Techn. Universiẗat, Diss., 2005--Dresden.

The Gesellschaft für Schwerionenforschung (GSI) facility in Darmstadt, Germany, will be upgraded to accommodate a new generation of physics experiments. The future accelerator facility will be called FAIR and one of the experimentsat the site will be PANDA, which aims at performing hadron physics investigations by colliding anti-protons with protons. The licentiate thesis consistsof three sections related to PANDA. The first contains energy resolutionstudies of PbWO4 crystals, the second light yield uniformity studies of PbWO4 crystals and the third reconstruction of the lambda-bar-polarisation in the PANDA experiment. Two measurements of the energy resolution were performed at MAX-Lab in Lund, Sweden, with an array of 3x3 PbWO4 crystals using a tagged photon beam with energies between 19 and 56 MeV. For the April measurement, the crystals were cooled down to -15 degrees C and for the September measurement down to -25 degrees C. The measured relative energy resolution, <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csigma" />/E, is decreasing from approximately 12% at 20 MeV to 7% at 55 MeV. In the standard energy resolution expression <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csigma" />/E = a/<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csqrt%5B%5D%7BE%7D" /> <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Coplus" /> b/E <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Coplus" /> c, the three parameters a, b and c seem to be strongly correlated and thus difficult to determine independently over this relative small energy range. The value of a was therefore fixed to that one would expect from Poisson statistics of the light collection yield (50 phe/MeV) and the results from fits were <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csigma" />/E=0.45%/<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csqrt%5B%5D%7BE_%7BGeV%7D%7D" /> <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Coplus" />0.18%/EGeV <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Coplus" />8.63% and <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csigma" />/E = 0.45%/<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csqrt%5B%5D%7BE_%7BGeV%7D%7D" /><img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Coplus" />0.21%/EGeV <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Coplus" />6.12% for the April and September measurements, respectively. The data from the September measurement was also combined with previous data from MAMI for higher energies, ranging from approximately 64 to 715 MeV. The global fit over the whole range of energies gave an energy resolution expression of <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csigma" />/E = 1.6%/<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csqrt%5B%5D%7BE_%7BGeV%7D%7D" /> <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Coplus" />0.095%/EGeV <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Coplus" />2.1%. Light yield uniformity studies of five PbWO4 crystals, three tapered and two non-tapered ones, have also been performed. The tapered crystals delivered a light output which increased with increasing distance from the Photo Multiplier Tube (PM tube). Black tape was put on different sides of one tapered crystals, far from the PM tube to try to get a more constant uniformity prole. It was seen that the light output profile depends on the position of the tape. Generally, the steep increase in light output at large distances from the PM tube could be damped. The third part of the thesis concerns the reconstruction of the lambdabar polarisation in the reaction <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?pbar+p%20%5Crightarrow%20lambdabar%20+%20lambda" />. Events were generated using a modied generator from the PS185 experiment at LEAR. With a 100% polarisation perpendicular to the scattering plane, a polarisation of (99±1.8)% was reconstructed. Slight non-zero polarisations along the axis determined by the outgoing hyperon as well as the axis in the scattering plane, were also reconstructed. These were (4.1±2.1)% and (2.6±2.0)% respectively. From this investigation it was shown that the detector efficiency was not homogeneous and that slow pions are difficult to reconstruct.

No description available.

According to the standard model, the size of CP violation in the weak decay Λ → pπ- is predicted to be ∼ 10-5. This means that a very high precision is needed if we want to be able to measure it. It turns out that the precision mainly depends on three factors: if the decay already has a large parity-breaking effect, if we take more measurements, or if the Lambda baryon has a large polarization. The uncertainty is inversely proportional to the root mean square of the polarization, √⟨P2⟩, of Λ. Polarization is a measurement of the fraction of particles with parallel spin. In this project, we simulated the kinematic properties of the three-body decay of the charmed lambda baryon Λc+  → Λπ+ π0 by assuming that Λ+ c decays 100% of the times via the ρ+-resonance, i.e. Λc+ decays like Λc+ → Λρ+ , and ρ+ subsequently decays ρ+ → π+π0. The invariant masses of the daughter particles were then plotted in a so-called Dalitz plot. The fact that the ρ+-meson is a vector meson is exploited to compute the helicity amplitudes HλΛ,λρ, where λ is the helicity of respective particle, meaning the projection of the spin in the direction of the ρ+-mesons momentum. The helicity amplitudes describe the helicities of the particles involved before and after the decay. These are then used to compute the polarization of Λ. After doing the simulation, the invariant masses are plotted in the Dalitz plot and the polarization in each point is computed. The root mean square polarization of Λ was √⟨P2⟩ = 0.81 ± 0.08. This result gives a large polarization and points to this process being a good baryon polarizer. When conducting more research into the three-body decay of Λc+ as a potential source of Λ with large polarization, the contributions of other resonances should be taken into account. In light of these results, it would also be interesting to investigate if three-body decays could work as a baryon polarizer for other baryons. / CP brytning i sönderfallet Λ → pπ-, via den svaga växelverkan, kommer enligt standardmodellen att ha en storlek runt ∼ 10-5. Det betyder att en väldigt hög precision är nödvändig om man ska kunna observera denna effekt. Det visar sig att precisionen av mätningarna främst beror på tre faktorer: om sönderfallet redan har stor paritestbrytande effect, om man tar fler mätningar, eller om Lambdabaryonerna har stor polarisation. Osäkerheten är proportionerlig mot inversen av det kvadratiska medelvärdet, √⟨P2⟩ av Lambdabaryonerna. Polarisationen av en mängd partiklar är ett mått på andelen partiklar med parallellt spinn. I detta projekt kommer vi simulera de kinematiska egenskaperna hos trekroppssönderfallet av den charmade Lambdabaryonen Λc+ → Λπ+π0 genom att anta 100% sönderfall via ρ+-resonansen, dvs att Λc+ först sönderfaller som Λc+ → Λρ+, och att ρ+ sedan sönderfaller som ρ+ → π+π0. De invarianta massorna av dotterpartiklarna plottades sedan i en s.k Dalitz-plot. Vi utnyttjade att ρ+-mesonen är en vektormeson för att beräkna helisitetsamplituderna HλΛ,λρ, där λ är helisiteterna av respektive partikel, dvs projektionen av partiklarnas spinn längst ρ+-mesonens rörelsemängdsriktning. Helisitetsamplituderna beskriver helisiteten hos partiklarna före och efter sönderfallet. Dessa användes sedan för att beräkna polarisationen av Λ. Efter att simuleringen gjorts, plottades de invarianta massorna i en Dalitz-plott och polarisationen beräknades i varje punkt. Det kvadratiska medelvärdet av polarisationen hos Λ var √⟨P2⟩ = 0.81 ± 0.08. Detta är en stor polarisation som indikerar att trekroppsönderfall kan vara en bra baryonpolarisator. För vidare forskning om trekroppssönderfallet av Λc+ som en källa av Λ med stor polarisation, borde bidragen från andra resonanser också medräknas. Vidare skulle det även vara intressant att undersöka om trekroppssönderfall kan fungera som en baryonpolarisator för andra baryoner.

The first-time measurement of the angular dependence of the beam-helicity asymmetry for $\gamma p \rightarrow pK^{+}K^{-}$ is shown and compared to $\gamma p \rightarrow p\pi^{+}\pi^{-}$. The data obtained were from the CLAS g12 experiment at Jefferson Lab. The experiment utilized a beam of circularly polarized photons with energies between 1.1 and 5.4 GeV incident on an unpolarized liquid hydrogen target, which produced an unprecedented number of strange hadrons in photoproduction. The production mechanism for strange hadrons is not well understood. The beam-helicity asymmetry is a polarization observable that provides information on interfering production mechanisms in the reaction. It is shown that the asymmetry is sensitive to several kinematic variables that are key in modeling the reaction dynamics. Furthermore, the comparison of the beam-helicity asymmetry between the kaon and pion channels serves as a platform for the investigation of flavor dependence. A partial wave analysis on the $pK^{-}$ system is also performed in a search for missing hyperon excitations.

A basic assumption in fundamental physics is that equal amounts of matter and antimatter were created after the Big Bang. When particles and antiparticles collide, they annihilate, i.e. disappear and produce photons. Nevertheless, the universe consists mainly of matter today. To explain why all matter did not disappear, violation of CP symmetry beyond the Standard Model is required. CP symmetry means that the laws of physics are the same if particles are interchanged with antiparticles and spatial coordinates of all particles are mirrored. CP symmetry is relatively poorly tested in baryon decays. A new method to study CP symmetry in hyperon-antihyperon pairs has been developed at Uppsala University. Hyperons are baryons with one or more strange quarks. The method allows determining the decay asymmetry parameters of the hyperon and antihyperon separately if the hyperon-antihyperon pair is polarized. If any significant difference between the magnitudes of these parameters is found, the process is CP violating. The particle physics experiment BESIII in China is a suitable experiment to conduct this kind of measurements because it is a high precision experiment and has collected large data samples of hyperon-antihyperon pairs. The goal of this project was to study statistical precisions of the physics parameters that can be obtained with the new method in cases of J/ψ meson decaying into ΛΛ, Σ+Σ− and Σ0Σ0. High statistical precision is required to detect CP violation, because CP violating processes are, if they exist, expected to be rare. The main focus was to study the process e+e− → J/ψ → Σ0Σ0 → ΛγΛγ → pπ−γpπ+γ. In this process, CP symmetry can be tested in two decay processes: electromagnetic decay Σ0 → Λγ and weak decay Λ → pπ−. Only the asymmetry parameter of Λ → pπ− was studied. The study served as a validity check of the new method and ongoing analyses at BESIII. The statistical precision was studied by simulations: Monte Carlo data samples were created and then a maximum-log-likelihood fit was applied to the samples. An important component when determining the asymmetry parameters turned out to be the relative phase ∆φJ/ψ. The relative phase is one of the parameters used for determining the relation between the electric and magnetic form factors. ∆φJ/ψ is also related to the polarization of the hyperon-antihyperon pair. The study showed that the value of ∆φJ/ψ has a large impact on the uncertainties of the hyperon and antihyperon asymmetry parameters. A low value of ∆φJ/ψ resulted in high uncertainties and strong correlations between the asymmetry parameters. The formalism is different for different processes, which affects the uncertainties as well. The formalism used for the Σ0Σ0 process gives poorer parameter precision of the asymmetry parameter related to the Λ → pπ− decay than the formalism used for the ΛΛ process. Therefore, the ΛΛ process is a much more suitable process for CP studies of the Λ → pπ− decay. / Ett grundantagande inom den fundamentala fysiken är att lika stora mängder av materia och antimateria skapades efter Big Bang. När partiklar och antipartiklar kolliderar, annihilerar de, dvs försvinner och producerar fotoner. Trots detta består dagens universum huvudsakligen av materia. För att förklara varför all materia inte försvann krävs ett brott mot CP-symmetrin bakom standardmodellen. CP-symmetrin innebär att fysikens lagar är desamma om man byter partiklar mot antipartiklar och speglar partikelns rumsliga koordinater. CP-symmetri i baryonsönderfall är relativt dåligt testad. En ny metod för att studera CP-symmetrin har utvecklats vid Uppsala universitet för hyperon-antihyperon par. Hyperoner är baryoner med en eller fler särkvarkar. Metoden gör det möjligt att bestämma asymmetriparametrar hos hyperon- och antihyperonsönderfall separat om hyperonantihyperonparet är polariserat. Om en signifikant skillnad mellan värden av dessa parametrar upptäcks, är processen CP-brytande. Partikelfysikexperimentet BESIII i Kina är ett lämpligt experiment för sådana här mätningar eftersom det är ett högpresicionsexperiment och har dessutom samlat in stora mängder data av hyperon-antihyperonpar. Målet för detta projekt har varit att studera de statistiska precisioner av fysikparametrar som kan nås när man använder den nya metoden i de fall där J/ψ mesonen sönderfaller till ΛΛ, Σ+Σ− och Σ0Σ0. Hög statistisk precision behövs för att upptäcka CP-brott, eftersom CP-brytande processer, om de existerar, är relativt sällsynta. Huvudfokuset var att studera processen e+e− → J/ψ → Σ0Σ0 → ΛγΛγ → pπ−γpπ+γ. I denna process kan CP-symmetri testas för två sönderfallsprocesser: det elektromagnetiska sönderfallet Σ0 → Λγ och det svaga sönderfallet Λ → pπ−. I denna rapport studerades bara asymmetriparametrarna av Λ → pπ−. Detta arbete har fungerat som validitetskontroll av den nya metoden och pågående analyser på BESIII. Den statistiska precisionen undersöktes med simuleringar: Monte Carlo datamängder skapades och sedan en maximum-log-likelihood-anpassning av datan genomfördes. En viktig komponent i bestämningen av asymmetriparametrarna visade sig vara den relativa fasen, ∆φJ/ψ. Den relativa fasen är en av de parametrar som används för att bestämma relationen mellan de elektriska och magnetiska formfaktorer. ∆φJ/ψ är också relaterad till hyperonens hyperon-antihyperonparets polarisation. I forskningsprojektet visades att ∆φJ/ψ har en stor inverkan på osäkerheterna av hyperon- och antihyperonasymmetriparametrarna. Ett lågt värde av ∆φJ/ψ resulterade i stora osäkerheter och starka korrelationer mellan asymmetriparametrarna. Formalismen är annorlunda för olika processer, vilket också påverkar osäkerheterna. Formalismen som används för Σ0Σ0-processen ger sämre parameterprecision av asymmetriparametern kopplad till sönderfallet Λ → pπ− än formalismen som används för ΛΛ-processen. Därför är ΛΛ-processen en mycket lämpligare process för att testa CP-symmetrin i Λ → pπ− sönderfallet.

The thesis comprises investigations of two topics related to the PANDA experiment. The first part is dedicated to energy resolution and light yield uniformity studies of PWO crystals for the electromagnetic calorimeter. The second part of the thesis is dedicated to simulation studies of the p̅p→Λ̅Λ and the p̅p→Λ̅Σ0 channels. Photon response measurements with one 3×3 crystals matrix of rectangular crystals and one 5×5 matrix with tapered crystals have been performed at the MAX-Lab synchrotron facility in Lund, Sweden. Photon energies in the range of 13-84 MeV were used. GEANT4 simulations were performed in order to find the most suitable method for adding the energy contributions and for calibration purposes. The widths, σ, of the resulting experimental summed energy peaks were fitted using the Novosibirsk distribution. The results show that the electromagnetic shower at 84 MeV is completely contained in the 5×5 matrix. The widths (σ) for the summed energy peaks were determined.Studies of the uniformity of the light yield were performed for crystals of three different shapes and different wrapping materials. The light yield as a function of distance between the point of energy depositions and the PM tube, was shown to be closely related to the crystal shape and the wrapping material. The p̅p→Λ̅Λ channel was studies at beam momenta of 1.64 GeV/c, 4 GeV/c and 15 GeV/c, while p̅p→Λ̅Σ0 was studied at 4 GeV/c. In the simulations, both phase space differential cross-sections and experimental differential cross-sections from PS185 were used as input. The purpose of the simulations was to show that the reaction channels can be reconstructed in the detector. Special interest was paid to the polarisation and spin correlations of the hyperons. The result is that there is acceptance of cosθ angular range over the full momentum range of the HESR for both channels. Using isotropic differential cross-sections, the polarisation for Λ̅ and Σ0 as well as spin correlations between Λ̅Λ and Λ̅Σ0 can be well reconstructed. Using the differential cross-sections from PS185, the particles are more likely to go forwards in their respective directions in the CM-system, thus making reconstruction over the full angular range more difficult at high beam momenta.

Das Kuehlersynchrotron COSY im Forschungszentrum Juelich kann u.a. einen guten Protonenstrahl fuer Experimente zur Verfuegung stellen. An einem der Experimentierplaetze steht das Flugzeitspektrometer COSY-TOF. Es eignet sich hervorragend zur Untersuchung von Reaktionen, bei denen wenige Teilchen enstehen. Solange die entstehenden Teilchen einen Winkel von hoechstens 70 Grad mit Strahlachse einschliessen, werden diese nachgewiesen. Damit sind im schwellennahen Energiebereich die Flugzeit und Flugrichtung aller langlebigen Teilchen bestimmbar. Zum Nachweis kurzlebiger Teilchen gibt es einige Module im Flugzeitspektrometer zur Rekonstruktion der Flugrichtung. Die Arbeit untersucht zwei Reaktionen, bei denen Hyperonen entstehen: pp--&amp;gt;nK+Sigma+ und pp--&amp;gt; pK0Sigma+ bei einem Strahlimpuls von 2,95 GeV/c. Im schwellennahen Energiebereich liegen bisher keine (in Zeitschriften) veroeffentlichten Daten vor. Die Kenntnis der Reaktionsmechanismen sind jedoch wuenschenswert, um die physikalsiche Beschreibung in vielen Bereichen zu verbessern. Dazu gehoeren die Eigenschaften der Teilchen (Strangenessgehalt des Nukleons) und die Wechselwirkungen zwischen den Teilchen (auch in extrem dichter Materie). In dieser Arbeit werden die zur Kalibrierung des Detektors notwendigen Schritte, die vom Autor durchgefuehrt wurden, erlaeutert. Im Anschluss wird die Analyse der Daten fuer ein Experiment ausfuehrlich vorgestellt. Durch einige Bedingungen wird erreicht, dass eine (nahezu) untergrundfreie Menge der gesuchten Ereignissen vorliegt. Die Anzahl der gefundenen Ereignisse laesst fuer die Reaktion pp--&amp;gt; nK+Sigma+ nur die Angabe eines totalen Wirkungsquerschnittes zu. Fuer die Reaktion pp--&amp;gt; pK0Sigma+ wird ebenfalls der totale Wirkungsquerschnitt bestimmt. Weiterhin werden Untersuchungen zum Reaktionsmechanismus durchgefuehrt. Dadurch wird ersichtlich, dass diese Reaktion einen starken resonanten Anteil im K0Sigma-System hat. Die Analyse wird auch auf zwei andere Experimente mit COSY-TOF angewendet. Das Auftreten hoeherer Drehimpulse ist erst bei groesseren Strahlimpulsen 3,2 GeV/c zu erkennen.

The prospects of studying multi-strange and charmed antihyperon-hyperon physics and CP violation in hyperon decays in the upcoming PANDA experiment at FAIR, Germany, have been studied in this thesis. The angular dependence on polarisation parameters in the decay of the spin 3/2 Omega hyperon was calculated using the density matrix formalism. Expressions for the angular distributions in the Ω -&gt; ΛK and the subsequent Λ -&gt; pπ decays were derived. Simulations were performed for the pbar p -&gt; Ξ+ Ξ-, pbar p -&gt; Ω+Ω- and pbar p -&gt; Λc-Λc+ reactions. Special attention was given to the reconstruction of spin variables. It is shown that PANDA will register tens of events per second for the pbar p -&gt; Ξ+Ξ- reaction. This should be compared to the previously existing data of a handfull of events. For the other two reactions the event rates will be lower, but still reasonably high. This will be the first measurements of these reactions. It is shown that spin variables can be reconstructed in all three reactions for all production angles of the hyperons. Simulations concerning the possibility to measure CP violation parameters in hyperon decays were also made for the reactions pbar p- &gt; Λbar Λ and pbar p -&gt; Ξ+Ξ-. It was found that false signals from detector asymmetries disappears if no particle identification criterium is used and the analysis is restricted to events were the hyperon decays occur close to the beam axis. The effect of the magnetic field in the PANDA detector on the measurement of hyperon spin variables was investigated for the case of pbar p -&gt; Λbar Λ. The effect was observed to be small for polarisation and negligible for spin correlations.

Techn. Universiẗat, Diss., 2003--Dresden.