Say we have a Lagrangian [tex]\mathcal{L}=\bar{u}i\kern+0.15em /\kern-0.65em Du+\bar{d}i\kern+0.15em /\kern-0.65em Dd-m_u\bar{u}u-m_d\bar{d}d,[/tex](adsbygoogle = window.adsbygoogle || []).push({});

where u and d are fermions. In Peskin&Schroeder p. 667 it says that if [tex]m_u[/tex] and [tex]m_d[/tex] are very small, we can neglect the last two terms of the Lagrangian.

I'd like to know a somewhat rigorous reason for this.

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# Neglecting terms in a Lagrangian

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