The next generation of particle physics is on the horizon as the world’s largest particle collider, the Large Hadron Collider (LHC) near Geneva, Switzerland, is set to undergo major upgrades over the next several years to increase the likelihood of finding new physics and learning more about the fundamental structure of the universe.

Thirty stories underground at the European Organization for Nuclear Research (CERN), two beams of protons race in opposite directions around two 17-mile long tubes at near the speed of light. The proton beams collide at interaction points located at each of the collider’s four detectors, spraying out an assortment of other particles for physicists to analyze.

The measurement of how many of these protons actually collide in a given amount of time is called “luminosity,” and is a critical measurement that will play a central role in bringing on the next era of particle physics research.

“It’s like taking two Swiss watches, smashing them together into tiny pieces, and trying to figure out how a watch works,” said David Stickland, senior research physicist at Princeton University and systems manager for a specialized project called Beam Radiation, Instrumentation and Luminosity (BRIL).

With leadership and contribution in several key areas from Princeton researchers, BRIL is an international collaboration in charge of maintaining and upgrading various components of one of the collider’s four detectors, the Compact Muon Solenoid (CMS) detector.

The BRIL project group, which consists of about 50 particle physicists, engineers and computer scientists based at various worldwide institutions, is in the midst of upgrading and replacing several instruments that are involved in measuring luminosity to prepare the detector for upcoming large-scale developments.

 

 

 

High-Luminosity LHC

To improve the chances of detecting rare particles and events, a project is underway that will significantly increase the rate of collisions in the LHC. In 2029, CERN will unveil the next generation of the collider, called “High-Luminosity LHC.”

When two protons collide, a number of things can happen. They can lightly graze and deflect off of one another, or they can collide violently and produce a range of new particles. The more proton collisions that happen, the higher the chances are of seeing something rare.

High-Luminosity LHC will deliver ten times more collisions in three years than the LHC has produced in its entire lifetime thus far. The project was announced as the top priority of the European Strategy for Particle Physics ten years ago, and researchers are eager to complete upgrades over the next several years.

“The analysis we do at CERN involves searching for rare events,” said Bennett Greenberg, Princeton University graduate student and member of BRIL. “But we don’t just want to know that these events happen, we want to know how often they happen. Luminosity is a very important component of almost everything we do because it tells us the ratios for how often certain events happen, which allows us to precisely test our models.”

The proton beams do not actually consist of a constant flow of particles, but of many small pockets, or bunches of protons staggered throughout the beam. Picture a flowing river – the water is equivalent to empty space, and the protons are like clusters of twigs or debris traveling in clumps.

Increasing the number of collisions in the LHC is not as simple as adding more protons, or increasing their speed. It means optimizing the density and overlap of the proton beams to promote more collisions within each beam interaction.

On average, there are 100 billion protons in each bunch. When two bunches from the opposite beams interact, only about 35 to 45 of them actually collide.

These bunch interactions happen 40 million times per second. In the High-Luminosity LHC era, the frequency of interactions will be the same, but the number of actual collisions will increase to about 150 to 200 collisions during each interaction.

“Each proton beam is squeezed to about the width of a human hair at the interaction points. But because they’re microscopic particles, that still leaves a lot of space between each proton,” said Andrés G. Delannoy, postdoctoral research associate and member of BRIL. “So when two bunches of protons interact, luminosity tells us how many of the protons collide, and how many cruise right past each other.”

The High-Luminosity LHC project relies on a long list of innovative technologies developed at CERN, such as more powerful focusing magnets to squeeze the proton bunches even tighter before they collide, reinforced machine protection to absorb stray particles, and new superconducting equipment that will tilt or rotate each particle bunch before they meet to increase the overlap area of the colliding bunches.

“We work with some of the most capable engineers in the world,” Delannoy said. “It’s not just cutting-edge physics. It’s cutting edge engineering, material science, computer science, and more.”

 

A sight to behold

Work on the actual detectors and collider can only be done when the LHC is turned off — during the scheduled Long Shut Downs after each three-year run, and during the annual Year End Shut Downs that allow for a few months of maintenance and repairs.

Since most of the work is done above ground – design, prototype testing, computer programming and physics analyses – seeing the CMS detector in person is a precious experience for the majority of scientists at CERN. Many of them, especially those early in their career who have not yet had the opportunity, jump at the chance to sign up for a coveted slot in an underground tour of the detector.

After crossing the border into France, entering an unsuspecting warehouse building and donning a protective helmet, visitors step into the elevator and are quickly taken 30 stories underground, what many of the researchers refer to as going “downstairs.”

Once underground, visitors step in front of the perfect “selfie spot” – as labeled by a sign – and look up at the monstrous, cylindrical hunk of metal, opened and spread apart in the middle like a slice cut from a doughnut.

Protruding through the center of the three-story tall detector is a much smaller pipe, less than two inches in diameter, which contains the actual proton beams. Everything surrounding this narrow beam pipe was built to track, control, monitor and analyze the beams of protons and the particles that spray out when they collide.

 

A really big project

This drastic increase in luminosity calls for new measurement techniques. More data means more room for error, and BRIL researchers are working to bring the capabilities of CMS up to par with the amount of collisions it will see.

“2029 sounds like it’s really far in the future, but we need many things to be finalized now,” Delannoy said.

Two instruments that measure luminosity will be replaced: the Pixel Luminosity Telescope (PLT) and the Beam Condition Monitor “Fast” (BCM1F).

One goal of these upgrades is to be able to measure luminosity in real time, as the collisions are happening, with the best possible quality and accuracy.

“Normally the detector collects data, and you spend months and months analyzing that data to determine the luminosity,” said Delannoy, who specializes on these instruments. “We want to be able to do that in real time, to take information from multiple instruments and directly publish it with the best possible accuracy, which is a big, big project.”

They are replacing both the PLT and the BCM1F with two brand new instruments. One of these is the Fast Beam Condition Monitor (FBCM) which, although its name is very similar to one of its predecessors, will have far more advanced capabilities. Its new and improved design will be based mostly off of BCM1F.

“BCM1F is a much simpler detector compared to the PLT, which means it’s somewhat easier to build and commission, to run and to understand the results,” Delannoy said. “There’s a reason the PLT was designed the way it was, but ultimately, BCM1F can achieve some of the same things in a simpler way.” 

Unlike PLT, its sister luminometer, BCM1F was not originally designed to primarily measure luminosity. 

Before a collision, other stray particles are mixed in with the protons that are about to collide. The initial role of BCM1F was to look at the incoming beam of protons just nanoseconds before the actual collision and determine the quality, or condition, of the beam.

“We have to make sure that the beam conditions are perfect for the CMS operation, and BCM1F helps to understand what is happening and provides the LHC operators with feedback," said Joanna Wanczyk, graduate student at the Swiss Federal Institute of Technology in Lausanne, Switzerland and member of BRIL.

“We later pivoted to using BCM1F for luminosity,” Delannoy said. “Turns out, we were very successful with this. BCM1F is ultimately more versatile, it delivers the same quality of luminosity readings as PLT while also providing very reliable beam induced background measurements.”

The other instrument to be introduced for High-Luminosity LHC is the Tracker Endcap Pixel Extension Disc 4 Ring 1 (TEPX-D4R1), which has a similar function to BCM1F, and will aid in providing real-time luminosity measurements.

Equally important to knowing the luminosity is knowing the possible error or bias in luminosity measurement.

“The error is extremely important because when you make a measurement, you need to be incredibly precise. You need to be able to prove beyond a reasonable doubt that you have a real signal for the particle you’re looking for,” said Sam Higginbotham, Princeton University postdoctoral researcher and member of BRIL. “Because our analysis techniques have become so advanced, the leading systematic error can often be in the luminosity measurement.”

Beyond the planned upgrades and replacements, BRIL researchers are continuously monitoring the health of the CMS detector. Equipment can be easily damaged from radiation, as well as from changes in humidity and temperature, so the machine must always be checked and double-checked that its data is accurate.

Radiation effects in materials and electronics are another significant source of error. Physicists must continuously measure and compensate for this to better calculate the luminosity and minimize its error.

“Radiation damage is just something that we need to accept happens during the run, and it needs to be predicted and planned for,” Greenberg said. “But if it gets too bad, then eventually more things need to be replaced.”

 

A global collaboration

"This is all enormously complicated, I’m always in awe,” Stickland said. “And it’s all built on an enormous number of people each doing a small piece of the work. Each person is dependent on 50 other pyramids of people in order to do their job.”

Stickland said that training the next generation of CERN researchers is vital for the longevity of particle physics. He believes that working with BRIL is one of the best opportunities for graduate students and postdoctoral researchers to grow because projects tend to be more short-term, allowing them to see a project through from start to finish in a matter of a few years.

“In order for the field to be strong and to survive, they have to actually understand how to build these things, and how to operate them, and how to build the next one,” Stickland said. “Many of the projects in BRIL provide that kind of learning opportunity.”

Wanczyk said that working with BRIL has been a great advantage during her time as a student because of the wide range of projects she’s worked on.

“Being a student and trying to figure out my future, BRIL has been very nice because you can do a lot on your own, with guidance and advice, and you touch upon a lot of different things, from hardware to analysis,” she said.

Higginbotham said that working in such a highly collaborative, international environment enables incredible accomplishments that wouldn’t otherwise be possible.

“You join a small team, but then you realize that you're part of something so much greater,” Higginbotham said. “We're really putting our heads together, getting over the language barrier and accomplishing something amazing, tackling the fundamentals of the universe.”