Baryon asymmetry and gravitational waves from pseudoscalar inflation

Abstract

In models of inflation driven by an axion-like pseudoscalar field, the inflaton, a, may couple to the standard model hypercharge via a Chern-Simons-type interaction, L ⊃ a/ (4Λ) FF .This coupling results in explosive gauge field production during inflation, especially at its last stage, which has interesting phenomenological consequences: For one thing, the primordial hypermagnetic field is maximally helical. It is thus capable of sourcing the generation of nonzero baryon number, via the standard model chiral anomaly, around the time of electroweak symmetry breaking. For another thing, the gauge field production during inflation feeds back into the primordial tensor power spectrum, leaving an imprint in the stochastic background of gravitational waves (GWs). In this paper, we focus on the correlation between these two phenomena. Working in the approximation of instant reheating, we (1) update the investigation of baryogenesis via hypermagnetic fields from pseudoscalar inflation and (2) examine the corresponding implications for the GW spectrum. We find that successful baryogenesis requires a suppression scale Λ of around Λ ∼ 3 × 10 17 GeV, which corresponds to a relatively weakly coupled axion. The gauge field production at the end of inflation is then typically accompanied by a peak in the GW spectrum at frequencies in the MHz range or above. The detection of such a peak is out of reach of present-day technology; but in the future, it may serve as a smoking-gun signal for baryogenesis from pseudoscalar inflation.Conversely, models that do yield an observable GW signal suffer from the overproduction of baryon number, unless the reheating temperature is lower than the electroweak scale.

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

“…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.…”

Inflationary Magnetogenesis with Helical Coupling

“…Primordial helical hypermagnetic fields may be responsible for generating baryon asymmetry of the universe [26][27][28][29][30][31]. This imposes a post-inflationary constraint (17) on the admissible values of 0 and m in our simple scenario of monotonic evolution of .…”

“…One of the most interesting effects of evolving helical hypermagnetic fields is generation of baryon number in the early hot universe [24,25]. This opens up an intriguing possibility of explaining the observed baryon asymmetry ( b = b / ∼ 10 −10 , where b is the baryon number density, and is the entropy density in the late-time universe) [26][27][28][29][30][31]. According to the recent estimates [30], the resulting baryon asymmetry, when expressed through the current strength 0 and coherence length 0 of (originally maximally helical) magnetic field, turns out to be (with some theoretical uncertainty in the exponent) b ∼ 10 −(9-12) 0 Mpc…”

“…Helical hypermagnetic fields source the baryon number in the hot universe [24,25], opening up an interesting possibility of baryogenesis [26][27][28][29][30][31]. The requirement of not exceeding the observed baryon number density then imposes additional constraints on the strength and coherence length of maximally helical primordial magnetic field.…”

Magnetic field transfer from a hidden sector

“…For example, maximally helical B-field can be generated by pseudo scalar inflation [21,22] and chiral instability [29,30] (see also Refs. [58,59]). Besides taking it as an assumption for generating large magnetic fields, the existence of helical intergalactic magnetic fields may even be indicated by data from the parity-violating correlations of the diffuse gamma ray flux [60][61][62][63].…”