= Solution
A <Majorana neutrino> is its own <antiparticle>, with a <mass> term that can violate <lepton number> by two units. It is not thereby a state of both readily observable <helicities> with equal weak rates: charged-current vertices still select <chirality>. In light-neutrino exchange, the needed <helicity> connection between two beta vertices supplies a neutrino-mass factor.
Ordinary two-neutrino <double beta decay> is $(A,Z)\to(A,Z+2)+2e^-+2\bar\nu_e$ and conserves <lepton number>. It can occur when the direct single-beta route is energetically forbidden or highly suppressed but the double change is allowed. <neutrinoless double beta decay>, $(A,Z)\to(A,Z+2)+2e^-$, violates <lepton number> by two. For the conventional <light-Majorana exchange in neutrinoless double beta decay> mechanism,
$$
[T_{1/2}^{0\nu}]^{-1}=G_{0\nu}|\mathcal M_{0\nu}|^2
\left|\frac{m_{\beta\beta}}{m_e}\right|^2,\qquad
m_{\beta\beta}=\left|\sum_iU_{ei}^2m_i\right|.
$$
Here $U$ is the <lepton mixing matrix>, $G_{0\nu}$ the phase-space factor in this convention and $\mathcal M_{0\nu}$ a <nuclear matrix element>. Complex phases can cancel contributions to the <effective Majorana mass>; oscillation <mass> differences alone do not fix it. Other lepton-number-violating mechanisms can also cause the process, so a rate is not a model-independent measurement of $m_{\beta\beta}$.
The experimental signature is a peak in the summed <electron> <energy> at the nuclear Q-value, apart from small recoil and <energy> losses. Two-neutrino decay instead gives a continuous sum-energy <radiation spectrum> because <neutrinos> carry away variable <energy>. Isotope enrichment, large exposure, good <energy resolution> and radiopure shielding improve sensitivity; gamma backgrounds, <radon>, surface events and the two-neutrino tail require <calibration> and sideband controls. Coincidence or track information can test whether the topology is compatible with two <electrons>. A later concrete example is GERDA's 2013 germanium-76 source-detector search, which used pulse-shape discrimination and background controls rather than identifying any isolated <energy> deposit as a decay.<Results on neutrinoless double beta decay of 76Ge from GERDA Phase I by the GERDA Collaboration>[experimental method and pulse-shape/background analysis; published after this exam]{ref}
\b[Observation would establish lepton-number violation and support a Majorana <mass> component; a null search constrains mechanisms and does not prove <neutrinos> are Dirac particles.]
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