[Paper Review] Searching for the mu into e gamma decay with MEG and MEG-II

G. Cavoto|arXiv (Cornell University)|Jul 31, 2014

Particle Detector Development and Performance1 references3 citations

TL;DR

This paper presents the MEG experiment's world-leading upper limit of 5.7 × 10⁻¹³ on the branching fraction of the rare lepton-flavor-violating decay μ⁺ → e⁺γ, achieved using 3.6 × 10¹⁴ stopped muons at PSI. The MEG-II upgrade, with enhanced detector resolution and reduced background, is projected to reach a sensitivity of 5 × 10⁻¹⁴ within three years, significantly advancing the search for new physics beyond the Standard Model.

ABSTRACT

The MEG collaboration searches for the mu to e gamma decay at the muon beam line PiE5 of the Paul Scherrer Institut in Switzerland. The world best upper limit has been set to be BR(mu to e gamma) < 5.7 10^-13 at 90% C.L. analyzing a data set of 3.6 10^14 stopped muons on target. An upgrade program of the detector (MEG-II) should lead to a sensitivity at a level of few 10^-14 within the next five years.

Motivation & Objective

• To search for the lepton-flavor-violating decay μ⁺ → e⁺γ as a probe for new physics beyond the Standard Model.
• To improve the experimental sensitivity to μ⁺ → e⁺γ by reducing background contributions and enhancing detector resolution.
• To set a new world-leading upper limit on the branching fraction of μ⁺ → e⁺γ using data from the MEG experiment.
• To develop and implement the MEG-II upgrade to achieve a sensitivity of ~5 × 10⁻¹⁴, enabling deeper exploration of new physics models.
• To reduce the dominant accidental background by improving positron and photon reconstruction, timing, and detector granularity.

Proposed method

• Utilized a continuous positive muon beam at PSI's πE5 beamline with a stopping rate of 3 × 10⁷ μ⁺/s on a thin target.
• Employed a spectrometer with a gradient magnetic field and low-mass drift chambers to track positrons, directing high-momentum tracks to fast scintillator arrays for timing.
• Used a 900-liter liquid xenon calorimeter with PMTs to detect photons, achieving ~60% photon detection efficiency.
• Applied a blind analysis procedure masking the signal region until PDFs for background and signal were finalized, using sidebands for background estimation.
• Performed a maximum likelihood fit in a defined analysis region: 48 < Eγ < 58 MeV, 50 < Ee < 56 MeV, |teγ| < 0.7 ns, |θeγ| < 50 mrad, |φeγ| < 50 mrad.
• Calibrated signal PDFs using π⁰ decays (for Eγ), Michel spectrum fits (for Ee), and RMD events (for teγ), with dedicated samples for angular resolution.

Experimental results

Research questions

• RQ1 What is the current world-leading upper limit on the branching fraction of the μ⁺ → e⁺γ decay?
• RQ2 How does the MEG experiment suppress the dominant accidental background from positron-photon coincidences?
• RQ3 What improvements in detector resolution and design are required to achieve a sensitivity of 5 × 10⁻¹⁴ in the MEG-II experiment?
• RQ4 How does the MEG-II upgrade reduce the accidental background, which scales with the square of the muon stopping rate and detector resolution?
• RQ5 To what extent can the MEG-II detector’s enhanced positron tracking, timing, and photon reconstruction improve sensitivity to μ⁺ → e⁺γ?

Key findings

• The MEG experiment set the world’s best upper limit on the branching fraction of μ⁺ → e⁺γ at 5.7 × 10⁻¹³ at 90% confidence level, based on 3.6 × 10¹⁴ stopped muons.
• The sensitivity of the current MEG detector is estimated at 7.7 × 10⁻¹³, with final results from a full data set expected to reach 5.0 × 10⁻¹³.
• MEG-II is projected to achieve a sensitivity of 5.0 × 10⁻¹⁴ after three years of nominal data-taking, with a muon stopping rate of 7 × 10⁷ μ⁺/s.
• The MEG-II upgrade includes a 2 m long single-volume drift chamber with 1200 cells, improving positron tracking resolution to 1.6 mm (Z) and 0.7 mm (Y) at the core.
• The positron timing resolution is expected to improve from 122 ps to 84 ps, and the photon energy resolution will be reduced to 1.1% (w < 2 cm) and 1.0% (w > 2 cm).
• The positron detection efficiency is expected to increase from 40% in MEG to 88% in MEG-II, with improved timing and reconstruction via segmented scintillator tiles and SiPM readout.