TeV-scale seesaw with non-negligible

left-right neutrino mixings

Yukihiro Mimura (National Taiwan University)

Based on arXiv:1110.2252 [hep-ph]

Collaboration with N. Haba, T. Horita, and K. Kaneta (Osaka U)

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Seminar at Academia Sinica (2012.1.13, Friday)

Introduction

Neutrinos are massive, but active neutrino masses are tiny, < O(1) eV

The simplest mechanism is type-I seesaw.

“Natural” scale for

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Q. Is there a chance to “observe” the right-handed neutrino?

Ex.

C.f.

Right-handed neutrino mass can be O(100) GeV,but, …..

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Left-right neutrino mixing:

Ex.

Negligibly small to create the Majorana neutrino at the collider.

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Q. Is it possible to make the left-right mixing large enough to detect the existence of the right-handed neutrino?

A. Yes, if generation structure is taken into account.

Buchmuller-Wyler, Buchmuller-Greub, Tommasini-Barenboim-Bernabeu-Jarskog,Gluza, Kerstern-Smirnov, Adhikari-Raychaudhuri,

Ma, He-Ma,

He-Oh-Tandean-Wen, Chen-He-Tandean-Tsai,

…. (sorry, incomplete list)

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What we have done:

1. Find a convenient flavor basis to describe the non-negligible left-right neutrino mixing.

2. Consider a flavor symmetry to obtaina sizable left-right neutrino mixing.

3. Experimental implication

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What we see in this talk:

1. Introduction (Done)

2. Convenient basis to describe theleft-right neutrino mixing

3. Experimental constraints

4. Flavor symmetry

5. Experimental implications

6. Summary

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=Diagonalization :

PMSN neutrino mixing matrix :

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Currenteigenstate

Masseigenstates

(approximate) active neutrino mixing matrixfor neutrino oscillations

Left-right neutrino mixing matrix

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Lesson : Two-generation case

Without loss of generality, we can choose (1,1) elements are zeroby rotation of left- and right-handed fields.

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In the limit b0, the matrix is rank 2.Features of this basis:

Easy to find a tiny active neutrino mass limit.

Left-right mixing is characterized by

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Multiplying from both sides,

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After all,

Diagonalization matrix of charged-lepton mass

Diagonalization matrix of right-handed Majorana mass

Note : precise experimental results require ~

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Three-generation caseWithout loss of generality, we can choose a basis:

In the limit b,d,e0, the 6x6 neutrino matrix is rank 3.Features of this basis:

Easy to find a tiny active neutrino mass limit.

Left-right mixing is characterized by

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After all,

Ex.

(T2K/MINOS/WCHOOZ/Daya Bay…)

LHC (same-sign muons)

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In old works in the literature, people works in the basis:

is required for tiny neutrino mass.

In our basis,

The above condition is satisfied simply due to

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Experimental constraints (Atre-Han-Pascoli-Zhang)

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1.Colliders

2.Tau and K, D meson decays

3.Precision electroweak data

4.Neutrino-less double beta decay

5. Lepton flavor violation

~(Fermi constant, lepton universality, invisible Z decay, …)

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Numerical Example

(Unit in GeV)

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small

Rank reduced

Key structure :

It can be realized by a flavor symmetry.

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Froggatt-Nielsen mechanism

U(1):

SU(2):

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Example:

Dirac Yukawa :

: B-L charged scalars which acquire VEV

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(x denotes non-zero entry.)

If both the Dirac and Majorana mass matrices are in the form :

the seesaw mass matrix is also in the form of

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Suppose that the mixings from the charged leptonare small, the Unitary matrix U is the MSN matrix.

From the condition:

we obtain ….

Next page

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(Only the case of Normal hierarchy gives solutions in the setup.)

Using the experimental data, we obtain 13 mixing as a prediction.

(Cubic equation of 13 mixing for given CP phase).

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Current experimental best fit point :

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Resonant production

Same-sign WW fusion

Same-sign di-lepton events at the LHC

(This is more important)

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Bare cross sections for same-sign di-muon

(Atre-Han-Pascoli-Zhang)

Datta-Guchait-Pilaftsis, Almeidia-Coutinho-Martins Simoes-do Vale,Panella-Cannoni-Carimalo-Srivastava, del Aguila-Aguilar-Saavedra,Chen-He-Tandean-Tsai, ….

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LHC sensitivity

(Atre-Han-Pascoli-Zhang)

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Same-sign di-electron is strongly constrainted by double beta decay :

Amplitude is proportional to . It can also controlled by a flavor symmetry.Same-sign di-electron can have a chance to be observed.

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Several special cases:

Two-lighter right-handed neutrino masses are degenerate.

Double beta decay vanishes.

Double beta decay and μ e γ vanish.

Two right-handed neutrino masses are degenerate.Lepton number(-like) symmetry remains.

Degeneracy of Majorana neutrino Merit of TeV-scale resonant leptogenesis

1

2

3

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Summary1. We consider a convenient basis to describe

the non-negligible left-right neutrino mixing.

2. Tiny neutrino masses can be realized even ifthe left-right mixing is sizable.

3. The neutrino mass structure can be controlled by a flavor symmetry.

4. Same-sign di-electron events may be observed as well as di-muon events, satisfyingthe constraint of neutrino-less double beta decay.