Radiative type III seesaw model and its collider phenomenology

We analyze the present bounds of a scotogenic model, the radiative type III seesaw, in which an additional scalar doublet and at least two fermion triplets of $SU(2{)}_L$ are added to the Standard Model. In the radiative type III seesaw, the new physics (NP) sector is odd under an exact global ${\mathbf{Z}}_2$ symmetry. This symmetry guaranties that the lightest NP neutral particle is stable, providing a natural dark matter candidate, and leads to naturally suppressed neutrino masses generated by a one-loop realization of an effective Weinberg operator. We focus on the region with the highest sensitivity in present and future LHC searches, with light scalar dark matter and at least one NP fermion triplet at the sub-TeV scale. This region allows for significant production cross sections of NP fermion pairs at the LHC. We reinterpret a set of searches for supersymmetric particles at the LHC obtained using the package CheckMATE, to set limits on our model as a function of the masses of the NP particles and their Yukawa interactions. The most sensitive search channel is found to be dileptons plus missing transverse energy. In order to target the case of tau enhanced decays and the case of compressed spectra, we reinterpret the recent slepton and chargino search bounds by ATLAS. For a lightest NP fermion triplet with a maximal branching ratio to either electrons or muons, we exclude NP fermion masses of up to 650 GeV, while this bound is reduced to approximately 400 GeV in the tau-philic case. Allowing for a general flavor structure, we set limits on the Yukawa couplings, which are directly related to the neutrino flavor structure.

Figure 1

One-loop neutrino mass generation in the RSIII via the exchange of a ${\mathbf{Z}}_2$-odd neutral scalar ${\varphi }^0=H^0$, $A^0$ and a ${\mathbf{Z}}_2$-odd fermion ${\mathrm{\Sigma }}_k^0$. ${\widehat{\nu }}_{\alpha }$, ${\widehat{\nu }}_{\beta }$ denote neutrino interaction eigenstates.

Figure 2

NH and IH solutions in flavor space (visualized as described in Ref. [60]). For every set of normalized Yukawa couplings squared $|{\widehat{Y}}_{\alpha }{|}^2$, $\alpha =e$, $\mu$, $\tau$, the lightest neutrino mass $m_{{\nu }_k}$ of the obtained solutions is averaged logarithmically.

Figure 3

Feynman diagrams contributing to ${\mu }^{-}\to e^{-}\gamma$. Here, ${\varphi }^0=H^0$, $A^0$. Not shown are the self-energy corrections leading to electron-muon mixing.

Figure 4

EWPO constraints in the $(m_{H^{+}}-m_{H^0})$, $(m_{A^0}-m_{H^0})$ plane. The regions allowed at 68% (green), 95% (yellow), and 99% (red) C.L. have been obtained from the new physics contributions to the oblique parameters $S$, $T$, $U$. The dashed line delimits from below the region which allows for the correct scalar DM relic density in the RSIII. All shown points correspond to scenarios which satisfy the constraints of Sec. 3 for $m_{H^0}<80\mathrm{GeV}$. The gray area corresponds to $m_{H^0}>m_{A^0}$, and the dotted lines are contours of constant $m_{A^0}-m_{H^0}$.

Figure 5

Constraints of the scalar sector of the RSIII in the ${\mathrm{\Omega }}_{\mathrm{DM}}{h}^2$, $m_{H^0}$ plane. Green points satisfy all constraints of Sec. 3. Dark green (light green) points represent scenarios with $m_{A^0}-m_{H^0}<8\mathrm{GeV}$ ($m_{H^{\pm }}-m_{H^0}<12\mathrm{GeV}$), in which $A^0-H^0$ ($H^{\pm }-H^0$) coannihilation is the dominant annihilation channel before freeze-out. The upper bound on the invisible Higgs decay width from the LHC (black curve) gives a lower bound on $m_{H^0}$ except for the dark green points ($H^0-A^0$ coannihilation scenarios). The remaining scenarios are excluded by Planck relic density measurement (light blue), LUX direct detection searches (yellow), Fermi-LAT indirect detection searches (red), and LHC Higgs decay to photons (purple). The bound for the invisible Higgs decay for a naive projection at the LHC Run-II (ILC expected sensitivity) is shown as a dashed (dotted) black curve. The horizontal lines represent the $2\sigma$ band on the measured relic density. A vertical dashed gray line shows the threshold of Higgs decay to DM pairs.

Figure 6

The left panel (a) shows the main production channel for pair ${\mathrm{\Sigma }}_1^{-}{\mathrm{\Sigma }}_1^{+}$ at the LHC. The right panel (b) shows the main decay channels of ${\mathrm{\Sigma }}_1^{\pm }$ to DM. Here, $q$ denotes quarks of the first generation and ${\ell }_{\alpha }=e$, $\mu$, $\tau$.

Figure 7

Contours of constant $m_{{\mathrm{\Sigma }}_1^{+}}$ for the present LHC exclusion sensitivity of the RSIII in the ${\mathcal{B}}_e$, ${\mathcal{B}}_{\mu }$ plane (a) and ${\mathcal{A}}_{e\mu },({\mathcal{B}}_e+{\mathcal{B}}_{\mu })$ plane (b), for $m_{H^0}=70\mathrm{GeV}$ and $m_{H^{\pm }}\approx m_{A^0}>m_{{\mathrm{\Sigma }}_1^{\pm }}$. The flavor symmetric scenario with ${\mathcal{B}}_e={\mathcal{B}}_{\mu }={\mathcal{B}}_{\tau }$ is shown with a star. The shaded triangle in (a) is not physical. Both figures show the same results. In (b) the area above each contour is excluded for the corresponding NP fermion mass.

Figure 8

Present LHC exclusion sensitivity in the $m_{{\mathrm{\Sigma }}_1^{+}},({\mathcal{B}}_e+{\mathcal{B}}_{\mu })$ plane for $m_{H^0}=70\mathrm{GeV}$ and $m_{H^{\pm }}\approx m_{A^0}>m_{{\mathrm{\Sigma }}_1^{\pm }}$, in the mu-phobic (red), e-phobic (blue), and e-mu-symmetric (green) scenarios. The flavor symmetric scenario with ${\mathcal{B}}_e={\mathcal{B}}_{\mu }={\mathcal{B}}_{\tau }$ is shown with a star. The region above each curve is excluded.

Figure 9

NLO production cross section for charged ${\mathbf{Z}}_2$-odd fermion pairs at the LHC with 8 TeV center of mass energy (blue line) as a function of the fermion mass. The corresponding 95% C.L. exclusion limits for the tau-philic case, when they decay exclusively to a tau and the DM scalar, are shown for $m_{H^0}=60\mathrm{GeV}$ (red line, black dots) and 80 GeV (yellow line, white dots). The limits have been obtained from those derived in Ref. [45] for stau pair production. Also shown is the LEP lower bound on $m_{{\mathrm{\Sigma }}_1^{\pm }}$.

Figure 10

Exclusion confidence level as a function of the NP fermion mass in the worst case scenario of Eq. (27). The dots correspond to the e-philic case ${\widehat{Y}}_e=1$ (red) and mu-philic case ${\widehat{Y}}_{\mu }=1$ (blue). The dashed lines simply connect the dots. Masses for which $1-\mathrm{C}.\mathrm{L}.<0.05$ are excluded.