Status of AKK Fragmentation Functions

In the framework of the QCD-improved parton model, the inclusive production of single hadrons is described by means of fragmentation functions (FFs) D a(x, μ ). At leading order, the value of D a(x, μ ) corresponds to the probability for the parton a produced at short distance 1/μ to form a jet that includes the hadron h carrying the fraction x of the longitudinal momentum of a. Unfortunately, it is not yet possible to calculate the FFs from first principles, in particular for hadrons with masses smaller than or comparable to the asymptotic scale parameter Λ. However, given their x dependence at some energy scale μ, the evolution with μ may be computed perturbatively in QCD using the timelike Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations [2]. This allows us to test QCD quantitatively within one experiment observing single hadrons at different values of center-of-mass energy √ s (in the case of ee annihilation) or transverse momentum pT (in the case of scattering). Moreover, the factorization theorem guarantees that the D a(x, μ ) functions are independent of the process in which they have been determined and represent a universal property of h. This enables us to make quantitative predictions for other types of experiments as well. Our previous determinations of light-hadron FFs, yielding the BKK [3], KKP [4], and AKK [5] sets, were based entirely on ee annihilation data in the large-x range, x > 0.1, where x = 2Eh/ √ s is the scaled hadron energy. In this presentation, we summarize the improvements from new theoretical input and additional experimental data implemented in the 2008 update of the AKK sets for charged pions, charged and neutral kaons, (anti)protons, and (anti)lambdas (AKK08) [6]. In Secs. 2 and 3, we discuss the most important improvements due to new theoretical and experimental inputs, respectively. In Sec. 4, we briefly mention how the AKK08 FFs compare with the AKK ones and those recently presented by Hirai, Kumano, Nagai, and Sudoh (HKNS) [7] and by de Florian, Sassot and Stratmann (DSS) [8]. In Sec. 5, we present an outlook.