Online b-tagging selection for the ATLAS experiment at the LHC

The second level trigger system (LVL2) of the ATLAS experiment at the CERN LHC is required to reduce the trigger rate from 100 kHz to about 1 kHz. The trigger decision has to be taken on average in 10 ms. The ability of identifying b-quark jets at trigger level opens many interesting physics opportunities, especially in the top quark and Higgs sectors. Additionally the acceptance for several supersymmetric channels is increased. Here we present two methods of tagging b-quark jets at trigger level. One is based on impact parameters, the other one on secondary vertex reconstruction. The algorithms and their performances are described and compared.