# A Short Overview on Low Mass Scalars at Future Lepton Colliders

^{1}

^{2}

## Abstract

**:**

## 1. Introduction

## 2. Models

#### 2.1. Singlet Extensions

#### 2.2. Two Higgs Doublet Models

#### 2.3. Other Extensions

#### 2.3.1. N2HDM

#### 2.3.2. Lepton-Specific Inert Doublet Model

#### 2.3.3. Scalar Triplet Model

## 3. Studies at 90 GeV

## 4. Studies at 240–250 GeV

#### 4.1. Dedicated Studies

#### 4.1.1. Light Scalars in $Zh$ Production

#### 4.1.2. Higgsstrahlung and Decay into Two Light Scalars

#### 4.1.3. Other Channels

#### 4.2. Cross Section Predictions

## 5. Other Center of Mass Energies

## 6. Conclusions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Note

1 | I thank V. Miralles for providing these plots. |

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**Figure 1.**Limits on the process in Equation (1), taken from [11]. This displays current constraints which can especially be easily reinterpreted in extended scalar sector models, in particular models where couplings are inherited via a simple mixing angle. In this figure, the lighter scalar is denoted by s, which corresponds to ${h}_{i}$ in the notation used in this manuscript.

**Figure 2.**Available parameter space in the TRSM, with one (high-low) or two (low-low) masses lighter than 125 GeV.

**Left**: light scalar mass and mixing angle, with $sin\alpha \phantom{\rule{0.166667em}{0ex}}=\phantom{\rule{0.166667em}{0ex}}0$ corresponding to complete decoupling.

**Right**: available parameter space in the $\left({m}_{{h}_{1}},\phantom{\rule{0.166667em}{0ex}}{m}_{{h}_{2}}\right)$ plane, with color coding denoting the rescaling parameter $sin\alpha $ for the lighter scalar ${h}_{1}$. Within the green triangle, ${h}_{125}\phantom{\rule{0.166667em}{0ex}}\to \phantom{\rule{0.166667em}{0ex}}{h}_{2}{h}_{1}\phantom{\rule{0.166667em}{0ex}}\to \phantom{\rule{0.166667em}{0ex}}{h}_{1}\phantom{\rule{0.166667em}{0ex}}{h}_{1}\phantom{\rule{0.166667em}{0ex}}{h}_{1}$ decays are kinematically allowed.

**Figure 3.**Allowed regions in the 2HDM, from a scan presented in [24].

**Figure 4.**Mixing angle and masses of different additional scalars in the aligned 2HDM, from the scan presented in [24]. For all additional scalars, regions exist where masses are $\lesssim \phantom{\rule{0.166667em}{0ex}}125\phantom{\rule{0.166667em}{0ex}}\mathrm{GeV}$, with absolute values of mixing angles such that $|cos\left(\tilde{\alpha}\right)|\lesssim \phantom{\rule{0.166667em}{0ex}}0.1$.

**Figure 5.**Scan results in the N2HDM, taken from [29]. There are regions in the models parameter space where either one or two of the additional scalars have masses $\lesssim \phantom{\rule{0.166667em}{0ex}}125\phantom{\rule{0.166667em}{0ex}}\mathrm{GeV}$.

**Figure 6.**Allowed regions in the parameter space of the model discussed in [30], taken from that reference, where squares denote allowed and bullets excluded regions in the models parameter space. CP-even neutral scalars with low masses are viable within this model.

**Figure 7.**Allowed regions in the parameter space of the model discussed in [31], taken from that reference. For neutral and charged new scalars, masses $\lesssim \phantom{\rule{0.166667em}{0ex}}125\phantom{\rule{0.166667em}{0ex}}\mathrm{GeV}$ are achievable.

**Figure 9.**Leading order production cross sections for $Z\phantom{\rule{0.166667em}{0ex}}h$ and $h\phantom{\rule{0.166667em}{0ex}}{\nu}_{\ell}\phantom{\rule{0.166667em}{0ex}}{\overline{\nu}}_{\ell}$ production at an ${e}^{+}\phantom{\rule{0.166667em}{0ex}}{e}^{-}$ collider with a com energy of 240 GeV(

**left**) and 250 GeV (

**right**) using Madgraph5 for an SM-like scalar h. Shown is also the contribution of $Z\phantom{\rule{0.166667em}{0ex}}h$ to ${\nu}_{\ell}\phantom{\rule{0.166667em}{0ex}}{\overline{\nu}}_{\ell}\phantom{\rule{0.166667em}{0ex}}h$ using a factorized approach for the Z decay.

**Figure 10.**Sensitivity predictions for an ILC with a com energy of 250 GeV from [39]. See text for details.

**Figure 11.**Upper bounds on the mixing angle for the model discussed in [41], in a comparison of different detector concepts and using the recoil method.

**Figure 12.**Ninety-five percent confidence bounds on branching ratios for Higgs decay into a pair of lighter particles, for a com energy of 240 GeVand $\int \mathcal{L}\phantom{\rule{0.166667em}{0ex}}=\phantom{\rule{0.166667em}{0ex}}5\phantom{\rule{0.166667em}{0ex}}{\mathrm{ab}}^{-1}$. Taken from [42].

**Figure 13.**Bounds on decay of the SM Higgs boson into two light scalars, with a 4 $\tau $ final state, at an ${e}^{+}{e}^{-}$ collider with a com energy of 240 GeV, with different assumptions on tracking efficiencies. Taken from [43].

**Figure 14.**Allowed rates for various Higgs–Strahlung processes with successive decays into two light scalars.

**Top**: $2\phantom{\rule{0.166667em}{0ex}}b\phantom{\rule{0.166667em}{0ex}}2\phantom{\rule{0.166667em}{0ex}}\tau $ final state.

**Bottom**: $4\tau $ final state. Expected upper bounds for various collider machines are also shown, with projections from [42]. Figure taken from [44].

**Figure 15.**Expected bounds on Higgs production via Higgs–Strahlung and subsequent decay into two light scalars, in the singlet extension scenario discussed in [49,50].

**Left**: Taken from [49].

**Right**: Results presented in [50], for CEPC at 250 GeV. The blue band denotes the region where a strong first-order electroweak phase transition is possible. We see that ${e}^{+}{e}^{-}$ Higgs factories are required on order to confirm or exclude such scenarios. LHC searches as well as projections stem from [51,52] $\left(b\phantom{\rule{0.166667em}{0ex}}b\phantom{\rule{0.166667em}{0ex}}\mu \mu \right)$, [53] $\left(b\phantom{\rule{0.166667em}{0ex}}b\phantom{\rule{0.166667em}{0ex}}\tau \tau \right)$, [54] $\left(4\phantom{\rule{0.166667em}{0ex}}b\right)$, and [55,56] $\left(\mu \mu \tau \tau \right)$.

**Figure 16.**Exclusion and discovery regions in the 2HDM type X model, in the $\left({m}_{A},\phantom{\rule{0.166667em}{0ex}}tan\beta \right)$ plane. The color region additionally explains the current ${g}_{\mu}-2$ discrepancy. Regions above the respective lines are excluded. Taken from [58].

**Figure 17.**Significances as a function of charged scalar mass and charm tagging efficiency at an 240 GeV CEPC, at an integrated luminosity of $1\phantom{\rule{0.166667em}{0ex}}{\mathrm{ab}}^{-1}$, within a 3HDM as presented in [60], considering a $c\overline{c}b\overline{b}$ final state. Figures taken from that reference.

**Figure 18.**

**Left**: Points in the 2HDMs that agree with both CMS and LEP excess and which can be probed at the ILC.

**Right**: predicted rates in the 2HDMS and N2HDM at 250 GeVusing full target luminosity.

**Figure 19.**As Figure 9, for a com of 160 GeV. We assume onshell final states.

**Figure 20.**Achievable rates for various light scalar production modes at an ${e}^{+}{e}^{-}$ collider with a com energy of 350 GeV, in various 2HDM variant models. Figure taken from [64].

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**MDPI and ACS Style**

Robens, T.
A Short Overview on Low Mass Scalars at Future Lepton Colliders. *Universe* **2022**, *8*, 286.
https://doi.org/10.3390/universe8050286

**AMA Style**

Robens T.
A Short Overview on Low Mass Scalars at Future Lepton Colliders. *Universe*. 2022; 8(5):286.
https://doi.org/10.3390/universe8050286

**Chicago/Turabian Style**

Robens, Tania.
2022. "A Short Overview on Low Mass Scalars at Future Lepton Colliders" *Universe* 8, no. 5: 286.
https://doi.org/10.3390/universe8050286