Show that in Fig. 14.2 , normal double (beta)-decay conserves lepton number but neutrinoless double (beta)-decay does

Question:

Show that in Fig. 14.2 , normal double $\beta$-decay conserves lepton number but neutrinoless double $\beta$-decay does not. Define a lepton number by $L \equiv n_{\ell}-n_{\bar{\ell}}$, where $n_{\ell}$ is the total number of leptons and $n_{\bar{\ell}}$ is the total number of antileptons.

Data from fig 14.2

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