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