Measuring the Higgs boson's parity using tau --> rho nu

TL;DR

The paper tackles the challenge of determining the CP properties of a light Higgs by exploiting Higgs decays to tau leptons, specifically as tau decays to rho nu and subsequent rho decays to pi pi0. It defines a CP-sensitive observable, phi*, in the rho+ rho- rest frame and enhances sensitivity by grouping events into energy-difference zones based on y1 and y2, using replacement tau rest frames to circumvent Higgs rest-frame reconstruction. Through Monte Carlo studies with TAUOLA and PANDORA/PYTHIA, including detector smearing, the authors show that with 500 fb-1 at a 500 GeV e+e- collider, the CP nature (scalar vs pseudoscalar) of a 120 GeV Higgs can be identified with high confidence (≈95% CL, with many samples above 3σ). The method is model-independent and production-mechanism independent, and the authors discuss potential improvements and applicability to other production modes and decay channels, underscoring the practical relevance for future collider programs.

Abstract

We present a very promising method for a measurement of the Higgs boson parity using the H/A -> tau^+ tau^- --> rho^+ nu rho^- nu --> pi^+ pi^0 nu pi^- pi^0 nu decay chain. The method is both model independent and independent of the Higgs production mechanism. Angular distributions of the tau decay products which are sensitive to the Higgs boson parity are defined and are found to be measurable using typical properties of a future detector for an e^+ e^- linear collider. The prospects for the measurement of the parity of a Higgs boson with a mass of 120 GeV are quantified for the case of e^+ e^- collisons of 500 GeV center of mass energy with an integrated luminosity of 500 fb^-1. The Standard Model Higgsstrahlung production process is used as an example.

Figures (2)

• Figure 1: The $\rho^+ \rho^-$ decay products' acoplanarity distribution angle, $\varphi^*$, in the rest frame of the $\rho^+ \rho^-$ pair. A cut on the differences of the $\pi^\pm$$\pi^0$ energies defined in their respective $\tau^\pm$ rest frames to be of the same sign, selection ${\cal C}_{bare}$, is used in the left plot and the opposite sign, selection ${\cal D}_{bare}$, is used for the right plot. No smearing is done. Thick lines denote the case of the scalar Higgs boson and thin lines the pseudoscalar one. Units valid for the 500 GeV $e^+e^-$ CMS (scalar 120 GeV mass) Higgsstrahlung production only. Otherwise arbitrary units.
• Figure 2: The $\rho^+ \rho^-$ decay products' acoplanarity distribution angle, $\varphi^\bullet$, in the rest frame of the $\rho^+ \rho^-$ pair. A cut on the differences of the $\pi^\pm$$\pi^0$ energies defined in their respective replacement $\tau^\pm$ rest frames to be of the same sign, selection ${\cal C}_{reco}$, is used in the left plot and the opposite sign, selection ${\cal D}_{reco}$, is used for the right plot. All smearing is included. Thick lines denote the case of the scalar Higgs boson and thin lines the pseudoscalar one. Units valid for the 500 GeV $e^+e^-$ CMS (scalar 120 GeV mass) Higgsstrahlung production only. Otherwise arbitrary units.