Abstract
We present a novel baryogenesis mechanism in which the asymmetry is sourced from heavy particles which either gain their mass or are created during bubble expansion in a strong first order phase transition. These particles then decay in a CP and baryon number violating way inside the bubble. The particles are inherently out-of-equilibrium and sufficiently dilute after wall crossing so the third Sakharov condition is easily met. Washout is avoided provided the reheat temperature is sufficiently below the scale of the heavy particles. The mechanism relies on moderate supercooling and relativistic walls which -- in contrast to electroweak baryogenesis -- generically leads to a sizable gravitational wave signal, although in the simplest realisations at frequencies beyond upcoming detectors. We present a simple example model and discuss the restrictions on the parameter space for the mechanism to be successful. We find that high reheat temperatures $T_{\rm RH} \gtrsim 10^{10}$ GeV are generally preferred, whereas stronger supercooling allows for temperatures as low as $T_{\rm RH} \sim 10^{6}$ GeV, provided the vacuum energy density is sufficiently suppressed. We briefly comment on using resonantly enhanced CP violation to achieve even lower scales.
Highlights
We present a novel baryogenesis mechanism in which the asymmetry is sourced from heavy particles which either gain their mass or are created during bubble expansion in a strong first order phase transition
In order to cast as wide a net as possible, we extend our results to smaller cvac—which is consistently possible for the mass gain (MG) mechanism—even with some suppression of YB through y ∼ 10−2 and Tn ∼ TRH=5
For the gravitational wave signal we use the latest—state of the art—numerical results for thick walled bubbles calculated by Cutting et al [99]. (For related studies see [100,101,102,103].) The spectrum depends on the bulk parameters of the phase transition (PT)
Summary
The possibility of a first order early Universe phase transition (PT) [1,2,3,4], together with associated implications for inflation [5,6], baryogenesis [7,8], dark matter (DM). If the wall velocity approaches the speed of light in EWBG, the yield of baryons approaches zero, due to suppressed particle diffusion back into the symmetric phase where the sphalerons are active [38] ( see [39,40]) This has consequences for the gravitational wave (GW) signal, as very strong PTs needed to produce a sizable signal typically lead to ultrarelativistic walls, as shown via the Bodeker and Moore criterion [16,17]. For the mass gain mechanism, we can have MΔ 1⁄4 0 in the symmetric phase and can eventually implement the PT in a classically scale invariant potential (as relevant in the aforementioned “neutrino option”) Such models almost automatically result in supercooled phase transitions which are desirable for our baryogenesis mechanism. We will consider the CP violation in our model only in the hierarchical limit, and limit ourselves to two-body decays for our heavy states
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