Sterile Neutrinos

1
Sterile Neutrinos
PASCOS 2004
Probing
with cosmology, astrophysics, ? experiments

• Marco Cirelli
• (Yale)

with G.Marandella, A.Strumia, F.Vissani hep-ph/040
3158 (--gt NPB) and with Yi-Zen Chu (in
preparation)
2
Introduction and Purpose
We want to study Sterile Neutrinos
- (light) spin 1/2 fermions, - neutral under all
SM forces, - have a mixing with active ?.
i.e.
Oscillations into ?s are now excluded as the
dominant solution in solar and atmospheric
neutrinos
(details)
(details)
3
Now the relevant issues become
- which subdominant role is still possible for ?s
?
- where can we detect the ?s ?
- how can we detect the ?s ?
4
right-handed neutrino
axino
goldstino
majorino

• Yes, new light neutral fermions in so many
• Beyond the SM constructions

branino
dilatino
radino
modulino
familino
-behave effectively as ?s -parameterize with ?s ,
?ms2
mirror fermions

pulsar kicks
r-process nucleosynthesis
Dark Matter
galactic ionization

• LSND

5
Pure ?e ?s solution for solar neutrinos
go to Sterile effect in sun
6
Pure ?? ?s solution for atmospheric neutrinos
7
(No Transcript)
8
If ?? ?s

• (1b) matter effects crossing the Earth
• reduce oscillations

9
If ?? ?s
(2) less NC events upward
10
go to Sterile effects in atmo neutrinos
11
4? mixing formalism
more details
12
In the following
?s has arbitrary mass m4 and it mixes with angle
?s with ?e OR ?? OR ??
OR ?1 OR ?2 OR ?3
Also take best-fit values for ?sun and ?atm ,
choose ?13 0, Normal Hierarchy.
13
Full 4? mixing formalism
We want a full 4? formalism.
14
4 neutrinos
add 3 angles ?14 ?24 ?34 (and add 2
phases)
15
Explicitly
16
In the following
?s has arbitrary mass m4 and it mixes with angle
?s with ?e OR ?? OR ??
OR ?1 OR ?2 OR ?3
Also take best-fit values for ?sun and ?atm ,
choose ?13 0, Normal Hierarchy.
17
Sun
CMB
BBN
SN
Early Universe
LSS
Astrophysics
?s
AGN
Atmo Experiments
Atmosphere
SBL
Combined Results
reactors
accelerators
18
Sterile effects in the Early Universe
Neutrinos in the Early Universe are (1) a lot
(as abundant as photons) (2) the main component
of the (relativistic) energy density that
sets the expansion rate (3) trapped in the dense
early plasma non trivial matter
effects (4) important for the outcoming chemical
composition
An extra ?s can make a big difference.
19
BigBang Nucleosynthesis
?ms2 , ?s
N?
??e ??? ??? ??s
??e ??? ??? ??s
(T 1 MeV)
BBN
4He D 3Li 3He
n/p
?b
(nuclear rates, n mean life, weak cross sections)
CMB (WMAP)
20
Roadmap ( What we do)
For every choice of ?ms2 , ?s , for T gtgt MeV
0.07 MeV follow
(BBN ends, les jeux sont fait)

• Assumptions
• no large lepton asymmetries
• neglect spectral distortions

Where does a ?s enter the BBN game?
(A) ?s production ? larger total energy density
? faster expansion
21
Neutrino kinetic equations
4x4 neutrino density matrix ?
Dolgov, 1981 Barbieri, Dolgov 1990
? thermal masses
22
What happens

• for T gtgt MeV, matter effects suppress mixing
  ??s 0

• meanwhile ? decouple at T few MeV, ee-
  annihilate

• Output ??e(T) , ??? (T) , ??? (T) , ??s (T)

23
n/p ratio
2
Bottom line where does a ?s enter the game?
24
Light elements production
3
A network of Boltzmann equations with up-to-date
nuclear rates
25
Bounds in the parameter space
4He25.8 N? 3.8
4He25.0 N? 3.2
26
Large Scale Structure
The primordial free streaming of massive
neutrinos affects the LSS power spectrum
observed today.
M.Tegmark webpage
27
Bounds in the parameter space
??h2 10 -2
??h2 10 -3
28
Cosmic Microwave Background
The primordial neutrino energy densities
affect the acoustic peaks of CMB
power spectrum.
Barger et al., PLB566, 2003
Bound on the effective N?CMB ? ??e , ??? , ??? ,
??s .
N? 1, 2.75, 5, 7
At present N?CMB 3?2
Bound on the ?ms2 , ?s (that determine the ??s).
29
All bounds from cosmology
4He25.8 N? 3.8
4He25.0 N? 3.2
??h2 10 -2
??h2 10 -3
30
LSND in or out?
Requires a new ( sterile) neutrino
How does the LSND ?s fit in cosmology?
?LSND ?es ??s
31
Sun
CMB
BBN
SN
Early Universe
LSS
Astrophysics
?s
AGN
Atmo Experiments
Atmosphere
SBL
Combined Results
reactors
accelerators
32
Sterile effects in SN
Neutrinos from SN (1) are a lot (99 of emitted
energy) (2) undergo extreme matter effects (3)
come from very far away (10 kpc) (4) have the
right energy (10 MeV) for present detectors
An extra ?s can make a big difference.
33
SK
??
SNO
??
?e
?-sphere
34
Matter eigenstates in the mantle
Output final fluxes of ?e, ?? and ?? on Earth .
35
Results percentual reduction of ?e events (in a
large Cerenkov detector)
(?e p ? ne)
Beware of theoretical uncertainties
36
The energy dependance of matter/vacuum
conversions causes spectral distortions
Possible very clear feature!
37
Sun
CMB
BBN
SN
Early Universe
LSS
Astrophysics
?s
AGN
Atmo Experiments
Atmosphere
SBL
Combined Results
reactors
accelerators
38
Neutrinos from extragalactic sources

• produced in high-energy astrophysical processes
• expected flavor ratios e ? ? 1 2 0 at
  production
• 
  1 1 1 after (active) oscillations
• if a ?s is introduced, a selective depletion can
  occur .

But

• initial fluxes totally unknown
• we tag ?? and ?? which nevertheless equiparate
  (atmo oscillations)

Not a very interesting probe.
39
Sun
CMB
BBN
SN
Early Universe
LSS
Astrophysics
?s
AGN
Atmo Experiments
Atmosphere
SBL
Combined Results
reactors
accelerators
40
Sterile effects in solar neutrinos
Neutrinos from the sun (1) are a lot, and very
well studied (2) undergo matter effects in the
sun and in the Earth (3) come from far away (150
Mkm)
An extra ?s can make a difference.
more details
41
Solar ?e spectrum
(production regions)
Gonzalez-Garcia, Nir, review 2002
Bahcall, Pinsonneault 2001
Evolution
-vacuum oscillations
-(matter oscillations in Earth)
42
Neutrino density matrix formalism
4x4 density matrix ?
at production (?e in the sun) is
mixing matrices in matter (Vm) are computed
diagonalizing the matter Hamiltonian
evolve ? with evolution matrix
at each ij matter level crossing rotates
of
with
(? m effective mixing angle in matter)
at detection (back to flavor basis)
E.g. P(?e??e) corresponds to ?ee
43
Results (with KamLAND)
excluded
effect in a low energy exper. (sub-MeV)
44
Spectral distortions - the energy dependance
in the (matter and vacuum) oscillations
distorts the original (well known) solar ?e
spectrum - a very distinctive feature! -
mainly at low energies
Pe? ?
Pe? e
Pe? s
45
The still allowed component of ?s in solar
neutrinos
46
Sun
CMB
BBN
SN
Early Universe
LSS
Astrophysics
?s
AGN
Atmo Experiments
Atmosphere
SBL
Combined Results
reactors
accelerators
47
Sterile effects in atmospheric neutrinos
Evidence for oscillations is disappearance of ??
from below.
Basics
SK coll.
Where do they go? ?? ?? , ?? ?s or a
combination?
3 sensitive probes to discriminate and put bounds
48
If ?? ?s

• larger flux of thru-muons
• (1b) larger number of PC events
• fewer NC-enriched events
• (3) tau appearance

We perform a global ?2 analysis of SK Macro
K2K data.
No improvements w.r.t. pure ?? ??
found ? no evidence for sterile neutrinos ?
excluded regions .
49
Results
excluded
5,1 effect on NC at MINOS
50
The still allowed component of ?s in
atmospheric neutrinos
51
Sun
CMB
BBN
SN
Early Universe
LSS
Astrophysics
?s
AGN
Atmo Experiments
Atmosphere
SBL
Combined Results
reactors
accelerators
52
Sterile effects in SBL neutrinos
Chooz Bugey CDHS CCFR Karmen Nomad
Chorus
Main constraint comes from no-disappearance.
excluded
future SBL at reactor?
53
Sun
CMB
BBN
SN
Early Universe
LSS
Astrophysics
?s
AGN
Atmo Experiments
Atmosphere
SBL
Combined Results
reactors
accelerators
54
Combined Results
55
Conclusions and Executive Summary

• the direct/easy way for sterile neutrinos to
  enter our world
• (solar anomaly, atmospheric anomaly) is now
  ruled out

• performing a general analysis, we looked
• at more subtle and more interesting
  manifestations

• we find no evidence for sterile neutrinos so far

• we set the present bounds (in particular LSND
  excluded by Standard Cosmology)

• cosmology, astro-ph and ? experiments probe
• different and complementary patterns

• measure better 4He and D (different physics,
  different systematics)
• (CMB
  and LSS)

• detect the next SN - improve standard theory
  models
• - look for non-standard ? fluxes and
  spectra

• measure better low energy solar neutrinos

combine data from different fields