[Paper Review] Latest results of MEG and status of MEG-II

F. Renga|arXiv (Cornell University)|Oct 17, 2014

Particle Detector Development and Performance2 references8 citations

TL;DR

This paper presents the latest results from the MEG experiment, which searched for the rare muon decay μ⁺→e⁺γ with a sensitivity down to 5.7×10⁻¹³ branching ratio, finding no signal. It also details the status and design of MEG-II, an upgraded experiment aiming for a sensitivity of 4×10⁻¹⁴ through improved detectors, including a new cylindrical drift chamber, SiPM-based timing and calorimetry systems, and enhanced data acquisition.

ABSTRACT

Within the Standard Model, in spite of neutrino oscillations, the flavor of charged leptons is conserved in very good approximation, and therefore charged Lepton Flavor Violation is expected to be unobservable. On the other hand, most new physics models predict charged Lepton Flavor Violation within the experimental reach, and processes like the $μ o e γ$ decay became standard probes for physics beyond the Standard model. The MEG experiment, at the Paul Scherrer Institute (Switzerland), searches for the $μ o e γ$ decay, down to a Branching Ratio of about $5 imes 10^{-13}$, exploiting the most intense continuous muon beam in the world and innovative detectors. In this talk I will present the latest results from MEG, and the status of its upgrade (MEG-II), aiming at an improvement of the sensitivity by one order of magnitude within this decade.

Motivation & Objective

• To search for charged lepton flavor violation (cLFV) in the μ⁺→e⁺γ decay, a key probe for new physics beyond the Standard Model.
• To improve the experimental sensitivity to cLFV processes, particularly μ→eγ, to test theoretical models like supersymmetry that predict observable rates.
• To develop and implement MEG-II, a major upgrade of the MEG experiment to achieve a sensitivity one order of magnitude better than the original MEG.
• To reduce background from accidental coincidences and radiative decays through advanced detector design and data analysis techniques.
• To achieve a 0.2% accuracy in energy calibration and sub-millimeter resolution in tracking and timing for enhanced signal detection.

Proposed method

• The MEG experiment uses a continuous muon beam of ~3×10⁷ μ⁺/s at PSI, with positrons and photons detected in a magnetic spectrometer and liquid xenon calorimeter.
• A fully digital trigger system based on DRS4 chips digitizes waveforms at 1 GS/s, enabling real-time selection using energy, time, and position information.
• Signal reconstruction relies on five discriminating variables: positron energy, photon energy, relative time, and projected angles (ϕₑᵧ, θₑᵧ), with PDFs derived from detector resolution and Monte Carlo simulations.
• Accidental background is modeled directly from data using sideband regions in the Eγ vs. Tₑᵧ plane, while radiative decay background is simulated using measured kinematics.
• A likelihood fit with profile-likelihood ratio is used to set a 90% confidence level upper limit on the branching ratio, incorporating systematic uncertainties from angular PDFs.
• MEG-II will replace the 16-planar drift chambers with a single cylindrical chamber using stereo wires and a He/Isobutane gas mixture to reduce material budget and improve tracking efficiency.

Experimental results

Research questions

• RQ1 What is the current experimental upper limit on the branching ratio for the μ⁺→e⁺γ decay, and how does it constrain new physics models?
• RQ2 How do accidental and radiative background contributions affect the sensitivity of the μ→eγ search, and what methods are used to model and suppress them?
• RQ3 What detector upgrades are required in MEG-II to achieve a sensitivity improvement by one order of magnitude compared to MEG?
• RQ4 How do the new SiPM-based timing and calorimetry systems in MEG-II enhance time and energy resolution compared to the original PMT-based design?
• RQ5 What is the expected performance of MEG-II in terms of momentum, angular, and energy resolution, and how do these impact the final sensitivity?

Key findings

• The MEG experiment set a 90% confidence level upper limit of 5.7×10⁻¹³ on the μ⁺→e⁺γ branching ratio using 36×10¹³ stopped muons collected from 2009 to 2011.
• The expected sensitivity for the full MEG dataset, including 2012–2013 data, is projected to reach below 5×10⁻¹³ due to improved analysis algorithms and background suppression.
• MEG-II is designed to achieve a projected sensitivity of 4×10⁻¹⁴ branching ratio, representing a factor of ten improvement over the original MEG experiment.
• The new cylindrical drift chamber in MEG-II is expected to achieve a spatial resolution of 120 μm (radial) and 1 mm (axial), with a momentum resolution of 130 keV and angular resolution of 5 mrad.
• The SiPM-based timing counter in MEG-II is expected to achieve a time resolution of 30 ps, significantly improving the rejection of accidental coincidences.
• The upgraded liquid xenon calorimeter with SiPMs will improve energy resolution to 1% and enhance detection of shallow conversion events and pileup photons.