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Register a new userHidden Extra U(1) at the Electroweak/TeV Scale
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We propose a simple extension of the Standard Model (SM) by adding an extra U(1) symmetry which is hidden from the SM sector. Such a hidden U(1) has not been considered before, and its existence at the TeV scale can be explored at the LHC. This hidden U(1) does not couple directly to the SM particles, and couples only to new SU(2)_L singlet exotic quarks and singlet Higgs bosons, and is broken at the TeV scale. The dominant signals at the high energy hadron colliders are multi lepton and multi b-jet final states with or without missing energy. We calculate the signal rates as well as the corresponding Standard Model background for these final states. A very distinctive signal is 6 high p_T b-jets in the final state with no missing energy. For a wide range of the exotic quarks masses the signals are observable above the background at the LHC.
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We investigate the possibility of electroweak phase transition in the minimal supersymmetric standard model (MSSM) with an extra $U(1)$. This model has two Higgs doublets and a singlet, in addition to a singlet exotic quark superfield. We find that at the one-loop level this model may accommodate the electroweak phase transitions that are strongly first-order in a reasonably large region of the parameter space. In the parameter region where the phase transitions take place, we observe that the lightest scalar Higgs boson has a smaller mass when the strength of the phase transition becomes weaker. Also, the other three heavier neutral Higgs bosons get more large masses when the strength of the phase transition becomes weaker.
A gauged $U(1)_X$ extension of the Standard Model is a simple and consistent framework to naturally incorporate three right-handed neutrinos (RHNs) for generating the observed light neutrino masses and mixing by the type-I seesaw mechanism. We examine the collider testability of the $U(1)_X$ model, both in its minimal form with the conventional charges, as well as with an alternative charge assignment, via the resonant production of the $U(1)_X$ gauge boson ($Z^prime$) and its subsequent decay into a pair of RHNs. We first derive an updated upper limit on the new gauge coupling $g_X$ as a function of the $Z$-boson mass from the latest LHC dilepton searches. Then we identify the maximum possible cross section for the RHN pair-production under these constraints. Finally, we investigate the possibility of having one of the RHNs long-lived, even for a TeV-scale mass. Employing the general parametrization for the light neutrino mass matrix to reproduce the observed neutrino oscillation data, we perform a parameter scan and find a simple formula for the maximum RHN lifetime as a function of the lightest neutrino mass eigenvalue ($m_{rm lightest}$). We find that for $m_{rm lightest}lesssim 10^{-5}$ eV, one of the RHNs in the minimal $U(1)_X$ scenario can be long-lived with a displaced-vertex signature which can be searched for at the LHC and/or with a dedicated long-lived particle detector, such as MATHUSLA. In other words, once a long-lived RHN is observed, we can set an upper bound on the lightest neutrino mass in this model.
The null results in dark matter direct detection experiments imply the present scalar dark matter (DM) annihilation cross section to bottom quark pairs through the Higgs boson exchange is smaller than about $10^{-31}$ cm$^3/$s for a wide DM mass range, which is much smaller than the required annihilation cross section for thermal relic DM. We propose models of a thermal relic DM with the present annihilation cross section being very suppressed. This property can be realized in an extra $U(1)$ gauge interacting complex scalar DM, where the thermal DM abundance is determined by coannihilation through the gauge interaction while the present annihilation is governed by Higgs bosons exchange processes. An interaction between DM and the extra $U(1)$ breaking Higgs field generates a small mass splitting between DM and its coannihilating partner so that coannihilation becomes possible and also the $Z$-mediated scattering off with a nucleon in direct DM search becomes inelastic. We consider scalar dark matter in $U(1)_{B-L}, U(1)_{(B-L)_3}$ and $U(1)_{L_mu-L_tau}$ extended models and identify viable parameter regions. We also discuss various implications to future DM detection experiments, the DM interpretation of the gamma-ray excess in the globular cluster 47 Tucanae, the muon anomalous magnetic moment, the Hubble tension and others.
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