Sterile neutrino searches at future colliders Stefan Antusch University of Basel Max-Planck-Institut für Physik Department of Physics (Werner-Heisenberg-Institut) NUFACT2017, Uppsala September 28, 2017
One of the big open questions in BSM physics: What is the origin of the observed neutrinos masses? 2
Outline Ø Introduction: Some basics on sterile neutrinos Ø Then: focus on EW scale sterile neutrinos. How can there be sterile neutrinos with mass M ~ Λ EW and “large” Yukawa couplings Y ν ? Ø Which observable effects allow to test such models? • M >> Λ EW : “Non-unitarity effects” (indirect tests) • M ~ Λ EW : On-shell heavy neutrino effects (direct tests at colliders, various promising signature processes) • M < m W : Very sensitive searches via “displaced vertices” at colliders Ø Discovery prospects at possible future colliders (ee, ep and pp) 3 Stefan Antusch University of Basel
Sterile (= right-chiral) neutrinos? There are no right- chiral neutrino states ( ν Ri ) in the Standard Model è ν Ri would be completely neutral under all SM symmetries Adding ν Ri leads to the following extra terms in the Lagrangian density: M: sterile ν mass matrix Y N : neutrino Yukawa matrix (Dirac mass terms) 4
Light neutrino masses via the seesaw mechanism Mass matrix of the (2+n) sterile Mass matrix of the (three) (= right-handed) neutrinos light neutrinos (masses of Majorana-type) Valid for v EW y ν << M R „Seesaw Formula“ P. Minkowski ('77), Mohapatra, From neutrino oscillation experiments Senjanovic, Yanagida, Gell-Mann, and mass searches: Ramond, Slansky, Schechter, Valle, … Neutrino Yukawa matrix |m 3 2 - m 1 2 | ≈ 2.4 · 10 -3 eV 2 m 2 2 - m 1 2 ≈ 7.5 · 10 -5 eV 2 all three m i below ~ 0.2 eV + measurements of the leptonic mixing Note: At least two sterile neutrinos are required angles (from neutrino osc. experiments) è generate masses for two of the light neutrinos (necessary for realizing the two observed mass splittings) 5 Stefan Antusch University of Basel
What do the measured light neutrino parameters tell us about the sterile neutrino parameters M, Y ν ? Stefan Antusch University of Basel 6
What do we know about the neutrino parameters? Getting started: 1 ν R , 1 ν L è Knowledge of m ν implies relation between y ν and M R “Naive” seesaw relation: y ν 2 < O(10 -13 ) (M / 100 GeV) 7 Stefan Antusch University of Basel
What do we know about the sterile neutrino parameters? Example of a Example 1: 2 ν R , 2 ν L small perturbation ε ε δ i2 ) è Also in this example: Knowledge of m ν i implies relation between y ν i and M R 8 Stefan Antusch University of Basel
What do we know about the sterile neutrino parameters? Similar: “inverse” seesaw, “linear” seesaw Example 2: 2 ν R , 2 ν L See e.g.: D. Wyler, L. Wolfenstein ( ’ 83), R. N. Mohapatra, J. W. F. Valle ( ’ 86), M. Shaposhnikov (‚07), J. Kersten, A. Y. Smirnov (‘07), M. B. Gavela, T. Hambye, D. Hernandez, P. Hernandez ( ’ 09), M. Malinsky, J. C. Romao, J. W. F. Valle (‘05), … ε Example of a small ε perturbation è In general: No “fixed relation” between y ν and M R , larger y ν possible! 9 Stefan Antusch University of Basel
What do we know about the sterile neutrino parameters? Example 2: 2 ν R , 2 ν L Similar: “inverse” seesaw, “linear” seesaw ε ε Example for “protective” symmetry: L α ν R1 ν R2 Note: Can be realized by symmetries, e.g. by an (approximate) “lepton number”-like symmetry Lepton-# +1 +1 -1 10 Stefan Antusch University of Basel
Possible values of M R and y ν |y ν | Also allowed! Possible if one of the light neutrinos has very small mass! M R Not considering experimental constraints 11 Stefan Antusch University of Basel
“Landscape” of sterile neutrino models Examples, schematic |y ν | EW scale sterile neutrino models” GUT models (often similar to ★ ★ ★★ example 2) ★ ★ ★ ★★ ★ ★ ★ ★ ★ ★ ★ ★ “Reactor ★ ★ ★ anomaly“, ★ ★ ★ LSND “keV sterile neutrino ★ warm dark matter“ M R Not considering experimental constraints 12 Stefan Antusch University of Basel
A benchmark model for EW scale sterile ν : SPSS (Symmetry Protected Seesaw Scenario) Consider 2+n sterile neutrinos (plus the three active) è with M and Y ν for two of the steriles as in example 2 due to some generic “lepton number”-like symmetry) + O( ε ) perturbations to generate the light neutrino masss (which we can often neglect for collider studies) Similar: “inverse” seesaw, “linear” seesaw Additional sterile neutrinos can exist, but have no effects at colliders (which can be realised easily, For details on the SPSS, see: e.g. by giving lepton number = 0 to them). S.A., O. Fischer (arXiv:1502.05915) 13 Stefan Antusch University of Basel
A benchmark model for EW scale sterile ν : Note: Since in the SPSS we allow for additional sterile neutrinos, M and y α SPSS (Symmetry Protected Seesaw Scenario) ( α =e,µ, τ ) are indeed free parameters (not constrained by m ν ). In specific models there are correlations among the y α . Strategy of the SPSS: study Consider 2+n sterile neutrinos (plus the three active) è with M and Y ν for two of how to measure the y α independently, the steriles as in example 2 due to some generic “lepton number”-like symmetry) in order to test (not a priori assume) such correlations! + O( ε ) perturbations to generate the light neutrino masss (which we can often neglect for collider studies) Similar: “inverse” seesaw, “linear” seesaw Additional sterile neutrinos can exist, but have no effects at colliders (which can be realised easily, For details on the SPSS, see: e.g. by giving lepton number = 0 to them). S.A., O. Fischer (arXiv:1502.05915) 14 Stefan Antusch University of Basel
Testing specific low scale seesaw models: Examples Stefan Antusch University of Basel 15
A benchmark model for EW scale sterile ν : Note: Since in the SPSS we allow for additional sterile neutrinos, M and y α SPSS (Symmetry Protected Seesaw Scenario) ( α =e,µ, τ ) are indeed free parameters (not constrained by m ν ). In specific models there are correlations among the y α . Strategy of the SPSS: study Consider 2+n sterile neutrinos (plus the three active) è with M and Y ν for two of how to measure the y α independently, the steriles as in example 2 due to some generic “lepton number”-like symmetry) in order to test (not a priori assume) such correlations! + O( ε ) perturbations to generate the light neutrino For example: Low scale seesaw with masss 2 sterile neutrinos: y α /y β given in tems (which we can of the PMNS parameters. E.g. for NO: often neglect for collider studies) For details on the SPSS, see: S.A., O. Fischer (arXiv:1502.05915) Cf.: Gavela, Hambye, D. Hernandez, P. Hernandez (‘09) 16 Stefan Antusch University of Basel
Further predictions in specific types of low scale seesaw mechanisms: Δ M of heavy ν ‘s *) Basis: ( ν L α , N 1 , N 2 ) lin ~ * lin inv lin inv ~ additional parameter, no contribution to light neutrino masses ) ( Perturbations O( ε ) generate the light neutrino masses and, e.g. in the case of the minimal linear seesaw model, lead to a prediction for the heavy neutrino mass splitting Δ M ( in terms of the light neutrino mass splittings ): Cf.: S.A., E. Cazzato, O. Fischer (arXiv:1709.03797) ... More about this later in my talk! 17 Stefan Antusch University of Basel
Recommend
More recommend
