Beyond the Higgs boson
The Large Hadron Collider amazed the world with its discovery of the Higgs boson. But what has it done lately? We catch up with Imperial physicists to explore the exciting new findings and breakthroughs unfolding at the world’s largest physics experiment.
When the discovery of the Higgs boson was announced in 2012, it was headline news around the world. Even if the details were beyond most of us, it was clearly a huge breakthrough.
Finding the Higgs boson required the Large Hadron Collider (LHC), the world's largest and most powerful particle accelerator, which is located at the CERN laboratory near Geneva. It accelerates bunches of approximately 100 billion protons to nearly the speed of light and smashes the bunches together 40 million times a second.
A tiny fraction of the protons collide head-on and interact in a way that breaks them apart to produce a variety of new particles, much like a miniature 'explosion'. These particles are then detected by highly sensitive experiments at four major collision points.
The cavern that contains the CMS experiment in CERN. Credit: CERN.
The cavern that contains the CMS experiment in CERN. Credit: CERN.
Researchers from one of these, the Compact Muon Solenoid (CMS) experiment, have compiled findings from many of the 1,500 scientific papers published by the Collaboration. Alongside new analyses of earlier data, these findings have been synthesised into review articles that highlight the achievements so far and the questions that remain open for exploration.
Researchers from Imperial College London have played a pivotal role in several of these reviews, which cover topics such as searches for dark matter and other mysterious particles in the hunt for new physics, as well as novel ways to deal with the huge volumes of data from the particle collisions.
Quick introduction: what is the CMS experiment?
The CMS detector works like a giant 3D camera, capturing 'photographs’ of the energetic particles produced in the proton collisions.
These 'images' are then analysed by physicists to understand particle interactions at the fundamental level. CMS is a ‘general-purpose’ detector, which means it can be used to study a broad range of physics phenomena, rather than being specialised like other experiments, such as LHCb, which focuses on particles called beauty quarks.
High energy collisions between two protons, inside the CMS detector. Credit: CERN.
High energy collisions between two protons, inside the CMS detector. Credit: CERN.
Imperial’s Professor Sir Tejinder ‘Jim' Virdee was one of five physicists who first proposed the CMS experiment in the 1990s and, along with several colleagues from Imperial, oversaw the development and construction, becoming a driving force behind many of the major technology decisions for the detector.
Professor Virdee recently was awarded the Royal Society's Royal Medal for “extraordinary leadership and profound impact on all phases of the monumental CMS experiment at the CERN Large Hadron Collider, including the crucial discovery of the Higgs boson through its decays to two photons”.
We’re finding new things we can do with this scientific instrument that we never imagined possible when it was designed – when it was just a twinkle in Jim Virdee's eye, thirty years ago.
The CMS experiment is now one of the largest international scientific collaborations in history, involving more than 4,000 particle physicists, engineers, technicians, students and support staff from around 240 institutes in more than 50 countries.
Dark matter searches
Not to start on a downer, but one thing the CMS hasn’t found so far is dark matter. But it’s not for lack of trying: the CMS Collaboration summary reviewed over 40 different searches for dark matter the experiment has conducted since 2015. And, on the plus side, there are still plenty more places it can look.
Observations of the cosmos tell us that dark matter makes up roughly 80% of all the matter in the Universe. However, no dark matter particle has ever been observed, because it’s expected to interact very weakly with ordinary matter.
There’s strong evidence that dark matter exists; we just haven’t found it yet. We need to continue to push and innovate by developing new ways to analyse the CMS data.
What an interaction might look like relies on what form dark matter takes – and there are plenty of theories. For example, dark matter can be searched for in space, in deep underground tanks of liquids, or, as at CMS, in particle collisions that may produce it.
Here, it could be found in three ways: through ‘visible’, ‘invisible’ or ‘long-lived’ signatures in the detector. Visible signatures arise when a dark matter particle quickly decays into other known particles, in predictable ways, and these known particles are then detected by the experiment.
There are many different dark matter theories studied by CMS scientists searching for each of these different types of signature, including fantastic names such as Axion-Like Particles, Dark Higgs bosons, Hidden Valleys, versions of Supersymmetry, and Hidden Abelian Higgs bosons. So far though, no evidence for any of these models has been found.
We’ve basically ruled out the ‘low-hanging fruit’ models that would produce obvious signatures, and now we’re looking for the more subtle signs. We’re narrowing down which theories are relevant and which are not.
The researchers say they won’t give up though – while the summary paper looks at what has been done so far, they’re also looking to the future. The LHC and CMS itself will be going through upgrades that will allow them to make new kinds of searches with larger data samples, and new colliders are being discussed – even a possible 100km circular collider, three times the size of the current LHC.
The vacuum tank of the CMS magnet system consists of inner and outer stainless-steel cylinders and houses the superconducting coil, which can help us produce collisions that may release dark matter. Credit: CERN.
The vacuum tank of the CMS magnet system consists of inner and outer stainless-steel cylinders and houses the superconducting coil, which can help us produce collisions that may release dark matter. Credit: CERN.
The crystals used in CMS's electromagnetic calorimeter are made of lead tungstate, a material heavier than steel! It is highly transparent and emits light when electrons and photons pass through it, making it sensitive to potential dark matter candidates. Credit: CERN.
The crystals used in CMS's electromagnetic calorimeter are made of lead tungstate, a material heavier than steel! It is highly transparent and emits light when electrons and photons pass through it, making it sensitive to potential dark matter candidates. Credit: CERN.
'Enriching the physics program of the CMS experiment via data scouting and data parking' by CMS Collaboration is published in Physics Reports. DOI: 10.1016/j.physrep.2024.09.006
'Dark sector searches with the CMS experiment' by CMS Collaboration is published in Physics Reports. DOI: 10.1016/j.physrep.2024.09.013
'Review of searches for vector-like quarks, vector-like leptons, and heavy neutral leptons in proton–proton collisions at √s = 13 TeV
at the CMS experiment' by CMS Collaboration is published in Physics Reports. DOI: 10.1016/j.physrep.2024.09.012
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