Indirect measurement of triple-Higgs coupling at an electron-positron collider with polarized beams

Abstract

We examine the possibility of using single-Higgs production at an $e^+e^-$ collider with polarized beams to measure, or constrain, indirectly a possible anomalous triple-Higgs coupling, which can contribute to the process via one-loop diagrams. In the dominant process $e^+e^- \to ZH$, longitudinally polarized beams can lead to an improvement in the cross section by 50\% for $e^-$ and $e^+$ polarizations of $-0.8$ and $+0.3$, respectively. This corresponds to an improvement in the sensitivity to the triple-Higgs coupling of about 18\% for a centre-of-mass energy of 250 GeV and an integrated luminosity of 2 ab$^{-1}$, making a strong case of beam polarization. This also implies that with polarized beams, the luminosity needed to get a particular sensitivity is less by about 33\% as compared to that needed with unpolarized beams. Even when only the $e^-$ beam is polarized $-0.8$, the improvement in the sensitivity is about 8\%. We also study the effect of longitudinal beam polarization on the sensitivity to the triple-Higgs coupling of Higgs production through the subdominant process $e^+e^- \to H \nu \bar \nu$ occurring through $WW$ fusion.