Sterile Neutrinos
Marco Cirelli
(Yale)
with G.Marandella, A.Strumia, F.Vissanihep-ph/0403158 (--> NPB)
and with Yi-Zen Chu (in preparation)
PASCOS 2004
Probing
with cosmology, astrophysics, experiments…
2
We want to study Sterile NeutrinosSterile Neutrinos
Oscillations into ss are now excluded as the dominant solutionin solar and atmospheric neutrinos:
Introduction and Purpose
- (light) spin 1/2 fermions, - neutral under all SM forces,- have a mixing with active .
i.e.
(details…)solar: e s s ? e ,no, (SNO)
(details…)atmo: ss? no, (SK, Macro)
3
Now the relevant issues become:
- which subdominant role is still possible for ss ?
- where can we detect the ss ?
- how can we detect the ss ?
• set present bounds
• identify future signals
• look for sterile evidence in present data. None.
Perform a complete analysis:
(2) including the established e-, and - mixing
(1) for any possible e,, - ss mixing pattern
(3) study all neutrino sources ( experiments, astrophysics, cosmology)
(= in a full 4 mixing formalism)
4
Are sterile neutrinos still interesting at all?
• Yes, “new light neutral fermions” in so many Beyond the SM constructions…
• Yes, sterile neutrinos invoked for so many “puzzles” … ( ?)
• …LSND
axino
braninodilatino
familino
goldstino
modulino
majorino
radino
-behave effectively as ss
-parameterize with s , ms2 …
pulsar kicks
Dark Mattergalactic ionization
r-process nucleosynthesis
…
mirror fermions
right-handed neutrino
11
4 mixing formalism
We want instead a full 4 formalism.
Present bounds are computed in a limited 2 formalism: l coss l’+sin s s .
A simple parametrization:
define a complex unit 3-vector n
n identifies the combination of active :
which mixes with ss with a single angle
more details
12
In the following:
ss 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.
( l ·n = ) ( i ·n = 2 )
ss
SN
Sun
Atmosphere
CMB
AGNLSS
BBN
SBL
accelerators
CombinedResults
reactors
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
(T ~ 1 MeV)
QuickTime™ and aGIF decompressor
are needed to see this picture.
BBN
(nuclear rates, n mean life,weak cross sections…)
n/p
4He D3Li3He
…
b
CMB (WMAP)
Ne se s
ms2 , s
Roadmap ( = What we do)
For every choice of ms2 , s ,
for T >> MeV 0.07 MeV follow:(BBN ends, les jeux sont fait)
Assumptions:• no large lepton asymmetries• neglect spectral distortions
1 kinetic equ.s for neutrino densities e(T), (T), (T), s (T)
2 equation for n/p
3 equations of light nuclei (4He , D) production 4 (4He , D) observations
Where does a s s enter the BBN game?(A) ss production larger total energy density faster expansion(B) mixing e -ss depletion of e effect on n p reactions
25
Bounds in the parameter space
4He=25.0%N = 3.2
4He=25.8%N = 3.8
Large Scale StructureThe primordial free streaming of massive neutrinosaffects the LSS power spectrum observed today. QuickTime™ and a
GIF decompressorare needed to see this picture.
2dF+WMAP :
Upper bound on:
M.T
egm
ark
web
page
ss contribute to bound on ms i.e. m2s .
but: if ss do not fully thermalize s << 1 weaker bound
1,2,3 and ss
27
Bounds in the parameter space
h2 = 10 -2
h2 = 10 -3
28
Cosmic Microwave BackgroundThe primordial neutrino energy densities affect the acoustic peaks of CMB power spectrum.
Bound on theeffective N
CMB e , , , s .
At present: NCMB
= 32
Bar
ger
et a
l., P
LB
566,
200
3
N = 1, 2.75, 5, 7
Bound on the ms2 , s (that determine the s).
29
All bounds from cosmology
4He=25.0%N = 3.2
4He=25.8%N = 3.8
h2 = 10 -2
h2 = 10 -3
30
LSND: in or out?
LSND ~ es s
LSND claims evidence for e with m2 m2sun, atm (if oscillations)
Requires a new (= sterile) neutrino: s e
How does the LSND s fit in cosmology?
ss
SN
Sun
Atmosphere
CMB
AGNLSS
BBN
SBL
accelerators
CombinedResults
reactors
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.
Overall picture confirmed by SN1987a Set present bounds
Thousands of events from future SN Propose future probes
33
core
mantle e
s
SNO
SK
-sp
here
Matter oscillations in the star mantle:
A.B
urro
ws
et a
l., 2
001,
200
2, 2
003
34
Matter eigenstates in the mantle:
Output: final fluxes of e, and on Earth .
At each crossing there is acrossing probability “lost”
35
Results: percentual reduction of e events (in a large Cerenkov detector)
Beware oftheoretical uncertainties…
Excluded bySN1987a
(e p ne+)
36
The energy dependance of matter/vacuum conversions causesspectral distortions:
Possible very clear feature!
ss
SN
Sun
Atmosphere
CMB
AGNLSS
BBN
SBL
accelerators
CombinedResults
reactors
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.
ss
SN
Sun
Atmosphere
CMB
AGNLSS
BBN
SBL
accelerators
CombinedResults
reactors
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.
Look for evidence of s effects around the LMA solution. None
Identify future probes
Set present boundsmore details
41
Active-sterile resonance
Solar e spectrum:(production regions)
Evolution:LMA e-, resonance
-crossings in sun matter
-vacuum oscillations
-(matter oscillations in Earth)
-input e flux
-output fluxes“lost”
Ba h
c all
, Pin
son n
eaul
t 200
1
Go n
zale
z -G
a rc i
a , N
ir, r
e vi e
w 2
002
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(ee) 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 e
Pe s
Pe
45
The “still allowed component” of s in solar neutrinos:
means the naïve limit e coss ,+sin s s .In our framework:
sin2s < 0.2
ss
SN
Sun
Atmosphere
CMB
AGNLSS
BBN
SBL
accelerators
CombinedResults
reactors
47
Sterile effects in atmospheric neutrinos
Basics:
SK
col
l.
Evidence for oscillations is disappearance of “from below”.
Where do they go? , s s or a combination?3 sensitive probes to discriminate and put bounds:
48
If ss :
(1) larger flux of thru-muons(1b) larger number of PC events(2) fewer NC-enriched events(3) tau appearance…
We perform a global 2 analysis ofSK + 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:
means the naïve limit coss +sin s s .In our framework:
sin2s <0.16
ss
SN
Sun
Atmosphere
CMB
AGNLSS
BBN
SBL
accelerators
CombinedResults
reactors
52
Sterile effects in SBL neutrinosChooz + Bugey + CDHS + CCFR + Karmen + Nomad + Chorus
future SBL at reactor?excluded
Main constraint comes from “no-disappearance”.
ss
SN
Sun
Atmosphere
CMB
AGNLSS
BBN
SBL
accelerators
CombinedResults
reactors
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