With the discovery of a Higgs boson with a mass of 125 gigaelectronvolts (GeV) at the Large Hadron Collider (LHC) at CERN in 2012, the Standard Model (SM) is complete, and despite intensive searches, no new fundamental particle has been observed since then. In fact, a discovery can be challenging without a predictive new physics model because different channels and observables cannot be combined directly and unambiguously. Furthermore, without supporting indirect hints, the signal space to be searched is huge, resulting in diluted significances owing to the look-elsewhere effect. Several LHC processes with multiple leptons in the final state point towards the existence of a new Higgs boson with a mass between 140 GeV to 160 GeV decaying mostly to $\textit{W}$ bosons. While the former strongly reduces the look-elsewhere effect, the latter indicates that it could be a Higgs triplet with zero hypercharge. Within this simple and predictive extension of the SM, we simulate and combine different channels of di-photon production in association with leptons, missing energy, jets, $\textit{etc}$... Using the full run-2 results by ATLAS, including those presented recently at the Moriond conference, an increased significance of 4 standard deviations is obtained for a $\approx$ 152 GeV Higgs. Due to the previously predicted mass range, the look-elsewhere effect is negligible, and this constitutes the highest statistical evidence for a new narrow resonance obtained at the LHC. Furthermore, the model predicts a heavier-than-expected $\textit{W}$ boson, as indicated by the global electroweak fit. If further substantiated, the discovery of a new Higgs would overthrow the SM, provide a compelling case for the construction of future particle colliders, and pave the way to a novel understanding of the known shortcomings of the SM. In particular, the triplet Higgs field can lead to a strong first-order phase transition and could thus be related to the matter anti-matter asymmetry in our Universe.

Switzerland

Andreas Crivellin

China

Saiyad Ashanujjaman

Sumit Banik

Guglielmo Coloretti

India

Siddharth P. Maharathy

South Africa

Bruce Mellado

Growing evidence for a Higgs triplet * The work of Andreas Crivellin is supported by a Professorship Grant from the Swiss National Science Foundation (PP00P21_76884). The work of Saiyad Ashanujjaman is partially supported by the National Natural Science Foundation of China (11835013). Siddharth P. Maharathy acknowledges using the SAMKHYA: High-Performance Computing Facility provided by the Institute of Physics, Bhubaneswar

In this article, we examine the Standard Model extended by a $\textit{Y}$ = 0 real Higgs triplet, the ∆SM. It contains a $\textit{CP}$-even neutral Higgs (∆ ) and two charged Higgs bosons (∆ ), which are quasi-degenerate in mass. We first study the theoretical constraints from vacuum stability and perturbative unitarity and then calculate the Higgs decays, including the loop-induced modes such as di-photons ($\textit{γγ}$) and $\textit{Zγ}$. In the limit of a small mixing between the SM Higgs and ∆ , the latter decays dominantly to $\textit{WW}$ and can have a sizable branching ratio to di-photon. The model predicts a positive definite shift in the $\textit{W}$ mass, which is compatible with the current global electroweak fit. At the Large Hadron Collider, it leads to a ($\textit{i}$) stau-like signature from $\textit{pp}$ → ∆ ∆ → $$ {\tau}^{+}{\tau}^{-}\nu \overline{\nu} $$ , ($\textit{ii}$) multi-lepton final states from $\textit{pp}$ → $\textit{γ}$ → ∆ ∆ → $\textit{W}$ $\textit{W}$ $\textit{ZZ}$ and $\textit{pp}$ → $\textit{W}$ → ∆ ∆ → $\textit{W}$ $\textit{ZW}$ $\textit{W}$ as well as ($\textit{iii}$) associated di-photon production from $\textit{pp}$ → $\textit{W}$ → ∆ (∆ → $\textit{γγ}$). Concerning ($\textit{i}$), the reinterpretation of the recent supersymmetric tau partner search by ATLAS and CMS excludes $$ {m}_{\Delta ^{\pm }} $$ $\textit{<}$ 110 GeV at 95% CL. From ($\textit{ii}$), some of the signal regions of multi-lepton searches lead to bounds close to the predicted cross-section, but electroweak scale masses are still allowed. For ($\textit{iii}$), the recast of the associated di-photon searches by ATLAS and a combined log-likelihood fit of signal and background to data find that out of the 25 signal regions, 10 provide relevant limits on Br(∆ → $\textit{γγ}$) at the per cent level. Interestingly, 6 signal regions show weaker than expected limits at around 152 GeV, leading to a preference for a non-zero di-photon branching ratio of about (0.7$\textit{±}$0.2)%.

Germany

Siddharth Maharathy

Anatomy of the real Higgs triplet model

The multi-lepton anomalies suggest the existence of a new scalar with a mass between 145 GeV and 155 GeV, which is compatible with indications of the associated production of a narrow resonance with a mass of ≈151 GeV. These anomalies require a sizable branching fraction of the new scalar to <italic>WW</italic>. However, because no <italic>ZZ</italic> signal at this mass has been observed, this implies that the new boson could be the neutral component of an <math altimg='si1.svg'><mi>S</mi><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>2</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>L</mi></mrow></msub></math> triplet with zero hypercharge. This field leads to a positive definite shift in the <italic>W</italic> mass and is produced via the Drell-Yan process <math altimg='si2.svg'><mi>p</mi><mi>p</mi><mo stretchy='false'>&#8594;</mo><msup><mrow><mi>W</mi></mrow><mrow><mo>&#8270;</mo></mrow></msup><mo stretchy='false'>&#8594;</mo><msup><mrow><mi mathvariant='normal'>&#916;</mi></mrow><mrow><mn>0</mn></mrow></msup><msup><mrow><mi mathvariant='normal'>&#916;</mi></mrow><mrow><mo>&#177;</mo></mrow></msup></math>. We use the side-bands of the ATLAS analysis <cross-ref refid='br0010' id='crf0170'>[1]</cross-ref> of the associated production of the Standard Model Higgs in the di-photon channel to search for this production mode of the triplet. Since the dominant decays of <math altimg='si10.svg'><msup><mrow><mi mathvariant='normal'>&#916;</mi></mrow><mrow><mo>&#177;</mo></mrow></msup></math> depend only on its mass, the effect in the 22 signal categories considered by ATLAS is completely correlated. Combining all channels in a likelihood ratio test, we find that a non-zero Br <math altimg='si4.svg'><mo stretchy='false'>[</mo><msup><mrow><mi mathvariant='normal'>&#916;</mi></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy='false'>&#8594;</mo><mi>&#947;</mi><mi>&#947;</mi><mo stretchy='false'>]</mo></math>, with a best-fit value of 0.66%, is preferred by &#8776;3 <italic>&#963;</italic> for a mass of &#8776;151.5 GeV. While this mass is consistent with the multi-lepton anomalies, there is also an excess at &#8776;127 GeV with a significance of 3.6 <italic>&#963;</italic>. However, the latter is inconsistent with the mass predicted by the multi-lepton anomalies and is in close proximity to the SM Higgs mass, suggesting the possibility of enhanced associated production of the SM Higgs.

Explaining the <italic>&#947;&#947;</italic>&#8239;+&#8239; <italic>X</italic> excesses at &#8776;151.5 GeV via the Drell-Yan production of a Higgs triplet

We explore the possibility that an <math><mrow><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mrow><mo>)</mo></mrow><mrow><mi>L</mi></mrow></msub></mrow></math> triplet scalar with hypercharge <math><mi>Y</mi><mo>=</mo><mn>0</mn></math> is the origin of the 95 GeV diphoton excess. For a small mixing angle with the Standard Model Higgs, its neutral component has naturally a sizable branching ratio to <math><mi>γ</mi><mi>γ</mi></math> such that its Drell-Yan production via <math><mi>p</mi><mi>p</mi><mo>→</mo><msup><mi>W</mi><mo>*</mo></msup><mo>→</mo><mi>H</mi><msup><mi>H</mi><mo>±</mo></msup></math> is sufficient to obtain the desired signal strength, where <math><msup><mi>H</mi><mo>±</mo></msup></math> is the charged Higgs component of the triplet. The predictions of this setup are (1) The <math><mi>γ</mi><mi>γ</mi></math> signal has a <math><mrow><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></mrow></math> spectrum different from gluon fusion but similar to associated production. (2) Photons are produced in association with tau leptons and jets, but generally do not fall into the vector-boson fusion category. (3) The existence of a charged Higgs with <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><msup><mrow><mi>H</mi></mrow><mrow><mo>±</mo></mrow></msup></mrow></msub><mo>≈</mo><mo>(</mo><mn>95</mn><mo>±</mo><mn>5</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> leading to <math><mrow><mi>σ</mi><mo>(</mo><mi>p</mi><mi>p</mi><mo>→</mo><mi>τ</mi><mi>τ</mi><mi>ν</mi><mi>ν</mi><mo>)</mo><mo>≈</mo><mn>0.4</mn><mtext> </mtext><mtext> </mtext><mi>pb</mi></mrow></math>, which is of the same level as the current limit and can be discovered with Run 3 data. (4) A positive definite shift in the <math><mi>W</mi></math> mass as suggested by the current global electroweak fit.

Anza-Tshilidzi Mulaudzi

<math><mrow><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mrow><mo>)</mo></mrow><mrow><mi>L</mi></mrow></msub></mrow></math> triplet scalar as the origin of the 95 GeV excess?

Despite a great deal of effort in searching for the tripletlike Higgses in the type-II seesaw model, evidence for their production is yet to be found at the LHC. As such, one might be in the balance regarding this model’s relevance at the electroweak scale. In this work, we peruse a scenario, akin to compressed mass spectra in supersymmetry, which might have eluded the experimental searches thus far. We perform a multivariate analysis to distinguish signals with a pair of same-sign leptons with low invariant mass from the Standard Model processes, including those accruing from fake leptons and electron charge misidentification, and find that a significant part of the hitherto unconstrained parameter space could be probed with the already collected run 2 13 TeV LHC and future HL-LHC data.

Probing compressed mass spectra in the type-II seesaw model at the LHC

Search for light (within the mass range 84–200 GeV) doubly charged Higgs bosons decaying into a pair of <math><mi>W</mi></math> bosons has been deemed challenging using the conventional LHC searches with leptons, jets, and missing transverse momentum in the final state. Such Higgses, together with slightly heavier singly-charged and neutral Higgses, when arranged in an <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>L</mi></msub></math> triplet as in the type-II seesaw model, have been recently shown to accommodate the recent measurement of the <math><mi>W</mi></math>-boson mass by the CDF Collaboration. When produced in a highly Lorentz-boosted regime, these tend to manifest as a single fat jet or a pair of adjacent same-sign leptons plus missing transverse momentum. First, we perform a multivariate analysis to discern such exotic jets from the standard model jets. Then, we present a novel search in the final state with an exotic jet and two same-sign leptons plus missing transverse momentum. We find that such low-mass doubly charged Higgses could be directly probed with the already collected Run 2 LHC data.

Kirtiman Ghosh

Rameswar Sahu

Low-mass doubly charged Higgs bosons at the LHC

While the tripletlike Higgses up to a few hundred GeV masses are already excluded for a vast region of the model parameter space from the LHC searches, strikingly, there is a region of this parameter space that is beyond the reach of the existing LHC searches, and doubly/singly charged and neutral Higgses as light as 200 GeV or even lighter are allowed by the LHC data. We study several search strategies targeting different parts of this LHC elusive parameter space at two configurations of <math><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math> colliders—500 GeV and 1 TeV center of mass energies. We find that a vast region of this parameter space could be probed with <math><mn>5</mn><mi>σ</mi></math> statistical significance indicating discovery with the early <math><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math> colliders’ data.

Finland

Katri Huitu

Type II seesaw model: Searching for the LHC-elusive low-mass triplet Higgs bosons at <math><msup><mi>e</mi><mo>−</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math> colliders

Exotic leptons in large gauge multiplets, appearing in many scenarios beyond the Standard Model (SM), can be produced at the LHC in pairs or association. Owing to their large masses, their eventual decay products — SM leptons and bosons — tend to be highly boosted, with the jets stemming from the SM bosons more likely to manifest themselves as a single fat-jet rather than two resolved ones. With the corresponding SM backgrounds being suppressed, final states with two or three leptons and one or two fat-jets are expected to be sensitive in probing exotic fermions much heavier than 1 TeV, and we propose and investigate an appropriate search strategy. To concentrate on the essential, we consider extensions of the SM by leptonic multiplets of a single kind (triplets, quadruplets or quintuplets), bearing in mind that such simplified models typically arise as low-energy limits of more ambitious scenarios addressing various lacunae of the SM. Performing a systematic and comprehensive study of nine such scenarios at the 13 TeV LHC, we find that the corresponding 5σ discovery reaches a range from 985 GeV to 1650 GeV (1345 GeV to 2020 GeV) for 300 (3000) fb −1.

Debajyoti Choudhury

Search for exotic leptons in final states with two or three leptons and fat-jets at 13 TeV LHC

The type-II see-saw mechanism based on the annexation of the Standard Model by weak gauge triplet scalar field proffers a natural explanation for the very minuteness of neutrino masses. Noting that the phenomenology for the non-degenerate triplet Higgs spectrum is substantially contrasting than that for the degenerate one, we perform a comprehensive study for an extensive model parameter space parametrised by the triplet scalar vacuum expectation value (VEV), the mass-splitting between the triplet-like doubly and singly charged scalars and the mass of the doubly charged scalar. Considering all Drell-Yan production mechanisms for the triplet-like scalars and taking into account the all-encompassing complexity of their decays, we derive the most stringent 95% CL lower limits on the mass of the doubly charged scalar for a vast model parameter space by implementing already existing direct collider searches by CMS and ATLAS. These estimated limits are stronger by approximately 50–230 GeV than those reported by CMS and ATLAS. Strikingly, we also find a specific region of the parameter space that is beyond the reach of the existing LHC search strategies. Then, we forecast future limits by extending an ATLAS search at high-luminosity, and we propose a search strategy that yields improved limits for a part of the parameter space.

Revisiting type-II see-saw: present limits and future prospects at LHC

The type-III see-saw model holding out a riveting rationale for the minuscule neutrino masses caters for a well-to-do phenomenology at the Large Hadron Collider (LHC). Several searches targetting the triplet fermions have been performed at the LHC. Not only are the signals for the leptonic final states considered in these searches suppressed by the branching fractions of the Standard Model (SM) bosons, but they are also beset with considerably large SM backgrounds. Thus, these searches are deemed not to be sensitive enough in probing the triplet fermions much heavier than 1 TeV. To this end, we perform a search for the triplet fermions in final states with multiple leptons and fat-jets that are cleaner than the usual LHC searches and allow kinematic reconstruction of the triplets. After performing a systematic and comprehensive analysis with seven distinct final states, we project the required luminosities for both 3σ and 5σ discoveries of the triplet fermions as a function of their mass. The triplet fermions with mass as large as 1265 (1380) and 1480 (1600) GeV could be discovered with 5σ (3σ) significance at 500 and 3000 fb$^{−1}$ luminosities, respectively.