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ALICE event display – decoded!

Diagram of the ALICE detector

Diagram of the ALICE detector

Most of the time, the Large Hadron Collider accelerates protons, particles so tiny they fit inside of atoms. But for about a month each year the LHC runs, the scientists load the machine with something a bit heartier: lead ions.

Heavy-ion season is the ALICE detector’s time to shine. Of the four detectors at the LHC, only ALICE was designed specifically to study these types of collisions. This post breaks down different components of the ALICE detector and explains how scientists use them to study matter as it formed after the big bang.

Heavy-ion collisions in the LHC can create quark-gluon plasma, a phase of matter scientists think only occurred in nature during the first millionths of a second of the universe’s birth. In this state, protons and neutrons melt into a hot soup of their constituent pieces, quarks and gluons. Earlier this year, physicists announced definitive evidence for having created QGP at Brookhaven National Laboratory's Relativistic Heavy
Ion Collider. In November, scientists at the LHC observed characteristics attributed to the quark-gluon plasma as well.

Members of the ALICE collaboration want to study the quark-gluon plasma to learn more about the first few moments that shaped our universe. The many components of the ALICE detector give them the information they need.

1: Silicon tracker
The silicon tracker is a cylinder made up of six layers of silicon detectors surrounding the beam pipe, where particles speed into one another and collide. The silicon tracker takes high-precision measurements of the properties and flight path of particles leaving collisions in the beam pipe.

Scientists need the highest precision measurements closest to the collision point to ensure that the particles they are tracking, there and elsewhere in the detector, actually come from a collision. Particles within the beam can scatter or decay on their own, providing distracting background particles the scientists do not want to confuse with the particles they’re trying to study.

ALICE event display

ALICE event display

2: Time projection chamber
Surrounding the silicon tracker is the time projection chamber, or TPC. It has a radius of 2.5 meters. The TPC is a container full of gas that allows scientists to measure the momenta and masses of charged particles. When a charged particle flies through the gas in the TPC, it pulls electrons from the gas molecules. A high-voltage electric field running through the center of the TPC causes those electrons to drift to the ends of the chamber, the endcaps.

The detector records where and when the electrons hit sensors on the endcaps. It’s like taking a series of photos of a finish line of a race from directly above. If you flip through the photos you took, you can see the order in which the runners arrived, how far apart they were and where they crossed the finish line. Scientists take information from the detector to find out how the electrons ran their race to the endcap. Using this information, they can figure out the path the particle that knocked them loose followed and how fast it was moving.

Scientists can also use the detector to determine the momentum of a particle. The entire chamber is in a magnetic field, so charged particles curve as they move through it. Positively charged particles curve in one direction and negatively charged particles curve in the opposite direction. Scientists can determine the momentum of a particle based on how much it curves. The field bends the path of slower particles more than it does faster ones.

Not pictured: Time-of-flight barrel
Surrounding the TPC is the time-of-flight barrel, not pictured here. The time-of-flight barrel is important, as it allows scientists to identify passing particles.

Scientists recognize a particle by its mass. The momentum of a particle is equal to its mass multiplied by its velocity. Since scientists already know the momentum of a particle based on the curvature of its path through the magnetic field in the TPC, all they need to know to calculate the mass is its velocity. That’s where the time-of-flight barrel comes in.

The time-of-flight barrel measures the time that each signal arrives, which lets the scientists know how long the particle took to travel through the TPC. The distance the particle traveled through the TPC divided by that time equals the velocity of the particle. Scientists calculate the mass of a particle by dividing its momentum by its velocity.

ALICE event display at an angle

ALICE event display at an angle

3: Electromagnetic calorimeter
The electromagnetic calorimeter, or EMCal, has the task of measuring neutral particles, such as photons or electrons. It earned its name because photons and electrons interact through the electromagnetic force.

The image here represents the data the ALICE detector collected from a collision of two lead ions. You can see the tracks left in the TPC by about 3,000 charged particles. Another about 1,500 neutral particles passed through the gas chamber unnoticed, as they did not ionize the gas inside it. The calorimeter measures the energy of those neutral particles by absorbing them.

The red bars pictured here represent particles that deposited a particularly large amount of energy. The height of the bar represents its energy. The smaller purple bars represent lower energy particles, most likely charged particles, which will most likely be considered background and no longer be measured once the tolerance of the detector is reset.

4: Transition radiation detector
This detector measures transition radiation, emitted when a particle traveling nearly the speed of light traverses a material. The transition radiation detector allows scientists to identify high-velocity particles. As you read above, the momentum of a particle is equal to its mass multiplied by its velocity. If two particles of similar mass both are moving at very high velocities, they will have similar momenta and be difficult to tell apart using only the TPC.

5: Photon spectrometer
The photon spectrometer, or PhoS, detects photons, which are neutral particles. Some particles that emerge from collisions break down into electron – positron pairs.

In this image, the sensitivity of the photon spectrometer was set lower than that of the calorimeter, so you don’t see as many background signals. The blue bars represent neutral particles that deposited energy.

ALICE event display with muon detector

ALICE event display with muon detector

6: Muon detector
Muons are the heavy cousins of electrons. Scientists want to identify particles that decay into muons, such as beauty and charm quarks. The last piece of the particle detector is the muon detector, pictured here in green.

The muon detector is made up of steel plates and tracking chambers. The steel absorbs most particles, but muons rarely interact with matter. So any charged particles hardy enough to make it through the entire detector and steel plates are likely to be muons, shown here as blue and green lines.

Other posts in this series include:
CMS event display -- decoded!, ATLAS event display -- decoded! and LHC Page 1 -- decoded!