There is nothing like observational data, or the lack thereof, to get physicists thinking. We have a fair bit of indirect evidence for dark matter, but we are not really sure what dark matter is. This uncertainty provides the sand for a theoretical physicist's playground. In a recent paper, a group of theorists, buckets and shovels in hand, have ventured into the playground and have created a pretty cool sandcastle. They have theorized that, if dark matter consists of weakly interacting massive particles, it may help create strange quark matter stars.

How does this work? Physics' Standard Model provides us with a set of basic particles and their interactions via various forces. During collisions, fundamental particles, like quarks, coalesce to form various other particles, like neutrons and protons. Most of these new particles are unstable and decay.

The probability of each type of particle being created in a certain collision is something that can be calculated. For instance, a collision between several particles has many possible outcomes, and the chance of any particular outcome depends on the circumstances of the collision.

Once the various flavors of quarks were discovered—or rather, inferred through theory and later found in experiments—and theorists went through these calculations, one result in particular attracted a lot of interest from cosmologists and astrophysics. A collision between up, down, and strange quarks could result in an absolutely stable form of matter—that is, it can't decay to any other form of matter. This strange matter is unlike neutrons and protons, which can decay into each other depending on the circumstances.

That leads to an interesting question: if a neutron star is big enough, are the internal forces sufficient to allow the neutrons to decay into strange matter, creating strange matter stars?

The answer is, apparently, maybe. The problem is that, for this conversion process to occur, you need a seed—one event to create the first bit of strange matter, which then allows the rest of the star's matter to undergo a similar transition with a much higher probability. Within the standard model of physics, the probability of such an event is thought to be rather low.

Luckily, for theoretical physicists, the standard model is not complete. So, the researchers took what we know about dark matter—which is mainly a list of things that we've excluded—and started calculating. The calculations involved understanding the balance between the gravitational attraction of the star, thermal evaporation of dark matter, and dark matter self-annihilation—two dark matter particles interact and decay into various new particles.

Now, thanks to rather good astronomical observations, we have upper limits on dark matter density and self-annihilation rates, and lower limits on the mass of dark matter particles. The theoreticians used the lower limit of the mass of dark matter particles to calculate the chances of different mass dark matter particles producing dark matter when they self-annihilated. They also calculated how much of a neutron star mass should be dark matter, but in this calculation they ignore dark matter density and, again, use the expected mass of dark matter particles.

In the end, they weren't quite brave enough to put these numbers together to come up with the half life of a neutron star to strange matter star conversion. That was a bridge too far for the more skeptical among us, I should imagine.

However, there are several interesting facets to this paper, most of which are basically mentioned in passing. First of all, neutron stars are crystalline in nature, although they contain defects. As these defects shift around, the rotation rate of the star changes. In the case of pulsars, this is as observed as a sudden change in the pulse frequency. Strange matter stars, on the other hand, are unlikely to be crystalline and will probably not exhibit sudden shifts in pulsar frequency.

The distribution of dark matter in galaxies is also rather well known. We should, therefore, find strange matter stars more often in the parts of the galaxy where dark matter is most dense. These are two great, albeit qualitative and tentative, predictions that could allow experimentalists to use existing all-sky surveys to either turn this sandcastle into something more lasting, or wash it back into the sands of possibility.

Physical Review Letters, 2010, DOI: 10.1103/PhysRevLett.105.141101

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