A Measurement of the ATLAS Di-Muon Trigger Efficiency in Proton-Proton   Collision at $\sqrt{s}=7$ TeV
  The B physics programme of the ATLAS experiment includes measurements of production cross sections, searches for rare B-decay signatures which are sensitive to new physics at the TeV energy scale and studies of CP violation effects in B-events, such as $B_{s}^{0}\rightarrow J/\psi \phi$ and $B_{d}^{0}\rightarrow J/\psi K_{s}^{0}$. The key to the detection of these B signals in ATLAS is to achieve a high trigger efficiency for low-$p_{T}$ di-muon events, whilst keeping an acceptable trigger rate. ATLAS developed two separate approaches for triggering on di-muon events from resonances such as $J/\psi$ and Upsilon ($\Upsilon$). The first approach is to start from a di-muon trigger selected at Level-1 while the second is based on dedicated Level-2 algorithms. The performance for these triggers has been studied using collision data at $\sqrt{s}=7$ TeV collected in 2011. 
A.Picazio (on Behalf of the ATLAS Collaboration)
hep-ex