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Register a new userWhat can a heavy $U(1)_{rm B-L}$ $Z^prime$ boson do to the muon $(g-2)_mu$ anomaly and to a new Higgs boson mass?
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The minimal $U(1)_{rm B-L}$ extension of the Standard Model (B-L-SM) offers an explanation for neutrino mass generation via a seesaw mechanism as well as contains two new physics states such as an extra Higgs boson and a new $Z^prime$ gauge boson. The emergence of a second Higgs particle as well as a new $Z^prime$ gauge boson, both linked to the breaking of a local $U(1)_{rm B-L}$ symmetry, makes the B-L-SM rather constrained by direct searches at the Large Hadron Collider (LHC) experiments. We investigate the phenomenological status of the B-L-SM by confronting the new physics predictions with the LHC and electroweak precision data. Taking into account the current bounds from direct LHC searches, we demonstrate that the prediction for the muon $(g-2)_mu$ anomaly in the B-L-SM yields at most a contribution of approximately $8.9 times 10^{-12}$ which represents a tension of $3.28$ standard deviations, with the current $1sigma$ uncertainty, by means of a $Z^prime$ boson if its mass lies in a range of $6.3$ to $6.5$ TeV, within the reach of future LHC runs. This means that the B-L-SM, with heavy yet allowed $Z^prime$ boson mass range, in practice does not resolve the tension between the observed anomaly in the muon $(g-2)_mu$ and the theoretical prediction in the Standard Model. Such a heavy $Z^prime$ boson also implies that the minimal value for a new Higgs mass is of the order of 400 GeV.
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After a brief review of the muon g-2 status, we analyze the possibility that the present discrepancy between experiment and the Standard Model (SM) prediction may be due to hypothetical errors in the determination of the hadronic leading-order contribution to the latter. In particular, we show how an increase of the hadro-production cross section in low-energy e^+e^- collisions could bridge the muon g-2 discrepancy, leading however to a decrease on the electroweak upper bound on M_H, the SM Higgs boson mass. That bound is currently M_H < ~ 150GeV (95%CL) based on the preliminary top quark mass M_t = 172.6(1.4)GeV and the recent determination Delta alpha_{rm had}^{(5)}(M_Z) = 0.02768(22), while the direct-search lower bound is M_H > 114.4GeV (95%CL). By means of a detailed analysis we conclude that this solution of the muon g-2 discrepancy is unlikely in view of current experimental error estimates. However, if this turns out to be the solution, the 95%CL upper bound on M_H is reduced to about 130GeV which, in conjunction with the experimental lower bound, leaves a narrow window for the mass of this fundamental particle.
The discovery of the Standard Model (SM) Higgs boson at the LHC completed the theory of electroweak and strong interactions. To determine the Higgs bosons intrinsic properties, more measurements on its various decay channels are still necessary. In this paper, we investigate $Hto ellbar{ell}Z$ and $Hto u_ellbar{ u}_ell Z$ (with $ell=e$, $mu$, $tau$) processes at the next-to-leading order electroweak accuracy. The total decay widths, and the differential decay rates with respect to various kinematic variables are obtained. For a typical choice of cut on the invariant mass of lepton pair, we find the branching ratios $mathcal{B}(Hto ellbar{ell}Z)=7.5times 10^{-4}$ (with $ell=e, mu$), $mathcal{B}(Hto tau^-tau^+ Z)=7.3times 10^{-4}$, and $mathcal{B}(Hto u_ellbar{ u}_ell Z)=1.5times 10^{-3}$ (with $ell=e, mu, tau$), which are attainable in the LHC experiments.
We derive perturbativity constraints on beyond standard model scenarios with extra gauge groups, such as $SU(2)$ or $U(1)$, whose generators contribute to the electric charge, and show that there are both upper and lower limits on the additional gauge couplings, from the requirement that the couplings remain perturbative up to the grand unification theory (GUT) scale. This leads to stringent constraints on the masses of the corresponding gauge bosons and their collider phenomenology. We specifically focus on the models based on $SU(2)_Ltimes U(1)_{I_{3R}} times U(1)_{B-L}$ and the left-right symmetric models based on $SU(2)_Ltimes SU(2)_Rtimes U(1)_{B-L}$, and discuss the implications of the perturbativity constraints for new gauge boson searches at current and future colliders. In particular, we find that the stringent flavor constraints in the scalar sector of left-right model set a lower bound on the right-handed scale $v_R gtrsim 10$ TeV, if all the gauge and quartic couplings are to remain perturbative up to the GUT scale. This precludes the prospects of finding the $Z_R$ boson in the left-right model at the LHC, even in the high-luminosity phase, and leaves only a narrow window for the $W_R$ boson. A much broader allowed parameter space, with the right-handed scale $v_R$ up to $simeq 87$ TeV, could be probed at the future 100 TeV collider.
We study the Higgs boson mass and the muon anomalous magnetic moment (the muon $g-2$) in a supersymmetric standard model with vector-like generations. The infrared physics of the model is governed by strong renormalization-group effects of the gauge couplings. That leads to sizable extra Yukawa couplings of Higgs doublets between the second and vector-like generations in both quark and lepton sectors. It is found with this property that there exist wide parameter regions where the Higgs boson mass and the muon $g-2$ are simultaneously explained.
We comprehensively study the charged-Higgs contributions to the leptonic $B^-_q to ell bar u$ ($q=u,c$) and semileptonic $bar B to X_q ell bar u$ ($X_u=pi, rho; X_c=D,D^*$) decays in the type-III two-Higgs-doublet model (2HDM). We employ the Cheng-Sher ansatz to suppress the tree-level flavor-changing neutral currents (FCNCs) in the quark sector. When the strict constraints from the $Delta B=2$ and $bto s gamma$ processes are considered, parameters $chi^u_{tq}$ from the quark couplings and $chi^ell_ell$ from the lepton couplings dictate the leptonic and semileptonic $B$ decays. It is found that when the measured $B^-_uto tau bar u$ and indirect bound of $B^-_c to tau bar u$ obtained by LEP1 data are taken into account, $R(D)$ and $R(pi)$ can have broadly allowed ranges; however, the values of $R(rho)$ and $R(D^*)$ are limited to approximately the standard model (SM) results. We also find that the same behaviors also occur in the $tau$-lepton polarizations and forward-backward asymmetries ($A^{X_q,tau}_{FB}$) of the semileptonic decays, with the exception of $A^{D^*,tau}_{FB}$, for which the deviation from the SM due to the charged-Higgs effect is still sizable. In addition, the $q^2$-dependent $A^{pi,tau}_{FB}$ and $A^{D,tau}_{FB}$ can be very sensitive to the charged-Higgs effects and have completely different shapes from the SM.
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