The Stuff that came in from the Cold

by Roger Dixon
Fermilab physicist,
CDMS Project Manager

Despite the best efforts of particle physicists and astrophysicists, most of the Universe is still missing. We know where it is, but we don’t know what it is. It is all around us, but we can only see it by looking far, far away. That is the challenge for the Cryogenic Dark Matter Search (CDMS).

In its quest to find some of the dominant but invisible component of matter in our universe, CDMS (E-891) will make the first serious incursion into the realm of parameter space where physicists believe they have a reasonable chance of detecting and studying dark matter in an earthbound laboratory. If we do find Weakly Interacting Massive Particles (WIMPs) thought to make up the dark matter, it will be largely due to state-of-the-art detectors, which give CDMS a substantial advantage over our competitors. The complex cryogenic system installed by a Fermilab team in the Soudan Laboratory is no small part of making those detectors work. This system pushes the edge of the envelope even by Fermilab standards.

Where most supercooled experiments at Fermilab run near 4.5 degrees Kelvin, the CDMS cryostat will chill our germanium and silicon detectors to 25 milli-Kelvin—25 thousandths of a degree above absolute zero. And it will reach this temperature frontier at the bottom of an old iron mine, nearly a half-mile underground in Soudan, Minnesota.

Rich Schmitt and his crew, including Bruce Lambin, Bryan Johnson, Rodney Choate, and Jeff Duncan, have done some of the most unusual work. They’ve put in long, hard hours in the north woods, for weeks at a time, in order to make this delicate system a dependable everyday reality. In late fall and winter, they sometimes don’t see the sun, or even a cloudy sky, for more than a week at a time. Other essential Fermilab contributions include design and fabrication of the front end electronics, an effort led by Mike Crisler with the help of Merle Haldeman. The Laboratory has also been responsible for the CDMS infrastructure that is being built at Soudan, a difficult job headed up by Lou Kula and Stan Orr. Finally, Fermilab has also made substantial contributions to the data acquisition system. Don Holmgren of the Computing Division has taken the Fermilab lead on CDMS for this task.

Thanks to these efforts, and the efforts of all the CDMS collaborators at 10 other institutions, we have a chance to answer the question Discovery magazine recently placed at the top of its list of “The 11 Greatest Unanswered Questions of Physics:” What is dark matter?

But first, a little history, ‘way back to 1996.

We were beginning to run CDMS I at Stanford and planning a version of the experiment, CDMS II for the Soudan Laboratory. At the time, physicists were beginning to feel that they understood the makeup of the universe. Most cosmologists believed that the universe was balanced at the critical density between expanding forever and re-collapsing: sort of like standing a pin on a table, and then balancing the universe on the head of the pin. That scenario is actually more plausible than it sounds (or maybe I have been in this business too long).

Dark matter had been an accepted concept for a long time because physicists wanted to believe that our notions of gravity were correct. We even had a good idea about how much of the universe was ordinary matter (baryons), and how much was something mysterious. In addition, observational evidence from astronomers, including the rotation curves of the galaxies, gave a good indication for how much dark matter there had to be to make the theories hold together. The inventory was something like one percent visible matter, five percent baryons, and 95 percent something else—dark matter. The picture seemed to be coming together—if we could just determine what the something else was.

But surprises are the motivators for experiment-alists. In 1998, two experiments (the Supernova Cosmology Project, based at Berkeley Lab and headed by Saul Perlmutter; and the High-Z Supernova Search Team led by Brian Schmidt of Australia’s Mount Stromlo and Siding Spring Observatories) set out to measure the density of the universe using Type IA supernovae as “distance candles.” They compared the luminosities of near and far supernovae to establish their distances independent of red shift—then compared the red shifts to see how fast each was moving away from us due to the expansion of the universe.

They expected to see the gravitational effect of the mass of the universe slowing the expansion, and, therefore, the supernovae. Instead, their data indicate that supernovae are not slowing down: they are speeding up. Everyone was skeptical at first, but for the experimentalists, it was a golden period. We were overjoyed to see the theorists sufficiently frazzled to arrive at work unshaven, with untied shoes and mismatched socks.

Looking back, it was actually fairly easy to incorporate the new information within existing theories, but it did mean a revision of the inventory of the universe. It was necessary to incorporate something that was causing this acceleration. To this day we don’t really understand what the “something” is, but that does not prevent us from naming it. Dark energy must make up 60 to 70 percent of the energy density of the Universe. This revision reduces the amount of dark matter to around 20 or 30 percent of the universe, with the baryon content remaining at around five percent—plenty of mysterious stuff left around for everyone.

The longstanding rotation curves tell us what the density of matter, and hence dark matter, must be for our own galaxy to rotate the way it does. Since these curves were used in the initial planning for CDMS, the new information from the supernova experiments did nothing to change our plans. The curves tell us the density of dark matter comes out to about 0.3 GeV per cubic centimeter. Now we don’t know precisely the mass of these dark matter particles, but accelerator experiments give good

indications that the mass is likely to be more than 50 GeV if they are supersymmetric. At 50 GeV we would have roughly 10,000 particles passing through the passenger compartment of a good-sized Buick at any given time. The germanium and silicon detectors, which are being fabricated at Stanford University, are designed to be sensitive enough to detect weakly interacting dark matter particles of such abundance. These detectors have already been used by the CDMS collaboration at a shallow site underneath the Stanford campus to set the best dark matter limits in the world. That bodes well for the sensitivity they will have at Soudan. The bottom line is that, if the dark matter interacts weakly, it is in our crosshairs. And if we do see something, then we can say: “Here it is, this is its cross-section, this is its mass, now let’s go study it in the Tevatron.” Furthermore, observational data from astronomers looking at the universe with infrared, visible light and x-rays continues to support the hypothesis that the dominant form of matter in the universe is dark matter. For example, the Sloan Digital Sky Survey has done some exquisite work with visible light and gravitational lensing to measure the distortion in faraway galaxies to show that there is lots of dark matter everywhere. In addition, many observations including recent results from the Chandra X-Ray Observatory confirm the abundance of dark matter. So what is dark matter, and how do we detect it in the Laboratory, and how will we know we’ve found what we’re looking for, when and if we find something? The possibilities for detecting it are two-fold and complementary. The big collider experiments at Fermilab will be looking for a mysterious form of matter, called supersymmetric particles, to show up in the collisions they observe. There are good theoretical reasons for believing, not only that this form of matter could exist, but that it could also be the dark matter. If it is first observed in the Collider, it will be up to CDMS to show that it exists in nature and has the correct properties. If it is first observed in CDMS, it will be left to the collider experiments to measure its more subtle properties and determine whether it is the supersymmetric particles or not. Either way, Fermilab has a great opportunity to take a giant step in our understanding of our universe, its evolution and its makeup.

Chandra Probes Nature of Dark Matter The cluster of galaxies EMSS 1358+6245 about 4 billion light year away in the constellation Draco is shown in this Chandra image. When combined with Chandra's X-ray spectrum, this image allowed scientists to determine that the mass of dark matter in the cluster is about 4 times that of normal matter. The relative percentage of dark matter increases toward the center of the cluster. Measuring the exact amount of the increase enabled astronomers to set limits on the rate at which the dark matter particles collide with each other in the cluster. This information is extremely important to scientists in their quest to understand the nature of dark matter, which is thought to be the most common form of matter in the universe. Scale Image is 3.8 arcmin on a side. Category Galaxy Cluster Coordinates (J2000) RA 13h 59m 50.60s | Dec +62º 31' 04" Constellation Draco Observation Date Sept 3-4, 2000 Observation Time 55 ksec Color Code Intensity Instrument ACIS (Courtesy Chandra X-Ray Observatory Center)

A Cryostat on the Edge