Investigation of baryons structure in the HADES experiment

project funded by National Science Center grant Harmonia 9 2017/26/M/ST2/00600

One of the key issues of modern hadron physics is investigation of excited states of nucleons, so-called resonances, to get the insight in the strong interaction, which influence the mass of the visible matter in the Universe. Nucleons are described as a bound state of three quarks surrounded by a sea of gluons and quark-antiquark pairs. The knowledge of the properties of different nucleon resonances is crucial for understanding of the Quantum Chromodynamics (QCD), the quantum field theory of strong interactions. There are many evidence showing that nucleon and its resonances cannot be regarded as simple states of three static quarks. A big role is played by dynamics of gluon interactions leading to the light quark (u, d, s) mass generation and the spontaneous chiral symmetry breaking. The latter leads to appearance of quark condensates which can be interpreted as a so-called meson cloud surrounding quarks and counterbalancing the vacuum pressure. Such clouds are visible in the baryon electromagnetic structure, charge distribution inside the baryon, and are called form factors. In this project we would like to focus on the energy range, where significant effects of mesonic cloud play an important role and are expected to show-up.

The physics of baryon resonances is a major challenge to hadronic physics due to the nonperturbative nature of QCD at these energies. While methods such as chiral perturbation theory are not inclined to N physics, lattice QCD has only recently begun to contribute to this field. Thus, most of the theoretical work on the nucleon excitation spectrum has been performed within quark models. These models predict a much richer resonance spectrum than has been observed in piN → piN scattering experiments, included in Particle Data Book. The obvious question is: where are the “missing” resonances?

One of the explanation is that the “missing” resonances may couple weakly to piN channel, which is a predominant source of knowledge about baryon properties. There are many experiments which investigate the baryon structure with the use of the pion, electron or photon beams. One of them is the HADES experiment, located at GSI Helmholtz Institute in Darmstadt, exploring baryonic matter at zero (vacuum) and moderate temperatures. HADES performs measurements with pion-nucleon, nucleon-nucleon and heavy ion collisions. One of the research objectives of HADES are studies of baryon resonance production and their hadronic and electromagnetic decays. Especially, by measuring Dalitz decays of baryon resonances and determining the baryon internal structure and the role of a pion cloud. This establishes the reference information for studies of the virtual photon radiation from hot and dense baryonic matter, and in particular, the understanding of masses of hadrons decaying into dilepton channel. HADES was measuring nuclear reactions with kinetic beam energies in the range of 1–3.5 GeV/nucleon. Once the SIS18 synchrotron upgrade is completed, it will become possible to reach even higher resonances excitations.

The prominent results and excellent detector performance led to decision of using the HADES spectrometer also with the future FAIR accelerator facility, currently under construction in Darmstadt. HADES is currently the only experiment in world which measures rare penetrating probes like dilectrons at moderate temperatures and large baryonic potential by means of a pion, proton, and heavy ion beams in energy range sqrt(s) = 1–4 GeV.


Research project objectives/Research hypothesis: Baryonic resonances are excited states of the nucleon , which alike states of hydrogen in QED, offer a rich laboratory to study QCD, the theory of strong interactions. Studies of electromagnetic transition between the levels reveal underlying distributions of currents of the nucleon constituents. The transition can be probed by means of real and virtual photons, characterized by four momentum squared q2 as well in time-like (q2>0) as in space-like (q2<0) regions. One of important questions addressed in these studies are quark and pion cloud contribution to the resonance wave functions and their evolution probed at different q2 scales. The project focuses on region of small q2 | (<1 GeV2), where significant effects of mesonic cloud surrounding a quark core are expected to show-up. For the first time we want to address space-like and time like region by combined analysis of existing data from electro-scattering experiments (q2<0 GeV2 ) with novel data on resonance decays into virtual massive photons (q2>0) converting to dielectron (e+e-) pairs recently gathered by HADES experiment from pion-nucleon reactions using secondary pion beams at GSI in Darmstadt (Germany).


Methodology: Partial Wave Analysis (PWA) of single and two-pion final states will be combined with analysis of dielectron production in pion induced reactions around sqrt(s)=1.5 GeV to separate various resonance contribution and extract their electromagnetic Transition Form Factors as a function of q2 (e+e- invariant mass). The analysis will combined with a global fit of pion production in electro-scattering experiments (inverse process) and real photon production to access time like and space like region simultaneously. The crucial input to the analysis are precise HADES data on baryon resonance Dalitz decays N*→Ne+e- at sqrt(s) ~1.5 GeV (so called second resonance region). New measurements at higher energy will be prepared and conducted. HADES is an universal magnetic spectrometer specialized in detection of rare dielectron decays (respective branching ratios are of the order 10-5) in various reaction type which in combination with secondary pion beam at GSI is a word unique facility for such studies.


Expected impact of the research project on the development of science: Electromagnetic Dalitz decays of baryon resonances have not been measured before, except for the Delta(1232) isobar recently published by HADES. Extraction of respective transition form factors in a time like region, proposed in this project, will be a significant development in the field extending studies in the space like region performed by means of electro-scattering experiments. Results of this project on hadronic decays of resonance, particularly in 2-pion channels, will have an impact on baryon spectroscopy providing precise results on the respective branching ratios. Furthermore, the results on vector meson (rho)-baryon couplings will verify Vector Meson Dominance ansatz in baryon sector essential for modelling of dielectron radiation from the compressed and hot nuclear matter (“emissivity of nuclear matter”) probed in relativistic heavy ion collisions, particularly important the program of Compressed Baryonic Matter experiments at FAIR. Last but not least, success of this project will be a strong argument for continuation of pion beam programme recently launched at GSI in FAIR as part of Phase-0 programme.


International collaboration: HADES is the international collaboration founded in 1995, consisting from 21 European institutions and about 148 members ( located at Geselschaft fuer Schwerionenforschung (GSI) near to Darmstadt. HADES is also a first experiment ready for future experiments in a new international FAIR (Facility for Antiproton and Ion Research), a biggest hadron physics facility in Europe which is now under construction ( . HADES is one of two major experiments (the second one is new CBM detector) of Compressed Baryonic Matter pillar at FAIR devoted to studies of baryonic matter. A Polish group from M. Smoluchowski Institute of Physics from Jagiellonian University (IPUJ) is a HADES member since very beginning (1995) . IPJU has made major contribution to the experiments in terms of detectors (Pre-Shower, Electromagnetic Calorimeter, Forward tracking system), Data Acquisition developments and physics analysis. Principal investigator of this project was the HADES spokesman in 2003-2013, deputy spokesperson in 1996-2003 and now is HADES Collaboration Board member.