Andrea Soddu

eV Seesaw with Four Generations and 3+3 scenario

ICHEP06

July 26 – August 2, 2006

Russian Academy of Sciences

Wei-Shu Hou, A.S.

Moscow

Weizmann Institute of Science

Phys. Lett. B 638, 2006

LSND data and eV-Seesaw

Can a forth generation be embedded in a eV-Seesaw scenario?

Decoupling the forth generation4+4 3+3

Fitting the oscillation data in 3+3

Outlook

Is a forth generation ruled out by EW precision tests?

MiniBooNE will be able to prove or disprove LSND anomaly

Additional “sterile” neutrinos involved in oscillations

Strong constraints from BBN and CMB

BBN constraints can be evaded in Non-Standard Cosmology

R. Foot and R. R. Volkas Phys. Rev. Lett. 75, 1995

LSND is not yet ruled out by cosmological observations

S. Hannestad JCAP 0305, 2003

If 4He systematic larger

After SSB describes 6 neutral Weyl fermions 6 neutrinos

The 6 neutrinos “fuse” into 3 Dirac fermions with

The 6 neutrinos split up into 3 light mostly active neutrinos

and 3 heavy, mostly sterile neutrinos with

Evidence for flux 30 meters away from a source of neutrinos produced in decay in flight, and the subsequent decay, at rest, of

appearance can be explained if

LSND results

eV Seesaw and LSND

Andre’ de Gouvea Phys. Rev. D72, 2005

Nothing experimental is really known about the magnitude of M except perhaps the LSND result, which is at eV scale.

In the basis where charged lepton mass matrix is diagonal

eV Seesaw and LSND

It is natural to have particles with mass of order EW scale

Existence of a 4TH generation: U, D, N, E

Existence of a 4TH generation with all fermions heavier than MZ strongly disfavored by EW precision data

Is a forth generation allowed ?

What about ?

In the Standard See-saw all the four active neutrinos are light

J. Erler and P. Langacker Phys. Lett. B 592, 2004

All EW precision data and mH > 90 GeV at 95 % CL from LEP II were used in the fit

M. Maltoni, V.A. Novikov, L.B. Okun,A.N. Razanov and M.I. Vysotsky

One and even two generations are allowed when neutral fermions are relatively light

N strongly decoupled from the three light neutrinos to avoid constraint

from LEP II

S,T,U parameterization valid for mNP >> MZ

Phys. Lett. B 476, 2000

has 6 zero eigenvalues plus two eigenvalues

4+4 scheme

Pseudo-Dirac neutrino N of mass

For the six null eigenvalues of one can apply perturbation theory with degeneracy

Having started in the basis where is diagonal, one still has the freedom to perform a rotation in the left sector.

4+4 scheme 3+3 scheme

3+1 scheme

is the fourth lightest neutrino and oscillations involving the 5th

and 6th states do not contribute in any significant way.

3+3 scheme

Democratic Structure

MNS

Mass Spectrum

3+3 Mass Matrix

3+3 Rotation Matrix

Upper Corner Solution

Lower Corner Solution

Lower Corner Solution

Upper Corner Solution

Conclusions

LSND not yet ruled out by MiniBooNE or cosmological observations

LSND eV scale could be an indication for the right-handed neutrino scale M

eV Seesaw can explain all the oscillations data

A sequential forth generation is well embedded in a eV Seesaw scenario

All EW precision measurements are satisfied with mN = 50 GeV

eV Seesaw with a forth generation can equally fit all the oscillations data

back up

Normal mass hierarchy for the three active neutrinos

Too small to explain LSND anomaly

Inverted mass hierarchy for the three active neutrinos

M. Sorel, J.M. Conrad and M.H. Shaevitz,

NSBLLSND

For (3+1) models poor agreement between NSBL and LSND data

3+2 schemeQuasi-degenerate neutrino masses

(2003)

U=0

J. Erler and P. Langacker hep-ph/0407097

Oblique Parameters S, T, U M. E. Peskin and T. Takeuchi

An extra generation of chiral fermions is excluded at 99.95% CL on the basis of the S parameter alone, if the new doublets are degenerate, T=U=0.For non degenerate doublets T>0. The data currently favor T<0 strengthening the exclusion limit.If the extra neutrino is close to its direct mass limit, S can be small or even negative but at the expense of too large contribution to T

Adequate when all particles of the forth generation are heavier than MZ

outside 95% C.L. S-T contour

i) decrease Sii) lift up Tiii) achieve both

Extra family gives large positive T and inmany cases also a sizable S>0 (case ii)

H.-J. He, N. Polonsky and S. Su hep-ph/0102144

(2000)

Modified Oblique Parameters S, T, U

Correlations among the contributions to the three oblique parameters S, T, U rather than the contribution to a particular one, provide the most significant bound.

A single extra chiral family with a constrained spectrum is consistent with precision data without requiring any New Physics source

(2005)

hep-ex/0509008

A four generation could still be alive and if yes it will appear soon