Possibility of observing Higgs bosons at the ILC in the lepton-specific 2HDM

The Higgs boson pair production at a linear $e^{+}{e}^{-}$ collider is analyzed in the $4\tau$ final state in the context of lepton-specific or type IV two Higgs doublet models (2HDMs). Both beams are assumed to be unpolarized. The Higgs boson pairs ($HA$) are produced through off-shell $Z^{*}$ production and decay to $\tau$ jets, which is the main decay channel for neutral Higgs bosons in 2HDM type IV. Using a simplified detector simulation based on the SiD detector at ILC, the $4\tau$ signal is studied through the $\tau$-jet pair invariant mass reconstruction. Several benchmark scenarios are considered for center-of-mass energies of 500 and 1000 GeV at integrated luminosity of $500{\mathrm{fb}}^{-1}$. Among standard model background processes, the main background is $e^{+}{e}^{-}\to ZZ$ followed by $Z\to \tau \tau$. This background is, however, well under control. With the luminosity assumed in the analysis, striking signals are obtained beyond the reach of the LHC. Such signals would allow for precise determination of masses and cross sections and already much lower luminosities are sufficient for discovery.

Figure 1

$\mathrm{BR}(\mathrm{H}/\mathrm{A}\to \mathrm{XY})$ as a function of $\tan \beta$. The selected points showing $H$ decays assume $m_A=150$, 250, 250, 300 GeV from top to bottom. Similarly, points showing $A$ decays assume $m_H=150$, 150, 250, 300 GeV.

Figure 2

$\mathrm{BR}(\mathrm{H}/\mathrm{A}\to \mathrm{XY})$ as a function of the Higgs boson masses.

Figure 3

Distribution of the effective center-of-mass energy ($\sqrt{s^{\prime }}$) including both beamstrahlung and ISR effects for the two cases of $\sqrt{s}=500$ and 1000 GeV. $\sqrt{s_{\mathrm{nom}}}$ is the nominal center-of-mass energy of the collider.

Figure 4

Signal of the $\tau$-jet pair invariant mass at $\sqrt{s}=500\mathrm{GeV}$ with and without $\mathrm{beamstrahlung}+\mathrm{ISR}$.

Figure 5

Signal of the $\tau$-jet pair invariant mass in different benchmark points at $\sqrt{s}=500\mathrm{GeV}$.

Figure 6

Signal of the $\tau$-jet pair invariant mass in different benchmark points at $\sqrt{s}=1000\mathrm{GeV}$.