Tuesday, February 18, 2014

7 keV sterile dark matter?

It's a good day when you wake up and see the U.S. medal in your favorite Winter Olympic sport (SBX), and you see a blog post at Resonannces with a good discussion about a topic of interest: dark matter.
The Resonnances blog post discusses a manuscript by Bulbul et al. recently uploaded to Arxiv about X-ray emission lines ~3.5 keV that can't be attributed to known atomic spectra. The authors of the manuscript attribute the emission to sterile dark matter particles with a mass of ~7 keV. Though, it should be noted that there are other, less likely, explanations to the emission at 3.5 keV. The manuscript discusses some of the other possible explanations. As seen below in the graph at the Resonannces website, the emission line at 3.5 keV is consistent with other experiments, and in region of parameter space that has yet to be ruled out.

(Image from http://resonaances.blogspot.com/)

What I'd like to add to the discussion is that this value of dark matter mass is very close to the 95% confidence window from computer simulations by  Horiuchi et al., whose 95% confidence window as 6-10 keV in one set of data and 8-13 keV in a second set of data. (shown below)

While there's still a large amount of uncertainty about what is the cause of dark matter, it appears that there is starting to be some convergence between experiments and computational simulations. And I hope that the recently submitted manuscript by Bulbul et al. will convince NASA to fund more research into analyzing X-rays in the ~0.5 keV to ~5 keV range as possible signals of sterile neutrinos decaying into fertile neutrinos. Of course, the term sterile and fertile neutrino are misnomers because sterile neutrinos aren't completely sterile (w.r.t. to the weak nuclear force) or else they wouldn't be able to decay to normal neutrinos, and it should be pointed out that normal neutrinos, electrons and quarks are not always fertile (w.r.t. to the weak nuclear force) because as they zig-and-zag, they go between being fertile and sterile.

I also want to point out that it does seem intuitively strange that "mostly" sterile neutrinos are heavier than the "mostly" fertile, normal neutrinos. This seems to violate the trend that the fundamental particles with more mass also have more forces with which they can interact. Therefore, it's important to point out that there is still a lot fundamental physics that we don't understand, even if it turns out that dark matter is ~7 keV sterile neutrinos.

Update: Here's a link to a paper by a separate group that also found a 3.5 keV signal in the X-ray spectra from two galaxies.

Wednesday, February 12, 2014

Evidence for Massive Neutrinos, which also Interact with the Earth

Just want to highlight the following research paper by physicists in the UK.

Massive neutrinos solve a cosmological conundrum

They estimate that the sum of the masses of neutrinos is 0.32 eV +/- 0.081 eV.
You have access to APS journals, you can find their paper here.

It's unclear to me what is the connection between this group's findings and the 2-10 keV  particle that seems to explain dark matter. So, I welcome feedback in the comments section.

I'd also like to highlight some recent research from Japan, showing that solar neutrinos interact with the Earth. In other words, as the solar neutrinos pass through the Earth, they can be convert from one type of neutrino into another type of neutrino faster than if the neutrinos were travelling through a vacuum.
Pretty cool that, once again, predictions using the Standard Model were confirmed experimentally!

Wednesday, February 5, 2014

Recent Experimental Measurements of the Weak Nuclear Force: Implications for the Arrow of Time

I wanted to highlight some recent experiments conducted at the Jefferson Lab in Virginia. The group measured the interactions of electrons with quarks, and was able to measure the weak nuclear interaction between these particles with greater precision than any previous experiments. (I'll link to the journal article as soon as it is published.)

They quantified the breaking of the mirror (P) symmetry of the weak nuclear force. Though, it should be point out that this type of measurement is not new. It has been know for a long time that the weak nuclear force violations P, as well as T & CP symmetry.
My main point in highlighting this research is that this measurement was much more precise than previous measurements and that this measurement is in agreement with the Standard Model of physics. (i.e. most data for the Standard Model and more data that reduces the likelihood that there is Beyond Standard Model Physics at the <10 p="" scale.="" tev="">
My secondary goal in highlighting this research is to highlight that the weak nuclear force is present in collisions between electrons and quark, which means that it's present any time molecules collide with sufficient velocity. This in turn means that the weak nuclear force is most likely the cause of the arrow of time.

Notice that we never see an arrow of time when there's only Boson particles or when Fermi particles are interacting only via Gravity, E&M or the Strong Nuclear Force.
(Try determining which way a movie is running for the following phenomena: superconductivity, superfluid helium, photons travelling in the vacuum of space, or planets orbiting a star.)
The arrow time only exists when there are Fermions interacting via the weak nuclear force.

As such, it's important for us to recognize that Boltzmann's assumption of molecular chaos is not required in order to obtain time-asymmetric equations of motion. You just need to include the weak nuclear force (which occurs only when Fermions collide with sufficient energy.)

I also wanted to let readers know that I'm working on a Socrates dialogue between a defender of Boltzmann's molecular chaos assumption and a defender of the theory that the weak nuclear force is the cause of the arrow of time. I'm hoping that, after reading this dialogue, one will be able to see the problems with the assuming that the reason for the arrow of time is that there is molecular chaos (i.e. randomization of velocities after collisions.) This assumption is quite useful for most problem of engineering interest; however, it's doesn't actual teach us what is the real cause of the arrow of time. (And therefore needs to be scrapped and replaced.)

The real cause of the (one and only) arrow of time is one time asymmetric term that shows up in the weak nuclear force. This means that the real way to determine rate-based coefficients (such as diffusivity, thermal conductivity, and electrical conductivity) is to include the weak-nuclear force into computer simulations of molecular models. Assuming molecule chaos gets us pretty close to the right answer, but it's likely that there are some cases where we can do a better job in predicting transfer coefficients using first-principles than in making Boltzmann's assumption of molecular chaos.