Dark Matter

Dark matter has been a hot topic in particle physics and cosmology for almost 80 years, since Swiss astronomer Fritz Zwicky first observed in 1933 a discrepancy between the amount of light emitted by a cluster of far-away galaxies, and the total amount of mass contained within that cluster as inferred from the relative motions of those galaxies, and simple application of Isaac Newton’s theory of gravitation.

The surprising result from Zwicky’s observation was that the vast majority of the mass in the galaxy cluster did not emit any light or electromagnetic radiation. This went contrary to the instinctive assumption that in such a large collection of galaxies, most of the mass would be carried by the stars. For instance, within our own solar system the Sun is more than 700 times heavier than all the planets and known objects orbiting it put together. It is not unreasonable to assume that the same would be true at the galactic and super-galactic scale.

Over 80 years of further observations, covering a wide range of cosmic scales and experimental techniques, have kept pointing out the same result and refining it. We now know with certainty that in the entire Universe, all the matter we know –the stars, planets, intergalactic gases, and other odd cosmic objects like black holes –can only account for less than 5% of the mass we know to be there. Five times more abundant is an unknown: a kind of matter that needs to be there to explain the gravitational behavior of galaxies but that we have never been able to see. It would be everywhere, permeating our galaxy and others like it, surrounding us at any given time with incredible density, yet stubbornly avoiding detection despite more than twenty years of dedicated efforts from particle physicists all over the world. Scientifically, this is both very frustrating and very exciting.

Many theories have been crafted to try to predict what this dark matter really is, and more are being proposed almost every week. Many of the most compelling predict the existence of a new kind of elementary particle, which would possess significant mass and interact with ordinary matter exclusively through gravity, and through the weak force, one of the four fundamental forces of nature which governs processes such as radioactivity. The weak force has an extremely short range, smaller than the size of a single nucleon, which would explain why dark matter particles are so difficult to detect: one has to literally bump head-on into an atomic nucleus of ordinary matter in order to leave any sign of its passage. Because an atom is essentially empty space (a nucleus surrounded by its electronic cloud is equivalent to a marble sitting at the center of a kilometer-wide sphere), and because a dark matter particle, albeit massive, is still extremely small, this is a very rare occurrence.

It is still statistically possible; the trick to dark matter detection is then to design a detector sensitive enough to register the tiny bump of a dark matter particle into an ordinary nucleus, but also discriminating enough to avoid confusing any other interaction with that of a dark matter particle. The latter part is extremely difficult, because of the overwhelming amount of radiation that every object is constantly subjected to, or indeed itself emits. The average human body emits thousands of gamma rays every second through natural radioactivity; an unacceptable background when one is looking for one tiny event, possibly as rare as a few per year in hundreds of kilograms of material.

Thankfully, most radiation can be stopped given a sufficient amount of shielding material, such as lead or even pure water. If one takes great care to select the materials used to build the detector (and the shield) for their very low radioactivity levels, pays attention to minimizing exposure during construction, and designs a shield of appropriate thickness, it is possible to build a very quiet detector.

The Large Underground Xenon Experiment

This shielding is a necessity when one is looking for very rare events such as dark matter particle interactions with ordinary matter. Unfortunately some backgrounds are much harder to shield against. At the surface of the Earth, cosmic rays originating from outer space are constantly bombarding us with a flux of about 100 per square meter per second. Those very high energy, charged subatomic particles are extremely penetrating and the only way to protect against them to the degree that is required for a rare event search such as a dark matter experiment is to put kilometers of material between them and the detector. That is why a location such as the Sanford Underground Research Facility (SURF) is particularly attractive. One mile underground, the flux of cosmic rays is reduced by a factor of a million compared to the surface, which makes them just manageable.

There are many additional design features one can employ to improve a dark matter detector’s sensitivity, and several technologies have been explored over the past 20 years. Currently the LUX dark matter detector is being operated at the Sanford Laboratory, in the new Davis laboratory which was completed in the spring of 2012. LUX is a time-projection chamber, a traditional detector design dating back to the 70s which allows 3D positioning of interactions occurring within its active volume. LUX uses as its target 368 kilograms of liquefied ultra-pure xenon, which is a scintillator: interactions inside the xenon will create an amount of light proportional to the amount of energy deposited. That light can be collected on arrays of light detectors sensitive to a single photon, lending the LUX detector a low enough energy threshold to stand a good chance of detecting the tiny bump of a dark matter particle with an atom of xenon.

Because the xenon is very pure, the amount of intrinsic background radiation originated within the target itself remains limited. Because xenon is three times as dense as water, it can stop a lot of the radiation originating from outside the detector before it can reach the very center; combined with the 3D positioning capabilities of a time-projection chamber, this allows the definition of a very quiet region in the middle of the target where to look undisturbed for those rare dark matter interactions. Because the LUX detector is larger than any other similar detector currently in operation, it can make maximal use of this “self-shielding” feature, while retaining sufficient active detector mass to accumulate statistics rapidly.

This is a key feature for current and future dark matter detectors. Since the early 90s, detectors have been getting bigger and more sensitive, as dark matter keeps eluding us and physicists are forced to look for ever more tenuous interactions. In order to reach the degree of sensitivity required for positive dark matter detection, an experiment must be able to pick out a few events per year in hundreds of kilograms of material. Without targets built at least on that scale, the amount of time required to stand a chance of even seeing one is simply prohibitive.

The first dark matter search results from LUX will be released before the end of 2013. The experiment will continue running and release further results into 2015.

This summary uses material first published in the DUSEL project newsletters for May 2011 and June 2011. © S. Fiorucci.

© Copyright 2013 Carlos H. Faham (chfaham-at-lbl-dot-gov)