There are many subsystems in ALICE, each of them with different purposes.  The main tracking detector in ALICE is a Time Projection Chamber (TPC).  The TPC is cylinder about 5m (16.4 ft) in diameter and 5 m long filled with gas – the largest time projection chamber in the world.  You can see above that a person can actually fit in it.  The basic principles behind a TPC are really simple:

• A charged particle in a magnetic field moves in a circle with a radius r = p/qB where p is the particle’s momentum, q is the charge of the particle, and B is the magnetic field.  The magnetic field in the TPC is roughly constant, as shown below:

  

  so that a charged particle will bend either clockwise or counterclockwise, depending on its charge:

  

• A charged particle moving through a gas will ionize the gas, knocking electrons (called secondary electrons) off the gas molecules
• The TPC has an electric field (400 V/cm), as shown below, that causes the secondary electrons to drift to the end of the TPC (the ends of the cylinder):

  

• We have pads on the ends of the TPC which collect the charge and each of these pads has to be read out for every event.  The position on the end of the cylinder (usually called x and y) is determined by where the charge hits the end of the TPC.  The position along the beam axis (usually called z) is determined by the time it took the charge to get there and the drift velocity of the TPC.  (This conversion from time to distance is what gives the TPC its name – a time projection chamber.)
• This gives us a bunch of “hits” – the position of a charged particle in the TPC at different times so what we actually see are the red dots below:
  We then have to figure out which hits belong to which track.  Not so difficult if there’s only a few tracks but for heavy ion collisions we expect to create a few thousand particles in each collision.

  

• We have to collect data fast enough that the detector is ready for the next collision.  Collisions occur several hundred times per second.

A lot of details have to be just right to get a TPC to work well.  We have to know the electric field and the magnetic field very precisely.  The amount of charge left by a particle is sensitive to the type of gas and to the temperature.  We have to keep the temperature constant to within 0.1 degree Celcius.  Because the TPC is so large, keeping everything constant and well calibrated is very difficult.  But ALICE has done it.

And not only is it the biggest TPC in the world, but it’s also the best, in my humble opinion.

Here you can see some tracks reconstructed in the TPC from a 7 TeV proton-proton collision:

You can see some more event displays here.  Some animations of event displays collisions at 7 TeV in ALICE are here, here, and here.  (You can see some of the other detectors in these displays – I left them out of the diagrams above for simplicity.)

ps – Thanks to Jim Thomas, one of the many members of the TPC team, for helping me find event displays, technical details, and editing!