Solving the strong CP problem with massless grand-color quarks
Ravneet Bedi (School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, 55455, USA)
; Tony Gherghetta (School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, 55455, USA)
; Keisuke Harigaya (Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba, 277-8583, Japan, Department of Physics, The University of Chicago, Chicago, Illinois, 60637, USA, Enrico Fermi Institute and Kavli Institute for Cosmological Physics, The University of Chicago, Chicago, Illinois, 60637, USA)
We propose a solution to the strong CP problem that specifically relies on massless quarks and has no light axion. The QCD color group SU(3) is embedded into a larger, simple gauge group (grand-color) where one of the massless, colored fermions enjoys an anomalous chiral symmetry, rendering the strong CP phase unphysical. The grand-color gauge group is Higgsed down to SU(3) × $$ {G}_{c^{\prime }} $$ , after which $$ {G}_{c^{\prime }} $$ eventually confines at a lower scale, spontaneously breaking the chiral symmetry and generating a real, positive mass to the massless, colored fermion. Since the chiral symmetry has a $$ {G}_{c^{\prime }} $$ anomaly, there is no corresponding light Nambu-Goldstone boson. The anomalous chiral symmetry can be an accidental symmetry that arises from an exact discrete symmetry without introducing a domain wall problem. Potential experimental signals of our mechanism include vector-like quarks near the TeV scale, pseudo Nambu-Goldstone bosons below the 10 GeV scale, light dark matter decay, and primordial gravitational waves from the new strong dynamics.