Recent results on heavy resonances at CMS

The Standard Model (SM) of particle physics provides a successful description of elementary particles and their interactions. However, it does not explain phenomena such as the hierarchy problem or the nature of dark matter. Many Beyond the Standard Model (BSM) theories provide answers to these open questions by adding new heavy resonances, which can be probed directly at colliders. This note summarizes recent resonance searches by the CMS Collaboration, focusing on final states containing top quarks and Higgs bosons.

💡 Research Summary

The note summarizes the most recent heavy‑resonance searches performed by the CMS Collaboration using the full Run 2 dataset (≈138 fb⁻¹ of 13 TeV proton‑proton collisions). The focus is on final states that contain top quarks or Higgs bosons, which are key signatures for many Beyond‑the‑Standard‑Model (BSM) scenarios such as two‑Higgs‑doublet models, Randall‑Sundrum extra dimensions, dark‑matter mediators, and generic spin‑0 or spin‑1 resonances.

Three complementary t t̄‑based analyses are presented. In the single‑lepton channel, events with one isolated electron or muon, missing transverse momentum and at least two jets are selected. Both resolved and boosted topologies are considered, with DeepJet and DeepAK8 used for b‑tagging and large‑R top‑tagging. The angular variable cos θ* (the cosine of the angle between the leptonically‑decaying top and the t t̄ system) exploits spin‑correlation differences between SM t t̄ production and BSM signals. No excess is observed; Z′ bosons with widths of 1 %, 10 % and 30 % are excluded below 4.3 TeV, 5.3 TeV and 6.7 TeV respectively, Randall‑Sundrum Kaluza‑Klein gluons below 4.7 TeV, and dark‑matter mediators below 3.2 TeV.

The fully hadronic channel reconstructs both tops as large‑R jets (p_T > 400 GeV) and uses the DeepAK8 tagger. The dominant multijet background is estimated with an ABCD method in the (m_top, m_t̄) plane. Limits improve to 5 TeV for KK gluons, 4.49–6.85 TeV for Z′ (depending on width) and 3.9 TeV for dark‑matter mediators.

A dedicated search for Z′ produced in association with a t t̄ pair (four‑top final state) uses ParticleNet‑tagged large‑R jets for the Z′ candidate and the single‑lepton reconstruction for the associated tops. For width‑to‑mass ratios of 10 %, 20 % and 50 %, Z′ masses below 564 GeV, 849 GeV and 1.125 TeV are excluded at 95 % CL, providing the most stringent constraints on a Z′ that couples exclusively to top quarks.

In the Higgs‑boson sector, a search for charged Higgs bosons (H^± → t b) employs a parametric deep neural network to discriminate signal from background across several jet‑multiplicity and b‑tag categories. Masses between 0.5 and 1 TeV are probed for scalar and pseudoscalar bosons with relative widths of 2.5–25 %. A mild local excess near 600 GeV (2.4 σ local, 0.1 σ global) is observed but is not statistically significant.

The photon‑plus‑H/Z searches target resonances decaying to a high‑p_T photon and a boosted H → b b̄ or Z → b b̄ system reconstructed as a single large‑R jet. ParticleNet mass‑decorrelated jet mass and the Particle Transformer tagger are used to separate H/Z from QCD jets. Backgrounds from γ+jets and γ+V are modeled with data‑driven parametric fits. For narrow (≈1 %) and wider (up to 5 %) scalar resonances, limits are set over the 0.7–3.5 TeV mass range. Z′ → Hγ is excluded up to 3.5 TeV and a scalar S → Zγ up to 1.1 TeV, improving previous CMS limits by factors of six to ten.

Overall, no significant deviation from the Standard Model is found. The analyses set the most stringent limits to date on a broad class of heavy resonances: Z′ bosons up to 6.85 TeV (depending on width), Kaluza‑Klein gluons up to 5 TeV, dark‑matter mediators up to 3.9 TeV, scalar/pseudoscalar Higgs bosons up to 1 TeV, charged Higgs bosons up to 1 TeV (with a small excess at 600 GeV), and photon‑plus‑H/Z resonances up to 3.5 TeV. These results significantly advance the sensitivity of LHC searches for new physics and provide a solid benchmark for future high‑luminosity runs.