The LHC accelerates two beams of protons in opposite directions around a 26.659 kilometre superconducting ring about 100 metres underground, then steers them into head on collisions inside four detectors. Superconducting magnets cooled to 1.9 kelvin bend the beams; radio frequency cavities push the protons to 99.999999 percent of light speed.
Start Small: The Injector Chain
No accelerator goes from zero to full energy in one machine. Protons begin as hydrogen, are stripped of their electrons, and climb an energy ladder built from CERN's older accelerators: a linear accelerator (Linac4), the Booster, the Proton Synchrotron from 1959, and the Super Proton Synchrotron. Each hands its beam to the next, and the SPS injects protons into the LHC at 450 GeV. The big ring then takes them the rest of the way, to 6.8 TeV per beam in Run 3 and 7 TeV in the coming Run 4.
The Ring: 1,232 Superconducting Dipoles
A proton moving near light speed does not turn willingly. Bending the beam around the 26.659 km circumference takes 1,232 dipole magnets, each 15 metres long, producing fields up to 8.3 tesla, about 100,000 times stronger than Earth's magnetic field. Fields that strong require superconductivity: the niobium titanium coils carry around 11,000 amps with zero electrical resistance, but only if kept absurdly cold.
Hence the cryogenics. Superfluid helium holds the magnets at 1.9 kelvin, colder than the 2.7 kelvin background temperature of outer space. The LHC is often called the coldest large scale installation on Earth, and with 36,000 tonnes of material at that temperature, the claim is fair. In between the dipoles, quadrupole magnets act as lenses, alternately squeezing the beam horizontally and vertically to keep it from spreading.
Speeding Up: The RF Cavities
Acceleration happens in radio frequency cavities that oscillate 400 million times per second. Each passing proton gets a small forward kick, and because a proton makes about 11,245 laps every second, small kicks compound fast. The cavities also organise the beam into bunches: not a continuous stream but up to about 2,800 packets per beam, each containing on the order of 100 billion protons, spaced 25 nanoseconds apart.
The Collision
At four points around the ring, the two counter rotating beams are focused down to a width measured in millionths of a metre and crossed. Even then, protons are so small that almost all of them miss; the machine's whole art is making "almost all" slightly less total. The rate of hits is what physicists call luminosity, and it is the number the entire High Luminosity upgrade exists to raise.
When protons do collide, their energy converts into new particles under E = mc2, spraying debris into the surrounding detector. Most of those particles are unstable and decay within fractions of a nanosecond, so what the detectors actually record is the shrapnel of the shrapnel.
The Four Detectors
- ATLAS and CMS: general purpose detectors the size of buildings, designed to see everything and cross check each other. Both discovered the Higgs independently in 2012.
- ALICE: specialised for lead ion collisions, which recreate the quark gluon plasma that filled the microsecond old universe.
- LHCb: an asymmetric detector built to study particles containing beauty quarks and the subtle matter antimatter differences they reveal.
The Energy Bill of a Discovery Machine
Running the coldest place on Earth next to the highest energy collisions on Earth is not cheap in any currency. When the accelerator complex is running flat out, CERN draws about 200 megawatts from the French grid, roughly a third of the city of Geneva's consumption, most of it feeding the cryogenics and the magnets. Each beam at full intensity stores hundreds of megajoules, and the magnets themselves hold gigajoules more, which is why the machine protection systems that monitor for a quench react in thousandths of a second. During the shutdown the power draw falls dramatically, one of the few upsides of a dark machine.
From Collision to Discovery
The detectors face an impossible firehose: hundreds of millions of collisions per second, far too many to record. Trigger systems, layers of electronics and software making microsecond decisions, keep only the few thousand most interesting events per second. Even after that filtering, the experiments write petabytes per year, distributed to computing centres worldwide for analysis. A discovery is not a flash on a screen; it is a statistical excess emerging from years of recorded collisions.
Right now the beams are off while Long Shutdown 3 rebuilds the collision regions. The timer tells you exactly how long until this whole chain lights up again.