Simplify the following text for a Grade 9 native English-speaking person.
Original: His work in theoretical particle physics exerted great influence on the development of the standard model in the late 20th century especially on the renormalization of elecro-weak model and the gauge theory .
Simplified: His work in theorical particle physics was very important to the development of the standard model in the late 20th century . His work mainly affected the renormalization and the charm quark .