The High Luminosity LHC (HL-LHC) is a major upgrade of the Large Hadron Collider designed to increase its luminosity, the rate at which it produces collisions. Over its lifetime it will deliver roughly ten times more collision data than the original LHC, reaching about 3,000 inverse femtobarns. First beams are targeted for June 2030.
Same Ring, New Heart
The HL-LHC does not raise the collision energy in any dramatic way. Energy is set by the bending magnets around the full 27 kilometre ring, and those stay. What changes is everything around the collision points. About 1.2 kilometres of accelerator either side of ATLAS and CMS is being removed and rebuilt during Long Shutdown 3, with hardware that squeezes the beams far harder and keeps them colliding efficiently for hours.
The result is measured in luminosity. The original LHC was designed for a peak of 1x1034 collisions per square centimetre per second. The HL-LHC will run levelled at five times that, with the machinery capable of seven and a half times.
The New Hardware
Niobium Tin Inner Triplets
The focusing quadrupoles closest to the collision points, called the inner triplets, are being replaced with magnets wound from niobium tin instead of niobium titanium. Niobium tin sustains higher magnetic fields, around 12 tesla in these magnets, which lets them squeeze the beam to a smaller spot at the collision point. These are the first niobium tin magnets ever installed in a working collider.
Crab Cavities
Because the two beams cross at an angle, the bunches normally overlap only partially, like two cars clipping mirrors instead of meeting head on. Crab cavities are superconducting radio frequency cavities that rotate each bunch sideways just before the crossing so the overlap is nearly complete, then rotate it back. The name comes from the sideways motion. No hadron collider has used them before.
Superconducting Links and New Collimators
Powering the new magnets means moving tens of thousands of amps from surface buildings down into the tunnel, done with flexible superconducting cables made of magnesium diboride, another first. Meanwhile the collimation system, the machine's crash barriers, is being reinforced, because the upgraded beams store roughly 700 megajoules, about the kinetic energy of a fully loaded high speed train.
Why More Data Instead of More Energy
The blunt answer: more energy requires a bigger tunnel or better dipole magnets across all 27 kilometres, and both belong to a future machine, not an upgrade. The scientific answer is that many of the most interesting measurements left at the LHC's energy are limited by statistics, not by reach. Rare processes occur in one collision out of billions or trillions. The only way to see them clearly is to produce far more collisions.
The flagship example is the Higgs boson's self interaction, the process where the field that gives particles mass interacts with itself. It is central to understanding why the universe's vacuum sits where it does, and it is so rare that the original LHC would never have collected enough events. Ten times the data puts it within reach.
What the Numbers Look Like
- Integrated luminosity target: around 3,000 fb-1 over the HL-LHC era, against roughly 450 fb-1 from the LHC's first fourteen years.
- Levelled luminosity: 5x1034 cm-2s-1, held constant for hours by gradually tightening the focus as the beams burn down.
- Pileup: up to about 140 to 200 overlapping proton collisions per bunch crossing, versus around 60 in Run 3. This is why ATLAS and CMS are rebuilding their trackers and adding timing detectors.
- Higgs bosons: on the order of 15 million produced per year at full rate.
Who Is Building It
The HL-LHC is CERN's project but not CERN's alone. The niobium tin magnet programme was shared with a consortium of United States national laboratories, which built roughly half of the new inner triplet magnets. Japan's KEK laboratory contributed corrector magnets and cavity work, Italy's INFN built other magnet families, and institutes across Europe, Canada and China supplied cavities, cryogenics and collimation hardware. Components were built and tested over a decade on three continents, then shipped to Geneva for the one four year window when the tunnel is open.
That distribution is normal for machines this size, and it is part of why the schedule is what it is: the restart date is really the sum of hundreds of delivery schedules, test stands and installation slots, all converging underground between now and 2030.
When You Can Watch It Happen
Installation runs through 2029, hardware commissioning follows, and first beams are targeted for June 2030, the date our LHC timer counts down to. The first data taking period of the new machine is Run 4, and the plan stretches to around 2041. The HL-LHC is not a victory lap for the LHC. It is the main event, with two thirds of the collider's lifetime data still ahead of it.