Fermilab

Explaining the D0 detector

Fermilab is the world's highest energy particle accelerator. We create beams of high energy protons and antiprotons and bring them into collision with each other. These collisions allow us to see what holds protons together and what they are made of; the collisions also pack sufficient energy into a tiny point of space to create entirely new particles.

The collision points are surrounded by arrays of instrumentation called detectors. D0 is a fairly typical high energy physics detector. It measures thirty feet tall and fifty feet long and consists of four major parts.

1. Tracking system
The point where the beams collide is surrounded by "tracking detectors" to record the tracks (trajectories) of the high energy particles produced in the collision. The measurements closest to the collision are made using silicon detectors. These are flat wafers of silicon chip material. They give very precise information, but they are expensive, so we concentrate them closest to the beam where they don't have to cover so much area. The information from the silicon detector can be used to identify b-quarks (like the ones produced from the decay of a Higgs particle).

Outside the silicon, D0 has an outer tracker made using scintillating fibers, which produce photons of light when a particle passes through. The whole tracker is immersed in a powerful magnetic field so the particle tracks are curved; from the curvature we can deduce their momentum.

2. Calorimeter
Outside the tracker is a dense absorber to capture particles and measure their energies. This is called a calorimeter. It uses uranium metal bathed in liquefied argon; the uranium causes particles to interact and lose energy, and the argon detects the interactions and gives an electrical signal that we can measure.

3. Muon System
The outermost layer of the detector detects muons. Muons are unstable particles but they live long enough to leave the detector. High energy muons are quite rare and a good sign of interesting collisions. Unlike most common particles they don't get absorbed in the calorimeter so by putting particle detectors outside it, we can identify muons. Because the muon system has to surround all of the rest of the detector, it ends up being very large, and it is the first thing that you see when looking at D0.

4. Trigger system
Proton-antiproton collisions happen inside the detector 2.5 million times every second. We cannot record all those events; at most, perhaps 20 events per second can be stored on computer tape. The trigger is the system of fast electronics and computers than has to decide, in real time, whether an event is interesting enough to be worth keeping.

Simulated Higgs event

Simulation of the characteristic signature of a Higgs: the tracks and energy deposits it would make in the D0 detector. The proton-antiproton collision produced a Higgs particle, which decayed to two b-quarks (seen in the detector as energy in the calorimeter, and indicated by the red arrows at 11 and 5 o'clock), together with a W boson, whose decay products are an electron (the green track at two o'clock) and a neutrino (inferred from an imbalance in overall momentum - the blue arrow at twelve o'clock). This kind of "associated production" of a W together with the Higgs is much easier to identify than the Higgs alone. Discovery of the Higgs will require the accumulation and study of hundreds of events like this one.