proposal2007_draft09.. - Henry A. Rowland Department of Physics ...

data but needs the same database infrastructure that must be developed for the standard algorithm.
The template algorithm is based upon the premise that the Pixelav simulation/electric field model
can be tuned to accurately describe the pixel sensors as they are gradually damaged by exposure to
the large LHC radiation field. This premise rests upon the demonstrated success in the modeling
of charge collection profiles measured with test sensors irradiated to several fluences. The actual
calibration procedure is to repeat the beam test measurements in-situ in CMS. This requires that
samples of large-η tracks and pixel clusters be recorded at a series of pixel bias voltages. It has
already been shown that, due to displaced primary vertices, it is possible to acquire such samples
even in the central barrel modules. We are planning to develop a suite of CMSSW software packages
to facilitate this. The template calibration will be a continuing responsibility of the JHU group
after LHC operation begins.
Pixel Hit Validation The template-based reconstruction technique performs chisquare fits
of the projected cluster shapes to simulated shapes. Goodness-of-fit information is generated and
can be used to determine the consistency of the measured projections with the expected shapes.
There are two distinct uses for this information. The first is to validate that the cluster is likely to
have been produced by the transit of primary charged particle. In the region 2.25 < |η| < 2.5, about
20% of all clusters are caused by secondaries and after track finding, about 8% of all muon tracks
contain a hit generated by a secondary particle. These are entirely eliminated by the application
of template probability cuts of 10 −3 . A second use of the goodness-of-fit information is to test
angle hypotheses during track seed generation. Pixel track seeds contain two or three pixel hits
and define track angles at each detector element. The template probabilities can be used to test
the consistency of these angles with the observed clusters. The elimination of inconsistent seeds
before the very cpu-intensive track-finding algorithm is invoked has the potential to significantly
reduce the computing resources needed for track reconstruction.
Fehling has recently begun to study the application of the template probabilities to seed-finding.
This has the advantage that it eliminates pixel hits due to secondary particles at the earliest stage of
track finding in addition to reducing the computing resources needed for track reconstruction. His
initial finding suggests that this procedure can reduce the total seed count by a factor of three with
an efficiency larger than 98.2% (it’s possible that the lost tracks are “fakes”). Careful study and
optimization are needed before the scheme can be used in the default CMS track reconstruction.
Higher Level Trigger The second-level or Higher-Level Trigger (HLT) uses pixel triplets
as primitives in the definition of various triggers. Pixel tracks are a crucial part of HLT triggers
that identify τ-leptons and b-quarks in the final state. New postdoc Rappoccio is taking over
maintenance of the trigger path designed to find two-τ final states from Z bosons and from neutral
Higgs-boson-like states. This trigger is also a natural place to test the influence of the template algorithm
which has already been shown to improve the impact parameter resolution of reconstructed
tracks [301].
Template-Based Pixel Simulation We know that the performance of the pixel tracker will
be significantly degraded by radiation damage for much of its useful lifetime. This will adversely
affect many physics analyses and it will be essential that the CMSSW simulation reflect the degraded
pixel performance. We have established that the Pixelav simulation can simulate irradiated sensors
but it requires approximately 1.5 s per cluster on a 2.5 GHz G5 processor and more than 3 s per
cluster on a 2.8 GHz Xeon processor (due to an inferior vector processing architecture). The official
CMSSW pixel simulation adequately models unirradiated sensors with a simplified charge drift
model and requires only about 12 ms per cluster on the 2.8 GHz Xeon processor. In order to retain
the speed of the CMSSW simulation and achieve the accuracy of the Pixelav simulation, Swartz
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