Unitarity Bounds on the Massive Spin-2 Particle Explanation of Muon g−2g-2 Anomaly

Motivated by the long-standing discrepancy between the Standard Model prediction and the experimental measurement of the muon magnetic dipole moment, we have recently proposed to interpret this muon $g-2$ anomaly in terms of the loop effect induced by a new massive spin-2 field $G$. In the present paper, we investigate the unitarity bounds on this scenario. We calculate the $s$-wave projected amplitudes for two-body elastic scatterings of charged leptons and photons mediated by $G$ at high energies for all possible initial and final helicity states. By imposing the condition of the perturbative unitarity, we obtain the analytic constraints on the charged-lepton-$G$ and photon-$G$ couplings. We then apply our results to constrain the parameter space relevant to the explanation of the muon $g-2$ anomaly.Comment: 23 pages, 7 figure

WW-boson Mass Anomaly from High-Dimensional Scalar Multiplets

In light of the recently discovered $W$-boson mass anomaly by the CDF Collaboration, we discuss two distinct mechanisms that could possibly explain this anomaly through the introduction of $SU(2)_L$ scalar multiplets. The first mechanism is by the tree-level $W$-boson mass correction, induced by the vacuum expectation values of one or more $SU(2)_L$ scalar multiplets with odd dimensions of $n\geq 3$ and zero hypercharge of $Y=0$ to avoid the strong constraint from measurements of the $Z$-boson mass. However, it remains ruled out by the electroweak precision data of the $\rho$ parameter. The second mechanism is by the one-loop level $W$-boson mass correction. In particular, we focus on the case with an additional scalar nonet with $Y=0$ or $Y=4$. As a result, we find that the model can interpret the $W$-boson mass anomaly without violating any other theoretical or experimental constraints

WW-Boson Mass Anomaly from a General SU(2)LSU(2)_{L} Scalar Multiplet

We explain the $W$-boson mass anomaly by introducing an $SU(2)_L$ scalar multiplet with general isospin and hypercharge in the case without its vacuum expectation value. It is shown that the dominant contribution from the scalar multiplet to the $W$-boson mass arises at one-loop level, which can be expressed in terms of the electroweak (EW) oblique parameters $T$ and $S$ at leading order. We firstly rederive the general formulae of $T$ and $S$ induced by a scalar multiplet of EW charges, confirming the results in the literature. We then study several specific examples of great phenomenological interest by applying these general expressions. As a result, it is found that the model with a scalar multiplet in an $SU(2)_L$ real representation with $Y=0$ cannot generate the required $M_W$ correction since it leads to vanishing values of $T$ and $S$. On the other hand, the cases with scalars in a complex representation under $SU(2)_L$ with a general hypercharge can explain the $M_W$ excess observed by CDF-\uppercase\expandafter{\romannumeral2} due to nonzero $T$ and $S$. We further take into account of the strong constraints from the perturbativity and the EW global fit of the precision data, and vary the isospin representation and hypercharge of the additional scalar multiplet, in order to assess the extent of the model to solve the $W$-boson mass anomaly. It turns out that these constraints play important roles in setting limits on the model parameter space. We also briefly describe the collider signatures of the extra scalar multiplet, especially when it contains long-lived heavy highly charged states.Comment: 36 pages, 13 figures. Minor modifications, references updated