750 GeV diphoton resonance as a singlet scalar in an extra dimensional model

We interpret the 750 GeV diphoton excess recently found in the 13 TeV LHC data as a singlet scalar in an extra dimensional model, where one extra dimension is introduced. In the model, the scalar couples to multiple vectorlike fermions, which are just the Kaluza-Klein modes of SM fermions. Mediated by the loops of these vectorlike fermions, the $\varphi$ effective couplings to gluons and photons can be significantly large. Therefore, it is quite easy to obtain an observed cross section for the diphoton excess. We also calculate the cross sections for other decay channels of $\varphi$, and find that this interpretation can evade the bounds from the 8 TeV LHC data.

Figure 1

Nonexistent decay $\varphi \to U^{(0)}{\overline{Q}}^{(0)}$.

Figure 2

Feynman diagrams for gluon-gluon fusion to $\varphi$ (a) and $\varphi \to \gamma \gamma$ decay (b) through the loops of fermion KK modes. Here $q^{(n)}=Q^{(n)}$, $U^{(n)}$, $D^{(n)}$, while ${\ell }^{(n)}=L^{(n)}$, $E^{(n)}$.

Figure 3

Branching ratios of $\varphi$ decay channels as functions of $y_l$ for $v_{\varphi }=M_{\mathrm{KK}}=1\mathrm{TeV}$ and $y_q=1$.

Figure 4

Contours of ${\sigma }_{\gamma \gamma }$ (red dashed lines) at 13 TeV in the $v_{\varphi }-y_{q,l}$ plane for $M_{\mathrm{KK}}=1\mathrm{TeV}$ assuming $y_l=y_q$. The gray, blue, pink, and yellow regions are excluded at 95% C.L. by the $pp\to \varphi \to ZZ$, $W^{+}{W}^{-}$, dijet, and $\gamma Z$ searches at the 8 TeV LHC.

Figure 5

Contours of ${\sigma }_{\gamma \gamma }$ (red dashed lines) at 13 TeV and the total width of $\varphi$ (blue solid lines) in the $M_{\mathrm{KK}}-y_{q,l}$ plane for $v_{\varphi }=1\mathrm{TeV}$ assuming $y_l=y_q$. The gray region is excluded at 95% C.L. by the $pp\to \varphi \to ZZ$ search at the 8 TeV LHC.

Figure 6

Contours of ${\sigma }_{\gamma \gamma }$ (red dashed lines) at 13 TeV in the $y_q-y_l$ plane for $M_{\mathrm{KK}}=v_{\varphi }=1\mathrm{TeV}$. The gray region is excluded at 95% C.L. by the $pp\to \varphi \to ZZ$ search at the 8 TeV LHC.