Many Facets of Strangeness Nuclear Physics with Stored Antiprotons

Stored antiprotons beams in the GeV range represent a unparalleled factory for hyperon-antihyperon pairs. Their outstanding large production probability in antiproton collisions will open the floodgates for a series of new studies of strange hadronic systems with unprecedented precision. The behavior of hyperons and -- for the first time -- of antihyperons in nuclear systems can be studied under well controlled conditions. The exclusive production of $\Lambda\bar{\Lambda}$ and $\Sigma^-\bar{\Lambda}$ pairs in antiproton-nucleus interactions probe the neutron and proton distribution in the nuclear periphery and will help to sample the neutron skin. For the first time, high resolution $\gamma$-spectroscopy of doubly strange nuclei will be performed, thus complementing measurements of ground state decays of double hypernuclei with mesons beams at J-PARC or possible decays of particle unstable hypernuclei in heavy ion reactions. High resolution spectroscopy of multistrange $\Xi$-atoms are feasible and even the production of $\Omega^-$-atoms will be within reach. The latter might open the door to the $|s|$=3 world in strangeness nuclear physics, by the study of the hadronic $\Omega^-$-nucleus interaction and the very first measurement of a spectroscopic quadrupole moment of a baryon which will be a benchmark test for our understanding of hadron structure.Comment: Proceddings of HYP201

Feasibility studies for the high precision X-ray spectroscopy of heavy Ξ− hyperatoms at PANDA using the PANda GErmanium Array PANGEA

Mit PANDA entsteht ein Hadronenphysikexperiment an der neuen Beschleunigeranlage FAIR in Darmstadt, Deutschland. Dabei ermöglicht der gespeicherte Antiprotonenstrahl des HESR Speicherrings die Produktion hoher Raten von Hyperon-Antihyperon-Paaren mit ein- und mehrfacher Strangeness. Dies erlaubt mehrere Experimente im Strangeness-Sektor, wobei besonders die Wechselwirkung von (Anti-)Hyperonen und Kernmaterie untersucht werden kann, die ein wichtiger Baustein ist, um das “Hyperon-Puzzle” im Zentrum von Neutronensternen zu lösen. Dabei wird die hohe Modularität des PANDA-Spektrometers benutzt, welche es erlaubt einzelne Detektorkomponenten für spezifische Experimente auszutauschen. Besonders das Hyperatom-sowie das Hyperkernexperiment machen davon Gebrauch. Diese Experimente benötigen zwin-gend einen hochauflösenden γ-Detektor: PANGEA. Das PANda GErmanium Array besteht aus insgesamt 60 hochreinen Germaniumkristallen in 20 Kryostaten. Die Optimierung der Anordnung dieser Detektoren sowie deren Integration innerhalb von PANDA werden in dieser Arbeit diskutiert. Zusätzlich wird der Einfluss des hadronischen Untergrunds während der Experimente untersucht. Insbesondere schnelle Neutronen führen zu Strahlenschäden innerhalb der hochreinen Germaniumkristalle, was die hohe intrinsische Energieauflösung verringert. Dieser Einfluss wurde experimentell in zwei Bestrahlungstests am COSY Protonenbeschleuniger in Jülich, Deutschland, mit bis zu 5,6*10^9 Neutronen/cm^2 untersucht. Durch die digitale Analyse der Pulsformen konnte ein großer Anteil der Auflösungsverluste bei der Energiemessung korrigiert werden. Das thermische Ausheilen der Kristalle nach der Bestrahlung im Labor konnte die ursprüngliche Energieauflösung wiederherstellen. Trotz alledem müssen die Einflüsse der Bestrahlung bei der Studie zur Machbarkeit des Hyperatomexperiments berücksichtigt werden. PANDA wird das erste Experiment weltweit sein, das die Wechselwirkung von Ξ−-Hyperonen und vornehmlich Neutronen in der neutronendominierten Peripherie von Ξ^−-Pb-208 Hyperatomen untersuchen kann. Simulationen zeigen, dass mehrere experimentelle Observablen eine Bestimmung sowohl des Real- als auch des Imaginärteils des optischen Potentials von Ξ^−-Hyperonen mit einer Genauigkeit von ±1 MeV ermöglichen.PANDA is a new experiment in hadron physics at the upcoming FAIR facility in Darmstadt, Germany. The combination of PANDA and its antiproton beam, provided by the antiproton storage ring HESR, yields high production rates of strange hyperon-antihyperon pairs. This enables multiple experiments in strangeness nuclear physics which allow to study the interaction of hyperons and antihyperons within nuclear matter. This is essential to understand the composition of neutron star matter and solve the “hyperon puzzle”. The modularity of PANDA allows to design and integrate a dedicated setup for the high resolution X-ray and γ spectroscopy of heavy Ξ^− hyperatoms and double Λ hypernuclei. The germanium detector array PANGEA (PANda GErmanium Array) is mandatory for these experiments. Its optimization and integration into the PANDA target spectrometer is discussed in this thesis. During the experiments at PANDA, the HPGe (High Purity Germanium) crystals of PANGEA will suffer from inevitable hadronic background. Especially fast neutrons will damage the lattice structure of the crystal and deteriorate its resolution. This effect has been experimentally studied in irradiation tests at the COSY accelerator in Jülich, Germany, with up to 5.6*10^9 neutrons/cm^2. A large fraction of the performance loss of the detector could be corrected by analyzing the pulse shape of the detector response. After the irradiation, the initial crystal performance could be restored by annealing of the crystal in the laboratory. The effects of the irradiation had to be kept in mind when the feasibility of the hyperatom experiment was studied. PANDA is unique in its ability to study the Ξ^− nucleon interaction in the neutron-rich periphery of Ξ^−-Pb-208 hyperatoms. As shown in simulations, multiple experimental observables will allow to measure the real and imaginary part of the Ξ^− optical potential with a precision of ±1 MeV

Has the neutral double hypernucleus ∧∧4n{}_{\wedge}^{}{}_{\wedge}^4\rm{n} been observed?

The BNL-AGS E906 experiment was the first fully electronic experiment to produce and study double hypernuclei with large statistics. Unfortunately, the interpretation of the measured $\pi^{−}−\pi^{−}$ momentum correlation is still blurry because the hypothesized production of ${}_{\Lambda }^{3}\rm{H}{+}_{\Lambda }^{4}\rm{H}$ pairs remains questionable. We show, that neither a scenario where the hypernuclei are produced after the capture of a stopped $\Xi^-$ by a $^9$Be nucleus nor interactions of energetic $\Xi^-$ with $^9$Be nuclei in the target material can produce a sufficient amount of such pairs. We have therefore explored the conjecture that decays of the $_{\Lambda}^{~}{}_{\Lambda}^4\rm{n}$ may be responsible for the observed structure. Indeed, the inclusion of $_{\Lambda}^{~}{}_{\Lambda}^4\rm{n}$ with a two-body $\pi^-$ branching ratio of 50% in the statistical multifragmentation model allows to describe the E906 data remarkably well

Has the neutral double hypernucleus nΛΛ4 been observed?

The BNL-AGS E906 experiment was the first fully electronic experiment to produce and study double hypernuclei with large statistics. Two dominant structures were observed in the correlated π−–π− momentum matrix at (pπ−H,pπ−L)=(133,114)MeV/c and at (114,104)MeV/c. In this work we argue that the interpretation of the structure at (133,114)MeV/c in terms of Λ3H+Λ4H pairs is questionable. We show, that neither a scenario where these single-Λ hypernuclei are produced after capture of a stopped Ξ− by a 9Be nucleus nor interactions of energetic Ξ− with 9Be nuclei in the target material can produce a sufficient amount of such twin pairs. We have therefore explored the conjecture of Avraham Gal that decays of the Image 1 may be responsible for the observed structure. Indeed, the inclusion of Image 1 with a two-body π− branching ratio of 50% in a statistical multifragmentation model allows to describe the E906 data remarkably well. On the other hand, a bound Λ3n nucleus would cause a striking structure in the momentum correlation matrix which is clearly inconsistent with the observation of E906. Keywords: Hypernuclei, Statistical decay mode

Exploring the neutron skin by hyperon–antihyperon production in antiproton–nucleus interactions

In this work we propose a new method to measure the evolution of the neutron skin thickness between different isotopes. We consider antiproton–nucleus interactions close to the production threshold of ΛΛ̅ and Σ− Λ̅ pairs. At low energies, ΛΛ̅ pairs are produced in p̅ + p collisions, while Σ−Λ̅ pairs can only be produced in p̅ + n interactions. Measuring these cross sections provides information on the neutron skin thickness