Abstract
In the type-I seesaw mechanism for neutrino masses, there exists a B − L symmetry, whose breaking leads to the lepton number violating mass of the heavy Majorana neutrinos. This would imply the existence of a new neutral scalar associated with the B −L symmetry breaking, analogous to the Higgs boson of the Standard Model. If in such models, the heavy neutrino decays are also responsible for the observed baryon asymmetry of the universe via the leptogenesis mechanism, the new seesaw scalar interactions with the heavy neutrinos will induce additional dilution terms for the heavy neutrino and lepton number densities. We make a detailed study of this dilution effect on the lepton asymmetry in three generic classes of seesaw models with TeV-scale B − L symmetry breaking, namely, in an effective theory framework and in scenarios with global or local U(1)B−L symmetry. We find that requiring successful leptogenesis imposes stringent constraints on the mass and couplings of the new scalar in all three cases, especially when it is lighter than the heavy neutrinos. We also discuss the implications of these new constraints and prospects of testing leptogenesis in presence of seesaw scalars at colliders.
Highlights
Local, there should exist a Higgs field that breaks the symmetry and the real part of that complex Higgs field will be an important new particle intimately linked with the seesaw mechanism
If the heavy RHNs are discovered at the LHC or future higher energy/luminosity colliders [8, 9], this will narrow the parameter region to search for the seesaw Higgs boson and its properties, which would provide a direct test of leptogenesis and existence of B − L symmetry in this scenario
The effective theory could be considered as the low energy simplified version of a more fundamental ultraviolet (UV)-completion at higher energy scale, such as the specific U(1)B−L models discussed in section 3 and 4, yet some of the key features of dilution of lepton asymmetry could already be seen and understood in the effective field theory (EFT) framework, cf. the Feynman diagram in figure 1 and the plots in figures 2–6
Summary
The lowest-order effective couplings of a CP-even (odd) scalar H (A) to the RHN can be parameterized as. In most of the realistic seesaw models, there might be more than one physical scalar, or even multiplets, that couple to the RHN; see for instance the global U(1)B−L model in section 3 and the local one in section 4 (in the local case the CP-odd component is eaten by the heavy ZR boson). Involved in the dilution of RHNs through the processes N N → Si → SjSk (i, j, k being scalar indices, see e.g. the diagrams in figure 7) and play an important role in leptogenesis, when the Yukawa coupling f is comparatively smaller. To capture the most important consequence of the presence of a (light) scalar/pseudoscalar in the type-I seesaw leptogenesis, we neglect the model-dependent scalar interactions and consider only the t-channel process N N → HH/AA, mediated by the Yukawa coupling f in eq (2.1), as shown in figure 1. In addition to the RHN mass mN (and the leptogenesis-relevant quantities such as the effective neutrino mass m), there are only two free parameters in the effective theory, i.e. the scalar mass mH (mA) and the effective Yukawa coupling fH(A) in eq (2.1)
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