Cosmic ΔB from lepton violating interactions at the electroweak phase transition

Abstract

We propose a new mechanism for late cosmological baryon asymmetry in models with first order electroweak phase transition. Lepton asymmetry arises through the decay of particles produced out of equilibrium in bubble collisions and is converted into baryon asymmetry by sphalerons. Supersymmetric models with explicitly broken R-parity may provide a suitable framework for the implementation of this mechanism. 0 The realization that the baryon (B) and the lepton (L) violating quantum effects in the standard electroweak theory are efficient at high temperature [1] has sparkled a widespread interest on the issue of late cosmological B asymmetry production. The problem is that any matter-antimatter asymmetry created at some superheavy scale [2] can be easily wiped out by the B-violating quantum effects, which are abundantly in equilibrium throughout all the history of the early Universe until its temperature drops down to the Fermi scale range [1]. If this is indeed the case, then we are faced with the vital problem of originating a new cosmological B asymmetry at the scale of the Electro Weak Phase Transition (EWPT). A key ingredient for the development of a sizeable late cosmological B asymmetry is that the EPTW be of first order. This seems necessary for the implementation of the so-called out of equilibrium condition which, together with B and CP violation, constitutes one of the essential requirements to generate a net ∆B [3]. There already exist several studies of ∆B production at the electroweak scale in the Standard Model (SM) or in minimal extensions of it [4]. In SM the B-violating quantum effects play a twofold role [5]: on one hand they are responsible for the washing of any preexisting B asymmetry, but, on the other hand, they are invoked for the production of the new ∆B at the Fermi scale. In general, in order for these scenarios at the electroweak scale to work, the B-violating quantum effects must be sufficiently suppressed after the accomplishment of the EWPT, or else they can wipe out the B asymmetry once again. This condition translates into an upper bound on the mass of the light neutral physical Higgs [5,6]. This limit is in contrast with or dangerously close to the present LEP lower bound on Higgs masses [7], depending on the details of the different analyses and whether the SM [5] or the Minimal Supersymmetric Standard Model (MSSM)

Weak scale baryogenesis in a supersymmetric scenario with R-parity violation

“…Let us start by the model proposed by Masiero and Riotto [52]. They study the generation of a lepton asymmetry through / R interactions at the EWPT, then convert it to the BAU using sphalerons, which therefore must be still efficient just after the EWPT.…”

“…The EWPT is assumed to be of first order, and in particular proceeds via percolation of subcritical bubbles, originated by thermal fluctuations. We will not discuss in detail the dynamics of bubble nucleation and collision (see [52]): the important point is that in the collisions the energy of the bubble can be released through the direct production of particles, whose distribution will be far from equilibrium. Neutralinos are produced through this mechanism, which therefore provides for Sakharov's condition.…”

Leptogenesis at the TeV scale

“…A preexisting L 3 asymmetry will be erased in the same way as the produced L 3 asymmetry mentioned above. A preexisting L 2 or L 1 asymmetry will be erased to a large extend at T ∼ m S by the effect of the S mediated 14 The neutrino mass hierarchy in this case is that the two heaviest neutrinos have masses differing by the small solar ∆m 2 solar and the lightest neutrino differ from the other two by the larger atmospheric ∆m 2 atmos . 15 Note that we could naively believe that if the f µτ couplings are small the produced L 1 asymmetry will also be small.…”

“…In this context it is important to look for other alternatives. Beside the electroweak baryogenesis mechanism in the context of more complicated models, leptogenesis [6][7][8][9][10][11][12][13][14][15] is probably the most attractive and simple alternative. It is based on a two step process.…”

R-Parity-violating supersymmetry

“…In the scenario considered by Masiero and Riotto [161], the lepton asymmetry is generated through the R p (and CP -violating) decays of the LSP, which is produced out of equilibrium in bubble collisions during the electroweak phase transition. The origin of CP violation is simply the presence of phases in the R p couplings λ ijk and λ ′ ijk .…”