Probing the seesaw mechanism at the 250 GeV ILC

Arindam Das,Nobuchika Okada,Satomi Okada,Digesh Raut

doi:10.1016/j.physletb.2019.134849

Abstract

We consider a gauged U(1)B−L (Baryon-minus-Lepton number) extension of the Standard Model (SM), which is anomaly-free in the presence of three Right-Handed Neutrinos (RHNs). Associated with the U(1)B−L symmetry breaking the RHNs acquire their Majorana masses and then play the crucial role to generate the neutrino mass matrix by the seesaw mechanism. Towards the experimental confirmation of the seesaw mechanism, we investigate a RHN pair production through the U(1)B−L gauge boson (Z′) at the 250 GeV International Linear Collider (ILC). The Z′ gauge boson has been searched at the Large Hadron Collider (LHC) Run-2 and its production cross section is already severely constrained. The constraint will become more stringent by the future experiments with the High-Luminosity upgrade of the LHC (HL-LHC). We find a possibility that even after a null Z′ boson search result at the HL-LHC, the 250 GeV ILC can search for the RHN pair production through the final state with same-sign dileptons plus jets, which is a “smoking-gun” signature from the Majorana nature of RHNs. In addition, some of RHNs are long-lived and leave a clean signature with a displaced vertex. Therefore, the 250 GeV ILC can operate as not only a Higgs Factory but also a RHN discovery machine to explore the origin of the Majorana neutrino mass generation, namely the seesaw mechanism.

Highlights

We consider a gauged U(1)B−L (Baryon-minus-Lepton number) extension of the Standard Model (SM), which is anomaly-free in the presence of three Right-Handed Neutrinos (RHNs)
In other words, the mass scale of the Majorana RHNs lies at the TeV scale or lower, RHNs can be produced at high energy colliders through the process mediated by the Z′ boson
One may think that the 250 GeV International Linear Collider (ILC) is not a powerful machine compared to the Large Hadron Collider (LHC) in exploring new physics if it is less related to the SM Higgs sector

Summary

Weak interactions of the neutrino mass eigenstates

In terms of the neutrino mass eigenstates, the charged current (CC) interaction can be written as LCC. Where lα (α = e, μ, τ ) denotes the three generations of the charged leptons, and PL The neutral current (NC) interaction is given by LNC g cos θW. Where θW is the weak mixing angle

Heavy neutrino decay to three SM fermions

Determining Rαi from the neutrino oscillation data

Long-lived heavy neutrinos