Physics analysis

The investigation of the properties of the proton is one of the major topics in modern hadron physics. The inner structure of the proton can be accessed via electromagnetic form factors. Of great interest is the time-like region ($q^2$ > 0), which can be investigated in reactions of $\bar{p}p \rightarrow {\ell}^+{\ell}^-$ (${\ell}$ = $e$, $\mu$, $\tau$).

This work reports on feasibility studies for the measurement of time-like electromagnetic form factors of the proton, $|G_E|$ and $|G_M|$, in reactions of $\bar{p}p \rightarrow {\mu}^+{\mu}^-$ at the future \PANDA experiment (FAIR, Darmstadt). The studies are performed at four different beam momenta between 1.5 and 3.3 GeV/$c$. The high luminosity, which will be available at \PANDA/FAIR, will allow for the collection of data with high statistics, which is a prerequisite for the separate extraction of the electromagnetic proton form factors from the signal angular distribution of the reconstructed ${\mu}^-$ (${\mu}^+$).

At first, the studies were performed for the conditions of the latest data taking phase at \PANDA (\PANDA Phase-3). The achievable accuracy of the form factors is determined by means of Monte-Carlo simulation and the subsequent data analysis for the signal reaction, assuming $R = |G_E|/|G_M|$ to be equal unity. Data samples for both the signal and for all relevant background channels have been generated and simulated using the PandaRoot software framework together with dedicated event generators.

For the most challenging background channel $\bar{p}p \rightarrow \pi^+\pi^-$, data sets of 10$^8$ events were generated and used for the determination of the suppression factor. The simulated data samples also allow for the calculation of the expected pion contamination statistics, which will remain in the signal data after the application of all selection criteria. The signal-to-background separation has been optimized through the use of multivariate classification methods (Boosted Decision Trees). A background subtraction will be necessary to remove the pion contamination from the reconstructed data at \PANDA. This effect has been taken into account in these feasibility studies. For this purpose, a method was developed in order to construct angular distributions for the pion contamination with both the expected statistics and a mostly realistic shape. The influence of this shape on the extracted precision of the form factors was investigated in a separate study. Systematic uncertainties were estimated as well.

It will be shown, that a sufficient suppression of all other relevant background channels (as for example $\bar{p}p \rightarrow \pi^+\pi^- \pi^0$, $\bar{p}p \rightarrow \pi^0\pi^0$, $\bar{p}p \rightarrow K^+K^-$, etc.) will be achieved at \PANDA.

Due to the high rest mass of the muon, QED radiative corrections in the signal $\bar{p}p \rightarrow \mu^+\mu^-$ are expected to be small compared to $\bar{p}p \rightarrow e^+e^-$, mainly caused by final state radiation. The influence of final state radiation on the signal statistics is studied using the PHOTOS software package.

Since \PANDA will be able to investigate both channels of $\bar{p}p \rightarrow {\ell}^+{\ell}^-$ (${\ell}$ =$e$, $\mu$), it is possible to determine the ratio of the effective form factor obtained with both channels. This allows to perform a sensitive test of lepton universality ($e$-$\mu$) at \PANDA.

A separate study was performed for the conditions of the first data acquisition time period of \PANDA (\PANDA Phase-1).

This thesis aims to study low-energy strong interaction effects of open-charm mesons. Our studies are based on the chiral Lagrangian supplemented by constraints from the heavy-quark spin symmetry. The pseudoscalar and vector open-charm-meson masses are calculated up to next-to-next-to-next leading order (N^3LO) corrections. Different assumptions on the counting rules are investigated. It is illustrated that a chiral expansion uniformly converges rapidly up to Goldstone boson masses as heavy as the kaon masses if formulated in terms of physical meson masses. First estimates of the relevant low-energy parameters are extracted from lattice QCD data on the quark mass dependence of the D meson masses. Such low-energy parameters are of crucial importance for the low-energy interaction of the Goldstone bosons with the D mesons.

To study rare processes, such as the formation of exotic particles, the PANDA experiment has been set up. To process the large dataflow, the subsystems preprocess the data. One example is the algorithm to search online for clusters in the data of the calorimeter, developed in this thesis. Due to the high interaction rates, the time is important. Optima have been determined to assign the data to (time)bunches (257, 97, and 15 ns) and hits to clusters (282, 122, and 40 ns), for interaction rates of 200 kHz and less, 2 MHz, and 20 MHz, respectively, and each hit must have a minimal energy of 3 MeV. Using these optima, the efficiencies for the investigated two- and seven-photon channels are 70-80% and 9-20%, respectively, depending on the interaction rate. In addition, it was investigated if the algorithm can run on the readout system, making use of the processing power in the data concentrators and the nodes in the data collection network. This was found to be possible, and, furthermore, the processing time using this approach is much lower. An important note to the efficiencies is that there were problems in creating the time stamps for the hits, which may cause the obtained results to deviate from reality. In addition, another step needs to be added to the reconstruction, where the event selection takes place, to determine the final efficiencies. It is recommended to proceed with the development of the cluster finding algorithm, as it delivers good performance at low costs.

Abstract
This thesis work, set in the framework of hadron physics, reports on a phenomenological and an experimental study dedicated to nucleon internal structure, both related to hadron electromagnetic form factor measurements in the time-like and in the space-like regions.
At the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, an experimental program aiming to determine the proton electromagnetic form factors in the time-like region is planned benefiting from the PANDA (anti-Proton Annihilation at Darmstadt) large acceptance detector through the annihilation reaction p̄p -> e+ e- and p̄p -> μ+ μ- in the energy range 2.25(1.5) ≤ √s(p L ) ≤ 5.47(15) GeV (GeV/c), where √s(p L ) is the total energy in the center of mass system (the beam momentum in laboratory frame). In such reactions, the difficulty will be to extract the signal of interest from the huge background coming from hadronic channels, mainly pion pair production. Phenomenological studies of meson pair production in p̄p annihilation is one of the subjects of this thesis. The goal is two-fold : to better evaluate the hadronic background and to improve our knowledge on the hadron internal structure as such reaction contains by itself useful information on the hadron quark content. For this purpose, an effective meson model has been developed to evaluate the cross section of p̄p annihilation into light meson pairs, taking into account mesonic and baryonic degrees of freedom, in the energy domain relevant to the PANDA experiment. A logarithmic form factor is introduced to account for the composite nature of the interacting hadrons. A comparison with the existing data for charged pion pair production and predictions for angular distributions and energy dependence are presented in the range 3.362(5) ≤ √s(p L ) ≤ 4.559(10.1) GeV (GeV/c). The model is applied to πp elastic scattering, using crossing symmetry, and to charged kaon pair production, on the basis of SU(3) symmetry. An exponential term is added to reproduce the energy dependence of the total cross section. The model is extended to different neutral channels π0π0, ηη, ηπ0 relying on SU(3) symmetry. The obtained angular distributions and energy dependences are presented and successfully compared to the existing data.
In the space-like region, the experimental effort, started years ago at Jefferson Laboratory (Newport News, VA, USA) and dedicated to precise measurements of the proton electromagnetic form factor ratio based on the recoil polarization method in elastic ep scattering, will be pursued up to higher momentum transfer squared, above 10 GeV2, and will be also applied to neutron. This method requires intense and highly polarized electron beams, as well as an accurate measurement of the polarization of the recoil particle, the proton or the neutron. In order to design and optimize the polarimetry in the GeV region, the determination of the efficiency and the analyzing powers, that combine into the figure of merit, is mandatory to conceive a polarimeter, as polarization experiments are very lengthy. In this context I contributed to the ALPOM2 experiment (JINR Dubna, Russia), the only experiment right now able to measure proton and neutron analyzing powers in the momentum range of interest for Jefferson Lab experiments and allowing considering different types of target. Experimentally, in the framework of ALPOM2, from the Nuclotron polarized deuteron beam (up to 13 GeV energy), polarized proton and neutron beams at 3.0, 3.75 and 4.2 GeV/c were obtained by deuteron break-up and the charged fragments from the collisions with C, CH2, CH and Cu targets were measured. For neutrons, in the beam momentum range from 1 to 6 GeV/c, two processes have been considered to determine the analyzing powers: np -> np (zero charge exchanged) and the charge exchange reaction np -> pn. From the comparison of the figures of merit associated to each of these two processes, the charge exchange reaction appears to be more efficient for polarimetry at high energy. Neutron analyzing powers for the charge exchange reaction on CH, and Cu targets have been measured up to 4.2 GeV/c for the first time. For the proton, more precise analyzing power data have been obtained. Experimental results of preliminary analysis are presented.

PANDA Management Quarterly Report 18Q3

Im ersten Teil dieser Arbeit wird die Analyse des Zerfalls J/ψ → ωπ⁺π⁻ präsentiert. Die Daten wurden dabei im Rahmen des BESIII-Experiments aufgenommen. Im Rahmen dieser Analyse wird eine Datenselektion samt ereignisbasierter Untergrundunterdrückung sowie eine Partialwellenanalyse zur Identifizierung der beitragenden Subzerfälle durchgeführt. Anschließend erfolgt eine Messung der Verzweigungsverhältnisse der Hauptbeiträge. Als signifikant beitragend mit einem Anteil von jeweils über 20% am Gesamtzerfall sind die Subzerfälle f­­­­­2(1270) → π⁺π⁻ und b1(1235)± → ωπ± identifiziert worden.

Im zweiten Teil werden die Entwicklungen der Front- und Seitenkühlung sowie der thermischen Isolierung der Vorwärtsendkappe des elektromagnetischen Kalorimeters des PANDA-Experiments vorgestellt, um die geforderte Betriebstemperatur von -25°C zu erreichen und einen geringen zeitlichen sowie räumlichen Temperaturgradienten innerhalb des Detektors zu gewährleisten.

One of the main topics of interest in physics is the composition and structure of matter. Especially for hadrons, which are bound states of quarks and gluons such as the proton and neutron, the description is incomplete. Quantumchromodynamics (QCD), the theory of the strong interaction, is in accordance with most of the results from measurements. However QCD as well as derived effective theories or lattice-QCD calculations are unable to fully explain the rich hadron spectrum. The isoscalar part of the hadron spectrum is particularly interesting, because states with a large gluon content are expected in this sector. In order to validate or invalidate theories, the precise measurement of the states is crucial. The mass and width of states can be measured by energy scans. With the method of partial wave analysis one is able to extract additional information such as the spin and parity of the states. In this respect, the BESIII experiment at IHEP in Beijing and the future PANDA experiment at FAIR in Darmstadt, will contribute with valuable information.

A key parameter for such experiments is the luminosity, which is needed for energy scans and the measurement of absolute cross sections. In this thesis the implementation of the versatile LuminosityFit software package is presented. It is responsible for the determination of the luminosity from the reconstructed tracks of the PANDA luminosity detector with high stability and an accuracy of below 0.5 %. Essential for the small systematic uncertainty is the correction of influences from the accelerator beam and the target. In consequence a high accuracy for line shape measurements of resonances is ensured with the highly accurate luminosity determination.

Furthermore this thesis aims towards the extraction of the isoscalar mesons in the reaction J/ψ → π⁰π⁰γ. The presence of many wide and interfering states makes a correct theoretical description difficult. To address this matter the implementation of the helicity formalism for the general partial wave analysis framework ComPWA is presented in this thesis. The goal of ComPWA is to establish the comparability of various theoretical models and formulations. The study of the isoscalar mesons can benefit from the direct comparison of different theories. This implementation of the helicity formalism takes the novel approach by using an expert system. An exhaustive validation of the implementation is performed on the basis of the J/ψ → π⁰π⁰γ reaction. With the availability of the helicity formalism in ComPWA, partial wave analysis can be performed with this first general model, that serves as a reference for comparisons with other implementations.