CERN’s Lucio Rossi and Michelangelo Mangano explain why the upgrade to the High Luminosity Large Hadron Collider is necessary, some of the challenges involved in this evolution, and what new physics they hope it will achieve.
When in 2010 the High Luminosity Large Hadron Collider (HL-LHC or ‘Hilumi’) Design Study took off, we had not yet discovered the Higgs boson. We were, of course, sure that we were soon going to discover it, and so there were, perhaps, certain questions that could be asked:
- Why propose an improvement to a machine that was just starting to prove fruitful?
- Why propose an improvement to the luminosity of the LHC and not an increase in energy? And
- Why not upgrade the LHC later, when we could know more about the Higgs boson and then orient the improvements around what we had discovered?
There are two dimensions (scientific and technological) to these questions. From the science perspective, it had been known since early studies conducted at the end of the 1990’s that, no matter what the LHC would discover, an extension of the nominal phase of operations would greatly add to our knowledge. Any LHC discovery, including the Higgs, was going to be a major revolution in physics. The LHC was designed and dimensioned to perform such discoveries. But the HL-LHC was to be tailored to achieve a thorough exploration of the properties of such newly-discovered phenomena.
The technological perspective
To appreciate the technological perspective, it is useful to start with a historical observation. The development of accelerators requires the deep study of innovative technologies. No less than 25 years are needed from the first concept of an accelerator to its construction. The first conceptual design of the LHC was done in 1983 and it took until 2008 for it to be completed.
The studies and developments for the LHC started when the LEP (the first machine installed in the present LHC tunnel) was not yet finished! However, the emergent technologies can often be used to extend the discovery capacity of the present machines. This was the case for the LEP, where the studies for the superconducting radiofrequency cavities (which were started around the end of the seventies) were used to boost the LEP and convert it in the LEP2, thereby doubling its energy.
When the LHC started its operation, there had been several studies on linear machines that could study the Higgs boson in detail, but it was also clear that, in much the same way as it happened for the LEP, the emerging technologies that could be used for future circular colliders could be used to boost the LHC. Increasing the energy would have required higher magnetic fields and much heavier investments, while increasing the luminosity was more realistic with ‘only’ 1.2km of LHC to redesign and upgrade. A proposed increase in luminosity also only implied the need to study several technological challenges that would have to be overcome in any case if we wanted to build a more powerful accelerator.
This was the starting point of the High Luminosity Large Hadron Collider study, launched by CERN in consortium with 15 European institutions and with the support of the European Commission’s FP7 Framework Programme. The decision to explore the technologies required to increase the luminosity of the LHC received a great boost in 2013 by the European Strategy for Particle Physics, which placed the HL-LHC as the ﬁrst priority project for the next decade.
Returning to the reason why the LHC was to be upgraded to the High Luminosity Large Hadron Collider: the LHC can produce up to one billion proton-proton collisions per second. The changes foreseen in the next two technical stops that will take place between 2019-2020 and 2024-2026 will allow 10 times more data to be obtained than during the current phase of operation. These data will improve the precision of the measurements, increase the sensitivity to rare phenomena, and extend the reach for the discovery of new heavy particles.
The additional precision will mostly benefit our understanding of the Higgs boson: does it behave as predicted by the Standard Model (SM) of particle physics, which predicted its existence, or does it deviate from it? Today’s data allows for the testing of only some of the Higgs properties, and with a precision limited to 10-20%. The question of whether the Higgs gives mass to all known particles, for example, is still open, since processes sensitive to its interactions with the lightest particles are extremely rare and beyond the LHC’s statistical reach. The HL-LHC data sample will improve the precision of the most prominent properties to the per-cent level, and will reveal some of the rarest Higgs interactions.
The search for new phenomena
The search for new phenomena beyond the Standard Model is not limited to possible deviations in the Higgs’ behaviour. New particles and interactions must exist to explain, amongst other things, the presence of dark matter in the cosmos or the asymmetry between matter and antimatter. Many theories exist to address these fundamental questions, and most of them predict visible consequences at the LHC. The High Luminosity Large Hadron Collider statistics will allow rare collisions in which individual quarks and gluons inside the protons carry the largest fraction of the proton energy, thus increasing the collision energy available for the creation of new particles. The tenfold increase in statistics will then increase the discovery reach at the highest masses by 30%.
Eight years have now passed since the beginning of the project, and in June this year a ceremony for the beginning of the civil engineering works took place. It has taken eight years to study, conceive and make prototypes of the equipment that will increase the luminosity and physics reach, i.e. the discovery potential, of the LHC; and it has taken eight years of collaborative effort, and the same will and enthusiasm are still needed to reach our goal: to turn on the new machine, the HL-LHC!
There are currently 29 institutes from 13 countries contributing to the construction of the HL-LHC components (almost 50 if we consider all types of contributions). The first study that culminated in 2015 with the first technical design report (TDR) gave a long list of components to replace. About 1.2km of the current LHC tunnel will be dismantled and reinstalled with new magnets, collimators, radio frequency cavities, vacuum systems, instrumentation to see the beam, and so on. In total, this will include more than 200 different components, added to which is the creation of an additional kilometre of tunnels, galleries and caverns. If we see this as a whole and not as an improvement of an existing accelerator, it is be the biggest accelerator currently under construction in the world.
HL-LHC’s luminosity increase
The luminosity increase will be achieved thanks to three different factors. The first is to improve the LHC injection chain in order to increase the number of protons circulating in the LHC. The improvement of the injectors is a project in itself (LIU, the LHC Injector Upgrade). But this is not sufficient: numerous pieces of equipment are being modified in the LHC to cope with the higher beam intensity, such as – but not limited to – collimators with low impedance materials, collimators in the cold regions coupled with new more powerful dipole (11 T, 30% above LHC field), more powerful absorbers, and more.
The second focuses on the components that currently limit the beam’s luminosity. To eliminate these limitations, new magnets will be built – in particular the so-called Inner Triplet quadruples which, in order to reach almost 12 T of peak field, use a superconducting cable in Nb3Sn, a superconductor that is more advanced than the Nb-Ti on which the LHC magnets are based.
‘Crab cavities’ will also be installed to maximise the superposition of the particle bunches at the collision points. These and other components will allow the operation of the beam to be changed and the packets of particles to be compressed at the interaction points (ATLAS and CMS) and go from the current 40 collisions taking place each time the two counter circulating beams cross each other in the detectors, to about 140.
Finally, the other key ingredient to increase the integrated luminosity is to improve the availability of the LHC. The more days the accelerator can be operated and the longer the beam can travel without unnecessary losses or stops, the more data can be taken. Therefore, High Luminosity Large Hadron Collider includes the relocation of equipment to make it more accessible for maintenance. Key elements for such relocation are an original double decker configuration for the new service galleries and new powerful 100 kA superconducting links.
All this work would be impossible without a close collaboration with industry. More than 1,000 suppliers from all over Europe are building components for the High Luminosity Large Hadron Collider. Taking into consideration the experience of the LHC, every component has been designed and analysed to maximise what can be done by industry and to ensure a timely and efficient transfer of knowledge from the laboratories to the production plants.
One of the most challenging factors we are facing concerns the small series to be produced for each piece of equipment. These are too big to be prototypes and too small to be series, and it has been challenging to convince industry to take an interest in the new technologies we are developing and to garner their early involvement in the prototyping phases.
The new components that will be built have been designed with their operability beyond the nominal values, and with technologies that will be necessary to build a new accelerator to increase the energy of the collisions not too far in future, in mind.
The HL-LHC will not only allow new physics to be achieved; it also explores and transfers to industry the technologies that will be necessary for new discoveries to be made in the future.
The High Luminosity Large Hadron Collider, however, is not limited to the major changes in the collider. All LHC detectors will be upgraded, especially the high luminosity detectors, ATLAS and CMS, which have an immense challenge ahead. Indeed, their challenges will be no less difficult than those experienced by the accelerator, especially when it comes to dealing with the 140 collision per bunch crossing and with the enormous quantity of data taking, storage and analysis.
Physicists and engineers from all over the world are designing the ‘eyes’ which will allow us to ‘see’ and understand the results of collisions.
As our theoretical friends say, we hope the High Luminosity Large Hadron Collider will be the new version of the ship that allowed Christopher Columbus to see that he had not, in fact, arrived at the West Indies, but rather had discovered a new continent!
This article will appear in SciTech Europa Quarterly issue 28, which will be published in September, 2018.