Abstract
We study the perspective to observe lepton number violating signatures from heavy Majorana neutrino decays at colliders in view of the requirement to explain the light neutrino masses via the seesaw mechanism. In the minimal model with only two heavy neutrinos and in the νMSM one can identify three distinct regions in the mass- mixing plane. For Majorana masses above the electroweak scale the branching ratio for lepton number violating processes at the LHC is generically suppressed. For masses well below the electroweak scale that are probed in displaced vertex searches or at fixed target experiments lepton number violation is the rule and can only be avoided at the cost of fine tuning. In between there is a mass regime where both possibilities coexist. In models with more than two heavy neutrinos the larger parameter space allows for more freedom, but our results remain qualitatively correct unless there is a mass degeneracy amongst more than two of the heavy neutrinos.
Highlights
While the sterile neutrinos νR are gauge singlets, the mass eigenstates after electroweak symmetry breaking Ni participate in all weak processes with amplitudes that are suppressed by θai,2 which makes it possible to search for them experimentally [15, 16]
We studied the perspectives to see lepton number violating (LNV) signatures in collider searches for heavy neutrinos Ni that interact with the SM exclusively via their mixings θai with the SM neutrinos, where a denotes a SM generation
Mixing angles θai that are large enough to yield observable production cross sections at the LHC can be made consistent with the smallness of the light neutrino masses if the heavy neutrino interactions respect the approximate conservation of a generalised lepton number L
Summary
Neutrino masses and approximate lepton number conservation. For the purpose of collider searches, the most important properties of the heavy neutrinos are their masses Mi and the mixing angles θai which determine the suppression of their weak interactions. The reason is that the heavy neutrinos do have a Majorana mass MM , and receive a mass from the Higgs mechanism that is of the same order as the masses of the light neutrinos Their physical masses after electroweak symmetry breaking are not given by the entries of MM , but by the square roots of the eigenvalues of the matrix MN† MN with [70]10. The second term in (2.11) dominates and we can parametrically estimate the size of the correction as The requirement that this correction remains smaller than the tree level contribution imposes an upper bound on ∆Mphys for given Mand U 2.
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