REVIEW
Singlet Scalar Resonances and the Diphoton Excess
High Energy Physics - Phenomenology · Submitted 16 Dec 2015 · 20 references
Pith's verdict: A competent early template paper; the width-tension argument and the monojet-to-invisible-width bound are the parts that outlast the excess itself.
T0 · AI review
Why Pith says this
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Referee report
The paper
The paper claims
A single neutral scalar can explain the 750 GeV diphoton bump, but its loop-induced decays are far too narrow to explain the width ATLAS prefers.
The reading
What carries the argument
Two pieces carry the argument. The first is the branching-ratio formula Eq. (4), which writes the diphoton fraction as a ratio of loop amplitudes weighted by the new fermion charges and by $q^4_L$ and $q^4_Q$, with the gluon channel enhanced by the strong coupling and a $K$-factor of 1.7. The second is the width formula Eq. (8), $S$ into $gg$ plus $S$ into $Z$ and $Z$, which evaluates to roughly 25 MeV times the squared coupling and the squared loop function, with the loop function bounded by 2 when on-shell decays are forbidden. Together these two expressions convert a measured event rate and a measured width into statements about how heavy and how strongly coupled the new fermions can be, and they show that the width and the rate pull in opposite directions. A third, simpler relation, $S$ into $Z$ and $Z$ divided by $S$ into $Z$ and $Z$ equals $2t_W^2$, makes the $Z$ plus photon channel a fixed fraction of the diphoton channel for any choice of parameters.
Extended reading notes
Core claim
The paper's central claim is that a gauge-singlet scalar $S$ coupled to a vector-like quark and a vector-like lepton is a generically viable origin for the diphoton excess, and that this same minimal structure fails on the width. Production proceeds through a $Q$ loop ($gg$ fusion) and decay through $Q$ and $L$ loops into $gg$, $Q$ and $L$ loops into $gg$, $Z$ and $Z$, $Z$ and $Z$; the ratio of the gluon and photon couplings is adjustable through the charges and masses of the two new fermions. The authors show that the observed event rate is reached for couplings of order one and fermion masses from a few hundred GeV up to beyond a TeV, with the diphoton branching fraction rising as the colored state gets heavier. The load-bearing quantitative statement is Eq. (8): the loop-induced total width is at most tens of MeV even in the strong-coupling limit, so a 45 GeV width requires an additional decay channel $X$. That channel cannot be invisible, since the monojet bound on $pp$ into $S$ into invisible is at most 3 pb while the required rate is about 9 pb. Finally, whatever the width, the model fixes $Z$ and $Z$ at $2t_W^2$ times the diphoton rate, about 0.6, so a $Z$ plus photon signal is unavoidable.
Load-bearing premise
The claim that the model avoids all other constraints rests on borrowing a dijet bound measured for a colored octet and applying it to a color-singlet scalar, and on the assumption that raising the colored fermion mass suppresses dijets without spoiling the diphoton rate — two requirements that the paper's own large-width contours push against each other.
Editorial extensions
If this is right
- A $Z$ plus photon resonance search at 750 GeV should see a bump at roughly 0.6 times the diphoton rate; the authors estimate about 600 fb$^{-1}$ of luminosity for a 3-sigma observation in the $\ell\ell\gamma$ final state, so the first 300 fb$^{-1}$ should give hints.
- If the large width is real, the model needs a new visible decay channel with an order-one branching fraction, which in turn implies additional light states and a richer hidden sector than the minimal two-fermion module.
- In the large-width case the new colored and charged fermions must sit below about 800 GeV, so they should be directly produced at the LHC and are targets for vector-like quark and lepton searches.
- The dijet rate can be suppressed by raising the colored fermion mass, but only in the narrow-width version of the model; the diphoton rate itself is largely insensitive to that mass once the branching fraction saturates.
- The 8 TeV diphoton and $Z$ plus photon searches are compatible with the required 13 TeV rate under the standard gluon-luminosity rescaling, so no conflict with earlier data arises.
Reading between the lines
- The fixed ratio between the $Z$ plus photon and diphoton channels is not special to this model: any loop-induced singlet interpretation of the bump shares it, so a measurement of that ratio is a clean discriminator between singlet-scalar explanations and spin-2 or direct-coupling ones.
- Because the same loop that makes the signal also caps the width, the observation of a broad resonance would be evidence not for a strongly coupled scalar but for a multi-state sector, which is a qualitatively different search target than a single narrow resonance.
- The paper's escape route from dijet bounds depends on pushing the colored fermion mass up, while a large width forces it down; if both the width and the diphoton rate persist, the two requirements squeeze the model into a corner that direct searches can close quickly.
- The monojet argument used here to exclude an invisible width is a template that applies to any resonance whose width is unexplained: a width that is invisible is easier to bound than a width that is merely unobserved.
Original abstract
ATLAS and CMS recently released the first results of searches for diphoton resonances in 13 TeV data, revealing a modest excess at an invariant mass of approximately 750 GeV. We find that it is generically possible that a singlet scalar resonance is the origin of the excess while avoiding all other constraints. We highlight some of the implications of this model and how compatible it is with certain features of the experimental results. In particular, we find that the very large total width of the excess is difficult to explain with loop-level decays alone, pointing to other interesting bounds and signals if this feature of the data persists. Finally we comment on the robust Z-gamma signature that will always accompany the model we investigate.
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
ATLAS and CMS recently released the first results of searches for diphoton resonances in 13 TeV data, revealing a modest excess at an invariant mass of approximately 750 GeV. We find that it is generically possible that a singlet scalar resonance is the origin of the excess while avoiding all other constraints. We highlight some of the implications of this model and how compatible it is with certain features of the experimental results. In particular, we find that the very large total width of the excess is difficult to explain with loop-level decays alone, pointing to other interesting bounds and signals if this feature of the data persists. Finally we comment on the robust Z-gamma signature that will always accompany the model we investigate.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | ATLAS and CMS recently released the first results of searches for diphoton resonances in 13 TeV data, revealing a modest excess at an invariant mass of approximately 750 GeV. | background 0.99 | 0.05 | 0.05 | asserted 0.99 |
| s1 | We find that it is generically possible that a singlet scalar resonance is the origin of the excess while avoiding all other constraints. | result 0.92 | 0.92 | 0.83 | hedged possibility 0.81 |
| s2 | We highlight some of the implications of this model and how compatible it is with certain features of the experimental results. | contribution 0.51 | 0.70 | 0.09 | asserted 0.69 |
| s3 | In particular, we find that the very large total width of the excess is difficult to explain with loop-level decays alone, pointing to other interesting bounds and signals if this feature of the data persists. | interpretation 0.68 | 0.91 | 0.10 | asserted 0.84 |
| s4 | Finally we comment on the robust Z-gamma signature that will always accompany the model we investigate. | result 0.74 | 0.75 | 0.07 | asserted 0.97 |
What was examined, and what was not
- arXiv listing abstract v1
- 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:1512.05326 by the authors.
References
How the literature uses it
85 arXiv papers cite this in 102 sentences
Papers citing it · 31 sentences without a confident label
Uses its data or values 2 papers
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Also the ATLAS experiment group [ref] reported $Γ_S=45$ GeV that is the best fit value of the decay width of $S$ to the two photons, and $Γ_S=5.3$ GeV is given as the experimental resolution obtained by the analysis [ref] .
Four-loop Neutrino Model Inspired by Diphoton Excess at 750 GeV · Introduction · 0.99
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The value of the parameters $α≈1/127$, $α_s ≈0.1$ and $K ≈1.7$ [ref] in which $A(x)$ is a loop function defined by
Implications of the diphoton excess on Left-Right models and gauge unification · Diphoton signal from a scalar resonance · 0.93
Cites it as background 65 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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Although the significance of this excess is only around $3 σ$ and more statistics is required to draw a firm conclusion, it has already triggered an avalanche of theoretical speculations in terms of a new resonance $X$; see e.g. [ref] .
Asymmetric Dark Matter in the Sun and the Diphoton Excess at the LHC · Introduction · 0.88
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Related studies considering gluon fusion partonic processes have been recently performed in Refs. [ref] , whereas photon fusion was considered in Refs. [ref] .
A 750 GeV Portal: LHC Phenomenology and Dark Matter Candidates · Two Different Scenarios for · 0.98
What it cites · 15 sentences without a confident label
Uses data or values from 2 papers
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Heavy Quarks: If the heavy quark $Q$ decays inside the instrument, for a vector-like quark with $q_Q=1/3, 2/3$, the bounds come from [ref] and [ref] , requiring $m_Q ≳750-920$ depending on the channel.
Search for pair-produced vectorlike B quarks in proton-proton collisions at sqrt(s) = 8 TeV · Other Experimental Constraints · 0.88
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Heavy Quarks: If the heavy quark $Q$ decays inside the instrument, for a vector-like quark with $q_Q=1/3, 2/3$, the bounds come from [ref] and [ref] , requiring $m_Q ≳750-920$ depending on the channel.
Search for vector-like charge 2/3 T quarks in proton-proton collisions at sqrt(s) = 8 TeV · Other Experimental Constraints · 0.88
Cites as background 1 paper
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Models of a scalar resonance which explain the excess can come from sectors with a wide variety of field content and quantum number assignments [ref] .
Composite Models for the 750 GeV Diphoton Excess · Introduction · 0.80
Discussion
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