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Ultra-Peripheral Collisions at STAR

S T A R. Ultra-Peripheral Collisions at STAR. Pablo Yepes, Rice University for the Collaboration. Workshop on Electromagnetic Probes of Fundamental Physics Erice (Sicily) Oct 16-21, 2001. Ultra-Peripheral Collisions (UPC). Au. g , P, or meson. Au.

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Ultra-Peripheral Collisions at STAR

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  1. S T A R Ultra-Peripheral Collisions at STAR Pablo Yepes, Rice University for the Collaboration Workshop on Electromagnetic Probes of Fundamental Physics Erice (Sicily) Oct 16-21, 2001 Pablo Yepes, Rice U.

  2. Pablo Yepes, Rice U.

  3. Ultra-Peripheral Collisions (UPC) Au g, P, or meson Au Coupling ~ nuclear form factor time • Nuclei “miss” each other (b > 2RA) • No hadronic interactions • Ions are sources of fields • Photons: ~ Z2  very high fluxes • Pomerons: A 2 (bulk)-A 4/3 (surface) • Fields couple coherently to ions • P < h/RA, ~30 MeV/c for heavy ions • P|| < gh/RA ~ 3 GeV/c at RHIC (Au Au, 200GeV/nucleon, University of Frankfurt) Pablo Yepes, Rice U.

  4. Nuclear Excitation Au*+n Au g g Au P Au*+n In addition to UPCproduction, nuclei can exchange one or more separate photons and become mutually excited. Process Au+Au -> Au*+Au* +UPC • It is thought to factorize as function of impact parameter • Decay yields neutrons at zero degrees wrt beam direction (ZDC) Pablo Yepes, Rice U.

  5. Some Physics Topics VM g Production occurs in/near one ion gs Za ~ 0.6; is Ng > 1? • Vector mesons: gA r0, w, f, J/y,… • Very large cross sections. Can we address physics of meson factories? • Production cross sections --> s(VN) • Vector meson spectroscopy (r*, w*, f*,…) • Wave function collapse • EM particle production: gge+e-,m+m-,mesons • Strong Field QED: Za ~ 0.6 • meson spectroscopy Ggg • Ggg ~ charge content of scalar/tensor mesons • particles without charge (glueballs) won’t be seen • Mutual Coulomb excitation (GDR & higher) • Luminosity measurement, impact parameter tag e+e-, qq,... Pablo Yepes, Rice U.

  6. Pablo Yepes, Rice U.

  7. A typical STAR events=200 GeV/A Pablo Yepes, Rice U.

  8. Year 2000 Samples • Data: • 400 K Minimum Bias: • Ntracks<10 • (Signal in both ZDC’s) • 7 h dedicated run with special trigger • Monte Carlo: • 100 K: r pp • 100 K di-electrons • Level 0 Trigger with Central Trigger Barrel: • Top-Bottom Veto • 1-2 hit North and 1-2 South • Output 20-40 Hz • Level 3 Trigger: • Vertex and Multiplicity • Output: 1-2 Hz Pablo Yepes, Rice U.

  9. Minbias 2 +/- Primary Tracks rvert< 15 cm, |zvert|<200 cm Transverse Angle > 2.7 rad Total pT<0.2 GeV Triggered As Minbias plus anti-cosmic cut: Opening Angle < 3 rad. r Analysis Selection Backgrounds: • incoherent photonuclear interactions • grazing nuclear collisions • beam gas Pablo Yepes, Rice U.

  10. r pT and y Acceptance and Efficiency From r MC sample |yr|<1 Pablo Yepes, Rice U.

  11. Opening Angle in Transverse Plane Minbias Triggered Cut Cut MC normalized to data Trigger simulation applied to MC! MC normalized to data Pablo Yepes, Rice U.

  12. Theta* with Pion Mass Assumption Minbias Triggered Theta*: decay angle relative to beam axis in center of mass mother MC normalized to data Trigger simulation applied to MC! MC normalized to data Pablo Yepes, Rice U.

  13. Triggered Sample r0 PT • Track dE/dx consistent with p • Sample divided: • No signal in ZDC: No nuclear breakup • Signal in ZDC: Nuclear breakup • peak for pT < 2h/g ~ 100 MeV/c Preliminary M(p+p-) Preliminary Pablo Yepes, Rice U.

  14. ‘Minimum Bias’ PT r0 PT Preliminary Pablo Yepes, Rice U.

  15. Direct p+p- production p- r0 p+ g p+ g gA -- > p+p- A gA -- > r0A -- > p+p- A p- • Direct p+p- is independent of energy • The two processes interfere: 1800 phase change at M(r0) • changes p+p- lineshape • good data for gp --> ppp (HERA + fixed target) • poor data for gA • p+p- fraction should decrease as A rises Pablo Yepes, Rice U.

  16. r0 lineshape STAR gAu --> (r0 + p+p- )Au ZEUS gp --> (r0 + p+p- )p Preliminary Set =0 for STAR Pablo Yepes, Rice U.

  17. A Look at gg e+e- Blue - all particles red - e+ e- pairs • ‘Minimum bias data set • 2 track Q=0 vertex • Find electrons by dE/dx • p< 140 MeV/c • Select identified pairs • pT peaked at 1/<b> p dE/dx (keV/cm) e P (GeV/c) Preliminary Events Pablo Yepes, Rice U. Pt (GeVc)

  18. Present and Future: 2001 Run • Dedicated trigger for UPC running in parallel for the last part of the run • Around 10 times more minbias events Minbias Triggered Very Preliminary Pablo Yepes, Rice U.

  19. Conclusions • For the first time, we have observed three peripheral collisions processes • Au + Au -- > Au + Au + r0 • Au + Au -- > Au* + Au* + r0 • Au + Au -- > Au* + Au* + e+e- • We see interference between r0 and direct p+p- • In 2001: more data, more triggers, more luminosity,more energy, more channels, more acceptance. Pablo Yepes, Rice U.

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