1. Diffractive Vector Meson Photoproduction in ultra-peripheral heavy ion collisions with STAR Exclusive  0 photoproduction in AuAu and dAu collisions  0 interferometry 4-prongs – the  *0 ? e + e - pair production Conclusions Akio Ogawa(BNL), Spencer Klein(LBL) For STAR Collaboration

2. A. Ogawa, BNL 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 n Coherence photon emission and scattering  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 u Incoherent scattering can also be studied Au 00  qq

3. A. Ogawa, BNL The Collaboration STAR ~ 400 collaborators 41 institutions 9 countries Solenoid Tracker At RHIC

4. A. Ogawa, BNL

5.  0 photo- production 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 Au 00  qq

6. A. Ogawa, BNL 200 GeV Exclusive  0 Enhancement at  p T < 2h/R A ~100 MeV/c 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

7. A. Ogawa, BNL 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 n n

8. A. Ogawa, BNL 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

9. A. Ogawa, BNL 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*

10. A. Ogawa, BNL 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

11. A. Ogawa, BNL Interference in AuAu 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

12. A. Ogawa, BNL 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 F Non-local wave function  non-factorizable :   +  -    +   - -- b (transverse view) --   ++ ++

13. A. Ogawa, BNL 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

14. A. Ogawa, BNL 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

15. A. Ogawa, BNL 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

16. A. Ogawa, BNL 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

17. A. Ogawa, BNL  d   0 pn n Topology trigger + ZDC for Au breakup u Clear single neutron signal M  well fit by  0 + direct    0 mass = 766 ± 1 MeV   = 159 ±13 MeV F ~ particle data book values   0 :direct  +  - ratio slightly lower than AuAu data n t spectrum is similar to ZEUS u slope b ~ 11.5 GeV -2 n Dropoff at small t u Too little energy to dissociate the deuteron t (GeV 2 ) Deuteron does not dissociate M  (GeV) Preliminary

18. A. Ogawa, BNL pTpT 4-prong analysis n Very preliminary n ‘Model’ reaction   A->   *(1450/1700) -->  +    +  - u Expect ~ 100 events n Follows 2-prong analysis u p T < 100 MeV/c  Excess seen for  +    +  -  Over  +    +  - F Only at low p T n Analysis on a fraction of data n Background subtracted mass spectrum peaks at ~1.5 GeV Neutral 4 pion combos Charged 4 pion combos Entries Net Signal   mass (GeV) Entries Preliminary

19. A. Ogawa, BNL 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)

20. A. Ogawa, BNL Conclusions & Outlook STAR has observed photonuclear  0 production in AuAu and dAu collisions  The  0 cross sections agree with theoretical predictions.  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. We observe coherent 4-prong events, likely the  *0. n The cross section for e + e - pair production is consistent with lowest order quantum electrodynamics. In 2004, we have multiplied our data sample, and hope to observe photoproduction of the J/ .

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22. A. Ogawa, BNL 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

23. A. Ogawa, BNL  0 production in dAu n The photon usually comes from the Au u The coherent (no breakup) reaction has a small contribution due to photons from the deuteron  d -->  0 d u Coherent, coupling to entire deuterons  d -->  0 pn u Incoherent, couples to individual nucleons n Both are ‘usually’ two photon processes u Factorization does not hold here The deuteron is small;  0 p T can be large

24. A. Ogawa, BNL  d   0 d? n No neutron detected   d   0 d F Deuteron form factor   d   0 pn where the neutron missed the ZDC F Simulations in progress   Au   0 Au F Mostly at p T < h/R au n Studies are in progress to understand these contributions  0 mass, width close to particle data book values Ratio of  0 : direct  similar to  d   0 pn t (GeV 2 ) M  (GeV) Preliminary