Serguey Petrov: Observables in Neutrino Mass Spectroscopy Using Atoms
The speaker started his talk by explaining that in order to make progress in neutrino physics we must determine the nature of neutrinos – Dirac or Majorana, and the type of the neutrino mass spectrum. Also, we direly need to determine the absolute scale of neutrino masses. Questions one wants to answer are also about the CP symmetry in the lepton sector. All these issues can in principle be addressed by a future experiment.
What is discussed in Petrov’s talk is the radiative emission of neutrino pairs (RENP) in atoms. This deexcitation from a metastable level the emission of neutrino pairs is accompanied by a photon. It is a combined weak and EM process, of the type E1xM1, with a change of angular momentum by two units. This possibility was investigated by Yoshimura in 2007, and a group has been formed at Okayama University in 2009 to study the feasibility of an experiment. An arxiv (1211.4904) paper was produced. Instead arxiv:1209.4808 describes what is discussed in Petrov’s talk.
The rate of the RENP process is very small, proportional to the energy difference of the levels (1 eV). One can overcome this by a macro-coherence amplification of the rate. Similar amplification is known to exist and observed e.g. in single photon emission (super-radiance), predicted by Dicke in 1954.
The proposed experimental method consist in measuring under irradiation of two counter-propagatin lasers, the continuous gamma energy spectrum below each of the six threshold energies corresponding to the production of the six different pairs of neutrinos, nu_1, nu_2, nu_3. One makes a scan in energy.
The photon energy needs to be measured with very high precision in order to satisfy the threshold requirements. The absolute neutrino mass scale can then be determined from the threshold behaviour of the effect. The nature of massive neutrinos (Dirac or Majorana) is instead more challenging to determine in this setup. Also the CPV phase measurement is challenging to perform, requiring a high statistics. One possible exception is the case of an inverted hierarchy spectrum where the difference between the spectral rates can reach 10%. For the normal hierarchy, the analogous difference is at most of a few percents.