engleski

Transverse single spin asymmetries for the production of neutral pions in proton collisions at $\sqrt{;s};$ = 200 GeV

The STAR collaboration has reported precision measurements on the transverse single spin asymmetries for the production of forward $\pi^0$ mesons from polarized proton collisions at $\sqrt{;s};=200\, $GeV. To disentangle the contributions to forward asymmetries, one has to look beyond inclusive $\pi^0$ production to the production of forward jets or direct photons. Present forward detector capabilities are not well matched to the complete reconstruction of forward jets, but do have sufficient acceptance for "jet-like" events. "Jet-like" events are the clustered response of an electromagnetic calorimeter that is primarily sensitive to incident photons, electrons and positrons. During the RHIC running in the year 2006, STAR with the Forward Pion Detector (FPD++) in place collected 6.8 pb$^{;-1};$ of data with an average polarization of 60\%. FPD++ was a modular detector prototype of the Forward Meson Spectrometer (FMS) that consisted of two detectors placed symmetrically with respect to the beam line at a distance of 7.4 m from the interaction point. Readout of the FPD++ was triggered when the sum of energies in the central module of the calorimeter used for $\pi^0$ measurements was larger than a threshold. This trigger minimizes the bias for "jet-like" events, making it appropriate to disentangling contributions to the forward transverse spin asymmetries. This thesis reports on the analysis, giving in specific detail all the calibration and analysis steps, as well as the final results. It has been demonstrated that the calibration procedures used earlier to calibrate the previous versions of the detector can be extended to the current setup. Results from earlier analyses were successfully reproduced and extended as well. It has been shown that the parts of the detector that were not used earlier have no systematic influence on the results. After the extraction of the "jet-like" events, their properties were carefully explored. The final result shows that the neutral pion asymmetry shows no dependence on the angle with respect to the "jet-like" event axis, thus showing that there is no net Collins contribution to the neutral pion asymmetries.

STAR; 200GeV; asymmetry

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