CESSY, France—As we drive through small villages in Eastern France, some for the second time, it's becoming increasingly clear that my tour guide is lost. She's got a stack of printed Google Maps, but, without a clear indication of where we actually are, it's tough to tell where in the stack she should be looking.

Eventually, on the other side of a corn field, she spots a building that looks out of place. Someone apparently ripped this structure out of an office park in New Jersey and dropped it into France. Before long, we step onto an elevator and step out into a scene that wouldn't look out of place in a science fiction movie. A building-sized heap of electronics dominates the underground cavern before us, and this contraption has an equally sci-fi-ish name: the Compact Muon Solenoid.

Best known for helping physicist Peter Higgs earn his Nobel Prize for the discovery of the Higgs boson, the CMS is one of two general purpose particle detectors built on the Large Hadron Collider (LHC) at CERN, which extends from Switzerland and into France. Normally when the LHC runs, the chambers housing the detectors experience intense radiation; security systems keep pretty much anyone from entering (and for good reason).

But for the past two years, the building hasn't been used for physics. Instead, physical repairs and improvements to all of the detectors, as well as the LHC itself, were made. A bit over a year ago, while the LHC was shut down and partly disassembled, I was lucky enough to be invited underground to see its massive detectors and walk the vast tunnels housing the particle accelerator itself.

CERN initially targeted March of 2015 for the return of protons to the LHC, but everyone knows the saying about best laid plans. According to Nature, metal debris in a diode box caused the most recent delay. That meant, following a planned two-year hiatus, the team behind LHC needed to fix one final short circuit before a triumphant return. In late March, they sent an electrical discharge through to burn away the problematic metal, and follow-up tests confirmed the fix worked.

Now, the LHC is finally gearing up for its second major scientific run. On the eve of its return, we're bringing you a rare look inside.

CMS

Ohio State University's Carl Vuosalo helped show us around the CMS, but first he had to shepherd us past higher security than I've ever experienced. To do so, he passed through a retina-scanning security system that simultaneously checked his weight (presumably to keep someone with a disembodied eyeball from making their way past the system). I passed it solely because of Vuosalo's ingenuity. He opened a door meant for the delivery of equipment, slipping me through as if I was a UPS shipment.

Even though the LHC was shut down, the team made safety the highest priority. They lectured on protocol; they issued me a hard hat. The greatest risk of death at the LHC, as it turns out, is suffocation. Liquid helium (120 tons of it, along with another 10,000 tons of liquid nitrogen) cool the accelerator hardware, while many parts of the giant detectors rely on liquid argon to track particles through them. Either of those will happily convert to a gas if let loose from their containers. In addition, the fire suppression system could fill the entire chamber the CMS resides in with foam in under a minute.

That says a lot. Despite its name, no one would call the CMS compact. At over 20 meters long and 15 meters across, it weighs about 14,000 tonnes—and the chamber itself is considerably larger. That's because the space has to be big enough to allow the detector to enter in pieces. Teams lowered the detector to the site in sections, and the sections were currently slid apart in order to allow workers to service the individual detectors. Despite the immense mass, CMS comes apart by floating on a cushion of air. Tubes allow compressed air to be pushed under the detector's feet, raising it just enough that its main parts can be slid around the chamber. Vuosalo said the noise involved is deafening even several stories above the chamber floor.

John TImmer

John Timmer



John Timmer



John Timmer



John Timmer



John Timmer

John Timmer

John Timmer

John Timmer

John Timmer



To some extent, compromise produced CMS. When first developing the site, the team discovered the ruins of a Roman village. This slowed things down, and some water leaking through the surrounding rock delayed things further. As a result, the cavern couldn't initially host the detector. Instead the team built everything on the surface and later lowered it into its home in sections (ATLAS, the other general purpose detector, was assembled in place). Vuosalo says they "made some budget compromises on construction" as a side effect.

With the long downtime, however, LHC can make corrections. Although the electronics are "one step below space grade," the ones closest to the site of collisions have a five-year lifespan. They need to be replaced.

The detector itself is a bit like an onion, with a layered structure. The inner-most layers house silicon-based particle detectors, which have very high spatial resolution. Outside of those sits a calorimeter, which measures the energies carried by electrons and photons. Beyond that, magnets cause charged particles to take a curved trajectory through the hardware, aiding in identification. Large areas of the detector are iron plates that help shape the resulting magnetic fields.

Next comes the hadron calorimeter, which measures the energy carried by particles composed of quarks—protons, neutrons, and their more exotic relatives. Muon detectors make up the outside, and these can be kept at some distance simply because muons are relatively stable particles (their 2.2 microsecond half-life is stretched out by relativity, since they're moving quickly relative to the detector hardware).

By tracking particles through the various detectors, it's possible to reproduce the tracks of everything that comes out of a collision and quickly figure out something about the physics. This is done at the detectors themselves, which have "triggers" that dictate whether collision data gets saved or not, based on whether it represents familiar behavior or something potentially interesting. Out of roughly 40 million collisions a second, the triggers ensure roughly 100 are kept, preventing a flood of data from overwhelming the entire system. Even with this heavy filtering, the LHC saves 15,000TB of data each year.

But aside from the Higgs, interesting phenomena have been hard to come by. Vuosalo is part of what's called the "exotica working group," a team looking for previously undiscovered particles. Sadly, there haven't been any hints of anything, he told Ars. "It's a bit scary that all we have is the Higgs." His hope is that we just haven't seen enough collisions yet—something the upgraded collider is intended to fix.

Another thing that got Vuosalo a bit nervous was when one of our group spotted a water bottle perched in the middle of a wall of electronics on the inner parts of the detector. He began to mutter under his breath and look for someone he could alert about its presence. But, as we continued to stare at the bottle, it became clear that it had a couple of unusual features: it was zip-tied to the detector, and a small tube ran out of the bottle and into the hardware. Eventually, Vuosalo agreed that it was probably put there for testing purposes and didn't represent a threat to the detector's health.

Listing image by John Timmer