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Flavor physics

The different stages of nucleus-nucleus collisions (parton scattering, thermalization, expansion and hadronization) will be studied by measuring the yield and spectral distribution of produced hadrons. The formation of a QGP might be revealed through an enhancement of the relative abundance of strange to non-strange quarks [[2]]. Considerable reduction of the ratio of strange to anti-strange baryon production is also expected with the formation of a QGP [[3]]. Measurements of multiply strange baryon ratios ( e.g. ) are very sensitive to enhanced strange quark density[[4], [5]] which might result from chiral symmetry restoration in the QGP. The addition of the SVT with its high position resolution close to the production vertex allows the elimination of most of the combinatoric background and drastically improves the detection efficiency for strange particles. Fig. 1a shows the tracking efficiency for secondary tracks in the SVT alone as a function of momentum.

To illustrate the positive impact of the excellent SVT position resolution we show in Fig. 1b an Armenteros scatter plot for and reconstructed from combined SVT and TPC tracks. Although no particle identification information is used in this plot, a clean separation of and is obtained.

Figure: a.) Secondary track reconstruction efficiency for the SVT alone
b.) Armenteros plot showing and separation based on SVT information

The capability of the combined SVT and TPC detector for reconstructing strange and multistrange particles is indicated by the invariant mass spectra in Fig. 2 (see also [[6]]). To obtain the spectrum a complete tracking algorithm in the SVT was employed, including momentum information from the TPC, matching efficiency, and detector resolution factors derived from simulations. The decay particles are identifiable via energy loss in the combined tracking system. The spectrum assumes ideal tracking. Based on the simulation for the , we expect a modest reduction of the signal to noise ratio for the in the case of realistic tracking.

The ability of the SVT+TPC system to reconstruct multi-strange baryons coupled with the ability of STAR to study global event observables makes the proposed detector system unique among the RHIC experiments.

Modification of the mass and decay widths of certain very short lived particles has been predicted to occur in extremely dense hadronic matter and might also signal the formation of a QGP and chiral symmetry restoration [[7]]. Because of its narrow width and simple decay schemes the meson has received particular attention [[8]]. The excellent momentum resolution and wide coverage of combined SVT and TPC detectors will allow the study of possible medium modifications of the meson mass and width through the decay channel.

Charm and in particular open charm production is currently believed to be an excellent indicator of the initial parton scattering stage of heavy ion collisions [[9]]. Because such particles must propagate through the medium they might also be sensitive to the production of the QGP. As the D-meson production cross section () and lifetime ( 200 m) are small and considering that most D decay channels are complex, the detection capability of open charm with STAR SVT and TPC is unclear at present. Strong enhancements in the D meson production, predicted recently [[10]], will produce a cleaner signal than that expected from previous estimates of the production cross section. It is apparent however that such a measurement cannot be achieved without the excellent position resolution afforded by the SVT [[11]].

Figure: Invariant mass spectra of reconstructed strange and multi-strange particles based on SVT and TPC tracking information .

next up previous
Next: Low physics Up: Physics Capabilities of Previous: Physics Capabilities of

Claude Andre Pruneau
Wed Aug 23 00:13:24 EDT 1995