Anomalous Couplings in Double Higgs Production

TL;DR

This paper provides a model-independent assessment of anomalous Higgs couplings in gluon-fusion double Higgs production, highlighting the ttbar hh non-linear coupling as a hallmark of Higgs compositeness and showing limited sensitivity to the trilinear coupling. Using a general coupling parametrization and exact one-loop matrix elements, the authors quantify how gg→hh cross sections respond to a, c, c2, and d3, finding strong leverage on c2. A parton-level analysis of the bbγγ final state at 14 TeV demonstrates meaningful discovery and measurement prospects for c2 and xi, especially in composite-Higgs scenarios like MCHM5, while noting that realistic detector effects will require further study. Overall, double Higgs production emerges as a powerful probe of Higgs non-linearities and compositeness, with complementary channels and benchmark scenarios shaping its experimental reach.

Abstract

The process of gluon-initiated double Higgs production is sensitive to non-linear interactions of the Higgs boson. In the context of the Standard Model, studies of this process focused on the extraction of the Higgs trilinear coupling. In a general parametrization of New Physics effects, however, an even more interesting interaction that can be tested through this channel is the (ttbar hh) coupling. This interaction vanishes in the Standard Model and is a genuine signature of theories in which the Higgs boson emerges from a strongly-interacting sector. In this paper we perform a model-independent estimate of the LHC potential to detect anomalous Higgs couplings in gluon-fusion double Higgs production. We find that while the sensitivity to the trilinear is poor, the perspectives of measuring the new (ttbar hh) coupling are rather promising.

Figures (9)

• Figure 1: Feynman diagrams for double Higgs production via gluon fusion (an additional contribution comes from the crossing of the box diagram). The last diagram contains the new non-linear Higgs interaction $t\bar{t} hh$.
• Figure 2: Invariant mass distribution of the two Higgs bosons in $pp \rightarrow hh$ at the LHC ($14\ {\rm TeV}$) for $m_h = 120\ {\rm GeV}$. The various curves correspond to different choices of Higgs couplings: $c = d_3 = 1$, $c_2 = 0$ (SM couplings, solid blue curve), $c = d_3 = 1$, $c_2 = -1$ (dashed purple curve), $c = 1$, $d_3 = c_2 = 0$ (dotted yellow curve). The dot-dashed green curve shows the distribution obtained in the approximation of infinite top mass with SM couplings. All curves have been normalized to unit area. The corresponding LO total cross sections are $15.2\,$fb (solid blue curve), $253\,$fb (dashed purple curve), $31.6\,$fb (dotted yellow curve).
• Figure 3: Left plot: total cross section in SM units as a function of $c_2 = \Delta$ (solid blue curve) and $d_3 = 1 + \Delta$ (dashed purple curve), with the other Higgs couplings set to their SM values and $m_h =120\,$GeV. Right plot: LO total cross section (in fb) in the SM as a function of $m_h$ as computed by means of the full one-loop matrix element (solid blue curve) and the infinite top mass approximation (dot-dashed green curve). In both plots the LHC center-of-mass energy has been set to 14 TeV.
• Figure 4: Value of the branching ratio $BR(hh\to \gamma\gamma b\bar{b})$ (on the left) and $BR(hh\to WW\gamma\gamma)$ (on the right) in SM units as a function of the ratio of Higgs couplings $c/a$. The dots show the prediction in the MCHM5, where $c/a = (1-2\xi)/(1-\xi)$, for various values of $\xi$. In both plots the Higgs mass is set to $m_h =120\,$GeV.
• Figure 5: Signal yield per fb$^{-1}$ predicted in the MCHM5 for the two final states $hh\to \gamma\gamma b\bar{b}$ (solid curves) and $hh\to WW\gamma\gamma \to l\nu q\bar{q} \gamma\gamma$ (dashed curves) as a function of $\xi$. The thick (thin) curves correspond to the LHC with 14 TeV (8 TeV) center-of-mass energy. The Higgs mass is set to $m_h =120\,$GeV. The rate has been computed using the cross section for $gg\to hh$ at LO in $\alpha_s$ (i.e. no $K$-factor is included) given by eq.(\ref{['eq:fitxsectot']}) and Table \ref{['tab:fit']}.