Finding new particles using the AFP detectors

What are CERN and the LHC ?

CERN is the European Council for Nuclear Research, an organisation dedicated to research into nuclear and particle physics. CERN was established in 1954 and is based near Geneva. It currently has 23 member states.

The Large Hadron Collider (LHC) is the world's largest and most powerful particle collider. It was built by CERN and is situated in a 27 km circular tunnel beneath the Franco-Swiss border. Magnets along the tunnel are used to accelerate protons at high speeds. When these high energy protons collide many particles are produced. Four detectors The 4 detectors around the LHC are ATLAS, CMS, LHCb and ALICE. are placed around the tunnel to detect the outgoing particles from these collisions. The data collected from these detectors is used by physicists to understand the fundamental building blocks of matter.

What are ATLAS and the AFP detectors?

ATLAS is the largest of the four detectors around the LHC. It detects the products from proton-proton The proton is a subatomic particle of positive charge made up of elementary particles called quarks. collisions. The figure on the right is a cross-section of the ATLAS detector. It is made up of multiple layers which are sensitive to different particles. The inner detector can detect charged tracks. In the figure we can see that electrically charged particles leave a track, whereas neutral particles such as photons do not. This is how we can differentiate an electron from a photon for example. Any particle that interacts with the detector leaves an energy deposit.

The Atlas Forward Proton (AFP) detector is on either side of the main ATLAS detector and is able to detect deflected protons. The figure below shows its location and how it can catch scattered protons.

Detecting ALPs using the AFP detector

Axion-Like Particles (ALP) are a type of particle that appear in some extensions of the Standard Model The Standard Model of particle physics is the theory that describes the elementary particles and their interactions with the electromagnetic, strong and weak forces. . They could in particular be a dark-matter Dark matter is an invisible type of matter which makes up most of our universe. candidate.

Usually at the LHC protons collide head-on as shown on the left figure below. This destroys the protons and produces new particles. However we are interested in a different process where the protons miss each other. This is shown in the right figure below. They still interact electromagnetically through their respective electric fields and are deflected. This interaction produces a pair of virtual Virtual particles cannot be observed directly but serve as intermediaries in particle interactions. photons. In turn the photons could scatter through an ALP into a pair of real photons which could be detected. This is shown in the Feynman diagram Feynman diagrams are a way to visualise particle interactions. Each line represents a particle and they interact at vertices. on the right. We can measure the energy of the scattered photon pair (in the main detector) as well as the energy of the deflected protons (in the AFP detector). We can search for signs of ALP production at a given mass by recording many photon-photon collisions and matching the energy observed in the different detectors.


Press the button below to access a simulation which will walk you through the process of detecting an ALP using the AFP detector. The ATLAS detector in the simulation is simplified consisting of a single layer. In this simulation, you will produce proton-proton collisions and identify photon pairs by looking at the tracks in the central detector. Then by looking at the energy loss in the central and AFP detectors you will be able to find the signal for an ALP.

Access simulation

Further reading

André Sopczak

CERN 2021 summer student
Antoine Vauterin