Underground Science: Exploring the Large Hadron Collider
My name’s Emma Hattersley, I study physics and I’m currently on a year-long industrial placement at the STFC Particle Physics Department. As part of my placement, I was given the chance to visit the iconic Large Hadron Collider at CERN for a few days…
Buried far beneath the French-Swiss border is one of the most iconic science experiments ever constructed — the Large Hadron Collider, or LHC. With a circumference of 27 kilometres, the LHC is the world’s largest and most powerful particle collider.
The ring may be hidden, but the offices, control centres and laboratories required to run it certainly aren’t. When I first arrived at CERN, the site of the LHC, I was slightly overwhelmed by its scale. More than 17,500 researchers of over 110 nationalities work on the LHC (although the majority aren’t based on site), and they fill the sprawling site in Geneva with a bustling, intensely collaborative atmosphere.
Working in the STFC Particle Physics Department (PPD) back in the UK, I spend a lot of my time learning about how our own subsection of those researchers contribute to the collider, alongside attempting to understand the associated physics. I’d come to CERN with a group of colleagues during the LHC winter shutdown to try and build a more complete picture of how the collider works.
70 years of CERN
CERN, the European Laboratory for Particle Physics, was set up in 1954 by 12 countries, including the UK. Originally standing for the ‘Conseil Européen pour la Recherche Nucléaire’ (‘European Council for Nuclear Research’), CERN was founded to focus on nuclear physics.
In the decades that followed, the laboratory moved to specialise in particle physics, but still maintained a broader scientific program. It discovered many new particles and physical effects, uncovering more about how the Universe behaves at its smallest scale.
Scientific progress also led to developments in technology, sometimes with unprecedented societal impact. As we walked through an office block on our way to lunch our tour guide stopped us suddenly to point out a plaque on the wall. We were standing where Tim-Berners Lee invented the World Wide Web back in 1989, designed to make collaboration between CERN researchers easier.
The LHC, a synchrotron or circular particle accelerator, was built by CERN between 1998 and 2008. In 2012 it completed its first aim of finding the Higgs boson, which with its associated Higgs field gives mass to almost all the particles in the Universe.
1. Electrons are stripped from hydrogen atoms and the remaining protons are accelerated to 99.93% the speed of light in a series of booster rings and the proton synchrotron.
2. They then move 40m underground to the Super Proton Synchrotron where they are accelerated around the ring’s 7km circumference to 99.9998% of the speed of light. Here the proton beams are fed in two streams into the LHC in opposing directions.
3. To get that last 0.0001991 of a percent toward the speed of light requires the protons to be pushed around the LHC’s 27km ring– the two beams are accelerated to 99.9999991% of the speed of light (covering about 11,000 laps of the ring every second).
4. The two beams are made to collide within four experiment areas (ATLAS, LHCb, ALICE, CMS). Detectors then trace and analyse the particles that emerge from the collisions.
Since then, LHC research continues to try to answer the big questions in physics and expand the Standard Model, a collection of every particle and physical process currently known to science. More than 20 UK universities and research institutes contribute to this research, with the STFC managing the UK’s subscription.
Keeping the LHC Colliding
First up on our tour was the CERN Control Centre (CCC), which ensures the LHC works as expected. As its name suggests, the LHC collides hadrons, which are a type of particle made of smaller particles called quarks.
A linear accelerator speeds up the hadrons a little, before they pass through circular accelerators called the Proton and Super Synchrotrons to make them even faster. Two beams of these particles are then injected into the much larger LHC, to travel around the ring in opposite directions and collide. This is done using carefully calibrated magnets which generate magnetic fields more than 100,000 times as powerful than the Earth’s.
The CCC is split into four islands, whose teams take responsibility for the LHC, the two smaller synchrotrons and Technical Infrastructure (which supports almost everything else at CERN).
All eyes will be on them in 2029, when they turn on the upgraded collider for the first time. Known as the Hi-Lumi (higher luminosity) LHC, the accelerator will collide more particles at higher energies, allowing the experiments which use it to explore new areas of physics.
Beauty is in the eye of the LHCb(eholder)
Four main experiments — ATLAS, CMS, LHCb and ALICE — lie on the ring itself, making precise measurements of particle collisions. After a short bus journey from the CCC I took a lift 100m underground to visit LHCb, and see the collider for the first time.
LHCb predominantly studies a particle known as the beauty (b) quark. It is hoped that this research could let us understand why our universe is made almost entirely of ‘ordinary’ matter, with very little of its opposite, antimatter.
When particles collide, they produce other particles. Measuring the paths, momentum and energy of this debris allows you to work out what particles were produced, and which physical processes took place throughout the collision.
Detectors in LHCb are placed to measure debris thrown in one direction by the collision. These include Rich Imaging Cherenkov (RICH) detectors, worked on by the LHCb team within PPD, which help the experiment identify higher energy charged particles.
Generally speaking
The next day I went back across the border to see ATLAS, based close to CERN’s main site in Switzerland. Naively, I had always assumed that all the detectors were fully built before being lowered underground, but there clearly wasn’t space for that at ATLAS, which stretched to the very edges of its cavern.
All underground experiments apart from CMS were actually built below the surface, which allowed ATLAS to fully make the most of the space available. Its the largest particle detector ever constructed at 46 m long, 25 m high and 25 m wide, with a volume of about 300 double decker buses.
ATLAS is a general-purpose detector, designed to search for a wide range of new processes and particles. Unlike LHCb, it measures particle debris travelling in all directions after a collision.
These measurements are taken using six different detection subsystems, arranged in layers around the collision point, and a large magnet system. A team at Rutherford Appleton Laboratory, where I work, are helping to design, build and test a new Inner Tracker, the detector closest to the collision point. This will be installed in time for Hi-Lumi LHC, as part of broader upgrades to all the LHC experiments.
Further round the ring
One disadvantage of the vast size of the LHC is that it takes a long time to travel from one side of the ring to the other, so I couldn’t see CMS or ALICE in person. Although CMS is also a general-purpose, all-direction detector with similar physics goals to ATLAS, it works very differently.
Smaller and heavier than ATLAS, CMS is built around a massive solenoid, or spring-shaped, magnet — the most powerful ever made. The coil is surrounded by detectors, with RAL focusing on the ones that run through the middle.
Having two, independently designed detectors allow them to verify each other’s results, as they did with the discovery of the Higgs boson.
ALICE, the final detector on the main ring, differs from the other three as it’s considered a nuclear physics experiment. This means it specialises in investigating heavier particles.
Coping with the data
Billions of particle collisions a second can take place within these experiments — far too many to be recorded. Each experiment uses ‘trigger’ systems, which tell the detector which events it should record, and which it can feel free to ignore.
However, these can only get rid of so much data. During its last run, the LHC produced more than 600 petabytes of data — equivalent to over 20 000 years of 24/7 HD video recording.
All this information has to be stored, analysed and made accessible to researchers all over the world, starting at the CERN Data Centre.
Filled with processors and servers, the system required to cool the centre is so noisy we had to put on ear plugs to safely walk through it. Hi-Lumi LHC is due to need 10 x as much computing power, so a second data centre has been built to help.
These centres are ‘Tier-0’ of the Worldwide LHC Computing Grid, which allows computers to share processing power and data storage. Data moves from here to over 10 Tier-1 centres, including one near my office at Rutherford Appleton Laboratory (RAL).
As with all Tier-1 centres, it has a direct network connection to CERN, as well as a complete copy of all the LHC data. I was surprised to learn that tape is still used for the LHC’s data archiving, as it’s more durable and cost effective than hard drives. At RAL, it’s the job of tape robots Asterix and Obelix to pick out the relevant information from thousands of tapes.
Over 160 Tier-2 sites worldwide work with these larger sites to process this data further, before researchers like my colleagues can access it to work out what the detectors have found.
Leaving CERN
In the airport waiting to go home, I bumped into one of my old university physics professors, heading back to Manchester from CERN. He’s one of the vast community of researchers that, I remembered, had initially stood out to me when I arrived on site — before I’d seen any of the LHC itself.
CERN draws people together to form an invaluable shared knowledge base, allowing tough scientific and technical problems to be gradually solved. With the new Hi-Lumi LHC approaching, who knows what the next 70 years might bring?
Story by: Emma Hattersley, PPD industrial placement student
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