Sterile Neutrinos

Marco Cirelli

(Yale)

with G.Marandella, A.Strumia, F.Vissanihep-ph/0403158

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.

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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

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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

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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).

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All bounds from cosmology

4He=25.0%N = 3.2

4He=25.8%N = 3.8

h2 = 10 -2

h2 = 10 -3

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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

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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

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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

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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