The SVT will further enhance the capabilities of the STAR baseline detector in areas where the TPC performance is improved by combining SVT and TPC information. For the simulations presented in this section, full tracking within the SVT and matching with TPC tracks were performed as in the analysis of real events, unless otherwise mentioned.
Primary track reconstruction: The SVT not only extends the range of primary particle reconstruction in STAR to lower momenta, but also enhances the tracking efficiency for momenta below 400 MeV/c compared to the TPC alone. Fig. 5a shows the primary track reconstruction efficiency for SVT and TPC as a function of the momentum. The plot corresponds to the present level of tracking capabilities. Improvements in both tracking software packages are expected in the future. Fig. 5b shows the number of ghost tracks in the SVT tracking as a function of momentum. This number is greatly reduced by matching tracks between SVT and TPC.
Figure: a.) Primary track reconstruction efficiency for SVT and TPC as a
function of momentum.
b.) classification of tracks in the SVT as a function of momentum
based on the tracking efficiency
Particle Identification:
The good de/dx resolution of the SVT will complement significantly
the particle identification capabilities of the TPC. Fig. 6a
shows the average energy loss as a function of 
for the combined
SVT+TPC system.
Fig. 6b shows a slice in momentum space. The addition of the SVT extends
the upper limit for 
separation to about 600 MeV/c and for 
to about 800 MeV/c.
The improvement in particle identification will aid in HBT analysis,
strangeness production
studies, and in overall event characterization.
Figure: energy loss in the combined SVT+TPC system
Two-track resolution: HBT analysis depends strongly on the ability to separate tracks that have small relative momenta. We show in Fig. 7a the result of a laboratory measurement of the minimum relative distance needed for two hit separation on a Silicon Drift Chamber as a function of the size of the signal [[32]]. The SVT also contributes to enhance the pion pair statistics on an event by event basis, by extending the momentum range for reconstruction of pion tracks. Fig. 7b shows the uncertainty of the rms radius as a function of two track separation and lower momentum cutoff. The dashed curves correspond to the TPC separation range projected back to the outer SVT radius, the solid curve reflects the SVT separation power at the same point. The improved two track resolution of the SVT-TPC system will increase the maximum source size to which STAR is sensitive. Fig. 7c shows that the SVT might enable HBT analysis on an event-by-event basis.
Figure: a.)Two hit separation as a function of signal width.
b.) Uncertainty in rms radius for inclusive pion pair spectra as a function
of lower momentum cutoff and two track separation at the outer radius
of the SVT. Dashed curves reflect a range for the TPC performance depending
on the drift gas, the solid curve is the result of the SVT analysis
c.) Same as b.) for single event pion pair spectra.
Secondary track reconstruction:
The superior position resolution of the SVT enhances the
identification and reconstruction of decay vertices, thus considerably
reducing the combinatoric background to obtain a statistically
significant sample of short lived particles such as 
,
and 
. This improvement results from the enhanced
impact parameter resolution (see Fig. 8). It is augmented
by a constant SVT-TPC track matching efficiency of 86% in the momentum
range from 200 MeV/c to 40 GeV/c. The high detection
efficiency thus achieved will enable STAR to examine the correlation between
the production cross section of these particles and other observables such
as the total multiplicity, the transverse energy production of the particle's
mean 
hence providing much more information than is available
from inclusive distributions alone.
High 
:
Momentum resolution for tracks with 
above 2 GeV/c is important for
studies of gluon shadowing and jet quenching in the heavy-ion program as well as
for some aspects of the pp spin physics
program, in particular, the high momentum electron-hadron separation
which
is crucial for 
and 
reconstruction and the study of
Drell-Yan 
pairs.
However, at high 
the momentum resolution of the TPC alone is
seriously impaired by its modest position resolution (
700 
m),
which limits the determination of the curvature of these nearly straight
tracks. The addition of the
SVT will provide vector information near the main vertex, greatly improving
the momentum resolution for high 
. Fig. 9 shows the
transverse momentum resolution as a function of 
for the
TPC alone, and the TPC+SVT system. Note that at 50 GeV/c the
improvement provided by the addition of the SVT is more than a factor of 5.
Inclusion of the main vertex, as determined by the combined tracking system,
will further improve the momentum resolution.
Figure: High 
resolution from the TPC alone and the SVT+TPC
system
Figure: Impact parameter resolution from the SVT and TPC
Primary vertex location:
The SVT improves the main vertex position
resolution
and in turn augments primary particle momentum resolution,
secondary particle identification, and high 
momentum
resolution.
A plot of primary vertex z resolution as a function of the
mass of symmetric systems is shown Fig. 10 for
the TPC and TPC+SVT systems. The addition of the SVT is of primary
importance for the lighter systems. There is also a corresponding improvement
in the two vertex separation from applying
the Raleigh criteria to the results of the previous plot. This improved
vertex resolution will be extremely useful
in p-p collision studies at high luminosity
where recorded events will include multiple primary vertices. It
should also enhance STAR's ability to reject multi-vertex
events when searching for events with rare multiplicity fluctuations.
Trigger:
The SVT will also provide third level trigger capabilities for
STAR. Using existing algorithms, it
should be feasible to achieve track reconstruction and particle
identification via dE/dx with the SVT within a
few tens of milliseconds. Thus, charged particle spectra are
accessible on the third trigger level. Conditions could be envoked on
e.g. particle ratio fluctuations and/or spectral shapes. One example of
particular interest is the low momentum pion enhancement. Fig. 11a shows the
pion mean 
calculated event-by-event for 200 events
for two different production mechanisms,
as discussed in the previous
chapter. The mean momentum is shifted by varying the chemical
potential 
in the Boltzmann distribution. A 
near the pion mass
corresponds to a condensation scenario in which the low momentum
component of the spectrum is enhanced.
Fig. 11b shows the difference in the mean of the two spectra, which can
be determined in time for a third level trigger decision.
Figure: Pion momentum distribution and mean for different mechanisms on an
event by event basis
Figure: Main vertex resolution measured by the SVT and TPC as a function of
system size