The High-Luminosity LHC (HL-LHC) is a major upgrade of the Large Hadron Collider (LHC). The LHC collides tiny particles of matter (protons) at an energy of 13 TeV in order to study the fundamental components of matter and the forces that bind them together. The High-Luminosity LHC will make it possible to study these in more detail by increasing the number of collisions by a factor of between five and seven.
Prototype of a quadrupole magnet for the High-Luminosity LHC. (Image: Robert Hradil, Monika Majer/ProStudio22.ch)
What is luminosity?
Luminosity, which is the measure of the number of potential collisions per surface unit over a given period of time, is an essential indicator of an accelerator’s performance. Integrated luminosity is measured in inverse femtobarns (fb−1); one inverse femtobarn equates to 100 million million collisions.
By the end of its first few years of operation at 13 TeV (at the end of 2018), the LHC should have produced 150 inverse femtobarns of data. The HL-LHC will produce more than 250 inverse femtobarns of data per year and will be capable of collecting up to 4000 inverse femtobarns.
The phenomena that physicists are looking for have a very low probability of occurring and this is why a very large amount of data is needed to detect them. Increasing luminosity produces more data, allowing physicists to study known mechanisms in greater detail and observe rare new phenomena that might reveal themselves. For example, the High-Luminosity LHC will produce at least 15 million Higgs bosons per year, compared to around three million from the LHC in 2017.
How will the High-Luminosity LHC work?
Increasing the luminosity means increasing the number of collisions: at least 140 collisions will be produced each time the particle bunches meet at the heart of the ATLAS and CMS detectors, compared to around 40 at present. To achieve this, the beam will need to be more intense and more focused than at present in the LHC. New equipment will need to be installed over about 1.2 of the LHC’s 27 kilometres.
- More powerful focusing magnets and new optics
New, more powerful superconducting quadrupole magnets will be installed on either side of the ATLAS and CMS experiments to focus the particle bunches before they meet. These magnets will be made of a superconducting compound, niobium-tin, used for the first time in an accelerator, which will make it possible to achieve higher magnetic fields than the niobium-titanium alloy used for the current LHC magnets (12 teslas as opposed to 8). Twenty-four new quadrupole magnets are currently in production. The use of niobium-tin magnets is an opportunity to test this technology for future accelerators. New beam optics (the way the beams are tilted and focused) will notably make it possible to maintain a constant collision rate throughout the lifespan of the beam.
- “Crab cavities” for tilting the beams
This innovative superconducting equipment will give the particle bunches a transverse momentum before they meet, enlarging the overlap area of the two bunches and thus increasing the probability of collisions. A total of sixteen crab cavities will be installed on either side of each of the ATLAS and CMS experiments.
- Reinforced machine protection
As the beams will contain more particles, machine protection will need to be reinforced. Around one hundred new, more effective collimators will be installed, replacing or supplementing the existing ones. These devices absorb particles that stray from the beam trajectory and might otherwise damage the machine.
- More compact and powerful bending magnets
Two of the current bending magnets will be replaced with two pairs of shorter bending
What civil engineering work is required for the High-Luminosity LHC?
The new equipment for the High-Luminosity LHC requires civil engineering work to be undertaken on the sites of the ATLAS experiment in Meyrin, Switzerland (LHC Point 1) and the CMS experiment in Cessy, France (LHC Point 5).
On each site, the underground constructions will consist of:
- A shaft around 80 metres deep
- An underground service hall that will notably house cryogenics equipment
- A 300-metre-long tunnel for electrical equipment (power converters)
- Four tunnels measuring around 50 metres in length, connecting the new structures to the accelerator tunnel. These will house specific hardware, such as radiofrequency equipment.
On each site, the surface work consists of constructing five new buildings, representing a total surface area of 2800 m2. These will house the cooling and ventilation equipment, as well as electrical equipment. These buildings will be constructed outside the current site perimeters at Cessy and on a site made available by the Swiss Confederation at Meyrin.
Who is carrying out the work?
Two consortia won the call for tenders for the civil engineering work. Each will employ up to 70 people on each site during peak periods.
What is the work schedule?
Work began in April 2018 and should last four years. The underground work (excavation of shafts, caverns and tunnels) will be carried out first and should be completed in 2021. The surface buildings will be constructed between 2020 and 2022. The surface work will be carried out only on working days.
What will happen to the excavated earth?
Around 100 000 m3 of earth will be excavated to create the underground structures. The excavated material will be analysed on the surface to check its quality. On the Meyrin site, much of the excavated material will be reused to create a platform on which to erect the buildings. The rest will be taken to a treatment centre. At Cessy, almost all of the excavated material will be taken to inert waste storage facilities located less than 20 km from the worksite, to limit the transport distance. On both sites, the topsoil will be reused for landscaping.
How will the work affect traffic?
- Road traffic
During the excavation period, lorries will transport the spoil to treatment or storage centres. A maximum of 10 lorries will come and go on the Meyrin site each day, and 10 to 15 on the Cessy site. Spoil will be transported only during working days (Monday to Friday) and, in Meyrin, during off-peak hours (9.30 a.m. to 12.00 p.m. and 1.00 p.m. to 4.30. p.m.).
- Footpaths and cycle paths
In Meyrin, the footpath linking the Maisonnex sports complex to the Chemin de la Berne (north-north-easterly direction) will be closed while the work is taking place; access to the complex will be possible via the Route de Meyrin and the path along the border.
In Cessy, the pedestrian and cycle path around the site (Chemin du Milieu and Chemin de Mouillets) will stay open and will be sheltered from the worksite.
How will the work affect the environment?
The contracts concluded with the two civil engineering consortia impose environmental restrictions and, notably, the hiring of experts to do environmental monitoring on the worksite.
The noise generated by the work will be limited in order to respect French and Swiss regulations. An acoustic system will monitor noise levels at various times. Measures will be taken to limit noise, such as the construction of a temporary building with noise barriers above