Particlelevel

Particlelevel is a term used in high-energy physics to describe the state of an event after parton showering and hadronization have occurred and all unstable particles have decayed, leaving a set of stable final-state particles that would be present in a collider detector. This level focuses on the particles that would contribute to observable signals, while excluding detailed detector effects such as finite resolution and efficiency.

At the particlelevel, the final-state particles typically include photons, charged leptons (electrons and muons), neutrinos, and a spectrum of hadrons produced by hadronization. The exact composition can depend on the analysis, and definitions may vary between experiments and simulation tools. Particle-level objects such as jets, missing transverse energy (MET), and isolated leptons are often reconstructed from these stable particles, providing a bridge between theory and what detectors ultimately observe.

Particlelevel sits between the parton-level and detector-level descriptions. Parton-level refers to the state before hadronization and decay, while detector-level represents the response of a specific experimental apparatus. By using particle-level descriptions, theorists can produce predictions that are closer to what is experimentally accessible, and experimentalists can perform unfoldings or compare results to theory in a common framework that abstracts away detector specifics.

Variations in particle-level definitions are common, and analysts should consult the specific analysis or collaboration documentation for the exact criteria used, such as which particles are included in MET calculations or how jets are clustered from stable particles.