The Compressed Baryonic Matter experiment at FAIR

Abstract

Abstract.The CBM experiment will investigate highly compressed baryonic matter created in A+A collisions at the new FAIR research center. With a beam energy range up to 11 AGeV for the heaviest nuclei at the SIS 100 accelerator, CBM will investigate the QCD phase diagram in the intermediate range, i.e. at moderate temperatures but high net-baryon densities. This intermediate range of the QCD phase diagram is of particular interest, because a first order phase transition ending in a critical point and possibly new highdensity phases of strongly interacting matter are expected. In this range of the QCD phase diagram only exploratory measurements have been performed so far. CBM, as a next generation, high-luminosity experiment, will substantially improve our knowledge of matter created in this region of the QCD phase diagram and characterize its properties by measuring rare probes such as multi-strange hyperons, dileptons or charm, but also with event-by-event fluctuations of conserved quantities, and collective flow of identified particles. The experimental preparations with special focus on hadronic observables and strangeness is presented in terms of detector development, feasibility studies and fast track reconstruction. Preparations are progressing well such that CBM will be ready with FAIR start. As quite some detectors are ready before, they will be used as upgrades or extensions of already running experiments allowing for a rich physics program prior to FAIR start. Mapping the high density region of the QCD phase diagram with CBMHeavy ion experiments at relativistic energies are the experimental tool in order to map the conjectured QCD phase diagram, see figure 1 (left), which is of fundamental interest for understanding the strong interaction and strongly interacting matter in particular in the non-perturbative regime. In dependence on the collision energy strongly interacting matter of varying temperature and baryon chemical potential is created. The exploration of matter at high temperatures but low baryon chemical potential in heavy-ion collisions at the highest energies at RHIC and LHC [1,3] reveals insight into the characteristics of the quark-gluon plasma and the phase transition to hadronic matter. Experimental results can be compared to lattice QCD calculations at µ B = 0 which predict a crossover from partonic to hadronic matter at temperatures around 160 MeV [4] in accordance to experimental findings. According to lattice QCD, the phase transition is driven by the energy density and occurs at a critical density on the order of 1 GeV/fm 3 . At large baryon densities the QCD phase diagram is uncharted territory as lattice calculations are not applicable, and as only few experimental measurements mainly of bulk