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Saturation, Low-x, and QCD at RHIC, Part 3

Saturation, Low-x, and QCD at RHIC, Part 3. T. Hallman Villaggio Guglielmo, Italy. Ultra-peripheral relativistic heavy ion collisions: a unique testing ground for QCD. The interest in UPC’s. What are Ultra-Peripheral Collisions:

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Saturation, Low-x, and QCD at RHIC, Part 3

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  1. Saturation, Low-x, and QCDat RHIC, Part 3 T. Hallman Villaggio Guglielmo, Italy 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 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 Z2 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: 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%. 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 r0 production rate with gold beams is 120 Hz! Although the ‘ground state’ r0, w, and f mesons are well understood, their excited states are not. For example, the particle data group points out that the r(1450) and r(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 g-A interactions will have a different shape from studies at e+e- colliders. These excited states have production rates ~ 1% of the ‘ground’ states. 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 Za ~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. 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 pT 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 pT 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. TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007

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

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

  10. 200 GeV r0 dataset (Preliminary) r0 • ppmass spectra showr0, interference with directppproduction • Accompanied by mutual Coulomb excitation (MCE) [left] and 1n1n MCE [right] Inteference Term ppInv. mass ppInv. mass TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007

  11. ds/dy: Data + 3 theoretical predictions Main Preliminary Results • ds/dy for r photoproduction • With and without mutual excitation • Mpp spectrum: r and direct pp • pT spectrum for r • Coherent + Incoherent production • r helicity analysis • Helicity is conserved in r photoproduction • Spectra & results for 1n,1n excitation Preliminary 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 Preliminary Ratio of r to direct pp similar toHERA And independent of photon energy pT spectrum TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007

  12. r0 interference paper TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007

  13. r0interference paper • Interference between r0 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 Minbias data, |y|<0.5 Preliminary Data Interference No interference t ~ t = pT2 (GeV2) TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007

  14. dAu --> r analysis (Preliminary data) Mpp • dAu --> dAu r & pn Au r • Photon mostly comes from Au • Breakup channel (pn) cleaner due to use of ZDC in trigger • Measured ds/dy, ds/dpT, ds/dMpp Preliminary ds/dy ds/dt Preliminary Preliminary TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007

  15. f2 Au Au --> Au* Au* n (East) • Nuclei may be electromagnetically excited to a Giant Dipole Resonance & decay by single neutron emission • Vector transition --> 1— • Neutron pT follows photon (transverse) polarization • b is the same for both nuclei • Convolute two cosines • Df = f1- f2 f1 n (West) (transverse view) Nucl. Phys. A729, 787 (2003) 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 (Nr/Ntrigger) is what we hope • based on best 2002 yields • ~ 50,000r • 600 4-prong (r’) • Other good stuff TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007

  17. Real Data, p+p Run V J/ytrigger in 2007 run • L0 trigger • Two EMC towers with E>~ 1.2 GeV • Separated by at least 600 in f • Low multiplicity in CTB • L2 not used for UPCs • Rate: 2,380 triggers as of June 6th • Expect ~ 3,100 total 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 Preliminary Preliminary ~100 events pT (GeV/c) 4pMass (GeV) 4-particle combinations 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 TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 2007

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