1. Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive  0 production  0 interferometry e + e - pair production (V. Morozov dissertation, 2003) Conclusions Spencer Klein, LBNL (for the STAR Collaboration)

2. S. Klein, LBNL Coherent Interactions n b > 2R A; u no hadronic interactions u ~ 20-60 fermi at RHIC n Ions are sources of fields u photons  N  ~ Z 2 u Pomerons or mesons (mostly f 0 ) F A 2 (bulk) A 4/3 (surface) u Fields couple coherently to ions  Photon/Pomeron wavelength = h/p> R A F amplitudes add with same phase F P  < 30 MeV/c F P || < 3 GeV/c F Strong couplings --> large cross sections Au Coupling ~ nuclear form factor , P, or meson

3. S. Klein, LBNL Unique Features of Ultra- peripheral collisions n Very strong electromagnetic fields   --> e + e - and  --> qq u Multiple interactions between a single ion pair n Unique Geometry u 2-source interferometer n Nuclear Environment u Particle Production with capture  Large  for e -

4. S. Klein, LBNL Exclusive  0 Production n A virtual photon from one nucleus fluctuates to a qq pair which scatters elastically from the other nucleus and emerges as a vector meson u Photon emission follows the Weizsacker-Williams method  For heavy mesons (J/  ), the scattering is sensitive to nuclear shadowing Coherence --> Rates are high   ~ 8 % of  (had.) for gold at 200 GeV/nucleon F 120 /sec at design luminosity u Other vector mesons are copiously produced Au 00  qq

5. S. Klein, LBNL Nuclear Excitation n Nuclear excitation ‘tag’s small b n Multiple Interactions are independent n Au* decay via neutron emission u simple, unbiased trigger n Higher order diagrams u smaller u Harder photon spectrum u Production at smaller |y| n Single (1n) and multiple (Xn, X>0) neutron samples Au  P Au*  00  0 with gold @ RHIC d  /dy y Exclusive - solid X10 for XnXn - dashed X100 for 1n1n - dotted

6. S. Klein, LBNL

8.  0 Analysis 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 Trigger u Back to back hits in Central Trigger barrel

9. S. Klein, LBNL 200 GeV Exclusive  0 n 1.5 Million topology triggers n 2 track vertex  non-coplanar;  < 3 rad to reject cosmic rays     and      model background shape      pairs from higher multiplicity events have similar shape u scaled up by ~2  Incoherent  0 (w/ p T >150 MeV/c) are defined as background in this analysis asymmetric M  peak M(     )  0 P T Signal region: p T <0.15 GeV Preliminary

10. S. Klein, LBNL 200 GeV XnXn data n 1.7 million minimum bias triggers n Select events with a 2 track vertex     and      model background n single (1n) and multiple (Xn) neutron production u Coulomb excitation F Giant Dipole Resonance n Rapidity distribution matches Soft Pomeron model calculation After detector simulation Soft Pomeron pTpT

11. S. Klein, LBNL M  M  spectrum includes  0 + direct  +  -  Same  0 :  +  - ratio as is observed in  p-->  +  - p at HERA --  ++ --  ++ 00 M(     ) XnXn sample ZEUS  p --> (  0 +  +  - )p e + e - and hadronic backgrounds M  d  /dM   b/GeV  STAR  Au --> (  0 +  +  - )Au*

12. S. Klein, LBNL Cross Section Comparison n 130 GeV data n Normalized to 7.2 b hadronic cross section n Systematic uncertainties: luminosity, overlapping events, vertex & tracking simulations, 1n selection, etc. Exclusive  0 bootstrapped from XnXn u limited by statistics for XnXn in topology trigger n Good agreement u factorization works

13. S. Klein, LBNL Interference n 2 indistinguishable possibilities u Interference!! n Like pp bremsstrahlung u no dipole moment, so u no dipole radiation n 2-source interferometer with separation b  is negative parity so   ~ |A 1 - A 2 e ip·b | 2 n At y=0   =  0 [1-cos(p  b)] n b is unknown u Reduction for p T Interference No Interference  0 w/ mutual Coulomb dissoc.  0.1< |y| < 0.6 t (GeV/c) 2 dN/dt

14. S. Klein, LBNL Entangled Waveforms  0 are short lived, with c  ~ 1 fm << b n Decay points are separated in space-time F Independent decays to different final states 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:   +  -    +   - -- b (transverse view) --   ++ ++

15. S. Klein, LBNL Interference Analysis Select clean  0 with tight cuts u Lower efficiency Larger interference when  0 is accompanied by mutual Coulomb dissociation n Interference maximal at y=0 u Decreases as |y| rises u 2 rapidity bins 0.1 < |y| < 0.5 & 0.5<|y|<1.0 F |y|<0.1 is contaminated with cosmic rays

16. S. Klein, LBNL t for 0.1 < |y| < 0.5 (XnXn) n 2 Monte Carlo samples: u Interference u No interference u w/ detector simulation F Detector Effects Small n Data matches Int n Inconsistent with Noint n Interference clearly observed n 973 events dN/dt Data (w/ fit) Noint Int Background STAR Preliminary t (GeV 2 ) = p T 2

17. S. Klein, LBNL XnXn Fitting the Interference n Efficiency corrected t n 1764 events total n R(t) = Int(t)/Noint(t) u Fit with polynomial n dN/dt =A*exp(-bt)[1+c(R(t)-1)] u A is overall normalization u b is slope of nuclear form factor F b = 301 +/- 14 GeV -2 304 +/- 15 GeV -2 u c=0 -- > no interference u c=1 -- > “full” interference F c = 1.01 +/- 0.08 0.78 +/- 0.13 n Data and interference model match dN/dt STAR Preliminary STAR Preliminary Data (w/ fit) Noint Int Data (w/ fit) Noint Int t (GeV 2 ) 0.1 < |y| < 0.5 0.5 < |y| < 1.0

18. S. Klein, LBNL Exclusive  0 n ~ 46 fm n 5770 events total n dN/dt = A*exp(-bt)[1+c(R(t)-1)] u A - overall normalization u b = 361 +/- 9 GeV -2/ 368 +/- 12 GeV -2 F Different from minimum bias data u c = 0.71 +/- 0.16 1.22 +/- 0.21 n Interference is present t dN/dt Data (w/ fit) Noint Int Data (w/ fit) Noint Int STAR Preliminary t STAR Preliminary 0.1 < |y| < 0.5 0.5 < |y| < 1.0

19. S. Klein, LBNL Combining the Data n The c values are consistent -- > take weighted mean u c= 0.93 +/- 0.06 (statistical only) u Data matches predictions The b’s for the exclusive  0 and breakup data differ by 20%  Exclusive  0 : 364 +/- 7 GeV -2 u Coulomb breakup: 303 +/- 10 GeV -2 u Photon flux ~ 1/b 2  More  0 production on ‘near’ side of target Smaller apparent size n Systematic Errors (in progress) u Change simulation input form factor slope b by 20% F 3% (2%) change in c(b) u No Detector simulation F 18% (1.4%) change in c(b) F If simulation is 75% ‘right--> 5% systematic error

20. S. Klein, LBNL Au Au  --> e + e - Au* Au* n e + e - pairs accompanied by nuclear breakup Z  EM ~ 0.6 u Higher order corrections? n Cross section matches lowest order quantum electrodynamics calculation u No large higher order corrections n p T peaked at ~ 25 MeV u Matches QED calculation F By Kai Hencken et al.  4  disagreement with equivalent photon (massless photon) calculation n V. Morozov PhD dissertation Preliminary Pair P t (GeVc) Pair Mass (GeV) STAR Preliminary STAR Preliminary

21. S. Klein, LBNL Conclusions & Outlook STAR has observed photonuclear  0 production.  The  0 cross sections agree with theoretical models.  Interference between  0 and direct     is seen. We observe 2-source interference in  0 production.  The interference occurs even though the  0 decay before the wave functions of the two sources can overlap. n The cross section for e + e - pair production is consistent with lowest order quantum electrodynamics. u The equivalent photon approximation does not describe the p T spectrum. n 2003 run:  Good dA -->  0 sample. u Meson production via double Pomerons from pp pending.