How the new Large Hadron Collider experiments could change physics forever

After a three-year-long nap, the world’s largest particle collider is awake and ready to help physicists probe the very edge of science itself, including the possible existence of a mysterious fifth force of nature.

Sam Harper has been a particle physicist and a collaborator on the LHC’s CMS (Compact Muon Solenoid) experiment since 2007. He says that new upgrades at the collider over the past three years are now poised to hopefully bring scientists to the brink of discovering revelations that could alter our understanding of the smallest parts of our universe forever.

“We’re really excited about following up on [previous] anomalies,” Harper tells Inverse. “[But,] we are also really nervous to get everything correct.”

What is the Large Hadron Collider?

Teetering between the borders of France and Switzerland, the LHC is the world’s largest (nearly 16-miles long) and most powerful particle accelerator. This giant donut-shaped collider uses superconducting magnets and beams of protons to smash known particles together at extremely high energies (e.g., 13.6 trillion electron volts).

This number may sound huge, but when converted to more everyday units of energy, like watts or joules, it’s not even enough to power a 100-watt light bulb for one hour (the LHC energy is equivalent to roughly 2.18 10^-6 Joules, whereas a 100-W bulb needs 360,000 Joules for one hour of light.)

But don’t think you’ve been bamboozled — that might not be much energy for a relatively heavy object like a lightbulb, but it can power incredibly light particles to velocities just below the speed of light.

Detectors dotted around the loop then collect data from these collisions to watch as particles break into smaller fragments, revealing lesser-understood areas of physics. These pieces can include things like quarks or even a class of particles called bosons. Bosons are a family of ultra-light particles that include photons and which are responsible for creating forces between particles, including the strong and weak nuclear forces and electromagnetism. In the case of the famous Higgs boson, it’s even responsible for giving particles mass.

Beyond the excitement and curiosity that comes with smashing things together, Harper says that the scientists use the LHC to explore the validity of particle physics' most important theory: the Standard Model. Since its development in the 1970s, this theory has described nearly all the behavior of subatomic particles that scientists have observed, but recent findings have thrown this supremacy into question, including a 2022 finding from FermiLab data which suggests a certain boson, called the W boson, may be much heavier than predicted by the standard model.

With new upgrades at the LHC, scientists may finally be able to get to the bottom of this mystery, Harper says. If data from LHC’s new run, Run 3, observes behavior not predicted by the Standard Model, that could be a tell-tale sign that there are forces or particles not yet known to the Standard Model.

“Voila, new physics discovered!” Harper says.

Why did the LHC stop running?

In the past, the operation of the LHC has been the subject of worry for onlookers who once feared a catastrophic accident at the collider would create a dangerous black hole (it wouldn’t have), but skeptics can rest easy knowing that the collider's three-year break was nothing but scheduled upgrades and maintenance.

In fact, this is not the first nor the last time such a shutdown will happen. According to an operation schedule, the LHC has two more planned shutdowns scheduled into the 2030s. The main objective of these shutdowns, says Harper, is to gradually upgrade the energy capabilities of the proton beams flung inside the collider to improve the chances of particles smashing together.

“Physicists want more collisions [and] more collisions,” Harper says. “The LHC and its detectors are being upgraded to supply and record as many as possible, which makes [for] happier physicists.”

The LHC sent out two test beams last week, and the team intends to begin collecting data in earnest for Run 3 later this summer. Aside from brief maintenance breaks along the way, Harper says Run 3 will last through the end of 2025.

What upgrades did the LHC get?

During its most recent shutdown, which started at the end of 2018, the LHC received two main upgrades:

• Increased energy capabilities for its instruments allowing researchers to create more and faster collisions
• More sensitive data collection software with improved capture rates to increase the number of collisions researchers can record and analyze

Together, these upgrades should create and record more collisions for the detectors. According to CERN, the detector that Harper works on (CMS) should expect to observe “more collisions during this physics run than in the two previous physics runs combined.” Other ongoing experiments, including ATLAS, ALICE, and LHCb, will likely see collisions up to fifty times previous numbers.

In addition to upgrading existing experiments, Run 3 will also feature two new experiments – FASER and SND@LHC – specifically designed to look for physics beyond the standard model.

What discoveries could the LHC make now?

For Harper, one of the most exciting discoveries LHC might make during Run 3 is digging deeper into an anomaly that LHCb observed at the end of the last run that seemed to point toward beyond standard model physics. In this Run 2 data, scientists saw a type of boson, called a B meson, break down into more electrons than predicted by the standard model.

If Harper and colleagues can confirm this trend with more data, scientists believe this might be evidence that a new fifth force is acting upon these particles.

“It’s too early to say anything [certain,] but it's got us very excited, and we are really looking forward to Run 3 being able to shed more light on this,” Harper says.

In addition to exploring this anomaly, LHC experimenters also hope to dig deeper into other mysteries, including the particles that make up dark matter, by looking for missing momentum data from proton collisions. FASER, in particular, will zero in on this hunt.

Yet despite having all this tantalizing data at their fingertips, Harper says there will still be some considerable lag time between collecting this data and actually coming to any conclusions from it. This might be the most challenging part of the whole endeavor for eager scientists like Harper.

“Unfortunately, we will have to wait while we collect and carefully analyze the data,” he says. “[It’s] tough for us as we are champing at the bit to see the results ourselves!”