Inside CERN’s Large Hadron Collider, a giant atom smasher located at the French-Swiss border, proton beams collide an astounding 40 million times per second. Smash after smash, the protons transform into other particles that shoot away from the collision point. Scientists rifle through the subatomic debris to pick out particles and patterns that help us answer long-standing questions about our universe.
Forty million collisions per second is no slouch, but if you could produce even more collisions, you’d have more opportunities for discovery. CERN is upgrading the LHC to increase the number of proton-proton collisions per beam crossing by as many as seven times — turning streams of postcollision particles into a torrent. Coping with this particle flood, generated by the future, revamped machine — called the High-Luminosity LHC — brings its challenges.
Fermilab scientist Artur Apresyan will tackle one of the biggest. The Department of Energy has awarded him $2.5 million over five years to help separate the countless particles emerging from the increased, rapid-fire collisions. He and a team of researchers are developing instruments that can read the particle data with unprecedented precision, enabling scientists to help tease out individual particle trajectories.
“Those collisions create clouds of particles everywhere, and you can easily confuse which particles come from where,” Apresyan said. “When the density is high, it’s difficult to disentangle.”
From long exposures to snapshots
At the LHC, protons collide at the center of stories-high particle detectors; the two largest are called ATLAS and CMS. The resultant particles zip and spiral at lightning speed through one layer of detector after another, leaving tracks that are recorded by the detector’s instruments.
Using sophisticated particle detectors, physicists are able to reconstruct the particles’ energies, travel paths and arrival times.
Measurements of energy and travel path have become fairly solid over the years. But measurements of arrival times? Not as far along.
Apresyan and his team will design new types of detectors that will record with extremely high precision the time that a particle passes through the detector layers.
That data will make it easier to see snapshots of collision remnants passing through the detector. Without timing information, the picture of particles passing through the detector would be similar to taking a photograph of a fireworks display using a slow shutter speed. The visual data “piles up” in the shot.
But with precise records of the particle arrival times, you can more clearly see which particle originates when. Rather than looking at thousands of tracks in the detector at once, you can get more of a moment-by-moment look. Your long-exposure, still shot of a firework display is now divided into shorter, faster frames.
“With the time-of-arrival info, we reduce pileup by a factor of five,” Apresyan said. “With the same number of particles traveling through the detector, you can pull 30 percent more data from the collisions for free.”
Clearer, more detailed data enables scientists to better identify which particle comes from where, helping to disentangle the messy outcome of proton-proton collisions. And that clearer data could reveal phenomena they’d otherwise not see.
“You can look for very rare, new physics phenomena, and in completely new ways,” Apresyan said. “We can, for example, search for long-lived particles, which are predicted by many theories.” In detectors, long-lived particles fly a little farther than the others before they decay into smaller pieces.
The target time resolution is around 30 picoseconds — a “shutter speed” of less than 30 trillionths of a second.
Apresyan is developing the detector components for CMS, and ATLAS is expected to use the same technology. He is also developing a complementary analysis technique for the CMS collaboration.
“Artur’s program not only advances the technology of particle detection, it also sets the stage for discovering new phenomena at the HL-LHC and future colliders,” said Fermilab scientist Anadi Canepa, head of the CMS Department at the laboratory. “We are really proud of Artur’s past achievements. He continues Fermilab’s tradition of excellence.”
Hardware for hard problems
The fast-timing sensor under development for CMS is made of planes of silicon. Particles that pass through the silicon generate signals that are sent to computers for analysis.
Silicon transmits signals quickly, and that’s critical for the High-Luminosity LHC, which will collide protons every 25 nanoseconds. Silicon is fully up to the task.
The bottleneck, Apresyan says, is the electronics that read the signals. That’s the area where he’ll focus his work.
“You need high-bandwidth electronics that will take advantage of the fast signals that the silicon produces,” Apresyan said.
While such high-performing electronic readout systems have been built for industry, no one has built one for the high-radiation environment inside a giant particle detector with millions of channels. Additionally, the electronics need to consume very little power, since the heat generated inside the detectors is difficult to remove. Developing these multichannel, low-power electronics to withstand radiation will be one of the engineering challenges.
Apresyan will develop the electronics with an expert team at Fermilab that focuses on application-specific integrated circuits, or ASICs.
“We have a leading ASIC engineering group that can really take a lead role in the development of these systems. And we have a world-class silicon detector facility,” he said. “It’s a unique place for knowledge and expertise in every aspect of the detector system and in combining them to make the final technology.”
Particle detection for future machines
The need for highly time-sensitive particle detection will only increase as the particle physics community plans its next big machine, envisioned for physics pursuits several decades from now. Currently under discussion, for example, are the Future Circular Collider, a roughly 60-mile-around collider sited for Europe, and the Circular Electron Positron Collider, a roughly 40-mile-around collider sited for China.
The higher energies of the collisions in these machines would create pileup effects that are 10 to 20 times greater than what the LHC experiences currently.
“Precision timing is the way of the future for colliders, and we’re getting better and better at it,” he said.
Apresyan’s DOE Early Career Award will fund his own work, one postdoc and several engineers and technicians.