Higgs Pt. 5: ATLAS and CMS – the biggest T&M devices on earth

-August 21, 2012

To review, in Part 1, I talked about what happens when protons collide in CERN’s LHC: a bunch of stuff comes out that’s measured by detectors. In Part 2, we described the stuff that comes out: electrons, photons, hadrons, muons, and neutrinos. Hadrons are made of quarks and gluons, the same sort of stuff as protons and neutrons. Muons are like really heavy electrons. Neutrinos barely interact with matter at all and go zipping through the detectors without leaving a trace. Part 3 described what the detectors measure: electric charge, energy, and momentum and Part 4, how the different stuff is identified in the detectors – I’ll revisit Part 3 and Part 4 today.

We’re finally ready to dive into the Test & Measurement technology, but first the acronyms. ATLAS is perhaps the silliest acronym ever devised; it stands for A Toroidal Lhc ApparatuS. CMS is more reasonable if less lyrical: Compact Muon Solenoid.

As stuff emerges from the proton collision, it penetrates the beam pipe and enters the inner detector. Both ATLAS and CMS use solid state detectors that provide high resolution. As charged particles pass through silicon, they ionize the medium and the collected charge lights up pixels that trace their paths. Two properties are measured in these high resolution solid state detectors: momentum and the location of any “secondary vertices.”

To measure momentum, both experiments have superconducting solenoid (i.e., cylindrical shell) magnets surrounding their inner detectors. The CMS magnet operates at 4 Tesla, the ATLAS solenoid operates at 2 Tesla. These fields are within the range of MRI magnets, though they cover about 1000 times the volume (for reference, a 1 Tesla magnetic field is about 25,000 times the strength of earth’s field). Recall from Part 3 that magnets curve the paths of charged particles. The radius of curvature yields a particle’s momentum (actually, just the component of momentum perpendicular to the proton beam, but let’s not get too caught up in details) and the direction that they curve reveals their charge. The larger CMS field provides greater momentum resolution at the expense of curling lower energy particles into tight circles that prevent them from being identified and measured by the detectors outside the central tracker.

Since the particles that make it into the detector are the decay products of whatever was formed in the proton collision – nothing like a Higgs or top quark survives long enough to get near the detector* – and since the collision results in particles of different lifetimes, some of the longer lived particles travel a few millimeters before decaying. The point at which these secondary decays occur is called a “secondary vertex.” The identification of secondary vertices indicates what types of particles were created in the collision, key information for discovering new phenomena.

As the particles emerge from the inner detectors they enter the calorimeters. The calorimeter that measures electron and photon energy is encountered first.

The ATLAS electromagnetic calorimeter is made of alternating layers of metal plates immersed in liquid argon. At room temperature and pressure, argon is an inert gas, so liquid argon has to be contained in a cryostat. It provides a clean dense medium that’s easily ionized by charged particles. When electrons and positrons reach the metal plates, they collide with the electrons in the metal and create showers of more electrons and positrons at ever smaller energies until they are absorbed and the shower ends. When photons hit the metal, they convert to pairs of electrons and positrons which cause showers of their own. The ionization left behind in the liquid argon is collected and compared to a calibration table to calculate the particle energies.

Installation of the ATLAS calorimeters. (Photo courtesy of CERN)

The CMS electromagnetic calorimeter is made of lead-tungstate, a dense crystal that generates light as charged particles decelerate. The light is detected by hybrid photo detectors attached to the end of each crystal. Hyrbid photodetectors have the light collecting front end of photo-multiplier tubes, light hits a cathode, releases an electron that is accelerated through a few stages, generates a small shower electric charge that is counted by a diodes. The amount of light recorded by the photodetectors is compared to a calibration table to calculate the particles’ energy.

Both the ATLAS and CMS hadronic calorimeters consist of alternating layers of absorber and plastic scintillator. ATLAS uses thick iron absorber oriented perpendicular to the beam line and CMS uses brass (an alloy of copper and zinc) absorber – most of which came from melted down World War II shell casings donated by the Russian navy.

Plastic scintillator is plastic that is doped with a fluorescent so that charged particles generate light as they pass through. The light tends toward the ultraviolet end of the spectrum, but the detectors are more efficient in the optical so wavelength shifting plastic fibers embedded in the scintillator both carry the light to detectors and shift its wavelength into the optical spectrum. ATLAS uses photomultiplier tubes to read out the light and CMS uses avalanche photo-diodes.

The CMS lead tungstate crystal calorimeter. Yes, it's transparent and green and more dense than stainless steel. (Photo courtesy of CERN)

There is an interesting subtlety in measuring electrons, positrons, and photons versus measuring hadrons. The two categories of particles interact in different ways with the calorimeter media. Hadrons are only stopped when they interact with an absorber nucleus but electrons (etc) are easily stopped by interacting with the absorber’s electrons. This difference means that converting the charge or light observed in the calorimeters into energy requires a different calibration table for each category of particles. And that means that misidentified particles’ energies are incorrectly measured. These, and all the other uncertainties and miss-measurements, have to be accounted for statistically before the experimental collaborations can contemplate any discovery.

Next time, I’ll cover the all important muon detectors and how the elusive neutrinos are “observed.”

(* The cavalier assumption that only known objects survive long enough to make it into the detector is mitigated by the experimentalists greatest desire: that they’re wrong. That something heretofore unobserved really will get into the detector!)

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