The LHC will be the first top quark factory. tt pairs will be produced at such a high rate that statistical error on the top quark mass measurement will soon become negligible. A precise top quark mass measurement is of fundamental importance because it is one of the fundamental parameter of the Standard Model with a value close to the electroweak symmetry breaking scale. Moreover, during the LHC start-up, it will be a high mass reference candle. In the ATLAS experiment, the most promising

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... sis will use tt pairs that decay semileptonically. The different steps of this analysis will be detailed and a quick overview of the other ongoing analyses will be given.