A lot of the articles in the news talk about the physics we’re trying to learn, but there’s not much discussion of how we do it. I spend most of my day thinking about our detectors, how they work, and how to interpret the data. Our detectors are the way we see, hear, feel, smell, and taste what goes on in proton-proton and lead-lead collisions. Just like the Mars rover explores places that are uninhabitable to people, our detectors explore places that are too small for us.
ALICE is made up of several subsystems, each of which helps us sense collisions in a different way. Below is a picture of ALICE with each subsystem labeled:
Some detectors are like our eyes – they help us see particles coming out of the collision. These are our tracking detectors. The main tracking detector is the Time Projection Chamber (TPC). We also have an Inner Tracking System (ITS), comprising three different silicon detectors, the Silicon Strip Detector, the Silicon Drift Detector, and the Silicon Pixel Detector. The ITS is a bit like our glasses – we can see particles with just the TPC, but with the ITS, the picture comes into sharper focus. Tracking detectors tell us the momentum and spatial location of particles that go through our detector.
Some detectors are our taste buds – they help us determine the flavor of the particles we’re measuring. A lot of different types of particles are produced in both a proton-proton collision and a lead-lead collision. The Time-Of-Flight (TOF), the Transition Radiation Detector (TRD), and the High Momentum Particle Identification Detector (HMPID) are all designed to identify particles. These detectors all work by measuring a particle’s velocity. Momentum is velocity times mass, so if we know the velocity and the momentum of a particle (which we can get from the tracking detectors), we can determine its mass and therefore figure out what kind of particle it is. The TOF measures the velocity of particles by measuring how long it takes for a particle to reach the TOF. Since velocity is the change in distance over time and the distance traveled is known, this measures the velocity of the particle. Here you can see one of our physics performance plots showing different particles identified by the TOF:
The x-axis is the momentum, as measured by the TPC and the ITS, and the y-axis is the ratio of the velocity to the speed of light in a vacuum. Pions (π) are the lightest particle (140 MeV/c2) so at a given momentum, they have the highest velocity. Protons (p) are the heaviest (938 MeV/c2) particle visible in the plot above so at a given momentum they have the lowest velocity.
The HMPID and the TRD both work on the same principle. The speed of light in a vacuum is constant, but the speed of light in a medium can be lower. For example, the speed of light is lower in water than in air – this is why images get distorted when you look through water. If a fast particle moves through a medium faster than the speed of light in that medium, it will emit photons – called Cherenkov radiation – until it slows down to the speed of light in the medium. At a given momentum, lighter particles go faster, so lighter particles will emit photons at a larger angle relative to their path. The medium in the TRD is optimized so that only electrons (0.5 MeV/c2) will radiate photons, so the TRD can be used to distinguish electrons from everything else. The HMPID is a ring imaging Cherenkov detector. The photons emitted by a particle are emitted in a cone and the radius of that cone depends on the velocity of the particle. The HMPID is optimized for distinguishing pions, kaons, and protons. Here you can see the signal from the HMPID:
The x-axis is the momentum and the y-axis is the angle of the cone of light emitted by the particle. At a given momentum, a pion is going faster than a kaon or a proton. The radius of the cone of light emitted by the particle is larger the further the particle’s speed is from the speed of light in the medium, so at a given momentum the pion band is above the kaon band, which is above the proton band.
The tracking detectors, the TPC and the ITS, can also identify particles. They work by measuring how much energy a particle loses as it travels through the detector. A heavier particle will loss more energy than a light particle. Think of one of those ball pits for kids. If you threw a tennis ball in, it would knock some of the balls out of the way. If you threw in a bowling ball, a lot of balls would get knocked around. We know the bowling ball lost more energy than the tennis ball because the lighter balls got knocked around more. We can distinguish between heavier particles and lighter particles like this. If the TOF, the HMPID, and the TRD are the way we taste the particles created in the collision, the ITS and the TPC help us smell them. Below you can see the signal from the TPC:
The x-axis is rigidity, which is the momentum over the charge. Charge is in units of the electron charge. All of the particles here have a charge of ±1. Positively charged particles are on the right and negatively charged particles are on the left. The y-axis is proportional to the energy lost by the particle in the TPC. We see the same three particles we saw before – pions, kaons, and protons – but now we also see deuterons and tritons. At a given momentum, heavier particles lose more energy, so as you go up the y-axis the mass of the particles increases.
My last post was on the Electromagnetic Calorimeter (EMCal). A calorimeter is used to measure particles’ energy. This is a way of feeling the collision – it’s like laying in the sun. When you lay in the sun, you don’t feel photons hitting you but when photons hit you, they warm you up. Particles hitting the calorimeter do the same thing – they hit the calorimeter and deposit their energy. (Everything loses energy except muons – muons travel right through the calorimeters.) We look at the energy deposited in the calorimeter to determine how much energy the particle had. (See my post on the electromagnetic calorimeter for more details.) We have two more calorimeters in ALICE, the Photon Spectrometer (PHOS) and the Zero Degree Calorimeter (ZDC). The PHOS is optimized to measure photons. The ZDC is a calorimeter very close to the beam pipe far away from the interaction point, at an angle close to zero degrees from the beam pipe. The ZDC is useful in lead-lead collisions for both measuring nucleons which did not participate in the collision. These particles are called “spectators” and are not deflected by the magnetics that keep the beam in the beam pipe because the spectators do not have the same charge to mass ratio as lead nuclei. We can figure out of the collision was head-on or just glancing using this information.
We hear the collision in the VZERO, a scintillator detector. When a particle hits it, the scintillator emits photons and we know there was a collision when we see these photons. Think of it as like a fire alarm – it’s what tells us there was a collision.
There’s a few detectors that don’t really fit into this metaphor but I want to mention them anyways. The Photon Multiplicity Detector (PMD) measures the multiplicity of photons at angles close to the beam pipe. The muon arm measures muons, the heavy cousin of the electron. The ALICE Cosmic Ray Detector (ACORDE) is designed to trigger on cosmic rays so that the rest of ALICE can be used to study cosmic rays. Cosmic rays were used to calibrate ALICE before the first collisions at the LHC.
Each of these detectors helps us understand proton-proton and lead-lead collisions in a different way. When we put them all together, we have a sort of Quantum Chromodynamics rover that helps us explore exotic places – the insides of protons and nuclei – that are near us all the time.