Experimental Hadron Structure (IKP-1)
Hadrons and Fundamental Symmetries
Our research activities focus on the nature of the strong interaction which manifests itself in the properties of hadrons and their mutual interactions. The study of these systems enables tests of fundamental symmetries, and thereby advances our basic understanding of particle physics. To pursue this objective, experiments with polarized proton and deuteron beams with momenta from 0.3 to about 3.5 GeV/c are being carried out at the COSY facility in Jülich. In addition, a comprehensive experimental program using phase space cooled antiproton beams with momenta from 1.5 to 15 GeV/c at the high energy storage ring HESR at the FAIR facility is being prepared.
The strange quark (s quark) is the lightest quark not contributing to the valence quantum numbers of stable matter. The study of strangeness as an additional degree of freedom is thus important to achieve a more complete understanding of the strong interaction. This concerns in particular the excitation spectrum of the nucleon, whose states have very different coupling strength to final states with strangeness, as well as the hyperon-nucleon interaction in comparison to the nucleon-nucleon interaction.
At COSY energies the production of strange hadrons is suppressed by about 2−3 orders of magnitude compared to hadrons consisting of light (u and d) quarks. Therefore, very selective detector systems are required to unambiguously identify final states with strange quark content. The delayed decay by a few cm of Λ und Σ+ hyperons as well as Ks mesons can be exploited as a characteristic signature of strangeness production. The COSY-TOF detector is an external experiment at COSY, which allows precise measurement of the particle trajectories close to the interaction point. Therefore, COSY-TOF is ideally suited to identify such delayed decays. This capability has recently been significantly enhanced by the implementation of the Straw Tube Tracker. A further strength of the detector is its large acceptance, in many cases corresponding to full 4π solid angle in the center of mass reference frame, as well as its azimuthal symmetry. Both of these properties are essential to measure complete angular distributions of the produced particles. Together with the high quality polarized proton beam this allows e.g. the nucleon resonance contributions to the pp→pK+Λ reaction to be determined, and the spin resolved pΛ scattering length to be measured (further information is found here).
A main approach in our understanding of physics principles is based on insight into the symmetries that are fulfilled or violated by the fundamental forces. For example, it has been known for more than 50 years that processes mediated by the weak interaction are not invariant with respect to parity transformation; in contrast both the strong and the electromagnetic interactions respect this symmetry. In addition, a tiny violation of the symmetry with respect to the combined charge conjugation (particle - antiparticle transformation) and parity transformation, i.e. CP transformation, has been observed in the decay of neutral K and B mesons. The origin of this violation is still not understood and thus an observation of CP violation in other systems or at least a significant shift of the experimental limit is essential to explain the absence of antimatter in our everyday world. The so-called isospin symmetry is another important issue: the strong interaction seems to be invariant with respect to the z component of the isospin (interchange of proton and neutron, or u and d quark) except for a small violation due to different masses of the light quarks. Determining the size of isospin breaking would thus constrain the u and d quark masses which have large uncertainties.
The study of rare decays of η und η' mesons constitutes a very promising way to the test of fundamental symmetries since these mesons have very small decay widths and thus higher relative probability for rare decays. All strong decays of the η meson are symmetry breaking and thus forbidden or suppressed. The investigation of rare η (and η') decays is the major topic within the physics program of the WASA-at-COSY experiment, which is capable of detecting photons as well as charged particles with almost 4π acceptance. Meanwhile a large number of η meson decays has been detected in pp→ppη and pd→³Heη reactions.
The charm quark (c quark) is the fundamental particle next higher in mass than the strange quark (s quark) taking part in the strong interaction. However, in contrast to u, d, and s quarks, the c quark mass of about 1.5 GeV/c² is so large that dynamical effects are comparably small. As a consequence the spectroscopic study of hadrons with charm delivers complementary information on the strong interaction. Both charm-anticharm (i.e. charmonium) states and open charm states consisting of a charm and a light quark (D and Ds mesons) are important in this respect. Furthermore, due to small width of Charmonium states, the search for states with gluonic excitations, i.e. quark gluon hybrid or purely gluonic states, in the mass region around 4 GeV/c² seems to be particularly promising. Exotic states beyond the quark model are allowed by quantum chromo dynamics but have not been observed so far. The study of hadrons with charm as well as the search for exotic states that cannot be reduced to quark-antiquark or three-quark states is a key topic within the physics program of the PANDA experiment at the antiproton storage ring HESR at FAIR.
The development of detectors and their read-out systems is a key issue for design and construction of PANDA. Within this task, detector development focuses on the micro vertex detector (MVD), on the straw tube tracker (STT) as central tracking detector, and on the luminosity monitor. The MVD consists of silicon pixel detectors in the inner part and of silicon strip detectors in the outer part. It is the PANDA sub-detector next to the interaction point, and thus in particular has the task to determine delayed decay vertices of hadrons with open charm. The goal is to achieve a resolution of about 50 μm. The STT consists of a closely packed cylindrical arrangement of about 4400 straw tubes, each of which have 1 cm diameter and 150 cm length. A planar straw tube detector based on the same light-weight technology is being successfully used in the COSY-TOF experiment. The luminosity monitor, a stack of four layers of silicon strip detectors, is located about 10 m downstream of the interaction point. Its task is to determine the luminosity during the experiments with an absolute precision of 3% based on the known elastic antiproton-proton cross section by measuring the rate of forward scattered antiprotons in the Coulomb-Nuclear interference region.
However, with the uncertainty of the existing antiproton-proton elastic scattering data the desired precision of the luminosity determination cannot be achieved. Therefore an independent experiment in the HESR ("day-1 experiment") is planned in order to determine the three relevant parameters σ (total cross section), ρ (ratio of imaginary to real part) and b (exponential slope of dσ/dt as function of the squared 4-momentum transfer t) for different beam momenta between 1.5 and 15 GeV/c. In the "day-1 experiment" the covered t range between 0.0008 and 0.1 GeV/c² considerably exceeds that of the PANDA luminosity detector. The polar angle of the scattered antiprotons and both polar angle and energy of the recoil protons will be measured. A preparatory experiment commissioning the setup will take place in the COSY ring using proton-proton elastic scattering.
Facilities and Opportunities for Cooperation
Detector Systems for Experiments with Hadrons
Two-dimensional fast scintillator detectors; three-dimensional wire chamber systems with very high resolution and extremely low mass in vacuum; silicon and germanium µ-strip detectors; magnetic spectrometers; cluster target systems in UHV; decay spectrometer and polarimeters for "strange" particles; liquid hydrogen target and Cerenkov counters.
Detector Simulation and Particle Transport
Monte Carlo simulation of particle transport and particle interaction for complex detector structures.