Non-derivative axionic couplings to nucleons at large and small N

Abstract

Among the possible CP-odd couplings of the axion to ordinary matter, the most relevant ones for phenomenology are the Yukawa couplings to nucleons. We analyze such non-derivative couplings within three complementary approaches: standard effective field theory, the Skyrme model and holographic QCD. In all the cases, the couplings can be related to the CP-odd non-derivative couplings to nucleons of the low-lying mesons and the η . Using the effective field theory approach we derive the expressions for the CP-odd interaction terms as functions of the parameters of the effective Lagrangian. Then, we compute the CP-odd couplings to nucleons of the axion, the η and the pseudo-Goldstone mesons in both the Skyrme and the holographic QCD model with N f = 2, 3 flavors. We relate the coefficients of the non-derivative axion-nucleon couplings to those of the derivative ones. The relations do not explicitly depend on model parameters. This allows us to provide quantitative estimates of these couplings. D. CP violating η -pion-pion coupling 56 E. Alternative numerical estimates of the couplings 57 Introduction and resultsThe Peccei-Quinn proposal [1] for a natural solution of the strong CP problem -the unnaturally small value of the QCD θ angle -implies the existence of a new light neutral pseudoscalar boson, the axion [2, 3]. The original theory, severely constrained by data, was not renormalizable because of the axion coupling to the QCD instanton density [4,5]. In renormalizable axion models, successively proposed in [6,7,8], the axions were also made very weakly interacting with ordinary matter, as required by experimental constraints. These "invisible axions", whose couplings to matter have been deduced using anomalies and the chiral Lagrangian [9]- [14], are nowadays considered among the most promising candidates as dark matter constituents, and also as possible realizations of the inflaton [15]- [20]. As of today, axion-like particles (ALPs) are ubiquitous and serve various purposes. The nomenclature has also evolved but still remains sometimes murky. Axions that solve the strong CP problem are typically called QCD axions. The term "axionlike particle" is often used to refer (only) to other types of axions.Generically, axions display a perturbative shift symmetry and couple to instanton densities. In any reliable quantum field theory (QFT) realization, the symmetry is broken (at best) to a discrete symmetry due to non-perturbative effects. Such effects, related to instantons in weakly-coupled models, induce a mass and, more generally, a potential term, for the QCD axion (see e.g. [13]).ALPs arise very commonly in string theory [21], the simplest example being the Ramond-Ramond (RR) axion of type IIB string theory. They can also arise, after compactification, from internal components of antisymmetric form gauge fields as well as from off-diagonal components of the metric. In these cases, ALPs are related to generalized gauge fields [22,23]. The corresponding gauge symmetries provide the perturbative Peccei-Quinn (PQ) sym...

Fate of false vacua in holographic first-order phase transitions

“…In the following we will also consider a more general setup where part (if not all) of the flavor branes are not antipodal. In general, there can be several distinct flavor brane pairs as it happens in the recently considered Holographic QCD axion scenario [38,39].…”

The holographic QCD axion

“…The couplings of the axion with the nucleons and the photon have no UV contributions and are entirely determined by the low energy action and the mixing with the pseudoscalars. The magnitude of the couplings to nucleons have been estimated in [14].…”

“…can be induced whenever the Peccei-Quinn mechanism is not precise and an effective θ parameter is left over. In [14] it has been found that 27) whereḡ η N N andḡ πN N are the linear in θ CP-odd couplings of η and pions with nucleons. 10 To leading order in the chiral limit they reduce tō…”

“…We thank Michele Redi for discussions about this point 9. Notice that the axion decay constant f a in the present paper differs from that in[14]: f a [here] = √ 2f a [there].…”

Emergent Axions

“…The role of the emergent axion as an inflaton may be investigated along the lines of[94] 12. See also[95].…”