Science

Overview

COBAND experiment searches for the cosmic background neutrino which is predicted to exist uniformly in the universe after it was produced at the joining of the Big Bang like the cosmic microwave background radiation. In the standard cosmology, the cosmic background neutrino became free 1 second after the Big Bang and exist with a density of 110 per cm³ at 1.9K in the present universe. The density and temperature of the cosmic background neutrino is close to those of the cosmic microwave background radiation but is liberated much earlier than 300,000 year after the Big Bang when the cosmic microwave background radiation is liberated. While the observation of the cosmic microwave background radiation has enabled us to understand the beginning of the universe deeply, the discovery of the cosmic background neutrino will give us to a probe to understand much earlier universe and will make great progress of the cosmology.

In particle physics, the neutrino physics have developed very rapidly since the neutrino oscillation was discovered in 1998. By the observation of the neutrino oscillation, the neutrino masses were found to be non-zero and the mass square differences and the mixing angles between different types of neutrino were measured very accurately. However the neutrino mass itself has not been measured yet. It is a very important subject to measure the neutrino mass to develop not only the neutrino physics but also the particle physics. The COBAND experiment measure the neutrino mass by observing the neutrino decay. Heavier neutrino decays into a lighter neutrino and a photon. By measuring the energy of this photon, we can determine the heavy neutrino mass.






Fig.1．History of the Universe. Cosmic background neutrino became free 1second after the Big Bang, and the cosmic microwave background radiation became 300,000 years later. The cosmic background neutrino has not been observed yet.




Experiment Plan

The energy distribution of the photon from the cosmc background neutrino decay has a distinctive cutoff at the high energy end as shown in Fig. 2. The cutoff energy depends on the neutrino mass,and is in the far-inrared photon energy range around tens of meV. In the COBAND experimnet, we measure the cosmic infrared photon energy spectrum continuously to find the energy cutoff from the cosmic background neutrino decay against the zodiacal emission which is the dominant background.






Fig. 2．Cosmic infrared photon energy distribution. About the energy measurement of the cosmic infrared background, COBE and AKARI satellite experiments measured 4 energy points. The energy distribution (red curve) of the decay photon of the cosmic background neutrino with a mass of 50meV and a lifetime of 10¹⁴ year has a cutoff at 25meV.

The present lower limit of the neutrino lifetime is 10¹² year obtained from the results of the cosmic infrared background measured by COBE and AKARI satellites. In Left-Right Symmetric Model, the neutrino lifetime is predicted to be 1.5x10¹⁷ year at minimum. The COBAND experiment aims at observing the cosmic background neutrino decay with a lifetime of 1.5x10¹⁷ year. First we will perform the rocket experiment with a lifetime sensitivity of 10¹⁴ year, and next perform the satellite experiment with a lifetime sensitivity of 10¹⁷ year. In the rocket experiment, we will measure the photon energy by the telescope with a diameter of 15cm and a viewing angle of 0.006°x 0.05°for 200 seconds. We can observe the photon from the cosmic background neutrino decay at 5σ significanse, if the lifetime of the neutrino is less than 10¹⁴ year as shown in Fig.3. In the satellite experiment with a telescope diameter of 20cm and a viewing angle of 0.1°for 100 days, we can observe the photon from the cosmic background neutrino decay at 5σ significanse, if the lifetime of the neutrino is less than 10¹⁷ year.








Fig.3．Simulation results of the rocket experiment. One example of the wavelength distribution of the decay photon of the neutrino with a mass of 50meV and a liftime of 10¹⁴ year (top figure). We made the simulation of the background (zodiacal emmision) plus the neutrino decay signal, and by fitting the simulation data to a sum of the expected signal curve plus the expected background curve, we obtained the wavelength distribution(top figure) after subtracting the background contribution. By doing 1000 simulation each for 10 lifetime points and 6 mass points, we obtained the lifetime where the neutrino decay can be observed at 5σ significance by the rocket expeiment as a function of the neutrino masses (biottom figure).