operator in (\[WT\]) with the identification of $L = L_e$ and $L' = L_{\mu}, L_{\tau}$ does indeed conserve this quantum number. However, in the string-theory motivated (top down) approach this conserved quantity arises as an [*accidental*]{} symmetry pertaining to the underlying geometry of the string compactification. It reflects the different geometrical location of the fields (in terms of orbifold fixed points) of the electron doublet from the muon and tau doublets.

To make contact with data it is necessary to consider the Yukawa interactions of the charged leptons as well. To that end, our string-inspired model should have a superpotential of the form $$\begin{aligned}
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W &=& \lambda_T L T L' + \lambda_1 H_1 T H'_1 + \lambda_2 H_2
\oline{T} H'_2 \nonumber \\
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 & & + \lambda_3 S_3 T \oline{T} + \lambda_4 S_4 H_1 H_2 +
 \lambda_5 S_5 H'_1 H'_2 \nonumber \\
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 & & + \wh{\lambda}_4 \wh{S}_4 H_1 H'_2 +
 \wh{\lambda}_5 \wh{S}_5 H'_1 H_2 \nonumber \\
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 & & + \lambda_6 S_6 L H_1 + \lambda_7 S_7 L' H'_1 \nonumber \\
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 & & + \wh{\lambda}_6 \wh{S}_6 L H'_1 + \wh{\lambda}_7 \wh{S}_7 L' H_1 ,
%
\label{Wfull} \end{aligned}$$ where generation indices have been suppressed. The terms proportional to 