Enhancements in the low momentum part of invariant cross section spectra were discovered in heavy ion reactions at CERN and AGS. Although several explanations were suggested by theorists, the phenomena related to pion spectra under 200 MeV/c is not fully understood [, , , ]. First results from the AGS for low kaons also reveal a very steep component at very low (below 50 MeV/c) which might be linked to a QGP phenomenon []. If these enhancements persist at the higher incident RHIC energies and in the baryon free rapidity region studied with STAR, explanations based on resonance models [] could be excluded in favor of more exotic models based on some form of phase transition []. The high position resolution and low acceptance of the SVT for pions, kaons and protons is mandatory for such studies at RHIC. Several collective effects have been hypothesized which predict changes (enhancements) in the low particle spectra as well as correlations among the low particles. `Van Hove bubbles' [], localized plasma formation, might lead to spatially correlated hadronization of low particles and can be verified by measuring fluctuations of the yield in rapidity. Chiral condensate models [] predict enhancements in the low particle spectra; disoriented chiral condensate models [] in addition predict anomalous ratios of charged to neutral pions on an event-by-event basis. Hints of this behavior may have been observed in cosmic ray collisions []. Information on the yield of low charged pions on an event-by-event basis, as well as their correlation behavior as a function of centrality may provide information about the hadronization of a chiral condensate []. Bose-Einstein condensation [] of pions is predicted to occur if a critical pion density can be reached at RHIC energies. If so, the majority of the pions will be emitted with very low momenta. In the case of coherent emission the mean momentum depends only on the source radius. SVT capabilities for detecting Bose-Einstein condensation effects in the RHIC environment are detailed in []. Fig. 3a shows the magnitude of the effect.
Figure: a.)The effect of Bose Einstein condensation and n-body symmetrization on the spectrum.
b.) momentum resolution for reconstructed in the SVT alone
Fig. 3b shows the pion momentum resolution for reconstructed tracks between 60 MeV/c and 250 MeV/c. The open dots represent the improvement achievable by including de/dx information from the SVT in the calculation. Although moderate, the resolution in fig. 3b is sufficient to permit momentum reconstruction, which preserves the shape of the spectrum at low p. The presence of a low p enhancement will not only be determined for a large ensemble of events but, given the charged pion multiplicities expected at RHIC, it is statistically feasible to measure a shift in the mean of the momentum spectrum, event-by-event, which could be used as an exotic third level trigger (see section 2.4).