Search for In-Medium Modifications of Hadrons:
Experiments at HADES/SIS and ANKE & TOF/COSY and Accompanying Theory
1. Physics Motivation
Chiral symmetry (and the corresponding symmetry breaking pattern) is one governing principle of strong interaction. It is anchored in the fundamental theory of strong interaction - quantum chromodynamics. Under normal conditions (i.e. in our physical vacuum), chiral symmetry is spontaneously and explicitly broken. In compressed and heated nuclear matter, however, chiral symmetry becomes partially restored. This causes a change of the properties of hadrons.
The origin of the masses of fundamental constituents of matter represents a major concern of contemporary particle physics. Hadrons (protons and neutrons) carry more than 99 % of the mass of luminous matter in the universe; the same holds for our natural environment, where protons and neutrons are bound to a large extent in nuclei.
2. HADES experiment: basic ideas
To understand the phenomenon of "mass of hadrons" various experiments are presently performed. One intriguing idea is to study the decay products of particularly suited hadrons within the environment of strongly interacting matter. For instance, rho, omega and phi mesons represent such suitable hadrons. They can be produced by elementary projectiles impinging on nuclei or in the course of heavy-ion collisions. The created rho, omega and phi-mesons decay, amongst other particles, into electron-positron (e+ e-) pairs which leave the ambient strongly interacting medium nearly undisturbed, thus offering a direct access to the original mass of their parent hadrons. In such a way, in-medium hadron spectroscopy is enabled.
To some extent, one is attempting to search for a change of the hadronic excitation spectrum by an external field (represented by its surrounding nuclear matter). This is analog to atomic physics where external electromagnetic fields change the spectrum, as verified by the Stark and Zeeman effects.
3. HADES: detector design
The electromagnetic decay of a vector meson is a rare process. Therefore one needs special and dedicated detectors to dig the wanted e+e- pairs out of a huge background of other particles. Additionally, the momentum of the electron (e- ) and the accompanying positron (e+) must be determined with high precision. These requirements are satisfied by a ring imaging Cherenkov counter (RICH) and highly position sensitive drift chambers (MDCs). The scheme of the HADES detector and further details of RICH, MDC and the superconducting magnet are explained separately. HADES is an acronym of H igh A cceptance D i-Electron S pectrometer; it is installed in cave H at GSI/Darmstadt (see picture). The heavy-ion synchrotron SIS delivers beams of ions (E d 2 GeV), protons (E d 4 GeV), and a pion beam (Pp d 2.8 GeV/c) is available too. Beside standard foil targets, also liquid hydrogen/deuterium targets may be used.
4. HADES: physics programme
The primary goal of HADES it the systematic and precise hadron spectroscopy of rho, omega and phi mesons via the e+e- decay channel. The physics programme may be grouped in
(i) heavy-ion experiments (probing the in-medium behavior in compressed nuclear matter),
(ii) proton and pion beam experiments with nuclear targets (probing the in-medium behavior in near ground state nuclear matter), and
(iii) proton and pion beam experiments on proton/deuterium targets (probing elementary reaction channels).
Besides the excellent opportunity to address many questions related to the e+e- channel of hadronic interactions, HADES may serve as hadron spectrometer with large solid angle acceptance.
The experiments of the first round are explained here .
5. Our contribution to HADES equipment
In the detector laboratory of the Institute of Hadron- and Nuclear Physics, the large (active surface 2.2m²) multi-wire mini-drift chambers (MDCs) have been built up. Six of these MDCs are arranged as hexagons and installed behind the superconducting magnet of HADES. Each MDC consists of 7 cathode layers and 6 anode layers. The wires (»250 per layer, 80 µm aluminum for cathodes, 100 µm Tungsten for field wires, 20 µm for sense wires of anodes) are inclined by relative angles of 20 degrees from layer to layer. The smallness of the drift cells (12x8mm 2), the accuracy of positioning the wires ( ± 20µm) and the mechanical stability of the complete chambers represent a technological challenge.
The meaning of the MDCs (two in front and two behind the magnet) is to have a position sensitive tool for precise particle tracking. This afterwards allows the determination of the electron/positron momentum and subsequently the mass of the parent hadron.
Our MDCs are like formula-1 race cars: the performance is running at the limits and they need permanent care.
6. Experiments at ANKE & TOF: strange meson probes
Hadrons with strangeness, in particular anti-kaons, are considered as alternative sensitive probes of in-medium properties. Anti-kaons (e.g. K-) suffer, according to theoretical predictions, a substantial decrease of their effective mass in a dense nuclear medium. This has inspired a series of heavy-ion experiments in the late nineties (see below). It turned out, however, that in heavy-ion reactions quite a lot of poorly known elementary reaction channels are operative; therefore it is up to now a matter of debate whether the predicted in-medium modifications directly offer themselves in the exciting findings. From this, supported by detailed calculations, the idea has arisen to study K- production in proton nucleus collisions. Here much fewer channels are operative and the nucleon density does not change as abruptly as in heavy-ion-collisions.
The spectrometer ANKE at the internal beam of the cooler synchrotron COSY/FZ Jülich offers the unique opportunity to study anti-kaon production, in conjunction with kaons, in proton-nucleus collisions near and below the threshold. Previously, we have built in our detector laboratory the large start and stop wire chambers for ANKE and supplied further special equipment ar well.
The time-of-flight spectrometer TOF uses the extracted beam from COSY. Together with colleagues from TU Dresden we built a start detector system and equipped the large barrel. At TOF, we are interested in measuring kaons in conjunctron with hyperons in proton-proton reactions.
7. The origin of our experimental expertise
While focusing now on hadron physical investigations at HADES/SIS and supplementary at ANKE & TOF/COSY, in the past our activities were spread over investigations of strangeness degrees of freedom and electro-magnetic signals in colliding hadron systems in various collaborations.
The theory group in the hadron physics department is accompanying these experimental activities. Predictions and interpretations of selected topics in the field of strange and rare electromagnetic probes are provided. Most of these calculations refer directly to the experiments performed with HZDR participation. Our tools are QCD sum rules, effective hadronic interaction Lagrangians and transport models. Additionally we deal with various phenomenological aspects of deconfined matter, as a state with chiral symmetry restoration in the strongly interacting medium, produced in high-energy heavy-ion reactions. Whereas the investigations of dense and hot hadronic matter, produced in medium-energy collisions, are of strong interest for the understanding of astronomical objects like supernovae and neutron stars, the studies of deconfinement effects are of relevance for more violent scenarios like the big bang.