We demonstrate that a vector-like fourth family of quarks induces a genuine chiral enhancement in $\textit{b}$ → $\textit{sγ}$, which is absent in the Standard Model (SM). The coexistence of doublet and singlet states allows the chirality flip to occur inside the loop, leading to contributions proportional to the heavy vector-like mass. The resulting amplitude is enhanced by a factor $$ {\overline{\lambda}}_d{v}_H/{m}_b $$ , which can be as large as $$ \mathcal{O}(40) $$ for moderate Yukawa couplings. This leads to sizable deviation from the SM prediction even for $$ \mathcal{O}\left(\mathrm{TeV}\right) $$ vector-like quark masses and small mixing angles. We find that $$ \mathrm{Br}\left(\overline{B}\to {X}_s\gamma \right) $$ provides the most stringent constraint on this scenario among a wide range of precision observables.

Japan

Junichiro Kawamura

Yuji Omura

Chiral enhancement in the vector-like fourth family: case of $\textit{b}$ → $\textit{sγ}$

We investigate the potential of leptonic meson decays $$ M\to \ell {\overline{\nu}}_{\ell } $$ , where $\textit{M}$ is a pseudo-scalar meson, as a probe of neutrino portal dark matter. The model of our focus features a neutral fermion $\textit{ψ}$ and scalar $\textit{ϕ}$, which are coupled predominantly to neutrinos in the form $$ \mathcal{L} $$ ⊃ $\textit{λ}$ $$ {\overline{\nu}}_L $$ $\textit{ϕ ψ}$ . This interaction generates two corrections to the $$ M\to \ell {\overline{\nu}}_{\ell } $$ observables. The first one is a novel three-body decay process $$ M\to \ell \overline{\psi}\phi $$ . This process is enabled by the splitting of the off-shell anti-neutrino $$ {\overline{\nu}}_{\ell } $$ into $\textit{ψ}$ and $\textit{ϕ}$ in the $$ M\to \ell {\overline{\nu}}_{\ell } $$ diagram. The helicity suppression in $$ M\to \ell {\overline{\nu}}_{\ell } $$ is absent in the three-body process, thereby forming a potentially large contribution to real experimental results, provided $\textit{ψ}$ and $\textit{ϕ}$ are invisible. The second one is one-loop radiative corrections to the weak vertex $$ W\ell {\overline{\nu}}_{\ell } $$ , which do not modify the charged lepton spectrum but lead to enhancement or suppression of the partial $$ M\to \ell {\overline{\nu}}_{\ell } $$ decay width. To demonstrate the ability of the leptonic meson decays to probe the neutrino portal dark matter, we compute two corrections analytically and compare the modified meson branching ratios with the experimental data on the lepton flavor universality of pion and Kaon decays. The resulting constraints turn out to surpass the existing bounds in a large part of parameter spaces.

South Korea

Shohei Okawa

Novel bounds on neutrino portal dark matter from leptonic meson decays

We study neutrino mass generation and dark matter in a left-right symmetric model. The model is based on an SU(3) × SU(2) × SU(2) × U(1) gauge theory with a softly broken parity symmetry. Masses of the charged leptons and neutrinos are generated radiatively at one-loop and three-loop level respectively, through their interactions with newly introduced neutral fermion and scalar particles. A mass hierarchy of those new particles is required to reproduce the observed patterns of the charged lepton spectrum and neutrino oscillation data. The resulting light particles, whose mass can be as light as GeV, serve as good dark matter candidates. The phenomenology of such dark matter candidates is governed by their interactions to left- or right-handed neutrinos. We study physics of dark matter with several benchmark parameter sets that reproduce the realistic neutrino mass matrix structure, and identify viable parameter spaces.

Spain

Keyun Wu

Dirac neutrino and dark matter in left-right symmetric models

In this paper, we study two Higgs doublet models with gauged $$U(1)_H$$ symmetry instead of the usual softly broken $$Z_2$$ symmetry, motivated by the excesses around 96 GeV reported by the CMS collaboration in the searches for light resonances decaying to two photons and two $$\tau $$ ’s. In this model, one Higgs doublet field is charged under the $$U(1)_H$$ symmetry, thereby one can avoid tree-level flavor changing neutral currents. The extra $$U(1)_H$$ gauge symmetry requires extra chiral fermions, in order to satisfy the anomaly-free conditions. We analyze the signals of the light resonances, taking into account the contribution of the extra fermions, and discuss the consistency with the experimental results in this model.

Seungwon Baek

P. Ko

Chaehyun Yu

96 GeV scalar boson in the 2HDM with U(1)

We study phenomenology of a light scalar dark matter (DM). In the model, there is an inert doublet scalar and a singlet Dirac fermion <math><mi>ψ</mi></math>, both charged under a global <math><msub><mi>Z</mi><mn>2</mn></msub></math> symmetry. The mass of the lightest inert scalar <math><mi>H</mi></math> can be lighter than 10 GeV by imposing appropriate relations between three scalar quartic couplings. The lightest <math><msub><mi>Z</mi><mn>2</mn></msub></math> odd particle is stable and DM. In this paper, focusing on the parameter space where <math><mi>H</mi></math> is lighter than <math><mi>ψ</mi></math> and is DM, we discuss DM physics related to relic density, direct detection, indirect detection, collider searches, and other cosmological observations. We clarify differences from the case where <math><mrow><mi>ψ</mi></mrow></math> is instead DM, which has been focused on in previous works.

Ryo Higuchi

Germany

Syuhei Iguro

Light mass window of inert doublet dark matter with lepton portal interaction

We discuss a model with a SU(2) L  doublet vector leptoquark (LQ), motivated by the recent experimental results relating to the lepton universality of   <math> <mover> <mi>B</mi> <mo>¯</mo> </mover> </math>  $$ \overline{B} $$  → D (*)   <math> <mi>τ</mi> <msub> <mover> <mi>ν</mi> <mo>¯</mo> </mover> <mi>τ</mi> </msub> </math>  $$ \tau {\overline{\nu}}_{\tau } $$ . We find that scalar operators predicted by the LQ are favored to explain the deviations, taking into account the recent LHCb result. We investigate the extensive phenomenology of the model and conclude that B  s  →   <math> <mi>τ</mi> <mover> <mi>τ</mi> <mo>¯</mo> </mover> </math>  $$ \tau \overline{\tau} $$ , B →   <math> <mi>Kτ</mi> <mover> <mi>τ</mi> <mo>¯</mo> </mover> </math>  $$ K\tau \overline{\tau} $$ , B  u  →   <math> <mi>τ</mi> <msub> <mover> <mi>ν</mi> <mo>¯</mo> </mover> <mi>τ</mi> </msub> </math>  $$ \tau {\overline{\nu}}_{\tau } $$  and high-p  T  di-τ lepton signatures at the LHC will probe the interesting parameter region in the near future.

A closer look at isodoublet vector leptoquark solution to the   <math> <msub> <mi>R</mi> <msup> <mi>D</mi> <mfenced> <mo>∗</mo> </mfenced> </msup> </msub> </math>  $$ {R}_{D^{\left(\ast \right)}} $$  anomaly

We investigate the di-Higgs production at the Large Hadron Collider in the two-Higgs doublet model (2HDM). In particular, we study the production of an extra neutral Higgs boson <math><mi>ϕ</mi></math> in association with the Standard Model (SM) Higgs boson <math><mi>h</mi></math> in the Higgs alignment limit. We analyze two scenarios where the additional Higgs <math><mi>ϕ</mi></math> is in a <math><mi>C</mi><mi>P</mi></math>-even or -odd state with a large top-Yukawa interaction. The leading contribution of this production comes from the top-quark loop-induced gluon-fusion channel <math><mi>g</mi><mi>g</mi><mo>→</mo><mi>h</mi><mi>ϕ</mi></math>. The measurement of the <math><mi>h</mi><mi>ϕ</mi></math> production can probe the quartic couplings in the Higgs potential as well as the top-Yukawa couplings. Imposing both theoretical constraints (from the perturbative unitarity and the vacuum stability bounds) and experimental bounds (from the SM Higgs and flavor physics measurements) on the 2HDM parameter space, we calculate the production cross section of <math><mi>g</mi><mi>g</mi><mo>→</mo><mi>h</mi><mi>ϕ</mi></math>. Furthermore, we scrutinize these processes in the parameter spaces where the CMS ditau and diphoton excesses around 100 GeV, and/or the muon <math><mi>g</mi><mo>−</mo><mn>2</mn></math> anomaly can be accommodated.

China

Teppei Kitahara

Hantian Zhang

Chasing the two-Higgs doublet model in the di-Higgs boson production

We examine the sensitivity of the Large Hadron Collider (LHC) to light lepton portal dark matter with its mass below 10 GeV. The model features an extra doublet scalar field and singlet Dirac dark matter, which have Yukawa interactions with left-handed leptons. To correctly produce the dark matter abundance via the thermal freeze-out, a large mass splitting among the extra scalars is required, thus providing a light neutral scalar below   <math> <mi>O</mi> <mfenced> <mn>10</mn> </mfenced> <mi>GeV</mi> </math>  $$ \mathcal{O}(10)\textrm{GeV} $$  and heavy neutral and charged scalars at the electroweak scale. In this paper, we focus on the electroweak pair-production of the extra scalars with subsequent model-specific scalar decays and evaluate the current constraints with the LHC Run 2 data and the discovery potential at the High Luminosity LHC (HL-LHC). It turns out that a large part of the theoretically allowed parameter space can be tested at the HL-LHC by taking into account complementarity between slepton searches and mono-Z plus missing transverse energy search. We also discuss same-sign charged scalar production as a unique prediction of the model, and the implication of the collider searches in the thermal dark matter scenario.

Light lepton portal dark matter meets the LHC

Recently, the CMS collaboration has reported a di-tau excess with a local significance of 2.6–3.1 $$\sigma $$  <math> <mi>σ</mi> </math>   where the invariant mass is  $$m_{\tau \tau } =95$$  <math> <mrow> <msub> <mi>m</mi> <mrow> <mi>τ</mi> <mi>τ</mi> </mrow> </msub> <mo>=</mo> <mn>95</mn> </mrow> </math>  –100 GeV. This excess can be interpreted as a light scalar boson that couples to the third generation fermions, particularly top and  $$\tau $$  <math> <mi>τ</mi> </math>  . Based on the simplest model that can account for the CMS di-tau excess, we evaluate experimental sensitivities to the additional light resonance, using the results reported by the ATLAS collaboration. We see that a search for the top-quark associated production of the SM Higgs boson that decays into  $$\tau {\bar{\tau }}$$  <math> <mrow> <mi>τ</mi> <mover> <mrow> <mi>τ</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> </mrow> </math>   sets a strong model-independent limit. We also find that the CP-even scalar interpretation of the light resonance is excluded by the ATLAS results, while the CP-odd interpretation is not.

Scrutinizing the 95–100 GeV di-tau excess in the top associated process

We study a lepton portal dark matter model, motivated by the deviation of the <math><mi>W</mi></math> boson mass reported by the CDF collaboration. We introduce vectorlike leptons and a scalar dark matter (DM) that exclusively couples to the extra leptons and muon. The one-loop corrections induced by the new particles can shift the <math><mi>W</mi></math> boson mass. Besides, the discrepancy in the muon anomalous magnetic moment and the DM density can simultaneously be explained by this setup, if the vectorlike lepton is lighter than 200 GeV and nearly degenerate with the DM particle. We also see that the constraints on such a light extra lepton from the collider experiments can be evaded due to the existence of the DM particle.