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Go4 developed at GSI is an analysis framework with a general purpose nonblocking Graphical User Interface (GUI). Go4 is based on ROOT. The GUI is implemented in Qt using GSIs QtROOT interface. Analysis and GUI run in separate tasks communicating through asynchronous object channels. Therefore, the framework is especially useful for on-line monitoring where an analysis should run continuously, but controlled at any time from the GUI. The analysis also should be able to update graphical objects in the display. The HADES experiment at GSI integrated an existing ROOT based monitoring analysis into Go4. A Go4 analysis may use any ROOT features. It can be organized in steps, which can be controlled from the GUI according to the user specifications. Each step is composed of event objects, the event processing, and event IO. Go4 composite event classes allow the construction of arbitrary complex events hierarchically composed of objects. The IO of the composite event objects to and from ROOT trees/branches is provided without explicit programming. Arbitrary hierarchy levels of composite events can be browsed by the Go4 tree viewer. The GUI provides hooks to attach user written GUI's. These GUI's have access to all objects of the analysis, i.e., events for asynchronous event display. Using the Qt designer the development of such GUI's is very efficient. The HADES experiment implemented a dedicated GUI for the on-line monitoring. The Go4 fit package (API and GUI) is a powerful and extendable tool to model and fit experimental data.

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