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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
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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?
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MiniBooNE will be able to prove or disprove LSND anomaly
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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
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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
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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.
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In the basis where charged lepton mass matrix is diagonal
eV Seesaw and LSND
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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
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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
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has 6 zero eigenvalues plus two eigenvalues
4+4 scheme
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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
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3+1 scheme
is the fourth lightest neutrino and oscillations involving the 5th
and 6th states do not contribute in any significant way.
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3+3 scheme
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Democratic Structure
MNS
Mass Spectrum
3+3 Mass Matrix
3+3 Rotation Matrix
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Upper Corner Solution
Lower Corner Solution
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Lower Corner Solution
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Upper Corner Solution
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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
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back up
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Normal mass hierarchy for the three active neutrinos
Too small to explain LSND anomaly
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Inverted mass hierarchy for the three active neutrinos
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M. Sorel, J.M. Conrad and M.H. Shaevitz,
NSBLLSND
For (3+1) models poor agreement between NSBL and LSND data
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3+2 schemeQuasi-degenerate neutrino masses
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(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
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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
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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
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(2005)
hep-ex/0509008
A four generation could still be alive and if yes it will appear soon