A ground-breaking ceremony was held on the 15th of June, to celebrate the start of civil engineering work for a major upgrade to the LHC . When completed in 2026, the HL-LHC will produce a five to seven times higher rate of proton-proton collisions, providing better statistics and powering new discoveries that could answer the fundamental questions about our universe. The highlight of the first run of the LHC was the observation of a Higgs boson in 2012, and it has also provided a wealth of measurements of electroweak and QCD processes. However, we have only scratched the surface of the ultimate physics potential of the LHC. There are still many open fundamental questions in particle physics that can be addressed by increasing the luminosity of the collider.

The main pillars of its physics programme are precision measurements of the Standard Model including the Higgs boson, searches for new physics through the study of rare processes, searches for new heavy states, and measurements of the properties of any newly discovered particles. The imbalance between matter and anti-matter in the universe is a big open issue for flavour physics. Moreover collider searches for new weakly interacting massive particles will continue in HL-LHC as these particles are good candidates of Dark Matter. If evidence of deviations from the Standard Model are seen before the upgrade, the HL-LHC will allow further scrutiny of the new landscape of particle physics. In the absence of any such hint, the ten-fold increase in data will push the sensitivity for new physics into uncharted territory. 

"The High-Luminosity LHC will extend the LHC's reach beyond its initial mission, bringing new opportunities for discovery, measuring the properties of particles such as the Higgs boson with greater precision and exploring the fundamental constituents of the universe ever more profoundly," mentioned CERN's Director-General Fabiola Gianotti during the ceremony. 

The increase in the number of collisions in HL-LHC, means more observations of rare phenomena and more chances for discovery. As an example, the upgrade will increase the number of Higgs bosons that can be produced by the LHC from 1.2 million to 15 million.  The HL-LHC data will enable the improvement of the precision on Higgs boson couplings by a factor of 2 to 3 with respect to the previous LHC running.  

Furthermore, extensive searches of SUSY particles have found no compelling evidence so far, thus putting stringent limits on the masses of the potential supersymmetric partners. Supersymmetric models motivate a large set of searches, including those for strongly produced supersymmetric particles. The higher luminosity of the LHC will allow to search for rare processes including third generation squarks, electroweak-inos and particles predicted by various compressed SUSY models. 

Other exotic models of new physics have been explored including particles to explain the observed light mass of the Higgs and signatures that could account for the mysterious dark matter. The LHC probes various types of interactions and dark matter can be detected by tagging SM particles in the detector and identifying the recoiling dark matter particles from a missing transverse energy signature. These searches can benefit from the higher statistics available at the HL-LHC - that will allow to discern signals from the noisy background - and will complement the direct astroparticle searches for dark matter. The tenfold larger dataset of HL-LHC results in a 20% approximately increase in mass reach while it would also allow detailed studies of any newly discovered particle in the next runs of the LHC. 

Moreover, HL-LHC will allow to push many of the technologies needed for any future energy and intensity frontier collider that could come after the completion of the LHC's scientific programme in 2036. Since 2010, scientists, engineers and technicians from all over the world, have been conducting R&D on new technologies that would make operations at the HL-LHC possible. The project will involve the replacement of high-tech accelerator components along 1.2 kilometres of the machine. Indeed High Luminosity LHC (HL-LHC), relies on a number of key innovative technologies, representing exceptional technological challenges, such as cutting-edge 11-12 tesla superconducting magnets, very compact superconducting cavities for beam rotation with ultra-precise phase control, new technology for beam collimation and 300 m long high-power superconducting links with negligible energy dissipation, able to carry currents of record intensities to the accelerator, up to 100,000 amps, over 100 metres. 

“We have to innovate in many fields, developing cutting-edge technologies for magnets, the optics of the accelerator, superconducting radiofrequency cavities, and superconducting links,” explained Lucio Rossi, Head of the High-Luminosity LHC project. HL-LHC will also see the construction of new buildings, shafts, caverns and underground galleries, as well as tunnels and halls to house the new cryogenic equipment, as well as power supplies and cooling and ventilation kit.

All these technologies have been explored since 2011 in the framework of the HiLumi LHC Design Study - partly financed by the European Commission's FP7 programme. HiLumi LHC brought together a large number of laboratories from CERN’s member states, as well as from Russia, Japan and the US. American institutes participated in the project with the support of the US LHC Accelerator Research Program (LARP), funded by the U.S. Department of Energy. Some 200 scientists from 20 countries collaborated on this first successful phase.

Moreover, the LHC experiments plan their major upgrades to fully exploit the opportunities offered by the increased luminosity. The expected average number of simultaneous proton-proton (pp) collisions (pile-up) will increase from 40 to up to 200, making each event much larger in size and much more complex to record and analyse. Faster detectors and readout electronics, as well as sophisticated trigger systems to efficiently identify physics signatures, will be required. Finally, the detectors will need to tolerate a substantial radiation dose.

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. “Audacity underpins the history of CERN and the High-Luminosity LHC writes a new chapter, building a bridge to the future,” said CERN’s Director for Accelerators and Technology, Frédérick Bordry. “It will allow new research and with its new innovative technologies, it is also a window to the accelerators of the future and to new applications for society.

Further reading:

1. HL-LHC Preliminary Design Report: https://cds.cern.ch/record/2116337?ln=el

2. HL-LHC website: http://hilumilhc.web.cern.ch