm{\Delta }})=(5,0.01),(10,0.01),(5,0.001)\mathrm{GeV}$, from the upper to the lower panels.

Figure 2

Branching ratios of $H^0\to b\overline{b}$ and $H^0\to t\overline{t}$ as a function of $M_{H^0}$ for various values of ${\lambda }_4$.

Figure 3

Left: Branching ratios of $H^0\to W^{+}{W}^{-},ZZ$ as a function of $M_{H^0}$ in the mass region 130–300 GeV. Right: Branching ratios of $H^0\to hh,ZZ$ as a function of $M_{H^0}$ in the mass region 200–1000 GeV.

Figure 4

Branching ratios of $H^0\to \gamma \gamma ,Z\gamma$ as a function of $M_{H^0}$ for various sets of ${\lambda }_{2,4}$ values.

Figure 5

Branching ratios of $A^0\to b\overline{b},t\overline{t}$ as a function of $M_{A^0}$ for various values of ${\lambda }_4$.

Figure 6

Branching ratios of $A^0\to Zh$ as a function of $M_{A^0}$ for various values of ${\lambda }_4$.

Figure 7

Branching ratios of $A^0\to \gamma \gamma$ and $A^0\to Z\gamma$ as a function of $M_{A^0}$ for various values of ${\lambda }_4$.

Figure 8

Branching ratios of $H^0/A^0$ as a function of $M_{H^0/A^0}$ at the benchmark point in Eq. (9).

Figure 9

Production cross sections for a pair of scalars at LHC14 versus $M_{\mathrm{\Delta }}$ for a degenerate spectrum. The black dashed line is for the SM $hh$ production.

Figure 10

Production cross sections for a pair of neutral scalars versus $M_{\mathrm{\Delta }}$ at LHC14 and with $\mathrm{\Delta }M=5\mathrm{GeV}$, $v_{\mathrm{\Delta }}=0.001\mathrm{GeV}$. Left: The red solid (dashed) line corresponds to $X_0$ ($X$). Right: The red line corresponds to $H^0{H}^0/A^0{A}^0$ from cascade decays $Y/Z$, and the green line to $H^0{A}^0$ from cascade decays ($X_C$). The shaded regions are filled by scanning over $\mathrm{\Delta }M$ and $v_{\mathrm{\Delta }}$.

Figure 11

Theoretical cross sections of $b\overline{b}\gamma \gamma$, $b\overline{b}{\tau }^{+}{\tau }^{-}$, and $b\overline{b}{\ell }^{+}{\ell }^{-}2\nu$ signal channels at LHC14. The red solid (dashed) line corresponds to the signal from $X_0$ ($X$), the green (blue) solid line corresponds to the signal from $Y(Z)$, and the purple dashed line shows the total cross section $S$ for the signal. The SM $hh$ cross section is shown for comparison. The lower right panel shows the enhancement factor $S/S_0$ in the three signal channels.

Figure 12

Distributions of $p_T^{b,\gamma },\mathrm{\Delta }{R}_{bb,\gamma \gamma },M_{bb,\gamma \gamma ,H^0{A}^0}$, and $E_T$ for the signal $b\overline{b}\gamma \gamma$ and its backgrounds before applying any cuts at LHC14.

Figure 13

Distributions of $p_T^{\tau },\mathrm{\Delta }{R}_{\tau \tau },M_{\tau \tau ,H^0{A}^0},E_T$, and ${\cancel{E}}_T$ for the signal $b\overline{b}{\tau }^{+}{\tau }^{-}$ and its backgrounds before applying any cuts at LHC14.

Figure 14

Distributions of $p_T^{\ell },\mathrm{\Delta }{R}_{\ell \ell },M_C^{\ell \ell },{\cancel{E}}_T,M_C$, and $E_T$ for the signal $b\overline{b}{\ell }^{+}{\ell }^{-}{\cancel{E}}_T$ and its backgrounds before applying any cuts at LHC14.

Figure 15

Left: Significance $\mathcal{S}(S,B)$ of the $b\overline{b}\gamma \gamma$ channel versus $M_{\mathrm{\Delta }}$ reachable at LHC14@300 (red region) and LHC14@3000 (green). Right: Required luminosity to reach a $3\sigma$ (red region) and $5\sigma$ (green) significance in the $b\overline{b}\gamma \gamma$ channel versus $M_{\mathrm{\Delta }}$ at LHC14. The solid line corresponds to the signal from $X_0$ alone, and the dashed line corresponds to the total signal including cascade enhancement.

Figure 16

Same as Fig. 15, but for the $b\overline{b}{\tau }^{+}{\tau }^{-}$ channel.

Figure 17

Same as Fig. 15, but for the $b\overline{b}{\ell }^{+}{\ell }^{-}{\cancel{E}}_T$ channel.