Information

How to identify various particles

To reconstruct what has happened within a decay in a particle detector, like the ATLAS detector at the Large Hadron Collider, each of the various particles which are seen have to be identified. To do this, there are various unique signatures which each particle has that can be used to separate them, telling scientists what each particle is. The following section focuses on five main particles which might be seen by a Higgs boson decay: photons, electrons, muons and pions. Neutrinos will also be discussed, but are unfortunately not seen within the detectors.

Below describes how you could differentiate a photon from an electron, muon or pion and vice-versa in any combination, with images showing how each particle will look within a cutout from a detector.

The Photon

Photon is one of the easier particles to identify as a photon will only leave energy in the ECAL (the electromagnetic calorimeter, or in this case, the first ring). This is the nicest way to separate them as only electrons and photons will leave energy only within the ECAL (pions deposit some energy in the ECAL and HCAL). To separate a photon from an electron, look for a tower of energy that has no track. Neutral particles do not interact with the tracking chamber, the innermost part of the detector. Since the photon is neutral, it shall leave no track.

The Electron

The electron will interact with the tracking chamber and the ECAL (first ring) only. Therefore, to identify an electron, you are looking for a tower within the ECAL that has a track from the centre where the decay occurred. Since electrons are charged they do interact with the tracking chamber, unlike photons.

The Pion

Pions are different to electrons and photons because they interact with both the ECAL and the HCAL (Hadronic Calorimeter, which is the second ring) and so will have energy deposited in both the electromagnetic calorimeter and the hadronic calorimeter. Their tell-tale signature is two energy towers and if the pion is charged, it shall have a track. However, pions can be positive, negative and neutral resulting in them not always leaving a track.

The Muon

Muons barely interact with most of the detector and so have a special apparatus added to the outside of the detector called the Muon Spectrometer. This ring is on the outside to avoid interference with other particles. This special apparatus makes muons simpler to identify as only muons will interact with the outer most ring, as well as the tracking chamber on the inside. To highlight this, they are also shown in blue.

A photon being detected.

The photon will be detected by the electromagnetic calorimeter only and hence expect only the one tower on the inner most ring. The photon is a neutral particle and will have no track. To therefore distinguish a photon from an electron, look for a particle with no track.

A muon being detected.

The muon will only be detected by the muon ring, the outer most ring within this decay. Hence they are fairly easy to spot due to their unique signature. Since muons are charged, they will also show a track within the tracking chamber.

An electron being detected.

The electron will only be detected within the electromagnetic calorimeter and hence only have the one tower of energy. Due to the electron being charged, it will show a track too.

A pion being detected.

The pion will have some energy measured within the electromagnetic calorimeter and the hadronic calorimeter. Hence there will normally be two towers for each pion and the energy from each calorimeter has to be accounted for.

Identifying a Higgs Boson and Neutrinos

Unfortunately for scientists, to identify the Higgs boson requires identifying the various particles discussed above, and then using them to find the energy and direction (amongst many other properties) of the Higgs boson. The same is true for neutrinos, particles which are very tricky to observe and measure within a detector like the one at CERN. From previous modules within the LPPP, you will have seen that when a particle with energy decays, the daughter particles (the particles produced), are emitted in the same direction with the energy of the mother (original) particle creating a cone structure for decays (the taus and b-quarks). If you change the energy that the Higgs boson is moving with, then you can see the particles being bent towards each other. This is similar to if you hit a snooker ball, the more force on the ball going in a certain direction, the more momentum the other ball has going in that direction after impact.

The combined energy of the Neutrinos

This arrow represents the combined directional energy of the neutrinos, not the magnitude of the combined energy.

As you will find out later on, neutrinos are difficult to detect without using specialised equipment, which generally takes up a large area. Due to this, neutrinos are not detected within the ATLAS detector which does result in some missing energy from what you would otherwise expect due to energy conservation.

Training Simulation

Below is a training simulation which shall walk you through the above information letting you identify some electrons, photons, muons and pions for yourself.