How is the Peccei-Quinn symmetry protected from gravitational breaking? · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
How is the Peccei-Quinn symmetry protected from gravitational breaking?
Plain
The axion solution needs a symmetry to be almost exact, but gravity is expected to break all such global symmetries slightly. Even a tiny breaking would spoil the solution.
Precise · changed · core
Revision 1: Planck-suppressed operators $O \sim \Phi^n / M_{\mathrm{Pl}}^{(n-4)}$ shift $\theta_{\mathrm{bar}}$ by $\sim f_a^n / (M_{\mathrm{Pl}}^{(n-4)} m_a^2 f_a^2)$; keeping $\mid\theta_{\mathrm{bar}}\mid < 1e-10$ for $f_a \sim 1e10\,\mathrm{GeV}$ requires n > ~ 10. An answer is a UV mechanism (gauge discrete symmetries, composite or extra-dimensional axions) that provably forbids operators up to the required n with predicted low-energy signatures.
Revision 2: Planck-suppressed operators $O \sim \Phi^n / M_{\mathrm{Pl}}^{(n-4)}$ shift $\theta_{\mathrm{bar}}$ by $\sim f_a^n / (M_{\mathrm{Pl}}^{(n-4)} m_a^2 f_a^2)$; keeping $\mid\theta_{\mathrm{bar}}\mid < 10^{-10}$ for $f_a \sim 10^{10}\,\mathrm{GeV}$ requires $n > \sim 10$. An answer is a UV mechanism (gauge discrete symmetries, composite or extra-dimensional axions) that provably forbids operators up to the required n with predicted low-energy signatures.
Settled by
A UV-complete model with the required protection and a distinct prediction, for example heavy axions or modified axion couplings, confirmed by experiment.
Kind
phenomenon
Answer type
mechanism
References
(empty)
Literature status
open
Status note
(empty)
Posed since
1992

Decisions