April 22nd, 2013

Compact Muon Solenoid (CMS)

The Compact Muon Solenoid (CMS) is located in an underground detector hall a hundred metres below the village of Cessy, France, almost directly across the diameter of the LHC from CERN headquarters and the ATLAS detector. Even before you walk into the building, it's apparent that CMS is “compact” only in a relative sense—everything you see is on a gargantuan scale.

Prof. Karsten Eggert of the TOTEM experiment gave us an introductory briefing in the CMS conference room, describing the LHC, CMS, TOTEM, and the science performed by the detectors we were about to see.

Since the LHC and CMS are in a two year shutdown for maintenance and upgrades, the CMS control room was a sea of blank screens.

Hats on! We wait for the elevator to take us down into the detector cavern. The cavern is 100 metres below the surface, so the elevator ride takes a while. This is one of the few places on Earth where people are counseled that in case of fire or release of nitrogen or helium (both of which are nontoxic but will cause asphyxiation by displacing breathable oxygen) to take the elevator to escape. The elevator is equipped with a positive pressure system for such emergencies. There is an emergency stairway, but the chance of escaping a major asphyxiation event by climbing the height of a thirty storey building would be nil without an auxiliary oxygen supply. Workers in the detector halls and tunnel carry an emergency breathing pack, but it is intended only to allow them to get to the elevator.

Going down—all aboard!

This is the shaft to the surface through which the pieces of the detector were lowered to be assembled in the cavern.

This door leads to the accelerator tunnel. Only a few people are allowed in there, most certainly not including us.

Having made our way from the elevator through a series of tunnels, we're about to enter the CMS detector hall.

The detector is surrounded by racks of electronics which perform the first-level signal processing and event filtering

The detector is in two pieces which can be separated, as presently, for maintenance. The beam pipe runs through the centre. Note the people on the work platform for scale. When assembled for operation the detector is 25 metres long, 15 metres in diameter, and weighs around 12,500 tonnes. The magnetic field of the superconducting solenoid magnet is 3.8 Tesla.

The right side contains an electron calorimeter, preshower detectors, half of the hadron calorimeter, muon tracking chambers, and the forward calorimeter.

The left side of the detector includes the interaction zone with its barrels of close-in silicon trackers, and detectors of the same kinds as in the right section to cover that part of the solid angle around the interaction zone. The left face of the detector is mostly occupied by muon chambers.

This door provides access to the detector hall. Note the retina scanner to the right, inside the double doors. When I visited the LEP in 1996, a system of tags, like those in mines, was used to control access and determine who was underground. People would sometimes lend their tags to others; the retina scanner puts an end to that (aside from James Bond scenarios, which somehow don't seem all that out of place in surroundings such as these).

Outside the detector hall racks and racks of electronics process signals in real time.

Silicon detectors like this detect particles which diverge from the point of collision. This is one petal of the “tracker end-cap”, which forms the circular ends of the cylindrical trackers closest to the interaction point.

These silicon “pixel” detectors run parallel to the beam pipe and are the first to detect particles created in a collision. Each detector segment alternates between detectors and electronics, with the other side having electronics and detectors offset so that most space is covered by a detector. I didn't ask about the radiation hardening requirements for solid state electronics to operate in this environment. This unit is part of the “tracker outer barrel” assembly.

Since the detector is too big to comprehend with human sight from close quarters, this model allows visitors to understand what they've just glimpsed. This document [PDF] provides technical details of the CMS detector.

This massive crane was used to lower pieces of the detector from the surface, where it was initially assembled and tested, down into the detector hall.

Now that's a cool vanity plate!

by John Walker May 12th, 2013

This document is in the public domain.