REVIEW
Testing the Muon g-2 Anomaly at the LHC
High Energy Physics - Phenomenology · Submitted 27 Feb 2014 · 32 references
Pith's verdict: A careful, useful enumeration of which minimal g-2 explanations survive LEP and 8-TeV data — the body is honest, but the abstract's 'almost all' oversells the 14-TeV reach.
T0 · AI review
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Referee report
The paper
The paper claims
The muon's magnetic moment can be used to predict what the LHC should already have seen, and the paper works out which candidate explanations survive.
The reading
What carries the argument
The load-bearing device is an exhaustive enumeration: every assignment of spin (0, $1/2$, 1) and weak isospin (singlet, doublet, triplet) to one or two new fields, with the couplings to muons that gauge invariance and the standard-model Higgs allow. Each case is reduced to a single number by projecting the one-loop $μ→μγ$ amplitude onto the magnetic form factor at zero momentum transfer, expanding in $m_μ$, and cross-checking with two independent computer implementations. The result is expressed as a required range of coupling over mass, typically $g/M$ or $Y/M$ between about 0.004 and 0.010 GeV$^{-1}$, which translates directly into a preferred mass window once perturbativity caps the coupling at $√{4π}$. Minimal flavor violation is the second piece of machinery: it is what keeps $μ→eγ$ and the muon mass under control, and it is what dictates how the new fermions are produced and how they decay, so that the recast of the experimental searches is well defined.
Extended reading notes
Core claim
The paper's claim is that the connection between the muon anomaly and collider data can be made model-independent enough to be decisive. Of the many candidate fields, most fail on their own: a neutral vector boson, a scalar doublet, and a scalar triplet are excluded or give the wrong sign, and single vector-like fermions are either negative or too small once Z-pole mixing limits are applied. Only a charged vector boson with right-handed couplings survives as a one-field explanation. Two mixing vector leptons can work, but only in a flavor scenario where the mixing among the new fermions is unconstrained, and then the viable masses run into the multi-TeV region the LHC cannot reach. The interesting cases are therefore two fields of different spin, and there the existing 8-TeV data already bite: the scalar-doublet-plus-fermion-doublet and scalar-doublet-plus-adjoint-fermion scenarios are excluded at two $\sigma$, with bounds such as $M_{φ_D}>395$ GeV, $M_{φ_A}>456$ GeV, $M_{ψ_D}>296$ GeV, and $M_V>398$ GeV. Projecting to 14 TeV with 300 fb$^{-1}$ pushes these to roughly 660, 760, 510, and 676 GeV, which the authors argue closes almost every remaining scenario. A secondary contribution is a set of general one-loop four-lepton contact-interaction bounds from LEP, applicable to any coupling of a lepton to two new weakly coupled fields.
Load-bearing premise
Every mass bound assumes each new particle decays only through the channels listed in the paper's summary table, with the stated branching fractions; the authors acknowledge that any additional decay mode would drain the signal and weaken the limits.
Editorial extensions
If this is right
- If the anomaly is real and the new physics is perturbative, at least one new particle should sit within a few hundred GeV, so a 14-TeV run with hundreds of inverse femtobarns should either produce a multi-lepton excess or close the minimal explanations.
- Any future multi-lepton excess at the LHC can be matched against the paper's table of viable spin and isospin assignments and their predicted mass orderings.
- The one-loop four-lepton contact-interaction limits give a general constraint on any lepton coupling to two new fields, usable in models outside the specific list tabulated here.
- A confirmed deviation in the electron magnetic moment of the size implied by the flavor-symmetric relation $δa_e/δa_μ=m_e^2/m_μ^2$ would remove the low-mass vector-lepton mixing window.
- The negative results are as useful as the positive ones: a confirmed muon anomaly would count against type-II seesaw scalars and simple neutral vector bosons as its source.
Reading between the lines
- Because every mass bound scales with the assumed branching fractions, a discovery in a decay channel not listed in the paper's table would shift the quoted bounds by tens of GeV and could reopen the marginal scenarios; the authors say as much, but the size of the effect is worth tracking explicitly.
- The 14-TeV projections assume selection efficiencies carry over unchanged from 8 TeV; if multi-lepton triggers and background rejection improve as they typically do, the real reach is likely to exceed the quoted 0.5 to 0.9 TeV rather than fall short of it.
- The same enumeration could be run in reverse as a discovery template: an excess in a specific final state would single out one row of the table, turning a null-search program into a particle-identification scheme.
- The paper's treatment of the electron magnetic moment as a constraint rather than a target suggests that improved electron $g-2$ measurements would squeeze the same parameter space from below, independently of collider reach.
Original abstract
The long-standing difference between the experimental measurement and the standard-model prediction for the muon's anomalous magnetic moment, $a_{\mu} = (g_{\mu}-2)/2$, may be explained by the presence of new weakly interacting particles with masses of a few 100 GeV. Particles of this kind can generally be directly produced at the LHC, and thus they may already be constrained by existing data. In this work, we investigate this connection between $a_{\mu}$ and the LHC in a model-independent approach, by introducing one or two new fields beyond the standard model with spin and weak isospin up to one. For each case, we identify the preferred parameter space for explaining the discrepancy of a_mu and derive bounds using data from LEP and the 8-TeV LHC run. Furthermore, we estimate how these limits could be improved with the 14-TeV LHC. We find that the 8-TeV results already rule out a subset of our simplified models, while almost all viable scenarios can be tested conclusively with 14-TeV 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
The long-standing difference between the experimental measurement and the standard-model prediction for the muon's anomalous magnetic moment, $a_{\mu} = (g_{\mu}-2)/2$, may be explained by the presence of new weakly interacting particles with masses of a few 100 GeV. Particles of this kind can generally be directly produced at the LHC, and thus they may already be constrained by existing data. In this work, we investigate this connection between $a_{\mu}$ and the LHC in a model-independent approach, by introducing one or two new fields beyond the standard model with spin and weak isospin up to one. For each case, we identify the preferred parameter space for explaining the discrepancy of a_mu and derive bounds using data from LEP and the 8-TeV LHC run. Furthermore, we estimate how these limits could be improved with the 14-TeV LHC. We find that the 8-TeV results already rule out a subset of our simplified models, while almost all viable scenarios can be tested conclusively with 14-TeV data.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The long-standing difference between the experimental measurement and the standard-model prediction for the muon's anomalous magnetic moment, $a_{\mu} = (g_{\mu}-2)/2$, may be explained by the presence of new weakly interacting particles with masses of a few 100 GeV. | background 1.00 | 0.05 | 0.05 | hedged possibility 1.00 |
| s1 | Particles of this kind can generally be directly produced at the LHC, and thus they may already be constrained by existing data. | background 0.99 | 0.05 | 0.02 | hedged possibility 0.97 |
| s2 | In this work, we investigate this connection between $a_{\mu}$ and the LHC in a model-independent approach, by introducing one or two new fields beyond the standard model with spin and weak isospin up to one. | method 0.62 | 0.89 | 0.05 | asserted 0.97 |
| s3 | For each case, we identify the preferred parameter space for explaining the discrepancy of a_mu and derive bounds using data from LEP and the 8-TeV LHC run. | method 0.83 | 0.95 | 0.06 | asserted 0.96 |
| s4 | Furthermore, we estimate how these limits could be improved with the 14-TeV LHC. | contribution 0.67 | 0.88 | 0.05 | hedged possibility 0.78 |
| s5 | We find that the 8-TeV results already rule out a subset of our simplified models, while almost all viable scenarios can be tested conclusively with 14-TeV data. | result 0.98 | 0.96 | 0.87 | asserted 1.00 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:1402.7065 by the authors.
References
How later papers used it
83 later papers cite it
1 later paper by other authors is listed below: 1 used its method or tool.
Uses its method or tool 1 of 4 papers with this label
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Followed the procedure outlined in this paper to derive the LEP RPV bounds.
Collider Constraints on a Dark Matter Interpretation of the XENON1T Excess (2020) · 0.99 · paraphrased: the licence does not allow quoting
How the literature uses it
82 arXiv papers cite this in 159 sentences
Papers citing it · 103 sentences without a confident label
Contests it 2 papers
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[equation] with $g_p2=±g_s2$ and $α=M_E^2/M_ϕ^+^2$, which disagrees by a factor of four with the result of Ref. [ref] .
New Physics Contributions to the Muon Anomalous Magnetic Moment: A Numerical Code · Neutron Lepton and Charged Higgs · contest 0.76
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We note that our result is opposite to the conclusion of existing literature [ref] for the same particle contents.
Hybrid Type-II and Type-III seesaw model for the muon $g-2$ anomaly · Introduction · contest 0.71
Builds on it 1 paper
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In this paper, extending the approach of [ref] , we try to answer the following question: in case a positive measurement of is obtained with large significance at Fermilab, what information can we infer on the couplings, masses, and quantum numbers of the new particles involved in the process, provided we require that the same physics also yields the relic density of dark matter in the Universe.
Expectations for the muon g-2 in simplified models with dark matter · Introduction · 1.00
Uses its method or tool 4 papers
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The formulae for those contributions are found in Ref. [ref] for the $W$ and $Z$ loop diagrams, while the reference [ref] is used for the Higgs one.
Muon $g-2$ and CKM Unitarity in Extra Lepton Models · Muon g-2 · 0.90
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Using the generic analytic formulas for the one-loop contributions to the lepton AMM [ref] in the limit of large LM masses with respect to the lepton mass $m_ℓ$ and the meson masses, these contributions can be written as [equation] where $m_S$ ($m_V$) is the scalar (vector) meson mass, $M$ ($M_0$) is the mass of the charged (neutral) LM, and the loop functions are [equation] where in the approximate expressions we neglected the terms of $𝒪(x^2)$.
Leptomeson contribution to the muon g-2 · Leptomesons in muon $g-2$ · 0.95
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The result is provided by the general expression [ref]
$g_\mu-2$ from Vector-Like Leptons in Warped Space · Charged VLL · 0.92
Uses its data or values 3 papers
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Though from the experimental side, where $W_R$ interacts only with right handed neutrinos, i.e for $g_a = g_v$ the LEP bound on $g_v/m_W_R$ reads as $g_v/m_W_R < 4.8 ×10^-3 GeV^-1$ [ref] .
Neutrino mass, mixing and muon $g-2$ explanation in $U(1)_{L_\mu-L_\tau}$ extension of left-right theory · Results and Discussion · 0.82
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Currently, the discrepancy between the experimental measurement and the SM prediction is $∼3.5 σ$ [ref] , where: [equation] This opens a window of opportunity for quantum corrections driven by BSM particles [ref] .
Connecting the Muon Anomalous Magnetic Moment and the Multi-lepton Anomalies at the LHC · Introduction · 0.82
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Searches in the regime where this new charged boson interacts only with right handed neutrinos, i.e when $g_a10=-g_v10$ give a 95% C.L bound from LEP using effective operators which reads $g_v10/M_W^' < 4.8 ×10^-3 GeV^-1$ [ref] .
New Physics Contributions to the Muon Anomalous Magnetic Moment: A Numerical Code · Singly Charged Vector · 0.98
Compares with it 12 papers
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The calculation in these cases has been presented in Ref. [ref] [With respect to their results, we get a discrepancy in the constant factor in both cases, while loop functions and $θ$-dependent terms are the same: [equation] We have adopted the notation of Ref. [ref] for the loop functions.
Minimal muon anomalous magnetic moment · Fermions · 0.91
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This paper's findings are reflected in Table [ref], except for the cases of the scalar doublet (see Sec.[ref]) and the neutral vector (see the discussion in Sec. [ref] at the end of this overview), where there is a lot of dedicated literature and it is known that breaking the assumptions of Ref. [ref] can change the result.
New physics explanations of $a_\mu$ in light of the FNAL muon $g-2$ measurement · Overview · 0.87
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These results were cross-checked against those in [ref] , and found agreement except for the numerical factors in Eq. (4.8) from Ref. [ref] for VLLs with doubly charged components.
A bridge to new physics: proposing new -- and reviving old -- explanations of $a_\mu$ · Two-field extensions · 0.98
Cites it as background 23 papers
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The introduction of a vector-like fermion and a scalar or a vector without any additional symmetries was dealt with by Ref. [ref] , considering different $SU(2)_L$ representations, namely singlets, doublets, triplets or adjoint triplets.
New physics explanations of $a_\mu$ in light of the FNAL muon $g-2$ measurement · Overview · 0.88
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Thus, the most straightforward new physics explanations assume additional contributions from new particles with masses near the EW scale that couple to the muon with EW interactions, for reviews see Refs. [ref] .
Highly enhanced contributions of heavy Higgs bosons and new leptons to muon $g-2$ and prospects at future colliders · Introduction · 0.92
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Scenarios involving the mixing between new heavy leptons and the muon via only the SM Higgs were first studied in the mass eigenstate basis in [ref] and revisited in [ref] in the SMEFT landscape.
Predictions for Muon Electric and Magnetic Dipole Moments from $h \rightarrow \mu^+ \mu^-$ in Two-Higgs-Doublet Models with New Leptons · 2HDM-II Effective Field Theory · 0.97
What it cites · 12 sentences without a confident label
Uses the method or tool of 2 papers
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For the 14-TeV projections, we follow the strategy of [ref] .
Projected Performance of an Upgraded CMS Detector at the LHC and HL-LHC: Contribution to the Snowmass Process · Projections for the 14-TeV LHC · 0.99
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Cross sections and event rates are computed at the parton level using the program CalcHEP [ref] .
CalcHEP 3.4 for collider physics within and beyond the Standard Model · Calculational techniques and tools · 1.00
Uses data or values from 2 papers
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The measurements of electroweak precision observables at LEP constrain the mixing with SM leptons to $|ϵ_S,D,T| 0.03$ and $|ϵ_N,A|0.05$ for flavor-universal couplings [ref] .
Effects of new leptons in Electroweak Precision Data · Two new mixed fermion fields · 0.89
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The current experimental bound $ℬ(μ→eγ) < 5.7×10^-13$ [ref] imposes strong constraints on $|Δ_μe|/Λ_FV^2$.
New constraint on the existence of the mu+-> e+ gamma decay · Constraints from $ℬ(μ→e γ)$ and the muon mass · 0.98
Compares with 1 paper
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Our results agree with [ref] for the case $ψ_D+ψ^±$.
Anthropic solution to the magnetic muon anomaly: the charged see-saw · New-physics contributions to $a_μ$ · 1.00
Cites as background 3 papers
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Details on vector lepton mixing and the resulting electroweak couplings in the context of $a_μ$ can be found, for instance, in [ref] .
Anthropic solution to the magnetic muon anomaly: the charged see-saw · One new field · 0.95
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Details on vector lepton mixing and the resulting electroweak couplings in the context of $a_μ$ can be found, for instance, in [ref] .
Explanation of the Muon g-2 Anomaly with Vectorlike Leptons and its Implications for Higgs Decays · One new field · 0.95
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We refer the reader to the pertinent literature for the MSSM [ref] , where these questions have been studied in detail.
Muon g-2 vs LHC in Supersymmetric Models · $tanβ$-enhanced corrections · 0.98
Discussion
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