Data compression in ALICE by on-line track reconstruction and space point analysis

Abstract

High resolution detectors in high energy nuclear physics deliver a huge amount of data which is often a challenge for the data acquisition and mass storage. Lossless compression techniques on the level of the raw data can provide compression ratios up to a factor of two. Higher compression ratios can be reached by introducing an appropriate model for the raw data and storing relevant information for the event reconstruction with respect to that model. In ALICE, a data compression technique has been developed for the Time Projection Chamber (TPC) to reach an overall compression factor suited for data taking in Heavy Ion collisions.The ALICE High Level Trigger provides online calculation of the TPC clusters from the raw data, followed by tracking, thus producing a fully reconstructed event. Storing the reconstructed cluster data in an appropriate compressed format for utilization in the off-line reconstruction allows to discard the original raw data of the TPC. In the presented solution, compression factors of four to six are achieved without significantly affecting the physics performance. By associating space points to reconstructed tracks, all relevant parameters can be further transformed into a format suitable for Huffman compression. In a first conservative approach, all reconstructed clusters are kept in the data.Data compression has been implemented for the ALICE TPC in 2011 for usage in the Heavy Ion data-taking. In this contribution the results on 2011 real data are presented for the first time.