Teaching New Tricks to a Particle Detector
Physicist Cristián Peña grew up in Talca, a small town a few hours south of Santiago, Chile. “The Andes cross the whole country,” he says. “No matter where you look, you still have the mountains.”
At 13, he first aspired to climb them.
Over the years, as his mountaineering skills have developed, so has his tool inventory. Ice axes, crampons and ropes have broadened his horizons.
In Peña’s work as a scientist at the US Department of Energy’s National Fermi Accelerator Laboratory, he applies that same mindset: he creates the tools his experience needs to explore new ground.
“The work of detection is essential,” he says.
Peña’s current focus is the CMS detector, one of the two large general purpose detectors at the Large Hadron Collider. Peña and his colleagues wish to use CMS to research a class of theoretical long-lived particles.
While working on the problem, they realized that an ideal long-life particle detector was already installed inside CMS: the CMS muon system. The question was whether they could hack it into doing something new.
When scientists designed the CMS detector in the 1990s, they had the most popular and mathematically resistant particle physics models in mind. To their knowledge, the most interesting particles would only live for a fraction of a fraction of a second before transforming into well-understood secondary particles, such as photons and electrons. CMS would pick up the signals from these secondary particles and use them as a trail back to the original.
The rapid decay hypothesis worked in the search for Higgs bosons. But scientists now realize that this “live fast, die young” model may not apply to all the interesting things that arise from a collision at the LHC. Peña says he sees this as a sign that it’s time for the experience to evolve.
“If you are a little kid and you walk a mile in the forest, everything is completely new,” he says. “Now we have more experience and want to push new frontiers.”
For CMS scientists, that means finding better ways to search for long-lived particles.
Long-lived particles are not a radically new concept. Neutrons, for example, live about 14 minutes outside the limits of an atomic nucleus. And protons have such a long lifespan that scientists have no idea if they decay. If undiscovered particles enter the detector before they become visible, they could be hiding in plain sight.
“Before, we hadn’t really thought about looking for long-lived particles,” says Christina Wang, a graduate student at Caltech working on the CMS experiment. “Now we have to find new ways to use the CMS detector to see them. “
A new idea
Peña was thinking about long-lived particles at a conference in Aspen, Colorado, in March 2019.
“There was a bunch of whiteboards, and we’re pitching ideas,” he says. “In this type of situation, you go with the vibe. There is a lot of creativity and you start to think outside the box.
Peña and her colleagues envisioned what an ideal long-life particle detector might look like. They would need a detector far from the point of collision. And they would need shielding to filter out the secondary particles that are the stars of the show in mainstream research.
“When you look at the CMS muon system,” says Peña, “that’s exactly what it is”.
Muons, often referred to as the heaviest cousins of electrons, are produced during high-energy collisions inside the LHC. A muon can travel long distances, which is why CMS and its sister experiment, ATLAS, have massive detectors in their outer layers solely dedicated to capturing and recording muon traces.
Peña performed a quick simulation to see if CMS’s muon system would be sensitive to firework-like signatures of long-lived particles. “It was quick and dirty,” he says, “but it seemed doable.”
After the conference, Peña resumed her usual activities. A few months later, Nathan Suri, a second year student at Caltech, joined Professor Maria Spiropulu’s lab as a summer student, working with Wang. Peña, who was also collaborating with the Spiropulu research group, told Suri about the idea of the muon detector as a summer project.
“I’ve always been encouraged to give ideas to talented young people and let them be inspired,” says Peña.
Suri was thrilled to take on the challenge. “I was in love with the originality of the project,” he says. “I couldn’t wait to put my teeth there. “
Test the concept
Suri began by analyzing the event displays of simulated long-lived particle decays to find shared visual patterns. He then explored the original engineering design report of the CMS Muon Detection System to see how sensitive it could be to these models.
“Looking at the detector’s unique design and the highly sensitive components, I was able to realize how powerful it is,” he says.
By the end of the summer, Suri’s work had shown that it was not only possible to use the muon system to detect long-lived particles, but that CMS scientists could use pre-existing data from the LHC to start the search.
“At this point, the floodgates opened,” says Suri.
In the fall of 2019, Wang took over as head of the project. Suri had shown that the idea was possible; Wang wanted to know if it was realistic.
So far, they had worked with processed data from the muon system, which was not suited to the type of research they wanted to do. “All of the reconstruction techniques used in the muon system are optimized to detect muons,” says Wang.
Wang, Peña, and Prof. Caltech Si Xie held a Zoom meeting with muon system experts to seek advice.
“They were really surprised that we wanted to use the muon system to deduce long-lived particles,” Wang said. “They were like, ‘This is not designed to do this.’ They thought it was a strange idea.
Experts suggested that the team should try looking at the raw data instead.
This would require extracting unprocessed information from the tapes, then developing new software and simulations that could reinterpret thousands of raw detector hits. The task would be difficult, if not impossible.
After the muon system experts left the call, Wang recalls, “We were still in the Zoom room and we were like, ‘Do we want to continue like this?
She said it wasn’t a serious question. Of course they did.
A trigger of their own
In the fall of 2020, Martin Kwok started a postdoctoral fellowship at Fermilab. “We are encouraged to talk to as many groups as possible and think about what we want to work on the most,” he says.
He met Fermilab researcher Artur Apresyan, who told him about working with Caltech to convert the CMS muon system into a long-lived particle detector. “It was immediately attractive,” Kwok says. “It’s not very often that we explore new uses for our detector. “
Wang and his colleagues had gone ahead with the idea, extracting, processing and analyzing the raw data recorded by the CMS muon system between 2016 and 2018.
It had worked, but the data set they had to study was not ideal.
The LHC generates around a billion collisions per second, far more than scientists can record and process. Scientists therefore use filters called triggers to quickly evaluate and sort new collision data.
For every billion crashes, only about 1,000 are rated “interesting” by triggers and recorded for further analysis. Wang and his colleagues had determined that the filters closest to what they were looking for were those programmed to look for signs of dark matter.
Apresyan explained to Kwok that he might design a new trigger, which is actually intended to look for signs of long-lived particles. They could install it in the CMS muon system before the LHC restarts in spring 2022.
With a dedicated trigger, they could multiply by 30 the number of events deemed “interesting” for long-lived particle searches. “says Kwok.
Kwok was up to the challenge. And it was a challenge.
“The price of doing something different, of doing something innovative, is that you have to invent your own tools,” says Kwok.
The CMS Collaboration is made up of thousands of scientists all using collective research tools that they have developed and refined over the past two decades. “It’s a bit like building with Legos,” Kwok says. “All the pieces are there, and depending on how you use and combine them, you can do almost anything.”
But developing this specialized trigger was less like picking the right Legos and more like creating a new Lego part from melted plastic.
Kwok has dug through the experience archives in search of his raw materials. He found old software that had been developed by CMS but rarely used. “This tool, which is no longer popular, has proven to be very practical,” he says.
Kwok and his collaborators also had to investigate whether the integration of a new trigger into the muon system was even possible. “There is only a limited amount of bandwidth in the electronics to send information upstream,” says Kwok.
“I am grateful that our collaborative ancestors designed the CMS muon system with a few unused bits. Otherwise, we would have had to reinvent the whole trigger scheme. “
What started as a feasibility study has now evolved into an international effort, with many other institutions helping to analyze the data and trigger R&D. The American institutions that contribute to this research are funded by the United States Department of Energy and the National Science Foundation.
“Because we don’t have dedicated long-lived particle triggers yet, we have low efficiency,” says Wang. “But we have shown that it is possible – and not only possible, but we are in the process of revising the CMS trigger system to further improve sensitivity.”
The LHC is expected to continue into the 2030s, with several major upgrades to accelerators and detectors along the way. Wang says that in order to continue probing nature at its most fundamental level, scientists must stay at the frontier of sensor technology and question every assumption.
“Then new areas to explore will naturally follow,” she says. “Long-lived particles are just one of these new areas. We’re just getting started.