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 []. 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