Light spin-1/2 or spin-0 Dark Matter particles

P. Fayet

REVIEW

Light spin-1/2 or spin-0 Dark Matter particles

P. Fayet

High Energy Physics - Phenomenology · Submitted 20 Mar 2004 · 29 references

Pith's verdict: A short, honest extension of Boehm–Fayet showing that a vector-coupled U gives light Majorana spin-1/2 dark matter the same v_dm^2-suppressed annihilation cross section as spin-0 — the algebra checks out, and the only real quibble is that the abstract's 'automatically' is conditional in the body.

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arXiv IDhep-ph/0403226 (version 1)

Pith IDFRXI22X3, Pith's permanent record of this paper and its review

CitationP. Fayet. "Light spin-1/2 or spin-0 Dark Matter particles." arXiv:hep-ph/0403226 (2004). https://pith.science/paper/FRXI22X3

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Editor's note

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Short version: this is a ten-page extension of Boehm–Fayet (hep-ph/0305261), and the one genuinely new thing in it is worth knowing — for a vector-coupled U, a light Majorana spin-1/2 particle has exactly the same annihilation cross section into e+e- as a light spin-0 particle, Eq. (32) = Eq. (13), with the same v_dm^2 threshold suppression. That widens the light-dark-matter candidate list at almost no cost, and it is the reason to read the paper rather than the earlier one alone. The derivation is clean and I checked it. The C-parity argument in §VI.A is right: the initial χχ S-wave state has C = +, angular momentum forces C' = + on the final state, and a vector current can only produce a C' = - pair, so there is no S-wave and the cross section goes as v_dm^2. The explicit trace in Eq. (33), with the 1/4 spin average and the phase-space factor, reproduces the spin-0 result. The Dirac/Majorana decomposition in Eq. (29) is standard and correctly applied. Nothing here is fitted; it is algebra, and the CPT relation between production and annihilation in §V is a nice way to get the same answer twice. Where I would push back is small. The abstract's 'automatically includes a v_dm^2 suppression' reads unconditional, but the body is explicit that it holds only for fV with fA = 0, and Eq. (35) is the paper's own counterexample: an axial coupling leaves an S-wave term proportional to m_f^2 and the suppression dies. The defense — that fA would give U an axionlike longitudinal mode [8] — is a real physics argument, but it is a constraint on the new sector, not a theorem, and mixing or anomaly diagrams could reintroduce an axial piece. That is a framing gap, not an error, and the paper flags it in §VI.A and §VI.B. The other soft spots are acknowledged: Appendix A calls the freeze-out treatment naive, and footnote 22 says the 511 keV cross-section estimate depends on the bulge density profile. Both are acceptable for what this is. The whole construction rests on an unobserved U with stipulated couplings, which is the price of the program, not a flaw in this installment. Who should read it: anyone working on light dark matter and the INTEGRAL line, or on light gauge bosons with vector couplings. It is a short, honest note that closes a gap in the candidate list. Send it out.

Referee report

Significance

The paper is a prescient and technically sound contribution to what became the light dark matter / dark photon program. The cross-section identity is a genuine algebraic result, verified independently by the explicit trace in Eq. (33), and the C-parity argument is correct. The paper is also transparent about its model-building assumptions: the U boson is introduced ad hoc (citing [8]), the vector-coupling requirement is stated as a stipulation motivated by the axionlike constraint, and the INTEGRAL connection is framed as a possibility ('could be responsible') rather than a derivation. The numerical estimates in Eqs. (16), (24), and Appendix A check out at the stated order of magnitude. The stress-test concern that the abstract's 'automatically includes a v_dm^2 suppression factor' reads as unconditional does not land: abstract:s2 explicitly conditions on the U being 'vectorially coupled to matter particles', and §VI.A and §VI.C state the axial-coupling counterexample (Eq. (35)) openly.

Major objections (2)

[§VI.A and Eq. (35)] The v_dm^2 suppression that makes the scenario viable is conditional on the U boson having no axial coupling to matter fermions. The paper states this assumption and motivates it via the axionlike constraint of [8], but does not quantify how small f_A must be. From Eq. (35), the S-wave term proportional to m_f^2 dominates over the P-wave term by a factor of order (3/4)(m_f/m_dm)^2/v_dm^2 at present-day velocities; for m_dm ≈ 3 MeV, m_f = m_e, and v_dm ≈ 3×10^-3, this factor is ≈ 2×10^3 even for f_A = f_V. The paper should state the resulting bound f_A/f_V ≲ 0.02 and check that it is compatible both with the axionlike constraint from [8] and with the required f_V from Eq. (16).
[§VI.B, Eqs. (20), (27), (28)] The relation (20) between production and annihilation cross sections is derived for spin-0 particles and contains a factor 1/4 from the ratio of spin degeneracies (g_phi = 1, g_e = 4). Applying it to spin-1/2 particles requires replacing this factor by 1 (g_psi = g_e = 4). The paper does not state this replacement, so the derivation of Eq. (28) from (20) and (27) appears to be off by a factor of 4. The final result (32) is correct, as confirmed by the direct trace calculation in Eq. (33), but the intermediate step should be clarified so that the reader does not have to reconstruct the modified degeneracy factor.

Minor objections (7)

[§VI.D] The statement that light spin-1/2 dark matter is 'more attractive' than spin-0 because its small mass is 'easier to understand' is a model-building preference, not a derived result, and should be framed as such.
[Eqs. (17)–(18)] The g-2 formulas assume m_U << m_mu for the muon and m_U >> m_e for the electron; the regime of validity should be stated explicitly, since the numerical coefficients change in the opposite limits.
[Eq. (19)] The neutrino-electron scattering constraint assumes that only U exchange contributes; loop-induced effects from the dark-matter coupling are neglected and should be noted.
[Appendix A] The freeze-out estimate is explicitly naive, and the factor ≈2 enhancement for v^2-suppressed cross sections is quoted from [14] without derivation. This is acceptable but should be flagged as an approximation when the resulting cross sections are compared with the INTEGRAL requirement.
[Notation] The symbols C_U, f_V, f_A, G_V, and G_A are introduced at different points without a summary table; a short glossary would help the reader keep track of which coupling enters which equation.
[Eq. (16)] The kinematic factor is stated as 'still to be multiplied'; giving the full expression with the factor included would make the numerical estimate self-contained.
[Eq. (35)] A numerical estimate for m_f = m_e would make the importance of the S-wave term concrete and would connect directly to the bound requested in the first major comment.

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The paper

The paper claims

A light spin-1/2 particle can annihilate into electron-positron pairs exactly as a light spin-0 particle does, provided a new vector-coupled U boson mediates the annihilation.

The reading

The paper asks how a dark matter particle much lighter than a GeV could annihilate strongly enough to leave the observed relic density, when ordinary weak interactions are far too feeble at such masses. Fayet's answer, developed earlier for spin-0 particles, is a new light neutral spin-1 boson $U$ that couples far more strongly to dark matter than to ordinary matter, and only very weakly to quarks and leptons. The new step is to show that the same mechanism works for a spin-1/2 (Majorana) particle, and that the resulting annihilation cross section into $e^+e^-$ is literally the same formula as the spin-0 one. The reason matters as much as the result: because $U$ couples vectorially to matter fermions, charge conjugation and angular-momentum conservation forbid the $S$-wave annihilation channel, so the cross section carries an automatic $v_{dm}^2$ suppression at threshold. That suppression is what keeps residual annihilations in today's Universe from overproducing gamma rays, while still allowing the freeze-out rate that sets the relic abundance. The payoff is that light spin-1/2 dark matter becomes a candidate for the bright 511 keV line seen by INTEGRAL from the galactic bulge, on the same footing as the spin-0 case.

What carries the argument

Two pieces of machinery carry the argument. The first is a charge-conjugation selection rule: the initial $χχ$ state is $C=+$ in an $S$ wave, and the final $fbar f$ state must also be $C=+$ by angular-momentum conservation, while a vectorially coupled $U$ can only produce a $C=-$ pair; hence no $S$-wave annihilation and an automatic $propto v_{dm}^2$ threshold behaviour. The same argument, applied to the axial coupling $f_A$, fails once $m_f eq 0$, producing the unwanted constant term of Eq. (35). The second is a bookkeeping trick relating the three cases: writing the Dirac field as $ψ=(χ-iχ')/sqrt{2}$ splits the axial current into two Majorana pieces, $barψγ^μγ^5ψ = frac{1}{2}barχγ^μγ^5χ + frac{1}{2}barχ'γ^μγ^5χ'$, so a Dirac particle with coupling $C_U$ annihilates like a Majorana particle with coupling $C_U/2$. Combined with the factor-four relation between spin-1/2 and spin-0 production cross sections in $e^+e^-$, this collapses all three cross sections into one formula. The freeze-out calculation, with $x_F$ between roughly 16 and 23 and the $4/11$ dilution factor from $e^+e^-$ annihilation, fixes the numerical targets those formulas must hit.

Extended reading notes

Core claim

The central claim is an identity: for a Majorana spin-1/2 dark matter particle $χ$ annihilating into $fbar f$ through a light $U$ boson vectorially coupled to matter fermions, the cross section $σ_{ann}v_{rel}(χχ→e^+e^-)$ equals, term for term, the spin-0 cross section $σ_{ann}v_{rel}(φbarφ→e^+e^-)$ of the earlier analysis. Fayet establishes this two ways — by a CPT-based relation between the annihilation rate and the corresponding production rate in $e^+e^-$ collisions, and by direct evaluation of the spin-summed squared amplitudes — and both routes land on the same expression, proportional to $v_{dm}^2$ times the standard kinematic factor $frac{3}{2}β_f - frac{1}{2}β_f^3$ for vectorial production of a fermion pair. The physical reason is that a two-Majorana $χχ$ state in an $S$ wave has $C=+$ and $J=S=0$, so a $C$-violating vector coupling cannot connect it to an $fbar f$ pair, and a vector current can only make a $C=-$ pair anyway; the $S$-wave amplitude therefore vanishes and the leading term is $P$-wave. The paper also quantifies the required cross sections: about 4–5 pb at freeze-out for self-conjugate (Majorana) dark matter, 8–10 pb for non-self-conjugate (Dirac fermion or complex scalar), corresponding to roughly $10^{-5}$ pb for annihilations of present-day relic particles moving at $v_{dm} approx 3 imes 10^{-3}$ — the order of magnitude needed for the INTEGRAL 511 keV signal.

Load-bearing premise

The whole result rests on the new U boson coupling vectorially, not axially, to matter fermions; if an axial piece is present, Eq. (35) shows a constant term proportional to $m_f^2$ reappears and the velocity suppression — and with it the freedom from excessive gamma rays — is lost.

Editorial extensions

If this is right

Light spin-1/2 dark matter in the MeV range becomes a viable relic-abundance candidate, not just the spin-0 case, because the required ~4–5 pb annihilation rate is reachable with a sufficiently strong $U$ coupling to dark matter.
The $v_{dm}^2$ threshold suppression keeps late-time annihilations roughly $10^{-5}$ times the freeze-out rate, so a light candidate need not overproduce gamma rays while still leaving the observed relic density.
Annihilations of such particles into $e^+e^-$ in the galactic bulge can supply the positrons needed for the 511 keV line observed by INTEGRAL, with the required present-day cross section of order $10^{-4}$ pb.
Because the dark-matter-to-$U$ coupling must exceed the $U$-to-matter coupling by a large factor, the same particles have significant $U$-mediated self-interactions, which lengthen the free-streaming scale and can be constrained by structure formation.
The same formulas convert directly into production cross sections for these particles in $e^+e^-$ collisions, giving a search channel — though one the paper expects to be very small at high energy.

Reading between the lines

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

The identity between spin-0 and spin-1/2 cross sections is a statement about the $S$-wave, not about the particles' spins as such; any dark matter candidate whose annihilation is forced to be $P$-wave by a $C$-parity mismatch should inherit the same rate, which suggests the result generalises to other $C$-odd mediators beyond the specific $U$ model.
If the 511 keV bulge flux is eventually resolved morphologically, the prediction $propto ρ_{dm}^2$ (versus a stellar or supernova source) would be the cleanest discriminator between this scenario and astrophysical positron sources, and the required cross section scales as $(m_{dm}/1,\mathrm{MeV})^2$, so the line's spatial profile and the inferred mass are linked.
The paper's own bound $|f_{Vν}f_{Ve}| lesssim G_F m_U^2$ means the neutrino coupling is squeezed from both sides: small enough for neutrino-electron scattering, yet the dark matter coupling must stay large; a future precision neutrino-electron measurement would probe precisely the corner of parameter space this scenario occupies.
Because the axial coupling destroys the suppression only through terms proportional to $m_f^2$, the failure mode is mass-dependent: a mixed vector/axial $U$ would still look clean for annihilation into electrons but could reintroduce an $S$-wave term for heavier final states, so the constraint on $f_A$ is weakest where the 511 keV signal is produced.

Original abstract

We recall and precise how light spin-0 particles could be acceptable Dark Matter candidates, and extend this analysis to spin-1/2 particles. We evaluate the (rather large) annihilation cross sections required, and show how they may be induced by a new light neutral spin-1 boson U. If this one is vectorially coupled to matter particles, the (spin-1/2 or spin-0) Dark Matter annihilation cross section into e+e- automatically includes a v_dm^2 suppression factor at threshold, as desirable to avoid an excessive production of gamma rays from residual Dark Matter annihilations. We also relate Dark Matter annihilations with production cross sections in e+e- scatterings. Annihilation cross sections of spin-1/2 and spin-0 Dark Matter particles are given by exactly the same expressions. Just as for spin-0, light spin-1/2 Dark Matter particles annihilating into e+e- could be responsible for the bright 511 keV gamma ray line observed by INTEGRAL from the galactic bulge.

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-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 5,080 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.17 · human participants 0.01 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.08 · compares with prior results 0.42 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; show every role.

We recall and precise how light spin-0 particles could be acceptable Dark Matter candidates, and extend this analysis to spin-1/2 particles. We evaluate the (rather large) annihilation cross sections required, and show how they may be induced by a new light neutral spin-1 boson U. If this one is vectorially coupled to matter particles, the (spin-1/2 or spin-0) Dark Matter annihilation cross section into e+e- automatically includes a v_dm^2 suppression factor at threshold, as desirable to avoid an excessive production of gamma rays from residual Dark Matter annihilations. We also relate Dark Matter annihilations with production cross sections in e+e- scatterings. Annihilation cross sections of spin-1/2 and spin-0 Dark Matter particles are given by exactly the same expressions. Just as for spin-0, light spin-1/2 Dark Matter particles annihilating into e+e- could be responsible for the bright 511 keV gamma ray line observed by INTEGRAL from the galactic bulge.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	We recall and precise how light spin-0 particles could be acceptable Dark Matter candidates, and extend this analysis to spin-1/2 particles.	contribution 0.88	0.52	0.11	hedged possibility 0.93
s1	We evaluate the (rather large) annihilation cross sections required, and show how they may be induced by a new light neutral spin-1 boson U.	contribution 0.92	0.87	0.12	hedged possibility 0.71
s2	If this one is vectorially coupled to matter particles, the (spin-1/2 or spin-0) Dark Matter annihilation cross section into e+e- automatically includes a v_dm^2 suppression factor at threshold, as desirable to avoid an excessive production of gamma rays from residual Dark Matter annihilations.	result 0.79	0.73	0.22	asserted 0.93
s3	We also relate Dark Matter annihilations with production cross sections in e+e- scatterings.	contribution 0.97	0.76	0.05	asserted 0.97
s4	Annihilation cross sections of spin-1/2 and spin-0 Dark Matter particles are given by exactly the same expressions.	result 0.99	0.76	0.27	asserted 1.00
s5	Just as for spin-0, light spin-1/2 Dark Matter particles annihilating into e+e- could be responsible for the bright 511 keV gamma ray line observed by INTEGRAL from the galactic bulge.	interpretation 0.85	0.31	0.26	hedged possibility 1.00

What kind of paper the classifier reports this is: theoretical result 0.99 · evidence from proof or derivation 0.94.

What was examined, and what was not

sources read

arXiv listing abstract v1

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

References

Works cited (29)

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P. Fayet, Phys. Lett. B95, 285 (1980); Nucl. Phys. B187, 184 (1981); Phys. Lett. B227, 127 (1989); Nucl. Phys. B347, 743 (1990); Class. Quant. Grav. 13, A19 (1996)

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P. Fayet, Phys. Lett. B69, 489 (1977); B70, 461 (1977); G. Farrar and P. Fayet, Phys. Lett. B76, 575 (1978)

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C. Boehm and P. Fayet, Nucl. Phys. B683, 219 (2004) (hep-ph/0305261)

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P. Fayet, Nucl. Phys. B149, 137 (1979); Proc. 1980 Karpacz School (Harwood, 1981) p.115; Phys. Lett. B142, 263 (1984); B159, 121 (1985)

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C. Boehm, T. Ensslin and J. Silk, J. Phys. G30 (2004) 279 (astro-ph/0208458)

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J. Kn¨ odlseder et al. , astro-ph/0309442; P. Jean et al. , Astron. Astrophys. 407 (2003) L55

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C. Bo ehm et al. , Phys. Rev. Lett. 92, 101301 (2004) (astro-ph/0309686)

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C. Boehm, P. Fayet and J. Silk, hep-ph/0311143, to be pub. in Phys. Rev. D

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P. Serpico and G. Raﬀelt, astro-ph/0403471

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R.C. Allen et al. , Phys. Rev. D47, 11 (1993); LNSD coll., Phys. Rev. D63, 112001 (2001)

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C. Boehm, P. Fayet and R. Scha eﬀer, Phys. Lett. B518, 8 (2001); Proc. “DARK 2002”, Cape Town (Springer, 2002), p. 333, astro-ph/0205406; C. Bo ehm et al. , astro- ph/0309652

View on Pith Pith review arXiv 2001
[18]

For heavier particles decoupling before muon annihila - tions, there is an extra dilution factor which may be writ- ten as 434/g′∗ , while for lighter particles decoupling after e+e− annihilations the factor 4/11 is absent

View on Pith
[19]

For comparison with other results, our equation (3) ﬁx- ing ρdm, with the extra dilution factor 434/g′∗ relevant for heavier Dark Matter particles included, may be rewrit- ten, using T 3◦ γ ≃ 1685 cm −3, as ρdm ≈ (2) 1.09 10 4 xF g′∗/√g∗ 1 mP l <σv> cm−3 , and, dividing by ρc/h2 , Ωdm h2 ≃ (2) xF g′∗/√g∗ 1.04 10 9 mP l × (1 GeV) <σv> , as in [14], for a v...

View on Pith
[20]

These formulas, which apply directly for mdm in the 10 MeV – 1 GeV range, can be immediately extended to heavier particles by including the extra factor √ g∗ (43/4) g′∗ (usually the same as √ (43/4)/g∗ ), and using the ap- propriate (slightly larger) value of xF . For comparison, in the familiar case of heavy Dark Matter particles, the signiﬁcantly larger...

View on Pith
[21]

Once Dark Matter particles and electrons have anni- hilated, the neutrino temperature gets related to the photon temperature by T 3ν T 3 ≃ g∗γ = 2(g∗ e+γ = 112 ) + g∗ dm ≃ 829, 36 or 30 , with g∗ dm = 74, 72 or 2 for a Majorana, Dirac or com- plex spin-0 particle, respectively. This leads to a total g∗ F ≃ 2.94, 2.71 or 2.90, respectively, instead of 3.36...

View on Pith
[22]

With a measured ﬂux of about 10 −3 photon cm −2 s−1, the required cross section may be estimated ∝ 10−4 pb, times ( mdm/1 MeV) 2, depending on the density dis- tribution within the galactic bulge. Note also that the rate of annihilation events is proportional to 12 N 2dm <σ annvrel> for self-conjugate Dark matter particles, and to ( Ndm2 )2 <σ annvrel> , ...

View on Pith
[23]

Furthermore in theories inspired from N = 2 extended supersymmetry, or higher-dimensional theories [7], the spin-0 ﬁeld ϕ may be a spin-0 photon ﬁeld, or com- panion of the weak hypercharge gauge ﬁeld Bµ, possibly described by the extra 5 th and 6 th components of the corresponding 6d-gauge ﬁelds, A ˆµ or B ˆµ

View on Pith
[24]

For example, α2/ (100 GeV) 2 ≃ 2 10 −36 cm2, which shows that appropriate values of < σannvrel/c > ≈ (2 to 5) 10 −36 cm2 may easily be obtained, from the ex- changes of heavy fermions F in the ∼ 100 – 1000 GeV mass range, with Yukawa couplings Cl and Cr remain- ing in the perturbative regime

View on Pith
[25]

The eﬀective Lagrangian density (8) is invariant under Charge Conjugation, allowing for a non-vanishing annihi- lation cross section at threshold of a ϕ ¯ϕ initial state with C = (−)L = + into a f ¯f state with C ′ = (−)(L′+S′) = +

View on Pith
[26]

In the presence of an axial contribution ( fA) to the U matter current, the longitudinal polarization state of the U would behave very much as an (unwanted) spin-0 ax- ionlike particle, having pseudoscalar couplings to quarks and leptons ≈ fAmq,l/mU [8]

View on Pith
[27]

v 2dm ≃ .16 c2 is associated with a kinetic energy ≃ .09 mdm, which would correspond roughly, to ﬁx ideas, to 3 2 TF = 32 xF mdm with xF ≃ 16 or 17

View on Pith
[28]

If, in addition, all fV ’s are of the same order, they should then be smaller than about .3 10 −5mU (MeV). Expres- sion (16) of <σ annvrel> then implies that the U cou- pling to Dark Matter cU should be larger than about .2 (m 2U − 4m 2dm)/mUmdm (for <σ annvrel> to be larger than 4 or 5 pb), in which case the self-interactions of Dark Matter would be sign...

View on Pith
[29]

As, also, for the decay of a massive spin-1 gauge boson into two “inos”, or the pair production (neglecting me) of photinos in e+e− annihilations, through (eﬀective) ax- ial couplings

View on Pith

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hep-ex 2018-02 accept novelty 5.0 of 10

No significant excess is observed in H to ZX or XX to four leptons at 13 TeV, and 95% CL upper limits are set near 0.1% for H to Z Zd, 0.01% for H to Zd Zd, and 1% for H to aa, depending on the new boson mass.
Search for new light gauge bosons in Higgs boson decays to four-lepton final states in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector at the LHC
hep-ex 2015-05 conditional novelty 5.0 of 10

No excess over the Standard Model appears in ATLAS four-lepton Higgs data, and 95% confidence upper limits are set on H to Z Zd and H to Zd Zd branching ratios and on the dark-sector couplings they probe.
Projections for Dark Photon Searches at Mu3e
hep-ph 2014-11 conditional novelty 5.0 of 10

Mu3e's large muon sample can search for 10 to 80 MeV dark photons in mu+ to e+ nu nu A', A' to e+e-, reaching epsilon squared near 10^-7 (phase I) and 10^-8 (phase II).
Exotic Decays of the 125 GeV Higgs Boson
hep-ph 2013-12 accept novelty 5.0 of 10

The paper maps about twenty exotic Higgs decay topologies onto the models that produce them and derives new Run 1 limits on several by reinterpreting existing ATLAS and CMS analyses.
Low Mass WIMP Searches with a Neutrino Experiment: A Proposal for Further MiniBooNE Running
hep-ex 2012-11 conditional novelty 5.0 of 10

Steering the MiniBooNE beam off target into an iron absorber reduces neutrino backgrounds enough to project sensitivity to 10-200 MeV WIMPs in the parameter region that also resolves the muon g-2 discrepancy.
Dark Side of Higgs Diphoton Decays and Muon g-2
hep-ph 2012-08 conditional novelty 5.0 of 10

A single dark U(1) sector with vector-like leptons can simultaneously enhance the Higgs diphoton rate and explain the muon g-2 discrepancy, predicting new Higgs decays to a photon plus a 20-100 MeV dark boson.
Signatures of sub-GeV dark matter beams at neutrino experiments
hep-ph 2012-05 conditional novelty 5.0 of 10

Neutrino beams at MiniBooNE, MINOS and T2K can carry a sub-GeV dark matter beam whose elastic scattering in near detectors yields picobarn-level sensitivity to dark matter nucleon scattering.
Rare meson decays into very light neutralinos
hep-ph 2009-05 accept novelty 5.0 of 10

In the MSSM with minimal flavor violation, kaon and B decays into light neutralinos are far below experimental reach, so a massless neutralino survives; with non-minimal flavor violation the same decays bound squark m...
Abelian Hidden Sectors at a GeV
hep-ph 2009-04 conditional novelty 5.0 of 10

Suppressed communication of supersymmetry breaking to a hidden U(1) sector naturally lands its mass scale near a GeV, and the resulting models can be viable dark matter sectors with collider signatures.
Dark Photon - ALP Freeze-in: 511 keV and H$\alpha$ Constraints
hep-ph 2026-07 conditional novelty 4.0 of 10

A nearly degenerate ALP–dark-photon freeze-in model can reproduce the relic density and the 511 keV line while passing Hα and other constraints only in a narrow window with Λ≈10^10–10^12 GeV and ϵ≈10^-24–10^-23.
The COHERENT Experimental Program
hep-ex 2022-04 unverdicted novelty 3.0 of 10

COHERENT has measured CEvNS on CsI and argon and lays out a multi-detector program to measure CEvNS on more nuclei and search for physics beyond the Standard Model.
Searches for light dark matter using condensed matter systems
hep-ph 2021-08 accept novelty 3.0 of 10

A self-contained review showing that sub-GeV dark matter detection reduces to computing or measuring a target's dynamic structure factor, with phonons for nuclear couplings and the dielectric loss function for electro...
Low Energy Measurements of the Weak Mixing Angle
hep-ex 2013-02 accept novelty 2.0 of 10

Low-energy determinations of the weak mixing angle test electroweak running and constrain TeV-scale new physics, with planned experiments aiming at ±0.1% precision.
Muonic hydrogen and the proton radius puzzle
physics.atom-ph 2013-01 accept novelty 2.0 of 10

The proton radius from muonic hydrogen is about 4% smaller than the electronic and scattering values, and this review lays out the evidence and candidate explanations for that discrepancy.

How the literature uses it

47 arXiv papers cite this in 71 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 · 46 sentences without a confident label

Cites it as support 1 paper

Even though the consideration of large $Δ$ values seems theoretically arbitrary, this regime of parameter space is of great interest, because it can simultaneously explain the muon $g-2$ anomaly and provide the correct DM relic abundance [ref] .

The High-Energy Frontier of the Intensity Frontier: Closing the Dark Photon, Inelastic Dark Matter, and Muon g-2 Windows · Inelastic Dark Matter · 0.97

Cites it as background 23 papers

Such models were first studied in [ref] and rephrased in the context of kinetically mixed U(1) models in [ref] .

Exploring Portals to a Hidden Sector Through Fixed Targets · 4. Probing a dark matter beam · 0.94
A possible explanation of the 511 keV line is from dark matter (DM) annihilations [ref] or decays [ref] , or new atoms binded [ref] .

Explanation of the 511 keV line: Cascade annihilating dark matter with the $^8$Be anomaly · Introduction · 0.94
For the Compton-like process to produce dark photon ($A'$): $γ e^- →A' e^-$, the interaction of an electron ($ψ_e$) with dark photon ($A'$) is described by the following term in the Hamiltonian: [equation] where $e$ is the electronic charge and $ϵ$ is the kinetic mixing parameter between the dark photon and the Standard Model photon [ref] .

Search for dark photon, axion-like particles, dark scalar, or light dark matter in Compton-like processes · Theory · 0.81

Discussion

Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.

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For heavier particles decoupling before muon annihila - tions, there is an extra dilution factor which may be writ- ten as 434/g′∗ , while for lighter particles decoupling after e+e− annihilations the factor 4/11 is absent

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For comparison with other results, our equation (3) ﬁx- ing ρdm, with the extra dilution factor 434/g′∗ relevant for heavier Dark Matter particles included, may be rewrit- ten, using T 3◦ γ ≃ 1685 cm −3, as ρdm ≈ (2) 1.09 10 4 xF g′∗/√g∗ 1 mP l <σv> cm−3 , and, dividing by ρc/h2 , Ωdm h2 ≃ (2) xF g′∗/√g∗ 1.04 10 9 mP l × (1 GeV) <σv> , as in [14], for a v...

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[20] [20]

These formulas, which apply directly for mdm in the 10 MeV – 1 GeV range, can be immediately extended to heavier particles by including the extra factor √ g∗ (43/4) g′∗ (usually the same as √ (43/4)/g∗ ), and using the ap- propriate (slightly larger) value of xF . For comparison, in the familiar case of heavy Dark Matter particles, the signiﬁcantly larger...

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[21] [21]

Once Dark Matter particles and electrons have anni- hilated, the neutrino temperature gets related to the photon temperature by T 3ν T 3 ≃ g∗γ = 2(g∗ e+γ = 112 ) + g∗ dm ≃ 829, 36 or 30 , with g∗ dm = 74, 72 or 2 for a Majorana, Dirac or com- plex spin-0 particle, respectively. This leads to a total g∗ F ≃ 2.94, 2.71 or 2.90, respectively, instead of 3.36...

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[22] [22]

With a measured ﬂux of about 10 −3 photon cm −2 s−1, the required cross section may be estimated ∝ 10−4 pb, times ( mdm/1 MeV) 2, depending on the density dis- tribution within the galactic bulge. Note also that the rate of annihilation events is proportional to 12 N 2dm <σ annvrel> for self-conjugate Dark matter particles, and to ( Ndm2 )2 <σ annvrel> , ...

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[23] [23]

Furthermore in theories inspired from N = 2 extended supersymmetry, or higher-dimensional theories [7], the spin-0 ﬁeld ϕ may be a spin-0 photon ﬁeld, or com- panion of the weak hypercharge gauge ﬁeld Bµ, possibly described by the extra 5 th and 6 th components of the corresponding 6d-gauge ﬁelds, A ˆµ or B ˆµ

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[24] [24]

For example, α2/ (100 GeV) 2 ≃ 2 10 −36 cm2, which shows that appropriate values of < σannvrel/c > ≈ (2 to 5) 10 −36 cm2 may easily be obtained, from the ex- changes of heavy fermions F in the ∼ 100 – 1000 GeV mass range, with Yukawa couplings Cl and Cr remain- ing in the perturbative regime

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[25] [25]

The eﬀective Lagrangian density (8) is invariant under Charge Conjugation, allowing for a non-vanishing annihi- lation cross section at threshold of a ϕ ¯ϕ initial state with C = (−)L = + into a f ¯f state with C ′ = (−)(L′+S′) = +

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[26] [26]

In the presence of an axial contribution ( fA) to the U matter current, the longitudinal polarization state of the U would behave very much as an (unwanted) spin-0 ax- ionlike particle, having pseudoscalar couplings to quarks and leptons ≈ fAmq,l/mU [8]

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[27] [27]

v 2dm ≃ .16 c2 is associated with a kinetic energy ≃ .09 mdm, which would correspond roughly, to ﬁx ideas, to 3 2 TF = 32 xF mdm with xF ≃ 16 or 17

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[28] [28]

If, in addition, all fV ’s are of the same order, they should then be smaller than about .3 10 −5mU (MeV). Expres- sion (16) of <σ annvrel> then implies that the U cou- pling to Dark Matter cU should be larger than about .2 (m 2U − 4m 2dm)/mUmdm (for <σ annvrel> to be larger than 4 or 5 pb), in which case the self-interactions of Dark Matter would be sign...

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[29] [29]

As, also, for the decay of a massive spin-1 gauge boson into two “inos”, or the pair production (neglecting me) of photinos in e+e− annihilations, through (eﬀective) ax- ial couplings

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