New constraint on the existence of the mu+-> e+ gamma decay

Abstract: The analysis of a combined data set, totaling 3.6 \times 10^14 stopped muons on target, in the search for the lepton flavour violating decay mu^+ -> e^+ gamma is presented. The data collected by the MEG experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and yield a new upper limit on the branching ratio of this decay of 5.7 \times 10^-13 (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.

• The paper establishes a new upper limit of 5.7×10⁻¹³ for the μ+→e+γ branching ratio, improving sensitivity fourfold over previous experiments.
• It details advanced detection methodologies including a high-efficiency DAQ system, precise Kalman filter-based positron track reconstruction, and effective photon pile-up discrimination.
• By analyzing data from 3.6×10^14 stopped muons, the study sets stringent constraints on flavor violating decays, challenging models beyond the Standard Model.

Overview of the Search for Lepton Flavor Violating Decay in the MEG Experiment

This paper presents an updated analysis of the search for the lepton flavor violating (LFV) decay, specifically the $\mu^+ \rightarrow e^+ \gamma$ decay ($\megsign$), as carried out by the MEG Collaboration at Paul Scherrer Institut. Utilizing a combined dataset amounting to $3.6 \times 10^{14}$ stopped muons, the authors report no observed excess events beyond what is expected from background processes. This investigation results in a new upper limit on the branching ratio for this decay: $5.7 \times 10^{-13}$ at a 90% confidence level. This finding provides a fourfold improvement in sensitivity compared to previous limits.

Experimental Setup and Methodology

The MEG experiment employs a specialized detector setup to identify signals indicative of the $\megsign$ decay. The detector system includes a positron spectrometer featuring drift chambers and timing counters, operated within a solenoid magnet, and a liquid xenon photon detector equipped with ultraviolet-sensitive photomultiplier tubes (PMTs). The key analyzable event signature is derived from a back-to-back, time-coincident photon-positron pair, arising from the two-body decay.

The paper describes advancements in the data acquisition (DAQ) approaches that improved the live time fraction to 99%, following the implementation of a new multiple buffer scheme, boosting the effective trigger efficiency to 97%. Additional enhancements in the algorithms for reconstructing positron paths and photon detection, including Kalman filters for positron tracks and waveform analysis for photon pile-up discrimination, significantly refined the data's resolution capabilities.

Results and Statistical Analysis

The dataset spans three years, comprising previously collected data from 2009 and 2010, along with newly acquired data from 2011. Each of these data batches underwent rigorous blind analysis procedures, ensuring unbiased results. Maximum likelihood methods were used extensively to estimate the number of potential $\megsign$ events, background radiative muon decays, and accidentals. Conclusive from the experiments is the absence of signal events surpassing statistical thresholds set against background noise, leading to the determination of a tightened upper limit on the decay's branching ratio.

Systematic uncertainties, monitored via multiple calibration techniques and internal alignments within the detector modules, were meticulously quantified and accounted for in the overall results. The confidence levels and sensitivity figures reported reflect these considerations, illustrating a robust approach to limit setting.

Implications for Particle Physics and Future Work

The MEG project's constraint on the $\megsign$ decay rate contributes significantly to the exploration of physics beyond the Standard Model (BSM). The absence of detected LFV decays places restrictions on new physics scenarios, challenging existing models that predict observable rates for such processes.

These findings set a precedent for future endeavors aimed at pushing the boundaries of LFV decay detection. Looking forward, the MEG Collaboration anticipates further improvements with additional data acquisition as well as planned detector upgrades, which could enhance sensitivity by another order of magnitude. Such developments are crucial for the continued exploration of BSM physics, potentially leading to unprecedented discoveries in fundamental particle interactions.