[Paper Review] Leptogenesis Constraints on $B-L$ breaking Higgs Boson in TeV Scale Seesaw Models

P. S. Bhupal Dev, Rabindra N. Mohapatra+1 more|arXiv (Cornell University)|Nov 21, 2017

Particle physics theoretical and experimental studies44 references3 citations

TL;DR

This paper investigates the cosmological constraints on a B-L breaking Higgs boson in TeV-scale type-I seesaw models, showing that its interactions with heavy Majorana neutrinos induce significant dilution of lepton asymmetry during leptogenesis. The study derives stringent bounds on the scalar’s mass and couplings—especially when lighter than the heavy neutrinos—using effective field theory and global/local U(1)_{B-L} scenarios, with implications for collider searches at the LHC.

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.

Motivation & Objective

• To investigate how the B-L breaking Higgs boson in TeV-scale seesaw models affects lepton asymmetry generation via leptogenesis.
• To derive constraints on the mass and couplings of the new scalar from successful leptogenesis in three classes of models: EFT, global U(1)_{B-L}, and local U(1)_{B-L}.
• To examine the dilution and washout effects of the seesaw scalar on lepton number density during leptogenesis.
• To assess the detectability of these constraints at the LHC and future hadron colliders.
• To provide model-independent bounds on the scalar parameters that are robust against details of the leptogenesis mechanism.

Proposed method

• Formulating the effective field theory (EFT) framework for B-L symmetry breaking with a new neutral scalar coupling to heavy Majorana neutrinos.
• Analyzing the dilution and washout effects on lepton asymmetry using Boltzmann equations that include interactions of the seesaw scalar with heavy neutrinos and lepton number.
• Deriving the Boltzmann equations for lepton number and heavy neutrino number densities, including the new scalar's contributions to decay and inverse processes.
• Computing the decay and inverse decay rates of heavy neutrinos mediated by the B-L scalar, including phase space and thermal averaging.
• Evaluating the impact of the scalar on the final lepton asymmetry through the Boltzmann equation solutions, with explicit expressions for the dilution factor.
• Applying the results to three model classes: EFT, global U(1)_{B-L} (singlet Majoron model), and local U(1)_{B-L} (gauged model), comparing constraints across all.
• Deriving partial widths for scalar decays into RHNs and gauge bosons (e.g., $Z_R$) to assess kinematic constraints and collider signatures.

Experimental results

Research questions

• RQ1 How does the presence of a B-L breaking Higgs boson affect the lepton asymmetry generated via leptogenesis in TeV-scale seesaw models?
• RQ2 What are the model-independent constraints on the mass and couplings of the B-L scalar imposed by the requirement of successful leptogenesis?
• RQ3 How do the dilution and washout effects from the seesaw scalar differ across EFT, global U(1)_{B-L}, and local U(1)_{B-L} scenarios?
• RQ4 Under what conditions does the scalar-induced dilution become dominant, and when can it be probed at the LHC?
• RQ5 What are the collider signatures of the B-L scalar, and how do they correlate with the success of leptogenesis?

Key findings

• The dilution of lepton asymmetry due to the B-L scalar imposes stringent constraints on its mass and couplings, especially when the scalar is lighter than the heavy Majorana neutrinos.
• In all three model classes—EFT, global U(1)_{B-L}, and local U(1)_{B-L}—successful leptogenesis requires the B-L scalar coupling to be sufficiently small or its mass sufficiently large to avoid excessive washout.
• For the local U(1)_{B-L} model, the scalar's coupling to $Z_R$ bosons and its decay width into $Z_R Z_R$ are constrained by kinematic thresholds and partial width calculations.
• The partial width for $H_3 o Z_R Z_R$ is derived with phase space suppression and threshold dependence, with $ heta(m_{H_3} - 2m_{Z_R})$ ensuring kinematic validity.
• The scalar's decay into two RHNs is governed by the width expression $rac{m_{H_3}^3}{64 au v_R^2} imes ext{phase space}$, with a threshold condition for on-shell decay.
• The LHC, especially in high-luminosity mode, can directly probe the RHN pair production and the associated scalar signatures, providing a testbed for leptogenesis constraints.