Thursday, October 24, 2013

Warm Dark Matter Still Works, But What Is It?

Increasingly overwhelming evidence from independent analysis of multiple, independent sets of astronomy data, by a substantial community of physicists, points to warm dark matter, with a mass that can be fit to a specific mass with considerable precision in model dependent indirect ways based on a good fit abundant data sets such as galactic rotation curves to a little bit more than 2 keV of mass, as the best fit solution to dark matter phenomena observed by astronomers in a non-baryonic dark matter particle paradigm.


Warm dark matter models work best with a very simple single fermionic dark matter particle type (i.e. spin 1/2, 3/2, 5/2 etc.) that interacts only via gravity, rather than mixed dark matter (at least in the current era), or a self-interacting dark matter species, although it isn't clear to me that one couldn't have like the other fermions, three generations of warm dark matter particles, only one of which is stable for more than a fraction of a second.

Precision electroweak high energy physics data from particle accelerators strongly disfavor the possibility that such a particle can be produced in the decay of W bosons (ca. 80 GeV) or Z bosons (90 GeV) and are on the verge of strongly disfavoring it is a possible decay product of a the Higgs boson (ca. 126 GeV). Thus, it is highly unlike that this particle interacts via the nuclear weak force. In these respect, warm dark matter particles would differ from every single other kind of massive particle (fermion or boson, composite or fundamental) in the Standard Model of Particle Physics.

Neither interactions via standard model electromagnetic interactions or nuclear strong force interactions are favored either.

It isn't at all obvious how one would from a practical perspective directly detect a particle with such a low cross-section of interation (virtually none except for Fermi contact forces that flow from the observation that two fermions can't be in the same place at the same time).

Warm Dark Matter requires beyond the Standard Model Physics

The warm dark matter mass favored by astronomy observations of about 2000 eV is far in excess of the experimentally favored masses of the known three neutrino mass states in the Standard Model, which are all in the vicinity of a range from 0.001 to 0.050 eV. And, they are far lighter than the lightest known fundamental fermion other than a neutrino, the electron which has a rest mass of about 510,000,000 eV. The lighest quarks (up and down) have masses similar in order of magnitude to the electron. All known and predicted composite particles in the Standard Model of Particle Physics are much heavier (the lightest is more than 100,000,000 eV in rest mass).

The Standard Model has no mechanism that would produce warm dark matter particles, and while one would describe them as "right handed neutrinos", the less presumptuous "sterile neutrino" description of these particles is a better fit as it doesn't imply that it is necessary part of the set of three generations each of four kinds of fermions of the Standard Model.

While it is trivial to use lamda CDM model constants to determine precisely how many such particles exist in the universe at that mass, this part of leptogenesis is strictly beyond the Standard Model physics.

Where could WDM fit in a BSM theory? Some conjectures.

I am rather inclined to see as promising ideas such as graviweak unification that seek a source for a fundamental warm dark matter particle in the gravitational side of a Theory of Everything, rather than the GUT side of a theory of everything (assuming that a GUT can be coaxed out of the Standard Model of Particle Physics).

For example, the notion of a spin-3/2 WDM counterpart to the spin-2 graviton is an attractive one with the anomalous spin both filling a gap in the roster of fundamental particles of the spin, and providing a supersymmetry-like counterpart to the graviton (the true spin-3/2 SUSY gravitino is basically excluded by LHC data, however, as are sterile neutrinos produced by active-sterile neutrino mixing), while possibly explaining why it cannot be produced (or at least detected) in decays of spin-1 particles like the W and Z, or spin-0 particles like the Higgs boson, while leaving open the potential for phenomenologically invisible weak force interactions of this particle.

In a simple sterile neutrino singlet model one might imagine it being produced in interactions between a high energy spin-2 graviton and a photon or Z boson (electric charge conservation rules out a W boson, color charge conservation would rule out a gluon) that produce two spin-3/2 WDM particles. The right conditions might arise frequently in the intense immediately post-singularity environment shortly after the Big Bang, but rarely thereafter, or only in the vicinity of supermassive black holes now.


Mitchell said...

I've noticed a few papers that are neutral or skeptical on WDM. It's a challenging subject...

andrew said...

The neutral paper is only looking at cosmological scales which is not what WDM is supposed to fix relative to CDM (I agree that gravity modification can do a lot of the same things).

The skeptical paper argues that Lyman alpha constraints demand a WDM mass heavier than the 2 keV give or take sweet spot of about 4-5 keV. I'm not convinced that the Lyman alpha constraints are really that stringent and other papers have argued for a Lyman alpha constraint of merely 2.3 keV or more, which is right in the sweet spot.

andrew said...

Also, both papers recognize the real problems with conventional CDM scenarios.

andrew said...

In terms of annihilation detection there are multiple issues there. Since WDM by definition have average relativistic momentum equal to or less than rest mass, an average annihilation energy, naively, would be less than 8 keV and due to the nearly collisionless nature of the WDM particles would be very sparse and distributed roughly proportionately to WDM generally, so virtually random in every direction. Preventing that signal from getting swamped by background would be extremely challenging and also requires assumptions about the kind of couplings that WDM particles make and their relative matter and antimatter composition that are nothing more than guesswork.

Even in the case of matter-antimatter pairs in the SM, not all annihilate into photons. For example, in the most analogous case of neutrinos and antineutrinos, they do not do so (directly at least), because they do not couple to photons since they lack electric charge. The equivalent to an annihilation reaction for neutrinos and antineutrinos would be a very Z boson decay, but only very energetic neutrino-antineutrino pairs have enough mass-energy for this to be a high frequency event.