Reconstructing the Higgs boson mass from its golden channel

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 stear protons around the ring at high speeds. When these high energy protons collide many particles are produced and among these, in rare cases, there may be a Higgs particle. 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 is ATLAS?

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.

Identifying a particle

To identify a particle, physicists often use the invariant mass. The invariant mass of a system is the part of its mass which is independent of the motion of the system. ATLAS can be used to detect the decay product of a particle and reconstruct its invariant mass.

Higgs golden channel

The Higgs boson golden channel is a specific decay mode in which the Higgs boson decays into a pair of Z bosons which in turn decay into two leptons each. Therefore, the final state of this decay is four leptons denoted by L in the figure below (we will only consider the case of light leptons: electrons and muons). From these four leptons we will be able to reconstruct the mass of the Higgs boson. From the energy and momentum of the leptons we are able to find the invariant mass of the four particle system which is the same as the particle that originally decayed (Higgs boson).

There is also another process which can produce four leptons: two Z bosons that are not decay products of a Higgs can decay to four leptons. We will call this the irreducible background as it is not the signal but cannot be removed. The data recorded by the ATLAS detector contains both of these processes. This is illustrated in the figure on the right. The goal of the simulation is to reproduce this plot and oberve the Higgs boson mass peak.


Press the button below to access a simulation which will take you through the process of reproducing the plot above and finding the Higgs boson mass. The ATLAS detector in the simulation is simplified, showing only the inner detector. In this simulation, you will produce proton-proton collisions and identify groups of four leptons by looking at the tracks in the ATLAS detector.

In the simulation, typical events as recorded by the ATLAS detector will be presented in a simplified way. An event contains a signal composed of four leptons, the irreducible ZZ background as well as an additional background of low transverse momentum (momentum perpendicular to the beam pipe) particles. Your goal is to remove the low transverse momentum background by setting a momentum cut. Only tracks with momentum larger than the applied cut are shown. You will be able to change the cut for each event to keep the four particles with highest transverse momentum. At CERN, physicists apply the same cut for all events. They choose it such that in most cases it removes most of the background and keeps the signal. However, in some cases the signal itself is removed.

Access simulation

Further reading

André Sopczak

CERN 2021 summer student
Antoine Vauterin