COherent Muon to Electron Transition (COMET)
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The aim of the COMET experiment is to search for coherent neutrino-less conversion of muons to electrons in a muonic atom (&mu--e- conversion), &mu- + N(A,Z) &rarr e- + N(A,Z), at a sensitivity of 10-16, at the Japanese Proton Accelerator Research Complex (J-PARC). This sensitivity goal is a factor of 10,000 better than that of current experimental limits. Several important features have been considered in order to improve this experimental limit, the most important of which being:
High Intensity Muon Source
The total number of muons needed is of the order of 1018 to achieve an experimental sensitivity of 10-16 . Therefore, a highly intense muon beam line has to be constructed. One method to increase the muon beam intensity is to use a novel system of collecting pions, which are parents of muons, with high efficiency. In muon collider and neutrino factory R&D, superconducting solenoid magnets (producing a high magnetic field surrounding the proton target) have been proposed and studied for pion capture over a large solid angle. With this pion capture solenoid system, approx. 8× 1020 protons of 8 GeV are necessary to achieve the required number of muons.
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Pulsed Proton Beam
There are several potential sources of electron background events in the energy region around 100 MeV, where the &mu--e- conversion signal is expected. One of them is beam-related background events. In order to suppress the occurrence of beam-related background events, a pulsed proton beam utilizing 'beam pulsing' is proposed. Since muons in muonic atoms have lifetimes of approx. 1 &mu s, a pulsed beam with beam buckets that are short, relative to muonic atomic lifetimes, would allow removal of prompt beam background events by allowing measurements to be performed in a delayed time window.
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Muon Transport Solenoid
The captured pions decay to muons, which are transported with high efficiency through a superconducting solenoid magnet system. Beam particles with high momenta would produce electron background events in the energy region of 100 MeV and, therefore, they must be eliminated with the use of curved solenoids where the centers of the helical motion of the electrons drift perpendicular to the plane in which their paths are curved, and the magnitude of the drift is proportional to their momentum. By using this effect and by placing suitable collimators at appropriate locations, beam particles of high momenta can be eliminated.
Curved Spectrometer Solenoid
In order to reject electron background events and reduce the probability of false-tracking owing to high counting rates, a curved solenoid spectrometer is considered to allow selection of electrons on the basis of their momenta. The principle of momentum selection is the same as that used in the transport system, but, in the spectrometer, electrons of low momenta which mostly come from muon decay in orbit (DIO) are removed. The tracking detector rate, including the direct hit of DIO electrons and the secondary electrons from the scattering and gammas, would be less than 1 MHz. This is almost two orders of magnitudes less than the expected detector rate at other proposed LFV experiments.
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Last Modified : 21:39:00 05 Jun 2009
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