Pasquale Di Bari

1
Neutrino Oscillation Workshop, Conca Specchiulla,
September 9-16, 2006
RH neutrinos in cosmology light vs.
heavy

• Pasquale Di Bari
• (Max Planck, Munich)

2
4 PROBLEMS from COSMOLOGY

• Matter - antimatter asymmetry
• Dark matter
• Accelerating Universe
• Inflation

Plus 1 from neutrino physcis
5. Neutrino Masses
3
Minimal RH implementation

• 3 limiting cases
• pure Dirac MR 0
• pseudo-Dirac MR ltlt mD
• see-saw limit MR gtgt mD

4
(Minkowski Yanagida Gell Mann,Ramond,Slansky
Glashow Barbieri,Nanopoulos,Morchio,Strocchi Mo
hapatra,Senjanovic Schechter,Valle)
In the rest of the talk we will concentrate on
this case !
5
See-saw mechanism

• 3 light LH neutrinos
• N?2 heavy RH neutrinos N1,
  N2 ,

(here we are barring the possibility of a
singular see-saw case where M1 0)

• considering that m? is almost entirely
  determined by the experiments
• ? 2 important quantities for the role of RH
  neutrinos in cosmology

• the see-saw pivot scale ?
• their number N

6
Pivot see-saw scale
? 1 GeV

• gt ? ? high pivot see-saw scale
• (the usual case only heavy RH
  neutrinos are possible)

• lt ? ? low pivot see-saw scale
• (light RH neutrinos are possible)

7
LSND,BBN and CMB

• Future possible scenarios
• MiniBoone disproves LSND and ?N? 0
• ? no evidence for ?0.1eV
• MiniBoone confirms LSND but ?N? 0
• ? evidence for ?eV an asimmetry
• L?? 10-4 can prevent the thermalization
• (Foot,Volkas95PDB,Lipari,Lusignoli99Chu,Cirell
  i06) and L? 10-2 can also reduce the
  Helium abundance (Fuller et al) this could be
  produced also by RH neutrino decays realizing a
  late leptogenesis (PDB)
• MiniBoone disproves LSND but ?N? ? 0
• then it can be still interpreted as evidence for
  
• for ?1 eV if RH neutrino are stable or larger if
  they decay after active neutrino decoupling
• Both MiniBoone confirms LSND and ?N? ? 0
• ? very nice interplay between Earth experiments
  and physics of the early Universe

• eV- see-saw
• can accomodate LSND realizing a 32 data fit
  (De Gouvea05)
• If L?0 then the 2 RH neutrinos thermalize
  problem for BBN and CMB !
• BBN (Helium)
• YBBNCMB 0.2489 - 0.3 L?e0.014 ?N?
• Yexp 0.241 0.004 (Steigman 05)
• 2 additional neutrinos (if L?e0) are too
  many even for a conservative experimental upper
  bound on Y.

?0.1eV

• CMB
• ?milt(0.17 0.9) eV (only active)
• (Seljak et alLesgourgues ,PastorTegmark et
  al.Fogli et al.Hannestad et al.)
• msterilelt (0.26-0.55) eV (31)
• (Dodelson,Melchiorri,SlosarSeljak,Slosar,McDonald
  )
• -0.3lt ?N? lt 1.6 (95 CL) (no Ly?)
• (Hannestad,Raffelt)
• 0.6lt ?N? lt 4.4 (95 CL) (with Ly?)
• (Seljak,Slosar,McDonald)

8
Lightest RH neutrino as Dark Matter candidate

• Even if the active-sterile neutrino mixing is
  typically very small
• ??s mD/M ltlt 1 ,
• the sterile neutrino production is enhanced
  by matter effects and
• (Dodelson,Widrow94Dolgov,Hansen01
• Abazajian,Fuller,Patel01)
• This condition can be fullfilled if m1lt10-5 eV
  and the Dark Matter RH neutrino is the lightest
  one (i.e. M1 O(KeV)
• (Asaka,Blanchet,Shaposhnikov05)
• Moroever, the oscillations between the 2 heavier
  RH neutrinos can be responsible for baryogenesis
  if M2? M3 10 GeV
• (Akhmedov,Rubakov,Smirnov99Asaka,Shaposhni
  kov 05)

9
Good and bad news

• Production for L?0

X-Ray Background measurements

• Good news. Also able to explain
• - Pulsar kicks (Kusenko,Segrè 97 )
• - Early reionization and star formation
• (Kusenko,Biermann 05 )

L?0.01

• Bad news
• - The same flavor-mixing mechanism
  describing the prduction, also lead to radiative
  decay N1 ? ?? ?
• ? ? sin2 2? M15 gtgt t0
• - SDSS Ly? M1 gt (10-14) KeV
• (Seljak et al. 06Lesgourgues et al)
• - However for L??0 producion is more efficient
  (resonant) and there is still some allowed region
  (Abazajian,Fuller,Paterl01)

10
Heavy RH neutrinos

• Just 2 but solid motivations
• See-saw original philosophy is not spoiled ?
  Mew , MRMGUT no need to introduce a new third
  fundamental scale to explain neutrino masses
• Leptogenesis from heavy RH neutrino decays
• it is simple and it works easily without
  requiring a particular tuning of parameters
• (talks by Blanchet,Ma,Petcov,.)
• Main criticisms
• How to prove it ?
• Can it explain other cosmological problems beyond
  the matter-antimatter asymmetry (with
  leptogenesis) ?

11
Cosmological Heavy RH neutrinos beyond
leptogenesis

• models to explain Dark Energy (and LSND) with
  Mass Varying Neutrinos varying the Universe
• MRMR(A) with A a scalar field coupled to
  matter
• (Kaplan,Nelson,Weiner 04)
• heavy RH neutrino as candidates of DM
• (Babu,Eichler,Mohapatra 89)
• unified models of leptogenesis and Dark Matter
• (Cosme et al. 05Saju,Urjit,Yajnik05Strumia
  06)

They all require significant departures from a
minimal extension of the SM ! A point in favor
of light RH neutrinos ??
12
Heavy vs. light which verdict ?

• Heavy
• solid motivations but difficult to test
• Light
• richer phenomenology with many different
  potential ways to discover them but
• original see-saw philosophy gets lost

13
Heavy and light together ?
Is it possible to have a unified picture joining
both virtues of heavy and light neutrinos ?

• Singular see-saw
• Due some symmetry S ? det(MR) , det(mD)0
• ? Taking out the zero eigenvalues from MR and
  mD leads to a see-saw formula just for 2 light
  neutrinos and 2 heavy neutrinos plus one massless
  RH neutrino and one massless LH neutrino
  (Glashow91,FukugitaYanagida91,Chun,Kim,Lee98)
• Weak breaking of S or loop effects can then
  lead to a nonzero mass for the RH neutrino that
  can explain LSND with a 31 spectrum (Mohapatra
  01)
• Double see-saw
• (Mohapatra,Valle86Ellis,Lopez,Nanopoulos 92)
• Recently proposed to have both KeV DM and usual
  leptogenesis at the same time
• (Kang,Kim06)

14
Conclusions

• Heavy and light RH neutrinos have both
  attractive, though complementary interest
• Light RH neutrinos have a rich cosmological
• phenomenology but (waiting for MiniBoone and
  more precise measurement of ?N?) we do not have
  today any compelling reason
• Heavy RH neutrinos have just 2 but solid (and
  traditional) motivations but difficulty in
  describing DM .but maybe it is not impossible !
  We are currently considering with A. Anisimov an
  intriguing possibility a lightest weakly coupled
  RH neutrino DM (M11 TeV) is produced from the
  oscillations of a heavier thermalized one while
  Leptogenesis can proceed from its decays
  (A.Anisimov,PDB)