We don’t really know yet what Dark Matter is made of. This begs the question: is there a tell-tale signature that we may expect from any kind of Dark Matter? Indeed, we can look for annual modulations of possible Dark Matter interactions in our detectors.

We can assume Dark Matter has certain properties by astronomical observations. For example, Dark Matter does not interact with us electromagnetically.  We cannot hold up a magnet and collect Dark Matter, just like we cannot trap it in an electric field.  The only way we can definitely “see” Dark Matter is gravitationally, through methods like gravitational lensing.  Although we can’t actually see the Dark Matter, we can see light bending around it, which produces multiple copies of a galaxy, or even a whole ring of images!

The elliptical galaxy in the center is so massive that light bends around it. Many of the oblong images are copies of objects located behind this elliptical galaxy. For example, the ring-like smear towards the bottom of the galaxy is actually one luminous object. This phenomenon is referred to as a partial Einstein ring. (Image courtesy of the Hubble Space Telescope)

This mysterious light-bending substance is all around us.  One popular idea states that Dark Matter is comprised of small particles called WIMPs — Weakly-Interacting Massive Particles. Imagine our galaxy sitting in the middle of a cloud of Dark Matter. Since the Dark Matter does not interact with us much, we would move through this Dark Matter cloud freely. In doing so, we would then encounter a Dark Matter “wind”. Imagine being in a convertible driving on the freeway on a clear, wind-free day. You feel like the wind is blowing in your face. The wind sensation is caused by you moving through the air very quickly, quick enough to feel a “wind”.  The same is true of Dark Matter, we move through it very quickly and would expect to feel a Dark Matter “wind”. The tricky bit is that Dark Matter particles can pass through Earth constantly and we won’t feel them much, due to their feeble interactions.

The Earth moving through the Galaxy. While Earth moves around the Sun, the Sun moves around the Galactic center. To an observer outside our galaxy, we move faster in the summer than in the winter.

Just like in the convertible, we move through the Dark Matter with our spaceship, Earth. However, we do not always move through the Dark Matter at the same speed. During the winter, we actually move slower than we do in the summer time. Why is this? Well, on Earth, we orbit the Sun, and the Sun moves around the Galactic center. During the summer we move in the same direction as the Sun, whereas in the winter time we move in the opposite direction. To an observer sitting outside of our galaxy, we move faster in the summer time than in winter, even though we would observe ourselves moving at roughly the same speed during the year, relative to the Sun.

We know that we will feel a constant wind of Dark Matter if we stand still, but we know we are moving at a variable rate, depending on what time of the year it is.  If we can catch Dark Matter particles, we will catch more of them during the summer because there is more of this Dark Matter “wind” than in the summer. Just like in the convertible – The faster you go, the more head wind you experience! If we make a graph of how much Dark Matter we expect to see over the course of years, we expect it to modulate: that’s the the idea of annual modulation.

This graph represents seven years of data collected by the DAMA experiment. The particles detected had an energy of 2-4 keV. The y-axis is the number of particles detected while the x-axis is the time over which it was collected. The Roman numerals represent the year number. Is this dark matter? You decide! (Figure from Barnabei et al. 2003)

Since we know Dark Matter should modulate the question becomes: does it? Experiments like DAMA and CoGeNT have reported seeing modulated signal rates. However, there are many other things that modulate, including (but not limited to) cosmic rays, temperatures, ground water levels, neutrons and some have even proposed modulating radioactive decay rates. How do we tell all of this modulating background from our modulating Dark Matter signal? Well, these detectors are of course shielded by many layers of different material (including being put under a mountain!), but this is the crux of one of the important research questions we are exploring right now.  Can the signals reported by these experiments actually be due to Dark Matter? While these experiments are meticulously designed and the data is analyzed very carefully, it signals might be confused with some other modulating background. This is the crux of one of our experiments — the Modulation experiment. I will leave the details of that one to a future post, so keep checking back!

Dark Matter is hypothesized to account for the vast majority of matter in the Universe. Despite the fact that Dark Matter cannot be observed directly because it does not absorb or emit electromagnetic radiation in any significant quantity, its existence is deduced by the observed gravitational effects on visible matter. Many experiments are in progress to search for the yet undiscovered subatomic particle that make up Dark Matter. These experiments are categorized into three groups: direct detection, the search for Dark Matter particles scattering off nuclei in a detector, indirect detection, the search for Dark Matter decay products, and experiments that aim to produce Dark Matter in particle colliders. Here at Purdue, we are working on the XENON direct dark matter search.

Usually, a direct detection experiment operates in an underground laboratory to shield it from cosmic radiation and reduce background. These laboratories include, just to name a few: the SNOLAB underground laboratory in Sudbury, Ontario; the Canfranc Underground Laboratory in Spain; the Deep Underground Science and Engineering Laboratory in South Dakota; and, of course, the Gran Sasso National Laboratory in Italy which hosts our XENON project. One of two primary types of detector technologies is used in each of these locations are the cryogenic detector or the noble liquid detector. The cryogenic detector, which functions at incredibly low temperatures just fractions of a degree above absolute zero temperature, detects heat from collisions with a crystal absorber. A noble liquid detector looks for scintillation light signals that are emitted from particle collisions with liquid noble elements such as xenon or argon. Liquid noble elements are the preferred target for direct Dark Matter searches. They are easily scalable to targets of variable mass and shield from external background, in addition to allowing discrimination between nuclear recoil and electronic recoil. Liquid xenon is an especially appealing choice in Dark Matter searches because its high mass and large atomic number allow for compact design of detector geometry. A Time Projection Chamber (TPC) is able to reconstruct the 3D position of an interaction within its target volume, permitting a high degree of background rejection.

Particles can interact either with the electron shell of an atom, or with its nucleus. Radioactive backgrounds mostly interact with electrons, causing electronic recoils. Dark Matter on the other hand is expected to be heavier and thus to mostly interact with the nuclei, causing nuclear recoils. Image courtesy of University of California, Berkeley.

In order to be consistent with the state of the early universe, any candidate for a Dark Matter particle should have a small, yet measurable interaction probability, called cross section, with ordinary matter. The cross section between a neutralino particle and nucleon, in particular, is on the order of the electro-weak scale. This interaction combined with the projected range of the mass of a Dark Matter particle, 10-1000 GeV, bestows the name Weakly Interacting Massive Particle (WIMP) to possible Dark Matter particles.

Our planet is colliding every second with a cluster of these particles called the dark halo of the Milky Way galaxy. Cassie will write more about that in the next blog entry. The electro-weak scale is precisely what allows WIMPs to interact with ordinary matter. WIMPs are expected to scatter elastically off atoms, producing nuclear recoils and depositing energy. As such, the goal of direct Dark Matter experiments is to measure the energy deposited in the detector through collisions and discern what types of particles caused them. Tens of thousands of Dark Matter particles may be going through you as you read this post! With our detectors, we try to catch a few of them to be able to study their properties.