High Level Trigger b-tagging

Contact: Ariel Schwartzman

The LHC will produce collisions at a rate of 40MHz. The purpose of the trigger system 
is to reduce the output event storage rate to about 300MB/s, or approximately 200Hz.
Since most of the collisions at the LHC center of mass energy will result in non-interesting 
physics events, such QCD multi-jet production, the main task of the trigger system is to 
reject QCD events while maintaining high efficiency for low cross section processes 
like Super-symmetry, Higgs, etc.

Tagging b-jets at the trigger level can expand the physics reach of ATLAS in many ways: 
it allows to lower the jet ET threshold to increase the acceptance of interesting physics 
events containing multiple b-jets (like bbH and ttH ), and can be used to reduce non-b 
backgrounds and improve the purity of Super-symmetry searches involving b-jets in the final 
state.

The SLAC group has implemented one of the two ATLAS b-tagging algorithms at level 2 trigger: 
the Impact Parameter CHI2 Probability tagger (IPChi2)
Current and future work involves the optimization of the operational parameters of the algorithm, 
determination of performance in real data events, on-line and off-line validation, and the 
investigation of new trigger menus involving b-quarks and missing transverse energy for 
Super-symmetry searches. 

The proposed project consists in the development of offline "validation" tools, to evaluate the
quality of the input variables and the trigger algorithm performance, and the development of 
a "global" double b-tagger trigger algorithm, by building and tuning a joint probability 
discriminator to simultaneously tag two jets in a given event.