[Paper Review] Searches for Lepton Number Violation and resonances in the $K^{\pm} o πμμ$ decays at the NA48/2 experiment

K. Massri|arXiv (Cornell University)|Jul 14, 2016

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a search for lepton number violating (LNV) decays and resonant new physics in $K^{\pm} \to \pi\mu\mu$ decays using 2003–2004 data from the NA48/2 experiment at CERN. It sets a new upper limit of $8.6 \times 10^{-11}$ on the branching ratio for the LNV decay $K^{\pm} \to \pi^{\mp}\mu^{\pm}\mu^{\pm}$ at 90% confidence level, improving the previous limit by over an order of magnitude, and establishes upper limits on products of branching ratios for resonances like heavy neutral leptons $N_4$ and inflatons $\chi$ in the $10^{-10}$ to $10^{-9}$ range for lifetimes below 100 ps.

ABSTRACT

The NA48/2 experiment at CERN collected a large sample of charged kaon decays into final states with multiple charged particles in 2003--2004. A new upper limit on the rate of the lepton number violating decay $K^{\pm} o π^{\mp} μ^{\pm} μ^{\pm}$ obtained from this sample is reported: $\mathcal{B}(K^{\pm} o π^{\mp} μ^{\pm} μ^{\pm})<8.6 imes 10^{-11}$ at 90\% CL. Searches for two-body resonances in the $K^{\pm} o πμμ$ decays (including heavy neutral leptons~$N_4$ and inflatons~$χ$) in the accessible range of masses and lifetimes are also presented. In the absence of a signal, upper limits are set on the products of branching ratios~$\mathcal{B}(K^{\pm} o μ^{\pm} N_{4})\mathcal{B}(N_{4} o π^{\mp}μ^{\pm})$ and $\mathcal{B}(K^{\pm} o π^{\pm} χ)\mathcal{B}(χ o μ^{+}μ^{-})$ as functions of the resonance mass and lifetime. These limits are in the $10^{-10}-10^{-9}$ range for resonance lifetimes below 100~ps.

Motivation & Objective

• To search for lepton number violating (LNV) decays in charged kaon decays, particularly $K^{\pm} \to \pi^{\mp}\mu^{\pm}\mu^{\pm}$, which are forbidden in the Standard Model.
• To probe the existence of new physics beyond the Standard Model, such as heavy neutral leptons ($N_4$) and inflatons ($\chi$), via resonant decays in $K^{\pm} \to \pi\mu\mu$ final states.
• To set improved upper limits on the branching ratios of LNV and resonant decay modes, providing constraints on models like the $\nu$ MSM and scalar extensions.
• To lay the foundation for future searches in the NA62 experiment, which aims to measure rare kaon decays with high precision.

Proposed method

• The NA48/2 experiment collected a large sample of charged kaon decays into final states with multiple charged particles using a 114 m long decay region and a magnetic spectrometer with drift chambers and a hodoscope.
• Invariant mass distributions of $\pi\mu$ and $\mu\mu$ pairs were scanned for narrow peaks indicative of resonant decays, such as $K^{\pm} \to \mu^{\pm} N_4$ followed by $N_4 \to \pi^{\mp}\mu^{\pm}$ or $K^{\pm} \to \pi^{\pm}\chi$ followed by $\chi \to \mu^{+}\mu^{-}$.
• Signal acceptance and background estimation were performed using Monte Carlo simulations, with corrections applied for detector efficiency and reconstruction effects.
• Upper limits on the products of branching ratios were derived using the number of observed events, expected background, and acceptance as a function of resonance mass and lifetime.
• The statistical significance of potential signals was evaluated using the Z-score formula: $ z = \frac{N_{\text{obs}} - N_{\text{exp}}}{\sqrt{\delta N_{\text{obs}}^2 + \delta N_{\text{exp}}^2}} $, with no signal exceeding 3σ.
• Systematic uncertainties were evaluated, with the largest contributions from Monte Carlo simulation accuracy (1.0%) and branching ratios of related decays.

Experimental results

Research questions

• RQ1 What is the upper limit on the branching ratio for the lepton number violating decay $K^{\pm} \to \pi^{\mp}\mu^{\pm}\mu^{\pm}$?
• RQ2 Can evidence for resonant decays involving heavy neutral leptons ($N_4$) be observed in the $K^{\pm} \to \pi\mu\mu$ decay channel?
• RQ3 What are the constraints on the product of branching ratios $\mathcal{B}(K^{\pm} \to \mu^{\pm} N_4) \mathcal{B}(N_4 \to \pi^{\mp}\mu^{\pm})$ as a function of $N_4$ mass and lifetime?
• RQ4 What are the upper limits on $\mathcal{B}(K^{\pm} \to \pi^{\pm}\chi) \mathcal{B}(\chi \to \mu^{+}\mu^{-})$ for inflaton-like resonances in the $K^{\pm} \to \pi\mu\mu$ decay?
• RQ5 How do these results compare to previous limits and what is their impact on models like the $\nu$ MSM and scalar extensions?

Key findings

• The upper limit on the branching ratio for the lepton number violating decay $K^{\pm} \to \pi^{\mp}\mu^{\pm}\mu^{\pm}$ is $8.6 \times 10^{-11}$ at 90% confidence level, representing an improvement of over one order of magnitude compared to the previous best limit.
• No significant signal was observed in the invariant mass spectra of $\pi\mu$ or $\mu\mu$ pairs, with local significances below 3 standard deviations across all mass hypotheses.
• Upper limits on $\mathcal{B}(K^{\pm} \to \mu^{\pm} N_4) \mathcal{B}(N_4 \to \pi^{\mp}\mu^{\pm})$ are in the range $10^{-10}$ to $10^{-9}$ for resonance lifetimes below 100 ps.
• Upper limits on $\mathcal{B}(K^{\pm} \to \pi^{\pm}\chi) \mathcal{B}(\chi \to \mu^{+}\mu^{-})$ are similarly in the $10^{-10}$ to $10^{-9}$ range for lifetimes below 100 ps.
• The total systematic uncertainty on the LNV branching ratio limit is 1.5%, dominated by Monte Carlo simulation accuracy (1.0%) and branching ratio uncertainties.
• The results provide strong constraints on models involving heavy neutral leptons and scalar inflatons, with implications for the $\nu$ MSM and electroweak symmetry breaking scenarios.