This four-lecture series will cover the concept of online event selection ("triggering") at LHC.  One of the prime challenges presented to the LHC experiments consists in selecting the very few interesting events, those which may hint to new physics and those which permit the measurement of fundamental parameters amidst the much more copious occurrence of well-understood and studied processes. Out of a billion interactions per second, no more than a hundred can be stored for further analysis. After discussing the design and implementation of online event selection strategies of the LHC experiments, we will describe reconstruction algorithms based on the concept of regional/partial reconstruction and there applications to event selection based on physics objects (jets, muons, electrons, etc.). Special emphasis will be given to tracking algorithms such as the Kalman).

The use of the algorithms discussed in the lectures will be demonstrated with real world examples.