Singlet-catalyzed electroweak phase transitions in the 100 TeV frontier

We study the prospects for probing a gauge singlet scalar-driven strong first-order electroweak phase transition with a future proton-proton collider in the 100 TeV range. Singlet-Higgs mixing enables resonantly enhanced di-Higgs production, potentially aiding discovery prospects. We perform Monte Carlo scans of the parameter space to identify regions associated with a strong first-order electroweak phase transition, analyze the corresponding di-Higgs signal, and select a set of benchmark points that span the range of di-Higgs signal strengths. For the $b\overline{b}\gamma \gamma$ and $4\tau$ final states, we investigate discovery prospects for each benchmark point for the high-luminosity phase of the Large Hadron Collider and for a future $pp$ collider with $\sqrt{s}=50$, 100, or 200 TeV. We find that any of these future collider scenarios could significantly extend the reach beyond that of the high-luminosity LHC, and that with $\sqrt{s}=100\mathrm{TeV}$ (200 TeV) and $30{\mathrm{ab}}^{-1}$, the full region of parameter space favorable to strong first-order electroweak phase transitions is almost fully (fully) discoverable.

Figure 1

Left: Distribution of SFOEWPT points in $m_2$ vs $\cos \theta$ space. Maximum (minimum) benchmark points are shown in green (magenta). Right: Maximum (minimum) cross section times branching ratio as a function of $m_2$ at a 100 TeV $pp$ collider, taken from Table 1 (Table 2), is displayed as a solid green (dashed magenta) line.

Figure 2

Signal and background distributions for the $b\overline{b}\gamma \gamma$ final state. The signal distributions correspond to ${\mathrm{BM}10}^{\max }$. The kinematic quantities shown are (top left) the invariant mass of the $b\overline{b}\gamma \gamma$ system, and (top right) the $p_T$ of the leading particle from among the photons and the $b$ quarks. Also shown are the distributions of the BDT output with uniform binning (bottom left) and optimized binning (bottom right).

Figure 3

The $N_{\sigma }$ Gaussian significance for rejecting the background-only hypothesis, obtained using the $b\overline{b}\gamma \gamma$ final state, for each benchmark point. Different collider scenarios of energy and integrated luminosity are compared. The vertical range corresponds to the maximum and minimum signal cross sections in the $h_2$ mass window.

Figure 4

Signal and background distributions for the $4\tau$ final state, where the signal corresponds to ${\mathrm{BM}10}^{\max }$. The kinematic quantities shown are (top left) the invariant mass of the $4\tau$ system, and (top right) the average di-$\tau$ pair mass in the event. Also shown are the distributions of the BDT output with uniform binning (bottom left) and optimized binning (bottom right).

Figure 5

The $N_{\sigma }$ Gaussian significance for rejecting the background-only hypothesis, obtained using the $4\tau$ final state, for each benchmark point. Different collider scenarios of energy and integrated luminosities are compared. The vertical range corresponds to the maximum and minimum signal cross sections in the $h_2$ mass window.

Figure 6

The $N_{\sigma }$ Gaussian significance for rejecting the background-only hypothesis, obtained using the combination of the $b\overline{b}\gamma \gamma$ and $4\tau$ final states, for each benchmark point. Different collider scenarios of energy and integrated luminosities are compared. The vertical range corresponds to the maximum and minimum signal cross sections in the $h_2$ mass window.

Figure 7

Additional kinematics distributions for the $b\overline{b}\gamma \gamma$ final state, used as inputs to the BDT. The signal distributions correspond to ${\mathrm{BM}10}^{\max }$.

Figure 8

Additional kinematics distributions for the $4\tau$ final state, used as inputs to the BDT. The signal distributions correspond to ${\mathrm{BM}10}^{\max }$.