REVIEW
Diphoton Signatures from Heavy Axion Decays at the CERN Large Hadron Collider
High Energy Physics - Phenomenology · Submitted 15 Dec 2015 · 19 references
Pith's verdict: A clean, readable template for the 750 GeV diphoton bump, but the two numbers it leans on — R > 1 for Q_F ≥ 2 and the 45 events — don't survive a calculator check.
T0 · AI review
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Referee report
The paper
The paper claims
The paper proposes that the diphoton excess reported by ATLAS and CMS near 750 GeV is a heavy axion decaying into two photons, produced through a loop of a new TeV-scale coloured vector-like fermion with integer hypercharge.
The reading
What carries the argument
The load-bearing mechanism is the chiral global anomaly of the heavy fermion: the triangle diagram with one pseudoscalar and two gauge bosons generates the dimension-five operators $aF_{mu u}tilde F^{mu u}$ and $aG^a_{mu u}tilde G^{a,mu u}$, with the loop function $F_P( au)$ of Steinberger, Adler, Bell and Jackiw evaluated at $ au_a = M_a^2/(4m_F^2) ll 1$, where $F_P approx 1$. This is what ties the production and decay of $a$ to the same two parameters, $h_F/m_F$, and what makes the photonic and gluonic widths proportional to each other with a ratio set only by the gauge quantum numbers of $F$. The ratio $R$ is the pivot of the argument: it is the statement that the model does not have the freedom to dial the diphoton branching fraction independently of the colour and charge of the new fermion.
Extended reading notes
Core claim
The central claim is that a single pseudoscalar $a$ of mass $M_a simeq 750$ GeV, coupled derivatively to a vector-like colour-triplet fermion $F$ of mass $m_F gtrsim 1.5$ TeV and integer hypercharge $Y_F = 1, 2, ldots$, can account for the observed diphoton rate. The anomaly-induced widths give a ratio of photonic to gluonic decays, $$R equiv frac{Gamma(a to gammagamma)}{Gamma(a to gg)} = frac{$N_C^{2}$ alpha_{em}^2 $Q_F^{4}$}{2 $alpha_s^{2}$ K_g^a},$$ which is independent of the Yukawa coupling $h_F$; for $Q_F ge 2$ it exceeds unity, so $a to gammagamma$ becomes the leading channel and the branching fraction is $B(a to gammagamma) approx R/(1+1.57R)$. Rescaling the Standard Model Higgs gluon-fusion cross section, the author estimates $sigma(pp to a) sim sigma_{SM}(pp to H) imes (h_F^2/m_F^2)(m_t^2)(M_a^2/M_H^2)$, and for $M_a = 750$ GeV, $m_F/m_t = 10$ and $h_F = 0.1$ finds $sigma(pp to a to gammagamma) sim 15$ fb $ imes B(a to gammagamma)$, i.e. roughly 45 signal events at $3$ fb$^{-1}$. The model further predicts $Gamma(a to ZZ)/Gamma(a to gammagamma) approx 0.082$ and $Gamma(a to Zgamma)/Gamma(a to gammagamma) approx 0.57$, with $a to W^+W^-$ negligible because that coupling arises only at two loops. The author also notes that the field content develops a Landau pole below about $10^{14}$ GeV for $Q_F ge 2$, so an extension would eventually be required.
Load-bearing premise
The event-rate claim rests on the naive rescaling of the Standard Model Higgs gluon-fusion cross section in Eq. (10), which the paper itself calls a naive estimate and which does not perform a dedicated parton-level calculation for the vector-like fermion loop.
Editorial extensions
If this is right
- For $Q_F ge 2$ the diphoton channel is the dominant decay of the new state, so the model predicts a resonance that is seen in photons and not in gluon-dominated final states.
- The $Zgamma$ channel should appear at about 57 percent of the diphoton rate and the $ZZ$ channel at about 8 percent, giving a definite multi-channel pattern to check against further data.
- The $W^+W^-$ channel is expected to be absent or vanishingly small, since its effective coupling is generated only at two loops.
- The heavy axion could additionally mediate the radiative generation of a TeV-scale Majorana mass for right-handed neutrinos at three loops, connecting the diphoton sector to neutrino mass generation.
- The same axion sector is offered as a possible ingredient in the solution of the strong CP problem, linking the 750 GeV resonance to the Peccei-Quinn mechanism.
Reading between the lines
- The prediction that $Zgamma$ and $ZZ$ sit at fixed fractions of the diphoton rate is the sharpest handle here: a resonance seen in photons with no accompanying $Zgamma$ at roughly half that rate would count against the anomaly-loop picture, independently of the overall normalization.
- Because the ratio $R$ is independent of $h_F$, a future measurement of the diphoton-to-dijet ratio at the resonance would translate directly into a constraint on the fermion's electric charge, effectively counting the new fermion's colour and hypercharge from decay fractions alone.
- The presence of a Landau pole below $10^{14}$ GeV for $Q_F ge 2$ suggests that the minimal version is a low-energy effective description, and that any ultraviolet completion would have to supply additional matter or a larger gauge structure around that scale.
- If the same pseudoscalar couples to right-handed neutrinos through the anomaly, the diphoton resonance and the seesaw scale would be tied to a common symmetry-breaking sector, which would make lepton-number observables an indirect probe of the 750 GeV state.
Original abstract
Recently, the LHC collaborations, ATLAS and CMS, have announced an excess in the diphoton channel with local significance of about $3\,\sigma$ around an invariant mass distribution of $\sim 750$ GeV, after analyzing new data collected at centre-of-mass energies of $\sqrt{s} = 13~{\rm TeV}$. We present a possible physical interpretation of such a signature, within the framework of a minimal UV-complete model with a massive singlet pseudo-scalar state $a$ that couples to a new TeV-scale coloured vector-like fermion $F$, whose hypercharge quantum number is a non-zero integer. The pseudo-scalar state $a$ might be a heavy pseudo-Goldstone boson, such as a heavy axion, which decays into two photons and whose mass lies around the excess region. The mass of the CP-odd state $a$ and its coupling to $F$ may be due to non-perturbative effects, which can break the original Goldstone shift symmetry dynamically. The possible role that the heavy axion $a$ can play in the radiative generation of a seesaw Majorana scale and in the solution to the so-called strong CP problem is briefly discussed.
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
Recently, the LHC collaborations, ATLAS and CMS, have announced an excess in the diphoton channel with local significance of about $3\,\sigma$ around an invariant mass distribution of $\sim 750$ GeV, after analyzing new data collected at centre-of-mass energies of $\sqrt{s} = 13~{\rm TeV}$. We present a possible physical interpretation of such a signature, within the framework of a minimal UV-complete model with a massive singlet pseudo-scalar state $a$ that couples to a new TeV-scale coloured vector-like fermion $F$, whose hypercharge quantum number is a non-zero integer. The pseudo-scalar state $a$ might be a heavy pseudo-Goldstone boson, such as a heavy axion, which decays into two photons and whose mass lies around the excess region. The mass of the CP-odd state $a$ and its coupling to $F$ may be due to non-perturbative effects, which can break the original Goldstone shift symmetry dynamically. The possible role that the heavy axion $a$ can play in the radiative generation of a seesaw Majorana scale and in the solution to the so-called strong CP problem is briefly discussed.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Recently, the LHC collaborations, ATLAS and CMS, have announced an excess in the diphoton channel with local significance of about $3\,\sigma$ around an invariant mass distribution of $\sim 750$ GeV, after analyzing new data collected at centre-of-mass energies of $\sqrt{s} = 13~{\rm TeV}$. | background 1.00 | 0.04 | 0.04 | asserted 1.00 |
| s1 | We present a possible physical interpretation of such a signature, within the framework of a minimal UV-complete model with a massive singlet pseudo-scalar state $a$ that couples to a new TeV-scale coloured vector-like fermion $F$, whose hypercharge quantum number is a non-zero integer. | contribution 0.83 | 0.82 | 0.20 | hedged possibility 0.80 |
| s2 | The pseudo-scalar state $a$ might be a heavy pseudo-Goldstone boson, such as a heavy axion, which decays into two photons and whose mass lies around the excess region. | interpretation 0.87 | 0.14 | 0.33 | speculative 0.95 |
| s3 | The mass of the CP-odd state $a$ and its coupling to $F$ may be due to non-perturbative effects, which can break the original Goldstone shift symmetry dynamically. | interpretation 0.69 | 0.16 | 0.06 | hedged possibility 1.00 |
| s4 | The possible role that the heavy axion $a$ can play in the radiative generation of a seesaw Majorana scale and in the solution to the so-called strong CP problem is briefly discussed. | interpretation 0.67 | 0.39 | 0.07 | hedged possibility 0.72 |
What was examined, and what was not
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References
How the literature uses it
86 arXiv papers cite this in 99 sentences
Papers citing it · 30 sentences without a confident label
Contests it 1 paper
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In the validation process of this work, we identified several results in the literature that deviate, often by an order of magnitude or more, in comparison to our results [ref] .
Precision tools and models to narrow in on the 750 GeV diphoton resonance · The diphoton and digluon rates beyond leading order · contest 0.82
Builds on it 1 paper
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Such a scenario has been considered independently in Refs. [ref] , but in this work we extend such models to a case where the new strongly coupled sector (dark QCD) has an exact dark $G$-parity and demonstrate that this model has a viable dark matter candidate as the lightest $G$-odd pion.
A 750 GeV Dark Pion: Cousin of a Dark G-parity-odd WIMP · Introduction · 0.99
Cites it as background 64 papers
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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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Several varieties of this model have already been explored [ref] , even before 13 TeV LHC data was available [ref] .
Quirky Explanations for the Diphoton Excess · Introduction · 0.88
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This could be an argument against the reality of the diphoton excess, meaning nothing but a statistical fluctuations; but if taken seriously, it put strong constraint on the implied model building [ref] [ Refs. [ref] on this subject appeared at the same time as this work.].
The Minimal Scalar-Stealth Top Interpretation of the Diphoton Excess · Introduction · 0.85
Discussion
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