Gauge field and fermion production during axion inflation

Abstract

We study the dual production of helical Abelian gauge fields and chiral fermions through the Chern-Simons (CS) coupling with a pseudo-scalar inflaton in the presence of a chiral anomaly. Through the CS term, the motion of the inflaton induces a tachyonic instability for one of the two helicities of the gauge field. We show that the resulting helical gauge field necessarily leads to the production of chiral fermions by deforming their Fermi sphere into discrete Landau levels. The population of the lowest Landau level leads to a chiral asymmetry as inferred from the chiral anomaly, while the higher levels are populated symmetrically through pair production. From the backreaction of the fermions on the gauge field production we derive a conservative but stringent upper bound on the magnitude of the gauge fields. Consequently, we find that the scalar perturbations sourced by these helical gauge fields, responsible for enhanced structure formation on small scales, get reduced significantly. We also discuss the fate of the primordial chiral asymmetry and of the helical gauge fields after inflation, and show that the instability in the chiral plasma tends to erase these primordial asymmetries. This result may impact scenarios where the baryon asymmetry of the Universe is connected to primordial magnetic fields.

Chiral anomaly, Schwinger effect, Euler-Heisenberg Lagrangian and application to axion inflation

“…where θ denotes the Heaviside function. From this equation, one can see that the dispersion relation for the left-/right-handed fermions (proportional to χ L s , χ R s respectively) is given by ω L/R 0 = ∓sΠ z , reproducing the well known result for the lowest Landau level [9,11]. This is also illustrated in Fig.…”

“…In the limit of massless fermions, the lowest (gap-less) energy level induces asymmetric particle production. This in turn is a beautiful realization of the chiral anomaly in quantum field theory (QFT) (Adler Bell Jackiw anomaly [6,7]) in the presence of helical electric and magnetic background fields [8][9][10], discussed first in [11] in the context of Weyl fermions in a crystal. These arguments have been extended to non-abelian gauge fields in [12].…”

Energy-momentum tensor and helicity for gauge fields coupled to a pseudoscalar inflaton

“…To complement and confirm these analytic estimates we now present numerical results based on the Einstein-Klein-Gordon equations (13), (14) and (17) including the averaged energy-momentum tensor and helicity of gauge fields where we allow ξ = gφ/2H to vary with time. In the case of the aforementioned linear potential,…”

“…where g is a coupling constant with a physical dimension of length, or inverse energy (we put = c = 1), leads to decay of the pseudo-scalar field into gauge fields modifying its background dynamics [1] and to a wide range of potentially observable signatures including primordial magnetic fields [2][3][4][5][6][7][8][9][10], preheating at the end of * mario.ballardini@gmail.com † matteo.braglia2@unibo.it ‡ fabio.finelli@inaf.it § giovanni.marozzi@unipi.it ¶ alstar@landau.ac.ru inflation [4,11,12] , baryogenesis and leptogenesis [13][14][15], equilateral non-Gaussianites [16][17][18], chiral gravitational waves in the range of direct detection by gravitational wave antennas [19][20][21][22], and primordial black holes (PBHs) [18,[23][24][25][26]. The decay of the inflaton into gauge fields due to the coupling in Eq.…”

Gravitational leptogenesis with kination and gravitational reheating

“…[51][52][53]). The leptogenesis in the present case, indeed, corresponds to the particle production through the chiral anomaly in the gauge field case, which can change the efficiency of the gauge field production [36][37][38]. In our study we do not take into account the back reaction on the gravitational wave production from the particle production.…”