COMET (COherent Muon to Electron Transition) is an experiment currently under construction at J-PARC, Tokai, Japan. In contrast to the usual muon decay to an electron and neutrino, COMET seeks to look for neutrinoless muon to electron conversion, where the electron flies away with an energy of 104.8 MeV. Muon to electron conversion is not forbidden in the Standard Model but the branching ratio is of the order of 10^-54 ) considering neutrino oscillations. In beyond the Standard Model approaches the muon to electron conversion process can be as high as 10^-15.
Neutrinoless muon to electron conversion is charged lepton flavor violating (CLFV) process and nowadays its branching ratio upper limit is set at the order of 10-^13. Aim of COMET experiment is to improve the upper limit by factor of 4 and set it at the order of 10^-17. Improving the sensitivity up to this level is a strong probe for New Physics, since neutrinoless muon decay involves existence of supersimetryc particles (or sparticles).
Since this is very challenging, COMET experiment will be conducted in two phase.
Aim of phase one is to improve sensitivity by two order of magnitude, up to 10^-15 and estimation of background processes, when the muon flux will be as hi\u10d2h as 10^9 muons per second
Phase one will be lasting 4 or 5 month.
After that phase II will start and it will be lasting for few years. By that time all the sub detectors will be completed and ready for data taking. COMET experiment will consist a
- Straw tracker
- Electromagnetic calorimeter
- Cosmic veto counters
Collaboration consisting more 200 scientists from 41 institutions of 17 countries are involved in a COMET experiment construction. Leading experts from these institutions are facing unique challenges to achieve the goals set before the collaboration. It has to be noted, that since sub detectors needs to measure various parameters with extremely high precision latest achievements in scientific instrumentation and innovative engineering solutions are employed in the experimental set up development.
As it was discussed earlier,three main system will be constructed:
The main detector of COMET experiment is straw tracker, placed in 1 Tesla magnetic field and detecting tracks of electrons moving on a spiral path. Using the tracking and magnetic field an electron momentum will be reconstructed. Since momentum of electrons of interest is very close to the background electron momentum, the straw tracker must satisfy following conditions
a) High spatial resolution
b) The active zone of detection must contain as less as possible material along the particle path
c) Perfect operation in vacuum
Straw detector consists five independent modules, which can be moved relatively each other at some distance. Each module has 4 plane of straw tubes and measures electron spatial coordinates. In combination with electromagnetic calorimeter , system must measure trajectory and energy of the electrons.
Main part of of straw tracker system is cylindrical shape, 20/12 um thick wall straw tube, which is basically a gas filled single channel drift tube with a conductive inner layer as cathode and a tungsten wire (25/15 um diamter) stretched along the axis as anode. Principle of operation is based on the ionization of the gas by traversing particle and inducing electric charge on a anode wire. Signal is read out by special read out electronics
Straw tubes for COMET experiment is made by the innovative ultrasound welding technology, which avoids the gluing the tubes and hence reduces material budget. Also, tubes produced by this technologies functions perfectly under the vacuum conditions and maintains is cylindrical shape easily
Innovative ultrasound welding allows to use a straw tube material 12um thick aluminized Mylar.
Very thin aluminized Mylar foil is fed to special shaper, which forms a cylindrical shaped tubes with aluminum layer inside and then a sonotrode, machine used for ultrasound welding, welds two overlapping edges, producing a straw tube.
Production technology was developed by Georgian scientists in collaboration with JINR group of COMET experiment. A new, innovative ultrasonic welding machine was manufactured and 1um wall thick and 5mm diameter straw tubes were produced.
Straw tube wall thickness for phase 1 is 20um, for phase 2 – 12 um respectively. Gas pressure inside the tubes – 1bar, tubes will placed in 1 Tesla magnetic field. For phase COMET 2700 straw tubes are already produced
Since signal processing is one of the most important components in the Straw tracker operation, we have started research in this direction and at the initial stage we are modeling the signal generation in Straw tubes and studying the different parameters of this signal using GARFIELD ++.
At this point, the study involves looking at different parameters of the signal at distance distance from signal wire and how the avalanche starts.