Axiogenesis

Abstract

We propose a mechanism called axiogenesis where the cosmological excess of baryons over antibaryons is generated from the rotation of the QCD axion. The Peccei-Quinn (PQ) symmetry may be explicitly broken in the early universe, inducing the rotation of a PQ charged scalar field. The rotation corresponds to the asymmetry of the PQ charge, which is converted into the baryon asymmetry via QCD and electroweak sphaleron transitions. In the concrete model we explore, interesting phenomenology arises due to the prediction of a small decay constant and the connections with new physics at the LHC and future colliders and with axion dark matter.Introduction.-One of the goals of fundamental physics is to understand the origin of the Universe. For this purpose, the Standard Model (SM) of particle physics needs an extension to explain the cosmological excess of matter over antimatter. Mechanisms to generate the baryon asymmetry have been intensively studied in the literature under the name of baryogenesis [1][2][3][4][5][6][7][8].The SM also needs an extension to explain the smallness of CP violation in QCD [9] which on theoretical grounds is expected to be large [10]. This is known as the strong CP problem and can be elegantly solved by the Peccei-Quinn (PQ) mechanism [11,12]. The so-called the PQ symmetry is spontaneously broken to yield a pseudo Nambu-Goldstone boson, the axion [13,14]. The PQ symmetry is explicitly broken by the quantum effects of QCD of the Adler-Bell-Jeckiw type [15,16]. The quantum effects give a potential to the axion and drive the axion field value to the point where CP symmetry is restored, solving the strong CP problem. The axion is also a dark matter candidate [17][18][19], which makes the PQ mechanism even more attractive.We discover that when the PQ mechanism is introduced into the SM, the baryon (B) and lepton (L) asymmetries are generated in a wide class of models. We call the following baryogenesis scheme as axiogenesis, which in general includes two main ingredients: 1) an asymmetry of the PQ charge is generated in the early universe as a coherent rotation in the axion direction and 2) the PQ asymmetry is later transferred to the B + L asymmetry via the QCD and electroweak sphaleron transitions. We contrast axiogenesis with other existing baryogenesis models after we introduce a concrete example. (We may convert the B + L asymmetry into the B − L asymmetry by some B − L breaking interaction. The investigation of such a scenario will be considered in a future work [20].)The PQ symmetry is an approximate global symmetry which is explicitly broken by the QCD anomaly. Given that the symmetry is not exact, it is conceivable that the PQ symmetry is significantly broken in the early universe, and the rotation of the axion is induced. In fact, it is expected that quantum gravity does not allow for a global symmetry [21][22][23][24][25] and the PQ symmetry is at best understood as an accidental symmetry explicitly broken by higher dimensional operators [26][27][28][29]. Even when one requires that t...

Axion Kinetic Misalignment Mechanism

“…As shown in Ref. [55], even after thermalization, P continues to rotate because it is energetically favorable to keep the charge asymmetry in the rotation rather than particle excitations. At thermalization, an elliptical trajectory becomes circular and the charge density n θ stays conserved up to cosmic expansion.…”

“…Other known production mechanisms in this mass range are (1) parametric resonance from a PQ symmetry breaking field [39,40], (2) anharmonicity effects [41][42][43] when θ i approaches π due to fine-tuning or inflationary dynamics [44,45], (3) decays of unstable domain walls [46][47][48][49][50][51][52][53], and (4) production during a kination era [54]. Contrary to these mechanisms, kinetic misalignment offers an exciting theoretical connection with the baryon asymmetry of the Universe through so-called axiogenesis [55].…”

“…2 showing that kinetic misalignment is successful over a wide range of parameters. Besides signals in axion searches in the mass range Oð0.1-100Þ meV, kinetic misalignment can provide a unified origin of dark matter and the cosmological excess of matter over antimatter [55].…”

Predictions for axion couplings from ALP cogenesis

“…Baryogenesis can result from a condensate carrying charge Q other than baryon number [14,15], although they require an interaction that violates both Q and B to be in thermal equilibrium. The baryon number violation by the electroweak sphaleron process [16,17] is utilized in leptogenesis [18,19], electroweak baryogenesis [17,20,21], and together with the strong sphaleron process in axiogenesis [22].…”

QCD baryogenesis

“…Since WW is connected to the baryon current density j μ B through the anomaly equation ∂ μ j μ B ¼ ðα W =8πÞTr½WW], the GG condensate generates an effective chemical potential [9,13,14] for baryons given by…”