1. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 1 Saturation, Low-x, and QCD at RHIC, Part 3 T. Hallman Villaggio Guglielmo, Italy

2. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 2 Ultra-peripheral relativistic heavy ion collisions: a unique testing ground for QCD

3. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 3 The interest in UPC’s What are Ultra-Peripheral Collisions: Electromagnetic interactions that occur at impact parameters b larger than twice the nuclear radius, where no hadronic interactions occur Flux of “almost real” virtual photons (a la Enrico Fermi in 1924) scales with Z 2 so heavy ions are an intense source of photons and cross sections can be large The study both two-photon and photonuclear interactions is possible With the Lorentz boosts, two-photon energies up to 6 GeV are achievable at RHIC At RHIC,  -Au cms energy of ~30 GeV/nucleon and and  -proton cms energy of 300 GeV are achievable Some topics of interest:

4. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 4 The interest in UPC’s The gluon distribution in heavy nuclei: Photoproduction of coherent J/  production (virtual photon fluctuates to a quark- anti-quark pair which interacts coherently with the entire nucleus) can provide a window in gluon distributions not measurable with DIS. Avoids problems in studies using hadronic interactions are subject to uncertainties due to higher order corrections and the Cronin effect. STAR will study the gluon distribution of heavy nuclei using two photoproduction channels: coherent J/  production, and open charm. The total cross section isn’t very sensitive to shadowing, but the rapidity distribution is. If gluon shadowing is “large”, as predicted in HIJING and the diffractive model of Frankfurt and Strikman, shadowing may reduce mid-rapidity J/  production by as much a 40%.

5. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 5 The interest in UPC’s Meson Spectroscopy Photonuclear interactions are a prolific source of vector mesons. At RHIC design luminosity, the exclusive  0 production rate with gold beams is 120 Hz! Although the ‘ground state’  0, , and  mesons are well understood, their excited states are not. For example, the particle data group points out that the  (1450) and  (1700) may be 1 particle or 2 Data from UPC’s at RHIC (and elsewhere) can resolve this question. If they are two particles, they will likely have different absorption cross sections, and consequently, the mass spectrum from  -A interactions will have a different shape from studies at e+e- colliders. These excited states have production rates ~ 1% of the ‘ground’ states.

6. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 6 Interest in UPCs Interferometry The symmetric geometry in heavy ion collisions allows for interesting tests of quantum mechanics. STAR has already observed the interference between when nucleus 1 emits a photon which scatters from nucleus 2, emerging as a vector meson, and vice versa. This process requires a non-local wave function which retains amplitudes for all possible particle decays even after the decay occurs. Electrodynamics in Strong Fields The usual picture of two-photon interactions is that each beam particle emits a photon, and the photons collide. Since the coupling constant Z  ~0.6 at RHIC, this picture may fail. Extensive theoretical effort to study e+e- pair production in UPCs at RHIC; these studies have found relatively small changes in the total cross section. Preliminary results support these calculations, finding cross sections close to the lowest- order prediction. However, the additional photons are likely to make their presence felt in other ways, by changing the angular distributions, and in the production of multiple pairs.

7. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 7 Photo production of the  0 in UP AuAu Collisions at RHIC The p T and p-p invariant mass spectrum of two-track events collected in 200 GeV Au-Au collisions with the minimum bias trigger, from coherent photoproduction of ro and direct p-p pairs accompanied by mutual Coulomb excitation. The p T spectrum is peaked below 100 MeV/c, as expected for a photon from the electromagnetic field of one nucleus scattering coherently from the other nucleus, emerging as a vector meson.

8. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 8   photoproduction  Two-source interference   photoproduction in dA  Coherent production of 4-prong final states  Correlations in mutual Coulomb excitation Ongoing analyses in STAR

9. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 9

10. 10 200 GeV  0 dataset (Preliminary)   mass spectra show  0, interference with direct  production –Accompanied by mutual Coulomb excitation (MCE) [left] and 1n1n MCE [right]  Inv. mass 00 Inteference Term

11. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 11 Main Preliminary Results d  /dy for  photoproduction –With and without mutual excitation M  spectrum:  and direct  p T spectrum for  –Coherent + Incoherent production   helicity analysis –Helicity is conserved in  photoproduction Spectra & results for 1n,1n excitation d  /dy : Data + 3 theoretical predictions p T spectrum Blue – coherent Red- incoherent  (AuAu → AuAu  ) = 509  34  107 mb Consistent with theory prediction based on Glauber but not parton saturation model  inc /  coh = 0.29  0.03  0.03 Ratio of  to direct  similar to  HERA And independent of photon energy Preliminary

12. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 12  0 interference paper

13. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 13  0 interference paper Interference between  0 production on two target nuclei observed –Fit spectra to –R(t) = Int (t)/Noint(t) Int(t), Noint(t) are t-spectra with & without interference c = 1 – ‘full’ interference c = 0 – no interference –c =96 +/- 5 +/ ?% –Systematic error still under evaluation t ~ t  = p T 2 (GeV 2 ) Data Interference No interference Minbias data, |y|<0.5 Preliminary

14. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 14 dAu -->  analysis (Preliminary data) dAu --> dAu  & pn Au  –Photon mostly comes from Au –Breakup channel (pn) cleaner due to use of ZDC in trigger Measured d  /dy, d  /dp T, d  /dM  M  d  /dy d  /dt Preliminary

15. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 15 Au Au --> Au* Au* Nuclei may be electromagnetically excited to a Giant Dipole Resonance & decay by single neutron emission –Vector transition --> 1 — –Neutron p T follows photon (transverse) polarization – –b is the same for both nuclei Convolute two cosines   =  1 -  2 Nucl. Phys. A729, 787 (2003) (transverse view) n (East) n (West) 22 11

16. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 16 UPC trigger & rates in 2007 run Trigger on low multiplicity + ZDC coincidence As of June 6th – 1.74M UPC triggers in 228 hrs –Mean rate 2 Hz (after prescale) Expect ~ 2.2M UPC triggers If trigger yield (N  /N trigger ) is what we hope –based on best 2002 yields –~ 50,000  –600 4-prong (  ’) –Other good stuff

17. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 17 J/  trigger in 2007 run L0 trigger –Two EMC towers with E>~ 1.2 GeV Separated by at least 60 0 in  –Low multiplicity in CTB L2 not used for UPCs Rate: 2,380 triggers as of June 6th –Expect ~ 3,100 total Real Data, p+p Run V

18. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 18 4-prong analysis Preliminary data) Coherent 4-particle (Q=0) production seen in 2004 data Use 2007data for publication –Better understanding of trigger efficiency, statistics 4-particle combinations p T (GeV/c) 4  Mass (GeV) ~100 events Preliminary

19. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007 19 Outlook UPC’s in heavy ion collisions at RHIC and in the future at LHC provide a fertile field for fundamental QCD studies which avoid problems with other measurements In STAR, particularly when the DAQ1000 upgrade becomes operational, this physics should really “come into its own”. It is a great time to think about this problem theoretically and publish your work