Higgs boson pair production at future hadron colliders: From kinematics to dynamics

The measurement of triple Higgs coupling is a key benchmark for the Large Hadron Collider (LHC) and future colliders. It directly probes the Higgs potential and its fundamental properties in connection to new physics beyond the Standard Model. There exist two phase space regions with an enhanced sensitivity to the Higgs self-coupling, the Higgs pair production threshold, and an intermediate top pair threshold. We show how the invariant mass distribution of the Higgs pair offers a systematic way to extract the Higgs self-coupling, focusing on the leading channel $pp\to hh+X\to b\overline{b}\gamma \gamma +X$. We utilize new features of the signal events at higher energies and estimate the potential of a high-energy upgrade of the LHC and a future hadron collider with realistic simulations. We find that the high-energy upgrade of the LHC to 27 TeV would reach a $5\sigma$ observation with an integrated luminosity of $2.5{\mathrm{ab}}^{-1}$. It would have the potential to reach 15% (30%) accuracy at the 68% (95%) confidence level to determine the Standard Model (SM) Higgs boson self-coupling. A future 100 TeV collider could improve the self-coupling measurement to better than 5% (10%) at the 68% (95%) confidence level.

Figure 1

Representative Feynman diagrams contributing to the leading Higgs pair production process via gluon fusion.

Figure 2

Total cross section for $pp\to hh$ production at NLO as a function of the $pp$ collider energy. The width of the curve reflects the 10% theoretical uncertainty.

Figure 3

Kinematic distributions (dashed lines with left vertical axes) and significance distribution (solid lines with right vertical axes) assuming a Higgs self-coupling with ${\kappa }_{\lambda }=0$, 1, 2. The significance describes the discrimination of an anomalous self-coupling ${\kappa }_{\lambda }\ne 1$ from the SM hypothesis ${\kappa }_{\lambda }=1$. The results are for the HE-LHC (upper row) and for the 100 TeV collider (lower row).

Figure 4

Higgs pair production cross section (red lines with left vertical axis) and maximum significance (black lines with right vertical axis) for discriminating an anomalous self-coupling ${\kappa }_{\lambda }\ne 1$ from the SM, as a function of the modified self-coupling. The results are for the HL-LHC, the HE-LHC, and a future 100 TeV collider, respectively. The HL-LHC results are taken from Ref. [17].

Figure 5

Representative Feynman diagrams contributing to Higgs pair production via gluon fusion including an ISR jet at hadron colliders.

Figure 6

Composition of the second hardest jet in the signal sample after the acceptance cuts of Eq. (5) for the HE-LHC and the 100 TeV future collider, respectively, with arbitrary units.

Figure 7

Higgs pair invariant mass for the signal and backgrounds based on realistic simulations for the HE-LHC (left) and the 100 TeV future collider (right). The $m_{\gamma \gamma }$ distribution is described by a Gaussian with width 0.75 GeV.

Figure 8

Luminosity required for a $5\sigma$ discover of Higgs pair production for the HE-LHC (dashed) and a 100 TeV collider (full). Left: sensitivity in terms of the total rate, demanding two $b$-tags among the two or three leading jets and assuming $|m_{\gamma \gamma }-m_h|<1\mathrm{GeV}$. Right: sensitivity for three mass windows $|m_{\gamma \gamma }-m_h|<1,2,3\mathrm{GeV}$. We assume the SM hypothesis with ${\kappa }_{\lambda }=1$ and use a binned log-likelihood analysis of the $m_{hh}$ distribution.

Figure 9

Confidence level for separating an anomalous Higgs self-coupling hypothesis from the Standard Model ${\kappa }_{\lambda }=1$.

Figure 10

Comparison of the final confidence level for separating an anomalous Higgs self-coupling hypothesis from the Standard Model ${\kappa }_{\lambda }=1$ for several efficiency choices. We display the results for 27 TeV (top panel) and 100 TeV (bottom panel).