Search for Higgs Boson and Observation of $Z$ Boson through Their Decay into a Charm Quark-Antiquark Pair in Boosted Topologies in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

A search for the standard model (SM) Higgs boson ($H$) produced with transverse momentum ($p_T$) greater than 450 GeV and decaying to a charm quark-antiquark ($c\overline{c}$) pair is presented. The search is performed using proton-proton collision data collected at $\sqrt{s}=13\mathrm{TeV}$ by the CMS experiment at the LHC, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$. Boosted $H\to c\overline{c}$ decay products are reconstructed as a single large-radius jet and identified using a deep neural network charm tagging technique. The method is validated by measuring the $Z\to c\overline{c}$ decay process, which is observed in association with jets at high $p_T$ for the first time with a signal strength of ${1.00}_{-0.14}^{+0.17}(\mathrm{syst})\pm 0.08(\mathrm{theo})\pm 0.06(\mathrm{stat})$, defined as the ratio of the observed process rate to the SM expectation. The observed (expected) upper limit on $\sigma (H)\mathcal{B}(H\to c\overline{c})$ is set at 47 (39) times the SM prediction at 95% confidence level.

Figure 1

The DDCvL and DDCvB performance for $H\to c\overline{c}$ identification versus QCD and $H\to b\overline{b}$ processes, respectively. No selection apart from the displayed $m_{\mathrm{SD}}$ and $p_T$ requirements is applied. The working points used in this search are marked with a cross. The AUC is the area-under-curve metric.

Figure 2

The observed and fitted $m_{\mathrm{SD}}$ distributions for the passing (left) and failing (right) regions, combining all $p_T$ categories and the three data-taking years. The fit is performed under the signal-plus-background hypothesis with a single inclusive $H\to c\overline{c}$ signal strength parameter. The $t\overline{t}$ background yields and the QCD background yields and shapes are estimated from data. The $t\overline{t}$ process constitutes the majority of contributions labeled “other.” The dashed line represents the $H\to c\overline{c}$ expectation, multiplied by a factor of 200. The steplike features at 166 and $180\mathrm{GeV}$ are due to the $\rho$ acceptance upper bound, which excludes bins with high $m_{\mathrm{SD}}$ and low $p_T$ from the analysis. The lower panel shows the residual difference between the data and the overall background (excluding $Z\to c\overline{c}$), divided by the statistical uncertainty in the data. The near perfect model agreement with data in the failing region (right) is by construction.