Speaker
Description
The measurement of the Higgs self-coupling is considered the next milestone
in the study of the Higgs boson properties. At future $e^+e^-$ facilities below
the double Higgs production threshold, this is extracted from the $Zh$
production cross-section, which is sensitive to the trilinear coupling at the
one-loop level. At the same perturbative order, potential effects beyond the
Standard Model (SM) may affect the Higgstrahlung rate and distort the
self-coupling determination. We study the question focusing especially on
contact interactions containing two electron and two top-quark fields. We
conclude that, in the context of FCC-ee and its planned runs at different
energies, $eett$ interactions change the Higgs self-coupling sensitivity below
the percent level. Even in the most pessimistic scenarios, we confirm a robust
sensitivity of the order of 17% at the 1$\sigma$ confidence level under the
assumption of otherwise SM-like Higgs couplings. A crucial role in these
results is played by the measurement of fermion pair production above the $Z$
resonance.