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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) 1 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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3 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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4 Left-right neutrino mixing: Ex. Negligibly small to create the Majorana neutrino at the collider.

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5 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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6 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 obtain a sizable left-right neutrino mixing. 3.Experimental implication

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7 What we see in this talk: 1.Introduction (Done) 2.Convenient basis to describe the left-right neutrino mixing 3.Experimental constraints 4.Flavor symmetry 5.Experimental implications 6.Summary

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8 = Diagonalization : PMSN neutrino mixing matrix :

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9 Current eigenstate Mass eigenstates (approximate) active neutrino mixing matrix for neutrino oscillations Left-right neutrino mixing matrix

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10 Lesson : Two-generation case Without loss of generality, we can choose (1,1) elements are zero by rotation of left- and right-handed fields.

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11 In the limit b 0, 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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12 Multiplying from both sides,

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13 After all, Diagonalization matrix of charged-lepton mass Diagonalization matrix of right-handed Majorana mass Note : precise experimental results require ~

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14 Three-generation case Without loss of generality, we can choose a basis: In the limit b,d,e 0, 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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15 After all, Ex. (T2K/MINOS/WCHOOZ/Daya Bay…) LHC (same-sign muons)

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16 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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17 Experimental constraints (Atre-Han-Pascoli-Zhang)

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18 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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19 Numerical Example (Unit in GeV)

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20 small Rank reduced Key structure : It can be realized by a flavor symmetry.

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21 Froggatt-Nielsen mechanism U(1): SU(2):

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22 Example: Dirac Yukawa : : B-L charged scalars which acquire VEV

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23 (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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24 Suppose that the mixings from the charged lepton are small, the Unitary matrix U is the MSN matrix. From the condition: we obtain …. Next page

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25 (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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26 Current experimental best fit point :

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27 Resonant production Same-sign WW fusion Same-sign di-lepton events at the LHC (This is more important)

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28 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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29 LHC sensitivity (Atre-Han-Pascoli-Zhang)

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30 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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31 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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32 Summary 1.We consider a convenient basis to describe the non-negligible left-right neutrino mixing. 2.Tiny neutrino masses can be realized even if the 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, satisfying the constraint of neutrino-less double beta decay.

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