Partonfördelningskvoten

Parton distribution functions, often abbreviated as PDFs, describe the momentum distribution of partons within a hadron. Partons are point-like constituents of hadrons, and in the context of quantum chromodynamics (QCD), they are primarily quarks and gluons. PDFs are not directly observable but are fundamental non-perturbative quantities that need to be determined experimentally. They are crucial for understanding the internal structure of hadrons and for making predictions in high-energy scattering experiments.

The concept of PDFs arose from the parton model, which was developed by Richard Feynman to explain the results of deep inelastic scattering experiments. In these experiments, high-energy leptons scatter off hadrons. The way the lepton's momentum is transferred to the hadron's constituents suggested that the hadron was composed of smaller, point-like particles carrying momentum.

Mathematically, a parton distribution function, denoted as $f_i(x, Q^2)$, gives the probability of finding a parton of type $i$ carrying a fraction $x$ of the hadron's momentum, at a given energy scale $Q^2$. The variable $x$ ranges from 0 to 1. The energy scale $Q^2$ is related to the momentum transfer in the scattering process and reflects the resolution at which the hadron's structure is being probed. As $Q^2$ increases, the hadron appears to be made of more partons, and their momentum distributions change. This evolution of PDFs with $Q^2$ is described by the DGLAP (Dokshitzer–Gribov–Lipatov–Altarelli–Parisi) evolution equations.

Experimentally, PDFs are extracted by fitting data from various high-energy colliders like the Large Hadron Collider (LHC) and the former Tevatron. Different experiments, such as deep inelastic scattering, Drell-Yan processes, and jet production, provide complementary information that allows for precise determination of these functions. These determined PDFs are then used in theoretical calculations to predict the outcomes of other experiments and to explore phenomena in particle physics.