Sphaleron Effects Research Articles

Quark seesaw mechanism, dark U(1) symmetry, and the baryon-dark matter coincidence

We attempt to understand the baryon-dark matter coincidence problem within the quark seesaw extension of the standard model where parity invariance is used to solve the strong $CP$ problem. The $SU(2{)}_{L}\ifmmode\times\else\texttimes\fi{}SU(2{)}_{R}\ifmmode\times\else\texttimes\fi{}U(1{)}_{B\ensuremath{-}L}$ gauge symmetry of this model is extended by a dark $U(1{)}_{X}$ group plus inclusion of a heavy neutral vector-like fermion ${\ensuremath{\chi}}_{L,R}$ charged under the dark group which plays the role of dark matter. All fermions are Dirac type in this model. Decay of heavy scalars charged under $U(1{)}_{X}$ leads to simultaneous asymmetry generation of the dark matter and baryons after sphaleron effects are included. The $U(1{)}_{X}$ group not only helps to stabilize the dark matter but also helps in the elimination of the symmetric part of the dark matter via $\ensuremath{\chi}\ensuremath{-}\overline{\ensuremath{\chi}}$ annihilation. For dark matter mass near the proton mass, it explains why the baryon and dark matter abundances are of similar magnitude (the baryon-dark matter coincidence problem). This model is testable in low threshold (sub-keV) direct dark matter search experiments.

MATTER–ANTIMATTER ASYMMETRY GENERATED AT LOW TEMPERATURES

We explore the possibility of having baryogenesis at temperatures below any dilution mechanism, such as inflation or sphaleron effects at the electroweak transition. We find that large infrared CPT violation (CPTV) effects are a necessary ingredient for such a baryogenesis. We give an explicit example of a theory supporting this scenario at the kinematical level in which such CPTV effects are suppressed at the level of CPT tests based on mass differences between particle and antiparticle.

On Higgs and sphaleron effects during the leptogenesis era

We discuss the effects of various processes that can be active during the leptogenesis era, and present the Boltzmann equations that take them into account appropriately. A non-vanishing Higgs number asymmetry is always present, enhancing the washout of the lepton asymmetry. This is the main new effect when leptogenesis takes place at $T>10^{12}$ GeV, reducing the final baryon asymmetry and tightening the leptogenesis bound on the neutrino masses. If leptogenesis occurs at lower temperatures, electroweak sphalerons partially transfer the lepton asymmetry to a baryonic one, while Yukawa interactions and QCD sphalerons partially transfer the asymmetries of the left-handed fields to the right-handed ones, suppressing the washout processes. Depending on the specific temperature range in which leptogenesis occurs, the final baryon asymmetry can be enhanced or suppressed by factors of order 20%--40% with respect to the case when these effects are altogether ignored.

Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology

We calculate and display the primordial light-element abundances as a function of a neutrino degeneracy parameter $\ensuremath{\xi}$ common to all flavors. It is the only unknown parameter characterizing the thermal medium at the primordial nucleosynthesis epoch. The observed primordial helium abundance ${Y}_{\mathrm{p}}$ is the most sensitive cosmic ``leptometer.'' Adopting the conservative ${Y}_{\mathrm{p}}$ error analysis of Olive and Skillman implies $\ensuremath{-}0.04\ensuremath{\lesssim}\ensuremath{\xi}\ensuremath{\lesssim}0.07$ whereas the errors stated by Izotov and Thuan imply $\ensuremath{\xi}=0.0245\ifmmode\pm\else\textpm\fi{}0.0092$ ($1\ensuremath{\sigma}$). Improved determinations of the baryon abundance have no significant impact on this situation. A determination of ${Y}_{\mathrm{p}}$ that reliably distinguishes between a vanishing or nonvanishing $\ensuremath{\xi}$ is a crucial test of the cosmological standard assumption that sphaleron effects equilibrate the cosmic lepton and baryon asymmetries.

Generation of cosmological large lepton asymmetry from a rolling scalar field

We propose a new scenario to simultaneously explain a large lepton asymmetry and a small baryon asymmetry. We consider a rolling scalar field and its derivative coupling to the lepton number current. The presence of an effective nonzero time derivative of the scalar field leads to $CPT$ violation so that the lepton asymmetry can be generated even in thermal equilibrium as pointed out by Cohen and Kaplan. In this model, the lepton asymmetry varies with time. In particular, we consider the case where it grows with time. The final lepton asymmetry is determined by the decoupling of the lepton number violating interaction and can be as large as order unity. On the other hand, if the decoupling takes place after the electroweak phase transition, a small baryon asymmetry is obtained from the small lepton asymmetry at that time through sphaleron effects. We construct a model in which a rolling scalar field is identified with a quintessence field.

Large lepton asymmetry fromQ-balls

We propose a scenario which can explain large lepton asymmetry and small baryon asymmetry simultaneously. Large lepton asymmetry is generated through Affleck-Dine (AD) mechanism and almost all the produced lepton numbers are absorbed into Q-balls (L-balls). If the lifetime of the L-balls is longer than the onset of electroweak phase transition but shorter than the epoch of big bang nucleosynthesis (BBN), the large lepton asymmetry in the L-balls is protected from sphaleron effects. On the other hand, small (negative) lepton numbers are evaporated from the L-balls due to thermal effects, which are converted into the observed small baryon asymmetry by virtue of sphaleron effects. Large and positive lepton asymmetry of electron type is often requested from BBN. In our scenario, choosing an appropriate flat direction in the minimal supersymmetric standard model (MSSM), we can produce positive lepton asymmetry of electron type but totally negative lepton asymmetry.

Baryogenesis in SU (5) can survive sphaleron effects

We briefly discuss a model, presented earlier, for efficient annihilation of monopoles that is known to accommodate scenarios for baryogenesis wherein B − L ≠ 0. It is shown that with the aid of this model an adequately large baryon asymmetry can also be produced within the framework of the simplest GUT, SU (5), that will survive the effect of sphaleron interactions: the final baryon asymmetry at temperatures smaller than the electroweak phase transition temperature can easily lie in the range allowed by nucleosynthesis constraints, and B − L = 0.

Sphaleron effects near the critical temperature.

We discuss one-loop radiative corrections to the sphaleron-induced baryon-number-violating transition rate near the electroweak phase transition in the standard model. We emphasize that in the case of a first-order transition a rearrangement of the loop expansion is required close to the transition temperature. The corresponding expansion parameter, the effective three-dimensional gauge coupling, approaches a finite \ensuremath{\lambda}-dependent value at the critical temperature. The \ensuremath{\lambda} (Higgs boson mass) dependence of the one-loop radiative corrections is discussed in the framework of the heat kernel method. Radiative corrections are small compared to the leading sphaleron contribution as long as the Higgs boson mass is small compared to the W mass. To one-loop accuracy, there is no Higgs boson mass range compatible with experimental limits where washing out of a B+L asymmetry could be avoided for the minimal standard model with one Higgs doublet.