Sunday, July 14, 2013

Is Dark Matter 2 keV Sterile Neutrinos?

In various previous posts, I've been summarizing the recent evidence for dark matter particles in the keV range. Last month, there was a conference in Paris (Chalonge Meudon Workshop 2013) devoted specifically to the question of whether "warm" dark matter could explain all of the current data collected by astronomers on the distribution of dark matter in the universe. The presentations from the conference can be found online here.

Here's my summary of the presentations from the conference:
(1) The evidence for dark matter particles with a rest mass of 2 keV is getting stronger by each day. The leading candidate particle that matches this rest mass would be a right-handed (sterile) neutrino. However, there are still many unanswered questions, such as how a right-handed, sterile neutrino has a rest mass that is heavier than left-handed neutrinos and how there are so many sterile neutrinos.
(2) It should also be pointed out that the evidence so far is indirect rather than direct evidence. "Direct evidence" would still be somewhat "indirect evidence" because dark matter particles are virtually non-interacting. But what I mean by "direct evidence" is evidence from Beta-decay experiments in lab in which one could potentially show that there is particle with a rest mass of 2 keV by comparing experimentally measured distributions of electrons energies exiting from the nucleus with models. What I mean by "indirect evidence" is the current astronomical data we have collected. This evidence is from the distribution of galaxies and the distribution of mass within galaxies. We can build models of what the distribution of mass would look like if the dark matter particle had various rest masses, and then we can compare the models with the experimental data. The conclusion from these models/experiments is that the rest mass of the dark matter particle is likely around 2 keV.

(3) We need still to collect data from Beta decays in order to determine the exact rest mass of the dark matter particle (see page 3 of the following set of slides from the conference.) These experiments should provide improved means of measuring the rest mass and to verify that the particle is a a sterile neutrino rather than some other type of possible dark matter particle. From the "indirect evidence" from distribution of dark matter in the universe, it's hard to get a firm grasp on the properties of the dark matter particle. The "direct evidence" from Beta decay will hopefully pin down important properties of the dark matter particle, such as rest mass, spin, and weak-nuclear-interaction parameters. While it may be possible to infer the values of rest mass, spin, and  weak-nuclear-interaction parameters from the astrophysical data, the results from Beta decay would provide corroboration. These properties are crucial for figuring out how to incorporate the dark matter particles into the Standard Model of Physics.

(4) Let me state why the evidence for cold GeV dark matter particles is vanishing day by day. First, we aren't finding evidence for dark GeV particles at particle accelerators, such as CERN. Second, the distribution of mass in the center of Galaxies predicted by cold dark matter theories is significantly different than experimental results. As such, we are getting closer and closer to being able to rule out supersymmetry and, hence, string theory (i.e. String Theory is Wrong, not just "Not Even Wrong.") Why is this good? I personally feel that most physicists have led themselves down a dead-end path of supersymmetry and string theory because the theories posited the existence of super-symmetric particles, for which there was absolutely no evidence. Over the last few years, we finally have collected the experimental data to prove these theories wrong. Now, we can re-focus our efforts into finding a single theory that explains General Relativity and the Standard Model of Particle Physics, i.e. focus efforts on theories that have measurable predictions and that don't predict that each particle has a partner.

If you all find more evidence for or against dark matter particles with rest masses of ~2 keV, please leaves links to the articles in the comment section below.

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