Experimental Considerations Motivated by the Diphoton Excess at the LHC

TL;DR

This paper investigates plausible near-term explanations for the 750 GeV diphoton excess and outlines concrete collider signatures for three broad scenarios. It analyzes a minimal natural model with a pseudoscalar $X_s= ext{(} ilde{ ight. ext{)}$ coupled to gauge bosons via heavy-vectorlike fermion loops, a suite of fake-photon models with boosted long-lived states or dark photons, and a quirks-based bound-state framework, detailing production mechanisms, branching ratios, lifetimes, and experimental handles. The authors emphasize leveraging existing Run 2 analyses (e.g., multijet, lepton+jets, and photon categories) and propose new search channels such as $t' o t X_s$ decays, collimated photon signatures, and HCAL-only jet–photon events, along with potential signatures from hidden glueballs and quirkonium spectra. Collectively, the work provides a flexible experimental roadmap to test the excess and related new-physics scenarios, offering concrete guidance on cross sections, lifetimes, and distinctive kinematic patterns that could reveal or constrain the proposed models.

Abstract

We consider the immediate or near-term experimental opportunities offered by some scenarios that could explain the new diphoton excess at the LHC. If the excess is due to a new particle $X_s$ at 750 GeV, additional new particles are required, providing further signals. If connected with naturalness, the $X_s$ may be produced in top partner decays. Then a $t'\bar t'$ signal, with $t'\to t X_s$ and $X_s\to gg$ dominantly, might be discovered by reinterpreting 13 TeV SUSY searches in multijet events with low MET and/or a lepton. If $X_s$ is a bound state of quirks, the signal events may be accompanied by an unusual number of soft tracks or soft jets. Other resonances including dilepton and photon+jet as well as dijet may lie at or above this mass, and signatures of hidden glueballs might also be observable. If the "photons" in the excess are actually long-lived particles decaying to photon pairs or to electron pairs, there are opportunities for detecting overlapping photons and/or unusual patterns of apparent photon-conversions in either $X_s$ or 125 GeV Higgs decays. There is also the possibility of events with a hard "photon" recoiling against a narrow isolated HCAL-only "jet", which, after the jet's energy is corrected for its electromagnetic origin, would show a peak at 750 GeV.

Figures (4)

• Figure 1: Left: Lower bound on the single $t^\prime$ (blue) and doublet $t^\prime,b^\prime$ mass $m_f$ (orange) as a function of $N_f$, from requiring the expected contribution of $t^\prime \to t \eta \to t\gamma\gamma$ to the diphoton bump is less than two events at ATLAS Run 2 (with luminosity of 3.2 fb$^{-1}$.) Right: Taking the minimal $m_f$ for each $N_f$ from the left plot, the bands show the required value of $y_f$ needed to match the diphoton signal, from Eq. (\ref{['eq:masstrivialmodel']}). The blue (orange) band is for $t^\prime$$(t^\prime, b^\prime)$ quarks. The rough perturbativity bound on $y^2N_f$ is shown as a dashed line.
• Figure 2: Left: an illustration of how a long lab-frame lifetime can reduce the angular separation $\Delta \eta(\gamma,\gamma)$ as measured in the ECAL. Right: The angular separation $\Delta \eta$ between two photons produced in the decay $a \to \gamma\gamma$ where the $a$ has a large boost. The orange solid contours correspond to $E_a = 375~{\rm GeV}$, as in the decay of a 750 GeV resonance, whereas the purple dashed contours correspond to $E_a = m_h/2 = 62.5$ GeV. The vertical axis shows the proper lifetime $\widehat{c\tau}$ in meters; we assume that the particle decays at a finite radius $L = \gamma \widehat{c\tau}$. In the blue shaded region, the particle produced in the decay of the 750 GeV resonance reaches the ECAL (assumed to be at 1.5 meter radius, appropriate for ATLAS) before it decays.
• Figure 3: Constraints on the scenario where a 750 GeV $s$ decays to $aa$ with $a \to \gamma\gamma$. The horizontal axis is the coefficient of $S^\dagger S H^\dagger H$ in the Lagrangian and the vertical axis is $f = \left<S\right>/\sqrt{2}$. The orange curves are contours of $\sigma(pp \to s) \times {\rm Br}(s\to aa)$ in fb. The blue dashed lines are contours of the exotic Higgs decay rate relative to the Standard Model Higgs to diphoton rate, $\Gamma(h \to aa)/\Gamma_{\rm SM}(h \to \gamma\gamma)$. We see that if $s$ production requires Higgs mixing, fitting the diphoton excess necessarily entails a significant Br$(h\to aa)$.
• Figure 4: For $m_a=2$ GeV and $c\tau=1.3$ mm, the angular separation $\Delta \eta$ and $\Delta R$ between two photons produced in the decay $a \to \gamma\gamma$ where the $a$ has a large boost. The blue curves correspond to the $s \to aa$ process with $m_s = 750~{\rm GeV}$, while the orange case is the less boosted decay $h \to aa$ of the Higgs boson. The dashed vertical lines correspond to the angular granularity 0.0174 and 0.025 of the CMS and ATLAS ECALs, respectively. The shaded gray region corresponds to the fine segmentation in $\Delta \eta$ available in the first layer of the ATLAS ECAL (which ranges between 0.003 and 0.006 depending on $\eta$). We see that Higgs decays have sufficient angular separation that the two photons together are unlikely to be classified as a single well-identified photon, while in the $s$ decay scenario the photons may be so collimated that they are observed as a single photon.