Measurements of transverse momentum dependent azimuthal asymmetries in SIDIS at COMPASS

Sbrizzai, Giulio
2011-03-29
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Abstract
The main topic of this thesis is the measurement of the transverse momentum dependent (TMD) azimuthal asymmetries in the inclusive hadron production in the deep inelastic scattering (DIS). This work has been carried on in the framework of COMPASS, a fixed target experiment at CERN's SPS, which started data taking in 2002. One of the main topic of COMPASS is the study of the nucleon spin structure using high energy (100-200 GeV) muon beam and either transversely and longitudinally polarized proton and deuteron target. Recent developments both from the experimental and theoretical side pointed out the relevance of the transverse spin effects and of new effects, related to the intrinsic transverse momentum of the quark and its correlation with the spin. The nucleon structure, in the non perturbative QCD formalism, can be described at first order, in the collinear approximation, by three parton distribution functions (PDF): the unpolarized PDF, the helicity PDF and the transversity PDF. When including the intrinsic transverse momentum of the quark other 5 PDFs are needed at first order. They are related to the correlation between the transverse momentum of the quark and the spin and are practically unmeasured. The semi inclusive DIS (SIDIS) allows to study all these functions. Indeed the the SIDIS differential cross section, taking into account the beam and target polarization, is characterized by 14 azimuthal modulations in independent linear combination of the nucleon spin angle and of the hadrons angle, as measured with respect to the virtual photon direction. Their amplitudes are related to the structure functions given by the convolution between the new PDFs and the hadron fragmentation functions. In this thesis the measurement of the azimuthal asymmetries which appears in the SIDIS cross section on the unpolarized and longitudinally polarized target. Of particular interest is the information that this measurement give on the Boer-Mulders TMD PDF, which is a very interesting topic. The analysed data have been taken using the 160 GeV positive muon beam on the deuteron target. The target configuration and the spectrometer are optimized for the spin asymmetry measurements and are described in the thesis as well as the data taking. Also, the data processing and the software used for the analysis and the SIDIS data selection are described. The measurement of the azimuthal asymmetries on the longitudinally polarized target have been done using the same data as the one used for the $\Delta$G (the contribution to the nucleon spin of the longitudinal spin of the gluons) measurement. To avoid acceptance effects in the asymmetries extraction, the data collected from the two oppositely polarized target cells (which form the target) have been opportunely combined. Other possible systematic effects have been studied in details and are described in the thesis. \\ The asymmetries have been extracted as functions of the relevant kinematical variables and separately for positive and negative hadrons. The final results, accepted for publication in European Journal C, are compared with the results produced by other experiments and with the calculations of theoretical models of the TMD functions. The measurement of the azimuthal asymmetries which appears in the spin independent part of the SIDIS cross section is more difficult since it cannot benefit from the periodically reversal of the target polarization. The measured hadron azimuthal distributions must be corrected for the possible effects introduced by the apparatus acceptance (geometrical acceptance, detectors and trigger efficiencies). The Monte Carlo simulation of the apparatus has been used to estimate those effects. Extensive studies of the description of the azimuthal acceptance of the apparatus have been done in the whole kinematical domain of interest using different SIDIS events generations. A reliable asymmetries extraction method has been developed and used. A considerable effort has been put into the study and the control of the systematics. Also in this case the asymmetries have been extracted as functions of the relevant kinematical variables and separately for positive and negative hadrons. \\ The strongest measured signals are related to the Cahn effect, which is a kinematical effect proportional to the intrinsic transverse momentum of the quarks, and to the Boer-Mulders TMD PDF, describing the correlation between the intrinsic transverse momentum of the quark and its transverse spin. \\ The results, in particular the kinematical and the charge dependencies of the asymmetries, are interpreted in terms of TMD quantities and higher effects in the non perturbative QCD. The comparisons with the results produced by the other experiments, and with the theoretical predictions are also discussed.