Quantum Diaries

As I was writing a talk for a conference, I ran into a bunch of pictures of ALICE that I thought y’all might like to see.  In my last post I gave an overview of ALICE and described each of the subsystems in ALICE.  I don’t have a picture of every subsystem, but to give you an idea of what these pieces look like:

This is the ALICE magnet (which used to be the L3 magnet) in 2001, before any of the detectors were installed inside of it.

This is the hole inside the TPC where the ITS sits now

This is the whole ITS when it was being installed

Here you can see the PMD team in front of the PMD

This shows the PMD after it was installed in ALICE

Here you can see piece of the TRD right before it was installed

This is the HMPID right before it was installed

This is one of the trays of the TOF

And here you can see one of the EMCal supermodels right before it was installed

And this is the ALICE collaboration in front of ALICE

(Note to ALICE collaborators – if you didn’t see a picture of your favorite subsystem, email it to me.  If I get enough cool ALICE pictures for another post, I’ll do a follow up with more gratuitous cool pictures of ALICE.)

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.

ALICE is made up of several detectors, each of them designed for a different purpose.  Here you can see a drawing of ALICE with the different detectors labeled:

In this post I introduced you to the Time Projection Chamber (TPC).  The US is mostly involved in the electromagnetic calorimeter (EMCal), which is positioned outside of the TPC (labeled “EMCAL”).  I work on the EMCal.

At some point in a chemistry or physics lab you probably did a lab with a calorimeter.  The standard calorimeter experiment involves heating something up, submerging it in cool water, and measuring the change in the temperature of the water in order to determine the specific heat of the object.  A calorimeter measures the energy deposited by an object.

In high energy physics calorimeters are used to measure the energy of particles.   When a high energy particle hits our detector, it initiates an electromagnetic shower, emitting electron/positron pairs and photons.  As the shower propagates through the calorimeter, it deposits more and more of its energy, until it either stops or comes out the other side of the calorimeter.   An electronic calorimeter is most sensitive to photons and electrons.

Of course sometimes a high energy particle will hit one of the nuclei of one of the atoms in the calorimeter and deposit some of its energy this way.  Hadrons – such as protons and pions – are more likely to do this than electrons because they interact strongly (through the strong force), unlike electrons and photons.  This is how a hadronic calorimeter works.  This would not happen in an ideal electromagnetic calorimeter, but no detector is ideal so some hadrons will leave a shower in our detector.

The TPC is used for measuring the tracks left by charged particles and can measure their momentum.  The EMCal can measure particles’ energy.  We can use the EMCal to measure photons, which the TPC cannot see.  We can also use the EMCal to distinguish electrons from hadrons because electrons will leave most of their energy in the calorimeter while hadrons will not.  The EMCal can be used to measure many neutral particles, whereas the TPC can only see charged particles.  By combining the information from both of these detectors, we can characterize events better.

A sampling calorimeter is made up of layers of something which produces the shower and something which can collect the energy from the shower.  In the ALICE EMCal we have 77 alternating layers of lead and scintillators.  A high energy particle produces an electromagnetic shower in the lead.  The photons released in the shower are collected by the layers of scintillators.  The light from the scintillators is collected by fiber optic cables, converted into an electrical signal, and read out as digital signals.

The ALICE EMCal is made up of 12,288 towers, each of which is about 6 cm².  It covers about 107° around the beam pipe.  This is what it looks like:

Each green square is one tower.  For scale, in this post our piece of the EMCAL is 4×4 towers.  We have to read out data from the EMCal towers.  This takes a lot of cables and what we call Front End Electronics – electronic boards which manage the data until we record it.  All of these pieces have to be carefully tested before they’re installed.  Here you can see me testing one of the Front End Electronics boards at CERN:

Right now only about 36% of the EMCal is installed.  We got great news recently – we’ll be able to install the rest of it in January!  This is wonderful – but it means we have a lot of work to do.  I’ll be spending November at CERN testing parts and helping prepare the EMCal for installation.  Luckily there are plenty of places to get a Thanksgiving turkey in Geneva!

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!