Sterile Particle Dark Matter Contradicts Observation

In a real 4th of July firework of a paper, the authors claim that dark matter particles that interact only via gravity is not consistent with observational evidence. 

Therefore, a gravity modification, a new 5th force, or a more complex dark matter particle model that includes dark matter that has some interactions with ordinary matter is required to explain the evidence. This very general finding, taken together with other exclusions of the dark matter particle parameter space (e.g. exclusions for MACHOs and SIDM theories), is a major boost for modified gravity theories that naturally produce a tight linkage between dark matter phenomena and ordinary matter distributions. 

The abstract and paper are as follows:

The more we go deep into the knowledge of the dark component which embeds the stellar component of galaxies, the more we realize the profound interconnection between them. We show that the scaling laws among the structural properties of the dark and luminous matter in galaxies are too complex to derive from two inert components that just share the same gravitational field. In this paper we review the 30 years old paradigm of collisionless dark matter in galaxies. We found that their dynamical properties show strong indications that the dark and luminous components have interacted in a more direct way over a Hubble Time. The proofs for this are the presence of central cored regions with constant DM density in which their size is related with the disk length scales. Moreover we find that the quantity ρDM(r,L,RD)ρ⋆(r,L,RD) shows, in all objects, peculiarities very hardly explained in a collisionless DM scenario.

Paolo Salucci and Nicola Turini, "Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017).

A December 2016 paper by Salucci acknowledges the strength of the correlations advanced by modified gravity proponents but asserts that it can have a particle rather than a modified gravity source (see also this 2016 paper he co-authors confirming the existence of a universal rotation curve for dwarf disk galaxies). Indeed, he is a co-author of a 2014 paper discussing the continued viable for R^N modified gravity theories.

A recent post at this blog surveys the evidence largely ruling out other particle dark matter theories, and largely confining viable theories remaining to those with stable or metastable dark matter particles that interact only via gravity (i.e. precisely the kind of theories that this paper rules out). Also, notably, the LHC ATLAS experiment has set strict bounds on Higgs channel dark matter production.

Salucci and Turini cling to the myth that a Dark Matter Particle theory rather than a modified gravity theory is necessary with no real scholarship research support, but their motivating exposition belies this claim (paragraph breaks and emphasis added):

The mass distribution in Spirals (and in any other galaxy) is largely dominated by a dark component. This comes from their kinematics and from weak and strong lensing effects that arise only in gravitational potentials dominated by such a component (Rubin (1983); Bosma (1981); Schneider (1996)). Moreover, the analysis of the CMB fluctuations spectrum and a number of cosmological measurements unavoidably point to a scenario in which a Dark Massive Particle is the responsible for the mass discrepancy phenomenon in Galaxies and Clusters of Galaxies (Planck Collaboration (2016)). Alternative scenarios to the Dark Matter do exist (e.g. Milgrom (1983)), but, in the light of the evidences reported above and of their inability to address the crucial issue of how galaxies did form, they are far less convincing than the DMP scenario. 

We associate, as usual, the huge local mass discrepancy in galaxies with the presence of surrounding halos made by a massive elementary particle that lays outside the HEP Standard Model (e. g.: see Bertone et al. (2010)). This particle also does not interact significantly with atoms, photons and with itself, through strong, weak and electromagnetic force. This does not strictly require that DMP must interact with the rest of the Universe only through gravitational force, but that, such eventual interaction must be much weaker with respect to the ordinary baryonic matter vs baryonic matter interaction. Moreover, no current observation prevents the existence of interactions between the dark and the luminous sector of elementary particles that result relevant in the galaxy formation context. However, so far, the simplest dark matter scenario has been routinely adopted, according to which the DM halos are made by WIMP particles, more precisely by collisionless cold dark massive particles that interact very feebly with themselves and atoms. These particles are thought to emerge in SuperSymmetric extensions of the Standard Model of Elementary Particles (e.g. Bertone et al. (2010)). 

Although large scale observations are in agreement with the predictions of this scenario, recently serious reservations are mounting against it. In fact, at the galactic scale masses of M < 10¹¹⁻¹²M , the predicted WIMP/ΛCDM dark matter halos are much more numerous than those detected and show very different structural properties with respect to those inferred by the internal motions of galaxies (e.g. see Salucci, F.-Martins & Lapi (2011)). The questioning issues for the WIMP particle are well known as the “missing satellites” ( Klypin et al. (1999)), the “too big too fail” (Boylan-Kolchin et al. (2011)) and the lack of a cuspy central density profiles in the DM halos (Gentile et al. (2004); Spano et al. (2008); Oh et al. (2011) and reference therein). There are proposals in which astrophysical processes could modify the predictions of the N-body ΛCDM models and the related density profiles to fit the observations (e.g. Vogelsberger et al. (2014); Pontzen & Governato (2012); Di Cintio et al. (2014); Read, Agertz, & Collins (2016)). However, this modelizations are growing in number and in diversity (Karukes & Salucci (2016)) and the cores formation via hypothetical strong baryonic feedbacks requires ad hoc fine tuning. Let us also remind that WIMP particles have not convincingly been detected in underground experiments (see e.g. Freese (2017)) and they have not emerged even in the most energetic LHC proton-proton collisions (e.g.CMS collaboration (2017)). Finally, the X and gamma ray radiation coming from annihilating WIMP particles at the center of our and nearby galaxies has not unambiguously been detected ( Freese (2017), e.g. Albert et al. (2016); Lovell et al. (2016)). Thus, to claim that ΛCDM is not anymore the forefront cosmological scenario for dark matter will bring no surprise. 

Recent alternative scenarios for dark matter point to a sort of significant self-interactions between the dark particles which seems suitable to explain the observational evidence which has created the ΛCDM crisis. Among those, the Warm Dark Matter, the axion as a BoseEinstein condensate and the self-interacting massive particles scenario (e.g. Freese (2017); Krishna et al. (2017); Suarez et al. (2014); de Vega, Salucci, & Sanchez (2014)) are the most promising. Their common characteristic is that, at galactic scales, dark matter stops to be collisionless and it starts to behave in a way which could make it compatible with observations. However, also these scenarios hardly explain the fact that we continue to find that in galaxies, dark and luminous matter are extremely well correlated (e.g. Gentile et al. (2009)). Thus, we have to envisage the possibility of a direct interaction between the dark particles and the galaxy atoms and photons leading to major cosmological/astrophysical consequences. In fact, the dark-luminous coupling that emerge in spirals is so intricate that it is extremely difficult to frame it in a scenario in which the dark and the luminous galactic components are completely separated but through their gravitational interaction.

Their pre-print concludes as follows, mostly proposing new kinds of dark matter particles not previously hypothesized.

Rotation curves studies on a large sample of spiral and dwarf galaxies show an extraordinary correlation between Dark and Luminous matter over many order of magnitude in halo masses. The idea of a dynamical evolution of galaxies, in the Hubble time period, driven only by gravity is failing the explanation of such a deep-rooted correlation. Moreover, the large discrepancy of DM densities in the inner part of the galaxies from the outcome of N-Body ΛCDM simulations and the actual measured data cannot be explained only by astrophysical phenomena without a fine tuned modelization, that very likely could not account for the mounting scenario of universal correlations. Warm Dark Matter models are certainly nearer to the latter, however they cannot account easily of the universality of its internal structure. In this paper we also show that the quantity ρ(r)ρ∗(r) tend to be almost the same on all the galaxies as the DM pseudo pressure reaches the maximum close to the Core Radius. The same Pressure has the same value no matter the galaxy dimension. This density product in fact is proportional to the interaction probability of the the two components, Luminous and Dark matter, and account for a direct interaction between them. This is hardly a coincidence, in that, the quantity like KSA = ρ 2 DM(r) which is proportional to the self interaction of the DM component is varying all over in galaxies and among galaxies. 

Therefore, we claim that the structure of the inner parts of the galaxies is driven by a direct interaction between Dark and Luminous components. The DM central cusp, foreseen for any heavy collisionless DM dark matter particle and also in many other cases, with an increase of DM pressure at lower radius, gets, as time goes by, progressively eaten up/absorbed by the dominant luminous component. The interaction flattens the density of DM and drops the pressure towards the center of the galaxy. 

SuSy, if ever appears, has a large energy scale that could in principle lead to neutralino masses in the TeV region. We estimated an absorption cross section for a heavy DM particle from the luminous component to be from 3 to 5 order of magnitude larger than foreseen by SuSy prediction. Current prediction for a minimal Dark Sector zoology based on a (U1) symmetry, with heavy Dark Fermions and a mediator Dark Photon, (e.g. Marciano (2015); Ringwald (2014); Alexander et al. (2016); Harigaya & Nomura (1996)) cannot address the full phenomenology described in this paper. A direct coupling between Dark Fermions and SM particles, in presence of luminous matter, should take place allowing the decay of the heavy Dark Matter particles into light ones, SM or other. Heavy Axion Like Particles (ALP) are another DM candidate. Finally There can be in principle also be mediators between the two sectors, such as the Higgs. 

The estimated absorption cross section is low enough to make direct DM detection experiments based on nucleus recoil fail, the recoil could not happen at all and the experiments should see large energy showers, that if they are initiated by electrons or photons can be confused with high energy neutrino showers. On the contrary, exotic searches at LHC, with the integrated luminosity already taken and foreseen in the next years, can confirm such picture. Production cross section at high energy should be big enough to have the DM particles produced in detectable quantity. Moreover, the positron excess in our galaxy, if from DM interaction, points to a TeV scale mass particle that might be related with our proposal. LHC searches are not calibrated to detect such type of invisible particles, the method based on missing transverse energy and momentum asymmetry, is not very sensitive if the production cross section is of the femtobarn order of magnitude. The large QCD background is masking all the events. Missing mass detection done with more sophisticated apparatus such as the forward spectrometers CT-PPS and AFP, respectively in CMS and Atlas, can enhance the detection probability if the production is initiated by photon-photon interaction. Also triggering algorithms can be optimized for such a search, right now they are focused on SuSy production and exotics searches are done with low efficiency. 

The DM scenario that arises from this picture points to a DM sector different from any predicted so far. Neutralino interaction cross sections foreseen by SuSy cannot account for the absorption rate we measure. So a new DMP or a more complex DM sector should appear. We yet don’t have any hint on how the coupling with matter is, surely nothing yet envisaged in the present DM theoretical panorama. Model independent searches have to be pursued in LHC. While in direct searches underground the detection could be made looking to the appearance of particle showers in large mass detectors as neutrino telescopes (Ice Cube, Antares...). A diffuse gamma ray signal with large energy, above 100GeV, with a cutoff spectrum, could be correlated to DMP absorption.

Bekenstein's extended MOND model (called TeVeS), Moffat's models, and the toy model proposed by Deur, in contrast, all offer viable and reasonable well motivated modified gravity theories to explain dark matter.

UPDATE July 7, 2017:

New exclusions from direct detection experiments have been placed on the parameter space of light WIMPS (under 3 GeV) in continuing evidence discrediting the WIMP hypothesis.