Hadron properties in nuclear matter with HADES and AMADEUS

Hadron properties within nuclear matter are predicted be modified. This is linked to the interaction of hadrons with nucleons at different densities and temperatures. For decades people have attempted to link possible changes of the spectral function of hadrons within nuclear matter to the restoration of chiral symmetry. Experimentally we can measure the mass distributions and velocities of hadrons produced colliding heavy or light nuclei at accelerators at energy around and below 1 GeV, such to either investigate matter at normal (ρ0) or supra-normal (2-3 ρ0) nuclear densities.
Our group is particularly interested in studying these modifications in terms of interactions between hadrons. Indeed, these interactions will characterize the Equation of State of the matter we are looking at and hence be relevant for the understanding of dense matter and maybe up to supra-dense objects like neutron stars.

The second pillar of our research evolves around the study of K,  K̅ and Λ hyperons within normal nuclear matter. There, we look for signatures of the mean value of the strange hadron interaction with many nucleons to get closer to the high density environment that characterize neutron stars.

For example, K̅ are known to experience an attractive interaction with nucleons. The K̅N interaction is so attractive to form the molecular state Λ(1405), as studied within the group in past years and eventually even form more involved bound states as K̅pp. We have investigated the existence of the latter in p+p collisions at 3.5 GeV measured by the HADES spectrometer and applied a sophisticated partial wave analysis to account for interferences among intermediate state So far we could only estimate upper limits.

 In general, the quest of in-medium properties of hadron is a complicated puzzle where many effects must be taken into account. Together with theoretical calculations, we try to compose a complete model that takes into account all possible processes that hadrons can undergo within normal nuclear matter: elastic scattering, inelastic scattering, formation of resonances, multi-step processes. Once all known processes are accounted for we see if something ‘else’ is left to be seen. In case collective effects as strong attraction or repulsion should be present, we hope to be able to see them in the kinematic variables of the observed hadrons. One can look at this kind of project the same way Michelangelo was trying to free his statues from the marble, cutting out all unnecessary and trivial marble around the core he was interested to disclose.

One of the best way to study the properties of nuclear matter is to exploit pion beams at energies of few GeV. Pions get absorbed close to the surface of the nucleus very easily and all most of the produced K,  K̅, Φ and Λ travel though the nucleus and undergo different reactions we want to investigate. Another advantage of pion-induced reactions is that one is dominated by primary reactions.
The absorption of mesons in nuclear matter is one of our main current interests. Here, we are especially addressing the K- absorption, since the microscopic optical potential used to describe the kaonic atoms still lacks good knowledge of its imaginary part which is related to the absorption. In general, the K- absorption is driven by strangeness exchange processes on one (K-N → Yπ) or more nucleons (K-NN → YN) with a hyperon Y (e.g.) in the final state. This process can be mediated by an intermediate resonance.

Within the AMADEUS collaboration  the capture of K- mesons on light nuclear targets (4He, 12C) is studied.
In detail the two, three and more nucleonic absorptions with the Σ0p final state have been measured, and the most precise search for the K-pp bound state in absorption experiments has been realized . The absorption of the K- by single nucleons have been studied as well by measuring the non-resonant production in the K-n→Λπ channel.

The goal of the pion-induced strangeness program of HADES is the comparison of the K+ and K- yield produced with different nuclei addressing the total absorption rate of the K-. Here, the K+ provide constraints on the strange hadron production, since due to strangeness conservation the K+, once produced, cannot be absorbed by surrounding nucleons.

Besides K-capture, we also interested in the Φ(s̅s) absorption. The Φ is linked to the K-via its dominant decay into K+K- pairs (BR ~50%) as can be seen in the side picture. According to the OZI rule quark exchange is suppressed in its interaction with ordinary (non-strange) baryonic matter. Consequently, the ΦN cross-section is assumed to be small. However, several experiments employing photon and proton beams impinging on light to heavy nuclear targets yield to a rather large in-medium ΦN absorption cross-section. So far, Φ absorption in different nuclei has not been studied in pion-induced reactions and hence, should provide complementary information to results obtained with photon- and proton-nucleus reactions.