Hadron physics with strangeness

Strangeness hadron physics tries to reveal the complex dynamics and phenomena of quarks and gluons, e.g., hadron properties in nuclear medium, symmetry breaking pattern and hadron mass generation, new forms of hadrons, which are described by the Quantum Chromo Dynamics (QCD). The "strong" QCD interaction is characterized by colour confinement and asymptotic freedom, namely the interaction becomes weak at short distances (at high energies) and can be treated perturbatively, on the contrary at long distances (at the temperature of the present universe) the interaction stays strong and colour charges are confined. The chiral symmetry and the heavy quark symmetry play a role in the QCD at the light quark sector (mu, md << ΛQCD) and at the heavy quark sector (mc, mb, mt >>ΛQCD), respectively. At the energy scale of the strange quark mass (≈ΛQCD), however, neither of the symmetries is good enough and the dynamics of the strange quark is thus very sensitive to the dynamics of QCD.

Exotic kaonic nuclear few-body bound states are predicted to exist by various calculations although the predictions of their masses and widths are rather scattered. Some theories predict that such kaonic nuclei might have significantly higher density than the normal nuclear density.

This could be understood with a picture that the quark composition of the antikaon (su) are different of the one of the nucleon (ud) and therefore a quark level Pauli excursion principle is suppressed.

E15 at J-PARC

The E15 experiment is located at J-PARC, Tokai, Japan aiming to search for the K-pp state with 3He(K-, n) reaction, complementary to the pp reaction measured with DISTO and FOPI, using a newly available high intensity, high quality kaon beam. The accelerators and the experimental areas came back in operation with a colossal effort from the substantial damage by the Great East Japan Earthquake on 11.3.2011.

The J-PARC E15 experiment is using the K1.8BR beam line of the J-PARC hadron experimental facility, with the K- beam, purified with an electrostatic separator. The K- are identified with an aerogel Cherenkov counter and confirmed off-line by a time-of-flight analysis. The beam momentum was analyzed with a beam-line spectrometer consisting of two sets of drift chambers installed across the dipole magnet. A typical K- yield was 1.5x105 per spill. To eliminate background caused by pile-up beam particles, it was requested that only one track exists in each beam-line chamber.

A cylindrical target cell 137 mm long and 68 mm in diameter was filled with liquid helium-3, with a target density of 0.081 g/cm3 at a temperature of 1.4 K.
The momentum of the forward neutron was measured by the time-of-flight method with a neutron counter (NC) placed at a distance of ~15 m from the target.
The analysis of the already taken data has been started.

The AMADEUS project

The strangeness sector of the non-perturbative regime of the low energy region of QCD is of main importance for the understanding of the hadron interactions and structure, and its in-medium modifications. The kaon-nucleon interaction, and specially the study of the antikaon-nucleon potential in nuclear matter is a current hot topic, and it is the main ingredient for understanding how a state of kaon condensate could appear in conditions of extreme density (being the most immediate example the core of the neutron stars) and its influence in the nuclear equation of state.

The AMADEUS project aims to extract new information from the most accurate available data (measured by the KLOE collaboration) from kaonic absorption by light nuclei. Low momentum and stopped kaons are absorbed in various components of the KLOE spectrometer, being of special interest are those produced in the drift chamber entrance wall, (mainly solid 12C) and those from the 4He gas filling of the drift chamber. Missing masses, momenta distributions, etc., are available for different channels (Σ0π0, Σ+π-, Λp, Λπ), in order to search for signals of exotic kaonic bound states and to study the formation of the Λ(1405) resonance.

Pushed by the obtained results, a high purity Carbon target (graphite) was installed inside the KLOE drift chamber, between the beam pipe and the DC entrance wall. The target was realised with the main aim to obtain an almost pure sample of absorbed stopped K-. These data were analysed and used as well to check possible effects of contamination of aluminium present in the top and bottom of the DC wall.

In the case of Λ(1405) the opportunity opened by the analysis of the carbon target data is even more obvious: now, a pure sample of stopped kaon events is available to study Λ(1405).  These data complement the 2.2 fb-1 collected by the first KLOE run, with in-flight absorptions of kaons at a momentum of around 120 MeV/c, which were also used for the study of Λ(1405).

DISTO and FOPI – the search for strange di-baryons Kpp

DISTO has investigated the formation of kaonic states in the 3-body final state pΛK+. A binary process pp→XK+ in which an exotic kaonic state is produced could in fact lead through this decay chain to the pΛK+ final state. DISTO has very large and pure statistics. The observation of an X resonance with a mass of 2265 MeV/c2, called X(2265) and the energy dependence study of its production have been performed using what-we-call the deviation method, that was very powerful, however, with a certain limitation. We are working for a reanalysis making use of a full acceptance matrix, which will allow to investigate further the nature of the X(2265) and to study the Σ decay channel as well.

FOPI as well has been investigating the kaonic states in the same reaction as DISTO, i.e. pp→pΛK+ but at the proton incident energy of 3.1 GeV. The FOPI data provides an independent check of the X(2265) found by DISTO, as well as an extended energy dependence study.