Becchi-Rouet-Stora-Tyutin (BRST)

The Becchi-Rouet-Stora-Tyutin (BRST) symmetry is a mathematical tool used in quantum physics to ensure that certain theories describing fundamental particles and forces remain consistent and free of inconsistencies. It introduces special auxiliary variables, called ghost fields, to maintain the symmetry of gauge theories, which are the frameworks for forces like electromagnetism and the weak and strong nuclear interactions. BRST symmetry helps to identify unphysical states and ensure that only meaningful, observable states contribute to calculations, making the theory both mathematically sound and predictive.