1 Pasquale Di Bari (Max Planck, Munich) Università di Milano, February 8, 2007 Can neutrinos help to solve the puzzles of modern cosmology ?

2 Outline A cosmological Standard Model ? A cosmological Standard Model ? Puzzles of Modern Cosmology Puzzles of Modern Cosmology Right-handed neutrinos in cosmology: light vs. heavy Right-handed neutrinos in cosmology: light vs. heavy Leptogenesis Leptogenesis

3 A cosmological Standard Model ?

WMAP

Large Scale Structure The Universe observed: Sloan Digital Sky Survey The Universe simulated : Open problems: cusps (too much Dark Matter in halo centers ?) Halo substructure issues (too many satellite galaxies ?) Halo and galaxy merging (too much galaxy merging ?)

Toward a Cosmological SM ?

The Mass-Energy budget today

The Universe is accelerating ! (  ,  M ) = (0,1) (  ,  M ) = (0, 0.3) q = 0 (  ,  M ) = (0.7, 0.3) Hubble diagram: High-redshift type Ia supernovae probe the expansion history and reveal accelerated expansion

Cosmological Concordance Clusters of galaxies are a laboratory for studying and measuring Dark Matter in a variety of ways: gravitational lensing effects, x-ray, radio, optical ….

Thermal history of the Universe

11 Puzzles of Modern Cosmology 1. Matter - antimatter asymmetry 2. Dark matter 3. Accelerating Universe 4. Inflation

Matter-antimatter asymmetry Symmetric Universe with matter- anti matter domains ? Excluded by CMB + cosmic rays )   = (  ±  x    >>   Pre-existing ? It conflicts with inflation ! (Dolgov ‘97) ) dynamical generation (baryogenesis) A Standard Model Solution ?   ¿   : too low ! New Physics is needed! CMB SM CMB (Sakharov ’67)

Dark Matter What do we need today to explain Dark Matter : a new particle … … or a new description of gravity ? Modification of Newtonian Dynamics (MOND) For accelerations a < a 0 ' cm s -2 usual Newton law is modified (Milgrom ’83) Relativistic tensor-vector-scalar field theory for MOND (Bekenstein ’04) However different observations (gravitational lensing, CMB, baryon acoustic oscillation peak, ‘bullet’ cluster, …) tend to exclude it and we will not consider it ! It is the most conservative option with many theoretical motivations: SUSY DM (neutralinos,gravitinos,…),extra DIM’s, Wimpzilla’s, sterile neutrinos,.. Today we know that the new particles have to be slowly moving at the matter-radiation equivalence (T ~ 3 eV )  Cold Dark Matter (M  10KeV) Particle Dark Matter Neutrinos behave as HOT Dark Matter 

Accelerating Universe C.C.  Why small ? -SUSY breaking (Weinberg ’87) - Anthropic principle (Weinberg ’87) - (Zel’dovich 67) - only the fluctuations of the vacuum energy contribute to  and not its absolute value (Zel’dovich 67) Quintessence ? A light scalar field still rolling down: w  in general Without Dark Energy modifying gravity At large distances, motivated in brane world scenarios (Dvali,Gabadadze,Porrati `00) without modifying gravity attempt to explain acceleration without new physics: acceleration would arise from inhomogeneities inside the horizon coincidence problem it would solve the coincidence problem but……..unfortunately it is unlikely to work ! With Dark Energy

Inflation It solves the well known problems of ‘old’ cosmology (horizon problem, flatness problem, initial conditions, spectrum of primordial perturbations…) supported by CMB data On the other hand it leads to serious problems that require to go beyond the SM: - where inflation comes from ? what is the inflaton ? - flatness of the potential - trans-Planckian scales inside the horizon - does not solve the problem of singularity (it is only shifted at earlier times) - cosmological constant problem (the large quantum vacuum energy of field theories does not gravitate today and thus we do not want it….but it is necessary for inflation !)

Which model beyond the Standard Model of Particle Physics can solve the cosmological puzzles ? In other words: cosmologists have cleaned their room but they swept away all the dust in the particle physicists lounge ! Experimental long-standing issues have been solved and the puzzles of modern cosmology are nicely expressed in a particle physics `language’ but they cannot be explained within the SM ! Some considerations

Neutrino masses: m 1 < m 2 < m 3

RH neutrinos in cosmology: light vs. heavy

Minimal RH neutrino implementation 3 limiting cases : pure Dirac: M R = 0 pseudo-Dirac : M R << m D see-saw limit: M R >> m D

See-saw mechanism 3 light LH neutrinos: 3 light LH neutrinos: N  2 heavy RH neutrinos: N 1, N 2, … N  2 heavy RH neutrinos: N 1, N 2, … m  M SEE-SAW - the `see-saw’ pivot scale  is then an important quantity to understand the role of RH neutrinos in cosmology

 * ~ 1 GeV m>  *  high pivot see-saw scale  `heavy’ RH neutrinos m<  *  low pivot see-saw scale  `light’ RH neutrinos

Light RH neutrinos and…. …..LSND A see-saw mechanism with  ~0.1eV can accommodate LSND with a ‘3+2’ data fit (De Gouvea’05) but potential problems with BBN and CMB …..CMB -0.3<  N < 1.6 (95% CL) (no Ly  ) (Hannestad,Raffelt) 0.6<  N < 4.4 (95% CL) (with Ly  ) (Seljak,Slosar,McDonald)  ~0.1eV A future 5 th cosmological puzzle ? It would be very interesting especially for neutrinos

…Dark Matter active-RH neutrino mixing:   N ~ m D /M << 1, the RH neutrino production is enhanced by matter effects and (Dodelson,Widrow’94;Dolgov,Hansen’01; Abazajian,Fuller,Patel’01) For `see-saw ‘ RH neutrinos the condition can be fullfilled if m 1 <10 -5 eV and the Dark Matter RH neutrino is the lightest one with M 1 ~ O(KeV) (Asaka,Blanchet,Shaposhnikov’05) Bad news: the same flavor-mixing mechanism describing the production, also lead to radiative decay: N 1   +  ”  >> t 0  M 1  10 KeV - SDSS Ly  : M 1 > (10-14) KeV (Seljak et al. ’06;Lesgourgues et al)

Heavy RH neutrinos 2 solid motivations: See-saw original philosophy is not spoiled:  ~ M ew, M R ~M GUT there is no need to introduce new fundamental scales to explain neutrino masses; Leptogenesis from heavy RH neutrino decays: it is simple and it works easily without requiring a particular tuning of parameters Objections: How to prove it ? Can one explain Dark Matter ?

CP asymmetry If  i ≠ 0 a lepton asymmetry is generated from N i decays and partly converted into a baryon asymmetry by sphaleron processes if T reh  100 GeV ! efficiency factors= # of N i decaying out-of-equilibrium efficiency factors = # of N i decaying out-of-equilibrium (Kuzmin,Rubakov,Shaposhnikov, ’85) M, m D, m are complex matrices  natural source of CP violation (Fukugita,Yanagida ’86) Leptogenesis

Kinetic Equations ``decay parameters´´ CP violation in decays Wash-out term from inverse decays Strong wash-out when K i  3 Weak wash-out when K i  3

flavor composition of leptons is neglected hierarchical heavy neutrino spectrum asymmetry generated from the lightest RH neutrino decays (N 1 -dominated scenario) The traditional picture It does not depend on low energy phases !

Neutrino mass bounds m 1 =0 ~ ( M 1 / GeV) M 1 (GeV)

N 2 -dominated scenario beyond the hierarchical limit flavor effects Beyond the traditional picture

Beyond the hierarchical limit 3 Effects play simultaneously a role for  2  1 : (Pilaftsis ’97, Hambye et al ’03, Blanchet,PDB ‘06) N 2 -dominated scenario (PDB’05) The lower bound on M 1 disappears and is replaced by a lower bound on M 2. The lower bound on T reh remains

(Barbieri et a l. ’01; Endo et al. ’04; Pilaftsis,Underwood ’05; Nardi,Roulet’06;Abada et al.’06;Blanchet,PDB’06) Flavor composition: Does it play any role ? However for lower temperatures the charged lepton Yukawa couplings, are strong enough to break the coherent evolution of the and of the, that are then projected on a flavor basis: ‘flavor’ is measured and comes into play ! Flavor effects It is then necessary to track the asymmetries separately in each flavor:

How flavor effects modify leptogenesis? The kinetic equations become : First effect: wash-out is suppressed by the projectors: Second effect: additional contribution to the ‘flavored’ CP asymmetries: Same as before! (Nardi et al., 06) The additional contribution depends on the low energy phases !

NiNi L NO FLAVOR NjNj Φ Φ LeLe LµLµ LτLτ

NiNi WITH FLAVOR NjNj Φ Φ LeLe LµLµ LτLτ

General scenarios (K 1 >> 1) –Alignment case –Democratic (semi-democratic) case –One-flavor dominance and big effect!

A relevant specific case Let us consider: Since the projectors and flavored asymmetries depend on U  one has to plug the information from neutrino mixing experiments m 1 =m atm  0.05 eV  1 = 0  1 = -  The lowest bound does not change! (Blanchet, PDB ‘06) Majorana phases play a role !!

Leptogenesis testable at low energies ? Let us now further impose  1 = 0 setting Im(  13 )=0 traditional unflavored case M 1 min More stringent lower bound but still successful leptogenesis is possible with CP violation stemming just from ‘low energy’ phases testable in:  0 decay (Majorana phases) and neutrino mixing (Dirac phase) Considering the degenerate limit these lower bounds can be relaxed ! (Blanchet,PDB 06)

When flavor effects are important ? (Blanchet,PDB,Raffelt ‘06) Consider the rate   of processes like It was believed that the condition   > H is sufficient ! This is equivalent to T  M 1  GeV In the weak wash-out regime this is true since H >  ID However, in the strong wash-out regime the condition   >  ID is stronger than   > H and is equivalent to If z fl  z B  W ID  1  M 1  GeV but if z fl > 1  much more restrictive ! This applies to the one-flavor dominated scenario through which the upper bound on neutrino masses could be circumvented.

Is the upper bound on neutrino masses be circumvented when flavor effects are accounted for ? 0.12 eV A definitive answer requires a genuine quantum kinetic calculation ! (Blanchet,PDB,Raffelt ‘06)

Conclusions the  CDM modelThe cosmological observations of the last ten years have pointed to a robust phenomenological model: (the  CDM model ) a cosmological SM ? 4 puzzles that can be solved only with ‘new physics’ Discovery of neutrino masses strongly motivate solutions of the cosmological puzzles in terms of neutrino physics and RH neutrinos in the see-saw limit are the simplest way to explain neutrino masses; Between light and heavy RH neutrinos the second option appears more robustly motivated; Leptogenesis Leptogenesis is one motivation and flavor effects open new prospects to test it in  0 decay experiments (Majorana phases) and neutrino mixing experiments (Dirac phase)