Speaker
Description
Since the observation of neutrino oscillations, lepton number conservation is known to be a non-exact symmetry of the Standard Model lagrangian: yet there is still no evidence of lepton flavour violating processes involving charged leptons (cLFV), such as $𝜇→e𝛾$, $𝜇→eee$ or $𝜇𝑁→eN$: according to minimal extensions of the Standard Model including neutrino masses, these processes are too rare to be observed experimentally (e.g. $BR(𝜇→𝑒𝛾)\sim\mathcal{O}(10^{-55})$). Nontheless, after more than 70 years of research the effort to observe these phenomena continues because any experimental observation would be a clear evidence of Physics beyond the Standard Model and most New Physics scenarios awaits for a positive result just below the current sensitivity limits.
Between these new experiments shines MEGII @Paul Scherrer Institut, designed to overtake the current measurement of
$BR(𝜇+→e+𝛾) ≤4.2×10^{-13}$ (MEG 2016), with a predicted sensitivity of $6×10−14$ (three years of data taking). MEGII experiment works in the following way: $𝜇^+$ of $28 \text{MeV/c}$ are stopped on a thin plastic target at the center of a detector system designed to detect the products of the two-body $𝜇→e𝛾$ decay, i. e. a coincident signal from anti-parallel $e^+$ and $𝛾$ with fixed energy ($\simeq52.8~\text{MeV}$). To achieve a sensitivity of $6×10^{-14}$, the experiment exploits the most intense muon beam in the world (up to $108~𝜇^+/s$) as well as newly conceived particle detectors to reduce background contamination and to improve the resolution of the particles' kinematic properties: the photon detector is a 900 l liquid Xenon detector instrumented with more than 4000 SiPM and hundreds of phototubes; the spectrometer to track the positron is composed of a ultra-light ($1.58×10^{-3} 𝑋^0$) and ultra-segmented (more than 1700 signal wires) cylindrical drift chamber, a highly segmented detector with scintillating tiles read by SiPM. These detectors are immersed in a non-solenoidal magnetic field.
Auxiliary detectors help to improve the background rejection.
A trigger and data acquisition system WaveDAQ allows to digitize the waveforms from each of the $\sim9000$ detector channels for offline event reconstruction, while simultaneously rejecting most of the background events: the event rate is reduced from $10^7~\text{Hz}$ down to $10~\text{Hz}$ while keeping a 97 % selection efficiency.
Currently, MEGII has ended its second year of data acquisition.
In this presentation In will review MEG~II's experimental concept and status.
