This paper presents the design of the LHCb trigger and its performance on data taken at the LHC in 2011. A principal goal of LHCb is to perform flavour physics measurements, and the trigger is ...designed to distinguish charm and beauty decays from the light quark background. Using a combination of lepton identification and measurements of the particles' transverse momenta the trigger selects particles originating from charm and beauty hadrons, which typically fly a finite distance before decaying. The trigger reduces the roughly 11\,MHz of bunch-bunch crossings that contain at least one inelastic \(pp\) interaction to 3\,kHz. This reduction takes place in two stages; the first stage is implemented in hardware and the second stage is a software application that runs on a large computer farm. A data-driven method is used to evaluate the performance of the trigger on several charm and beauty decay modes.
Phys.Rev.D83:054003,2011 Using 9.32, 5.88 million Upsilon(2S,3S) decays taken with the CLEO-III
detector, we obtain five product branching fractions for the exclusive
processes Upsilon(2S) => gamma ...chi_{b0,1,2}(1P) => gamma gamma Upsilon(1S) and
Upsilon(3S) => gamma chi_{b1,2}(1P) => gamma gamma Upsilon(1S). We observe the
transition chi_{b0}(1P) => gamma Upsilon(1S) for the first time. Using the
known branching fractions for BUpsilon(2S) => gamma chi_{bJ}(1P), we extract
values for Bchi_{bJ}(1P) => gamma Upsilon(1S) for J=0, 1, 2. In turn, these
values can be used to unfold the Upsilon(3S) product branching fractions to
obtain values for BUpsilon(3S) => gamma chi_{b1,2}(1P) for the first time
individually. Comparison of these with each other and with the branching
fraction BUpsilon(3S) => gamma chi_{b0} previously measured by CLEO provides
tests of relativistic corrections to electric dipole matrix elements.
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