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 +  0  0 production with nuclear excitation Direct  +  - production & interference e + e - pair production Conclusions Results

Coherent Interactions n b > 2R A u no hadronic interactions n Ions are sources of fields u photons F ~ Z 2 --> very high fluxes u Pomerons or mesons (mostly f 0 ) F A 2 (bulk)- A 4/3 (surface) u Fields couple coherently to ions F P  < h/R A, ~30 MeV/c for heavy ions  P || <  h/R A ~ 3 GeV/c at RHIC Au Coupling ~ nuclear form factor , P, or meson

Specific Topics n Vector meson production  A -- > ’   , , , J/ ,… A  Production cross sections -->  (VN)  Vector meson spectroscopy (  ,  ,  ,…) u Wave function collapse u Vector Meson superradiance n Electromagnetic particle production   leptons,mesons u Strong Field QED  Z  ~ 0.6  meson spectroscopy      ~ charge content of scalar/tensor mesons F particles without charge (glueballs) won’t be seen n Mutual Coulomb excitation (GDR & higher) u Luminosity measurement, impact parameter tag Production occurs in/near one ion  VM e + e -, qq,... ss Z  ~ 0.6; is N  > 1?

Exclusive  0 Production in STAR n One nucleus emits a photon n The photon fluctuates to a qq pair u vector meson dominance --> treat as vector meson n The pair scatters elastically from the other nucleus u also Photon- meson contribution n qq pair emerges as a vector meson  is large: 380 mb for Au at 130 GeV/nucleon u 5% of hadronic cross section u 120 Hz production rate at RHIC full energy/luminosity Au 00  qq

Interference n 2 possibilities u Interference!! n Similar to pp bremsstrahlung u no dipole moment, so u no dipole radiation n 2-source interferometer u separation b , , , J/  are J PC = n Amplitudes have opposite signs  ~ |A 1 - A 2 e ip·b | 2 n For p T << 1/b u destructive interference No Interference Interference   p T (GeV/c)

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

A typical STAR event 200 GeVnucleon cm)

Analysis Approach n Exclusive Channels   0 and nothing else F 2 charged particles F net charge 0 n Coherent Coupling   p T < 2h/R A ~100 MeV/c u back to back in transverse plane n Nuclear breakup possible n Backgrounds: u incoherent photonuclear interactions u grazing nuclear collisions u beam gas

Exclusive  0 n Trigger on low-multiplicity events u back-to-back topology n ~ 7 hours of data u prototype trigger n 2 track vertex u in interaction diamond  non-coplanar,  < 3 rad F reject cosmic rays track dE/dx consistent with  n No neutrons in ZDC peak for p T < 2h/  ~ 100 MeV/c     and      model background  0 P T M(     ) Preliminary

Nuclear Excitation n Multiple Interactions are possible  P(  0, b=2R) ~ 0.5% u P(2GDR, b=2R) ~ 30% u Factorization should hold F Diagram on rt. should dominate n Au* decay mostly by neutron emission Au  P Au*  00

‘Minimum Bias’ Dataset n Trigger on >1 neutron signal in both zero degree calorimeters n ~800,000 triggers n Event selection same as peripheral u no ZDC cuts     and      model background u phase space in p T u small  0 P T M(     ) Preliminary

Direct  +  - production Direct  +  - is independent of energy n The two processes interfere  phase change at M(  0 )  changes  +  - lineshape good data for  p -->  p (HERA + fixed target) poor data for  A   +  - fraction should decrease as A rises --  ++ --  A -- >  0 A -- >  +  - A ++  A -- >  +  - A 00

 0 lineshape Data Fit 00 +-+- Fit all data to  0 +  +  - interference is significant  +  - fraction is high (background?) ZEUS  p --> (  0 +  +  - )p Set =0 for STAR STAR  Au --> (  0 +  +  - )Au Preliminary

A peek at  --> e + e - n ‘Minimum bias dataset u 2 track Q=0 vertex n Find electrons by dE/dx u p< 140 MeV/c n Select identified pairs n p T peaked at 1/  e Blue - all particles red - e + e - pairs Preliminary P t (GeVc) P (GeV/c) dE/dx (keV/cm) Events

Conclusions n For the first time, we have observed three peripheral collisions processes  Au + Au -- > Au + Au +  0  Au + Au -- > Au* + Au* +  0 u Au + Au -- > Au* + Au* + e + e - We see interference between  0 and direct     n Peripheral collisions is in it’s infancy u next year: more data, more triggers, more luminosity,more energy, more channels, more acceptance, more... The  0 p T spectrum is sensitive to whether particle decay triggers wave function collapse