The Neutrino

Neutrinos have been a subject of intense study from decades. Neutrino was first proposed by W. Pauli in 1930 and term "neutrino" coined by E. Fermi in 1934. Neutrino was first detected in 1956 by F. Reines and C. Cowan. It is the most abundant matter particle and can be produced from wide range of physical phenomenon.

Experimental effort to probe the neutrino properties are spread across all horizons; Underground based, surface based and space based. Similarly, the neutrino properties are tightly knit in three regime: nuclear, particle and cosmological. Neutrinoless double beta decay (0νββ) decay if observed would then simultaneously address the multiple perplexing questions associated to neutrino,

Observed neutrino properties:

(i) Interact only via weak force

(ii) Zero electrical charge

(iii) Spin 1/2 fermion

(iv) Neutrino appears in three flavors νe, νμ, ντ and corresponding flavors of antineutrinos 

(v) Neutrino flavor oscillates, with the measured mixing angles θ12, θ23 and θ13 

(vi) Flavor oscillation manifests non vanishing neutrino mass, mν is not equal to 0 

(vii) Neutrinos are left-handed and antineutrinos are right-handed

UnObserved neutrino properties:

(i) Absolute mass scale of neutrino
(ii) Neutrino mass hierarchy, i.e. normal or inverted

(iii) Neutrinos distinguishable to its anti-particle i.e. Dirac or indistinguishable i.e. Majorana

(iv) Total lepton number remains conserved since flavor lepton number violates in neutrino propagation

(v) Origin of neutrino mass, i.e. seesaw mechanism

(vi) CP violating phase (δCP )

(vii) Matter- antimatter asymmetry, i.e. leptogenesis

(viii) Additional neutrino flavors beyond 3 active flavors i.e. sterile neutrino

(ix) Neutrino electromagnetic properties, i.e. neutrino magnetic moment, neutrino millicharge, neutrino, charge radius

Determination of Neutrino Mass

The phenomenon of neutrino flavor oscillation (να → νβ , where α,β = e,μ, τ ), observed by Super-Kamiokande and Sudbury Neutrino Observatory leads to the indication of non zero value of the neutrino mass. Standard Model (SM) of elementary particles unable to interpret this anomaly since SM presumes massless neutrinos. Massive neutrinos are the strongest indication of the presence of the physics beyond Standard Model (BSM).

Bounds on Neutrino Mass comes from Four different Physical Phenomenon:

(I) Neutrino Oscillations

(II) Cosmology

(III) β Decay

(IV) 0νββ Decay