2 The CDF Experiment at Fermilab Contents - Harvard University ...

Section 2: Run II Triggers 79
2.4.5 Multijet Trigger for W=Z + h
(Prof. Huth, Dr. Maksimovic, Dr. Riegler & Mr. Medvigy)
In his search for the Standard Model Higgs boson produced in association with a vector
boson (W or Z) in the Run I multijet data, Jorge Troconiz (then a post-doc in our group)
encountered problems with the old multijet trigger. The trigger was optimized for tt ! 6
jets, and involved only the second level of CDF's three-level trigger system. In Run Ib, the
multijet trigger had to satisfy the following criteria:
at least four Level 2 calorimeter clusters with E T > 15 GeV
total Level 2 calorimeter energy (sum over all clusters) P E T > 125 GeV
The fundamental challenge for Troconiz's Higgs search was that the above criteria were
rather inecient for the light SMHiggs. Furthermore, the trigger eciencies for H + W=Z
signal corresponding to Higgs masses between 90 and 140 GeV (that the search considered)
were found to vary signicantly { i.e., the Run I search ended up right on a steep part of
the trigger eciency curve.
In this situation, there clearly is room for improvement, and the natural place to do so is
in the calorimeter requirements of the multijet trigger. Professor Huth initiated this project
by advocating the idea of using displaced tracks found by the secondary vertex tracker (SVT)
P as a means of controlling the increase in the trigger rates caused by the lowering of the E T
requirement.
In the redesign of the multijet trigger { so that it is appropriate for a H + W=Z search
{ our goal is to dene trigger criteria that are ecient for the type of events we would like
to collect after all selection cuts, including both trigger and oine. For instance, in order to
form the invariant mass of a Higgs candidate, we need to be able to identify the b jets in a
multi-jet event; therefore we expect that we will have to employ some form of b-tagging. The
events that fail the oine version of the b-tagging (currently approximated by SECVTX)
are therefore of no use for us, and we ignore them while tuning the multijet trigger. We
therefore try to
1. keep the eciency of two b-tags at the oine level as high as possible;
2. keep the trigger rates (dominated by background) as low as possible.