Seminar 
Title: Form Invariance, Topological Fluctuations and Mass Gap of Yang-Mills Theory 
Speaker: Dr. Ya-Chao Qian (钱雅超) 
Time: 3:00PM, Dec. 13th, 2017 (Wednesday) 
Place: Theoretical Physics Division，319 

Abstract: In order to have a new perspective on the long-standing problem of the mass gap in Yang-Mills theory, we study the quantum Yang-Mills theory in the presence of topologically nontrivial backgrounds. The topologically stable gauge fields are constrained by the form-invariance condition and the topological properties. Obeying these constraints, the known classical solutions to the Yang-Mills equation in the 3- and 4-dimensional Euclidean spaces are recovered, and the other allowed configurations form the nontrivial topological fluctuations at quantum level. Together, they constitute the background configurations, upon which the quantum Yang-Mills theory can be constructed. We demonstrate that the theory mimics the Higgs mechanism in a certain limit and develops a mass gap at semi-classical level on a flat space with finite size or on a sphere.