We explore the phenomenology of models containing one Vector-Like Quark (VLQ), t′, which can decay into the Standard Model (SM) top quark, t, and a new spin-0 neutral boson, S, the latter being either a scalar or pseudoscalar state. We parametrise the underlying interactions in terms of a simplified model which enables us to capture possible Beyond the SM (BSM) scenarios. We discuss in particular three such scenarios: one where the SM state is supplemented by an additional scalar, one which builds upon a 2-Higgs Doublet Model (2HDM) framework and another which realises a Composite Higgs Model (CHM) through partial compositeness. Such exotic decays of the t′ can be competitive with decays into SM particles, leading to new possible discovery channels at the Large Hadron Collider (LHC). Assuming t′ pair production via strong interactions, we design signal regions optimised for one t′ → S t transition (while being inclusive on the other $$ \overline{t} $$ t ¯ ′ decay, and vice versa), followed by the decay of S into the two very clean experimental signatures S → γ γ and S → Z (→ ℓ ⁺ ℓ ⁻ )γ. We perform a dedicated signal- to-background analysis in both channels, by using Monte Carlo (MC) event simulations modelling the dynamics from the proton-proton to the detector level. Under the assumption of BR(t′ → S t) = 100%, we are therefore able to realistically quantify the sensitivity of the LHC to both the t′ and S masses, assuming both current and foreseen luminosities. This approach paves the way for the LHC experiments to surpass current VLQ search strategies based solely on t′ decays into SM bosons (W ^± , Z , h).

In this study, we focus on the bosonic decays of light charged Higgs bosons in the 2-Higgs Doublet Model (2HDM) Type-I. We quantify the Branching Ratios (BRs) of the \(H^\pm \rightarrow W^\pm h\) and \(H^\pm \rightarrow W^\pm A\) channels and show that they could be substantial over several areas of the parameter space of the 2HDM Type-I that are still allowed by Large Hadron Collider (LHC) and other experimental data as well as theoretical constraints. We suggest that \(H^\pm \rightarrow W^\pm h\) and/or \(H^\pm \rightarrow W^\pm A\) could be used as a feasible discovery channel alternative to \(H^\pm \rightarrow \tau \nu \).

We suggest an explanation for and explore the consequences of the excess around 95 GeV in the di-photon and di-tau invariant mass distributions recently reported by the CMS collaboration at the Large Hadron Collider (LHC), together with the discrepancy that has long been observed at the Large Electron-Positron (LEP) collider in the $$ b\overline{b} $$ b b ¯ invariant mass. Interestingly, the most recent findings announced by the ATLAS collaboration do not contradict, or even support, these intriguing observations. Their search in the di-photon final state similarly reveals an excess of events within the same mass range, albeit with a bit lower significance, thereby corroborating and somewhat reinforcing the observations made by CMS. We demonstrate that the lightest CP-even Higgs boson in the general 2-Higgs Doublet Model (2HDM) Type-III can explain simultaneously the observed excesses at approximately 1.3 σ C.L. while satisfying up-to-date theoretical and experimental constraints. Moreover, the 2HDM Type-III predicts an excess in the $$ pp\to t\overline{t}{H}_{\textrm{SM}} $$ pp → t t ¯ H SM production channel of the 125 GeV Higgs boson, H _SM. This effect is caused by a up to 12% enhancement of the H _SM tt Yukawa coupling in comparison to that predicted by the Standard Model. Such an effect can be tested at the High Luminosity LHC (HL-LHC), which can either discover or exclude the scenario we suggest. This unique characteristic of the 2HDM Type-III makes this scenario with the 95 GeV resonance very attractive for further theoretical and experimental investigations at the (HL-)LHC and future colliders.

In this paper, we examine the consistency of the Large Hadron Collider (LHC) data collected during Runs 1 and 2 by the ATLAS and CMS experiments with the predictions of a 2-Higgs doublet model embedding vectorlike quarks (VLQs) for $pp\to H$, $A$ production and $H$, $A\to \gamma \gamma$ decay mechanisms, respectively, of (nearly) degenerate $CP$-even ($H$) and $CP$-odd ($A$) Higgs bosons. We show that a scenario containing one single VLQ with electromagnetic charge $2/3$ can explain the above ATLAS and CMS data for masses in the region $350\mathrm{GeV}\le m_{\mathrm{VLQ}}\le 1.5\mathrm{TeV}$ or so, depending on $\tan \beta$, and for several values of the mixing angle between the top quark ($t$) and its VLQ counterpart ($T$). We then perform a global fit onto the model by including all relevant experimental as well as theoretical constraints. The surviving samples of our analysis are discussed within $2\sigma$ of the LHC measurements. Additionally, we also comment on the recent anomalous result reported by CMS using Run 2 data on the associated Standard Model Higgs boson production with top quark pairs $pp\to t\overline{t}h$ with an observed significance of $3.3\sigma$. Other than these specific examples, we also present a phenomenological analysis of the main features of the model, including the most promising $T$ decay channels.

At the Large Hadron Collider (LHC), both the ATLAS and CMS Collaborations have been searching for light charged Higgs bosons via top (anti)quark production and decays channels, like pp → t $$ \overline{t} $$t ¯ with one top (anti)quark decaying into a charged Higgs boson and a b (anti)quark, when the decay is kinematically open (i.e., when $$ {m}_{H^{\pm }} $$m H ± ≲ m_t). In this paper, we propose new searches at the LHC involving light charged Higgs bosons via their pair production channels like pp → H^±h/A and pp → H⁺H⁻ in the 2-Higgs Doublet Model (2HDM) Type-I and -X scenarios. By focusing on the case where the heavy H state plays the role of the Standard Model (SM)-like Higgs boson with a mass near 125 GeV, we study the aforementioned Higgs boson pair production channels and investigate their bosonic decays, such as H^± → W^±h and/or H^± → W^±A. We demonstrate that for a light charged Higgs boson state, with $$ {m}_{H^{\pm }} $$m H ± ≲ m_t, at the LHC, such di-Higgs production and decay channels can give rise to signatures with event rates much larger than those emerging from pp → t $$ \overline{t} $$t ¯ → t $$ \overline{b} $$b ¯ H⁻ + c.c. We specifically study h/A → b $$ \overline{b} $$b ¯ and τ⁺τ⁻ decays. We, therefore, claim that the discussed combination of new production and decay modes can result in an alternative discovery channel for charged Higgs bosons lighter than the top (anti)quark at the LHC within the above two 2HDM Types. Finally, in order to motivate experimentalists in ATLAS and CMS to search for such signatures, we propose 16 Benchmark Points (BPs) which are compatible with both theoretical and experimental constraints.

We analyse a light charged Higgs boson in the 2-Higgs Doublet Model (2HDM) Type-I, when its mass satisfies the condition M_H±< M_t + M_b and the parameter space is consistent with theoretical requirements of self-consistency as well as the latest experimental constraints from Large Hadron Collider (LHC) and other data. Over such a parameter space, wherein the Standard Model (SM)-like state discovered at the LHC in 2012 is the heaviest CP-even state of the 2HDM, it is found that the decay modes of the charged Higgs boson are dominated by H^±→ W^±(∗)h. Furthermore, the light neutral Higgs boson h dominantly decays into two photons. Under these conditions, we find that the production and decay process pp → H^±h → W^±(∗)hh → ℓν_ℓ + 4γ (ℓ = e, μ) is essentially background free. However, since the W^±(∗) could be largely off-shell and the h state is very light, so that both the lepton coming from the former and the photons coming from the latter could be rather soft, we perform here a full Monte Carlo (MC) analysis at the detector level demonstrating that such a W^± + 4γ signal is very promising, as it would be yielding significant excesses at the LHC with an integrated luminosity of L = 300 fb⁻¹ at both $$ \sqrt{s} $$s = 13 and 14 TeV.