Welcome to DCBA Home Page

Welcome to DCBA

DCBA is the acronym of Drift Chamber Beta-ray Analyzer. It could also be easily read as the inverted ABCD.
Here we describe the experimental and physical objectives of the DCBA experiment and its overviews.

DCBA experiment and its backgrounds

The neutrino was first conceived and proposed by W. Pauli in 1930, in order to explain the energy balance and the spin-angular momentum balance in the nuclear beata-decay events. At the beginning when conceived, Pauli himself was in quite skeptical, but in 1956 the evidence of the neutrino-existence was confirmed by F. Reines and C. L. Cowan Jr., through their experiments on neutrino interaction using a nuclear reactor.
The mass of the neutrino was first considered to be zero on the basis of the so-called Standard Model for elementary particles, since no evidence has been found so far against this interpretation. Theoretically, however, Z. Maki, M. Nakagawa and S. Sakata had pointed out in 1962 that if there are different kinds of neutrino species with different masses, a kind of neutrino oscillation, where a species of neutrino could change into another one, could be observed eventually. This was because that a mass of weak-interacting neutrino was expressed as the mixing of mass eigen-values of neutrinos. The mixing matrix is presently called Maki-Nakagawa-Sakata matrix (MNS matrix).
On the other hand, later in the mid-1960's, R. Davis, Jr. had been showing that a much less, about some half or so neutrino-fluxes than thoretically expected, were observed in their soler neutrino chemical experiment. And the period from 1980 through 1990, another evidence of the deficits of some atmospheric neutrino-fluxes was shown by the KAMIOKANDE-II collaboration. This solar neutrino puzzle was confirmed during the period from 1980 through the beginning of 2000, by those experiments of KAMIOKANDE-II, Super Kamiokande, and SNO. In order for explaining these experimental observations, it becomes more and more necessary to assume the existance of neutrino oscillation. Following these observations, thereafter, two big experiments, one called "K2K Experiment" by using KEK-PS and Super-Kamiokande, and the other one called "KamLAND Experiment" using neutrinos from nuclear reactor power plant, have been carried out. And through these experiments, it has been given that the neutrino-oscillation really exists, and at the sametime there could exist the mass differences between three neutrino species. Therefore, what the absolute mass scale is about the neutrino, is the next big problem. Through the experimental observations carried out so far in the wold, it is most widely understood that the absolute mass of the neutrinos could most likely be smaller than 300 meV. As described below, Our "DCBA Experiment" and its successively planning "MTD Experiment" can possibly explore to find the mass values down to 30 meV.~ Turning to the different point of view, it is well known that E. Majorana in 1937 proposed the existence of a neutral massive particle with the character of no difference between a particle and an anti-particle. This kind of particle is presently called Majorana particle. The observation of neutrino-oscillation means that neutrinos have to have masses, and the possibility for neutrinos being Majorana particles is getting more realistic than ever before. And then, if the neutrino has Majorana nature, then in principle, neutrinoless double beta-decay (the double beta-decay without an emission of any neutrinos) is possibly exsisting and so to be observed by the "DCBA Experiment", which has excellent capability to eliminate background events. In another word, an experiment to search for neutrinoless double beta-decay, such as "DCBA" is a realistic and powerfull experiment to search for this Majorana nature of neutrinos.

Principal Objectives of DCBA Experiment

The principal objectives of DCBA experiment can be expressed, in short, by the following single phrase that it is to search for a possibility of the neutrino for being a Majorana particle, and to measure a possible absolute mass value of the neutrinos, and through these efforts, the project is to try to get a useful information for a new fundamental theoretical overview beyond the Standard Model. But this explanation may be too symbolized and not to be easily understood clearly in general, ad so, the following more simplified and detailed explanations may be in order, as described below.

Neutrino is a kind of elementary particles of the matter.

The neutrino is one of the members of the elementary particle group, so-called Leptons, which constitute the matters together with another elementary particle group, called Quarks. It is now well known that a matter consists of atoms, and the atom itself consists of its nucleus and its associated electrons. Further in details, a nucleus is now known to be consisted of its associated proton and neutrons combined strongly. Up until early 1970, these proton and neutron were considered to be fundamental particles of the nature, but, it was revised and verified experimentally in 1978, that both proton and neutron are in fact a compound of more fundamental particles called Quarks.
As a matter of fact, even earlier in 1970, Quarks were theoretically predicted and conceived. And in 1973, Makoto Kobayashi and Toshihide Maskawa proposed the Six-Quark model which requests that there should be six-quarks in the nature in order to explain the newly observed CP-violation phenomena found in neutral K particle decay.~ At present, as shown in Fig.1, the elementary particles constituting the nature are classified as composed with Quarks and Leptons.

Fig. 1Fundamental particles of matter, Quarks and Leptons, and gauge particles connecting these matter particles.

In this Standard Theory, the quarks have different characters from leptons due to the obedience to the Strong Interaction that results in fractional charges of -1/3 and +2/3 with the unit of electron as -1. And the quarks have additional quantum numbers, so-called three kinds of quark-color, namely, red, green and blue. In addition, a quark can never be shown up experimentally for itself singly and could only exist as a compound of two or more quarks conbined into a color-less particles such as pions and protons and so on.

Fig.2 Picture of DCBA-T2. Tracking detector is installed in the cylindrical magnet.

Majorana nature of neutrino

Neutrino, that is a principal object-particle of our experimental research and analysis, belongs to the members of leptons, together with electron, mu- and tau-particles. The essential difference between neutrino and electron (mu- or tau particle) is in its electric charge, namely electron (mu- or tau-particle)has a unit-electron charge but neutrino is never charged. Being non-charged for neutrino has another additional distinctive character different from other fundamental particles. This special distinctive character can be expressed in another way such as in a character which gives no-existing difference between particle and anti-particle. This kind of character, defined by E. Majorana, is called Majorana nature, and accordingly such a particle is called a Majorana Particle. When a particle is charged, there is always an anti-particle which is also charged but with opposite-signed charge. This sort of a particle is called a Dirac Particle. As a matter of fact, only neutrino is of a special kind of particles that have a possibility being of a Majorana Particle.
If the neutrino is assumed to be of a Majorana Particle, it could theoretically have extremely small mass, compared to other leptonic particles such as electron, mu- and tau-particles, which have reasonable masses. This kind of theoretical expectation (or speculation) was first reported by Tsutom Yanagida at a KEK workshop, and was published in the KEK-report (KEK-79-18, 1979). This kind of study on neutrino mass character has also been made by M. Gell-Mann, P. Ramond and R. Slansky at the same time, independently, and has been called as a See-saw mechanism, these days. The reason why this theoretical hypothesis is called See-saw mechanism, is due to its balancing mechanism, which expects two types of neutrinos in its basic expectation. Namely, one type is a neutrino known presently with very light mass, and the other one is yet unobserved owing to its extremely heavy mass. The theory expects that the product of these two neutrino masses is to be equal to the square of Dirac particles such as charged leptons and quarks.
In general, a mass of a particle is observed along with a particle and its anti-particle. And so, if neutrino is a Majorana Particle, the neutrino could be its anti-particle for itself, and accordingly there could independently exist two kinds of neutrinos; one with an extremely light mass and the other with a very heavy mass. In this conjecture, Majorana Particle has to have a mass, and so if this is the case, the so-called Standard Theory that expects neutrino is a massless particle, become now unacceptable. And also, it has been recently observed that there exists a neutrino-oscillation phenomenon, which was originally proposed by the Maki-Nakagawa-Sakata Model, if neutrinos are massive particles. Accordingly, as a result due to this observation, it is now more and more plausible that neutrinos could be of Majorana Particles.

Neutrino and Double Beta Decay Experiment

Here, let us take the nucleus of Nd-150 as an example. As we understand, Nd-150 decays into Sm-150 by emitting two electrons. In general, an electron-ray emitted from a nucleus is called a Beta-ray, and so the case of the above mentioned Nd-150 decay emitting two electrons is called a Double Beta Decay. In a normal Double Beta Decay, two electrons and two anti-electrons are emitted, and as a result in this transition, the lepton numbers of the system are conserved and to be zero. On the other hand, if the neutrino has Majorana nature, in some probabilities a transition, where the electrons are emitted without any emission of electron-neutrinos, could happen. In this case of the transition, the lepton number in the initial system is zero, but the one after the decay becomes two, and as a result of this transition, the lepton numbers are not conserved. This kind of a transition is called specifically as Neutrinoless Double Beta Decay. Accordingly an observation of neutrinoless double beta decay in an experiment can lead to a confirmation of the speculation that neutrinos are Majorana particles.