Neutrino Seesaw Models at the LHC Xiao-Gang He Department of Physics and Center for Theoretical Sciences National Taiwan University Talk at USTC, April 15 The Seesaw Mechanism Type II Seesaw at the LHC Small light and heavy mixing in Type I and III at the LHC New possibility, large mixing, for Type-I and III seesaw at LHC Conclusions

1. The Seesaw Mechanism

Minkowski; Yanagida; Gell-Man, Ramond and Slansky…

What LHC can do for neutrino physics? A lot. One of them is to probe the heavy degree of freedom in models producing neutrino masses and mixing. Production of new particles, the most direct test! T.~Han and B.~Zhang, Heavy neutrino t LHC [arXiv:hep-ph/0604064]; M. Nebot et al., Zee Model arXiv:0711.0483 [hep-ph]]. R.~Franceschini et al.,Type-III [arXiv:0805.1613 [hep-ph]]. F.~del Aguila etal., Type I, II, III [arXiv:0808.2468 [hep-ph]]. P.~Fileviez Perez et al., Type-II [arXiv:0805.3536 [hep-ph]]; A.~Arhrib et al.,Type I+III Seesaw, arXiv:0904.2390 [hep-ph]; W.~Chao et al., Type I +II [arXiv:0804.1265 [hep-ph]]; X.-G. He et al., Type I and III, arXiv: 0908.1607[hep-ph] X.-G. He and Ernest Ma, arXiv:0907.2737[hep-ph] T. Li and X.-G. He., Type III, arXiv:0907.4193[hep-ph].

2. Type II Seesaw at the LHC

Del Aguila and Aguilar-Saavedra

P. Perez et al.

3. Small light and heavy mixing in Type I and III at the LHC Some features about Type I and III Seesaw Models

Type-I seesaw at LHC

V_{lN} ~ (m_\nu/m_N)^{1/2} < 10^{-6} for one generation. Too small!

Neutrino Mixing Schwetz et. al, Gonzalez-Garcia et. Al, Goswami et. al Neutrino Mixing Schwetz et.al, Gonzalez-Garcia et. Al, Goswami et.al. Strumia et al.

Real angle wij -> small mixing solution Real angle wij -> small mixing solution. Complex angle, can have large mixing solution.

Even with small V_{lN} Type-III seesaw can be tested at the LHC Tong Li and Xiao-Gang He, arXiv:0907.4193[hep-ph]

4. New possibility, large mixing, for Type-I and III seesaw at LHC X 4. New possibility, large mixing, for Type-I and III seesaw at LHC X.-G. He et al., arXiv:0907.1607[hep-ph] (earlier work: Smirnov and Kersten)

Some sample solutions

For U_{\nu U} of order 0.03, the cross section can be as large as 1 fb for m_N up to 150 GeV. Not hopelessly small! For Type III Seesaw, no displayed vertex!

If m_N < m_h, affect Higgs decay Br If m_N < m_h, affect Higgs decay Br. Park, Wang and Yanagida, arXiv:0909.2937; Chen et al. arXiv:1001.5215

Model realization for large light and heavy mixing X. -G. He and E Model realization for large light and heavy mixing X.-G. He and E. Ma, arXiv:0907.2737[hep-ph]

5. Conclusions Seesaw models are the most economy models to produce naturally small neutrino masses. If the seesaw scale is of order a TeV, LHC may be able to directly produce the heavy degrees of freedom (pair producing) providing direct test for type II and type III (small light and heavy neutrino mixing) models. There are solutions with sizeable mixing between light and heavy neutrinos. In this case the single N (Type-I, III) and E (Type-III) production cross sections can be as large as 1 fb with masses as heavy as 150 GeV and be tested at the LHC. Signature to distinguishing small and large mixing: displaced vertex for Small mixing and no displaced vertex for large mixing.