Matter and antimatter in the universe

2012, New Journal of Physics

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20 pages

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Abstract

We review observational evidence for a matter-antimatter asymmetry in the early universe, which leads to the remnant matter density we observe today. We also discuss bounds on the presence of antimatter in the present day universe, including the possibility of a large lepton asymmetry in the cosmic neutrino background. We briefly review the theoretical framework within which baryogenesis, the dynamical generation of a matter-antimatter asymmetry, can occur. As an example, we discuss a testable minimal particle physics model that simultaneously explains the baryon asymmetry of the universe, neutrino oscillations and dark matter.

Figures (2)

Figure 1: An illustration of fermionic level crossing: The changing bosonic background fields can modify the fermionic energy levels, leading to fermion number violation when a level raises above the surface of the Dirac sea. Mspn ~ Mw/aw. The sphaleron is the field configuration of maximal energy along the path of minimal energy in field space that connects two minima in the classical potential [57]. RELEASE That Nw ITO: | SEUAS Ere Py —_—_—Or—o—eeresn ES Oe NNT ON ee ee ne eee eee en eee eee ee Ne IN LACE |, SONNY, On Ov oo rr L J In quantum field theory, the early universe can be described as a thermodynamic ensemble characterised by a density matrix 0 = exp(—H/T), where H is the Hamiltonian. At low tempera- tures T’ < Tew, only configurations with field expectation values near the minima of the effective potential are significantly populated. Tunneling through the potential barrier is the only process that can change Neg from such an initial state. The tunneling rate at T = 0 [15] is suppressed by exp(—<) ~ 107!© | effectively forbidding B-violation in vacuum. Near T ~ Ty the Boltz- mann suppression exp(—H/T) is less efficient and Ncg can change by a classical transition due to thermal fluctuations. The probability for these baryon number violating processes is governed by the sphaleron rate [53]

Figure 2: Constraints on sterile neutrino mass M and mixing U? = tr(6'@) in the vMSM as found in [98, 99] for normal (left panel) and inverted (right panel) hierarchy of active neutrino masses. In the regions below the black dashed ”seesaw” line there exists no choice of vy MSM parameters that is in accord with experimental constraints on the active neutrino mixing matrix. In the region below the black dotted BBN line, the lifetime of N2,3 particles in the early universe is larger than 0.1s, yielding the danger that their decay spoils the agreement between BBN calculations and observed light element abundances. The regions above the green lines of different shade are excluded by the NuTeV [108], CHARM [109] and CERN PS191 [110] experiments, as indicated in the plot. In the region between the blue lines, a CP-asymmetry that explains the observed BAU can be produced during the thermal production of N2,3. In the region within the red line, thermal production of N, (resonant and non-resonant) is sufficient to explain all observed DM. The CP-violating phases that maximise the efficiency of baryogenesis and DM production are different. They were chosen independently for the blue and red line displayed here. The region in which Qg and Qpm can be explained simultaneously almost coincides with the area inside the red line, see [99].