Speaker
Dr
Marco Chiappini
(Università di Siena & INFN Pisa)
Description
The aim of MEG experiment is looking for the $\mu^{+}\rightarrow e^{+}\gamma$ at the Paul Scherrer Institute (PSI) near Zurich in Switzerland. The decay $\mu^{+}\rightarrow e^{+}\gamma$ is an extremely rare process of the violation of the flavour of charged-leptons and it is forbidden in the Model Standard that foresees an experimentally unattainable branching ratio$BR\approx10^{-55}$. Anyway some Super-Symmetric extensions to the Standard Model predict the decay $\mu^{+}\rightarrow$ $e^{+}\gamma$ in the range $10^{-11}$ - $10^{-14}$. MEG has already determined the world best upper limit on the branching ratio: BR$<$4.2$\times10^{-13}$@90\%CL with the full data set collected in the years 2009-2013.
The new positron tracker is a high transparency single volume, full stereo cylindrical Drift Chamber (CDCH), filled with a gas mixture of helium-isobutane. It is composed of 9 concentric layers, divided in 12 identical sector, each layer consisting of a sense wires plane between two field wires planes at alternating signs stereo angles. Each layer is composed of 192 drift cells (16 for sector), the single drift cell is approximately square, with a $20~\mu$m gold plated W sense wire surrounded by $40~\mu$m silver plated Al field wires in a ratio of 5:1. For equalizing the gain of the innermost and outermost layers, two guard layers have been added at proper radii and at appropriate high voltages. The total number of wires amounts to 11904.
We will present the CDCH commissioning operations performed at PSI after that it was moved from INFN-Pisa. The HV tests and conditioning of the chamber are presented, aiming at reaching the HV working point, as obtained from gas gain simulations. The first signals with cosmics rays are shown, before the description of the CDCH integration into the MEG II experimental apparatus.
Finally we will talk about the 2018 engineering run with CDCH fully integrated in the experimental apparatus for the first time. The subsequent the CDCH re-opening operations during the first half of 2019 to find the final stable working point configuration and of the pre-commissioning run of this year. Starting from a description of the read out configurations, the HV scans with cosmics and Michel positrons and the $\mu^+$ beam intensity scans are shown.
Primary authors
Mr
Gianluigi Chiarello
(INFN Roma1)
Dr
Giovanni Francesco Tassielli
(INFN Lecce & University of Salento)
Dr
Marco Chiappini
(Università di Siena & INFN Pisa)
