Year of publication
- 2009 (1) (remove)
- Di-electron spectroscopy in HADES and CBM : from p + p and n + p collisions at GSI to Au + Au collisions at FAIR (2009)
- The study of the electromagnetic structure of hadrons plays an important role in understanding the nature of matter. In particular the emission of lepton pairs out of the hot and dense collision zone in heavy-ion reactions is a promising probe to investigate in-medium properties of hadrons and in general the properties of matter under such extreme conditions. The first experimental observation of an enhanced di-electron yield in the invariant-mass region 0:3 - 0:7 GeV/c2 in p+Be collisions at 4:9 GeV/u beam energy  was announced by the DLS collaboration . Recent results of the HADES collaboration show a moderate enhancement above n Dalitz decay contributions for 12C+12C at 1 and 2 GeV/u [3, 4] confirming the DLS results. There are several theoretical explanations of this observation, most of them focusing on possible in-medium modifications of the properties of vector mesons. At low beam energies the question whether the observed excess is related to any in-medium effects remains open because of uncertainties in the description of elementary di-electron sources. In this work the di-electron production in p+p and d+p reactions at a kinetic beam energy of 1:25 GeV/u measured by the HADES spectrometer is discussed. At Ekin = 1:25 GeV/u, i.e. below the n meson production threshold in proton-proton reactions, the delta Dalitz decay is expected to be the most abundant source above the pi 0 Dalitz decay region. The observed large difference in di-electron production in p+p and d+p collisions suggests that di-electron production in the d+p system is dominated by the n+p interaction. In order to separate delta Dalitz decays and np bremsstrahlung the di-electron yield observed in p+p and n+p reactions, both measured at the same beam energy, has been compared. The main interest here is the investigation of iso-spin effects in baryonic resonance excitations and the off-shell production of vector mesons . We indeed observe a large difference in di-electron production in p+p and n+p reactions. Results of these studies will be compared to recent calculations. We will also present our experimentally defined cocktail for heavy-ion data. At much higher beam energies experimental results of the CERES  and NA60  collaborations also show an enhancement in the invariant mass region 0:3 - 0:7 GeV/c2, in principle similar to the situation in DLS. A strong excess of lepton pairs observed by recent high energy heavy-ion dilepton experiments hint to a strong influence of baryons, however no data exist at highly compressed baryonic matter, achievable in heavy-ion collisions from 8 - 45 GeV/u beam energy. These conditions would allow to study the expected restoration of chiral symmetry by measuring in-medium modifications of hadronic properties, an experimental program which is foreseen by the future CBM experiment at FAIR. The experimental challenge is to suppress the large physical background on the one hand and to provide a clean identification of electrons on the other hand. In this work, strategies to reduce the combinatorial background in electron pair measurements with the CBM detector are discussed. The main goal is to study the feasibility of effectively reducing combinatorial background with the currently foreseen experimental setup, which does not provide electron identification in front of the magnetic field.