A measurement of the ATLAS di-muon trigger efficiency in proton-proton collisions at √s= 7 TeV.

The B physics programme of the ATLAS experiment includes the study of the production cross sections, searches for rare B decays signatures which are sensitive to new physics at the TeV energy scale and measurements of CP violation effects in B-events, such as B_S⁰→J /ψ φ and B_d⁰ →J /ψ K_S⁰. 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, keeping an acceptable trigger rate. ATLAS developed two approaches for triggering on di-muon events from a resonance such as J/ψ and Y. The first approach is to start from a di-muon signature at Level 1, whilst the second is based on a dedicated Level 2 algorithm. These trigger scheme has been studied using collision data at √s= 7 TeV. Results are shown and compared to Monte Carlo predictions.