Charming Higgs bosons: constraint on the Higgs-charm coupling from a search for Higgs boson decays to charm quarks with the ATLAS detector

A search for a Standard Model Higgs boson decaying to a pair of charm quarks is presented. The analysis focuses on the associated production with a W or Z boson performed by the ATLAS collaboration using data collected between 2015 and 2018. Flavour tagging algorithms are used to identify jets originating from the hadronisation of charm quarks. The analysis method is validated with the simultaneous measurement of the WW, WZ and ZZ production, with observed (expected) significances of 2.6 (2.2) standard deviations for the Z decay to a pair of charm quarks and 3.8 (4.6) standard deviations for the W decay to a charm quark and a lighter quark. The search for the Higgs boson decay to a pair of charm quarks yields an observed (expected) limit of 26 (31) times the predicted cross-section times branching ratio, corresponding to an observed (expected) constraint on the charm Yukawa coupling modifier |𝜅c| < 8.5 (12.4), at 95% confidence level. A combination with the ATLAS measurement of the Higgs boson decay to a pair of bottom quarks is performed, allowing a constraint to be set on the ratio 𝜅c / 𝜅b without assumptions on the width of the Higgs boson. The ratio is constrained to be |𝜅c / 𝜅b| < 4.5 at 95% confidence level, smaller than the ratio of the bottom and charm quark masses and therefore determines the Higgs-charm coupling to be weaker than the Higgs-bottom quark coupling at 95% confidence level.