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

TL;DR

The paper targets the charm Yukawa coupling by probing Higgs production in association with a top-quark pair, via $t\bar{t}H$ with $H\rightarrow c\bar{c}$ and $H\rightarrow b\bar{b}$. It leverages CMS data at $\sqrt{s}=13$ TeV and employs advanced jet-flavour tagging using ParticleNet and a transformer-based classifier ParT to separate signal from $t\bar{t}$+jets in the fully hadronic, semi-leptonic, and dileptonic channels. A binned likelihood fit to ParT discriminants yields a SM-consistent $t\bar{t}H(H\rightarrow b\bar{b})$ signal strength of $\mu=0.91^{+0.26}_{-0.22}$ and sets a 95% CL upper limit on $t\bar{t}H(H\rightarrow c\bar{c})$ of $7.8$ times SM (8.7 expected), corresponding to $\kappa_c<3.0$ (3.3). Interpreting the results in the $\kappa$-framework and combining with CMS results tightens the charm Yukawa constraint to $\kappa_c<3.5$ (2.7), advancing the search for second-generation Higgs couplings with top-associated production.

Abstract

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 ($H\rightarrow c\bar{c}$). The same process with a Higgs boson decay to bottom quarks, $t\bar{t}H(H\rightarrow b\bar{b})$, 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 $t\bar{t}H(H\rightarrow\rightarrow b\bar{b}$ cross section relative to its prediction from the standard model (SM) is $0.91\,^{+0.26}_{-0.22}$. The observed (expected) results are compatible at the 95% confidence level with a $t\bar{t}H(H\rightarrow c\bar{c})$ 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.

Figures (2)

• Figure 1: Jet flavour categories derived from two ParticleNet scores, separating charm- from bottom-flavoured jets (vertically) and heavy from light jets (horizontally). The contours represent the distributions of the differently flavoured jets in the plane. CMS:2025dsh
• Figure 2: Left: measured and expected $\mathup{{{t}}{} _{ {}} ^{ {}}}$$\mathup{{ \overline{ {{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}}$$\mathup{{{H}}{} _{ {}} ^{ {}}}$ (${ \mathup{{{H}}{} _{ {}} ^{ {}}} }\xspace{}\rightarrow { \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace { \mathup{{ \overline{ {{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace$) signal strengths in the three channels and combined. Right: likelihood scan of the Yukawa coupling modifier $\kappa_c$ for fixed or floating $\kappa_b$. CMS:2025dsh