Experimental studies of the ground state QED : corrections in H- and He-like uranium

In summary, the cooled heavy-ion beams of the ESR storage ring offer excellent experimental conditions for a precise study of the effects of QED in the groundstate of high-Z one- and two-electron ions. This has been demonstrated within the series of experiments conducted at the electron cooler device as well as at the gasjet target. In this work we have used a recently developed experimental approach to obtain the first direct measurement of the two-electron contributions to the ground state binding energy of helium-like uranium. By employing our method, all one-electron contributions to the binding energy such as finite-nuclear size corrections and the one-electron self energy cancel out completely. Note, this is a distinctive feature of this particular kind of QED test and is in contrast to all other tests of bound state QED for high-Z ions such as 1s Lamb shift (in one-electron systems), g-factor of bound electrons, or hyperfine splitting. Compared to former investigations conducted at the superEBIT in Livermore we could already substantially improve the statistical accuracy and extend studies to the higher-Z regime. Moreover, our result has reached a sensitivity on specific two-electron QED contributions. Our value agrees with the theoretical predictions within the experimental uncertainty. Similar to the superEBIT experiment possible sources of systematic errors are essentially eliminated and the final result is limited only by counting statistics. For the case of the 1s Lamb shift in hydrogen-like uranium, the achieved accuracy of +- 4.2 eV is a substantial improvement by a factor of 3 compared to the most precise value up to now [44] (see Fig. 5.6). Our result already provides a test of the first-order QED contributions at the 1.5% level and only a slight improvement is required in order to achieve a sensitivity to QED contributions beyond first-order SE and VP.Für spektroskopische Untersuchungen an den schwersten Ein- und Zwei-Elektronensystemen bietet die Experimentier-Speicherringanlage für hochgeladene Ionen -ESR - bei GSI in Darmstadt ideale Voraussetzungen. Bei denen im Rahmen dieser Arbeit am ESR durchgeführten Experimenten lag das Hauptaugenmerk auf einer präzisen Vermessung des Beitrags der Elektron-Elektron-Wechselwirkung zum Ionisationspotenzial im He-ähnlichen Uran, d.h den schwersten Zwei-Elektronensystemen, die gegenwärtig Experimenten zur Verfügung stehen. Diese repräsentieren die einfachsten atomaren Vielteilchensysteme, in denen aber auf Grund der hohen Kernladung zugleich relativistische Effekte, Korrelationen und QED-Beiträge eine wichtige Rolle spielen. Auf Grund des gewählten Experimentieraufbaus konnten zudem die Grundzustandübergänge für H-artiges Uran vermessen werden, so dass ebenso präzise Informationen für die Grundzustandsenergie des Urans mit nur einem Elektronen gewonnen werden konnten. Für den Zwei-Elektronenbeitrag zum Ionisationspotenzial im He-ähnlichen Uran konnte aus dem Experiment ein Wert von 2248 eV gewonnen werden, wobei eine Genauigkeit von 9 eV erreicht wurde. Für den Grundzustand in He-ähnlichen Ionen stellt dieses Ergebnis die bislang genaueste Bestimmung des Zwei-Elektronenbeitrags dar, deren Sensitivität erstmals an die Größe der spezifischen Zwei-Elektronen-QED Beiträge heranreicht. Generell sei hier betont, dass es im Rahmen dieser Arbeit erstmals gelang, die vorliegenden Daten für das Ionisationspotenzial schwerer He-ähnlicher Ionen auf U90+ zu erweitern. Der Vergleich mit der theoretischen Vorhersage liefert eine gute Bestätigung für die Theorie der QED für Vielteilchensysteme im Bereich extrem starker Felder. Zudem erfolgte auch eine Bestimmung für die 1s-Lamb-Verschiebung für das H-ähnliche Uran. Hierzu diente vor allem eine Auswertung der sehr intensiven charakteristischen 2P3/2 -> 1s1/2 Ly-alpha1 Strahlung. Aus einem Vergleich der Messergebnisse mit den Vorhersagen der Dirac-Theorie für einen punktförmigen Urankern folgt für die 1s-Lamb-Verschiebung ein Wert von 460 eV mit einer Genauigkeit von 4.2 eV. Dieses sehr präzise Ergebnis, das frühere Ergebnisse um einen Faktor Drei an Genauigkeit übertrifft, befindet sich in sehr guter Übereinstimmung mit neuesten theoretischen Modellrechnungen. Insbesondere kann durch das vorliegende Ergebnis die 1s-Lamb-Verschiebung in wasserstoffähnlichem Uran auf dem Niveau von 1.5% als gesichert angesehen werden. Hierdurch erfährt die Theorie der Quantenelektrodynamik in sehr starken Coulomb-Feldern eine hervorragende Bestätigung

Searching a Dark Photon with HADES

The existence of a photon-like massive particle, the γ‘ or dark photon, is postulated in several extensions of the Standard Model to explain some recent puzzling astrophysical observations, as well as to solve the sofar unexplained deviation between the measured and calculated values of the muon anomaly. The dark photon, unlike the conventional photon, would have mass and would be detectable via its mixing with the latter. We present a search for the e+e− decay of such a hypothetical particle, also named U vector boson, in inclusive dielectron spectra measured by HADES in the p (3.5 GeV) +p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared (ϵ2) at 90% CL has been obtained for the mass range M(U)= 0.02 - 0.55 GeV/c2 and is compared with the present world data set. Furthermore, an improved upper limit of 2.3 × 10−6 at 90% CL has been set on the branching ratio of the helicity-suppressed direct decay of the η meson η → e+e−

Constraints on the kinetic mixing parameter ϵ2\epsilon^2 for the light dark photons from dilepton production in heavy-ion collisions in the few-GeV energy range

The vector $U$-bosons, or so called 'dark photons', are one of the possible candidates for the dark matter mediators. They are supposed to interact with the standard matter via a 'vector portal' due to the $U(1)-U(1)^\prime$ symmetry group mixing which might make them visible in particle and heavy-ion experiments. While there is no confirmed observation of dark photons, the detailed analysis of different experimental data allows to estimate the upper limit for the kinetic mixing parameter $\epsilon^2$ depending on the mass $M_U$ of $U$-bosons which is also unknown. In this study we present theoretical constraints on the upper limit of $\epsilon^2(M_U)$ in the mass range $M_U \le 0.6$ GeV from the comparison of the calculated dilepton spectra with the experimental data from the HADES Collaboration at SIS18 energies where the dark photons are not observed. Our analysis is based on the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which reproduces well the measured dilepton spectra in $p+p$, $p+A$ and $A+A$ collisions. Additionally to the different dilepton channels originating from interactions and decays of ordinary matter particles (mesons and baryons), we incorporate the decay of hypothetical $U$-bosons to dileptons, $U\to e^+e^-$, where the $U$-bosons themselves are produced by the Dalitz decay of pions $\pi^0\to \gamma U$, $\eta$-mesons $\eta \to \gamma U$ and Delta resonances $\Delta \to N U$. Our analysis can help to estimate the requested accuracy for future experimental searches of 'light' dark photons by dilepton experiments.Comment: 11 pages, 5 figures, extended version, to be published in Phys. Rev.

Relativistic calculations of the x-ray emission following the Xe-Bi83+^{83+} collision

We study the x-ray emission following the collision of a Bi$^{83+}$ ion with a neutral Xe atom at the projectile energy 70 MeV/u. The collisional and post-collisional processes are treated separately. The probabilities of various many-electron processes at the collision are calculated within a relativistic independent electron model using the coupled-channel approach with atomic-like Dirac-Fock-Sturm orbitals. The analysis of the post-collisional processes resulting in the x-ray emission is based on the fluorescence yields, the radiation and Auger decay rates, and allows to derive intensities of the x-ray emission and compare them with experimental data. A reasonable agreement between the theoretical results and the recent experimental data is observed. The role of the relativistic effects is investigated.Comment: 11 figures, 2 table