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Experimental Considerations Motivated by the Diphoton Excess at the LHC

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Prateek Agrawal, JiJi Fan, Ben Heidenreich, Matthew Reece, Matthew Strassler

High Energy Physics - Phenomenology · Submitted 17 Dec 2015 · 55 references

Pith's verdict: A useful, honest menu of search strategies built on a signal that has since evaporated, with one abstract-level signature that the body itself disclaims.

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arXiv ID1512.05775 (version 2)
Pith IDHAZOBMVL, Pith's permanent record of this paper and its review
CitationBen Heidenreich, JiJi Fan, Matthew Reece, Matthew Strassler, Prateek Agrawal. "Experimental Considerations Motivated by the Diphoton Excess at the LHC." arXiv:1512.05775 (2015). https://pith.science/paper/HAZOBMVL
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The paper

The paper claims

If the 750 GeV diphoton bump is real, companion signals should already be in LHC data, and this paper says where to look for them.

The reading

The paper takes the small excess of diphoton events near 750 GeV reported by ATLAS and CMS in the first Run 2 data and asks a practical question: if that excess is a real particle, what else must exist, and where should experimentalists look for it now? Its starting observation is that the Standard Model cannot supply the needed couplings — every known particle is lighter than half the resonance mass, so a loop of known fermions would come with a tree-level decay into fermion pairs that would have swamped the diphoton channel. New particles are therefore mandatory, and each way of adding them leaves a distinct footprint. The authors work through three families of explanation — a pseudoscalar coupled to heavy top partners, ``fake photons'' from long-lived light particles decaying to photon or electron pairs, and a bound state of quirks — and for each they name a concrete search that existing or near-future data could already support. The value of the paper is as a search menu: it converts a two-sigma bump into a set of questions that have answers.

What carries the argument

The load-bearing object is a dimension-five coupling of a new pseudoscalar $X_s = s$ or $X_s = a$ to the gluon and photon field strengths, $g_s^2$-normalized as $g_s^2/(16π^2 f_a) a G^a_{μν} G̃^{a μν}$ in the axion case, with the effective couplings $κ_g = (α_s/(4π)) Σ_f y_f t_f / m_f$ and $κ_γ = (α/(4π)) Σ_f y_f N_c Q_f^2 / m_f$ generated at one loop by heavy vector-like quarks of mass $m_f$, Yukawa coupling $y_f$ to $X_s$, and Dynkin index $t_f$. Those formulas fix the two numbers that matter: the diphoton branching ratio, $Br(X_s → γγ) ≈ 0.4%$ for $N_f$ copies of an $SU(2)$-singlet $t'$ of charge $2/3$ and $≈ 0.15%$ for $N_f$ copies of a doublet $(t',b')$, and the production rate $σ(pp → X_s) ≈ 400$ fb $(y_f N_f^{eff} v/m_f)^2$, which sets how heavy the new quarks can be for a given multiplicity. For the fake-photon scenarios the carrying calculation is geometric: the opening angle of a boosted decay is $θ ≈ 2m_a/E_a$, and a particle decaying after travelling $γ ĉτ$ toward a calorimeter at radius $R$ lands with separation $Δη ≈ θ(1 - γĉτ/R)$, so masses near or above 1 GeV with lifetimes around a millimetre put both photons inside a single calorimeter cell. For quirkonium the carrying calculation is the annihilation branching ratio $(C_h α_h)^2 : (4α_s/3)^2 : 8Q^4α^2$ into hidden gluons, Standard Model gluons, and photons, which decides whether the diphoton rate, a dijet resonance, or a photon-plus-jet resonance dominates.

Extended reading notes

Core claim

The claim is that the 750 GeV diphoton excess, if real, is not an isolated fact but the visible tip of a sector that must contain additional particles, and that the rest of that sector is likely to be reachable with data already on tape. The argument has two halves. First, a no-go: the required gluon and photon couplings cannot come from loops of Standard Model particles, because any such particle is light enough that $X_s$ would decay to it at tree level and drown the diphoton rate, so new colored and charged states are compulsory. Second, a menu: depending on what those states are, the LHC should also see top-antitop plus four hard jets from top-partner pair production with $t'$ decaying to $tX_s$; or, if the ``photons'' are actually collimated or displaced pairs from a light pseudoscalar or a dark photon, unusual overlapping photons and photon conversions whose vertices trace the lifetime distribution rather than the detector material; or, if $X_s$ is quirkonium, extra soft tracks and soft jets from de-excitation, plus dijet and photon-plus-jet resonances at or above 750 GeV. The authors also flag a low-background channel: a hard photon recoiling against a narrow, isolated, trackless hadronic-calorimeter-only jet, which after correcting the jet energy for its electromagnetic origin would show a peak at 750 GeV.

Load-bearing premise

The paper assumes the excess is a genuine 750 GeV resonance produced in gluon fusion with no accompanying hard activity, an assumption it takes directly from ATLAS's report that the events in the peak do not look different from the diphoton background, and that assumption is what fixes both the 3 to 6 fb rate window and the choice of scenarios considered.

Editorial extensions

If this is right

  • Multijet plus low missing energy and single-lepton-plus-jets searches at 13 TeV can be reinterpreted as $t'\bar t' \to t\bar t X_s X_s$ with $X_s \to gg$; the rate is a good fraction of what pair-produced 1.2 TeV gluinos would give, so existing data may already be sensitive.
  • If the ``photons'' are displaced or collimated pairs, the apparent conversion points in the inner detector should trace the long-lived particle's decay distribution rather than the known distribution of detector material, and the reconstructed resonance should look wider than the 8 GeV expected for pristine photons.
  • A hard photon recoiling against a narrow, isolated, trackless HCAL-only jet, with the jet energy corrected for electromagnetic origin, should reconstruct to a peak at 750 GeV with essentially no Standard Model background.
  • In the quirk interpretation, dijet and photon-plus-jet resonances near or above 750 GeV are expected and are not yet excluded, and additional weaker diphoton peaks at lower mass could appear in events with two or more jets from hidden glueball decays.
  • Diphoton events inside the peak should be compared with events outside it on soft-track multiplicity, small missing transverse momentum, and long-lived-particle content; any excess of activity inside the peak would be a direct handle on the underlying sector.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The same logic runs in reverse as a diagnostic: because the Standard Model cannot fake the required couplings without a visible fermion-pair resonance, any confirmed diphoton resonance at the LHC is evidence for a whole new tier of particles, not one particle, and the tier's mass scale is bounded above by perturbativity of the loop couplings.
  • The proposal to compare in-peak and out-of-peak events on soft activity is cheap and model-independent; it could be run on the existing 13 TeV diphoton sample without any new theory input, and would either sharpen the case for quirks or close that window.
  • If the excess is really a statistical fluctuation, the searches proposed here are still worth doing, since each is sensitive to electroweak-scale new physics on its own terms; the paper's framing makes the diphoton bump a prompt rather than a precondition.
  • A discrepancy between ATLAS and CMS in the apparent size or shape of the excess would itself be a clue under the fake-photon scenarios, since the two detectors have different calorimeter segmentation and photon definitions, and could be checked by comparing the two collaborations' photon identification efficiencies on collimated pairs.
Original 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.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 6,480 input tokens.

What the abstract itself states, as probabilities: claims first 0.02 · states limitation 0.09 · human participants 0.01 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.29 · compares with prior results 0.06 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; .

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.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We consider the immediate or near-term experimental opportunities offered by some scenarios that could explain the new diphoton excess at the LHC. contribution 0.74 0.30 0.18 hedged possibility 0.59
s1 If the excess is due to a new particle $X_s$ at 750 GeV, additional new particles are required, providing further signals. interpretation 0.53 0.06 0.11 hedged possibility 0.61
s2 If connected with naturalness, the $X_s$ may be produced in top partner decays. interpretation 0.52 0.05 0.04 hedged possibility 1.00
s3 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. future work 0.49 0.13 0.10 speculative 0.98
s4 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. contribution 0.31 0.06 0.03 hedged possibility 1.00
s5 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. future work 0.47 0.06 0.06 hedged possibility 0.83
s6 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. future work 0.62 0.05 0.04 hedged possibility 0.91
s7 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. future work 0.58 0.06 0.05 hedged possibility 0.96

What kind of paper the classifier reports this is: position or commentary 1.00 · evidence from argument 0.98.

What was examined, and what was not

sources read
  • arXiv listing abstract v2
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Left: Lower bound on the single t 0 (blue) and doublet t 0 , b0 mass mf (orange) as a function of Nf , from requiring the expected contribution of t 0 → tη → tγγ to the diphoton bump is less than two events at ATLAS Run 2 (with luminosity of 3.2 fb−1 .) Right: Taking the minimal mf for each Nf from the left plot, the bands show the required value of yf needed to match the diphoton signal, from Eq. (2.6). The blue (orange) band is for t 0 (t 0 , b0 ) quarks. The rough perturbativity bound on y 2Nf is shown as a dashed line. Our main point is that this scenario leads to a sign
Figure 2. Left: an illustration of how a long lab-frame lifetime can reduce the angular separation ∆η(γ, γ) as measured in the ECAL. Right: The angular separation ∆η between two photons produced in the decay a → γγ where the a has a large boost. The orange solid contours correspond to Ea = 375 GeV, as in the decay of a 750 GeV resonance, whereas the purple dashed contours correspond to Ea = mh/2 = 62.5 GeV. The vertical axis shows the proper lifetime cτb in meters; we assume that the particle decays at a finite radius L = γcτb . In the blue shaded region, the particle produced in the decay of…
Figure 3. Constraints on the scenario where a 750 GeV s decays to aa with a → γγ. The horizontal axis is the coefficient of S †SH†H in the Lagrangian and the vertical axis is f = hSi / √ 2. The orange curves are contours of σ(pp → s) × Br(s → aa) in fb. The blue dashed lines are contours of the exotic Higgs decay rate relative to the Standard Model Higgs to diphoton rate, Γ(h → aa)/ΓSM(h → γγ). We see that if s production requires Higgs mixing, fitting the diphoton excess necessarily entails a significant Br(h → aa). As shown in
Figure 4. For ma = 2 GeV and cτ = 1.3 mm, the angular separation ∆η and ∆R between two photons produced in the decay a → γγ where the a has a large boost. The blue curves correspond to the s → aa process with ms = 750 GeV, while the orange case is the less boosted decay h → 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 ∆η available in the first layer of the ATLAS ECAL (which ranges between 0.003 and 0.006 depending on η). We see…

Figures reproduced from arXiv:1512.05775 by the authors.

References
How the literature uses it

91 arXiv papers cite this in 116 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 52 sentences without a confident label

Contests it 1 paper
  • However, we show in this paper that the conclusions of [ref] and [ref] are not complete , the subtlety on whether the tracks from displaced decay can be properly constructed plays an important role.

    Faking SM Photons by Displaced Dark Photon Decays · Introduction · contest 0.85

Uses its data or values 1 paper
  • A statistical combination of all these results (performed external to either collaboration) yields 4$σ$ local, with a best fit cross section of $σ×BR_γγ ≃3.8 fb$ at 750 GeV [ref] , see also [ref] which discuss interpretations of the data.

    750 GeV Diphotons from Supersymmetry with Dirac Gauginos · Introduction · 0.83

Compares with it 4 papers
Cites it as support 1 paper
  • While the experiments have strong discriminating variables to reject e.g. photon pairs coming from neutral hadron decays, the studies [ref] show that there are regions of parameter space where the photon jets are expected to pass the tight photon selection.

    Photons, Photon Jets and Dark Photons at 750 GeV and Beyond · Introduction · 0.99

Cites it as background 56 papers

What it cites · 45 sentences without a confident label

Builds on 1 paper
Uses the method or tool of 2 papers
Uses data or values from 7 papers
Cites as background 10 papers
  • Quirks are particles that carry SM quantum numbers and hidden quantum numbers, and are bound by a flux tube that, unlike those in QCD, cannot easily break.Particles of this type have been considered for decades [ref] ; the modern study of these objects dates to [ref] , who gave them their modern name, and to [ref] .

    Macroscopic Strings and "Quirks" at Colliders · Quirks · 0.96

  • Quirks are particles that carry SM quantum numbers and hidden quantum numbers, and are bound by a flux tube that, unlike those in QCD, cannot easily break.Particles of this type have been considered for decades [ref] ; the modern study of these objects dates to [ref] , who gave them their modern name, and to [ref] .

    Echoes of a Hidden Valley at Hadron Colliders · Quirks · 0.96

  • Their phenomenology has been considered in various contexts in [ref] .

    Signals of New Physics in the Underlying Event · Quirks · 0.97

Discussion

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