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A SHOT IN THE DARK

Physicists are still searching for dark matter—the universe’s missing puzzle piece 

                                Professor Christopher Carone tries to modify the standard model of physics—a set of equations
                                                            that describes particle interactions—in order to explain the presence of dark matter.

Particle physicist Christopher Carone hopes to solve the mysteries of the universe without leaving his office. He doesn’t have a lab coat or high-tech science equipment. He wears a hooded sweatshirt and a martial arts wristband when he works. “I don’t do heavy-duty computational work, most of it’s pencil and paper,” he says.

Carone studies dark matter, a barely-detectable substance that explains the unexpectedly fast rotations of galaxies. The laws of gravitation determine the orbital velocities of the stars, dust and gases within a galaxy, but astronomers have observed that many galaxies spin faster than their equations predict. This suggests that these galaxies have matter that we cannot see—known as dark matter—providing the necessary mass to reach their high velocities.

Even though dark matter is five times more abundant than ordinary matter, it is difficult to study because it does not reflect light or react to electric charge. Since its existence was proposed in 1933, no human has been able to directly observe dark matter. Scientists know almost nothing about its composition, behavior, or mass—merely that it must have some mass in order to hold galaxies together.

How can Carone study such an elusive substance without leaving his office? He is one of many scientists attempting to modify the standard model of particle physics—a series of equations describing the properties of subatomic particles—in a way that would make room for dark matter. Particle physicists across the world propose modifications to the standard model on online preprint archives, but none have been experimentally confirmed. Around thirty articles are uploaded to these websites every day, and Carone has noted that a substantial portion usually involves dark matter. The absence of hard facts about dark matter leaves a lot of room for speculation. “Not much is known,” he admits. “This is why there are tons of theories.”

How do physicists sift through this abundance of theories? The Large Underground Xenon (LUX) experiment in South Dakota is currently underway, and it may disprove some of the existing dark matter theories. In the LUX experiment, scientists are observing an 800 pound bath of liquid xenon. Since dark matter is thought to be all around us, a stray particle may bump into the bath. Xenon is chosen for its large nucleus—a large target is easier to hit. Detectors surrounding the bath wait for a nucleus to bounce off a dark matter particle. Although dark matter is not detectable, scientists can reveal information about its mass, density, and interaction behavior by observing the movement of the scattered xenon particles.

Isolating dark matter is not easy. To a detector, a dark matter collision is indistinguishable from the random collisions of xenon atoms. Moreover, naturally occurring radiation can trigger signals in the detectors. The experimenters at LUX did their best to reduce false positives. First, they lowered the temperature of the bath to -160 degrees Fahrenheit, which slowed down the xenon particles and minimized collisions between them. Then they placed the xenon in a 70,000 gallon tank of water, which they placed in a mine shaft one mile below the ground, shielding the experiment from background radiation.

No experiment is perfect, though. Even with these precautions, xenon atoms bump into each other and some radiation slips through. “You need to show there is an excess over what could plausibly be due to background,” Carone explains. “And if you have an excess you have to have enough to show that statistically it’s not just a fluctuation in the background noise.”

After three months of data collection, the LUX experiment has seen nothing that can be differentiated from background radiation. “It’s a little frustrating,” Carone admits, “When you’re young and you go into science you hope that you will be there to witness some great revolution in our understanding of the universe. Really in particle physics it’s been a slow slog.”

Nonetheless, negative results from the LUX experiment help scientists rule out certain theories of dark matter. Most of these theories make predictions about the strength of collisions between dark matter and ordinary matter, so a negative result from the LUX experiment allows physicists to reject or constrain theories that anticipate a positive result.

As a theorist, Carone is dependent on experiments like LUX to confirm or reject his predictions. Without data to back up his ideas, a theorist is left with a bunch of pretty equations. “There are too many beautiful mathematical structures,” warns Carone, “You can go off on a tangent and spend your whole life following some idea that’s based on mathematical beauty. But there’s far more mathematical beauty than there is a correct theory of the universe.” For this reason, he focuses on topics like dark matter, where his theories will be testable in his lifetime.

Although the LUX experiment might rule out many popular dark matter models, Professor Carone is not particularly worried—in fact, he would be even happier if the data ruled out all of the leading theories. “It would be great if the findings forced us to throw away our preconceptions and come up with some completely different framework,” he says. While it may be tempting to hope that future experiments will confirm one of his theories, Carone realizes that the greatest scientific breakthroughs come from unexpected results.

 * This story was written as part of a course in the Environmental Science & Policy program at the College of William & Mary, ENSP 249: Science Communication. For more information on the course, please email the course instructor, Dr. Ibes at dcibes@wm.edu.

 

Sources

 Akerib, D., et al. 2014. First Results from the LUX Dark Matter Experiment at the Sanford Underground Research Facility. Physical Review Letters, Mar 4, 2014.

 Dark Matter: What we know and why you should care. The Washington Post. Nov 7, 2013.

 Bjaelde, O., S. Das, and A. Moss. 2012. Origin of Delta N-eff as a Result of an Interaction Between Dark Radiation and Dark Matter. Journal of Cosmology and Astroparticle Physics 10(17).

 Carone, C., Professor of Physics, College of William & Mary. cdcaro@wm.edu, 757.221.2451. Personal communication, in-person interview, Feb 6, 2014.

 Carone, C. and R. Primulando, 2011. Froggatt-Nielsen Model for Leptophilic Dark Matter Decay. Physical Review D 84(3).

 Griswold, Britt. What is the Universe Made of? National Aeronautics and Space Administration.