Blog Post 9, WIMPs

In Worksheet 3.1, Milky Way Rotation, we looked at the reason behind dark matter theories. If we calculate the amount of mass in the Milky Way Galaxy using Kepler’s Third Law and the observed orbital rotation velocity of objects within the galaxy, we find that the mass of the galaxy should be approximately \(10^{12} M_\odot\). However, the stellar mass of the galaxy as observed is approximately \(10^{10} M_\odot\), or only around 1% of the expected mass. The conclusion, therefore, is either that our estimation using Kepler’s 2rd law and Newtonian mechanics is wrong, or there must be some form of matter hidden in the galaxy that we cannot detect, but gravitationally affects all other matter in the galaxy. This “hidden” matter is known as dark matter. For this blog post, I was interested in learning about one of the major theories for what dark matter is – WIMPs.

WIMPs

WIMP stands for weakly interacting massive particles. The idea is that a WIMP is a non-baryonic particle (i.e. not composed of protons or neutrons), with a mass in a range from 1 GeV – 1 TeV, that only interacts through the electroweak interaction. I’ve never taken a course in particle physics, but from my understanding this means that these particles are extremely hard to detect, because they don’t interact with any of the matter we observe.

The theory behind the existence of WIMPs is that in the early universe, there were equal amounts of photons and WIMPs, or particles and antiparticles, and because of the extreme high temperature of the universe, these particles could be created and annihilated freely. As the temperature cooled to a point below the mass of the WIMPs, they could no longer be created, but could still be annihilated by joining with another WIMP. The density of WIMPs dropped until they had a density such that it was extremely unlikely that they would run into another WIMP, and that’s what the density is now.

From what I’ve read so far, the really fascinating part about WIMPs is that they align very coincidentally with the current theory of supersymmetry. Supersymmetry, from my understanding, is the theory that for every known particle in the universe, there is an “antiparticle” or supersymmetric paired particle. For example, the pair to a photon is photino, and the pair of the quark is the squark. Apparently, the predicted density of dark matter needed to fill the matter gap between our observations and our calculations fits the predicted density of the WIMPs that supersymmetry predicts.

As WIMPs supposedly interact through the weak force, and not the electromagnetic force, they have yet to be detected in the form that supersymmetry predicts. There are several types of experiments that are attempting to detect these particles.

One method attempts to directly detect WIMPs. WIMPs interact through the weak force, and are thus not usually detectable, even though there are theoretically several hundred going through one's body at any given moment [Source 2]. However, every now and then a WIMP might interact with an atomic nucleus by "scattering" [Source 1] off of it, causing the nucleus to recoil and possibly vibrate. This scattering is extremely difficult to detect, however, because this interaction is extremely rare and unpredictable.

Source: http://cdms.berkeley.edu/Education/DMpages/science/directDetection.shtml

Another method attempts to indirectly detect WIMPs. By the current theory, as WIMPs have been around for quite a long time, and therefore gravity must have caused some WIMPs must have settled at the core of the Earth and the Sun by now. As the density of the WIMPs at these cores increases, the possibly of these WIMPs colliding and annihilating to form neutrinos increases as well. Thus, scientists are attempting to detect the presence of neutrinos, formed by this annihilation, streaming out from the Earth’s and Sun’s cores.

Neither of these methods have yet to confirm the existence of WIMPs, but as scientists across the globe continue to experiment, they continue to explore and eliminate possibilities for the locations of WIMPs, hopefully drawing closer to their real location (or not, in which case it’s on to another theory).

Sources:

1. https://www.astro.umd.edu/~ssm/darkmatter/WIMPexperiments.html

2. http://www.astro.caltech.edu/~george/ay20/eaa-wimps-machos.pdf