LHC Run 4: The First High Luminosity Run

Where Run 4 Sits

The LHC's life alternates between runs and long shutdowns: Run 1 found the Higgs boson, Run 2 measured it, Run 3 pushed the dataset to records, and each pause between them rebuilt something. Run 4 follows Long Shutdown 3, the pause that turns the machine into the HL-LHC. The full sequence from 2008 to 2041 is on our timeline page.

When Run 4 Starts and How Long It Lasts

First beams are targeted for June 2030. After beam commissioning, physics data taking begins and continues for roughly four to five years before the next scheduled stop, Long Shutdown 4, in the mid 2030s. Precise year boundaries tend to shift as CERN tunes the schedule, which is why our timer tracks the one date that anchors everything else: first beams.

What Is Different From Run 3

Luminosity, Levelled

In earlier runs, luminosity peaked at the start of each fill and decayed as protons burned off. Run 4 introduces luminosity levelling as the standard mode: the machine starts with the beams deliberately under focused, then tightens the squeeze continuously to hold the collision rate constant at 5x10³⁴ for hours. The result is a steadier torrent of data and much less wasted beam.

Pileup

Each bunch crossing will contain up to around 140 to 200 simultaneous proton collisions, compared with about 60 in Run 3. Every interesting event is buried in dozens of mundane ones that happened in the same 25 nanosecond window.

New Detector Eyes

That pileup is why the experiments spent LS3 rebuilding themselves. ATLAS replaced its inner detector with an all silicon tracker. CMS replaced its tracker and installed a high granularity endcap calorimeter with millions of channels. Both added dedicated timing layers that stamp each track to a few tens of picoseconds, letting software separate collisions that happened in the same place but fractions of a nanosecond apart. Trigger and readout systems were rebuilt to skim this flood in real time.

The Physics Menu

• Higgs self coupling. The headline goal. Producing pairs of Higgs bosons probes how the Higgs field interacts with itself, a number tied to the stability of the vacuum. It needs the full HL-LHC dataset, and Run 4 starts the clock.
• Precision as discovery. Measuring known processes so precisely that any deviation from the Standard Model's predictions becomes visible. With ten times the data, percent level measurements become per mille level.
• Rare decays. Processes the Standard Model says should be vanishingly rare are exactly where new physics would distort the odds. LHCb's upgraded detector is built for this hunt.
• Direct searches, continued. More data extends reach for heavy or weakly interacting new particles, especially in hard to trigger corners the experiments could not previously afford to record.

Run 4 vs Run 3, by the Numbers

• Collision energy: 13.6 TeV becomes 14 TeV, the machine's original design value, reached at last with the consolidated magnet circuits.
• Peak luminosity: roughly 2x10³⁴ in Run 3 becomes a levelled 5x10³⁴, held flat for hours instead of decaying.
• Pileup: around 60 simultaneous collisions per crossing becomes up to 140 to 200.
• Stored beam energy: about 400 megajoules becomes roughly 700, which is why the collimation system was rebuilt.
• Data per year: the experiments expect to collect in roughly two years what Run 3 gathered in four.

Behind each of those numbers sits shutdown hardware: the triplets and crab cavities for luminosity, the new trackers and timing layers for pileup, and rebuilt trigger farms for the data rate.

What Run 4 Is Not

It is not a big energy jump. Collision energy rises modestly from 13.6 TeV to the design value of 14 TeV; the transformation is in rate, not reach. And it is not the end of the story. After Run 4 comes another shutdown and then Run 5, carrying the HL-LHC programme to around 2041, by which time decisions about the next collider will be well underway. Until first beams, the countdown is the scoreboard.