REVIEW
Quirky Explanations for the Diphoton Excess
High Energy Physics - Phenomenology · Submitted 17 Dec 2015 · 70 references
Pith's verdict: A clean, candid hybrid quirk-scalar model whose abstract oversells discoverability and whose motivating excess has since evaporated.
T0 · AI review
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Referee report
The paper
The paper claims
Charging the diphoton excess's mediating fermion under a new confining force could hide it from existing searches while predicting glueball, dilepton and dijet signals at the LHC.
The reading
What carries the argument
The load-bearing object is the quirk: a fermion charged under both an unbroken confining gauge group and the SM, so that the string between a pair cannot break by pulling light quarks out of the vacuum. The pair de-excites by soft emission, then annihilates from quirkonium states whose widths and branching fractions are adapted from quarkonium, with a spin-singlet piece giving $γγ$, $Zγ$, $ZZ$ and $gg$ and a spin-triplet piece mixing with the SM $γ/Z$ to give fermion pairs. The second ingredient is the pure-glue hidden valley: hidden gluons hadronize into about twelve glueball states, the lightest a $0^{++}$ of mass $m_0 ≈ 7Λ_v$, which decay to SM gauge bosons through dimension-8 operators generated by loops of the bifundamental $X$. The ratio $α_v/α_s$ at $μ = 2m_X$ is tied to $m_0$ by pure-gauge running, which is what connects the diphoton rate to the glueball lifetime and hence to displaced-vertex phenomenology.
Extended reading notes
Core claim
The paper proposes two models in which a fermion $X$ is a fundamental of both QCD and a new confining $SU(3)_v$. In the first, $X$ alone explains the 750 GeV diphoton excess: pair-produced through its color charge at $m_X ≈ 370$ GeV, it forms a quirky bound state (quirkonium) that promptly annihilates, with spin-singlet branching fractions giving $γγ$, $Zγ$, $ZZ$, $gg$ and hidden-glueball final states at rates that can match the observed event counts for $Q_X ∼ 1/3$–$1/2$. The authors show this minimal version is at best marginal, because the same annihilation rate would have shown up at 8 TeV, and because displaced dijet and dilepton searches bound the hidden-glueball and spin-triplet fractions to roughly the $10^{-3}$ level. They therefore propose a hybrid: keep the standard singlet scalar $φ$ with coupling $y_X φ X̄X$, but charge $X$ under $SU(3)_v$. Loops of $X$ then generate effective $φgg$, $φγγ$ and $φg'g'$ couplings, so $φ$ is produced by gluon fusion and decays to two photons with the right rate for $m_X/y_X ∼$ TeV and $Q_X ∼ 1.5$, while the colored fermion is sequestered into quirky bound states that evade the long-lived charged particle and cosmology problems.
Load-bearing premise
The annihilation branching fractions and widths of the quirkonium bound state are taken from spin-singlet or democratically mixed singlet-triplet estimates, and the authors themselves say the strong dynamics are too uncertain to determine which case is realized.
Editorial extensions
If this is right
- The 750 GeV scalar in the hybrid model must be extremely narrow, so a confirmed width of tens of GeV would disfavour this minimal setup.
- Quirk annihilation should produce additional diphoton, dijet or dilepton resonances at $\sim 2m_X$, i.e. around 2 TeV or above, reachable with 20 or 300 fb$^{-1}$ of run 2 data.
- Decays of $φ$ into hidden gluons, and hidden gluons from quirk annihilation, yield glueballs whose decays give displaced vertices, lepton pairs, photon pairs and $Z$ bosons.
- In the pure-quirk version, run 2 data with 20 fb$^{-1}$ should already reveal a dijet or dilepton resonance peak or a displaced signal.
- The quirky binding makes the colored fermion cosmologically safe and removes the need for it to be stable or to escape the detector as a long-lived charged particle.
Reading between the lines
- The quirk trick is more general than this particular excess: any diphoton explanation that needs a colored, electrically charged mediator can use a confining hidden group to convert that mediator's direct-search signature into displaced glueball signatures, so the strategy transfers to other mediator models.
- Because the glueball lifetime and the diphoton rate are both controlled by $α_v/α_s$ at $2m_X$, measuring a displaced-vertex rate together with the diphoton cross section would over-constrain the hidden confinement scale rather than merely test the model.
- A null result in every listed channel at 300 fb$^{-1}$ would not retire the underlying $φ X̄X$ framework, only its quirk embedding, since the minimal toy model reproduces the diphoton rate without any hidden gauge group.
- The model's own discovery timeline makes it cheap to falsify: if the resonance itself is not confirmed with more data, the entire construction loses its motivation regardless of what the glueball sector does.
Original abstract
We propose two simple quirk models to explain the recently reported 750 GeV diphoton excesses at ATLAS and CMS. It is already well-known that a real singlet scalar $\phi$ with Yukawa couplings $\phi \bar X X$ to vector-like fermions $X$ with mass $m_X > m_\phi/2$ can easily explain the observed signal, provided $X$ carries both SM color and electric charge. We instead consider first the possibility that the pair production of a fermion, charged under both SM gauge groups and a confining $SU(3)_v$ gauge group, is responsible. If pair produced it forms a quirky bound state, which promptly annihilates into gluons, photons, v-gluons and possibly SM fermions. This is an extremely minimal model to explain the excess, but is already in some tension with existing displaced searches, as well as dilepton and dijet resonance bounds. We therefore propose a hybrid Quirk-Scalar model, in which the fermion of the simple $\phi \bar X X$ toy model is charged under the additional $SU(3)_v$ confining gauge group. Constraints on the new heavy fermion $X$ are then significantly relaxed. The main additional signals of this model are possible dilepton, dijet and diphoton resonances at $\sim$ 2 TeV or more from quirk annihilation, and the production of v-glueballs through quirk annihilation and $\phi$ decay. The glueballs can give rise to spectacular signatures, including displaced vertices and events with leptons, photons and $Z$-bosons. If the Quirk-Scalar model is responsible for the 750 GeV excess it should be discovered in one of these channels with 20 or $300 \mathrm{fb}^{-1}$ of LHC run 2 data.
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.
The abstract, sentence by sentence
We propose two simple quirk models to explain the recently reported 750 GeV diphoton excesses at ATLAS and CMS. It is already well-known that a real singlet scalar $\phi$ with Yukawa couplings $\phi \bar X X$ to vector-like fermions $X$ with mass $m_X > m_\phi/2$ can easily explain the observed signal, provided $X$ carries both SM color and electric charge. We instead consider first the possibility that the pair production of a fermion, charged under both SM gauge groups and a confining $SU(3)_v$ gauge group, is responsible. If pair produced it forms a quirky bound state, which promptly annihilates into gluons, photons, v-gluons and possibly SM fermions. This is an extremely minimal model to explain the excess, but is already in some tension with existing displaced searches, as well as dilepton and dijet resonance bounds. We therefore propose a hybrid Quirk-Scalar model, in which the fermion of the simple $\phi \bar X X$ toy model is charged under the additional $SU(3)_v$ confining gauge group. Constraints on the new heavy fermion $X$ are then significantly relaxed. The main additional signals of this model are possible dilepton, dijet and diphoton resonances at $\sim$ 2 TeV or more from quirk annihilation, and the production of v-glueballs through quirk annihilation and $\phi$ decay. The glueballs can give rise to spectacular signatures, including displaced vertices and events with leptons, photons and $Z$-bosons. If the Quirk-Scalar model is responsible for the 750 GeV excess it should be discovered in one of these channels with 20 or $300 \mathrm{fb}^{-1}$ of LHC run 2 data.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We propose two simple quirk models to explain the recently reported 750 GeV diphoton excesses at ATLAS and CMS. | contribution 0.99 | 0.87 | 0.21 | asserted 0.72 |
| s1 | It is already well-known that a real singlet scalar $\phi$ with Yukawa couplings $\phi \bar X X$ to vector-like fermions $X$ with mass $m_X > m_\phi/2$ can easily explain the observed signal, provided $X$ carries both SM color and electric charge. | background 1.00 | 0.03 | 0.04 | asserted 0.89 |
| s2 | We instead consider first the possibility that the pair production of a fermion, charged under both SM gauge groups and a confining $SU(3)_v$ gauge group, is responsible. | contribution 0.84 | 0.43 | 0.05 | hedged possibility 0.78 |
| s3 | If pair produced it forms a quirky bound state, which promptly annihilates into gluons, photons, v-gluons and possibly SM fermions. | method 0.66 | 0.22 | 0.04 | asserted 0.80 |
| s4 | This is an extremely minimal model to explain the excess, but is already in some tension with existing displaced searches, as well as dilepton and dijet resonance bounds. | limitation 0.99 | 0.39 | 0.05 | asserted 0.90 |
| s5 | We therefore propose a hybrid Quirk-Scalar model, in which the fermion of the simple $\phi \bar X X$ toy model is charged under the additional $SU(3)_v$ confining gauge group. | contribution 0.91 | 0.73 | 0.16 | asserted 0.80 |
| s6 | Constraints on the new heavy fermion $X$ are then significantly relaxed. | result 0.74 | 0.58 | 0.08 | asserted 1.00 |
| s7 | The main additional signals of this model are possible dilepton, dijet and diphoton resonances at $\sim$ 2 TeV or more from quirk annihilation, and the production of v-glueballs through quirk annihilation and $\phi$ decay. | result 0.58 | 0.59 | 0.19 | hedged possibility 0.57 |
| s8 | The glueballs can give rise to spectacular signatures, including displaced vertices and events with leptons, photons and $Z$-bosons. | interpretation 0.50 | 0.30 | 0.07 | hedged possibility 0.87 |
| s9 | If the Quirk-Scalar model is responsible for the 750 GeV excess it should be discovered in one of these channels with 20 or $300 \mathrm{fb}^{-1}$ of LHC run 2 data. | interpretation 0.73 | 0.43 | 0.50 | asserted 0.60 |
What was examined, and what was not
- arXiv listing abstract v3
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Showing 3 of the paper's 5 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:1512.05753 by the authors.
References
How the literature uses it
61 arXiv papers cite this in 67 sentences
Papers citing it · 17 sentences without a confident label
Compares with it 2 papers
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This is in contrast to similar recent work which featured bound states with colored constituents [ref] .
Diphoton Signals from Colorless Hidden Quarkonia · Introduction · 0.99
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Our scenario may also be contrasted with many explanations with composite states proposed so far, by noting that no new strong dynamics (e.g., [ref] ) or new confining forces (e.g., [ref] ) are involved in our setup.
Resonances from QCD bound states and the 750 GeV diphoton excess · Summary and Discussion · 0.91
Cites it as background 48 papers
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A lot of studies have been done here [ref] .
The Fourth Generation Quark and the 750 GeV Diphoton Excess · Introduction · 0.97
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In this letter we study the regime where the $𝒬$ are heavier than the confinemant scale $Λ_TC$, see [ref] for early work.
Back to 1974: The $\mathcal Q$-onium · Introduction · 0.87
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The possibility that the new scalar particle is a bound state of exotic charged fermions has also been considered, e.g. [ref] .
Explaining the 750 GeV diphoton excess with a colored scalar charged under a new confining gauge interaction · Introduction · 0.92
What it cites · 38 sentences without a confident label
Uses data or values from 3 papers
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The sensitivity of dilepton resonance searches at 750 GeV from LHC run 1 searches [ref] is around $10^-3$ pb.
Search for high-mass dilepton resonances in pp collisions at sqrt(s) = 8 TeV with the ATLAS detector · Introduction · 0.98
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The sensitivity of dilepton resonance searches at 750 GeV from LHC run 1 searches [ref] is around $10^-3$ pb.
Search for physics beyond the standard model in dilepton mass spectra in proton-proton collisions at sqrt(s) = 8 TeV · Introduction · 0.98
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In cases where the hadronic matrix elements have not been computed on the lattice we employ the estimates of [ref] varied within a factor of $1/3 - 3$.
Pure-glue hidden valleys through the Higgs portal · Introduction · 0.89
Cites as background 37 papers
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Several varieties of this model have already been explored [ref] , even before 13 TeV LHC data was available [ref] .
Interpreting a 750 GeV Diphoton Resonance · Introduction · 0.88
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Several varieties of this model have already been explored [ref] , even before 13 TeV LHC data was available [ref] .
Knocking on New Physics' door with a Scalar Resonance · Introduction · 0.88
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Several varieties of this model have already been explored [ref] , even before 13 TeV LHC data was available [ref] .
Diphoton Signatures from Heavy Axion Decays at the CERN Large Hadron Collider · Introduction · 0.88
Discussion
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