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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)
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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.