[Paper Review] Prospects of probing new physics in bottomonium decays and spectroscopy

Miguel-Angel Sanchis-Lozano|ArXiv.org|Mar 26, 2005

Particle physics theoretical and experimental studies22 references3 citations

TL;DR

This paper proposes that a non-standard, light CP-odd Higgs boson could induce observable lepton universality breaking in bottomonium leptonic decays, particularly in the tauonic channel of Upsilon resonances, via virtual exchange mediated by soft photons. A significant 10% deviation in the tauonic branching fraction from lepton universality would signal such new physics, with a sharp peak in the photon spectrum serving as a 'smoking gun' signature for Higgs mass determination.

ABSTRACT

A non-standard CP-odd Higgs boson could induce a slight (but observable) lepton universality breaking in Upsilon leptonic decays. Moreover, mixing between such a pseudoscalar Higgs boson and $η_b$ states might shift their mass levels, thereby modifying the values of the $M_{Υ(nS)}-M_{η_b(nS)}$ hyperfine spplitings predicted in the standard model. Besides, $η_b$ resonances could be broader than expected with potentially negative consequences for discovery in both $e^+e^-$ and hadron colliders. A scenario with a CP violating Higgs sector is also considered. Finally, further strategies to search for a light Higgs particle in bottomonium decays are outlined.

Motivation & Objective

• To investigate whether deviations from lepton universality in Upsilon leptonic decays could signal a light CP-odd Higgs boson beyond the Standard Model.
• To assess the impact of mixing between a pseudoscalar Higgs and ηb states on hyperfine splittings and resonance widths.
• To evaluate the detectability of ηb resonances in e+e− and hadron colliders under new physics scenarios with enhanced widths.
• To explore the potential of Upsilon decays as a probe for light Higgs bosons, especially in scenarios with CP violation in the Higgs sector.
• To provide a strategy for identifying a light Higgs boson through monoenergetic photon signatures and τ± polarization measurements.

Proposed method

• The analysis uses the effective decay amplitude γsA⁰ → ℓ⁺ℓ⁻, where A⁰ is a light CP-odd Higgs boson and γs is a soft, undetected photon, to model non-standard contributions to Upsilon leptonic decays.
• The ratio Rτ = [Γ(Υ → γsτ⁺τ⁻)] / Γ(em)ee is defined to quantify deviations from lepton universality, with Rτ = 0 expected in the Standard Model.
• Mass-dependent decay widths from the Higgs contribution are derived, leading to an apparent breakdown of lepton universality, especially in the τ⁺τ⁻ channel.
• The model considers mixing between the CP-odd Higgs and ηb(nS) states, modifying the MΥ(nS) − Mηb(nS) hyperfine splitting beyond SM predictions.
• Theoretical estimates of ηb widths are extended to include Higgs-mediated decays, predicting broadening that could hinder detection at hadron colliders.
• A scenario with CP violation in the MSSM Higgs sector is analyzed, allowing for a light H₁ state with suppressed Z coupling, consistent with LEP constraints and new physics signals.

Experimental results

Research questions

• RQ1 Can a light CP-odd Higgs boson explain a 10% deviation from lepton universality in Upsilon leptonic decays?
• RQ2 How does mixing between a pseudoscalar Higgs and ηb states affect the MΥ(nS) − Mηb(nS) hyperfine splitting?
• RQ3 What are the implications of enhanced ηb widths for the discovery of ηb resonances at e+e− and hadron colliders?
• RQ4 Can the observation of (quasi)monoenergetic photons in τ⁺τ⁻ decay samples serve as a definitive signature for a light Higgs boson?
• RQ5 To what extent can Upsilon decays constrain the parameter space of light Higgs bosons in models like the NMSSM or CP-violating MSSM?

Key findings

• A non-standard light CP-odd Higgs boson could induce a 10% breakdown of lepton universality in Upsilon decays, particularly in the τ⁺τ⁻ channel, as quantified by Rτ ≠ 0.
• Mixing between the CP-odd Higgs and ηb states can shift the MΥ(nS) − Mηb(nS) hyperfine splitting beyond Standard Model predictions, depending on the relative mass ordering.
• Large widths for ηb resonances are expected for large tanβ values, potentially reducing branching fractions by an order of magnitude in hadron collider decay modes.
• The detection of (quasi)monoenergetic photons in ~10% of τ⁺τ⁻ decay events would serve as a 'smoking gun' signal for a light Higgs boson and allow direct mass measurement.
• Polarization analysis of final-state τ± could help determine the CP quantum numbers of the mediating Higgs boson.
• If lepton universality is confirmed in Upsilon decays, this would close mass windows for light Higgs bosons in scenarios below open bottom threshold, excluding regions otherwise hard to probe by other experiments.