The mott based double turn method for the MEG II spectrometer characterisation

Abstract

The MEG experiment has set the latest limit of 4.2 × 10 −13 at 90 % Confidence Level (C.L.) to the branching ratio of the charged Lepton Flavour Violating (cLFV) decay μ + → e + γ . The MEG II experiment will be more sensitive by an order of magnitude. High resolutions in terms of the kinematic variables used to identify a μ + → e + γ event are mandatory among other aspects to achieve this sensitivity. Thus careful calibration and monitoring of the experimental apparatus are required. One of the dedicated calibration methods involves the Mott scattering process of a mono-energetic positron beam on the MEG II target. As a result positrons with energies around the MEG II signal energy get scattered into the drift chamber. The calibration and extraction of resolution can then be done by the double turn method presented in this article. • Simulation of Mott events provides similar results as the expected MEG II signal. • Beam quality and details of scattering process hardly affect the outcome. • The Double Turn Method provides the detector resolution up to an expected factor. • Resolution in momentum and angular variables can be estimated.