Higgs Pt. 6: ATLAS and CMS – catching muons and neutrinos

-August 28, 2012

In Part 5, electrons, positrons, photons, and hadrons are all measured, accounted for, and absorbed within the central detectors leaving muons and neutrinos and, possibly, something heretofore never observed.

Muons are charged objects that don’t interact with matter enough to be stopped and absorbed even by the tons material that compose collider detectors. That they pass all the way through the detector leaving ionized trails behind makes them easy to identify. Being easy to identify means that electrons or hadrons are almost never mistaken for muons and that means Higgs decays to final states with muons have high signal to noise ratios. That high SNR means that muons play a key role in discovery so the detectors are designed with a lot of attention to them.

The CMS approach is simple. The extremely high magnetic field in the central detector, 4 Tesla, allows precise measurement of muon momentum. The muons propagate all the way through both calorimeters, the solenoid magnet, and out to the cleverly named “muon chambers.” The CMS muon chambers consist of four layers of thick iron plates interspersed with tracking electronics. The tracking electronics includes drift tubes (sealed regions of ionizing gas), cathode strip chambers, and resistive place chambers.

ATLAS takes a completely different approach to measuring muon momentum and energy. Where CMS uses an intense central magnetic field to measure muon momentum, ATLAS has a special muon spectrometer: a toroidal magnet wrapped around the central detector. A toroid has the geometry of a donut. Think of the central detector as the donut hole and the muon toroid as the donut.

The ATLAS muon spectrometer is based on the toroidal magnetic field generated by the coils in the pipes shown here (photo courtesy of CERN).

The outer toroid (donut) consists of big superconducting coils; the big pipes in the photo at the outer diameter of the ATLAS detector. The magnetic field is a closed circle contained within the toroid with just a bit of leakage. Thousands of long sealed tubes parallel to the axis of the detector (i.e., parallel to the donut hole) within the toroidal field detect the ionized gas trails of passing muons. The trajectory through the magnetic field yields their momentum and energy.

The detection of neutrinos is dicey in the best of circumstances. You and I are constantly bombarded by millions of neutrinos. Don’t fret, though, the probability that any interact with your body is negligible plus, there’s no way to shield them.

Neutrino logic gets kind of circular: there’s no way to shield neutrinos because they hardly interact with matter and because they hardly interact, they’re very difficult to detect.

Detecting neutrinos requires huge dedicated devices that operate on long timescales and only observe a small fraction of the neutrinos that pass through. ATLAS and CMS have to read out their millions of channels of electronics at 40 MHz so neutrinos are detected by ATLAS and CMS through their very absence: If something’s missing, it must be a neutrino.

Here’s how it works. Having measured the momentum and energy of everything else, and knowing that the initial state of the colliding protons has no momentum perpendicular to the beam, any observation of “missing momentum” indicates something invisible.

Many theories, including supersymmetry (an intimate descendent of superstrings), predict other invisible particles, but neutrinos are the only known invisible particle so the first assumption when something is missing is that it is a neutrino.
Stay tuned, in Part 7 we can finally look at a few of the collisions believed to have formed Higgs bosons. We’ll also ask how do we know if it’s the Higgs?

(The author, Ransom Stephens, very nearly made a career of building muon tubes for the ATLAS experiment. “Missed me by that much.”)

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