Strong Double Higgs Production at the LHC

TL;DR

This work analyzes a strongly coupled electroweak sector in which a light Higgs emerges as a pseudo-Goldstone boson, leading to energy-growing amplitudes in VV scatterings and enhanced double-Higgs production via vector-boson fusion. By parametrizing deviations with a, b, c, d3, d4 and applying realistic LHC-level analyses to three leptonic final states (S3, S2, S4), the authors quantify the discovery prospects for strong dynamics signatures at 14 TeV with 300 fb^-1 and project sensitivities for higher luminosities. They show that, for representative composite-Higgs realizations (MCHM4/5 with ξ close to unity), the three-lepton and same-sign dilepton channels can achieve significant evidence or discovery with substantial integrated luminosity, while the four-lepton channel provides complementary information. The study also discusses how Higgs mass, coupling deviations, and future collider upgrades influence observability and how one could extract model parameters a^2−b and d3 from high-mass hh final states, offering guidance for distinguishing pseudo-Goldstone Higgs from a dilaton.

Abstract

The hierarchy problem and the electroweak data, together, provide a plausible motivation for considering a light Higgs emerging as a pseudo-Goldstone boson from a strongly-coupled sector. In that scenario, the rates for Higgs production and decay differ significantly from those in the Standard Model. However, one genuine strong coupling signature is the growth with energy of the scattering amplitudes among the Goldstone bosons, the longitudinally polarized vector bosons as well as the Higgs boson itself. The rate for double Higgs production in vector boson fusion is thus enhanced with respect to its negligible rate in the SM. We study that reaction in pp collisions, where the production of two Higgs bosons at high pT is associated with the emission of two forward jets. We concentrate on the decay mode hh -> WW^(*)WW^(*) and study the semi-leptonic decay chains of the W's with 2, 3 or 4 leptons in the final states. While the 3 lepton final states are the most relevant and can lead to a 3 sigma signal significance with 300 fb^{-1} collected at a 14 TeV LHC, the two same-sign lepton final states provide complementary information. We also comment on the prospects for improving the detectability of double Higgs production at the foreseen LHC energy and luminosity upgrades.

Figures (26)

• Figure 1: Leading diagrams for the $V_LV_L\to V_LV_L$ (upper row) and $V_LV_L \to hh$ (lower row) scatterings at high energies.
• Figure 2: Higgs decay branching ratios as a function of $\xi$ for SM fermions embedded into fundamental representations of $SO(5)$ for two benchmark Higgs masses: $m_h=120$ GeV (left plot) and $m_h=180$ GeV (right plot). For $\xi=0.5$, the Higgs is fermiophobic, while in the Technicolor limit, $\xi \to 1$, the Higgs becomes gaugephobic.
• Figure 3: The full set of diagrams for $qq\to WWqq$ at order $g_W^4$. The blob indicates the sum of all possible $WW\to WW$ subdiagrams. It is understood that the bremsstrahlung diagrams (second and third diagrams) correspond to all possible ways to attach an outgoing $W$ to the quark lines.
• Figure 4: Cross section for the hard scattering $W^+W^+\to W^+W^+$ as a function of the center of mass energy for two different cuts on $t$ and $m_h = 180$ GeV. The left plot shows the almost inclusive cross section with $-s+ 4 M_W^2 < t < -M_W^2$. The right plot shows the hard cross section with $-3/4 < t/s < -1/4$.
• Figure 5: Differential cross section for longitudinal versus transverse polarizations for $a =0$ (left plot) and in the Standard Model ($a=1$, right plot). The different normalization reflects the different naive expectation in the two cases: in the SM, both differential cross sections scale like $1/s^2$ at large energy, whereas for $a=0$ the longitudinal differential cross section stays constant, see Eq. (\ref{['eq:diffxsecs']}).