Abstract
We present the status and the perspectives of the MEG experiment which has been searching for the Lepton Flavour Violating decay μ + → e+ γ for several years. In a dataset corresponding to 3.6 × 1014 positive muons stopped on target we didn’t find any evidence for this decay and established an upper bound on the μ + → e+ γ branching ratio of 5.7 × 10−13 at 90% C.L., with a sensitivity of 7.7 × 10−13 , which improves our world best limit by a factor of 4 and the best previous limit by a factor of 20. Our final dataset is about twice as large than the analyzed sample and the expected final sensitivity of the experiment is ∼ 5 × 10−13 . We also discuss the status of the upgrade of the experiment (MEGII) which aims to improve the sensitivity by a further order of magnitude.
Highlights
Introduction to Lepton FlavourViolation and μ+ → e+γ decayThe Lepton Flavour Violation (LFV) in the charged lepton sector is almost forbidden in the Standard Model (SM), even including neutrino oscillations and mixing [1]
We present the status and the perspectives of the MEG experiment which has been searching for the Lepton Flavour Violating decay μ+ → e+γ for several years
The expected B for μ+ → e+γ decay ranges from ∼ 10−14 to ∼ 10−12; here we show the results and the perspectives of the MEG experiment, which has improved the best world upper limit on this process by a factor of 20 and can reach a sensitivity which covers a large fraction of the allowed parameter space of New Physics models (NP) schemes
Summary
Decay μ+ → e+νeνμγ (RMD), whose rate is proportional to the muon stopping frequency Rμ and the ACCidental Background (ACCB), given by the random coincidence of energetic positrons from the SM Michel decay with photons from RMD, e+-e− annihilation-in-flight or bremsstrahlung. The experiment employs a Liquid Xenon (LXe) detector for the measurement of the photon energy, arrival time and first interaction point as well as a magnetic spectrometer for the measurement of the positron momentum vector and timing. The spectrometer is composed by 16 Drift Chambers (DC), each one formed by two staggered layers of sense wires and cathodic foils, and by a double-array of scintillation Timing Counter (TC) located inside a superconducting solenoid (COnstant Bending RAdius, COBRA) with a gradient magnetic field along the beam axis. The readout is performed by a custom-made chip (Domino Ring Sample, DRS), developed at PSI, with a maximum sampling speed of 5 GHz and 12 bit voltage digitization [10]
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