Our Standard Model of particle physics describes nature to extraordinary 
precision, but cannot be the full picture. The Standard Model lacks an 
explanation for both dark matter and neutrino masses, and at times relies 
on fine tuning of parameters to match experimental evidence. Many resolutions 
of these problems point towards the existence of new symmetries at higher 
energies. Since these proposed symmetries are not manifest at energy scales 
we have probed, they must be hidden in our low energy regime.

An intriguing signal of such hidden symmetry is a new light particle: 
a pseudo-Goldstone boson. In this talk, I will first explain what a 
pseudo-Goldstone boson is, and then I will give an example of one we 
have seen in nature: the pion. Next, I will illustrate how a pseudo-Goldstone 
boson called the axion could help solve the so-called, “Strong CP problem,” 
explaining why charge and parity (CP) symmetry violation is missing in the 
strong nuclear force. Then, I will present models of heavy axions and 
strategies to search for them.