UNICOR - experiment independent two-particle correlation analysis

Two-particle correlation functions provide experimental access to space-time extension of the particle source in nuclear collisions [1]. A typical correlation analysis involves i) looping over events, ii) looping over track pairs within the event, iii) calculating the momentum difference, iv) filling the pair histogram at the bin corresponding to this difference, v) repeating all the above but with a double loop over events and taking the two particles from different events (event mixing), and vi) dividing the true pair distribution by the mixed one to get the correlation function. The part related to reading events and applying experimental cuts is an order of magnitude less laborious (in terms of the number of lines of code) than pairing, event mixing, kinematics, and histogramming. A natural question arises whether the analysis software can be split into the experiment specific and the experiment independent pieces, such that the same analysis can be run on various data sets by changing only the interface part. I report here on a twoparticle analysis package that was developed along these lines. UNICOR is written in C++ and is strongly based on the ROOT data analysis framework [3]. It can be at present used to analyze data from CERES, ALICE, and CBM; interface to FOPI is to be added. The scheme is shown in Fig. 1. For each experiment an interface is implemented as a class inheriting from the event class with which the events were stored on one side, and from the DEvent class on the other, the latter being used by the subsequent analysis which calculates and histograms track pair variables. Some event and single track variables are being histogrammed as well. The only trace of the data source remains in the pseudorapidity range of the resulting histograms. Multidimensional histograms are the second special feature of UNICOR. In a typical correlation analysis threedimensional (three components of the momentum difference vector) correlation functions are produced for several pair-pt and rapidity bins. Azimuthal analyses [4] require,