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NEXT Report Summary

Project ID: 339787
Funded under: FP7-IDEAS-ERC
Country: Spain

Mid-Term Report Summary - NEXT (Towards the NEXT generation of bb0nu experimets)

Laboratory experiments show that in elementary collisions matter and antimatter are produced in the same proportions. Why then the universe is made only of matter? The almost total absence of antimatter in the universe is one of the greatest problems yet unsolved in physics.
One of the most attractive possibilities to explain this asymmetry would be the presence in the primitive universe of a type of heavy neutrino capable of decaying both to matter (electrons) and antimatter (positrons), but favoring slightly the former versus the later. Such a particle could have introduced a small excess of matter in the, otherwise symmetric, early universe. After the almost-total annihilation of matter and antimatter, the known universe is the result of this small excess.
For this explanation to work, however, such a particle would have had to be able to decay to matter and antimatter, which would have been only possible if the early neutrino was its own antiparticle.
If that was the case, then the light neutrinos that constitute the second most abundant particle in the universe (after the photons of light) could also be their own antiparticles. This fact can be demonstrated experimentally by observing a rare nuclear reaction called neutrinoless double beta decay (bb0nu).
The so-called “beta decay” is a radioactive process in which one of the neutrons of an unstable isotope decays into a proton, and electron and a neutrino. A few rare isotopes, which are stable against conventional beta decay, can, however experience double beta decay (bb2nu), a second-order process in which two neutrons decay simultaneously to two protons, two electrons and two neutrinos.
If the neutrino is its own antiparticle, an even more exotic decay is possible. The two neutrons decay to two protons and two electrons, with no emission of neutrinos (bb0nu). A naïve way to picture the decay is to imagine that the two neutrinos “annihilate each other before even being produced” something only possible if the neutrino can behave as matter and antimatter.
The NEXT experiment is searching for bb0nu events in xenon, in order to demonstrate the the neutrino is its own antiparticle. NEXT uses xenon gas at high pressure. The gas is enriched at 90% in the isotope xenon-136, which has the same chemical properties than ordinary xenon (a mixture of several isotopes) but can decay bb2nu and perhaps bb0nu. Xenon is an excellent candidate for the observation of the bb0nu since it responds to ionizing particles by emitting scintillation light, which in turn can be used as a detection signal. The NEXT detector is, in practice, an “electronic bubble chamber” (or an electronic 3D camera).
The detector consists of a pressure-tank holding up 100 kg of xenon at 15 atmospheres. Operation requires intense electric fields, and the use of many thousands of sophisticated optical sensors read-out by state-of-the-art electronics. The detector is built with ultra-radiopure materials and requires massive shielding to protect the target area from the contamination of natural radioactivity. In addition, the experiment must occur in an underground lab to shield the target from the copious background originated by cosmic rays.
NEXT is currently taking data at the Canfranc underground laboratory, after an initial period devoted to the construction of the infrastructures and the first phase of the detector. Initial operation will demonstrate the unique capabilities of the experimental technique and will measure the bb2nu mode. It will be followed by a full search for the bb0nu mode.
The technology developed in NEXT has already found a potentially ground-breaking application to Medical Imaging, based in the possibility to use liquid xenon equipped with silicon photomultipliers to build a full-body, high resolution PET scanner which could provide improved images with both reduced costs and reduced radioactive doses to the patients.

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