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Probing the Higgs Self-Coupling with an XFEL Compton $γγ$ Collider at $\sqrt{s} = 380$ GeV

Santiago Ampudia Castelazo, Umar Sohail Qureshi, Tim Barklow, Ariel Schwartzman

Abstract

We present a study probing the Higgs self-coupling with the X-ray free-electron laser Compton $γγ$ Collider (XCC) concept. The analysis is performed considering the $γγ\to HH \to bb\overline{bb}$ channel, and results are then extrapolated to obtain a projection on the Higgs self-coupling sensitivity that ranges between 7% and 12%. An ensemble of boosted decision trees is trained to discriminate between signal and backgrounds, paired with a genetic algorithm optimizer to combine the final classifier outputs. This study suggests that an X-ray FEL-based $γγ$ collider is a powerful tool to probe the mechanism of electroweak symmetry breaking, complementary to $e^+e^-$ Higgs factories and future high-energy hadron colliders.

Probing the Higgs Self-Coupling with an XFEL Compton $γγ$ Collider at $\sqrt{s} = 380$ GeV

Abstract

We present a study probing the Higgs self-coupling with the X-ray free-electron laser Compton Collider (XCC) concept. The analysis is performed considering the channel, and results are then extrapolated to obtain a projection on the Higgs self-coupling sensitivity that ranges between 7% and 12%. An ensemble of boosted decision trees is trained to discriminate between signal and backgrounds, paired with a genetic algorithm optimizer to combine the final classifier outputs. This study suggests that an X-ray FEL-based collider is a powerful tool to probe the mechanism of electroweak symmetry breaking, complementary to Higgs factories and future high-energy hadron colliders.
Paper Structure (14 sections, 6 equations, 18 figures, 5 tables)

This paper contains 14 sections, 6 equations, 18 figures, 5 tables.

Table of Contents

  1. Introduction
  2. The XFEL Compton Photon-Photon Collider
  3. Higgs Self-coupling Measurement at an XFEL Photon-Photon Collider
  4. Monte Carlo Samples and Event Reconstruction
  5. Event Generation
  6. Signal and Background Processes
  7. Detector Simulation and Event Reconstruction
  8. Analysis Strategy
  9. Results
  10. Conclusions
  11. Pileup Mitigation with a Transformer Neural Network
  12. Samples and Simulation
  13. Transformer Architecture
  14. Results

Figures (18)

  • Figure 1: Schematic of XCC including cryogenic RF injector, Linac, electron beam final focus (FF), and XFEL.
  • Figure 2: Close-up of the IP, illustrating the Compton process using only one side of the collider. The diagram can be mirrored to obtain the other side.
  • Figure 3: Luminosity spectra for the two most important initial state particle combinations: $\gamma\gamma$ collisions (blue) and $e^+e^-$ collisions (gold) for the center-of-mass energy range $150\ \mathrm{GeV} < \sqrt{\widehat{s}} < 380\ \mathrm{ GeV}$. Events due to $e^-\gamma$ and $e^-e^-$ collisions make up only 1.5% of the total background after the final signal-background separation.
  • Figure 4: Luminosity spectrum for collisions of $\gamma\gamma$ (blue), $e^-\gamma$ (red) $e^+e^-$ (gold) and $e^-e^-$ (green) for the full center-of-mass energy range $0\ \mathrm{GeV} < \sqrt{\widehat{s}} < 380\ \mathrm{ GeV}$.
  • Figure 5: Feynman diagrams for di-Higgs production at XCC at $\sqrt{s} = 380$ GeV and linear $e^+e^-$ colliders at $\sqrt{s} = 500$ GeV. Left: Tree-level $e^+e^- \to ZHH$ process typical of linear colliders such as the ILC, which includes an on-shell $Z$ boson in the final state. Right: Loop-level $\gamma\gamma \to HH$ process at the XCC, mediated by virtual $W$ bosons and top quarks.
  • ...and 13 more figures