The particle hypothesis consists of the information about a particle necessary for accurate track reconstruction. This is the

• absolute PDG
• mass
• absolute charge

This is redundant, as the absolute PDG can be used to look up mass and absolute charge, but used as a caching mechanism and can be an opportunity to use different, not necessarily physical values.

Internally, ACTS will use the absolute PDG to determine the energy loss or multiple scattering distribution for the particle. mass parameterizes these functions and is used for time transport. absolute charge is used as a conversion factor between the track parameter $\frac{q}{p}$ and $p$ whenever required.

The implementation consists of a generic, templated class Acts::GenericParticleHypothesis which depends on the charge type. There are four different charge types

• {struct}Acts::Neutral
• {struct}Acts::SinglyCharged
• {class}Acts::NonNeutralCharge
• {class}Acts::AnyCharge The reason for these different charge types is that in special cases it is not necessary to carry the absolute charge as it is already described by the type.

Ultimately there is a collection of classes which fill the charge type into the Acts::GenericParticleHypothesis for the user and provide a set of common particle hypotheses.

• {class}Acts::SinglyChargedParticleHypothesis
• {class}Acts::NeutralParticleHypothesis
• {class}Acts::NonNeutralChargedParticleHypothesis
• {class}Acts::ParticleHypothesis

Internally, ACTS will use {class}Acts::ParticleHypothesis, if not specified otherwise. This is the most generic one which is convertible from all the other hypotheses.