[Paper Review] Determination of the muon Yukawa coupling at high energy e+e- linear colliders

M. Battaglia, A. De Roeck|arXiv (Cornell University)|Nov 23, 2001

Particle physics theoretical and experimental studies6 citations

TL;DR

This paper proposes measuring the muon Yukawa coupling via the rare Higgs decay $H^0 \to \mu^+\mu^-$ at high-energy $e^+e^-$ linear colliders. Using Higgs-strahlung and $WW$ fusion processes at $\sqrt{s} = 0.8$ TeV and $3.0$ TeV, with 1 ab$^{-1}$ and 5 ab$^{-1}$ luminosity, respectively, the study demonstrates $5\sigma$ signal significance for $M_H \leq 155$ GeV and achieves 4%–11% accuracy in determining $g_{H\mu\mu}$, confirming Higgs-mediated mass generation for leptons.

ABSTRACT

The perspectives for the observation of the rare decay H -> mumu decay and the determination of the muon Yukawa coupling at a TeV-class and at a multi-TeV e+e- linear colliders are discussed. The signal for the decay can be obtained at 0.8 TeV and a first estimate of the coupling derived. A linear collider operating at 3 TeV, with high luminosity, is able to improve the accuracy on this couplings to 4% to 11% for 120 GeV < M_H < 150 GeV.

Motivation & Objective

• To test the Higgs mechanism of mass generation by measuring the muon Yukawa coupling $g_{H\mu\mu}$.
• To determine the branching fraction of the rare Higgs decay $H^0 \to \mu^+\mu^-$ at high-energy $e^+e^-$ linear colliders.
• To extend the precision test of Higgs couplings to the lepton sector, verifying scaling with fermion mass.
• To compare the performance of TeV-class and multi-TeV $e^+e^-$ colliders for this measurement.
• To assess the feasibility of observing the $H^0 \to \mu^+\mu^-$ decay against dominant backgrounds.

Proposed method

• Signal events are generated using Pythia-6 Monte Carlo for $H^0 \to \mu^+\mu^-$ in $WW$ fusion and Higgs-strahlung processes.
• Background processes, including $WW \to \mu\nu\mu\nu$, $ZZ \to \mu\mu\nu\bar{\nu}$, and $\mu\mu\nu\bar{\nu}$, are simulated using CompHEP and Pythia.
• Detector response is modeled using the Simdet parametric simulation tuned to TeV and multi-TeV LC expectations, with $\delta p/p^2 = 5 \times 10^{-5}$ GeV$^{-1}$ muon momentum resolution.
• Signal and background cross sections are computed using CompHEP, and branching fractions are calculated with HDecay.
• The di-muon invariant mass distribution is fitted with a Gaussian signal and polynomial background, enabling extraction of signal events.
• The muon Yukawa coupling is extracted from the measured product $\sigma(e^+e^- \to H\nu\bar{\nu}) \times \text{BR}(H \to \mu^+\mu^-)$, assuming 2% uncertainty in production cross section.

Experimental results

Research questions

• RQ1 Can the rare Higgs decay $H^0 \to \mu^+\mu^-$ be observed at $\sqrt{s} = 0.8$ TeV with 1 ab$^{-1}$ of integrated luminosity?
• RQ2 What is the achievable accuracy in determining the muon Yukawa coupling $g_{H\mu\mu}$ at a 3 TeV $e^+e^-$ linear collider with 5 ab$^{-1}$?
• RQ3 How does the performance of a multi-TeV $e^+e^-$ collider compare to a muon collider or hadron collider for measuring $g_{H\mu\mu}$?
• RQ4 Can the $H^0 \to \mu^+\mu^-$ decay be efficiently reconstructed and separated from backgrounds at high-energy $e^+e^-$ colliders?
• RQ5 What is the maximum Higgs mass for which a $5\sigma$ signal significance can be achieved for $H^0 \to \mu^+\mu^-$ at $\sqrt{s} = 3.0$ TeV?

Key findings

• At $\sqrt{s} = 0.8$ TeV with 1 ab$^{-1}$, the $H^0 \to \mu^+\mu^-$ signal is observed with more than 5$\sigma$ significance for $M_H \simeq 120$ GeV.
• The statistical accuracy on $\sigma(e^+e^- \to H\nu\bar{\nu}) \times \text{BR}(H \to \mu^+\mu^-)$ reaches 7.2% for $M_H = 120$ GeV at $\sqrt{s} = 3.0$ TeV with 5 ab$^{-1}$.
• The accuracy on the muon Yukawa coupling $g_{H\mu\mu}$ improves to 4.0% for $M_H = 120$ GeV and 6.2% for $M_H = 140$ GeV at $\sqrt{s} = 3.0$ TeV with 5 ab$^{-1}$.
• For $M_H = 150$ GeV, the accuracy on $g_{H\mu\mu}$ is 10.6%, indicating a performance limit near the $WW$ threshold.
• The $H^0 \to \mu^+\mu^-$ signal is visible up to $M_H \simeq 155$ GeV at $\sqrt{s} = 3.0$ TeV with 5 ab$^{-1}$, maintaining $>5\sigma$ significance.
• The accuracy of $g_{H\mu\mu}$ determination at a 3 TeV $e^+e^-$ collider is comparable to that of a muon collider at the Higgs peak and more than twice as good as at a 200 TeV VLHC.