The theoretical and experimental issues relevant to neutrinoless double beta decay are reviewed. The impact that a direct observation of this exotic process would have on elementary particle physics, nuclear physics, astrophysics, and cosmology is profound. Now that neutrinos are known to have mass and experiments are becoming more sensitive, even the nonobservation of neutrinoless double beta decay will be useful. If the process is actually observed, we will immediately learn much about the neutrino. The status and discovery potential of proposed experiments are reviewed in this context, with significant emphasis on proposals favored by recent panel reviews. The importance of and challenges in the calculation of nuclear matrix elements that govern the decay are considered in detail. The increasing sensitivity of experiments and improvements in nuclear theory make the future exciting for this field at the interface of nuclear and particle physics.

/pdf/double-beta-decay-majorana-neutrinos-and-neutrino-mass-2tfhqvhys8.pdf

Double Beta Decay, Majorana Neutrinos, and Neutrino Mass

Weak-interaction and nuclear-structure aspects of nuclear double beta decay

Neutrinoless double-beta decay, which is a very old and yet elusive process, is reviewed. Its observation will signal that the lepton number is not conserved and that the neutrinos are Majorana particles. More importantly it is our best hope for determining the absolute neutrino-mass scale at the level of a few tens of meV. To achieve the last goal certain hurdles must be overcome involving particle, nuclear and experimental physics. Nuclear physics is important for extracting useful information from the data. One must accurately evaluate the relevant nuclear matrix elements--a formidable task. To this end, we review the sophisticated nuclear structure approaches which have recently been developed, and which give confidence that the required nuclear matrix elements can be reliably calculated employing different methods: (a) the various versions of the quasiparticle random phase approximations, (b) the interacting boson model, (c) the energy density functional method and (d) the large basis interacting shell model. It is encouraging that, for the light neutrino-mass term at least, these vastly different approaches now give comparable results. From an experimental point of view it is challenging, since the life times are long and one has to fight against formidable backgrounds. One needs large isotopically enriched sources and detectors with high-energy resolution, low thresholds and very low background. If a signal is found, it will be a tremendous accomplishment. The real task then, of course, will be the extraction of the neutrino mass from the observations. This is not trivial, since current particle models predict the presence of many mechanisms other than the neutrino mass, which may contribute to or even dominate this process. In particular, we will consider the following processes: The neutrino induced, but neutrino-mass independent contribution. Heavy left and/or right-handed neutrino-mass contributions. Intermediate scalars (doubly charged, etc). Supersymmetric (SUSY) contributions. We will show that it is possible to disentangle the various mechanisms and unambiguously extract the important neutrino-mass scale, if all the signatures of the reaction are searched for in a sufficient number of nuclear isotopes.

/pdf/theory-of-neutrinoless-double-beta-decay-2c6bzk0uf6.pdf

Theory of neutrinoless double-beta decay.

Double beta (ββ) decay is a rare transition between two nuclei with
the same mass number that changes the nuclear charge number by two
units. It has been long recognized as a powerful tool for the study of
lepton conservation in general and of neutrino properties in particular.
Because the lifetimes of ββ decay are so long, the experimental study
of ββ decay is particularly challenging and has spawned a whole field
of experiments requiring very low background.

Double beta decay

Tables of double beta decay data

Double beta decays of 100Mo

The high sensitivity β-γ spectrometer ELEGANTS V (ELEctron GAmma-ray NeuTrino Spectrometer V) has been developed primarily for studying rare double-beta decays. It is a low background high sensitivity detector for low energy beta (β), electron (e) and gamma (γ) decays. ELEGANTS V is shown to be useful for the study of ultrarare nuclear β(e), ββ(ee), β(e)γ and ββ(ee)γ decays with decay rates of the order of T ∼ 10 −27–30 s −1 (t 1 2 ∼ 10 20–23 yr ) .

The high sensitivity beta-gamma spectrometer ELEGANTS V for rare β(e) and ββ(ee) decays

Task completion times of sighted and blind users were measured with two kinds of Web sites: sites marked up appropriately with heading elements and sites with the same visual appearance but with no heading elements marked up. The experiment was carried out with user agents that could navigate through heading elements. The results showed that 1) task completion time was reduced by as much as one half with marked up heading elements, 2) the benefits of markup on task completion time were greater for blind users, and 3) the overall difference in response time between sighted and blind users diminished with sites that were appropriately marked up.

Experimental evaluation of usability and accessibility of heading elements

An ultra low background β-γ spectrometer, ELEGANTS (electron gamma neutrino spectrometer) is developed primarily for studying neutrino-less double β decays (0νββ) of 76 Ge to both 0 + and 2 + states in 76 Se. It consists of a high resolution Ge β detector surrounded by a 4π NaI γ counter. Selection of tru β events by referring to accompanied γ events is shown to be very powerful. It shown extremely high sensitivity to both the 0 + → 0 + and 0 + → 2 + 0 νββ events. It is applied for studying other 0νββ and 2νββ decays, very rare β and / or γ decays and very low radioactivities.

An ultra low background beta-gamma spectrometer

Purpose. To evaluate if marking up heading elements of Web sites improves usability and accessibility.Method. Task completion times and questionnaire responses were collected from sighted and blind users for two kinds of Web sites: sites marked up appropriately with heading elements, and sites with the same visual appearance but with no heading elements marked up. The experiment was carried out with user agents that could navigate through heading elements.Results. (1) Heading elements improved usability both for sighted and blind users in terms of significantly reduced task completion time and also higher user satisfaction with the structured site. (2) A significant difference in accessibility (i.e., if reduction of task completion time is larger for blind users than sighted users) was not observed when heading elements were marked up. The lack of significant effect in this case might have been due to blind users making use of the user-agent's navigation functions to find the target information efficientl...

Takayuki Watanabe

Papers

Double beta decays of 100Mo

The high sensitivity beta-gamma spectrometer ELEGANTS V for rare β(e) and ββ(ee) decays

Experimental evaluation of usability and accessibility of heading elements

An ultra low background beta-gamma spectrometer

Experimental evaluation of usability and accessibility of heading elements.