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Andrea Giuliani: Neutrinoless Double Beta Decay

March 14, 2013

Double beta decay is a very rare nuclear process which normally sees the emission of two electrons and two neutrinos. Then there is the neutrinoless one, never observed; and then one can also hypothesize the emission of a Majoron ( a light neutral boson) with the two electrons.

There are a few nuclei that may undergo 2B decay, with mass number in the 100-150 range and Q values of 2-3 MeV. There is a plethora of mechanisms whcih may induce DBD, like exchange of massive light or heavy neutrinos, right-handed W bosons, SUSY superparnters, etcetera. But the generic process is the same. If we observe neutrino-less double beta decay we would infer the Majorana nature of neutrinos.

The mechanism goes on by exchange of a virtual neutrino, which is only possible if the neutrino is massive. The process is connected to the neutrino mixing matrix and masses in case of the process induced by the light neutrino exchange. The rate depends on an axial vector coupling constant to the fourth power, and nuclear matrix elements times an effective squared majorana mass. If one plots the latter as a function of the lightest neutrino mass one gets two bands. One corresponds to the normal hierarchy of neutrino masses, and another corresponding to the inverted hierarchy. So one can probe the hierarchy with this process.

For inverted hierarchy, it is is possible to have a cancellation of terms with sterile neutrinos, which changes the allowed region of the two-parameter space, making the allowed region wider at low values of masses.

There is a claim of evidence (Klapdor) at the level of 0.2 eV. One must investigate large masses to have sensitivity to test that claim and explore the inverted hierarchy region.

The signal will be a peak in the energy spectrum of two electrons. Then there is the continuum spectrum from events with two neutrinos, laying at lower energies. The sensitivity is the lifetime corresponding to the minimum detectable number of events over background, at a given confidence level. The sensitivity scales with the square root of the exposure, and inverse of background contamination and energy resolution. It improves very weakly with the typical experimental parameters, and one can only make a very wise choice of the neuclides. Rate depends on the nuclear matrix elements. There is no obvious choice for nuclei, but 76Ge and 150Nd are very interesting.

There are other parameters important for the choice of nuclide. The isotopic abundance is one: one must enrich the sample with the right isotope, and this is not easy for all of them.

Experimentally there are two approaches: one is to have the source coincide with the detector (a calorimetric technique, with scintillation, phonon-mediated detection, and solid-state devices. The other is to have the two distinct.

The end points of 222Rn induced radioactivity in Q-value guides the choice of nuclei. For the source=detector approach there are excellent technologies available. But these are less favourable in terms of background. The three isotopes that are really bgr-free because of their high Q-value, have low isotopic abundance and problematic enrichment. Then there is an intermediate region with isotopes that are free from natural gamma background but populated by degraded alphas. Here is the realm of scintillating bolometers.

The speaker then discussed how healthy is the Klapdor evidence. He said it is problematic but not dead. The experimental range of the Klapdor half-life can be converted into half-life for other isotopes taking into account the uncertainty related to the extrapolation in nuclear natrix elements. Xenon is coming in the game. KamLand has partially covered the region claimed by Klapdor. EXO-200 is a tpc with 200kg of liquid xenon.  Kamland-Zen has 13 tons of liquid scintillator loaded with 300kg of 136Xe in a nylon balloon. In the region of DBD there is contamination from radionuclides and the limit on the neutrinoless double beta decay is affected by the large backround. Kamland-Zen is thus interesting but should not be left alone in this measurement, because of that.

In another way EXO presented the data as the half-life of Germanium versus the half-life of Xenon. One can take the Klapdor band and translate it into limits for the other models. The limit obtained by combining EXO and KamLand seems to show a significant tension with the Klapdor signal region.

Then three is Gerda, Germanium diodes in liquid Argon. This expects to achieve 5 signal counts for the region corresponding to the Klapdor central value, with a background of 1.8 events. If there is a signal, the Klapdor affair is solved, but if no signal is seen the situation may remain controversial based on this. In phase two, Gerda can however reach a conclusive answer.

Cuore0 is running, and a calibration is ongoing. This is based on 11kg of 130Te. Cuore0 will examine the region of Klapdor in two years. Cuore will by far exceed the signal region. Operation of Cuore will start in 2014.

Another experiment is SuperNemo. This is the only experiment among those exploring the nine isotopes that use source separated by the detector. The demonstrator has sensitivity right in the region of the Klapdor claim.

Scintillation bolometers constitute a very interesting technique. Lucifer and Lumineu are in R&D. They use 82Se and 100Mo, respectively. 10 kg masses of solid-state crystals. The proof of principle of these has been largely demonstrated with 0.4kg masses. Backgrounds at the level of 0.0001 level seems reachable, which means that ton-sized detectors are thinkable.

The inverted hierarchy band can be reached by experiments that are already being built. The speaker showed how the various experiments can investigate different regions of the parameter space plot.

To conclude, the speaker looked at the big picture: we are today exploring the quasi-degenerate region with 8 experiments. If we observe the decay with these experiments we can do precision measurements on 9 isotopes, with several techniques on the 100-kg scale of active masses. However, if we do not observe a signal, we need to explore the inverted hierarchy region with at least two experiments at the ton scale. If these also fail, we would have to go to >10 ton detectors.

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