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An insight into the rare $Z\rightarrow b \bar{b}γ$ at the HL-LHC

T. Thallapalli, A. Alpana, A. M. Iyer, S. Sharma

TL;DR

This work investigates the HL-LHC’s ability to probe the rare decay Z -> Φ γ with Φ -> bb, considering Φ as a spin-0 or spin-2 state with mass below mZ. It develops a universal collider strategy focusing on two low-$p_T$ $b$-tagged jets and an isolated photon, and compares Boosted Decision Trees and Graph Neural Networks for discriminating signal from background. Across five benchmark Φ masses, the study derives upper bounds on BR(Z -> γ Φ), with the strongest reach near mΦ ≈ 60 GeV (~10^-5 at 68–95% CL), underscoring the importance of improving soft $b$-tagging. The results demonstrate robust ML performance across architectures and outline practical paths—enhancing low-$p_T$ $b$-tagging and mass-specific selections—for tighter constraints at the HL-LHC and future colliders.

Abstract

Studies at the $Z$-pole have played an important role in developing our understanding of the Standard Model (SM). Continuing the explorations in this regime, we consider the possibility of the production of two $b$-quarks and a photon in proton-proton collisions at the HL-LHC. While such a final state is possible in the SM by means of the process $Z\rightarrow b\bar b$ decay with a radiated photon, the focus is on extracting its possible origins due to beyond Standard Model (BSM) physics. The signal topology can be broadly identified as $Z\rightarrow Φγ\rightarrow b\bar bγ$, where $ Φ$ can either be a spin-0 or spin-2 state with a mass less than that of the $Z$ boson.The analysis is characterised by two relatively low $p_T$ jets that are required to be $b$-tagged jets and an isolated photon. The study provides a quantitative framework for their identification and highlights the potential challenges associated with this final state. A range of machine learning architectures is employed to demonstrate the stability and reliability of the discriminating variables. This study highlights the importance of the low-$p_T$ objects in searches at the HL-LHC, hence the need to pay special attention to their identification and efficiencies.

An insight into the rare $Z\rightarrow b \bar{b}γ$ at the HL-LHC

TL;DR

This work investigates the HL-LHC’s ability to probe the rare decay Z -> Φ γ with Φ -> bb, considering Φ as a spin-0 or spin-2 state with mass below mZ. It develops a universal collider strategy focusing on two low- -tagged jets and an isolated photon, and compares Boosted Decision Trees and Graph Neural Networks for discriminating signal from background. Across five benchmark Φ masses, the study derives upper bounds on BR(Z -> γ Φ), with the strongest reach near mΦ ≈ 60 GeV (~10^-5 at 68–95% CL), underscoring the importance of improving soft -tagging. The results demonstrate robust ML performance across architectures and outline practical paths—enhancing low- -tagging and mass-specific selections—for tighter constraints at the HL-LHC and future colliders.

Abstract

Studies at the -pole have played an important role in developing our understanding of the Standard Model (SM). Continuing the explorations in this regime, we consider the possibility of the production of two -quarks and a photon in proton-proton collisions at the HL-LHC. While such a final state is possible in the SM by means of the process decay with a radiated photon, the focus is on extracting its possible origins due to beyond Standard Model (BSM) physics. The signal topology can be broadly identified as , where can either be a spin-0 or spin-2 state with a mass less than that of the boson.The analysis is characterised by two relatively low jets that are required to be -tagged jets and an isolated photon. The study provides a quantitative framework for their identification and highlights the potential challenges associated with this final state. A range of machine learning architectures is employed to demonstrate the stability and reliability of the discriminating variables. This study highlights the importance of the low- objects in searches at the HL-LHC, hence the need to pay special attention to their identification and efficiencies.
Paper Structure (8 sections, 6 equations, 14 figures)

This paper contains 8 sections, 6 equations, 14 figures.

Table of Contents

  1. Introduction
  2. Theoretical Origins
  3. Discriminants and Efficiencies
  4. Classification Schemes
  5. Results and Discussions
  6. Conclusions
  7. Impact of choice of threshold
  8. Comparison of distributions for the true positives and false negatives for $m_{\Phi}=60$ GeV

Figures (14)

  • Figure 1: Signal and background Feynman diagrams.
  • Figure 2: $\Delta\eta$ distribution between the two $b$-quarks (left), $\Phi$ and $\gamma$ (right) for both the spin-0 and spin-2 hypothesis.
  • Figure 3: Transverse momentum of isolated photon across different masses (left) and efficiency due to requiring photon $p_T > 10$ GeV (right)
  • Figure 4: Efficiencies of the events with at least 2 b-tagged jets (left), at least 2 b-tagged jets and at least 1 photon (right)
  • Figure 5: Distribution of the different variables for the background (orange) and different signal benchmarks
  • ...and 9 more figures