[Paper Review] Measurements of $γγ o \mbox{Higgs}$ and $γγ o W^{+}W^{-}$ in $e^{+}e^{-}$ collisions at the Future Circular Collider

D. d’Enterria, P. Rebello Teles+1 more|arXiv (Cornell University)|Dec 19, 2017

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper investigates the feasibility of measuring two-photon Higgs boson and W⁺W⁻ pair production in e⁺e⁻ collisions at the Future Circular Collider (FCC-ee) using the equivalent photon approximation. By tagging scattered electrons within 1° of the beam using near-beam detectors, the study demonstrates a 5σ observation of γγ → Higgs with ~75 signal events at √s = 240 GeV, and up to 6,600 W⁺W⁻ pairs at 350 GeV, enabling improved constraints on anomalous quartic gauge couplings by factors of 2–10 over current LHC limits.

ABSTRACT

The measurements of the two-photon production of the Higgs boson and of $W^{\pm}$ boson pairs in $e^{+}e^{-}$ collisions at the Future Circular Collider (FCC-ee) are investigated. The processes $e^{+}e^{-}\xrightarrow{γγ}e^+\,{ m H}\,e^-,e^+\,{ m W^+W^-}\,e^-$ are computed using the effective photon approximation for electron-positron beams, and studied in their ${ m H} o b\bar{b}$ and ${ m W^+W^-} o 4j$ decay final-states including parton showering and hadronization, jet reconstruction, $e^\pm$ forward tagging, and realistic experimental cuts. After selection criteria, up to 75 Higgs bosons and 6600 $ m W^{\pm}$ pairs will be reconstructed on top of controllable continuum backgrounds at $\sqrt{s} = $240 and 350 GeV for the total expected integrated luminosities, by tagging the scattered $e^\pm$ with near-beam detectors. A 5$σ$ observation of $γγ o$H is thereby warranted, as well as high-statistics studies of triple $ m γWW$ and quartic $ m γγWW$ electroweak couplings, improving by at least factors of 2 and 10 the current limits on dimension-6 anomalous quartic gauge couplings.

Motivation & Objective

• To assess the feasibility of observing two-photon Higgs boson production in the b¯b decay channel at FCC-ee.
• To study the production of W⁺W⁻ pairs in their fully-hadronic 4-jet final state via γγ fusion at FCC-ee.
• To improve constraints on dimension-6 anomalous quartic gauge couplings (aQGC) beyond current LHC limits.
• To evaluate the impact of electron tagging in near-beam detectors on signal acceptance and background suppression.
• To enable high-statistics studies of γγ → H and γγ → W⁺W⁻ for precision tests of the Standard Model electroweak sector.

Proposed method

• Event generation using superchic 2.04 and pythia 8.226 with exact kinematics for scattered e⁺e⁻ and parton showering/hadronization.
• Application of the equivalent photon approximation (EPA) to model quasireal photon fluxes from e⁺e⁻ beams with Q² < 2 GeV².
• Jet reconstruction using the kT and Durham algorithms with cuts on pT > 10 GeV, |η| < 2.5, and ∆Rjj > 0.4.
• Implementation of kinematical selection criteria including mjj ≈ 76.5–84.5 GeV for W⁺W⁻ → 4j to enhance signal-to-background ratio.
• Use of near-beam e⁺ and e⁻ tagging within 1° of the beam line to identify γγ fusion events and improve acceptance.
• Simulation of realistic detector effects, including mis-tagging efficiencies and jet energy resolution, using pythia 6/8 and fastjet.

Experimental results

Research questions

• RQ1 Can γγ → Higgs production be observed with 5σ significance at FCC-ee using e± tagging and b¯b final states at √s = 240 GeV?
• RQ2 What is the expected event yield for γγ → W⁺W⁻ → 4j at √s = 240 and 350 GeV with realistic detector cuts and e± tagging?
• RQ3 To what extent can the γγ → W⁺W⁻ process improve limits on dimension-6 anomalous quartic gauge couplings compared to LHC results?
• RQ4 How does near-beam e± tagging within 1° affect the acceptance and signal visibility for γγ fusion processes?
• RQ5 What are the dominant backgrounds for γγ → H and γγ → W⁺W⁻ in e⁺e⁻ collisions, and how effectively can they be suppressed?

Key findings

• At √s = 240 GeV with 5 ab⁻¹ of integrated luminosity, approximately 75 Higgs bosons are expected to be reconstructed via γγ → H → b¯b, with a 5σ observation possible after selection.
• At √s = 350 GeV and 5 ab⁻¹, up to 6,600 W⁺W⁻ pairs are expected to be reconstructed in the 4-jet final state after kinematical cuts.
• The signal-to-background ratio for γγ → W⁺W⁻ → 4j improves significantly after applying cuts on jet transverse momentum, pseudorapidity, and dijet invariant mass.
• Near-beam e± tagging within 1° increases acceptance to 95% at √s = 240 GeV and 98.5% at 350 GeV, significantly improving signal visibility.
• The measurement of γγ → W⁺W⁻ at FCC-ee can improve current LHC limits on dimension-6 anomalous quartic gauge couplings by at least factors of 2 and 10 for parameters a₀^W/Λ² and a₀^c/Λ².
• Preliminary studies indicate that the γγ → W⁺W⁻ process at FCC-ee can achieve 1σ and 2σ limits on aQGC parameters of (-0.47; +0.47) × 10⁻⁶ GeV⁻² at √s = 240 GeV with 5 ab⁻¹.