Ju n 20 09 Future perspectives at SIS-100 with HADES-at-FAIR 1

Currently, the HADES spectrometer undergoes un upgrade pro gram to be prepared for measurements at the upcoming SIS-100 synchrotron at FAIR. We de scribe the current status of the HADES di-electron measurements at the SIS-18 and our future plans for SIS-100. 1 Current experimental status The experimental determination of hadron properties insid e trongly interacting media (normal nuclear or hot and dense matter) is one of the very interestin g challenges in hadronic physics (see [1] for a review). As the strong coupling αs becomes very large below the QCD scale ΛQCD ≈ 200 MeV, non-perturbative effects as confinement and the breaki ng of chiral symmetry rule the nature of strong interactions in the universe (besi de extreme cases like neutron stars) solely. Experimentally, the approach to get a deeper unders tanding of these features is to create nuclear matter under extreme conditions in the laboratory w hich can be done by employing heavy ion reactions, only. During the evolution of the fireball, created in the course of heavy ion collisions, the properties of nuclear matter change drastically (see [2] for a deta iled discussion): starting as a hot and dense intermediate state eventually with “free” ( i.e., only partly or weakly bound) quarks and gluons the system cools down until the exchange of resonance species stops (chemical freezeout) and finally the particles do not scatter any longer elast ic lly (thermal freeze-out). Our goal is, however, to gather information of the early phase, i.e. before the chemical freeze-out occurs. One of the main probes (if not the only one) which carry undist orted information over the entire history are di-leptons ( ee orμμ) as they are not hampered by final-state interactions. This included the first-chance radiation (bremsstrahlung) , radiation from short-lived resonances (the real “messengers” which have a life time shorter then th ose of the fireball) and post-freezeout sources, such as the late π and η Dalitz decays, which can be subtracted if their yields and distributions at the freeze-out point are precisely kno wn. Now, after a successful decade of measurements of electromagnetic probes (summarized in [ 3, 4]) there is common agreement that only by systematic studies in various systems over a wid e energy range, conclusions of the Future perspectives at SIS-100 with HADES-at-FAIR 3 following questions can be drawn: when does the onset of deco nfinement appear, and how it is related to chiral symmetry restoration? Recently, HADES [5] has added valuable measurements of di-e lectron mass spectra for light A + A [6] systems as well as for (quasi-free) p + N collisions [7] at kinetic beam energies of 1-2 AGeV and 3.5 GeV, which has triggered a lot of theoretic al activities [8]. One of the main features of HADES is that at the same time it measures als o h drons: pions for absolute normalization [9] and hadrons containing strangeness [10] . To summarize the setup at this point, HADES is a magnetic spec trometer, consisting of up to 4 planes of Mini Drift Chambers (MDC) with a toroidal field c reated by a superconducting magnet. Particle identification is based on momentum and tim e-of-flight measurements. In addition, a Ring Imaging Cherenkov detector (RICH) and an elec tromagnetic Pre-Shower detector provide electron identification capabilities. HADES will continue its program at its current place at SIS-1 8, and then move to the upcoming FAIR accelerator complex. Here, HADES will continue its experimental program up to kinetic beam energies per nucleon of 8 GeV at SIS-100. This is one of the main reason for upgrading the HADES detector and its trigger and readout sys tem, which will be outlined in the following. 2 The upgrade program for HADES-at-FAIR