Double peak searches for scalar and pseudoscalar resonances at the LHC

Many new physics models contain a neutral scalar resonance that can be predominantly produced via gluon fusion through loops. In such a case, there could be important effects of additional particles, that in turn may hadronize before decaying and form bound states. This interesting possibility may lead to novel signatures with double peaks that can be searched for at the LHC. We study the phenomenology of double peak searches in diboson final states from loop-induced production and decay of a new neutral spin-0 resonance at the LHC. The loop-induced couplings should be mediated by particles carrying color and electroweak charge that after forming bound states will induce a second peak in the diboson invariant mass spectrum near twice their mass. A second peak could be present via loop-induced couplings into $gg$ (dijet), $\gamma \gamma$ and $Z\gamma$ final states as well as in the $WW$ and $ZZ$ channels for the case of a pseudoscalar resonance or for scalars with suppressed tree-level coupling to gauge bosons.

Figure 1

Examples of a bound state peak accompanying a 1 TeV pseudoscalar resonance in the diphoton invariant mass spectrum with $50{\mathrm{fb}}^{-1}$ of integrated luminosity at 13 TeV. An 800 GeV charge $5/3$ VLQ is assumed, leading to a bound state peak (blue) near $m_{\gamma \gamma }\sim 1.6\mathrm{TeV}$. The signal strength of the pseudoscalar resonance is fixed by its Yukawa coupling to the VLQ, which is 0.3 (0.1) in the magenta solid (dashed) curve. We assume a 1% resolution on each peak. The red curve represents an estimate of the diphoton background, scaled from LHC data [16].

Figure 2

Top panel: Diphoton signals from $\overline{\mathrm{\Psi }}\mathrm{\Psi }$, QCD bound states of VLQs, at $\sqrt{s}=13\mathrm{TeV}$ as a function of the lowest bound state mass $M$ for the representation $(3,1,5/3)$. The signal strength is computed from the Coulomb potential calculations in Ref. [14]. The lower curve of the orange band corresponds to choosing $N=1$, while the top curve corresponds to $N=2$. Bottom panel: Diphoton resonant cross section at 13 TeV from production through a loop of VLQ in the representation $(3,1,5/3)$ with mass 1 TeV, as a function of the resonance mass, for $\lambda =0.3$ (magenta, solid line) and $\lambda =1$ (red, dashed line). The gray (blue) curve in both panels is the current diphoton cross section limit from ATLAS (CMS).