REVIEW
X-ray lines from R-parity violating decays of keV sparticles
High Energy Physics - Phenomenology · Submitted 21 Mar 2014 · 66 references
Pith's verdict: A timely, honest feasibility study whose real content is the gravitino no-go and the explicit admission that the relic abundance is the unsolved part.
T0 · AI review
Why Pith says this
Highlight any text to comment on it.
Referee report
The paper
The paper claims
A 7 keV supersymmetric particle could produce the 3.5 keV X-ray line
The reading
What carries the argument
The machinery is the loop-level decay of a neutral gaugino into a neutrino-photon pair, $B$ (or the mixed photino) $to nu gamma$, mediated either by a trilinear R-parity-violating coupling $lambda L L E$ or by a sneutrino vacuum expectation value induced by the bilinear term $mu' L H_u$. The decay rate scales as $lambda^{2} m^{3}/tilde{m}^{4}$ (or as $langle tilde{nu} rangle^{2}$ times the analogous factors), so a keV mass and a tiny coupling naturally give a lifetime in the $10^{27}$-$10^{28}$ s range. Two consistency constraints do real work: the sneutrino vev feeds into the neutrino mass as $Delta m_{nu} simeq g^{2} langle tilde{nu} rangle^{2}/m_{B}$, which keeps that vev at or below roughly 10 eV to 1 MeV depending on the case, and the hidden photino's mixing with the bino, $epsilon sim 10^{-7} (chi)(m_{gamma'}/10~{keV})(100~{GeV}/m_{chi})$, is naturally small because of the mass hierarchy. The same small mixing that makes the lifetime right also keeps the hidden sector out of thermal contact, which is what makes the abundance discussion tractable at all.
Extended reading notes
Core claim
The central claim is that R-parity-violating decays of a keV-scale superpartner can reproduce the lifetime inferred from the 3.5 keV cluster line, and that among the standard candidates the bino and the hidden-sector photino work while the gravitino does not. For a bino of mass $m_{B}$ decaying through the trilinear operator $LL E$, the lifetime is $5$ times $10^{27}$ s for coupling $10^{-8}$, slepton mass $2$ TeV, and $m_{B}=7$ keV, essentially the observed value. For a hidden photino mixed with the bino at level $10^{-7}$, a comparable lifetime follows even with a much larger R-parity-violating coupling, because the mixing suppresses the rate; a sneutrino-vev-mediated decay gives a similar result. The gravitino case is closed off: parameterising the R-parity-violating effect by a suppression factor $F$, the lifetime is $6$ times $10^{29}$ s times $(1/F)(7~{keV}/m_{3/2})^{3}$, which stays too long even at $F=1$. The authors also note that a keV axino is generically too long-lived unless parameters sit at the edge of experimental exclusion.
Load-bearing premise
The load-bearing premise is that a 7 keV bino or hidden photino can actually be brought down to the observed dark matter abundance; the paper states that standard thermal production overproduces them and offers only qualitative escape routes (assimilation with an exotic asymmetric species, or preferential inflaton reheating with suppressed freeze-in) without computing a final abundance.
Editorial extensions
If this is right
- A confirmed 3.5 keV line whose flux fixes the dark matter lifetime would single out a keV-scale gaugino-like state over a gravitino or axino, since those are too long-lived for any reasonable parameter choice.
- Because the bino lifetime depends on the fourth power of the slepton mass, a lifetime measurement combined with a coupling estimate would translate into a rough upper range on the slepton mass scale.
- The hidden-photino version tolerates much larger R-parity-violating couplings than the bino version, so the two scenarios are separated by whether ordinary R-parity-violating processes should be near their experimental limits.
- If keV-scale thermal relics of this kind exist, they contribute to the relativistic energy density, so a percent-level rise in the effective number of neutrino species would accompany the line and distinguish this from axion or misalignment-produced dark matter.
- The scenarios require a reheat temperature below the superpartner mass scale, so a confirmed line would indirectly say something about how the universe reheated.
Reading between the lines
- The two working candidates make opposite predictions for laboratory R-parity-violating searches: the bino needs $lambda sim 10^{-8}$, far below current bounds, while the photino allows $lambda$ near $10^{-2}$, so a future null result or signal in rare decays would discriminate between them.
- The abundance problem is the real bottleneck, and the sketched fixes point to a testable corner: the assimilation mechanism needs a particle asymmetry of order $6$ times $10^{-4}$, about a thousand times the baryon asymmetry, which would leave its own imprint on the expansion history and on the relic abundances of the exotic states involved.
- If the line is real but no keV gaugino exists, the same lifetime-versus-abundance tension would apply to any decaying thermal relic at this mass, making the 3.5 keV line a general probe of whether keV dark matter was ever in thermal equilibrium.
- A useful check the authors do not perform is a full Boltzmann calculation for the photino in the low-reheat, low-mixing regime; that would turn the qualitative escape in Eqs. 28 to 30 into a definite allowed region or exclude it.
Original abstract
If R-parity is only mildly violated then the lightest supersymmetric particle (LSP) can be stable over cosmologically time-scales and still account for the dark matter relic density. We examine the possibility of generating detectable X-ray lines from R-parity violating decays of keV-scale LSP dark matter to neutrino-photon pairs. Specifically, we consider scenarios in which the LSP is a light gravitino, bino, or hidden sector photino. Potential signals are discussed in the context of recent claims of an unidentified 3.5 keV X-ray line in studies of stacked galaxy clusters. We comment on the difficulties in obtaining the observed relic density for keV scale bino or hidden photino dark matter and some possible resolutions.
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
If R-parity is only mildly violated then the lightest supersymmetric particle (LSP) can be stable over cosmologically time-scales and still account for the dark matter relic density. We examine the possibility of generating detectable X-ray lines from R-parity violating decays of keV-scale LSP dark matter to neutrino-photon pairs. Specifically, we consider scenarios in which the LSP is a light gravitino, bino, or hidden sector photino. Potential signals are discussed in the context of recent claims of an unidentified 3.5 keV X-ray line in studies of stacked galaxy clusters. We comment on the difficulties in obtaining the observed relic density for keV scale bino or hidden photino dark matter and some possible resolutions.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | If R-parity is only mildly violated then the lightest supersymmetric particle (LSP) can be stable over cosmologically time-scales and still account for the dark matter relic density. | background 1.00 | 0.08 | 0.06 | hedged possibility 0.88 |
| s1 | We examine the possibility of generating detectable X-ray lines from R-parity violating decays of keV-scale LSP dark matter to neutrino-photon pairs. | contribution 1.00 | 0.67 | 0.15 | hedged possibility 0.86 |
| s2 | Specifically, we consider scenarios in which the LSP is a light gravitino, bino, or hidden sector photino. | contribution 0.89 | 0.43 | 0.04 | asserted 0.86 |
| s3 | Potential signals are discussed in the context of recent claims of an unidentified 3.5 keV X-ray line in studies of stacked galaxy clusters. | background 0.52 | 0.10 | 0.07 | asserted 0.47 |
| s4 | We comment on the difficulties in obtaining the observed relic density for keV scale bino or hidden photino dark matter and some possible resolutions. | limitation 0.63 | 0.53 | 0.07 | asserted 0.59 |
What was examined, and what was not
- arXiv listing abstract v2
- 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
Figures reproduced from arXiv:1403.5580 by the authors.
References
How the literature uses it
36 arXiv papers cite this in 39 sentences
Papers citing it · 17 sentences without a confident label
Contests it 1 paper
-
This was also pointed out in Ref. [ref] where it was concluded that although there are several sparticles capable of explaining the line signal in $R$-parity-violating supersymmetry, the gravitino is not one of them; however, as we will see below, this conclusion changes if we also take trilinear $R$-parity violation into account.
3.5 keV x-ray line from decaying gravitino dark matter · $R$-parity-violating decay of gravitino dark matter · contest 0.74
Compares with it 1 paper
-
Finally we note that our mechanism for generating a 3.5 keV line as well as the implications of the model are very different from other models that have recently been proposed [ref] .
3.5 keV Galactic Emission Line as a Signal from the Hidden Sector · Conclusion · 0.97
Cites it as background 20 papers
-
Light gravitino explanation [ref] (see also [ref] ) needs quite low reheating temperature.
SUSY in the sky or a keV signature of sub-GeV gravitino dark matter · Introduction · 0.88
-
A few attempts have been already taken in order to explain this excess of X-ray signal by (i) decaying dark matter or annihilating dark matter, [ref] , (ii) magnetic dipolar dark matter (via upscattering of dark matter into an excited states and subsequent decay of excited states into dark matter and an X-ray photon), [ref] , (iii) others [ref] .
Dipolar dark matter in light of 3.5 keV X-ray Line, Neutrino mass and LUX data · Introduction · 0.92
-
Since the fact provides a lot of implications on the nature of DM, many works have been studied [ref] .
X-ray line in Radiative Neutrino Model with Global $U(1)$ Symmetry · Introduction · 0.82
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.