Article

Dark matter searches going bananas: the contribution of Potassium (and Chlorine) to the 3.5 keV line

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Abstract

We examine the claimed excess X-ray line emission near 3.5 keV with a new analysis of XMM-Newton observations of the Milky Way center and with a re-analysis of the data on M31 and clusters. In no case do we find conclusive evidence for an excess. We show that known plasma lines, including in particular K XVIII lines at 3.48 and 3.52 keV, provide a satisfactory fit to the XMM data from the Galactic center. We assess the expected flux for the K XVIII lines and find that the measured line flux falls squarely within the predicted range based on the brightness of other well-measured lines in the energy range of interest. We then re-evaluate the evidence for excess emission from clusters of galaxies, including a previously unaccounted for Cl XVII line at 3.51 keV, and allowing for systematic uncertainty in the expected flux from known plasma lines and for additional uncertainty due to potential variation in the abundances of different elements. We find that no conclusive excess line emission is present within the systematic uncertainties in Perseus or in other clusters. Finally, we re-analyze XMM data for M31 and find no statistically significant line emission near 3.5 keV to a level greater than one sigma.

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... In this paper, we will dwell on the possibility that DM is a fermion with a mass of only a few keV. Such DM candidates, which often go by the name of "sterile neutrinos" are well known for their potential to improve predictions for small scale structure (see for instance ref. [3] and references therein), their clean observational signatures in the form of X-ray lines [3][4][5][6][7][8], and their tendency to evoke animated discussions among physicists [9][10][11][12][13]. Many such discussions were incited by recent claims for a yet-unidentified line at ∼ 3.5 keV in the X-ray spectra from galaxies and galaxy clusters [7,[9][10][11][12][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. ...
... Such DM candidates, which often go by the name of "sterile neutrinos" are well known for their potential to improve predictions for small scale structure (see for instance ref. [3] and references therein), their clean observational signatures in the form of X-ray lines [3][4][5][6][7][8], and their tendency to evoke animated discussions among physicists [9][10][11][12][13]. Many such discussions were incited by recent claims for a yet-unidentified line at ∼ 3.5 keV in the X-ray spectra from galaxies and galaxy clusters [7,[9][10][11][12][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Whether the origin of this lines is indeed related to DM physics [13,, or simply to imperfect modeling of atomic physics effects [9,12,32], the controversy surrounding it cannot belie the fact that precision observations of the X-ray sky are a prime tool to search for keVscale DM. ...
... Many such discussions were incited by recent claims for a yet-unidentified line at ∼ 3.5 keV in the X-ray spectra from galaxies and galaxy clusters [7,[9][10][11][12][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Whether the origin of this lines is indeed related to DM physics [13,, or simply to imperfect modeling of atomic physics effects [9,12,32], the controversy surrounding it cannot belie the fact that precision observations of the X-ray sky are a prime tool to search for keVscale DM. While the overwhelming majority of studies on this topic focus on line signals from DM decay, we will in the following explore the possibility that such signals arise from DM annihilation. ...
Article
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A long time ago (in 2014), in galaxies and galaxy clusters far, far away, several groups have reported hints for a yet unidentified line in astrophysical X-ray signals at an energy of 3.5\,keV. While it is not unlikely that this line is simply a reflection of imperfectly modeled atomic transitions, it has renewed the community's interest in models of keV-scale dark matter, whose decay would lead to such a line. The alternative possibility of dark matter annihilation into monochromatic photons is far less explored, a lapse that we strive to amend in this paper. More precisely, we introduce a novel model of fermionic dark matter $\chi$ with $\mathcal{O}(\text{keV})$ mass, annihilating to a scalar state $\phi$ which in turn decays to photons, for instance via loops of heavy vector-like quarks. The resulting photon spectrum is box-shaped, but if $\chi$ and $\phi$ are nearly degenerate in mass, it can also resemble a narrow line. We discuss dark matter production via two different mechanisms -- misalignment and freeze-in -- which both turn out to be viable in vast regions of parameter space. We constrain the model using astrophysical X-ray data, and we demonstrate that, thanks to the velocity-dependence of the annihilation cross section, it has the potential to reconcile the various observations of the 3.5\,keV line. We finally address the $\phi$-mediated force between dark matter particles and its possible impact on structure formation.
... Recently, few independent detections of a weak X-ray emission line at an energy of ∼ 3.5 keV have been found in a stacked XMM-Newton spectrum of 73 galaxy clusters with redshifts in the range 0.01 -0.35, in Chandra ACIS-I and ACIS-S spectra of Perseus [50] and in the XMM-Newton spectra of Andromeda galaxy and Perseus galaxy cluster [51]. Evidences of this emission have been also searched in Milky Way with data from XMM-Newton [52,56] and Chandra [53], in deep Suzaku X-ray spectra of the central regions of Perseus, Coma, Virgo and Ophiuchus clusters [54,55], in the stacked XMM-Newton spectra of dwarf spheroidal galaxies in the vicinity of Milky Way [57] and in the stacked spectra of a sample of galaxies selected from Chandra and XMM-Newton public archives [58]. Also, a re-analysis of XMM-Newton data of M31 was done in [56]. ...
... Evidences of this emission have been also searched in Milky Way with data from XMM-Newton [52,56] and Chandra [53], in deep Suzaku X-ray spectra of the central regions of Perseus, Coma, Virgo and Ophiuchus clusters [54,55], in the stacked XMM-Newton spectra of dwarf spheroidal galaxies in the vicinity of Milky Way [57] and in the stacked spectra of a sample of galaxies selected from Chandra and XMM-Newton public archives [58]. Also, a re-analysis of XMM-Newton data of M31 was done in [56]. Until now, none of these searches were able to establish precisely if the origin of this line is related to any known atomic transition in thermal plasma, with special interest in Potassium and Chlorine atomic transitions [52,53,56], or it is a signature of decaying dark matter [51]. ...
... Also, a re-analysis of XMM-Newton data of M31 was done in [56]. Until now, none of these searches were able to establish precisely if the origin of this line is related to any known atomic transition in thermal plasma, with special interest in Potassium and Chlorine atomic transitions [52,53,56], or it is a signature of decaying dark matter [51]. The evidence (or lack of evidence in some cases) of this emission line in different astrophysical sites placed strong constraints on both hypothesis, raising several controversies on this subject [59][60][61]. ...
Article
Few independent detections of a weak X-ray line at an energy of ~ 3.5 keV seen toward a number of astrophysical sites have been reported. If this signal will be confirmed to be the signature of decaying DM sterile neutrino with a mass of ~ 7.1 keV, then the cosmological observables should be consistent with its properties. In this paper we make a coupled treatment of the weak decoupling, primordial nucleosynthesis and photon decoupling epochs in the sterile neutrino resonant production scenario, including the extra radiation energy density via Neff. We compute the radiation and matter perturbations including the full resonance sweep solution for να/α → νs flavor conversion in the expanding Universe.We show that the cosmological measurements are in agreement with subdominant Dark Matter sterile neutrino resonant production with following parameters (errors at 95% CL): mass mνs=6.08 ± 3.22 keV, mixing angle sin2 2θ < 5.61 × 10−10, lepton number per flavor L4 = 1.23 ± 0.04 (L4 ≡ 104 Lνa) and sterile neutrino mass fraction fνs< 0.078.Our results are in good agreement with the sterile neutrino resonant production parameters inferred in ref. [1] from the linear large scale structure constraints to produce full Dark Matter density.
... The first was initially put forth by Ref. [1], proposing the exciting possibility that the line is due to the decay of sterile neutrino dark matter. The second possibility is astrophysical, suggesting that the size and impact of uncertainties in modeling the complex multi-phase plasma structure in clusters had been previously underestimated, and that one or more weak plasma transition lines could in fact be responsible for the signal [3]. Below we summarize these ongoing developments and present a novel spatial analysis method which aims to discriminate the two scenarios using existing archival XMM X-ray data. ...
... While relevant, Chandra observations of the Galactic center are significantly shallower than available archival XMM observations. In Ref. [3], two of us (TJ & SP) analyzed XMM observations of the center of the Galaxy, and discovered a line at an energy of about 3.5 keV. We pointed out that the detected line signal was compatible, at face value, with a dark matter decay origin, and calculated the corresponding preferred sterile neutrino lifetime and mixing angle 2 . ...
... In Ref. [3] we also re-analyzed, in the 3-4 keV energy range, archival XMM data from M31, and found no statistical evidence for a line at 3.5 keV. In addition, we pointed out that the procedure employed in Ref. [1] to predict the K XVIII line flux and the flux of other atomic transition lines was not, contrary to what stated in that paper, maximally conservative. ...
Article
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We test the origin of the 3.5 keV line photons by analyzing the morphology of the emission at that energy from the Galactic Center and from the Perseus cluster of galaxies. We employ a variety of different templates to model the continuum emission and analyze the resulting radial and azimuthal distribution of the residual emission. We then perform a pixel-by-pixel binned likelihood analysis including line emission templates and dark matter templates and assess the correlation of the 3.5 keV emission with these templates. We conclude that the radial and azimuthal distribution of the residual emission is incompatible with a dark matter origin for both the Galactic center and Perseus; the Galactic center 3.5 keV line photons trace the morphology of lines at comparable energy, while the Perseus 3.5 keV photons are highly correlated with the cluster's cool core, and exhibit a morphology incompatible with either dark matter decay or with axion-like particle conversions in the cluster's magnetic fields. The template analysis additionally allows us to set the most stringent constraints to date on lines in the 3.5 keV range from dark matter decay.
... After that, the same group [19] has presented another evidence for extra line at ∼3.5 keV by looking at the central part of our Galaxy. This study has been accompanied by claims of several other groups [17,18,20,21] that have not detected the extra line at ∼3.5 keV in several different datasets of dark matter objects. Basic properties of all these datasets are summarized in Table 1. ...
... But, these conclusions of [1,2] were questioned in [18], in which the authors argue that a) it is possible to explain new line in central part of our Galaxy and in combined dataset of [1] with contibution of K XVIII and Cl XVII lines 4 and b) extra line from M31 center seen by [2] can be lowered to <90% confidence by adjusting X-ray continuum over small energy range near the line (3-4 keV). ...
... The criticism of [18] have stimulated the immediate comment of [22]. Here, claim b) of [18] is repudiated by showing both that X-ray continuum of [18] selected at 3-4 keV is significantly overestimated at larger energies, and that the extra line flux is at least an order of magnitude less than expected from astrophysical lines near 3.5 keV. ...
Article
Full-text available
Recent works of [1402.2301,1402.4119], claiming the detection of extra emission line with energy ~3.5 keV in X-ray spectra of certain clusters of galaxies and nearby Andromeda galaxy, have raised considerable interest in astrophysics and particle physics communities. A number of new observational studies claim detection or non-detection of the extra line in X-ray spectra of various cosmic objects. In this review I summarize existing results of these studies, overview possible interpretations of the extra line, including intriguing connection with radiatively decaying dark matter, and show future directions achievable with existing and planned X-ray cosmic missions.
... In [3], a search was performed in the spectrum of Chandra observations of the central region of the Milky Way, finding no evidence of such line emission. More recently, two further analyses of the galactic centre spectrum using XMM-Newton data have been carried out [4,5]. These papers find mutually consistent results, but differ significantly in interpretations. ...
... Both analyses find evidence for a line around 3.5 keV. The authors of reference [4] argue that this line should be attributed to K XVIII emission at 3.47 and 3.51 keV, while the authors of reference [5] find that the line is consistent with a dark matter interpretation, while agreeing that the complicated galactic centre environment does not allow for definitive exclusion of astrophysical interpretations. In both analyses, the line flux per arcmin 2 observed with XMM-Newton is above the upper bound set by non-observations with Chandra in [3]. ...
... Furthermore, reference [4] also challenged the existence of a line in M31 (claimed in [2]), finding only 1σ support for such a line, and questioned the existence of an unexplained line from clusters, arguing that by including K XVIII lines at 3.47 and 3.51 keV and a Cl XVII line at 3.51 keV, no significant excess around 3.5 keV could be established after allowing for systematic uncertainties. In response, the authors of reference [2] pointed out in [6] that the lower significance of the 3.5 keV line for the M31 analysis in [4] was due to a restriction to an inappropriately narrow fitting interval of 3-4 keV, resulting in a relatively poorer fit for the index of the power law background and an inevitably lower significance for the line signal. ...
Article
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Motivated by the possibility of explaining the 3.5 keV line through dark matter decaying to axion-like particles that subsequently convert to photons, we study ALP-photon conversion for sightlines passing within 50 pc of the galactic centre. Conversion depends on the galactic centre magnetic field which is highly uncertain. For fields at low or mid-range of observational estimates (10--100 $\mu$G), no observable signal is possible. For fields at the high range of observational estimates (a pervasive poloidal mG field over the central 150 pc) it is possible to generate sufficient signal to explain recent observations of a 3.5 keV line in the galactic centre. In this scenario, the galactic centre line signal comes predominantly from the region with $z > 20$ pc, reconciling the results from the Chandra and XMM-Newton X-ray telescopes. The dark matter to ALP to photon scenario also naturally predicts the non-observation of the 3.5 keV line in stacked galaxy spectra. We further explore predictions for the line flux in galaxies and suggest a set of galaxies that is optimised for observing the 3.5 keV line in this model.
... [2,3] reported detection of a monochromatic line at 3.55 keV in galaxy clusters and in the Andromeda galaxy, which may be a signal of dark matter. The line has not been observed by several other studies [4][5][6][7], and its significance has been debated in the literature based on different estimates of systematics uncertainties on the X-ray background [5,8,9]. This detection, although still tentative, highlights the importance of and the opportunities in searching for dark matter via X-rays. ...
... [2,3] reported detection of a monochromatic line at 3.55 keV in galaxy clusters and in the Andromeda galaxy, which may be a signal of dark matter. The line has not been observed by several other studies [4][5][6][7], and its significance has been debated in the literature based on different estimates of systematics uncertainties on the X-ray background [5,8,9]. This detection, although still tentative, highlights the importance of and the opportunities in searching for dark matter via X-rays. ...
Article
Full-text available
We present a simple model of weak-scale thermal dark matter that gives rise to X-ray lines. Dark matter consists of two nearly degenerate states near the weak scale, which are populated thermally in the early universe via co-annihilation with slightly heavier states that are charged under the Standard Model. The X-ray line arises from the decay of the heavier dark matter component into the lighter one via a radiative dipole transition, at a rate that is slow compared to the age of the universe. The model predicts observable signatures at the LHC in the form of exotic events with missing energy and displaced leptons and jets. As an application, we show how this model can explain the recently observed 3.55 keV X-ray line.
... The expected line signal from ALPs is set by the product τ M 2 , where τ is the dark matter to ALP decay time. 3 In [5], it was shown that reproducing the observed signal strength of [1] from the sample of stacked galaxy clusters requires: ...
... Since this paper was written, the paper[3] claims that there is no significant line emission in M31, while in response[4] argues that this analysis is incorrect and the original claim of a 3.5 keV line in M31 still stands. It is beyond the scope of this paper to arbitrate between these competing claims.2 ...
Article
Full-text available
We further explore a scenario in which the recently observed 3.55 keV photon line arises from dark matter decay to an axion-like particle (ALP) of energy 3.55 keV, which then converts to a photon in astrophysical magnetic fields. This ALP scenario is well-motivated by the observed morphology of the 3.55 keV flux. For this scenario we study the expected flux from dark matter decay in the galactic halos of both the Milky Way and Andromeda (M31). The Milky Way magnetic field is asymmetric about the galactic centre, and so the resulting 3.55 keV flux morphology differs significantly from the case of direct dark matter decay to photons. However the Milky Way magnetic field is not large enough to generate an observable signal, even with ASTRO-H. In contrast, M31 has optimal conditions for ALP to photon conversion and the intrinsic signal from M31 becomes two orders of magnitude larger than for the Milky Way, comparable to that from clusters and consistent with observations.
... Hence, the DM parameter space opens up due to the absence of astrophysical X-ray limits [16][17][18][19]. Let us note that, in such a framework, the controversial 3.5 keV line [20][21][22][23][24] does not have a DM origin and the atomic physics explanation is favored [19,[25][26][27]. ...
Preprint
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We study the scenario in which the Standard model is augmented by three generations of right-handed neutrinos and a scalar doublet. The newly introduced fields share an odd charge under a $\mathbb{Z}_2$ parity symmetry. This model, commonly known as "Scotogenic", was designed to provide a mechanism for active neutrino mass generation as well as a viable dark matter candidate. In this paper we consider a scenario in which the dark matter particle is at the keV-scale. Such particle is free from X-ray limits due to the unbroken parity symmetry that forbids the mixing between active and right-handed neutrinos. The active neutrino masses are radiatively generated from the new scalars and the two heavier right-handed states with $\sim \mathcal{O}(100)$ GeV masses. These heavy fermions can produce the observed baryon asymmetry of the Universe through the combination of Akhmedov-Rubakov-Smirnov mechanism and recently proposed scalar decays. To the best of our knowledge, this is the first time that these two mechanisms are shown to be successful in any radiative model. We identify the parameter space where the successful leptogenesis is compatible with the observed abundance of dark matter as well as the measurements from the neutrino oscillation experiments. Interestingly, combining dark matter production and successful leptogenesis gives rise to strict limits from big bang nucleosynthesis which do not allow the mass of dark matter to lie above $\sim 10$ keV, providing a phenomenological hint for considered low-scale dark matter. By featuring the keV-scale dark matter free from stringent X-ray limits, successful baryon asymmetry generation and non-zero active neutrino masses, the model is a direct analogue to the $\nu$MSM model proposed by Asaka, Blanchet and Shaposhnikov. Therefore we dub the presented framework as "The new $\nu$MSM" abbreviated as $\nu\nu$MSM.
... Other explanations for the line are given in refs. . Irrespective whether the hints for the X-ray line persist [76,77] or disappear [78][79][80], keV sterile neutrinos are excellent WDM candidates, including keV sterile neutrinos with a mass below 10 keV. Thus we are studying the production of a keV sterile neutrino from a frozen-in scalar without restricting the frozen-in scalar mass to be larger than the Higgs mass. ...
Article
Sterile neutrinos with a mass of a few keV can serve as cosmological warm dark matter. We study the production of keV sterile neutrinos in the early universe from the decay of a frozen-in scalar. Previous studies focused on heavy frozen-in scalars with masses above the Higgs mass leading to a hot spectrum for sterile neutrinos with masses below 8 − 10 keV. Motivated by the recent hints for an X-ray line at 3.55 keV, we extend the analysis to lighter frozen-in scalars, which allow for a cooler spectrum. Below the electroweak phase transition, several qualitatively new channels start contributing. The most important ones are annihilation into electroweak vector bosons, particularly W -bosons as well as Higgs decay into pairs of frozen-in scalars when kinematically allowed.
... Finally, similar feature of systematic origin should be detected in the blank-sky dataset [69], and should have different radial behavior in the outskirts of Perseus cluster [69, 80]. On the other hand, the explanation of the new line with the K XVIII line complex at ∼3.5 keV suggested by [75] (see also an extensive discussion in [68, 86, 91, 92]) is still possible, at least for Galactic Centre region and galaxy clusters, contrary to initial claims of [68, 69]. The reason is that the emission flux from the K XVIII line complex at ∼3.51 keV suggested by [75] is highly uncertain due to large uncertainties of the Potassium abundance , see e.g. ...
Article
Full-text available
A number of observational studies claim detection or non-detection of the extra line in X-ray spectra of various cosmic objects dominated by dark matter -- gravitationally interacting substance that constitutes the major fraction of non-relativistic matter in the Universe. In this review I summarize results of these studies and especially the status of the detection of new emission line at ~3.55 keV in spectra of nearby galaxies and galaxy clusters, overview possible interpretations of this line, including an intriguing connection with radiatively decaying dark matter, and show directions achievable with existing and upcoming X-ray cosmic missions.
... An X-ray line signal has been observed at 3.5 keV [25,26]. We note that there is currently no consensus about the interpretation of this observation as arising from a dark matter signal [27][28][29][30][31][32][33][34][35][36][37][38]. Nevertheless, a large number of DM models have been proposed to explain this signal. ...
Article
Full-text available
We show that a couplet, a pair of closely spaced photon lines, in the X-ray spectrum is a distinctive feature of lepton flavored dark matter models for which the mass spectrum is dictated by Minimal Flavor Violation. In such a scenario, mass splittings between different dark matter flavors are determined by Standard Model Yukawa couplings and can naturally be small, allowing all three flavors to be long-lived and contribute to the observed abundance. Then, in the presence of a tiny source of flavor violation, heavier dark matter flavors can decay via a dipole transition on cosmological timescales, giving rise to three photon lines. The ratios of the line energies are completely determined in terms of the charged lepton masses, and constitute a firm prediction of this framework. For dark matter masses of order the weak scale, the couplet lies in the keV-MeV region, with a much weaker line in the eV-keV region. This scenario constitutes a potential explanation for the recent claim of the observation of a 3.5 keV line. The next generation of X-ray telescopes may have the necessary resolution to resolve the double line structure of such a couplet.
... ALPs that in turn decay into photons [42]. It has to be said that a study with the Chandra data of X-ray of the Milky Way did not show a conclusive evidence for the 3.5 keV line [43], and that other interpretations for the 3.5 keV line in terms of some specific Potassium and Chlorine lines were also suggested [44]. On the other hand it is argued in [45] that the interpretation of dark matter decay as the origin of the 3.5 keV line is consistent with the XXM-Newton dataset of the Milk Way center. ...
Article
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Light pseudoscalars known as axion like particles (ALPs) may be behind physical phenomena like the Universe transparency to ultra-energetic photons, the soft $\gamma$-ray excess from the Coma cluster, and the 3.5 keV line. We explore the connection of these particles with the inverse seesaw (ISS) mechanism for neutrino mass generation. We propose a very restrictive setting where the scalar field hosting the ALP is also responsible for generating the ISS mass scales through its vacuum expectation value on gravity induced nonrenormalizable operators. A discrete gauge symmetry protects the theory from the appearance of overly strong gravitational effects and discrete anomaly cancellation imposes strong constraints on the order of the group. The anomalous U$(1)$ symmetry leading to the ALP is an extended lepton number and the protective discrete symmetry can be always chosen as a subgroup of a combination of the lepton number and the baryon number.
... For more recent observations and analyses of the 3.55 keV line see[6][7][8][9][10][11][12][13][14] and the review[15]. Also see[16] for a discussion of compatibility of the DM→ a → γ model with these observations.2 ...
Article
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We further study a scenario in which a 3.55 keV X-ray line arises from decay of dark matter to an axion-like particle (ALP), that subsequently converts to a photon in astrophysical magnetic fields. We perform numerical simulations of Gaussian random magnetic fields with radial scaling of the magnetic field magnitude with the electron density, for both cool-core 'Perseus' and non-cool-core 'Coma' electron density profiles. Using these, we quantitatively study the resulting signal strength and morphology for cool-core and non-cool-core clusters. Our study includes the effects of fields of view that cover only the central part of the cluster, the effects of offset pointings on the radial decline of signal strength and the effects of dividing clusters into annuli. We find good agreement with current data and make predictions for future analyses and observations.
... A spectral feature of this kind has been recently identified in the combined spectrum of a large set of X-ray galaxy clusters [9] as well as in the combined observation of the Perseus Cluster and the M31 galaxy [10]. This signal can be accounted for by a rather broad variety of models of decaying DM as well as annihilating DM [40,41] although an astrophysical/instrumental origin is still feasible [42,43] as the line has not been seen in stacked spheroidal galaxies or galaxy groups [44,45]. ...
Article
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We investigate the possibility of reproducing the recently reported $3.55\,\mbox{keV}$ line in some simple decaying dark matter scenarios. In all cases a keV scale decaying DM is coupled with a scalar field charged under SM gauge interactions and thus capable of pair production at the LHC. We will investigate how the demand of a DM lifetime compatible with the observed signal, combined with the requirement of the correct DM relic density through the freeze-in mechanism, impacts the prospects of observation at the LHC of the decays of the scalar field.
... Recently, the detection of an unidentified spectral line at about 3.5 keV has been reported from two independent data sets [45,46]. Arguments in favour or against the dark matter decay interpretation of the signal can be found in [47,48,49,50]. If confirmed, that signal would provide compelling evidence for keV-scale dark matter and, in particular, for dark matter in the form of sterile neutrinos. ...
Article
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We show that novel paths to dark matter generation and baryogenesis are open when the Standard Model is extended with three sterile neutrinos $N_i$ and a charged scalar $\delta^+$. Specifically, we propose a new production mechanism for the dark matter particle --a multi-keV sterile neutrino, $N_1$-- that does not depend on the active-sterile mixing angle and does not rely on a large primordial lepton asymmetry. Instead, $N_1$ is produced, via freeze-in, by the decays of $\delta^+$ while it is in equilibrium in the early Universe. In addition, we demonstrate that, thanks to the couplings between the heavier sterile neutrinos $N_{2,3}$ and $\delta^+$, baryogenesis via leptogenesis can be realized close to the electroweak scale. The lepton asymmetry is generated either by $N_{2,3}$-decays for masses $M_{2,3}\gtrsim$ TeV, or by $N_{2,3}$-oscillations for $M_{2,3}\sim$ GeV. Experimental signatures of this scenario include an X-ray line from dark matter decays, and the direct production of $\delta^+$ at the LHC. This model thus describes a minimal, testable scenario for neutrino masses, the baryon asymmetry, and dark matter.
... The measured fluxes by XMM-Newton from Perseus (without the core), M31, and the MW are shown in Table I. The initial analyses have motivated a great deal of follow-up activity, including supporting evidence, null results, and proposed explanations in terms of line emission from ions [32][33][34][35][36]. ...
Article
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We consider a simple supersymmetric hidden sector: pure SU(N) gauge theory. Dark matter is made up of hidden glueballinos with mass $m_X$ and hidden glueballs with mass near the confinement scale $\Lambda$. For $m_X \sim 1~\text{TeV}$ and $\Lambda \sim 100~\text{MeV}$, the glueballinos freeze out with the correct relic density and self-interact through glueball exchange to resolve small-scale structure puzzles. An immediate consequence is that the glueballino spectrum has a hyperfine splitting of order $\Lambda^2 / m_X \sim 10~\text{keV}$. We show that the radiative decays of the excited state can explain the observed 3.5 keV X-ray line signal from clusters of galaxies, Andromeda, and the Milky Way.
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Sterile neutrinos with a mass of a few keV can serve as cosmological warm dark matter. We study the production of keV sterile neutrinos in the early universe from the decay of a frozen-in scalar. Previous studies focused on heavy frozen-in scalars with masses above the Higgs mass leading to a hot spectrum for sterile neutrinos with masses below 8-10 keV. Motivated by the recent hints for an X-ray line at 3.55 keV, we extend the analysis to lighter frozen-in scalars, which allow for a cooler spectrum. Below the electroweak phase transition, several qualitatively new channels start contributing. The most important ones are annihilation into electroweak vector bosons, particularly W-bosons as well as Higgs decay into pairs of frozen-in scalars when kinematically allowed.
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We present a Vector Dark Matter (VDM) model that explains the 3.5 keV line recently observed in the XMM-Newton observatory data from galaxy clusters. In this model, dark matter is composed of two vector bosons, $V$ and $V^\prime$, which couple to the photon through an effective generalized Chern-Simons coupling, $g_V$. $V^\prime$ is slightly heavier than $V$ with a mass splitting $m_{V^\prime}-m_V\simeq 3.5$~keV. The decay of $V^\prime$ to $V$ and a photon gives rise to the 3.5~keV line. The production of $V$ and $V^\prime$ takes place in the early universe within the freeze-in framework through the effective $g_V$ coupling when $m_{V^\prime}<T<\Lambda $, $\Lambda$ being the cut-off above which the effective $g_V$ coupling is not valid. We introduce a high energy model that gives rise to the $g_V$ coupling at low energies. To do this, $V$ and $V^\prime$ are promoted to gauge bosons of spontaneously broken new $U(1)_V$ and $U(1)_{V^\prime}$ gauge symmetries, respectively. The high energy sector includes milli-charged chiral fermions that lead to the $g_V$ coupling at low energy via triangle diagrams.
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The origin of neutrino masses and the nature of dark matter are two of the most pressing open questions of the modern astro-particle physics. We consider here the possibility that these two problems are related, and review some theoretical scenarios which offer common solutions. A simple possibility is that the dark matter particle emerges in minimal realizations of the see-saw mechanism, like in the majoron and sterile neutrino scenarios. We present the theoretical motivation for both models and discuss their phenomenology, confronting the predictions of these scenarios with cosmological and astrophysical observations. Finally, we discuss the possibility that the stability of dark matter originates from a flavour symmetry of the leptonic sector. We review a proposal based on an A4 flavour symmetry.
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