IMPRSGK Young Scientist Workshop July 23 27 2007

IMPRS/GK Young Scientist Workshop, July 23 -27, 2007 Ringberg Castle Leptogenesis & low-energy CP violation Steve Blanchet Max-Planck-Institut für Physik, Munich July 25, 2007

Outline « One of the great cosmological puzzles: Baryon Asymmetry of the Universe (BAU) « The see-saw mechanism « What is leptogenesis? « Modern view of thermal leptogenesis □ Unflavored leptogenesis □ Fully-flavored leptogenesis « CP violation in the see-saw « Dirac –phase leptogenesis (±-leptogenesis) Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 2

The baryon asymmetry of the Universe « From two independent probes (BBN and CMB), one knows that « In a baryon symmetric Universe, one cannot avoid the baryon annihilation catastrophe: number density 9 orders of magnitude too low ! « If you don´t believe in cosmology: maybe the Universe is symmetric, but with matter and anti-matter domains? no trace in the cosmic ray spectrum and conflict with CMB Conclusion: one needs a baryon asymmetry! Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 3

The baryon asymmetry of the Universe « Pre-existing asymmetry? Conflict with inflation! Dynamical generation of the BAU needed! « Necessary conditions to generate a baryon asymmetry [Sakharov, 1967] : □ Baryon number violation □ C and CP violation □ Departure from thermal equilibrium « Standard Model contains a priori all ingredients: Electroweak baryogenesis… Unfortunately, Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 4

The see-saw mechanism « A fact: neutrinos have masses and mix Sol. + Reac. Atm. + Acc. « From cosmology we know that the absolute neutrino mass scale should be small: « A natural explanation for this smallness is provided by the see-saw mechanism, based on the following Lagrangian: Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 5

The see-saw mechanism « The see-saw assumption is to take which case , in 1 st order « Leptogenesis is the cosmological consequence of the see-saw. Two seemingly unrelated problems find their solution in the same simple extension of the Standard Model! Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 6

What is leptogenesis? « Historically [Fukugita and Yanagida, 1986] , leptogenesis is the generation of a lepton asymmetry by the decay of heavy right-handed neutrinos , and its subsequent conversion into a baryon asymmetry by the sphaleron processes. At T~1010 Ge. V, i. e. t~10 -26 sec: L generated But in equilibrium violate B+L B-L and hence B are generated! conserve B-L Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 7

Modern view of leptogenesis « The heavy RH neutrinos decay and are repopulated by inverse decay thanks to the Yukawa interaction « However, if the lepton-lepton interactions coming from are fast (i. e. in equilibrium ), they impose a different description of leptogenesis. [Barbieri, et al, 99; Nardi, et al. , 06; Davidson, et al. , 06] 1 L 1 is the good quantum state: Unflavored leptogenesis applies. 2 The flavored states Lα have to be considered: fully-flavored lep. applies. Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 8

1 NO FLAVOR EFFECTS N 1 Φ L 1 Φ Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 9

2 WITH FLAVOR EFFECTS Lτ τR Lμ L 1 Φ N 1 Φ Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 10

Unflavored leptogenesis « The fundamental Boltzmann equations are CP violation Out-of-equilibrium condition Sphalerons conserve B-L ! « The CP violation parameter is given by « the important decay parameter. Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 11

Unflavored leptogenesis « One speaks about a strong wash-out when about a weak wash-out when and « The strong wash-out is theoretically favored, e. g. because there is no dependence on the initial conditions. « In the hierarchical limit (HL) , there is a lower bound on the mass of the lightest RH neutrino [Davidson, Ibarra, 02] « In the HL, a stringent upper bound on the absolute neutrino mass scale has been found [Buchmüller, Di Bari, Plümacher, 02] Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 12

Fully-flavored leptogenesis [Barbieri, et al, 99; Nardi, et al. , 06; Davidson, et al. , 06] « The flavor content of the leptons now matters! One has to track the flavored lepton numbers « Introducing the projectors [Barbieri, Creminelli, Strumia, Tetradis, 99] New source of CP violation! « The flavored CP asymmetries are indeed given by Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 13

Fully-flavored leptogenesis « Pictorially, the two sources of CP violation can be seen as follows 1) 2) e+μ τ τ e+μ « Altogether, the new Boltzmann equations are: Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 14

CP violation and leptogenesis « The see-saw has many new parameters (18!) compared to the Standard Model, among which 6 are CP-violating phases. « A useful parametrization is given by [Casas, Ibarra, 01] 3 low-energy (measurable) phases: 2 Majorana phases and 1 Dirac phase ± 3 high-energy (unmeasurable) phases « In the unflavored picture, only the high-energy phases play a role. On the other hand, in the fully-flavored one leptogenesis only from low-energy phases is possible. [SB, Di Bari, 06] Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 15

±-leptogenesis [Anisimov, SB, Di Bari, ar. Xiv: 0707. 3024] « Assume from now on that only the Dirac phase ± is turned on. This is a minimal condition on the necessary CP violation because this phase appears only in combination with the small µ 13 angle (<0. 2 at 3 ¾). « It turns out that it is possible to explain the BAU in the hierarchical limit but it is quite constrained and in the weak wash-out: Go to the degenerate limit (DL), , where the CP asymmetry can be resonantly enhanced! Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 16

±-leptogenesis « In the DL, there is still a nice link between low-energy parameters (µ 13, mass hierarchy, absolute neutrino mass scale, Dirac phase) and the BAU. Allowed regions (Far) future sensitivity Theoretical uncertainty The smaller the upper bound on µ 13, the smaller the mass must be! Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 17

Conclusions « Thermal leptogenesis is an attractive way to explain the BAU. « The latest developments (flavor effects) impose a description of leptogenesis where low-energy phases play an important role. « When the Dirac phase acts as the only source of CP violation (±-leptogenesis), the situation is quite constrained in the HL. « However, in the DL, it works well, and the explanation of the BAU implies interestingly an upper bound on m 1 which depends on the angle µ 13. « If ± is discovered, then we will know for the first time that there is a sufficient source of CP violation at hand to explain the BAU. Steve Blanchet, IMPRS/GK Workshop, Ringberg, 25. 07 18