PDG entries - HADES aims at precision
The HADES experiment has been designed to study di-electron emission in heavy ion reactions. However, its powerful particle identification capabilities and excellent momentum resolution also allows precise measurements of hadron properties in elementary collisions. Below we present two recent results, which are cited in the Particle Data Group (PDG 2012).
Rare η→e+e- decays
Experimental searches for the rare decays of the η meson are essential for testing the validity of the Standard Model (SM). Among other decay channels the direct decay of the η meson intoan electron-positron pair is of particular interest. In the SM the respective decay is mediated by two virtual photons with an internal electron line which is suppressed by helicity conservation.
Fig. 1 Expected signal corresponding to the η → e+e- decay with a branching ratio of 4.9 × 10-6 superimposed on e+e- invariant mass distribution measured by HADES in proton-proton collisions at 3.5 GeV.
The branching ratio (BR) for such a process can be estimated from the known η→γγ decay and leads to a lower limit of BR(η → e+e-) ≥ (1.77 ± 0.009) × 10-9. A larger value, (2.4 ± 0.33) × 10-9, can be derived from the observed η → μ+μ- channel assuming that the ratio between the real and the imaginary parts of the decay amplitude is the same for both decays. On the other hand, the experimental upper limit for the η → e+e- branching ratio is 2.7×10-5 , i.e.four orders of magnitude larger then theory suggests. Thus, experiment at the moment cannot exclude contributions to the decay amplitude from processes beyond the SM. For example, it has been suggested that direct decays of pseudo-scalar meson into electron-positron pairs can be enhanced by a coupling to a new undiscovered vector meson responsible for the annihilation of a neutral light dark matter particle . Such searches become of large interest because of the enhanced π0 → e+e- transition rate reported by the KTeV collaboration . Recently, the HADES collaboration reported dielectron measurements in proton proton interactions at 3.5 GeV kinetic beam energy . For the first time at this beam energy the inclusive production cross sections of π0, η, ρ and ω mesons have been measured. From the smooth spectral distribution of electron positron pairs in the η mass region an improved upper bound for the η → e+e- decay could be deduced lowering the old value by a factor of about 6 . The new value of BR = 5.6 ×10-6 will be quoted in the 2012 edition of the PDG as the best estimated upper limit for the η → e+e– decay.
Λ and Σ Hyperons in p+p Collisions
The measurement of Λ, Σ hyperons below the K-N threshold is of particular interest for understanding Kaon-Nucleon interaction in different environments.For example, widely distputed questions like of existence of kaonic clusters (bound states of K- and nucleons) or the lowering of the K- mass in dense nuclear matter are strongly related to the understanding of the holy grail of Kaon-Nucleon interaction: the Λ(1405). However, about the production of the lambda hyperon in N-N collisions very little is known  . Indeed, to separate it from the overlapping Σ(1385), which has a comparable width of 40-50 MeV and decays into similar channels, is very difficult. HADES has recently measured the production of both hyperons in proton-proton collisions at a kinetic beam energy of 3.5 GeV [6,7]
Thanks to the excellent momentum resolution of HADES and capability for reconstruction of weak decays through their secondary decay vertices, both resonances could be identified and separated from other, much more abundant reaction channels. In particular the Σ(1385)+ decay into the Λ(1115) and π+ was analyzed and allowed to extract the Σ+ spectral function with high precision, including the evaluation of the systematic errors, the latter not being available for older measurements reported in the PDG so far.
 WASA collaboration, M. Berłowski et al., Phys. Rev. D77(2008) 032004
 Y. Kahn, M. Schmitt, and T. M. P. Tait, Phys. Rev. D78 (2008) 115002, arXiv:0712.0007 [hep-ph].
 KTeV Collaboration, E. Abouzaid et al. Phys. Rev. D75 (2007) 012004
 L.S. Geng E. Oset, EPJA34 (2007) 405
 HADES Collaboration, Eur. Phys. J. A48, 64 (2012).
 HADES Collaboration, Phys. Rev. C 85, 035203 (2012)
 L. Fabbietti, E. Epple et. al (HADES Coll). arXiv:1202.023