Laboratory for High Energy Physics
LHEP is an active member of the EXO collaboration, a multi-phase experiment looking for neutrinoless double beta decay in the 136Xe isotope.
The two neutrino double beta decay is a rare nuclear process allowed in even-even isotopes where two neutrons are simultaneously converted to protons resulting in the emission of two electrons and two neutrinos. This process is very similar to the ordinary beta decay but much less frequent. It has been observed in various nuclei, typical half-lives being larger than 1020 years. Neutrinoless double beta decay in which two electrons only are emitted but no neutrinos, is forbidden in the Standard Model, but predicted in certain extensions accomodating finite neutrino masses. In the favored light neutrino scenario, the decay probability is proportional to the square of an effective neutrino mass, a linear combination of the light masses. It has so far not been unambiguously observed.
Current data from neutrino oscillation experiments, using solar, atmospheric and reactor generated neutrinos, clearly indicate that the neutrinos have indeed finite masses, but only determine differences of squared masses. In contrast the study of neutrinoless double beta decay may shed light on the absolute mass scale. Neutrinoless double beta decay requires the neutrino to be its own anti-particle, i.e. the neutrino has to be a Majorana fermion.
During the initial phase of the EXO experiment a 200 kg liquid xenon prototype will be deployed at WIPP (Waste Isolation Pilot Plant) in the experimental area. A series of class-100 cleanroom modules have been installed in the salt bed at a depth of about 300 meters and will host the experimental setup. By installing the detector underground and by using very low radioactive background components and materials, the collaboration has insured that the EXO-200 prototype will have a minimal background from natural activities and of cosmic origin. The sensitivity will be exploited to its full potential. The detector is composed of a TPC (Time Projection Chamber) filled with liquid xenon enriched to 80% of 136Xe and the ionization charge is collected by a X-Y wire grid. Scintillation light is also measured using APD (Avalanche Photodiodes) and it provides the event time zero reference as well as it helps to improve the energy resolution of the detector. The TPC chamber is hosted in a cryostat, filled with HFE-7000 coolant, which has been designed and build by our group. The EXO-200 experiment will allow us to extend the sensitivity towards smaller values of the neutrino effective mass. It offers also the opportunity to observe for the first time the allowed double beta decay in 136Xe.
The techniques developed while working with the 200 kg prototype are essential to the design and implementation of the following phase of the project, i.e. a ton scale, very low background, detector that has a realistic discovery potential for the neutrinoless double beta decay. The collaboration performs in parallel research and development work on innovative approaches to background reduction and energy resolution improvements. 136Ba++ ions are unique products of the double beta decay in 136Xe and therefore one can envisage the background reduction advantage of a detection scheme that incorporates barium ions tagging. Preliminary work done by our collaborators from Stanford University has shown that barium ions can be clearly identified using specific frequency lasers tuned to excite resonant light scattering in Ba+. The ions have to be captured and transported to a RF cage before laser irradiation and imaging. Various techniques are presently explored to insure an efficient ion collection for a large scale detector. Also, in-situ barium ion tagging is under investigation.
The allowed double beta decay is the ultimate background for the neutrinoless double beta decay but the two processes have very different energy spectra when considering the sum of the energy of the emitted electrons. The power of a detector to focus on the neutrinoless double beta decay and reject the background depends strongly on its energy resolution. Therefore, different techniques to increase the energy resolution, like the joint collection of charge and light, are actively explored and the interference with other objectives, like the tagging of barium ions, are to be considered. Presently, the collaboration proposes two options for the implementation of the ton scale detector, i.e. an extension of the liquid TPC with improved energy resolution and barium tagging and a high pressure gas TCP with similar features. Future research will allow us to choose the optimal solution.
The main responsibilities of the Bern group are:
We also participate regularly to the installation shifts for the EXO-200 project and will soon contribute to the detector operation and monitoring on site at WIPP.