The UNC-Higgs boson connection
Science class is where we are introduced to the standard model of particle physics, or what makes up matter.
Glossy posters point to small spheres of atoms and the particles inside them, like protons and electrons. These particles, known as fermions, relate to matter and live alongside other particles called bosons, which relate to force in matter.
Jack Ng, Kenan Professor of Physics, displays this poster on his office door. As a particle physicist, Ng looks to the model for almost everything that happens in the physical world. But the poster, like the model it represents, has been incomplete.
While an electron (a fermion) carries charge and a photon (a boson) transmits light, no particle has ever been associated with generating mass. While doing research at UNC in 1965, Peter Higgs theorized about a boson for mass, but no one could prove it.
This explains why on July 4 social networks, news channels and physics enthusiasts erupted in celebration when the European Organization for Nuclear Research (CERN) announced the discovery of what was probably the Higgs boson, a never-before proven particle that explains why matter has mass.
Ng (pronounced ING) was at a physics conference in Stockholm, Sweden, when the events were interrupted for one of the most important announcements of his career.
“It was a joyful scene,” Ng said. “The discovery of the Higgs boson is a bridge to the future, to more exciting things so that we can understand, even better, nature at the fundamental level.”
What is it?
While the science community celebrated, the general public tried to grasp the significance.
Before thinking of the Higgs boson, Ng said, think of the field associated with the boson. Higgs first theorized a field permeating all space with which particles would interact to get mass.
Ng uses metaphors to paint a picture of the abstract idea.
“When a famous rock star walks through a crowd and draws a lot of attention, people crowd around him, making him move very slowly,” he said. “On the other hand, if I walk through a crowd, I get no attention or crowd, so I can move very freely. A person who moves slowly because of their interaction with a crowd has more mass. I, the unknown, would have little mass.”
The crowd is the Higgs field, he said. The rock star is the particle in question. The bigger crowd represents a stronger field imparting a greater mass.
In another metaphor, Ng said if you think of the Higgs field as a wave, this Higgs boson would be the ripple.
“If finding the Higgs boson means there is a Higgs field, it is now conceivable that a similar type of background field could have existed in the early universe. That background field would eventually give rise to all matter that we have today.”
This could lead to later discoveries, maybe even what kind of background field could have catalyzed the “big bang.”
“It’s so much fun to be a physicist. We are all so proud,” Ng said. “People ask what the use of finding the Higgs particle is. Can we make an application of it? Not yet. But where mass comes from is something fundamental about nature, and we’d really like to understand our place in the universe. It’s like art; is has aesthetic value.”
Before Higgs left the University of Edinburgh for a year of research at UNC’s Bahnson Institute of Field Physics, he published two short papers on what is known as the Higgs mechanism, the idea that a background field permeating all matter was responsible for mass.
He was just warming up, Ng said.
At Carolina, he put a revolutionary idea to paper: A sub-atomic elementary particle, a boson, was somehow involved with this mechanism. Ng said a handful of other physicists theorized about a similar kind of mechanism, but not one explicitly proposed a massive particle associated with the field imparting mass to the different particles, giving matter mass.
“Higgs is the only one who said it, and he said it right here in Chapel Hill,” Ng said.
At first the theory was regarded as junk, he explained. But as Higgs’s paper made the rounds, he spoke at Princeton and Harvard, changing minds. The paper made him famous.
There’s an unsung hero in all this, Ng said. The work of Julian Schwinger, a Nobel Prize winner and mentor of Ng’s at Harvard University, inspired the Higgs mechanism. His ideas were shot down so firmly by contemporaries that he hung up his dream of discovering the origin of mass.
If he’d kept going, Ng said, we’d all be talking about the Schwinger boson.
“There’s a moral to this story for all scientists,” he said. “If you have a good idea, trust your intuition. Be bold enough to rear it.”
Experimental physicists had been hunting the Higgs boson since the theory became popular. The theory made sense, but there was no smoking gun, no evidence. The boson was so frustratingly elusive, it drew a profane nickname that was later abbreviated to the “God Particle.”
The popular colloquialism had nothing to do with religion, Ng said, or that its discovery could answer questions about the origin of the universe. It had to do with angry physicists swearing at a particle they could not seem to find.
The world’s largest atom smasher at CERN in Geneva, Switzerland, repeatedly smashed highly energetic protons together at great speeds, trying to see which other particles or products might be uncovered. Ng said it took nearly 500 trillion collisions to make the discovery.
Over time, scientists recorded what they started to believe were the byproducts of the Higgs boson, which appears only for a nanosecond – so unstable it quickly disintegrates into other products.
Those products are like footprints. Little by little, scientists collected the data that would suggest a boson nearly identical to the one Higgs described had been briefly showing its face. In December 2011, the chance this boson had appeared due to statistical flukes was 1 in 1,000. By the time of the announcement, it was 1 in 3.5 million.
Scientists are reluctant to say for sure that this is the Higgs boson. “But if it looks like a duck and walks like a duck,” Ng said, “it’s probably a duck.”
Although he’s glad the standard model is complete, if it’s all that CERN finds, Ng will be disappointed. “Everything seems to work so well with this standard model because it can describe almost anything, but we have to know it’s not the ultimate. There are still so many things we don’t know,” he said.
As for Ng’s beloved poster of the standard model of physics – it will need updating.
A physicist’s journey
Ng, whose lively lecture style and pure joy for physics won him a William Friday Teaching Award in 2006, is eager to bring the Higgs boson to his students at the start of the academic year.
“This is going to make a big impact on my teaching,” he said. “I’m very excited to share all these stories with my students.”
One of those stories is how this celebration of discovery takes Ng back to making his own mark on the standard model of particle physics three decades ago.
While at Stanford in the 1970s, Ng and two other physicists (and, independently, a group at CERN) proposed a way to uncover the gluon, the boson that binds quarks together. Experimental physicists later worked with the group’s suggested methods and proved the gluon in 1979. It was the second boson added to the standard model, after the photon.
“When I learned of the diagnostic discovery of gluons, I was so excited,” he said.
He didn’t get much credit – that went to the experimentalists – but he really didn’t mind.
“The collaboration between experimentalists and theorists is what keeps physics going,” Ng said.