*** Original post can be found here. ***
Dark matter is a term that elicits both curiosity and confusion.
First posited in the 1930s, the case for dark matter has grown in the ensuing decades, as measurements consistently found that galaxies were forming and moving as if they had more mass than was visible. This invisible mass was dubbed, naturally, “dark matter,” because it neither absorbs nor gives off light.
Astronomers now believe that dark matter is the substance that gives the universe structure, guiding “normal,” visible matter into galaxies and larger structures, as well as dictating its motions. Although it can’t be seen directly, dark matter leaves an obvious mark through its gravitational influence on the cosmos. Dark matter’s gravity also acts as a magnifying lens, bending and distorting light as it passes in an effect called gravitational lensing.
The current standard cosmological model states that dark matter comprises about 27 percent of the known universe, outnumbering the normal particles of matter we’re able to observe and manipulate by six to one. That means all the information humans have ever been able to collect about the universe in which we live has only focused on 5 percent of that universe to date.
But how do you find something invisible?
The Hunt For Dark Matter
The Hunt For Dark Matter is a new CuriosityStream film that focuses on the current cutting edge of dark matter detection, one of the most important undertakings in astrophysics today. Premiering on May 4, it follows the progress of dark matter research, from the earliest indications of dark matter’s existence to the observational and experimental efforts ongoing today to definitively identify and characterize it.
Despite the fact that dark matter is required to produce the universe we see, no experiment has yet found this elusive substance. But from mountaintops thousands of feet above sea level to facilities tens of stories underground, researchers continue to search for signs, looking for the proof needed to fill in the missing piece of the puzzle. The Hunt For Dark Matter takes viewers on a comprehensive journey, exploring how particle physicists and observational astronomers alike are using every tool at their disposal to better understand the nature of dark matter and its influence on the universe around us.
Dark Matter and the Higgs Boson
Dr. Joseph Incandela of the University of California at Santa Barbara is part of the team that discovered the Higgs boson. He is associated with the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN). Incandela was the leader of the experiment during the Higgs discovery, which he announced alongside Fabiola Gianotti, leader of the A Toroidal LHC ApparatuS (ATLAS) experiment, on July 4, 2012.
But now that we know it exists, “to study the Higgs boson, we need more data,” he says. That requires increasing the beam intensities of the experiment — a condition that the current CMS detector can’t handle, in terms of coverage, resolution, and hardiness in such a high-radiation environment.
What does the Higgs boson have to do with dark matter?
As it turns out, plenty. The mass of the Higgs boson, while at a value that satisfies the needs of the standard model of particle physics, is nevertheless incredibly unlikely when basic quantum physics is considered. But invoking a theory called supersymmetry may explain why. Supersymmetry states that there is a “partner particle” for every normal particle we see. Although I won’t go into detail here, these partner particles provide an important balance that explains why the Higgs boson is as light as it is. And supersymmetry also calls for a lightest supersymmetric particle that is both electrically neutral and interacts only weakly with the matter around it — exactly the nature of dark matter.
If dark matter is a light supersymmetric particle, it might be created in collisions at particle accelerators, such as the LHC. While dark matter by nature will not interact with normal particles created during a collision, nor will it register on detectors (made of normal matter), it will still carry away energy and momentum, removing it from the equation. Thus, if researchers are able to very accurately measure the resulting mass and energy following an experimental collision, they may be able to spot “missing” energy attributable to the escape of dark matter.
That’s where the High Granularity Calorimeter (HGC) comes in. The HGC, which Incandela and his associates are developing, incorporates 40 layers of silicon and tungsten, and readout cells less than one square centimeter each. That’s a huge improvement over current detectors, which only have three to four layers and readout cells of two to three centimeters. The stunning capabilities of the HGC, both in terms of data accumulation and radiation survivability, will not only make it possible to probe areas of particle physics currently out of reach to delve deeper into the nature of the Higgs boson and other particles, it will also improve the capability to spot the absence of energy left behind from any particular proton-proton collision that leads to the production of dark matter when particle beams collide.
“The HGC will give us a view of things we’ve never had before,” says Incandela. “It’s almost like opening another wavelength or window, allowing us to see energy flow in unprecedented detail.”
The HGC is only one part of a larger project meant to upgrade the CMS experiment. Including labor, the estimated costs of these upgrades are nearly a billion dollars, with the HGC taking up roughly a quarter of that cost. But, as Incandela stresses, the HGC will fill several current needs at the facility, including the ability to spot dark matter produced in collisions. It will serve as a general workhorse in the discovery of new physics — one of CERN’s highest priorities.
A worldwide effort
Finding dark matter is a worldwide — and cross-disciplinary — effort. The search for dark matter has brought astronomers and particle physicists together, Incandela says, as researchers in both fields seek a common goal.
The CMS experiment represents only one branch of dark matter searches: the production of dark matter particles. In total, there are three branches, which include direct and indirect detection of dark matter. Direct detection experiments are perhaps the most commonly envisioned when the public hears about dark matter. These are the experiments buried deep underground, which search for signals such as bursts of phonons that result from dark matter particles hitting material in the apparatus. Indirect detections come from astronomical observations, in which astronomers search for signatures of dark matter-related processes in the cosmos.
Direct detection experiments, Incandela notes, can access a wide range of possible masses but need to make assumptions about the distribution of dark matter particles in our vicinity. By contrast, production-based experiments “have the potential to control just about everything,” he says, starting with the amount of energy that goes into the experiment but require much more expensive machines and detectors. If dark matter production is detected in an accelerator-based experiment, then there is the potential to really study its properties in more detail, especially with the abilities of new detector technologies like the HGC.
Why, then, is it worth maintaining so many different types of dark matter experiments?
Ultimately, all of these experiments are complementary, each contributing information that will bring to light the nature of dark matter. If a direct detection is made, researchers will know they’ve seen dark matter, but be unable to measure its properties in very great detail. Conversely, production-based experiments may create dark matter and measure its properties precisely, but may not know it’s dark matter they’ve found without a direct detection.
Right now, there are a dozen or more experiments actively searching for dark matter, and Incandela predicts that there may be many more experiments running in the next several years.
A final question: Why pour so much money, effort, and hours of research into such a feat?
Incandela stresses that “85 percent of the matter in the universe is unknown.” But he also believes “in just a few to 25 years, we’ll start interacting with [dark matter]. It could open up a new revolution in our understanding of nature.” He adds that “it’s an important part of human nature and culture to ask where we are and how we got here.” And dark matter, though elusive, is a vital part of the answer.
The hunt is on.