Melting Protons at CERN

By Andy Fell
Published on September 18 in Driven by Curiosity

Physics Professor Manuel Calderón de la Barca Sánchez and his students are using the largest and most complex machine ever built to probe the strongest force in nature at the tiniest of scales.

“In a nutshell we’re trying to recreate a state of matter that existed about a microsecond after the Big Bang,” he said.

Calderón de la Barca Sánchez and graduate students Graham Waegel, Jared Jay, Ota Kukral and Santona Tuli are among a team of about 100 scientists running “heavy ion” experiments with the Compact Muon Solenoid detector, part of the Large Hadron Collider at CERN, Switzerland. They are trying to create a state of matter called the quark-gluon plasma by smashing nuclei of lead atoms into each other.

Physics Professor Manuel Calderón de la Barca Sánchez (center left) and (left to right) Graham Waegel, Ota Kukral and Jared Jay, all physics graduate students in the College of Letters and Science, are studying the strongest force of nature at the Large Hadron Collider at CERN, Switzerland. (UC Davis photo)
Calderón de la Barca Sánchez (center) and graduate student Santona Tuli (foreground) working on data at CERN. (Secrets of the Universe Production Photo)


Their research, supported by the National Science Foundation, is the focus of a new IMAX movie, “Secrets Of The Universe,” which premiered in July 2019. Calderón de la Barca Sánchez narrates the movie as it takes viewers inside CERN and on a journey through science.

The Ancient Greeks proposed that matter was made up of “indivisible” atoms. Hundreds of years later, scientists showed that atoms were not indivisible at all: electrons could be pulled away from them. In the twentieth century physicists showed that the atomic nucleus could itself be split up into protons and neutrons, and then that these particles could themselves be broken up into fundamental particles called quarks.

Calderón de la Barca Sánchez during filming of “Secrets Of The Universe,” a new IMAX movie that explores the leading edge of modern science. (Secrets of the Universe Production Photo)

Quarks are bound together by the “strong force.” Carried by particles called gluons, it’s the strongest force of nature but operates only at the tiniest of scales.

“What we are trying to do is heat up matter such that we are able to study things that are inside protons and neutrons, so we can see how quarks and gluons acted at the time of creation,” Calderón de la Barca Sánchez said. This melted state is called the quark-gluon plasma.

Melting a proton requires enormously high temperatures – a hundred thousand times that of the core of the sun, but on a tiny scale. That can only be achieved with a couple of colliders in the world: CERN’s Large Hadron Collider, and the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York.

Colliding heavy ions

Most of the time, the Large Hadron Collider smashes streams of protons into each other. That high-energy work led to the discovery of the Higgs boson in 2011 and continuing searches for dark matter particles and evidence of a new theory called supersymmetry. A number of UC Davis physicists are involved in those efforts, alongside thousands of scientists from around the world.

Calderón de la Barca Sánchez’s team has a different approach. Their “heavy ion” experiments accelerate and collide streams of lead nuclei. These collisions cannot reach the same overall energy as proton-proton collisions, but because the nucleus of a lead atom has 82 protons and 126 neutrons, you get a lot more individual collisions when two lead nuclei hit each other.

The five-story, 14,000 tonne Compact Muon Solenoid is one of four main detectors in the Large Hadron Collider at CERN, Switzerland. (CERN photo)

Those collisions are detected with the Compact Muon Solenoid, a five-story, 14,000 tonne device that is one of four major detectors in the Large Hadron Collider. The CMS is the primary experiment for U.S. researchers working at CERN. Physicists from the UC Davis College of Letters and Science were among the first to sign on to the CMS experiment back in 1992 and have collaborated in designing and building the machine.

The large number of collisions allows the researchers to study the thermodynamics of quark-gluon plasma. Calderón de la Barca Sánchez compared it to studying water freezing or melting.

“If you want to study the transition from solid to liquid water, you would look at a lot of water molecules, not just one molecule,” he said.

A heavy ion experiment from the Compact Muon Solenoid. Colliding streams of lead nuclei creates a soup of quarks and gluons. (CERN photo)

The strong force is different from other fundamental forces such as electromagnetism and gravity because it gets stronger with distance and weakens as objects get closer. So scientists had thought that at a very small scale, the quark-gluon plasma should act like a gas. Instead, they found that it’s more like a liquid.

“What we are making is quark soup,” Calderón de la Barca Sánchez said. “We’ve been trying to unravel what this means for the past ten years.”

It’s the most basic of fundamental research, but the work at CERN also brings real-world benefits. Most of the world’s particle accelerators are now in hospitals, where they are used to treat tumors or in medical imaging. CERN scientists created the original software for the World Wide Web as a way to collaborate and organize documents: there are numerous other spinoffs, especially in technology and computing.

“We know that applications will come out, but we don’t necessarily know what they will be until we do the basic research,” Calderón de la Barca Sánchez said.