{"id":{"repo_id":"freiburg-diss","oai_identifier":"oai:freidok.uni-freiburg.de:2309"},"canonical_url":"https://search.dev.ndltd.org/etd/freiburg-diss/oai:freidok.uni-freiburg.de:2309","repository":{"repo_id":"freiburg-diss","name":"University of Freiburg","base_url":"https://freidok.uni-freiburg.de/oai/oai2.php"},"display":{"title":"First measurement of the gluon polarisation in the nucleon using D mesons at COMPASS","abstract":"The complicated structure of the nucleon has been studied with great success
in deep-inelastic lepton-nucleon scattering (DIS) experiments at CERN, SLAC and
DESY. As a result the unpolarised structure functions have been measured
accurately over a wide kinematic range. From these measurements it is
possible to determine the gluon density in the nucleon with good accuracy via
a so-called QCD fit.
In the case of the spin structure of the nucleon the situation is different. Even after decades of experimental and theoretical efforts it remains to be understood how the spin of the nucleon of 1/2 in units of h-bar is to be
accounted for in terms of contributions from the quarks and gluons inside the
nucleon.
Of particular interest is the question whether the polarised gluon density
can explain the unexpected smallness of the quark contribution to the nucleon
spin.
The QCD fit, which worked well in the unpolarised case, yields a polarised
gluon density Delta G which is only badly constrained. This is due to
the fact that the information on the polarised structure functions is only
available in a rather small kinematic range, since the corresponding measurements are so far exclusively performed by fixed-target experiments.
The limited knowledge of the structure functions aggravates the difficulty that
the gluon distribution enters only in next-to-leading order and is thus
suppressed by the strong coupling constant.
A direct measurement of Delta G is therefore needed to get a clearer picture.
This direct measurement is the major aim of the COMPASS experiment at CERN, where polarised muons are scattered off a polarised fixed target.
The process on which this measurement is based is the photon-gluon fusion
(PGF). It can result in the production of a charm-anti-charm-quark pair and
is then tagged by detecting the charmed mesons in the final state.
This approach is used in this thesis and results in a theoretically very clean sample of PGF events, assuming that the intrinsic charm content in the nucleon is negligible.
Since the charm tagging of the PGF process is very much statistically limited,
it profits directly from improvements of the reconstruction efficiency.
A major contribution was made in the course of this work concerning the reconstruction of the momentum of the beam muons.
The efficiency of this reconstruction was initially limited by the fact that information from the contributing detectors, installed about 100\\,m apart along the beam line, could only be associated by using the time stamp of the
corresponding measurements. The main improvement is that now also space information is taken into account, based on the knowledge of the transfer matrix of the beam line.
In total the fraction of non-reconstructable events could be decreased from
19% to 6%.
The polarised target used in the COMPASS experiment is a large solid-state
target which does not allow the use of dedicated vertex detectors. The identification of the charmed mesons thus has to proceed via the reconstruction of their invariant mass. In order to control the combinatorial background, the reconstruction relies heavily on the RICH detector.
This work contributed to the understanding of how this detector and the
spectrometer in general can be used best in order to extract the cleanest
possible signal of charmed mesons.
Especially the tagging of D-star decays results in a very clear signal and
thus makes COMPASS the first polarised fixed-target experiment which is able
to extract the gluon polarisation via the open charm approach.
The analysing power needed for the extraction is given by the ratio of spin dependent and spin averaged muon-nucleon cross sections, which have been
calculated to first order in the strong coupling constant.
These cross sections cannot be computed exactly in the analysis, since they depend on parton kinematics, which are poorly known due to the fact that typically only one of the two charmed mesons produced in the PGF process is detected.
Thus a parametrisation based on Monte Carlo simulations was prepared, which maps the parton kinematics onto measurable quantities. This makes the analysing power
available on an event-by-event basis with sufficient accuracy and thus allows a statistically optimal analysis.
The final result for the gluon polarisation is extracted at a mean momentum
fraction carried by the gluon of eta=0.15 and at a scale mu^2=13(GeV/c)^2 and was found to be Delta G / G =-0.31+/-0.44(stat.)+/-0.07(syst.).
The systematic error is mostly due to the uncertainty on the mass of the charm quark, possible false asymmetries and the background contribution to the asymmetry.
This result for Delta G/G is consistent with parametrisations obtained from QCD fits to polarised DIS data.","abstract_html":"The complicated structure of the nucleon has been studied with great success <br>in deep-inelastic lepton-nucleon scattering (DIS) experiments at CERN, SLAC and <br>DESY. As a result the unpolarised structure functions have been measured <br>accurately over a wide kinematic range. From these measurements it is <br>possible to determine the gluon density in the nucleon with good accuracy via <br>a so-called QCD fit. <br>In the case of the spin structure of the nucleon the situation is different. Even after decades of experimental and theoretical efforts it remains to be understood how the spin of the nucleon of 1/2 in units of h-bar is to be <br>accounted for in terms of contributions from the quarks and gluons inside the <br>nucleon. <br>Of particular interest is the question whether the polarised gluon density <br>can explain the unexpected smallness of the quark contribution to the nucleon <br>spin. <br>The QCD fit, which worked well in the unpolarised case, yields a polarised <br>gluon density Delta G which is only badly constrained. This is due to <br>the fact that the information on the polarised structure functions is only <br>available in a rather small kinematic range, since the corresponding measurements are so far exclusively performed by fixed-target experiments. <br>The limited knowledge of the structure functions aggravates the difficulty that <br>the gluon distribution enters only in next-to-leading order and is thus <br>suppressed by the strong coupling constant. <br>A direct measurement of Delta G is therefore needed to get a clearer picture. <br>This direct measurement is the major aim of the COMPASS experiment at CERN, where polarised muons are scattered off a polarised fixed target. <br>The process on which this measurement is based is the photon-gluon fusion <br>(PGF). It can result in the production of a charm-anti-charm-quark pair and <br>is then tagged by detecting the charmed mesons in the final state. <br>This approach is used in this thesis and results in a theoretically very clean sample of PGF events, assuming that the intrinsic charm content in the nucleon is negligible. <br>Since the charm tagging of the PGF process is very much statistically limited, <br>it profits directly from improvements of the reconstruction efficiency. <br>A major contribution was made in the course of this work concerning the reconstruction of the momentum of the beam muons. <br>The efficiency of this reconstruction was initially limited by the fact that information from the contributing detectors, installed about 100\\,m apart along the beam line, could only be associated by using the time stamp of the <br>corresponding measurements. The main improvement is that now also space information is taken into account, based on the knowledge of the transfer matrix of the beam line. <br>In total the fraction of non-reconstructable events could be decreased from <br>19% to 6%. <br>The polarised target used in the COMPASS experiment is a large solid-state <br>target which does not allow the use of dedicated vertex detectors. The identification of the charmed mesons thus has to proceed via the reconstruction of their invariant mass. In order to control the combinatorial background, the reconstruction relies heavily on the RICH detector. <br>This work contributed to the understanding of how this detector and the <br>spectrometer in general can be used best in order to extract the cleanest <br>possible signal of charmed mesons. <br>Especially the tagging of D-star decays results in a very clear signal and <br>thus makes COMPASS the first polarised fixed-target experiment which is able <br>to extract the gluon polarisation via the open charm approach. <br>The analysing power needed for the extraction is given by the ratio of spin dependent and spin averaged muon-nucleon cross sections, which have been <br>calculated to first order in the strong coupling constant. <br>These cross sections cannot be computed exactly in the analysis, since they depend on parton kinematics, which are poorly known due to the fact that typically only one of the two charmed mesons produced in the PGF process is detected. <br>Thus a parametrisation based on Monte Carlo simulations was prepared, which maps the parton kinematics onto measurable quantities. This makes the analysing power <br>available on an event-by-event basis with sufficient accuracy and thus allows a statistically optimal analysis. <br>The final result for the gluon polarisation is extracted at a mean momentum <br>fraction carried by the gluon of eta=0.15 and at a scale mu^2=13(GeV/c)^2 and was found to be Delta G / G =-0.31+/-0.44(stat.)+/-0.07(syst.). <br>The systematic error is mostly due to the uncertainty on the mass of the charm quark, possible false asymmetries and the background contribution to the asymmetry. <br>This result for Delta G/G is consistent with parametrisations obtained from QCD fits to polarised DIS data.","abstract_has_math":false,"creators":["Hodenberg, Martin von"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":["Königsmann, Kay"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":null,"date_issued":"","date_published":null,"updated_at":"2026-07-24T02:22:40Z","subjects":["Spinstruktur","Gluonpolarisation","Nukleonenstruktur","spinstructure","gluon polarisation","COMPASS","CERN","D mesons"],"languages":[],"rights":[],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"https://freidok.uni-freiburg.de/data/2309","outbound_label":"Repository record","outbound_source":"source_url"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Königsmann, Kay"]},{"key":"dc:creator","label":"Author","values":["Hodenberg, Martin von"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:type","label":"Dc Type","values":["DoctoralThesis"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Spinstruktur","Gluonpolarisation","Nukleonenstruktur","spinstructure","gluon polarisation","COMPASS","CERN","D mesons"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["The complicated structure of the nucleon has been studied with great success
in deep-inelastic lepton-nucleon scattering (DIS) experiments at CERN, SLAC and
DESY. As a result the unpolarised structure functions have been measured
accurately over a wide kinematic range. From these measurements it is
possible to determine the gluon density in the nucleon with good accuracy via
a so-called QCD fit.
In the case of the spin structure of the nucleon the situation is different. Even after decades of experimental and theoretical efforts it remains to be understood how the spin of the nucleon of 1/2 in units of h-bar is to be
accounted for in terms of contributions from the quarks and gluons inside the
nucleon.
Of particular interest is the question whether the polarised gluon density
can explain the unexpected smallness of the quark contribution to the nucleon
spin.
The QCD fit, which worked well in the unpolarised case, yields a polarised
gluon density Delta G which is only badly constrained. This is due to
the fact that the information on the polarised structure functions is only
available in a rather small kinematic range, since the corresponding measurements are so far exclusively performed by fixed-target experiments.
The limited knowledge of the structure functions aggravates the difficulty that
the gluon distribution enters only in next-to-leading order and is thus
suppressed by the strong coupling constant.
A direct measurement of Delta G is therefore needed to get a clearer picture.
This direct measurement is the major aim of the COMPASS experiment at CERN, where polarised muons are scattered off a polarised fixed target.
The process on which this measurement is based is the photon-gluon fusion
(PGF). It can result in the production of a charm-anti-charm-quark pair and
is then tagged by detecting the charmed mesons in the final state.
This approach is used in this thesis and results in a theoretically very clean sample of PGF events, assuming that the intrinsic charm content in the nucleon is negligible.
Since the charm tagging of the PGF process is very much statistically limited,
it profits directly from improvements of the reconstruction efficiency.
A major contribution was made in the course of this work concerning the reconstruction of the momentum of the beam muons.
The efficiency of this reconstruction was initially limited by the fact that information from the contributing detectors, installed about 100\\,m apart along the beam line, could only be associated by using the time stamp of the
corresponding measurements. The main improvement is that now also space information is taken into account, based on the knowledge of the transfer matrix of the beam line.
In total the fraction of non-reconstructable events could be decreased from
19% to 6%.
The polarised target used in the COMPASS experiment is a large solid-state
target which does not allow the use of dedicated vertex detectors. The identification of the charmed mesons thus has to proceed via the reconstruction of their invariant mass. In order to control the combinatorial background, the reconstruction relies heavily on the RICH detector.
This work contributed to the understanding of how this detector and the
spectrometer in general can be used best in order to extract the cleanest
possible signal of charmed mesons.
Especially the tagging of D-star decays results in a very clear signal and
thus makes COMPASS the first polarised fixed-target experiment which is able
to extract the gluon polarisation via the open charm approach.
The analysing power needed for the extraction is given by the ratio of spin dependent and spin averaged muon-nucleon cross sections, which have been
calculated to first order in the strong coupling constant.
These cross sections cannot be computed exactly in the analysis, since they depend on parton kinematics, which are poorly known due to the fact that typically only one of the two charmed mesons produced in the PGF process is detected.
Thus a parametrisation based on Monte Carlo simulations was prepared, which maps the parton kinematics onto measurable quantities. This makes the analysing power
available on an event-by-event basis with sufficient accuracy and thus allows a statistically optimal analysis.
The final result for the gluon polarisation is extracted at a mean momentum
fraction carried by the gluon of eta=0.15 and at a scale mu^2=13(GeV/c)^2 and was found to be Delta G / G =-0.31+/-0.44(stat.)+/-0.07(syst.).
The systematic error is mostly due to the uncertainty on the mass of the charm quark, possible false asymmetries and the background contribution to the asymmetry.
This result for Delta G/G is consistent with parametrisations obtained from QCD fits to polarised DIS data.","Die komplizierte Struktur der Nukleonen ist mittels tief-inelastischer Lepton-Nukleon Streuung (DIS) mit grossem Erfolg in Experimenten am CERN, SLAC und DESY untersucht worden. Daraus resultieren sehr genaue Kenntnisse der unpolarisierten Strukturfunktionen ueber einen weiten kinematischen Bereich.
Im Falle der Spinstruktur des Nukleons sieht die Lage anders aus. Selbst nach Jahrzehnten an experimentellen und theoretischen Bemuehungen versteht man noch nicht wie der Spin-1/2 des Nukleons zustande kommt und wie er sich aus den Beitraegen der Quarks und Gluonen im Nukleon zusammensetzt.
Der QCD-Fit, der im unpolarisierten Fall genaue Ergebnisse geliefert hat, ergibt im polarisierten Fall eine polarisierte Gluondichte Delta G, die nur wenig eingeschraenkt ist. Dies ist auf die Tatsache zurueckzufuehren, dass die polarisierten Strukturfunktionen nur in einem relativ kleinen kinematischen Bereich bekannt sind, da die entsprechenden Daten bisher exklusiv aus Fixed-Target Experimenten stammen.
Die spaerlich vorhandene Information ueber die Strukturfunktionen verstaerkt die Schwierigkeit, dass die Gluondichte erst in hoeherer Ordnung eingeht und daher durch die starke Kopplungskonstante unterdrueckt wird, eine direkte Messung von Delta G ist daher wuenschenswert.
Diese direkte Messung ist eines der Hauptziele des COMPASS Experimentes am CERN, wo ein polarisierter Myonenstrahl an einem polarisierten stationaeren Target gestreut wird.
Der Prozess auf dem diese Messung beruht ist die Photon-Gluon-Fusion (PGF).
Dieser Prozess kann in der Bildung eines Charm-Anticharm-Quark Paares resultieren und wird dann ueber die Rekonstruktion der charmhaltigen Mesonen im Endzustand nachgewiesen.
Dieser Ansatz wird in dieser Dissertation verwendet und ergibt unter der Annahme, dass der Beitrag von intrinsischem Charm im Nukleon vernachlaessigbar ist, eine theoretisch saubere Menge von PGF Ereignissen.
Da der Nachweis des PGF Prozesses ueber die charmhaltigen Mesonen statistisch limitiert ist, profitiert die Analyse direkt von Verbesserungen der Rekonstruktionseffizienz.
Ein wesentlicher Beitrag hierzu ist im Rahmen dieser Arbeit gemacht worden, indem die Impulsrekonstruktion der Strahlteilchen verbessert worden ist.
Die Effizienz war bisher limitiert durch die Tatsache, dass Informationen von verschiedenen Detektoren kombiniert werden mussten, die etwa 100 m voneinander entfernt entlang der Strahlfuehrung liegen. Dies geschah ausschliesslich mittels der Zeitmessungen der Detektoren.
Die wesentliche Verbesserung liegt nun darin, dass jetzt auch Ortsinformationen verwendet werden, basierend auf der Kenntniss der Transfermatrix der Strahloptik.
Alles in allem konnte der Anteil der nicht rekonstruierbaren Ereignisse von 19% auf 6% verringert werden.
Das verwendete ausgedehnte Festkoerpertarget verhindert den Einsatz von dedizierten Vertexdetektoren.
Der Nachweis der charmhaltigen Mesonen wird daher ueber die Rekonstruktion ihrer invarianten Masse gefuehrt.
Um den kombinatorischen Untergrund unter Kontrolle zu halten, benoetigt diese Methode eine gute Teilchenidentifikation mittels des RICH Detektors.
Zu dem Verstaendnis wie dieser Detektor und das Spektrometer im Allgemeinen am besten eingesetzt wird um ein moeglichst sauberes Signal von charmhaltigen Mesonen zu erhalten, hat diese Arbeit beigetragen.
Speziell die Moeglichkeit ueber den Nachweis von D* Zerfaellen Untergrund zu unterdruecken resultiert in einem sehr sauberen Signal und macht COMPASS zu dem ersten Fixed-target Experiment, das in der Lage ist die Gluonpolarisation ueber den Open-Charm Ansatz direkt zu messen.
Fuer die Bestimmung der Gluonpolarisation wird die Analysierstaerke benoetigt, welche durch den Quotienten aus spin-abhaengigem und spin-gemitteltem Myon-Nukleon Wirkungsquerschnitten gegeben ist, die bis zur ersten Ordnung in der starken Kopplungskonstanten bekannt sind.
Diese Wirkungsquerschnitte koennen in der Analyse nicht exakt berechnet werden, da sie von der Partonkinematik abhaengen, die aufgrund der Tatsache, dass meist nur ein charmhaltiges Meson des Endzustandes rekonstruiert wird nur unzulaenglich bekannt ist.
Daher wurde basierend auf Monte Carlo Simulationen eine Parametrisierung erstellt, welche die Partonkinematik auf messbare Groessen abbildet. Durch diesen Ansatz steht die Analysierstaerke fuer jedes Ereignis zur Verfuegung und erlaubt dadurch eine optimale Auswertung.
Das Endergebnis fuer die Gluonpolarisation ist bei einem mittleren Impulsbruchteil des Gluons am Nukleonimpuls von eta=0.15 und einer Skala von mu^2=13(GeV/c)^2 zu Delta G / G =-0.31+/-0.44(stat.)+/-0.07(syst.) bestimmt worden.
Der systematische Fehler ist hauptsaechlich gegeben durch die Unsicherheit auf die Charmquark-Masse, moegliche falsche Asymmetrien und Beitraege des Untergrundes zur Asymmetrie.
Dieses Resultat fuer Delta G/G ist konsistent mit Parametrisierungen, die aus QCD-Fits an polarisierte Daten entstammen."]},{"key":"dc:format.medium","label":"Dc Format Medium","values":["application/pdf"]},{"key":"dc:title","label":"Title","values":["First measurement of the gluon polarisation in the nucleon using D mesons at COMPASS","Erste Messung der Gluon Polarisation im Nukleon mittels D Mesonen bei COMPASS"]}]}],"canonical_facts":{"dc:contributor":["Königsmann, Kay"],"dc:creator":["Hodenberg, Martin von"],"dc:description.abstract":["The complicated structure of the nucleon has been studied with great success
in deep-inelastic lepton-nucleon scattering (DIS) experiments at CERN, SLAC and
DESY. As a result the unpolarised structure functions have been measured
accurately over a wide kinematic range. From these measurements it is
possible to determine the gluon density in the nucleon with good accuracy via
a so-called QCD fit.
In the case of the spin structure of the nucleon the situation is different. Even after decades of experimental and theoretical efforts it remains to be understood how the spin of the nucleon of 1/2 in units of h-bar is to be
accounted for in terms of contributions from the quarks and gluons inside the
nucleon.
Of particular interest is the question whether the polarised gluon density
can explain the unexpected smallness of the quark contribution to the nucleon
spin.
The QCD fit, which worked well in the unpolarised case, yields a polarised
gluon density Delta G which is only badly constrained. This is due to
the fact that the information on the polarised structure functions is only
available in a rather small kinematic range, since the corresponding measurements are so far exclusively performed by fixed-target experiments.
The limited knowledge of the structure functions aggravates the difficulty that
the gluon distribution enters only in next-to-leading order and is thus
suppressed by the strong coupling constant.
A direct measurement of Delta G is therefore needed to get a clearer picture.
This direct measurement is the major aim of the COMPASS experiment at CERN, where polarised muons are scattered off a polarised fixed target.
The process on which this measurement is based is the photon-gluon fusion
(PGF). It can result in the production of a charm-anti-charm-quark pair and
is then tagged by detecting the charmed mesons in the final state.
This approach is used in this thesis and results in a theoretically very clean sample of PGF events, assuming that the intrinsic charm content in the nucleon is negligible.
Since the charm tagging of the PGF process is very much statistically limited,
it profits directly from improvements of the reconstruction efficiency.
A major contribution was made in the course of this work concerning the reconstruction of the momentum of the beam muons.
The efficiency of this reconstruction was initially limited by the fact that information from the contributing detectors, installed about 100\\,m apart along the beam line, could only be associated by using the time stamp of the
corresponding measurements. The main improvement is that now also space information is taken into account, based on the knowledge of the transfer matrix of the beam line.
In total the fraction of non-reconstructable events could be decreased from
19% to 6%.
The polarised target used in the COMPASS experiment is a large solid-state
target which does not allow the use of dedicated vertex detectors. The identification of the charmed mesons thus has to proceed via the reconstruction of their invariant mass. In order to control the combinatorial background, the reconstruction relies heavily on the RICH detector.
This work contributed to the understanding of how this detector and the
spectrometer in general can be used best in order to extract the cleanest
possible signal of charmed mesons.
Especially the tagging of D-star decays results in a very clear signal and
thus makes COMPASS the first polarised fixed-target experiment which is able
to extract the gluon polarisation via the open charm approach.
The analysing power needed for the extraction is given by the ratio of spin dependent and spin averaged muon-nucleon cross sections, which have been
calculated to first order in the strong coupling constant.
These cross sections cannot be computed exactly in the analysis, since they depend on parton kinematics, which are poorly known due to the fact that typically only one of the two charmed mesons produced in the PGF process is detected.
Thus a parametrisation based on Monte Carlo simulations was prepared, which maps the parton kinematics onto measurable quantities. This makes the analysing power
available on an event-by-event basis with sufficient accuracy and thus allows a statistically optimal analysis.
The final result for the gluon polarisation is extracted at a mean momentum
fraction carried by the gluon of eta=0.15 and at a scale mu^2=13(GeV/c)^2 and was found to be Delta G / G =-0.31+/-0.44(stat.)+/-0.07(syst.).
The systematic error is mostly due to the uncertainty on the mass of the charm quark, possible false asymmetries and the background contribution to the asymmetry.
This result for Delta G/G is consistent with parametrisations obtained from QCD fits to polarised DIS data.","Die komplizierte Struktur der Nukleonen ist mittels tief-inelastischer Lepton-Nukleon Streuung (DIS) mit grossem Erfolg in Experimenten am CERN, SLAC und DESY untersucht worden. Daraus resultieren sehr genaue Kenntnisse der unpolarisierten Strukturfunktionen ueber einen weiten kinematischen Bereich.
Im Falle der Spinstruktur des Nukleons sieht die Lage anders aus. Selbst nach Jahrzehnten an experimentellen und theoretischen Bemuehungen versteht man noch nicht wie der Spin-1/2 des Nukleons zustande kommt und wie er sich aus den Beitraegen der Quarks und Gluonen im Nukleon zusammensetzt.
Der QCD-Fit, der im unpolarisierten Fall genaue Ergebnisse geliefert hat, ergibt im polarisierten Fall eine polarisierte Gluondichte Delta G, die nur wenig eingeschraenkt ist. Dies ist auf die Tatsache zurueckzufuehren, dass die polarisierten Strukturfunktionen nur in einem relativ kleinen kinematischen Bereich bekannt sind, da die entsprechenden Daten bisher exklusiv aus Fixed-Target Experimenten stammen.
Die spaerlich vorhandene Information ueber die Strukturfunktionen verstaerkt die Schwierigkeit, dass die Gluondichte erst in hoeherer Ordnung eingeht und daher durch die starke Kopplungskonstante unterdrueckt wird, eine direkte Messung von Delta G ist daher wuenschenswert.
Diese direkte Messung ist eines der Hauptziele des COMPASS Experimentes am CERN, wo ein polarisierter Myonenstrahl an einem polarisierten stationaeren Target gestreut wird.
Der Prozess auf dem diese Messung beruht ist die Photon-Gluon-Fusion (PGF).
Dieser Prozess kann in der Bildung eines Charm-Anticharm-Quark Paares resultieren und wird dann ueber die Rekonstruktion der charmhaltigen Mesonen im Endzustand nachgewiesen.
Dieser Ansatz wird in dieser Dissertation verwendet und ergibt unter der Annahme, dass der Beitrag von intrinsischem Charm im Nukleon vernachlaessigbar ist, eine theoretisch saubere Menge von PGF Ereignissen.
Da der Nachweis des PGF Prozesses ueber die charmhaltigen Mesonen statistisch limitiert ist, profitiert die Analyse direkt von Verbesserungen der Rekonstruktionseffizienz.
Ein wesentlicher Beitrag hierzu ist im Rahmen dieser Arbeit gemacht worden, indem die Impulsrekonstruktion der Strahlteilchen verbessert worden ist.
Die Effizienz war bisher limitiert durch die Tatsache, dass Informationen von verschiedenen Detektoren kombiniert werden mussten, die etwa 100 m voneinander entfernt entlang der Strahlfuehrung liegen. Dies geschah ausschliesslich mittels der Zeitmessungen der Detektoren.
Die wesentliche Verbesserung liegt nun darin, dass jetzt auch Ortsinformationen verwendet werden, basierend auf der Kenntniss der Transfermatrix der Strahloptik.
Alles in allem konnte der Anteil der nicht rekonstruierbaren Ereignisse von 19% auf 6% verringert werden.
Das verwendete ausgedehnte Festkoerpertarget verhindert den Einsatz von dedizierten Vertexdetektoren.
Der Nachweis der charmhaltigen Mesonen wird daher ueber die Rekonstruktion ihrer invarianten Masse gefuehrt.
Um den kombinatorischen Untergrund unter Kontrolle zu halten, benoetigt diese Methode eine gute Teilchenidentifikation mittels des RICH Detektors.
Zu dem Verstaendnis wie dieser Detektor und das Spektrometer im Allgemeinen am besten eingesetzt wird um ein moeglichst sauberes Signal von charmhaltigen Mesonen zu erhalten, hat diese Arbeit beigetragen.
Speziell die Moeglichkeit ueber den Nachweis von D* Zerfaellen Untergrund zu unterdruecken resultiert in einem sehr sauberen Signal und macht COMPASS zu dem ersten Fixed-target Experiment, das in der Lage ist die Gluonpolarisation ueber den Open-Charm Ansatz direkt zu messen.
Fuer die Bestimmung der Gluonpolarisation wird die Analysierstaerke benoetigt, welche durch den Quotienten aus spin-abhaengigem und spin-gemitteltem Myon-Nukleon Wirkungsquerschnitten gegeben ist, die bis zur ersten Ordnung in der starken Kopplungskonstanten bekannt sind.
Diese Wirkungsquerschnitte koennen in der Analyse nicht exakt berechnet werden, da sie von der Partonkinematik abhaengen, die aufgrund der Tatsache, dass meist nur ein charmhaltiges Meson des Endzustandes rekonstruiert wird nur unzulaenglich bekannt ist.
Daher wurde basierend auf Monte Carlo Simulationen eine Parametrisierung erstellt, welche die Partonkinematik auf messbare Groessen abbildet. Durch diesen Ansatz steht die Analysierstaerke fuer jedes Ereignis zur Verfuegung und erlaubt dadurch eine optimale Auswertung.
Das Endergebnis fuer die Gluonpolarisation ist bei einem mittleren Impulsbruchteil des Gluons am Nukleonimpuls von eta=0.15 und einer Skala von mu^2=13(GeV/c)^2 zu Delta G / G =-0.31+/-0.44(stat.)+/-0.07(syst.) bestimmt worden.
Der systematische Fehler ist hauptsaechlich gegeben durch die Unsicherheit auf die Charmquark-Masse, moegliche falsche Asymmetrien und Beitraege des Untergrundes zur Asymmetrie.
Dieses Resultat fuer Delta G/G ist konsistent mit Parametrisierungen, die aus QCD-Fits an polarisierte Daten entstammen."],"dc:format.medium":["application/pdf"],"dc:subject":["Spinstruktur","Gluonpolarisation","Nukleonenstruktur","spinstructure","gluon polarisation","COMPASS","CERN","D mesons"],"dc:title":["First measurement of the gluon polarisation in the nucleon using D mesons at COMPASS","Erste Messung der Gluon Polarisation im Nukleon mittels D Mesonen bei COMPASS"],"dc:type":["DoctoralThesis"]},"updated_at":"2026-07-24T02:22:40Z"}