Measurement of Hadron Multiplicities in Deep Inelastic Muon-Nucleon Scattering

In deep-inelastic muon-nucleon scattering, a single quark can be ejected out of the nucleon by the absorption of a high-energy photon. Such a free isolated quark has never been observed in nature. In quantum chromodynamics (QCD), coloured objects, such as a single quark, create additional quark anti-quark pairs out of the colour field and the final state comprises a jet of hadrons. The hadronisation process can be described by fragmentation functions D_q^h, the probability that a quark with the flavour q turns into a hadron of the type h. Similar to the parton distribution function, the fragmentation functions are fundamental, universal and process-independent quantities. The fragmentation functions are measured with the COM- PASS spectrometer in muon-nucleon scattering. The observables are the hadron multiplicities M_h. The COMPASS experiment consists of a two-stage magnetic spectrometer located at the M2 beam line of the Super Proton Synchrotron at CERN and uses a polarised muon beam on a nuclear fixed target. The scattered muon and the final-state hadrons are measured with a large number of tracking detectors. The identification of the hadrons is done using a ring imaging Cherenkov detector. An essential part of the spectrometer is the trigger system. It mainly consists of scintillator hodoscopes and triggers on scattered muons coming from the target. The trigger acceptance was increased towards larger four-momentum transfers Q2 with the implementation of a new pair of hodoscopes, triggering on larger scattering angles. The trigger system is also used to determine spectrometer specific quantities such as efficiencies and luminosities. In a fixed target experiment, the luminosity is given to the incident particle flux and the target density. The flux is measured with two methods, by counting beam tracks using a true random trigger and by counting the hits in a plane of the beam telescope. For the determination of the fragmentation functions, a measurement taken in 2006 with a 160 GeV/c muon beam impinging on an isoscalar lithium-deuterid target is used. The scattered muon and additional hadrons are measured. The hadron multiplicities Mh are the average number of hadrons per deep inelastic scattering event and are determined in a three- dimensional binning in the Bjorken scaling variable x, the relative virtual photon energy y and the hadron energy fraction z for pions, kaons and unidentified hadrons. The DIS events are selected by requiring an invariant mass W larger than 5GeV/c2, thus excluding events coming from nucleon resonances. The charged hadron multiplicities are corrected for the particle identification efficiency using real data. Further corrections, e. g. the spectrometer acceptance and electron/positron contamination, are determined using Monte Carlo simulations. The fragmentation functions are extracted in two ways. The first method is the direct ex- traction point-by-point. The second method uses a simultaneous leading-order perturbative QCD fit on π+ and π ́ multiplicities. The results are compared to existing parametrisations.