750 GeV Diphoton Excess May Not Imply a 750 GeV Resonance

We discuss nonstandard interpretations of the 750 GeV diphoton excess recently reported by the ATLAS and CMS Collaborations which do not involve a new, relatively broad resonance with a mass near 750 GeV. Instead, we consider the sequential cascade decay of a much heavier, possibly quite narrow, resonance into two photons along with one or more additional particles. The resulting diphoton invariant mass signal is generically rather broad, as suggested by the data. We examine three specific event topologies—the “antler,” the “sandwich,” and the two-step cascade decay—and show that they all can provide a good fit to the observed published data. In each case, we delineate the preferred mass parameter space selected by the best fit. In spite of the presence of extra particles in the final state, the measured diphoton $p_T$ spectrum is moderate due to its anticorrelation with the diphoton invariant mass. We comment on the future prospects of discriminating with higher statistics between our scenarios, as well as from more conventional interpretations.

Figure 1

The event topologies with two photons $\gamma$ (wavy lines) and up to two additional particles ${\chi }_i$ (dashed lines) under consideration in this Letter: (a) antler, (b) sandwich, and (c) two-step cascade decay. Solid lines correspond to heavier resonances ($A$, $B_i$).

Figure 2

Upper panel: The ATLAS diphoton data (black dots) and our fit results with the antler and sandwich event topologies, Eq. (2). The red solid curve represents the best-fit signal plus background distribution. The blue dashed (green dashed) curve represents the best-fit Monte Carlo event distribution in the antler (sandwich) case after incorporating the ATLAS analysis cuts. Lower panel: The same but for the two-step cascade decay of Fig. 1.

Figure 3

Left panels: The allowed mass regions at $1\sigma$, selected by the best fit. Right panels: Temperature plots showing the correlation between $m_{\gamma \gamma }$ and $p_T^{\gamma \gamma }$, using parton-level Monte Carlo events with a representative mass spectrum consistent with the best-fit values in Eqs. (10) and (11).

Figure 4

Left: $p_T^{\gamma \gamma }$ distribution of events (in a single pseudoexperiment) near the bump, $m_{\gamma \gamma }\in (700,800)\mathrm{GeV}$, for the event topology of Fig. 1, with the mass spectrum from Fig. 3. Solid lines show the expected distributions. Right: Unit-normalized photon energy distributions for the conventional scenario with a heavy resonance of mass 750 GeV and width 45 GeV (black dotted line), and the three cascade decay scenarios: the antler topology (red solid), the sandwich topology (blue solid), and the two-step cascade (green dotted). The dashed vertical lines mark the expected energy peaks.