A major upgrade of the world’s largest science experiment, the Large Hadron Collider (LHC) at CERN in Geneva, has begun with the eventual aim of increasing the number of collisions in the large experiments by over five times and thus boosting the probability of the discovery of new physics phenomena and expanding our understanding of the Universe.
UK researchers, physicists and engineers are playing a key role in this work that will, by 2026, have considerably improved the performance of the LHC. This includes work on the construction of the accelerator, the machine contributions in HL-LHC-UK to the high intensity collimation system, the crab cavity system, the advanced beam diagnostics and cold powering.
Lancaster University’s Dr Graeme Burt, who is also from the Cockcroft Institute, is the HL-LHC-UK Project Manager.
He said: “We don’t create Higgs boson in every collision, in fact they are very rare, so we need billions of collisions to make each Higgs. For rarer particles at the current collision rate it will take decades to create enough particles to study. HL-LHC will allow us to have much more collisions and we can bring that timescale down to a few years.
“HL-LHC will use new technologies that needed to be developed specially for the machine. Two of the most important are the use of compact “crab” cavities to rotate the beams into a perfect collision, and using special superconducting cables to take the huge electrical currents from the surface to the tunnel without losses. The UK along with CERN have led the development of these technologies and we are currently investigating if these can be made in the UK for the project.”
Professor Roger Jones, Head of the Department of Physics at Lancaster University, is also involved.
He said: “Key to the new discoveries from the HL-LHC is the much larger data volumes and the rates with which the data are produced. This is posing exciting challenges to upgrade our software and computing, challenges that the UK is meeting with enthusiasm. We hope the lessons we learn in the process can help other major scientific projects.”
While the LHC is able to produce up to 1 billion proton-proton collisions per second, the HL-LHC will increase this number, referred to by physicists as “luminosity”, by a factor of between five and seven, allowing about 10 times more data to be accumulated between 2026 and 2036. This means that physicists will be able to investigate rare phenomena and make more accurate measurements.
Luminosity is a key performance indicator of an accelerator, as it tells you the number of particles colliding in a certain amount of time. Since discoveries in particle physics are based on collecting large amounts of data, then the greater the number of collisions the greater the chance hysicists have of seeing a new particle.
Professor Tara Shears, Professor of Physics at the University of Liverpool explains that:
“HL-LHC isn’t to study what we already know, it’s to discover what we don’t. There are so many mysteries; dark matter, antimatter, gravity. HL-LHC will reveal the universe in intricate detail and, we hope, give us some answers.”
In 2012 the LHC allowed physicists to unearth the Higgs boson, thereby making great progress in understanding how particles acquire their mass. The HL-LHC upgrade will allow the Higgs boson’s properties to be defined more accurately, and to measure with increased precision how it is produced, how it decays and how it interacts with other particles. In addition, scenarios beyond the Standard Model will be investigated, including supersymmetry (SUSY), theories about extra dimensions and quark substructure (compositeness).
The secret to increasing the collision rate is to squeeze the particle beam at the interaction points so that the probability of proton-proton collisions increases. To achieve this, the HL-LHC requires about 130 new magnets, in particular 24 new superconducting focusing quadrupoles to focus the beam and four superconducting dipoles. Both the quadrupoles and dipoles reach a field of about 11.5 tesla, as compared to the 8.3 tesla dipoles currently in use in the LHC.
Sixteen brand-new “crab cavities” will also be installed to maximise the overlap of the proton bunches at the collision points. Their function is to tilt the bunches so that they appear to move sideways – just like a crab. Much of this initial work has been carried out by a UK team and they successfully tested the new ‘crab cavities’ technology and rotated a beam of protons for the first time in May this year.
"The High Luminosity upgrade of the LHC is a massive step forward in the sensitivity and precision that we will have to explore the building blocks of the universe. It's a very exciting time for fundamental science" said Dr Rob Appleby, Manchester University/Cockcroft Institute and HL-LHC-UK Spokesperson.
Professor Jon Butterworth, Professor of Physics at UCL, added: "The LHC is our best microscope for studying the smallest constituents of the nature. The HL-LHC is effectively an increase in resolution, and I'm excited by what it might reveal."
The HL-LHC project started as an international endeavour involving 29 institutes from 13 countries. It began in November 2011 and two years later was identified as one of the main priorities of the European Strategy for Particle Physics, before the project was formally approved by the CERN Council in June 2016. After successful prototyping, many new hardware elements will be constructed and installed in the years to come. Overall, more than 1.2 km of the current machine will need to be replaced with many new high-technology components such as magnets, collimators and radiofrequency cavities and UK scientists will have a key role to play in contributing to that work.
Another key ingredient in increasing the overall luminosity in the LHC is to enhance the machine’s availability and efficiency. For this, the HL-LHC project includes the relocation of some equipment to make it more accessible for maintenance. The power converters of the magnets will thus be moved into separate galleries, connected by new innovative superconducting cables capable of carrying up to 100 kA with almost zero energy dissipation.
To allow all these improvements to be carried out, major civil-engineering work at two main sites is needed, in Switzerland and in France. This includes the construction of new buildings, shafts, caverns and underground galleries. Tunnels and underground halls will house new cryogenic equipment, the electrical power supply systems and various plants for electricity, cooling and ventilation.
The LHC started colliding particles in 2010. Inside the 27-km LHC ring, bunches of protons travel at almost the speed of light and collide at four interaction points. These collisions generate new particles, which are measured by detectors surrounding the interaction points. By analysing these collisions, physicists from all over the world are deepening our understanding of the laws of nature.
During the civil engineering work, the LHC will continue to operate, with two long technical stop periods that will allow preparations and installations to be made for high luminosity alongside yearly regular maintenance activities. After completion of this major upgrade, the LHC is expected to produce data in high-luminosity mode from 2026 onwards. By pushing the frontiers of accelerator and detector technology, it will also pave the way for future higher-energy accelerators.
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