Observation of a pseudoscalar excess at the top quark pair production threshold

Abstract: A search for resonances in top quark pair ($\text{t}\bar{\text{t}}$) production in final states with two charged leptons and multiple jets is presented, based on proton-proton collision data collected by the CMS experiment at the CERN LHC at $\sqrt{s}$ = 13 TeV, corresponding to 138 fb$^{-1}$. The analysis explores the invariant mass of the \ttbar system and two angular observables that provide direct access to the correlation of top quark and antiquark spins. A significant excess of events is observed near the kinematic $\text{t}\bar{\text{t}}$ threshold compared to the nonresonant production predicted by fixed-order perturbative quantum chromodynamics (pQCD). The observed enhancement is consistent with the production of a color-singlet pseudoscalar ($^1$S$^{[1]}_0$) quasi-bound toponium state, as predicted by nonrelativistic quantum chromodynamics. Using a simplified model for $^1$S$^{[1]}_0$ toponium, the cross section of the excess above the pQCD prediction is measured to be 8.8$^{+1.2}_{-1.4}$ pb.

Observation of a Pseudoscalar Excess at the Top Quark Pair Production Threshold

The paper "Observation of a Pseudoscalar Excess at the Top Quark Pair Production Threshold" presents a detailed analysis of top quark pair ($t\bar{t}$) production using collision data gathered by the CMS experiment at CERN's LHC. Operating at a center-of-mass energy of $\sqrt{s} = 13 \, \text{TeV}$, the study investigates over 138 $\text{fb}^{-1}$ of integrated luminosity collected between 2016 and 2018. The analysis employs final states comprising two charged leptons and multiple jets to probe potential resonances in $t\bar{t}$ production.

Key Findings

• Pseudoscalar Excess: The study reports a significant excess of events near the $t\bar{t}$ production threshold that challenges nonresonant production predictions from fixed-order perturbative Quantum Chromodynamics (pQCD). This excess is consistent with the hypothesis of producing a color-singlet pseudoscalar toponium state, as theorized by nonrelativistic Quantum Chromodynamics (NRQCD).
• Measured Cross Section: Using a simplified toponium model, the excess's cross section is quantified, showing a marked deviation from the pQCD baseline.
• Spin Correlation Observables: The analysis leverages angular observables that reveal the $t\bar{t}$ spin correlation, particularly with enhanced sensitivity near the kinematic threshold. These observables suggest a distinctive spin-correlation pattern supporting the presence of a pseudoscalar state.

Methodology

The CMS detector's capabilities allow precise reconstruction of events, utilizing sophisticated algorithms for electron, muon, photon, and jet identification. Events are reconstructed using a strategy that aligns with expected $t\bar{t}$ decay products, facilitating accurate mass and angular observable measurements. The study employs a comprehensive MC simulation, integrating both NLO and NNLO modeling techniques for cross-validation against observed data.

Statistical and Systematic Considerations

The study applies a 3D maximum likelihood fit to assess the compatibility of data with theoretical models, addressing both the statistical and systematic uncertainties inherent in collider data analysis. The fit explores angular distributions and $t\bar{t}$ invariant mass, leveraging these dimensions to isolate the pseudoscalar signal despite underlying model uncertainties, such as parton distribution functions and higher-order corrections.

Implications and Future Directions

The observation indicates potential new dynamics in $t\bar{t}$ production around the threshold, offering insights into beyond-Standard Model physics that could include new pseudoscalar particles. The results, while compelling, necessitate further theoretical scrutiny and more granular experimental validation. The findings add a layer of complexity to our understanding of quantum chromodynamics, encouraging enhanced theoretical frameworks and potential explorations into alternative physics models, such as extended Higgs sectors or other exotic particles.

In conclusion, the research significantly enhances our understanding of top quark pair production dynamics. It highlights the synergy between advanced experimental techniques and rigorous theoretical models, emphasizing the need for continued research into the fundamental interactions governing particle physics at unprecedented energy scales. As the study paves the way for future investigations, it challenges us to refine our models and confront the possible avenues new physics might present.