Classically conformal U(1)' extended standard model, electroweak vacuum stability, and LHC Run-2 bounds

摘要

We consider the minimal U(1)' extension of the standard model (SM) with theclassically conformal invariance, where an anomaly-free U(1)' gauge symmetry isintroduced along with three generations of right-handed neutrinos and a U(1)'Higgs field. Since the classically conformal symmetry forbids all dimensionalparameters in the model, the U(1)' gauge symmetry is broken by theColeman-Weinberg mechanism, generating the mass terms of the U(1)' gauge boson($Z$' boson) and the right-handed neutrinos. Through a mixing quartic couplingbetween the U(1)' Higgs field and the SM Higgs doublet field, the radiativeU(1)' gauge symmetry breaking also triggers the breaking of the electroweaksymmetry. In this model context, we first investigate the electroweak vacuuminstability problem in the SM. Employing the renormalization group equations atthe two-loop level and the central values for the world average masses of thetop quark ($m_t=173.34$ GeV) and the Higgs boson ($m_h=125.09$ GeV), we performparameter scans to identify the parameter region for resolving the electroweakvacuum instability problem. Next we interpret the recent ATLAS and CMS searchlimits at the LHC Run-2 for the sequential $Z$' boson to constrain theparameter region in our model. Combining the constraints from the electroweakvacuum stability and the LHC Run-2 results, we find a bound on the $Z$' bosonmass as $m_{Z'} \gtrsim 3.5$ TeV. We also calculate self-energy corrections tothe SM Higgs doublet field through the heavy states, the right-handed neutrinosand the $Z$' boson, and find the naturalness bound as $m_{Z'} \lesssim 7$ TeV,in order to reproduce the right electroweak scale for the fine-tuning levelbetter than 10\%. The resultant mass range of $3.5$ TeV $\lesssim m_{Z'}\lesssim 7$ TeV will be explored at the LHC Run-2 in the near future.