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Ultra-peripheral Collisions at RHIC Spencer Klein, LBNL for the STAR collaboration

Ultra-peripheral Collisions at RHIC Spencer Klein, LBNL for the STAR collaboration. Ultra-peripheral Collisions: What and Why Interference in Vector Meson Production Au + Au --> Au + Au + r 0 r 0 production with nuclear excitation Direct p + p - production & interference

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Ultra-peripheral Collisions at RHIC Spencer Klein, LBNL for the STAR collaboration

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  1. Ultra-peripheral Collisions at RHICSpencer Klein, LBNL for the STAR collaboration Ultra-peripheral Collisions: What and Why Interference in Vector Meson Production Au + Au --> Au + Au + r0 r0 production with nuclear excitation Direct p+p- production & interference e+e- pair production Conclusions Results

  2. Au g, P, or meson Au Coupling ~ nuclear form factor Coherent Interactions • b > 2RA • no hadronic interactions • Ions are sources of fields • photons • ~ Z2 --> very high fluxes • Pomerons or mesons (mostly f0) • 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

  3. VM g Production occurs in/near one ion gs Za ~ 0.6; is Ng > 1? Specific Topics • Vector meson production gA -- > ’ r0, w, f, J/y,… A • Production cross sections --> s(VN) • Vector meson spectroscopy (r*, w*, f*,…) • Wave function collapse • Vector Meson superradiance • Electromagnetic particle production gg -- > leptons,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,...

  4. Au g qq Au r0 Exclusive r0 Production in STAR • One nucleus emits a photon • The photon fluctuates to a qq pair • vector meson dominance --> treat as vector meson • The pair scatters elastically from the other nucleus • also Photon- meson contribution • qq pair emerges as a vector meson • s is large: 380 mb for Au at 130 GeV/nucleon • 5% of hadronic cross section • 120 Hz production rate at RHIC full energy/luminosity

  5. No Interference Interference r0 pT (GeV/c) Interference • 2 possibilities • Interference!! • Similar to pp bremsstrahlung • no dipole moment, so • no dipole radiation • 2-source interferometer • separation b • r,w, f, J/y are JPC = 1- - • Amplitudes have opposite signs • s ~ |A1 - A2eip·b|2 • For pT << 1/b • destructive interference

  6. Entangled Waveforms e+ J/Y • VM are short lived • decay before traveling distance b • Decay points are separated in space-time • no interference • OR • the wave functions retain amplitudes for all possible decays, long after the decay occurs • Non-local wave function • non-factorizable: Yp+ p- Yp+Yp- • Example of the Einstein-Podolsky-Rosen paradox e- + b J/Y 0 - (transverse view)

  7. A typical STAR event200 GeVnucleon cm)

  8. Analysis Approach • Exclusive Channels • r0 and nothing else • 2 charged particles • net charge 0 • Coherent Coupling • SpT < 2h/RA ~100 MeV/c • back to back in transverse plane • Nuclear breakup possible • Backgrounds: • incoherent photonuclear interactions • grazing nuclear collisions • beam gas

  9. Exclusive r0 r0 PT • Trigger on low-multiplicity events • back-to-back topology • ~ 7 hours of data • prototype trigger • 2 track vertex • in interaction diamond • non-coplanar, q < 3 rad • reject cosmic rays • track dE/dx consistent with p • No neutrons in ZDC • peak for pT < 2h/g ~ 100 MeV/c • p+p+ andp-p- model background Preliminary M(p+p-)

  10. Nuclear Excitation • Multiple Interactions are possible • P(r0, b=2R) ~ 0.5% • P(2GDR, b=2R) ~ 30% • Factorization should hold • Diagram on rt. should dominate • Au* decay mostly by neutron emission Au* Au g g r0 P Au Au*

  11. ‘Minimum Bias’ Dataset r0 PT • Trigger on >1 neutron signal in both zero degree calorimeters • ~800,000 triggers • Event selection same as peripheral • no ZDC cuts • p+p+ andp-p- model background • phase space in pT • small Preliminary M(p+p-)

  12. p- r0 p+ g p+ g gA -- > p+p- A gA -- > r0A -- > p+p- A Direct p+p- production 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

  13. Fit Data r0 p+p- r0 lineshape ZEUS gp --> (r0 + p+p- )p STAR gAu --> (r0 + p+p- )Au Preliminary Fit all data to r0 + p+p- interference is significant p+p- fraction is high (background?) Set =0 for STAR

  14. A peek at gg --> e+e- Blue - all particles red - e+ e- pairs p • ‘Minimum bias dataset • 2 track Q=0 vertex • Find electrons by dE/dx • p< 140 MeV/c • Select identified pairs • pT peaked at 1/<b> dE/dx (keV/cm) e P (GeV/c) Preliminary Events Pt (GeVc)

  15. 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- • Peripheral collisions is in it’s infancy • next year: more data, more triggers, more luminosity,more energy, more channels, more acceptance, more... • The r0 pT spectrum is sensitive to whether particle decay triggers wave function collapse

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