The axion quality problem: global symmetry breaking and wormholes

Abstract

Continuous global symmetries are expected to be broken by gravity, which can lead to important phenomenological consequences. A prime example is the threat that this poses to the viability of the Peccei-Quinn solution to the strong CP problem. In this paper, we explore the impact of wormholes as a source of global symmetry breaking by gravity. We review the current status of wormholes and global symmetries and note that, surprisingly, the axion has a quality problem within non-perturbative Einstein gravity. Although these wormholes lead to a large breaking of global symmetries, we show that their effect is nonetheless relevant for the model building of gauge protected axions. We also find wormhole solutions within two scenarios: (i) an extended global symmetry group within Einstein gravity, and (ii) U(1) wormholes within the low-energy limit of an open String Theory. The former allows us to show that the concept of a global symmetry in General Relativity is somewhat ill-defined. The latter illustrates that for motivated values of the string coupling constant, axions appear to have a quality problem within the open String Theory we consider.

The hubble tension as a hint of leptogenesis and neutrino mass generation

“…L is a global symmetry, a pseudo-Goldstone boson appears on the spectrum: the majoron φ [56]. After SSB, it is convenient to parametrize 2 Global symmetries are also expected to be explicitly broken by quantum gravity [64][65][66][67][68][69]. This has two important consequences.…”

“…Upon becoming non-relativistic these majorons decay back into neutrinos, producing a net enhancement in N eff [54,60]. Ensuring the effects of the majoron occur at the correct epoch requires a mass m φ near the eV scale; this scale, however, is not arbitrary, and can easily be motivated should global lepton number be explicitly broken by physics at the Planck scale [62,63], as might be expected in theories of quantum gravity [64][65][66][67][68][69]. In order to generate a sufficient level of damping one requires neutrino-majoron couplings λ ∼ 10 −13 [54,55].…”

An even lighter QCD axion

“…The threat posed on traditional QCD axion models by quantum non-perturbative gravitational corrections [56][57][58][59][60][61][62][63][64][65] may also affect the models discussed here, as f a is not very far from the Planck scale. These contributions are usually parametrized via effective operators, suppressed by powers of the Planck mass, that could explicitly violate the PQ symmetry and thus spoil the solution to the strong CP problem [56][57][58][59].…”

Shining ALP Dark Radiation

“…1 They can be either heavy or light but are unlikely to be massless since quantum gravity is argued to break any global symmetry, e.g. [21][22][23][24][25][26][27][28][29][30] 2 (for a nice introduction of the string landscape see, e.g. [31]).…”