Ultra-peripheral Collisions at RHIC Spencer Klein, LBNL for the STAR Collaboration Ultra-peripheral Collisions: What and Why Vector meson interferometry STAR Results: Au + Au --> Au + Au +  0  0 production with nuclear excitation Direct  +  - production & interference e + e - pairs 2001 Plans - STAR, PHENIX, PHOBOS Conclusions

Coherent Interactions n b > 2R A; u no hadronic interations u ~ 40 fermi n Ions are sources of fields u photons F Z 2 u Pomerons or mesons (mostly f 0 ) F A 2 (bulk) A 4/3 (surface) u Fields couple coherently to ions F P  < h/R A, ~30 MeV/c for heavy ions  P || <  h/R A ~ 3 GeV/c at RHIC u large cross sections n Enormous variety of reactions Au Coupling ~ nuclear form factor , P, or meson

Specific Channels n Vector meson production  A -- > ’   , , , J/ ,… A  Production cross sections -->  (VN)  Vector meson spectroscopy (  ,  ,  ,…) u Wave function collapse u Multiple vector meson production n Electromagnetic particle production   leptons,mesons u Strong Field QED  Z  ~ 0.6  meson spectroscopy      ~ charge content of scalar/tensor mesons F particles without charge (glueballs) won’t be seen n Mutual Coulomb excitation (GDR & higher) u Luminosity measurement, impact parameter tag Production occurs in/near one ion  VM e + e -, qq,... ss Z  ~ 0.6; is N  > 1?

Exclusive  0 Production n One nucleus emits a photon n The photon fluctuates to a qq pair n The pair scatters elastically from the other nucleus u also Photon- meson contribution n qq pair emerges as a vector meson  is large: 380 mb for  with Au at 130 GeV/nucleon u 5% of hadronic cross section 120 Hz  production rate at full RHIC energy/luminosity u 10% of hadronic cross section  , , ,  * rates > 5 Hz  J/  ’,  *,  *, all copiously produced,  a challenge Au 00  qq

Interference n 2 indistinguishable possibilities u Interference!! n Similar to pp bremsstrahlung u no dipole moment, so u no dipole radiation n 2-source interferometer u separation b , , , J/  are J PC = n Amplitudes have opposite signs  ~ |A 1 - A 2 e ip·b | 2 n b is unknown u For p T F destructive interference No Interference Interference         p T (GeV/c) y=0

Entangled Waveforms n VM are short lived u decay before traveling distance b n Decay points are separated in space-time F no interference u OR F the wave functions retain amplitudes for all possible decays, long after the decay occurs n Non-local wave function  non-factorizable:   +  -    +   - n Example of the Einstein-Podolsky-Rosen paradox e+e+ e-e- J/  ++ -- b (transverse view) ++

Multiple meson production P(  0 ) ~ 1% at b=2R A n w/ Poisson distribution  P(  0  0 ) ~ (1%) 2 /2 at b=2RA  ~ 10 6  0  0 /year n Identical state enhancement (ala HBT) for production from same ion (away from y=0)   p < h/R A u Like production coherence reqt. u Large fraction of pairs n Stimulated decays? b (fermi) P(b)

Looking further ahead….  A --> ccX, bbX, ttX (at LHC) u sensitive to gluon structure function u high rates u leptons/mass peak/separated vertex. u backgrounds from grazing hadronic ints. n Double-Pomeron interactions in AA u prolific glueball source u narrow range of b F small to moderate cross sections F harder p T spectrum u experimental signature? n Double-Pomeron interactions in pp u p T is different for qq mesons vs. exotica? u Tag pomeron momenta with Roman pots Production occurs in one ion QQ--> open charm Transverse view: a narrow range of impact parameters  g glueballs

Triggering Strategies n Ultra-peripheral final states u 1-6 charged particles u low p T u rapidity gaps n small b events u ‘tagged’ by nuclear breakup

Nuclear Excitation n Nuclear excitation ‘tag’s small b n Multiple Interactions probable  P(  0, b=2R) ~ 1% u P(2EXC, b=2R) ~ 30% n Factorization appears valid   EXC  F useful for triggering u select small b for interference studies n Au* decay via neutron emission Au  P Au*  (1+) 00 Preliminary

Experimental Studies 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 Nuclear breakup possible n Backgrounds: u incoherent photonuclear interactions u grazing nuclear collisions u beam gas

Peripheral Trigger & Data Collection n Level 0 - prototype, circa 2000 u hits in opposing CTB quadrants u Rate /sec n Level 3 (online reconstruction) u vertex position, multiplicity u Rate 1-2 /sec n 7 hours of data ~ 30,000 events n Backgrounds u cosmic rays u beam gas u upstream interactions u out-of-time events

Exclusive  0 n 2 track vertex u vertex in diamond  non-coplanar,  < 3 rad F reject cosmic rays track dE/dx consistent with  n No neutrons in ZDC n peak for p T < 100 MeV/c asymmetric M  peak     and      model background u matches pairs from higher multiplicity events u scaled up by 2.1 u flat/phase space in p T  mostly at low M(     )  0 P T M(     ) Preliminary

‘Minimum Bias’ Dataset n Trigger on >1 neutron signal in both zero degree calorimeters n ~800,000 triggers n Event selection same as peripheral u but no ZDC cuts     and      model background u phase space in p T u small  0 P T M(     ) Preliminary

Direct  +  - production Direct  +  - is independent of energy n The two processes interfere  phase shift at M(  0 )  changes  +  - lineshape good data with  p (HERA + fixed target)  +  - fraction should decrease as A rises --  ++ --  A -- >  0 A -- >  +  - A  ++  A -- >  +  - A 00

 0 lineshape Data Fit 00 +-+- Fit all data to  0 +  +  - interference is significant  +  - fraction is high (background?) ZEUS  p --> (  0 +  +  - )p Set =0 for STAR STAR  Au --> (  0 +  +  - )Au Preliminary

P (GeV/c) A peek at  --> e + e - n ‘Minimum bias dataset n Select electrons by dE/dx u in region p< 140 MeV/c n Select identified pairs n p T peaked at 1/ Events P t (GeVc) dE/dx (keV/cm) Blue - all particles red - e + e - pairs e  Preliminary

2001 n RHIC - higher luminosity, energy, longer run u larger cross sections n STAR u improved triggering F trigger in parallel with central collisions program F X ‘topological’ data u expanded solid angle (FTPC, MWC,…) u 10X minimum bias data n PHOBOS u “interest in pursuing it, but will require major trigger reconfiguration and enhancements before we can get serious data”

Coherent Peripheral Collisions in PHENIX Study coherent two-photon and photonuclear interactions in coincidence with mutual Coulomb excitation of the nuclei. Two central tracking arms |  |  0.35,  = 2  90° Global detectors (used for triggering): Zero-Degree Calorimeters, Beam-Beam Counters Trigger: Level 1 – ZDC coincidence, BBC veto Level 2 – 1-5 tracks (hits) in each tracking arm Goals for 2001: Observe  0 production in the reaction Au+Au  Au * +Au * +  0 ;  0   +  -, A total integrated Au+Au luminosity of 300  b -1 should give a sample of about 20,000  0 ’s, Study the factorization of  0 production and Coulomb excitation, Try to see the 2-source interference in  0 production.

Physics Recap RHIC is a high luminosity  and  A collider n Coherent events have distinctive kinematics n Ultra-peripheral collisions allow for many studies  Measurement of  (VA) u Vector meson spectroscopy  A measurement of the  0 p T spectrum can test if particle decay triggers wave function collapse u multiple vector meson production u Strong field QED tests u Studies of charge content of glueball candidates

Conclusions n STAR has observed three peripheral collisions processes  Au + Au -- > Au + Au +  0  Au + Au -- > Au* + Au* +  0 u Au + Au -- > Au* + Au* + e + e - Interference between  0 and direct     is seen n Next year: higher energy, more luminosity, more data, more experiments, more detector subsystems,... u More channels, cross sections, p T spectra,... n Ultra-peripheral collisions is in it’s infancy u It works!!! u come back next year!