Search for single production of a vector-like T quark decaying to a top quark and a neutral scalar boson in the lepton+jets final state in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Technical Summary: Search for Single Production of a Vector-Like T Quark Decaying to a Top Quark and a Neutral Scalar Boson

Problem and Motivation
This paper presents a search for the single production of a vector-like quark (VLQ) with charge $2e/3$, denoted as $T$, decaying into a top quark ($t$) and a neutral scalar boson ($\phi$). The scalar boson is considered either as the Standard Model (SM) Higgs boson ($H$) or a new beyond-the-SM scalar boson ($\phi$). While precision measurements constrain the existence of a fourth generation of chiral quarks, vector-like quarks remain a viable prediction of various Standard Model extensions addressing the hierarchy problem. Unlike chiral quarks, VLQs acquire mass independently of Yukawa couplings. Previous searches have largely focused on pair production or single production in decay channels involving $Wb$, $Zt$, or $tH$ (with $H \to$ diphotons or fully hadronic). This analysis targets the specific decay topology $T \to t\phi$ (where $\phi \to b\bar{b}$) in the lepton+jets final state, a channel previously unexplored for single $T$ production into a new neutral scalar.

Methodology
The analysis utilizes proton-proton collision data at a center-of-mass energy of $\sqrt{s} = 13$ TeV collected by the CMS experiment during 2016–2018, corresponding to an integrated luminosity of 138 fb$^{-1}$.

• Signal Topology: The signal is characterized by a single $T$ quark produced in association with a bottom quark ($bTq$ production). The $T$ decays to $t\phi$, where the top quark decays leptonically ($t \to Wb \to \ell\nu b$) and the scalar boson decays to a bottom-antibottom pair ($\phi \to b\bar{b}$). The final state consists of one isolated lepton ($e$ or $\mu$), missing transverse momentum ($p_T^{miss}$), and multiple jets, including a forward light-flavor jet and a boosted $b\bar{b}$ system.
• Event Reconstruction:
  • Scalar Boson ($\phi$): Reconstructed as a large-radius jet (AK8, $R=0.8$) with $p_T > 500$ GeV. A mass-decorrelated (MD) deep neural network, PARTICLENET-MD, is employed to tag jets originating from $X \to b\bar{b}$ decays ($Xbb$ tagger).
  • Top Quark: Reconstructed from a lepton, an AK4 jet, and $p_T^{miss}$. A machine-learning approach using a multiclass Boosted Decision Tree (BDT) is developed to distinguish true top quark candidates ($t_{true}$) from combinatorial background ($t_{QCD}$ and $t_{oth}$). Separate BDTs are trained for merged and resolved configurations, lepton flavors, and $p_T$ ranges.
• Event Categorization: Selected events are classified into mutually exclusive regions based on the quality of top quark and $Xbb$ tagging:
  • Signal Regions (SRs): Events with tight top tagging (TopT) and tight $Xbb$ tagging (XbbT), or loose top tagging (TopL) and tight $Xbb$ tagging.
  • Validation Regions (VRs): Used to verify data-background agreement (e.g., TopT/XbbL, TopL/XbbL).
  • Control Regions (CRs): Used to calibrate tagging efficiencies and mistag rates (e.g., TopT/XbbV, LepT/XbbT).
• Statistical Analysis: A simultaneous maximum likelihood fit is performed on the invariant mass distribution of the reconstructed $T$ quark candidate ($M_{\ell\nu bJ}$) across the SRs and VRs. Systematic uncertainties (theoretical and experimental) are treated as nuisance parameters. The $CL_s$ method is used to set upper limits.

Key Contributions

• Novel Decay Channel: This work provides the first exclusion limits on the single production of a $T$ quark decaying into a top quark and a new neutral scalar boson ($T \to t\phi$).
• Optimized Reconstruction: The analysis introduces a specialized machine-learning-based top quark tagging algorithm (BDT) optimized for boosted topologies, significantly improving identification efficiency over cut-based methods (approx. 30% relative improvement at 6% background misidentification).
• Comprehensive Mass Scan: The search covers a wide range of $T$ quark masses ($1.3$–$3.0$ TeV) and scalar boson masses ($25$–$250$ GeV).
• SM Higgs Interpretation: The analysis also interprets results for the $T \to tH$ channel, providing updated constraints on this specific decay mode.

Results
No significant excess of events over the expected Standard Model background was observed. Consequently, 95% confidence level (CL) upper limits were set on the product of the production cross section and branching fraction ($\sigma \times \mathcal{B}$).

• $T \to t\phi$ Channel: For a new scalar boson $\phi$, the observed limits on $\sigma \times \mathcal{B}(T \to t\phi \to t b\bar{b})$ vary between 14.7 fb and 0.1 fb for $T$ masses between 1.3 and 3.0 TeV, depending on the $\phi$ mass. Specifically, for $m_\phi = 25$ GeV, values greater than 2.3 to 0.1 fb are excluded for $m_T \in [1.0, 3.0]$ TeV. For $m_\phi = 250$ GeV, values greater than 14.7 to 0.3 fb are excluded for $m_T \in [1.3, 3.0]$ TeV.
• $T \to tH$ Channel: For the decay into the SM Higgs boson, values greater than 100 to 1.0 fb are excluded at 95% CL for $m_T$ between 1.0 and 3.0 TeV.

Significance
The paper claims that these results represent the first exclusion limits set on the production of a single $T$ quark decaying into a top quark and a new neutral scalar boson. Furthermore, for the $T \to tH$ decay channel, the analysis provides the best limits on production cross sections to date for $T$ quark masses above 2 TeV. The results improve upon all previous CMS searches for single $T$ production in the $tH$ channel for $m_T \ge 1.3$ TeV. The analysis demonstrates particular sensitivity to large Lorentz boosts of the decay products, making it effective for high-mass $T$ quark scenarios.