Have We Observed the Higgs (Imposter)?

TL;DR

This work classifies the 125 GeV LHC resonance as a CP-even scalar under custodial symmetry $SU(2)_C$, considering four candidate electroweak structures: a dilaton, a singlet imposter, a custodial singlet Higgs, and a custodial 5-plet triplet. It develops both model-independent diboson-rate ratios and model-dependent all-channel fits to ATLAS/CMS/Tevatron data, using observables like $D_{W/Z}$, $D_{\gamma/Z}$, $D_{Z\gamma/Z}$ and $P_{g/V}$, alongside Lagrangian parameters $c_V$, $c_g$, $c_\gamma$, and fermionic couplings. The analysis disfavors the dilaton and singlet imposters, while a custodial singlet Higgs or the SM Higgs provide good fits; the triplet imposter shows some tension, particularly from $b\bar{b}$ and $\tau\tau$ channels. A modest enhancement in the diphoton width, compatible with SM-like production, can reconcile the data, and future, more precise measurements of $D_{W/Z}$, $D_{\gamma/Z}$, and fermionic channels will be crucial to decisively distinguish between the Higgs doublet and triplet scenarios.

Abstract

We interpret the new particle at the Large Hadron Collider as a CP-even scalar and investigate its electroweak quantum number. Assuming an unbroken custodial invariance as suggested by precision electroweak measurements, only four possibilities are allowed if the scalar decays to pairs of gauge bosons, as exemplified by a dilaton/radion, a non-dilatonic electroweak singlet scalar, an electroweak doublet scalar, and electroweak triplet scalars. We show that current LHC data already strongly disfavor both the dilatonic and non-dilatonic singlet imposters. On the other hand, a generic Higgs doublet give excellent fits to the measured event rates of the newly observed scalar resonance, while the Standard Model Higgs boson gives a slightly worse overall fit due to the lack signal in the tau tau channel. The triplet imposter exhibits some tension with the data. The global fit indicates the enhancement in the diphoton channel could be attributed to an enhanced partial decay width, while the production rates are consistent with the Standard Model expectations. We emphasize that more precise measurements of the ratio of event rates in the WW over ZZ channels, as well as the event rates in b bbar and tau tau channels, are needed to further distinguish the Higgs doublet from the triplet imposter.

Figures (4)

• Figure 1: (a)$\chi^2$ from fitting $D_{\gamma/Z}$ and $D_{W/Z}$ using one single parameter $\kappa_W/\kappa_B$, which is above the 95% C.L. limit. (b) The predicted $D_{Z\gamma/Z}$ using current data. The 95% C.L. exclusion limit is derived from measurements of SM diboson production in the $Z\gamma$ channel, while the 95% C.L. band for $\kappa_W/\kappa_B$ is derived from comparing $\Delta \chi^2$ with the best-fit value in (a).
• Figure 2: (a) Predictions of Higgs boson and the triplet imposter on the $D_{W/Z}$-$D_{\gamma/Z}$ plane. The custodial singlet is within $1\sigma$ range while the custodial triplet is within the $2\sigma$ range of the measured value. The feature at the bottom of the $2\sigma$ contour is due to asymmetric uncertainties. (b) The predicted $P_{g/V}$ for the dilaton and the singlet imposters. The dilaton imposter predicts a $P_{g/V}$ that is strongly disfavored.
• Figure 3: Two-dimensional contours for four pairs of effective couplings. $(c_V, c_g)$ enters into the decays into $WW$ and $ZZ$ from gluon fusion production. $(c_\gamma, c_g)$ enters into the decays into diphotons from gluon fusion production. $(c_\gamma, c_v)$ enters into the decays into diphotons from vector boson fusion production. $(c_b, c_V)$ enters into the decays into $b\bar{b}$ from associated production with $W/Z$.
• Figure 4: Two-dimensional $\chi^2$ contours for the triplet imposter. There are only three pairs of effective couplings, which enter into the event rates in the $V_1V_2$ channel. The triplet imposter does not decay into $b\bar{b}$ and $\tau\tau$ final states by assumptions.