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Diffractive Vector Meson Photoproduction in ultra-peripheral heavy ion collisions with STAR

Diffractive Vector Meson Photoproduction in ultra-peripheral heavy ion collisions with STAR. Exclusive r 0 photoproduction in AuAu and dAu collisions r 0 interferometry 4-prongs – the r *0 ? e + e - pair production Conclusions.

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Diffractive Vector Meson Photoproduction in ultra-peripheral heavy ion collisions with STAR

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  1. Diffractive Vector Meson Photoproduction in ultra-peripheral heavy ion collisions with STAR Exclusive r0 photoproduction in AuAu and dAu collisions r0 interferometry 4-prongs – the r*0? e+e- pair production Conclusions Akio Ogawa(BNL), Spencer Klein(LBL)For STAR Collaboration

  2. Au g qq Au r0 Exclusive r0 Production • A virtual photon from one nucleus fluctuates to a qq pair which scatters elastically from the other nucleus and emerges as a vector meson • Photon emission follows the Weizsacker-Williams method • For heavy mesons (J/y), the scattering is sensitive to nuclear shadowing • Coherence photon emission and scattering • Rates are high • s(r) ~ 8 % of s(had.) for gold at 200 GeV/nucleon • 120 /sec at design luminosity • Other vector mesons are copiously produced • Incoherent scattering can also be studied A. Ogawa, BNL

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

  4. A. Ogawa, BNL

  5. Au g qq Au r0 r0 photo- production • 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 • Trigger • Back to back hits in Central Trigger barrel A. Ogawa, BNL

  6. 200 GeVExclusive r0 Signal region: pT<0.15 GeV • Enhancement at SpT < 2h/RA ~100 MeV/c • 1.5 Million topology triggers • 2 track vertex • non-coplanar; q< 3 rad to reject cosmic rays • p+p+ andp-p- model background shape • p+p- pairs from higher multiplicity events have similar shape • scaled up by ~2 • Incoherent r0 (w/ pT>150 MeV/c) are defined as background in this analysis • asymmetric Mpp peak r0 PT Preliminary M(p+p-) A. Ogawa, BNL

  7. Au* Au g 2+g r0 P Au Au* Nuclear Excitation n • Nuclear excitation ‘tag’s small b • Multiple Interactions are independent • Au* decay via neutron emission • simple, unbiased trigger • Higher order diagrams • smaller <b> • Harder photon spectrum • Production at smaller |y| • Single (1n) and multiple (Xn, X>0) neutron samples n r0 with gold @ RHIC ds/dy y Exclusive - solid X10 for XnXn - dashed X100 for 1n1n - dotted A. Ogawa, BNL

  8. 200 GeV XnXn data • 1.7 million minimum bias triggers • Select events with a 2 track vertex • p+p+ andp-p- model background • single (1n) and multiple (Xn) neutron production • Coulomb excitation • Giant Dipole Resonance • Rapidity distribution matches Soft Pomeron model calculation pT Soft Pomeron After detector simulation A. Ogawa, BNL After detector simulation

  9. p- p- r0 p+ p+ g g XnXn sample Mpp STAR gAu --> (r0 + p+p- )Au* ds/dMpp (mb/GeV) • Mpp spectrum includes r0 + direct p+p- • Same r0: p+p- ratio as is observed in gp--> p+p- p at HERA M(p+p-) ZEUS gp --> (r0 + p+p- )p e+e- and hadronic backgrounds Mpp A. Ogawa, BNL

  10. Cross Section Comparison • 130 GeV data • Normalized to 7.2 b hadronic cross section • Systematic uncertainties: luminosity, overlapping events, vertex & tracking simulations, 1n selection, etc. • Exclusive r0 bootstrapped from XnXn • limited by statistics for XnXn in topology trigger • Good agreement • factorization works A. Ogawa, BNL

  11. Interference in AuAu • 2 indistinguishable possibilities • Interference!! • Like pp bremsstrahlung • no dipole moment, so • no dipole radiation • 2-source interferometer with separation b • r is negative parity so • s ~ |A1 - A2eip·b|2 • At y=0 • s=s0[1-cos(pb)] • b is unknown • Reduction for pT <<1/<b> • r0w/ mutual Coulomb dissoc. • 0.1< |y| < 0.6 Interference No Interference dN/dt t(GeV/c)2 A. Ogawa, BNL

  12. Entangled Waveforms + r0 • r0 are short lived, with ct ~ 1 fm << b • Decay points are separated in space-time • Independent decays to different final states • 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- - b + r0 - (transverse view) A. Ogawa, BNL

  13. Interference Analysis • Select clean r0 with tight cuts • Lower efficiency • Larger interference when r0 is accompanied by mutual Coulomb dissociation • Interference maximal at y=0 • Decreases as |y| rises • 2 rapidity bins 0.1 < |y| < 0.5 & 0.5<|y|<1.0 • |y|<0.1 is contaminated with cosmic rays A. Ogawa, BNL

  14. 0.1 < |y| < 0.5 XnXn Fitting the Interference Data (w/ fit) Noint Int dN/dt • Efficiency corrected t • 1764 events total • R(t) = Int(t)/Noint(t) • Fit with polynomial • dN/dt =A*exp(-bt)[1+c(R(t)-1)] • A is overall normalization • b is slope of nuclear form factor • b = 301 +/- 14 GeV-2 304 +/- 15 GeV-2 • c=0  no interference • c=1  “full” interference • c = 1.01 +/- 0.08 0.78 +/- 0.13 • Data and interference model match STAR Preliminary t (GeV2) 0.5 < |y| < 1.0 Data (w/ fit) Noint Int dN/dt STAR Preliminary t (GeV2) A. Ogawa, BNL

  15. Exclusive r0 0.1 < |y| < 0.5 Data (w/ fit) Noint Int dN/dt • <b> ~ 46 fm • 5770 events total • dN/dt = A*exp(-bt)[1+c(R(t)-1)] • A - overall normalization • b = 361 +/- 9 GeV-2/ 368 +/- 12 GeV-2 • Different from minimum bias data • c = 0.71 +/- 0.16 1.22 +/- 0.21 • Interference is present STAR Preliminary t 0.5 < |y| < 1.0 Data (w/ fit) Noint Int dN/dt STAR Preliminary t A. Ogawa, BNL

  16. Combining the Data • The c values are consistent -- > take weighted mean • c= 0.93 +/- 0.06 (statistical only) • Data matches predictions • The b’s for the exclusive r0 and breakup data differ by 20% • Exclusive r0 : 364 +/- 7 GeV-2 • Coulomb breakup: 303 +/- 10 GeV-2 • Photon flux ~ 1/b2 • More r0 production on ‘near’ side of target • Smaller apparent size • Systematic Errors (in progress) • Change simulation input form factor slope b by 20% • 3% (2%) change in c(b) • No Detector simulation • 18% (1.4%) change in c(b) • If simulation is 75% ‘right--> 5% systematic error A. Ogawa, BNL

  17. gd  r0pn • Topology trigger + ZDC for Au breakup • Clear single neutron signal • Mpp well fit by r0 + direct pp • r0 mass = 766 ± 1 MeV • G = 159 ±13 MeV • ~ particle data book values • r0:direct p+p- ratio slightly lower than AuAu data • t spectrum is similar to ZEUS • slope b ~ 11.5 GeV-2 • Dropoff at small t • Too little energy to dissociate the deuteron Mpp (GeV) Mpp (GeV) Preliminary Deuteron does not dissociate t (GeV2) A. Ogawa, BNL

  18. Entries 0 pppp mass (GeV) 4-prong analysis Neutral 4 pion combos Entries Charged 4 pion combos • Very preliminary • ‘Model’ reaction • gA->r0*(1450/1700) --> p+p-p+p- • Expect ~ 100 events • Follows 2-prong analysis • pT < 100 MeV/c • Excess seen for p+p-p+p- • Over p+p+p+p- • Only at low pT • Analysis on a fraction of data • Background subtracted mass spectrum peaks at ~1.5 GeV Preliminary pT Net Signal A. Ogawa, BNL

  19. Au Au gge+e- Au* Au* • e+e- pairs accompanied by nuclear breakup • ZaEM ~ 0.6 • Higher order corrections? • Cross section matches lowest order quantum electrodynamics calculation • No large higher order corrections • pT peaked at ~ 25 MeV • Matches QED calculation • By Kai Hencken et al. • 4s disagreement with equivalent photon (massless photon) calculation • V. Morozov PhD dissertation Pair Mass(GeV) Preliminary Pair Pt (GeVc) A. Ogawa, BNL

  20. Conclusions & Outlook • STAR has observed photonuclear r0 production in AuAu and dAu collisions • The r0 cross sections agree with theoretical predictions. • Interference between r0 and direct p+p- is seen. • We observe 2-source interference in r0 production. • The interference occurs even though the r0 decay before the wave functions of the two sources can overlap. • We observe coherent 4-prong events, likely the r*0. • 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/y. A. Ogawa, BNL

  21. Back up

  22. t for 0.1 < |y| < 0.5 (XnXn) Data (w/ fit) Noint Int Background dN/dt • 2 Monte Carlo samples: • Interference • No interference • w/ detector simulation • Detector Effects Small • Data matches Int • Inconsistent with Noint • Interference clearly observed • 973 events STAR Preliminary t (GeV2) = pT2 A. Ogawa, BNL

  23. r0 production in dAu • The photon usually comes from the Au • The coherent (no breakup) reaction has a small contribution due to photons from the deuteron • gd --> r0d • Coherent, coupling to entire deuterons • gd --> r0pn • Incoherent, couples to individual nucleons • Both are ‘usually’ two photon processes • Factorization does not hold here • The deuteron is small; r0 pT can be large A. Ogawa, BNL

  24. gd r0d? • No neutron detected • gd r0d • Deuteron form factor • gd r0pn where the neutron missed the ZDC • Simulations in progress • gAu r0Au • Mostly at pT < h/Rau • Studies are in progress to understand these contributions • r0 mass, width close to particle data book values • Ratio of r0: direct pp similar to gd r0pn Preliminary Mpp (GeV) t (GeV2) A. Ogawa, BNL

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