[Paper Review] Search for Higgs Boson Decay to a Charm Quark-Antiquark Pair in Proton-Proton Collisions at $\sqrt{s}$=13 TeV

CMS Collaboration, Adam, Wolfgang+2378 more|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies13 citations

TL;DR

This paper presents a search for Higgs boson decay to a charm quark-antiquark pair (H → cc) in proton-proton collisions at √s = 13 TeV using 138 fb⁻¹ of data from the CMS experiment. Employing advanced machine learning-based charm jet identification and novel analysis techniques, the study sets the most stringent limit to date on the Higgs-charm Yukawa coupling, with observed (expected) 95% CL upper limits of |κc| < 5.5 (|κc| < 3.4), corresponding to 14 (7.6+3.4−2.3) times the SM prediction.

ABSTRACT

A search for the standard model Higgs boson decaying to a charm quark-antiquark pair, H $ o \mathrm{c\bar{c}}$, produced in association with a leptonically decaying V (W or Z) boson is presented. The search is performed with proton-proton collisions at $\sqrt{s}$ = 13 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 138 fb$^{-1}$. Novel charm jet identification and analysis methods using machine learning techniques are employed. The analysis is validated by searching for Z $ o \mathrm{c\bar{c}}$ in VZ events, leading to its first observation at a hadron collider with a significance of 5.7 standard deviations. The observed (expected) upper limit on $\sigma$(VH)$\mathcal{B}$(H$ o \mathrm{c\bar{c}}$) is 0.94 (0.50$^{+0.22}_{-0.15}$) pb at 95% confidence level (CL), corresponding to 14 (7.6$^{+3.4}_{-2.3}$) times the standard model prediction. For the Higgs-charm Yukawa coupling modifier, $\kappa_\mathrm{c}$, the observed (expected) 95% CL interval is 1.1 $\lt\vert\kappa_\mathrm{C}\vert \lt$ 5.5 ($\vert\kappa_\mathrm{c}\vert\lt$ 3.4), the most stringent constraint to date.

Motivation & Objective

• To search for the Standard Model Higgs boson decay into a charm quark-antiquark pair (H → cc) in VH associated production at the LHC.
• To overcome the challenges of identifying charm jets in dense QCD environments and distinguishing them from bottom quark jets.
• To improve sensitivity using machine learning-based jet reconstruction and identification techniques.
• To set the most stringent experimental constraints on the Higgs-charm Yukawa coupling modifier, κc, beyond the Standard Model.

Proposed method

• Utilizes 138 fb⁻¹ of proton-proton collision data at √s = 13 TeV collected by the CMS detector.
• Employs two jet types: small-R (R = 0.4) and large-R (R = 1.5) jets, with pileup mitigation via charged hadron subtraction and PUPPI algorithms.
• Applies a graph neural network (PARTICLENET) to improve large-R jet mass resolution by ~50% using PF candidates and secondary vertices.
• Uses machine learning-based charm jet identification to enhance signal sensitivity in merged-jet and resolved-jet topologies.
• Analyzes three mutually exclusive channels: 0L (Z → νν), 1L (W → ℓν), and 2L (Z → ℓℓ), with event selection based on triggers and isolation criteria.
• Performs signal modeling using NLO and NNLO QCD + NLO EW accurate event generators, with detector response simulated via GEANT4.

Experimental results

Research questions

• RQ1 What is the sensitivity of the CMS experiment to Higgs boson decay into charm quarks (H → cc) in VH associated production at 13 TeV?
• RQ2 How effective are machine learning-based jet identification techniques in distinguishing charm jets from light and bottom quark jets in high-multiplicity environments?
• RQ3 What is the observed upper limit on the product of the production cross section σ(VH) and branching fraction B(H → cc) at 95% confidence level?
• RQ4 What constraint does this analysis place on the Higgs-charm Yukawa coupling modifier, κc, compared to the Standard Model prediction?
• RQ5 Can the Z → cc decay mode be observed in VZ events at a hadron collider using advanced jet reconstruction and identification?

Key findings

• The Z → cc decay mode is observed in VZ events for the first time at a hadron collider with a significance of 5.7 standard deviations.
• The observed (expected) upper limit on σ(VH) × B(H → cc) is 0.94 (0.50+0.22−0.15) pb at 95% confidence level.
• The observed (expected) 95% CL interval for the Higgs-charm Yukawa coupling modifier is 1.1 < |κc| < 5.5 (|κc| < 3.4), representing the most stringent constraint to date.
• The analysis improves large-R jet mass resolution by approximately 50% using a PARTICLENET-based regression algorithm compared to traditional grooming techniques.
• The merged-jet topology, though contributing less than 5% of the signal cross section, provides significant sensitivity due to enhanced reconstruction efficiency for boosted Higgs bosons.
• The study sets a new benchmark in sensitivity for H → cc searches, reducing the upper limit on the signal strength relative to the SM by a factor of ~7.6 in the expected case.