Earlier in May this year, researchers at the Large Hadron Collider (LHC) detected the creation of gold nuclei inside the world’s largest atom smasher. The team at the ALICE detector at the LHC, composed of scientists from the University of Kansas (KU), made this detection possible by developing a technique that has been overlooked before.
The 17-mile (27 km) long particle accelerator at the French-Swiss border needs no introduction. Designed to safely carry out collisions of particles travelling at high speeds in opposite directions, the LHC has a series of detectors that take in data to understand the aftermath of these collisions.
A Large Ion Collider Experiment (ALICE) is one of the four big detectors at the LHC and has played a significant role in the detection of the Higgs Boson, which is a critical component of the Standard Model of physics.
While much of the attention during the experiments is focused on studying the aftermath of the collisions, there are also instances when particles do not collide. This is precisely what the scientists at the University of Kansas were interested in and is referred to as ultra-peripheral collisions.
Ultra-peripheral collisions
“Usually in collider experiments, we make the particles crash into each other to produce lots of debris,” explained Daniel Tapia Takaki, a professor of physics at KU’s group at ALICE.
“But in ultra-peripheral collisions (UPCs), we’re interested in what happens when the particles don’t hit each other. These are near misses. The ions pass close enough to interact — but without touching. There’s no physical overlap.”
Takaki added that ions in the LHC are heavy with many protons in their nuclei and can generate powerful electric fields. When these heavy nuclei are accelerated, they emit photons or light.
When electric charges are accelerated to near light speeds, they start shining with enough energy to take a picture of another ion in the accelerator.
“When that light is energetic enough, it can probe deep inside the other nucleus, like a high-energy flashbulb,” Takaki said in the press release.
When photons from both ions interact, they are called photon-photon collisions. These are extremely clean and do not create a spray of particles that detectors at the LHC are typically designed to detect.
Spotting clean collisions
Takaki and his team are pioneering new techniques to spot these clean collisions, even though they were not a popular subject.
“This light is so energetic, it can knock protons out of the nucleus,” Takaki further added in the press release. “Sometimes one, sometimes two, three, or even four protons. We can see these ejected protons directly with our detectors.”
Each ejected proton changes the nature of the element, from lead to thallium, to mercury and even gold. This was exactly what the researchers noticed when they spoke about the detection of gold in the LHC.
The technology developed at KU aided in detection of these short-lived particles that do not always decay but also hit the collider, triggering safety alarms.
With plans to build colliders up to 60 miles (100 km) long, researchers are keen to understand the nature of these byproducts generated inside the LHC.