Search for $H\to c\bar{c}$ and measurement of $H\to b\bar{b}$ via $t\bar{t}H$ production

A search is presented for Higgs boson production in association with a top quark-antiquark pair ($t\bar{t}H$) with the Higgs boson decaying to a charm quark-antiquark pair ($H\to c\bar{c}$). The same process with a Higgs boson decay to bottom quarks, $t\bar{t}H(H\to b\bar{b})$, is measured simultaneously. The analysis uses data from proton-proton collisions at 13 TeV collected with the CMS detector in 2016-2018, corresponding to an integrated luminosity of 136 fb${}^{-1}$. The observed $t\bar{t}H(H\to \to b\bar{b}$ cross section relative to its prediction from the standard model (SM) is $0.91{}_{-0.22}^{+0.26}$. The observed (expected) results are compatible at the 95% confidence level with a $t\bar{t}H(H\to c\bar{c})$ cross section that is at most 7.8 (8.7) times larger than the SM expectation. Assuming all other Higgs boson couplings to be SM-like, this sets an observed (expected) upper bound on the charm quark Yukawa coupling modifier of 3.0 (3.3) times its SM value.