PHYS08200604017 Manimala Mitra - Homi Bhabha National Institute

Figure 2.1: The possible neutrino spectra: (a) normal mass hierarchy (b) inverted mass
hierarchy.
observation. Measurement of WMAP 5 year analysis [16] give this following bound on
the sum of the neutrino masses
∑
mi ≤ 0.19eV (2.4)
There are other direct tests on neutrino mass, such as beta decay, neutrinoless double
beta decay.
Beta Decay
In the beta decay experiments measuring the distortion in the end point spectrum, neutrino
masses can be directly tested. In the following decay
d → u+e − + ¯ν e , (2.5)
the energy of the electron is E e = Q−E ν , which is maximal for E e = Q−m νe , where m νe
is m νe = m 1 |U e1 | 2 +m 2 |U e2 | 2 +m 3 |U e3 | 2 . Here Q represents the energy released in the β
decay. The energy spectrum of the electron is ∝ √ (Q−E e ) 2 −m 2 ν e
, and so m νe ≠ 0 will
imply adeviation fromthelineQ−E e . So far, thebest constraint comes fromMAINZ [17]
and TROITSK [18] experiments:
m νe ≤ 2.2eV. (2.6)
The future beta decay experiment, Katrin [19], which is scheduled to begin data taking
on 2010, is expected to reach a sensivity of 0.2eV to neutrino masses.
Neutrinoless Double-Beta Decay
Neutrinoless double-beta decay [20] experiment (0νββ) is very important for neutrino
physics. The observation of such a process would imply that neutrinos are Majorana particles.
In Fig.2.2, the Feynman diagram for this process has been shown. The neutrinoless
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