Doubly charged Higgs boson production at hadron colliders II: a Zee-Babu case study

Abstract

Motivated by searches for so-called leptonic scalars at the LHC and the recent measurement of the W boson’s mass at the Tevatron, we revisit the phenomenology of the Zee-Babu model for neutrino masses and the ability to differentiate it from the Type II Seesaw model at the LHC. We conclude that this task is much more difficult than previously believed. All inputs equal in the two scenarios, we find that total and differential rates for producing pairs of doubly and singly charged scalars are identical in shape and only differ in normalization. The normalization is given by the ratio of hadronic cross sections and can be unity. Differences in cross sections are small and can be hidden by unknown branching rates. This holds for Drell-Yan, γγ fusion, and gg fusion, as well as observables at LO and NLO in QCD. This likeness allows us to reinterpret Run II limits on the Type II Seesaw and estimate projections for the HL-LHC. Using updated neutrino oscillation data, we also find that some collider observables, e.g., lepton flavor-violating branching ratios, are now sufficiently precise to provide a path forward. Other means of discrimination are also discussed. As a byproduct of this work, we report the availability of new Universal FeynRules Object libraries, the SM_ZeeBabu UFO, that enable fully differential simulations up to NLO+LL(PS) with tool chains employing MadGraph5_aMC@NLO.

Search for doubly charged Higgs boson production in multi-lepton final states using 139 fb$$^{-1}$$ of proton–proton collisions at $$\sqrt{s}$$ = 13 TeV with the ATLAS detector

“…The resulting NLO-to-LO K -factor was then applied to the H ±± R LO production cross-section, σ LO R . The cross-sections for the Zee-Babu model were calculated using the same simulation set-up at NLO accuracy [19] and agree well with the K × σ LO R value, as expected. Eleven samples were simulated with different masses of the H ±± particle decaying into light leptons, and an additional 11 samples with at least one τ -lepton in the final state were produced.…”

“…For t t V production and processes producing three or more top quarks, the uncertainty due to initial-state radiation (ISR) was estimated by comparing the nominal event sample with two samples where the up/down variations of the A14 tune were employed. The theoretical uncertainty of the NLO crosssection for pp → H ++ H −− is reported to range from a few per cent at low H ±± masses to approximately 25% [18,19] for the highest signal mass points studied in the analysis. It is not included in a fit as a nuisance parameter but is drawn as an uncertainty band around the theoretical curves in the exclusion limit plots in Figs.…”

“…The grey line shows the limit using the asymptotic approximation [95], and the cyan dashed line shows the combined observed limit obtained analysing the first 36.1 fb −1 of Run 2 [21]. The theoretical signal cross-section predictions, given by the NLO calculation [18,19], are shown as blue, orange and red lines for the left-handed H ±± L , right-handed H ±± R (which is the same as the Zee-Babu k ±± ), and a sum of the two LRSM chiralities, respectively, with the corresponding uncertainty bands uncertainty of the FNP background yield varies between 10% and 20%.…”

“…The surrounding green and yellow bands correspond to the ±1 and ±2 standard deviation (±1σ and ±2σ ) uncertainty around the combined expected limit, respectively, as estimated using the frequentist approach, where toy experiments based on both the background-only and signal+background hypotheses are generated for this purpose. The theoretical signal cross-section predictions, given by the NLO calculation [18,19], are shown as blue, orange and red lines for the left-handed H ±± L , right-handed H ±± R (which is the same as the Zee-Babu k ±± ), and a sum of the two LRSM chiralities, respectively, with the corresponding uncertainty bands while the statistical uncertainties in each bin are uncorrelated. The statistical uncertainty of the fake factors is added in quadrature to the total systematic uncertainty.…”

“…While the analysis allows for H ±± decays into all lepton flavour combinations, it only studies electrons and muons in the final states electroweak production is the same. 1 It was recently shown that for the Drell-Yan production mechanism studied in this analysis, cross-sections and differential scalar distributions in the Zee-Babu and type-II seesaw models differ at most by a normalisation factor when all theoretical inputs are the same [19]. If not stated explicitly, H ±± represents any of the H ±± L , H ±± R , or k ±± particles throughout the paper.…”