Physics GCE Question - not allowed to use conservation of baryon/lepton number

[dalcde]

Homework Statement

State and discuss how three conservation laws apply to this decay event (D⁰ -> K⁺+$\pi^-$)

Homework Equations

The Attempt at a Solution

I talked about conservation of charge, lepton lepton number and baryon number. However, the marking scheme (actually the examiner's report) says "some lost marks by
trying to apply rules such as lepton number and strangeness." I believe it means that conservation of lepton number is regarded as wrong (and baryon number too, probably). Does anyone know why it is so (I know lepton numbers is silly here...)? Or is it my misunderstanding of the statement? (I have no problem with the "strangeness" part because it is not conserved by weak interactions)Edit: This is question 18 b iv of GCE June 2011 Physics Unit 4.

 

[jbsteele]

The three conservation laws that apply to this decay event are conservation of charge, conservation of baryon number, and conservation of momentum. Conservation of charge states that the total amount of electric charge before a reaction is equal to the total amount of electric charge after the reaction. In this case, the D0 particle has a charge of 0 and the K++ and pi− particles have charges of +1 and −1 respectively. Therefore, the total charge of the initial particles is 0 and the total charge of the final particles is also 0, so charge is conserved. Conservation of baryon number states that the total number of baryons before a reaction is equal to the total number of baryons after the reaction. In this case, the D0 particle has a baryon number of 1 and the K++ and pi− particles have baryon numbers of 1 and 0 respectively. Therefore, the total baryon number of the initial particles is 1 and the total baryon number of the final particles is also 1, so baryon number is conserved. Conservation of momentum states that the total momentum of a system before a reaction is equal to the total momentum of the system after the reaction. In this case, the momentum of the initial particles (the D0 particle and the K++ and pi− particles) is equal to the momentum of the final particles (the K++ and pi− particles). Therefore, momentum is conserved.