Quantum Interference in r 0 Production in Ultra-peripheral Heavy Ion Collisions

1. Au* Au g r0 P Au g qq Au r0 Au Au 2+g g r0 Au P Au Au Au* Quantum Interference in r0 Production in Ultra-peripheral Heavy Ion Collisions S. Klein, LBNL, for the Collaboration Comparing data and Monte Carlo Vector Meson Production The electromagnetic fields of a relativistic nucleus act like a field of virtual photons. The photon spectrum is: For gold, Za ~ 0.6, so the photon flux is very high. The maximum photon energy gh/RA ~ 3 GeV (lab frame). At RHIC, this corresponds to a 600 GeV fixed target experiment; at the LHC, the fixed target equivalent is almost 1 PeV! The photons fluctuate into quark-antiquark pairs, with probability ~ a ~ 1/137. The qq pairs elastically scatter from the other nucleus and emerge as real vector mesons. Because of the high photon flux and coherent scattering, the r0 production cross section is large, ~ 600 mb with gold at RHIC, rising to 5.2 b at the LHC. Mpp (GeV) r0 y The rapidity and pp invariant mass spectra of the simulations match the data. This shows that the soft-Pomeron model does a good job of describing the photoproduction, and gives us confidence that the simulation output is useful for measuring the interference. Interferometry with short-lived particles Interference Results The reaction can occur two different ways Nucleus 1 can emit a photon which scatters from nucleus 2 Nucleus 2 can emit a photon which scatters from nucleus 1 These possibilities are indistinguishable, so the amplitudes add Vector Mesons are negative parity --> The amplitudes subtract: The cross section is Where the second expression holds at mid-rapidity The impact parameter b is unknown, so we must integrate over b; r0 production is suppressed for For exclusive r0 production with gold at RHIC , <b> ~ 46 fm. It is also possible to select other impact parameter ranges. The interference is seen most clearly in dN/dt where t ~ t = pT2 . The two production points are well separated in space-time gbct << <b> so r0 decay before the wave functions from the two sources can overlap The two decays are independent, and the r0 can decay differently, as at the right. But, only identical final states can interfere. How is interference possible? Either There is no interference Or p+ XnXn Exclusive r0 r0 Data (w/fit) Interference MC No-Interference MC Data (w/fit) Interference MC No-Interference MC p- 0.1 < |y| < 0.5 b p- p+ Data (w/fit) Interference MC No-Interference MC Data (w/fit) Interference MC No-Interference MC The post-decay wave functions must include amplitudes for all possible decays, long after the decay occurs. These all-component wave functions overlap, and collapse sometime after they overlap, likely when the decay products are detected in STAR. 0.5 < |y| < 1.0 Trigger and Event Selection t=pT2 (GeV/c) 2 t=pT2 (GeV/c) 2 Two triggers were used: Minimum Bias – At least 1 neutron in each zero degree calorimeter. This selects r0 with mutual Coulomb dissociation Topology Trigger. This uses the central trigger barrel, a scintillator slat array surrounding the STAR TPC. The trigger requires hits on opposite (horizontal) sides of the interaction region. The top and bottom are vetos, to reject cosmic rays. Dips are clearly evident at small t; the interference exists. The spectrum is fit to a is the normalization b is an exponential slope, with (roughly) b ~ 4RA2. c gives the degree of interference. R(t) = Interference(t)/Nointerference(t) is the ratio of the spectrum with interference, divided by that without. So, c=0 --> No interference c=1 --> Expected Interference For the 4 samples, c= 1.01±0.08, 0.78±0.13, 0.71±0.16 and 1.22 ±0.21. We estimate the experimental systematic errors as 8%, and the uncertainties in the calculations to be 15%. The 4 c values are all consistent ;we combine them to find c=0.93 ± 0.06 ± 0.08 ± 0.15. Impact Parameter Tagging Selecting events with mutual Coulomb dissociation selects events with smaller median impact parameter <b> Conclusions Exclusive r0 <b> ~ 46 fm r0 w/ mutual Excitation <b> ~ 19 fm The topology trigger A typical r0 event Short-lived particles can interfere, even after they decay. Particle decay does not induce wave function collapse. The analysis selected events with exactly two tracks in the TPC These two triggers should have very different interference spectra This different <b> lead to different interference spectra.