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.

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Neglecting terms in a Lagrangian

**Physics Forums | Science Articles, Homework Help, Discussion**