Mulan: a part-per-million measurement of the muon lifetime and determination of the Fermi constant

The part-per-million measurement of the positive muon lifetime and determination of the Fermi constant by the MuLan experiment at the Paul Scherrer Institute is reviewed. The experiment used an innovative, time-structured, surface muon beam and a near-4\piπ, finely-segmented, plastic scintillator positron detector. Two in-vacuum muon stopping targets were used: a ferromagnetic foil with a large internal magnetic field, and a quartz crystal in a moderate external magnetic field. The experiment acquired a dataset of 1.6 \times 10^{12}1.6×1012 positive muon decays and obtained a muon lifetime \tau_{\mu} = 2\, 196\, 980.3(2.2)τμ=2196980.3(2.2)~ps (1.0~ppm) and Fermi constant G_F = 1.166\, 378\, 7(6) \times 10^{-5}F=1.1663787(6)×10−5 GeV^{-2}−2 (0.5~ppm). The thirty-fold improvement in \tau_{\mu}τμ has proven valuable for precision measurements in nuclear muon capture and the commensurate improvement in G_FF has proven valuable for precision tests of the standard model.

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Improved Measurement of the Positive-Muon Lifetime and Determination of the Fermi Constant

Physical Review Letters ◽

10.1103/physrevlett.99.032001 ◽

2007 ◽

Vol 99 (3) ◽

Cited By ~ 42

Author(s):

D. B. Chitwood ◽

T. I. Banks ◽

M. J. Barnes ◽

S. Battu ◽

R. M. Carey ◽

...

Keyword(s):

Positive Muon ◽

Muon Lifetime

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The Search for μ+ → e+γ with 10–14 Sensitivity: The Upgrade of the MEG Experiment

Symmetry ◽

10.3390/sym13091591 ◽

2021 ◽

Vol 13 (9) ◽

pp. 1591

Author(s):

Alessandro M. Baldini ◽

Vladimir Baranov ◽

Michele Biasotti ◽

Gianluigi Boca ◽

Paolo W. Cattaneo ◽

...

Keyword(s):

Particle Physics ◽

Rate Capability ◽

Muon Beam ◽

Paul Scherrer Institute ◽

Current Status ◽

Elementary Particle Physics ◽

Lepton Flavor ◽

The Standard Model ◽

Stringent Limit ◽

Paul Scherrer

The MEG experiment took data at the Paul Scherrer Institute in the years 2009–2013 to test the violation of the lepton flavor conservation law, which originates from an accidental symmetry that the Standard Model of elementary particle physics has, and published the most stringent limit on the charged lepton flavor violating decay μ+→e+γ: BR(μ+→e+γ) <4.2×10−13 at 90% confidence level. The MEG detector has been upgraded in order to reach a sensitivity of 6×10−14. The basic principle of MEG II is to achieve the highest possible sensitivity using the full muon beam intensity at the Paul Scherrer Institute (7×107 muons/s) with an upgraded detector. The main improvements are better rate capability of all sub-detectors and improved resolutions while keeping the same detector concept. In this paper, we present the current status of the preparation, integration and commissioning of the MEG II detector in the recent engineering runs.

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Towards a New μ→eγ Search with the MEG II Experiment: From Design to Commissioning

Universe ◽

10.3390/universe7120466 ◽

2021 ◽

Vol 7 (12) ◽

pp. 466

Author(s):

Marco Chiappini ◽

Marco Francesconi ◽

Satoru Kobayashi ◽

Manuel Meucci ◽

Rina Onda ◽

...

Keyword(s):

State Of The Art ◽

New Physics ◽

Muon Beam ◽

Current Status ◽

Beyond The Standard Model ◽

Experimental Apparatus ◽

The Standard Model ◽

Sensitivity Level ◽

Paul Scherrer ◽

Lepton Flavour

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating μ+→e+γ decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR (μ+→e+γ)≤4.2×10−13 at 90% confidence level, imposing one of the tightest constraints on models predicting LFV-enhancements through new physics beyond the Standard Model. An upgrade of the MEG experiment, MEG II, was designed and it is presently in the commissioning phase, aiming at a sensitivity level of 6×10−14. The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG (7×107 μ+/s). This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years.

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