February 26, 2024

Scientists expand search for new particles at the Large Hadron Collider

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The CMS detector is one of the experiments at the Large Hadron Collider. CMS scientists upgraded the detector’s trigger to expand the search for long-lived particles. Credit: CERN

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The CMS detector is one of the experiments at the Large Hadron Collider. CMS scientists upgraded the detector’s trigger to expand the search for long-lived particles. Credit: CERN

Since the 1960s, scientists have discovered more than a dozen fundamental particles. They all fit perfectly into the theoretical framework known as the Standard Model, the best description physicists have of the subatomic world.

The Higgs boson, which was co-discovered by the CMS and ATLAS experiments at CERN’s Large Hadron Collider in 2012, was the last fundamental particle predicted by the Standard Model.

Despite this major discovery, scientists still have many questions about the fundamental building blocks of the universe. Researchers know that the Standard Model is incomplete and cannot explain many physical phenomena – dark matter being a notable example.

Scientists around the world are pushing the limits of the Standard Model and searching for new particles that could help explain outstanding questions about the inner workings of the Universe.

“Our goal is to find new particles,” said Cristian Peña, organizer of the CMS exotic particle group and scientist at the U.S. Department of Energy’s Fermi National Accelerator Laboratory. “That’s what we’re here for.”

Peña and other Fermilab scientists recently collaborated with their international colleagues at CMS to create a new tool that allows them to explore particles that can travel about 1 to 10 meters before decaying into more stable byproducts.

Now scientists are analyzing the new dataset produced by this tool. According to Peña, they will find new physics or set the most stringent limits in the search for long-lived particles: a class of theoretical particles that can travel deep into the detector before creating visible signals.

“Our data set no longer doubles every six months as it did at the beginning of the program,” says Sergo Jindariani, senior scientist at Fermilab. “The places where we could still make rapid discoveries are the ones we have never looked at before, and long-lived particles are an example of that.”

When scientists built the experiments for the LHC, they assumed that the new particles would behave like those they had discovered in the past and would decay very quickly. For example, the top quark, discovered at Fermilab in 1995, has a lifetime of approximately 5×10−25 seconds. This is so short that the top quarks decay before they can travel the length of a hydrogen atom. But now more and more scientists are questioning this assumption.

“We’ve looked everywhere and haven’t found anything so far,” Peña said. “We know we can do better using particle lifetimes.”

Scientists already know that particles have a wide range of lifetimes. For example, bottom quarks can travel a few millimeters before decaying, and muons can travel a few hundred meters. Today, scientists ask: what if there are new particles that fall somewhere in between?

Even though these long-lived particles are extremely rare, CMS will still have a good chance of seeing them if they are being produced by the LHC.

“The CMS muon system has a lot of material, so if long-lived particles are decaying inside our detector, we should see a shower of particles in the muon chambers,” Peña said. “The signature is very powerful.”

But the question was whether scientists would be able to find these unexpected showers of particles hidden in their data. The LHC produces about a billion proton-proton collisions every second. Because more than 99.99% of collisions generate uninteresting particles and physical phenomena, scientists use data classification devices called triggers. Triggers choose 0.01% of the top events to be processed and stored in the LHC’s Worldwide Computational Grid and discard the rest.

“CMS is an extremely successful detector,” said Jindariani. “It actually does the physics it was designed for. But long-lived particles weren’t something people had in mind when they were designing the CMS trigger system.”

The team realized that if they wanted to improve their chances of finding long-lived particles with the CMS experiment, they would need to update the CMS trigger to look for the striking and peculiar signature that these particles are expected to leave in the detector.

“With a dedicated trigger, we saw that we could gain an order of magnitude in the sensitivity of these searches,” said Jindariani.

But updating the trigger is always a complicated task. Help and expertise from researchers and engineers was needed throughout the CMS collaboration. Jindariani highlighted that the trigger system relies on multiple data streams from different parts of the detector. These data streams operate like roads in a city and allow data to flow from the outermost parts of the detector to the “downtown” processing center, where the data is compiled and quickly evaluated by algorithms. Adding a new data stream is like adding a bike lane to an already busy metropolitan area.

“It would need to coexist with other triggers,” said Jindariani. “This is a delicate move; we don’t want to damage what is already in place.”

After extensive analysis of the CMS trigger and discussions with the collaboration, the team realized it was possible, thanks to some unused bits left over from the original design. But then came the challenge of actually implementing his new trigger into the experiment’s data processing.

“Once everyone agreed on the conceptual implementation, we needed to get into the firmware and software,” Jindariani said.

Firmware provides basic machine instructions that allow the hardware – in this case, Field Programmable Gate Arrays – to function according to the programmed algorithm. FPGAs can be very fast, but they are often not very dynamic.

“FPGAs have a limited amount of processing power, and CMS trigger algorithms are resource-intensive,” Jindariani said. “We needed to be smart about not overloading the capabilities of the FPGAs.”

Since the LHC makes protons collide every 25 nanoseconds, its new trigger also needed to be fast.

“We are stuck in time slots,” Jindariani said. “The algorithm needs to run in a few hundred nanoseconds. If it takes longer, it’s not good enough. This work was only possible through a strong team of scientists and engineers working together.”

Even after resource and time management challenges were resolved, the team still had to resolve some unexpected setbacks. During the testing phase, they saw that the trigger was activated with each collision. Upon further analysis, they discovered that this was because the transmitter of one of the muon systems was faulty.

“This was a problem that existed before, but the other triggers didn’t notice it because they weren’t looking for it,” says Jindariani.

After all glitches were resolved, the trigger evaluated all LHC collisions that happened within the CMS detector between 2022 and 2023 – about 1016or 10 million billion – and collected a data set of about 108 events. Scientists are currently analyzing this new dataset and hope to get the first results this summer.

“This trigger is one of the great innovations of the CMS”, says Peña. “Either we will find new particles or – if nature doesn’t want it – we will set stricter limits on long-lived particles.”

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