As a small child, physicist Chiara Oppedisano read the legend of King Midas with her mom.
“It remains very vivid in my memory,” she says. “Everything he touched turned to gold, but he was still unhappy. It was a fun story but also made me think.”
Recently, Oppedisano and her colleagues on the ALICE experiment examined the Midas touch happening inside the Large Hadron Collider: lead ions transmuting into thallium, mercury and gold.
Scientists have been able to transform heavy elements into gold using particle accelerators since the 1940s. This result was unique because it was the first time scientists had seen new chemical elements appearing in a type of collision in which the two nuclei never actually came into contact—a Midas touch without the touch.
But as King Midas realized, not everything is better when turned into gold.
Near miss
ALICE is designed to study quark-gluon plasma, the hottest and densest form of matter in the universe.
“Ultimately, we're trying to understand how matter behaves at the most extreme conditions,” says Anthony Timmins, an ALICE physicist at the University of Houston.
But only head-on lead-lead collisions in the LHC have the energy to produce a quark-gluon plasma. The majority of particle interactions during the LHC’s lead-ion run—more than 98%—result from what are actually near misses.
In those cases, even though the two nuclei never touch, strange things can still happen.
While lead atoms are neutral, the lead ions inside the LHC have been stripped of all their electrons so that only the bare nuclei collide. This means there is nothing to balance out the ions’ 82 protons, whose whopping positive charge generates an extremely strong electromagnetic field. This field is even more concentrated by the LHC’s extreme speeds and effects of Einstein’s special relativity, which compresses the ions into pancakes.
“This compression is so strong that the resulting magnetic and electric fields are the strongest known in the universe, although they act only over very short times,” says John Jowett, a member of the ALICE collaboration and a former accelerator physicist at CERN.
When two lead ions cross paths, their electromagnetic fields can produce photons. If a photon from one lead nucleus is absorbed by the other lead nucleus, it can pop out several neutrons and—if the energy is high enough—protons.
Losing neutrons only changes the isotope of the lead (that is, they are still lead nuclei but have a new atomic weight). But losing protons fundamentally transforms the lead into an entirely new element. One lost proton transforms lead into thallium; two lost protons, mercury; and three lost protons, gold.
The amount of gold generated by the LHC is tiny: It would take billions of years of continuous operation to produce enough gold to make a simple wedding band.
For researchers, these new elements and isotopes present a problem. Each new nucleus has a unique charge-to-mass ratio. Because the LHC is only tuned to steer lead-208, the new nuclei take idiosyncratic paths through the accelerator magnets. Depending on how close their charge-to-mass ratio is to lead-208, the new nuclei can travel hundreds—sometimes thousands—of meters inside the LHC beampipe. If scientists detect too many stray nuclei, the LHC’s protection systems will automatically dump the beams.
"If you start colliding lead ions, and then within the first few minutes the beams are dumped because you are over-protective, then you never do any physics," Jowett says. “That’s the best-case scenario. The worst-case scenario is that you are not protective enough and damage the machine.”
When configuring the LHC, accelerator physicists use elaborate simulations to predict and manipulate the trajectories of the most common unwanted particles so that these atomic interlopers can be safely intersected and absorbed. But scientists on ALICE noted that the experimental verification of one key component of these models—the proton emission from the near-miss lead collisions—was still lacking.
“Sometimes the models use scarce data from a long time ago, that has never been remeasured,” says Uliana Dmitrieva, who worked on this analysis as an ALICE graduate student.
Metamorphosis
Like Oppedisano, Dmitrieva was also struck by examples of transformation throughout her childhood, in a small town in the Russian countryside. She remembers planting seeds in the garden and watching them grow into strawberries, cucumbers and tomatoes, which she would pluck and eat for breakfast.
“I was always surrounded by processes of metamorphosis,” Dmitrieva says. “But what about more fundamental processes? This really interested me.”
As an undergraduate, Dmitrieva mentored with an ALICE theorist who models lead’s transmutation into other elements inside particle accelerators. “I wanted to work on the experimental side, and my advisor’s studies were more theoretical,” she says. “It was a good opportunity for us.”
For this recent study, Dmitrieva and her advisor collaborated with their colleagues on ALICE to count the number of protons and neutrons emitted by the non-colliding lead nuclei and estimate the probabilities of lead transmuting into lighter elements.
While simple in theory, it was extremely difficult in practice.
First, they needed a deep understanding of ALICE’s zero-degree calorimeter, a detector located along the LHC beampipe that Oppedisano helped design and build as a graduate student.
“Now when I see the spectrum of my detector, it's like it's talking to me,” Oppedisano says.
Next, they needed to simulate these kinds of events. This was perhaps the most challenging part of the analysis.
“Neutrons have no electrical charge; they move in a straight line, and we can predict where they will end up,” Oppedisano says. “But protons are charged and will be bent by the magnetic fields of the LHC. It was very tricky, and Uliana was really stubborn and dedicated to the task.”
Using theoretical frameworks and Monte Carlo simulations, Dmitrieva modeled the propagation of the neutrons and protons from several millions of near-miss collisions. From this, she was able to estimate how often their detectors would see and register the emitted protons and neutrons.
“That was really difficult because the detectors we are using are more than 100 meters away from the interaction point,” Dmitrieva says. “But it provided crucial information about our detector efficiencies for these kinds of measurements.”
When they finally analyzed the data and compared it to the simulations and models, they had a surprise: While the models did a pretty good job predicting how often thallium, mercury and gold are produced, they did not accurately predict which isotopes.
“In some cases, the model was off by more than a factor of two,” Dmitrieva says. “We need to fully understand this process if we want to build bigger and more powerful colliders.”
According to Jowett, the production rates of the new ions at the LHC collision points are low enough that scientists don’t need to be concerned about potential beam dumps or damage. But that is not true for future colliders, such as the proposed FCC-hh, which is a super-sized version of the LHC.
“For the FCC, you will have to take very special measures beyond those implemented in the LHC to absorb these ions,” he says. “The power in these beams will be tens of kilowatts rather than the hundreds of watts that we have learned to handle at the LHC.”
While characterizing these near-miss collisions is vital to the designs of future colliders, Jowett notes that these results are scientifically interesting in their own right.
“The heavy-ion program at the LHC has given access to many interesting phenomena beyond what was originally anticipated,” he says. “Among them, there is this fascinating interplay between the nuclear physics studied by the experiments and the accelerator and beam physics limiting the performance of the collider.”
