Abstract
Since the discovery of neutrino oscillation in atmospheric neutrinos by the Super-Kamiokande experiment in 1998, study of neutrinos has been one of exciting fields in high-energy physics. All the mixing angles were measured. Quests for 1) measurements of the remaining parameters, the lightest neutrino mass, the CP violating phase(s), and the sign of mass splitting between the mass eigenstates m3 and m1, and 2) better measurements to determine whether the mixing angle theta23 is less than pi/4, are in progress in a well-controlled manner. Determining the nature of neutrinos, whether they are Dirac or Majorana particles is also in progress with continuous improvement. On the other hand, although the ideas of detecting cosmic neutrino background have been discussed since 1960s, there has not been a serious concerted effort to achieve this goal. One of the reasons is that it is extremely difficult to detect such low energy neutrinos from the Big Bang. While there has been tremendous accumulation of information on Cosmic Microwave Background since its discovery in 1965, there is no direct evidence for Cosmic Neutrino Background. The importance of detecting Cosmic Neutrino Background is that, although detailed studies of Big Bang Nucleosynthesis and Cosmic Microwave Background give information of the early Universe at ~a few minutes old and ~300 k years old, respectively, observation of Cosmic Neutrino Background allows us to study the early Universe at $\sim$ 1 sec old. This article reviews progress made in the past 50 years on detection methods of Cosmic Neutrino Background.
Highlights
According to the Big Bang theory, neutrinos decoupled from other particles earlier than Cosmic Microwave Background (CMB)
The direct detection methods can further be divided into two classes: measurements of energy/momentum transfer from CνB neutrinos to target materials in terms of acceleration of the targets through torsion unbalance or possibly through calorimetry, and measurements of energy of outgoing electrons produced by neutrino capture by β-decaying nuclei
With the current available technology, none of the proposed methods that are described in this review are close to be reality, except for the promising Project 8 and PTOLEMY experiment, unless local neutrino overdensity is much larger than expected
Summary
According to the Big Bang theory, neutrinos decoupled from other particles earlier than Cosmic Microwave Background (CMB). Detecting these neutrinos will give direct information of the earliest possible epoch of the Universe we can observe after the Big Bang. From Equation (1) for a relativistic boson of type i its number density is ζ (3)giT3/π 2 and for a relativistic fermion (3/4)ζ (3)giT3/π 2 where ζ is the Riemann zeta function. The energy density is (π 2/30)giT4 for a boson and (7/8)(π 2/30)giT4 for a fermion [3]. From the Friedman equation for radiation-dominant epoch, the Hubble parameter is
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