When Only a Clunky Old Laser Will Do
A Coherent 599 tunable dye laser in Nick Hutzler’s lab. Credit: Lance Hayashida
How a laser from the 1970s could help physicist Nick Hutzler’s team solve the mystery of antimatter.
By Omar Shamout
Sometimes, only a 50-year-old laser will do.
Nick Hutzler (BS ’07), an assistant professor of physics at Caltech, says the Coherent 599 tunable dye laser in this photograph, which was manufactured in 1977, is a surprisingly useful tool for an initial step in his lab’s analysis of electron excitations in three species of metal-containing molecules: ytterbium, strontium, and radium hydroxide. The researchers hope their experiments will one day reveal a new fundamental force (or forces) that will explain why the universe is almost entirely made of matter rather than antimatter particles.
This physics mystery has its roots in a nearly 100-year-old discovery made in Caltech’s Guggenheim Aeronautical Laboratory, which stands adjacent to the building that houses Hutzler’s lab in the George W. Downs Laboratory of Physics and Charles C. Lauritsen Laboratory of High Energy Physics.
In 1932, Carl Anderson (BS ’27, PhD ’30), then an assistant professor at Caltech, made the first observation of an antimatter particle—the positron—in a lab on the third floor of Guggenheim. Positrons are identical to electrons except that they have a positive rather than negative charge. However, they only exist in tiny quantities throughout the universe despite theoretical predictions that matter and antimatter should have been created in nearly equal amounts after the big bang. Anderson’s discovery reshaped our view of the universe and earned him the Nobel Prize in Physics in 1936. Since then, physicists have been puzzled by the question of what happened to these positron antimatter particles after the big bang.
“If you go to CERN, in Switzerland, and use the Large Hadron Collider to smash particles together and use the energy to create new particles, you get matter and antimatter particles in the same amount,” Hutzler says. “But clearly something happened where we got a lot more of one than the other, and we don’t know what that is.”
The Hunt for Asymmetries
The standard model of particle physics tells us that matter particles must behave symmetrically in electromagnetic fields—specifically, that they must interact with electric fields as though they were perfectly round with no asymmetric charge distribution. “So, one way to explain the lack of antimatter is if regular matter particles, which are easily studied in the laboratory compared to antimatter, have a slight asymmetry in the way they individually respond to an electric field,” Hutzler explains. “We’re trying to observe those symmetry violations that could only be explained by undiscovered fundamental particles and forces—something outside the standard model.”
After Hutzler’s team creates the ytterbium, strontium, and radium hydroxide molecules for their experiments, they use the Coherent dye laser to excite the molecules’ electrons. Because the dye laser emits light over a broad bandwidth, this enables the team to capture a wide range of the electronic excitation spectrum at once, albeit at low resolution. The electrons sit in this excited state for a few nanoseconds before reradiating back down to the ground state and emitting light either at the same wavelength or a different wavelength.
Broad-bandwidth lasers have fallen out of fashion, Hutzler says, because researchers often prefer to zero in on a more specific frequency range at higher resolution using narrow bandwidth lasers. “It sounds weird, but the reason we like this laser is because it’s really bad,” Hutzler says. In other words, if the team were to use a more popular narrow-bandwidth laser at this stage, they would have to take a series of scans to cover the same range.
The team does later employ narrow-bandwidth lasers to take more refined measurements once they identify which spectral features they would like to investigate further to learn more about the behavior of the electrons that emitted the light.
Undiscovered Particles?
In 1967, Russian physicist Andrei Sakharov proposed that whatever as-yet-undiscovered particle or force causes preferential treatment between matter and antimatter in the universe could also modify the electromagnetic properties of regular matter. And it was Caltech physicist and Nobel laureate Richard Feynman who later figured out that all subatomic particles, including any undiscovered ones, would have to exist in a cloud of virtual particles surrounding the electron.
“So, this difference in treatment between matter and antimatter will cause matter particles such as electrons to interact with electric fields, for example, in a symmetry-violating way,” Hutzler says. “What’s actually happening is the electromagnetic field is interacting with that undiscovered symmetry-violating particle in the cloud around the electron and then doing something very particular to the electron.”
The hope is that further analysis will uncover these symmetry violations in electron features such as spin orientation. “Ultimately, we identify some particular feature in the spectrum that should be very sensitive to the effect we’re looking for and then study that one feature as precisely as we can,” Hutzler adds. “For example, does changing the tilt of the electron spin in the molecule change the energy levels? The standard model would say no, but the new particles and forces we are looking for would say yes.”
The idea to combine this laser with Hutzler’s measurement approach came from Timothy Steimle, a visitor in physics at Caltech and an emeritus professor of chemistry at Arizona State University. Hutzler says his team has learned a lot from the chemistry community, which has experienced a revival in studying the structure of these types of metal-containing molecules.
Hutzler’s group is unusual in the realm of particle physics because its tests do not require the kind of massive particle accelerators found at Fermilab and CERN. “One of our main selling points is that all this stuff is small,” he says. “It’s all laboratory scale.”
But what happens if the 50-year-old laser breaks? “It’s not a very precise instrument,” Hutzler says. “We’ve had mirrors and mounts on them break, and we just eyeballed it and made a new part, which you couldn’t do for a modern laser. If it really came down to it, we probably could just build a whole new one.”
Editor’s note: This story originally included a photo of the Coherent 699 dye laser in error.