Authors:	Giarra, Johannes Vincenzo
Title:	Deeply virtual compton scattering at COMPASS
Online publication date:	28-Nov-2022
Year of first publication:	2022
Language:	english
Abstract:	Studying hard exclusive reactions in lepton-nulceon scattering like for e.g. hard exclusive photoproduction in Deeply Virtual Compton Scattering (DVCS), allows to study the 3-dimensional nucleon structure in the framework of Generalized Parton Distribution functions (GPDs). From the DVCS cross section, combinations of Compton Form Factors (CFFs) can be extracted, which are used to further constrain the related GPDs. In 2016 and 2017, COMPASS measured DVCS on a 2.5 m long liquid hydrogen target using high energy (160 GeV), positively and negatively charged polarized muon beams provided by the M2 beamline of the SPS at CERN. The scattered muons and the produced real photons were detected by the COMPASS spectrometer, which was supplemented by an additional electromagnetic calorimeter for the detection of large-angle photons. To perform an exclusive measurement of the full final state, the target was surrounded by a barrel-shaped time-of-flight system to detect the recoiling target protons. The exclusive photon event sample is extracted using a dedicated event selection to identify all participating particles. This event selection includes the use of exclusivity conditions like for e.g. restricting the missing mass and is further improved by a kinematic fit under the assumption of the topology of a exclusive photon event. Its quality is used as an additonal selecting criterium. The fit also improves the resolution of the event kinematics, in particular on the square of the four-momentum transfer to the proton |t|. The selected data sample does not only include DVCS events, but also contributions by radiative photons produced in the Bethe-Heitler (BH) process and a background contamination by photons originating from the decay of neutral pions. These contributions are determined using dedicated Monte-Carlo samples and are subtracted from the data sample to obtain the pure DVCS contribution. The DVCS cross section is extracted in bins of |t|, separately for each beam charge and polarization. These cross section values are averaged over both beam charges, which allows to extract the |t|-dependence of the charge spin cross section sum. Studying this dependence allows to extract the slope parameter B by a binned log-likelihood fit using an exponential ansatz of the from: e^{-B|t|}. The slope parameter is dominated by the contribution of the imaginary part of CFF H. At the COMPASS kinematics, which covers the x_{Bj}-domain of sea quarks, it is related to the transverse extension of the proton due to the quark distributions. In the present analysis about 2/3 of the available data in 2016 are used. The 2017 sample is expected to provide two to three times higher statistics, which will allow to extend the analysis and extract the dependence of the slope parameter on x_{Bj}.
DDC:	500 Naturwissenschaften 500 Natural sciences and mathematics 530 Physik 530 Physics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-8316
URN:	urn:nbn:de:hebis:77-openscience-b4c17501-4236-428b-b014-7b757202063b9
Version:	Original work
Publication type:	Dissertation
License:	CC BY-ND
Information on rights of use:	https://creativecommons.org/licenses/by-nd/4.0/
Extent:	xi, 251 Seiten ; Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen