Ultra-peripheral Collisions at RHIC Spencer Klein, LBNL for the STAR Collaboration

1. Ultra-peripheral Collisions at RHICSpencer Klein, LBNLfor the STAR Collaboration Ultra-peripheral Collisions: What and Why Vector meson interferometry STAR Results: Au + Au --> Au + Au + r0 r0 production with nuclear excitation Direct p+p- production & interference e+e- pairs 2001 Plans - STAR, PHENIX, PHOBOS Conclusions

2. Coherent Interactions Au • b > 2RA; • no hadronic interations • <b> ~ 40 fermi • Ions are sources of fields • photons • Z2 • Pomerons or mesons (mostly f0) • A 2 (bulk)A 4/3 (surface) • Fields couple coherently to ions • P < h/RA, ~30 MeV/c for heavy ions • P|| < gh/RA ~ 3 GeV/c at RHIC • large cross sections • Enormous variety of reactions g, P, or meson Au Coupling ~ nuclear form factor

3. VM g Production occurs in/near one ion Specific Channels • Vector meson production gA -- > ’ r0, w, f, J/y,… A • Production cross sections --> s(VN) • Vector meson spectroscopy (r*, w*, f*,…) • Wave function collapse • Multiple vector meson production • Electromagnetic particle production gg -- > leptons,mesons • Strong Field QED • Za ~ 0.6 • meson spectroscopy Ggg • Ggg ~ charge content of scalar/tensor mesons • particles without charge (glueballs) won’t be seen • Mutual Coulomb excitation (GDR & higher) • Luminosity measurement, impact parameter tag e+e-, qq,... gs Za ~ 0.6; is Ng > 1?

4. Exclusive r0 Production Au g qq Au • One nucleus emits a photon • The photon fluctuates to a qq pair • The pair scatters elastically from the other nucleus • also Photon- meson contribution • qq pair emerges as a vector meson • s is large: 380 mb for r with Au at 130 GeV/nucleon • 5% of hadronic cross section • 120 Hz r production rate at full RHIC energy/luminosity • 10% of hadronic cross section • r, w, f, r* rates > 5 Hz • J/y , Y’, f*, w*, all copiously produced, U a challenge r0

5. Interference • 2 indistinguishable possibilities • Interference!! • Similar to pp bremsstrahlung • no dipole moment, so • no dipole radiation • 2-source interferometer • separation b • r,w, f, J/y are JPC = 1- - • Amplitudes have opposite signs • s ~ |A1 - A2eip·b|2 • b is unknown • For pT << 1/<b> • destructive interference No Interference Interference y=0 r0 -->p+p- pT (GeV/c)

6. Entangled Waveforms e+ J/Y • VM are short lived • decay before traveling distance b • Decay points are separated in space-time • 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- • Example of the Einstein-Podolsky-Rosen paradox e- + b J/Y + - (transverse view)

7. Multiple meson production • P(r0) ~ 1% at b=2RA • w/ Poisson distribution • P(r0r0) ~ (1%)2/2 at b=2RA • ~ 106r0r0 /year • Identical state enhancement (ala HBT) for production from same ion (away from y=0) • Dp < h/RA • Like production coherence reqt. • Large fraction of pairs • Stimulated decays? P(b) b (fermi)

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

9. Triggering Strategies • Ultra-peripheral final states • 1-6 charged particles • low pT • rapidity gaps • small b events • ‘tagged’ by nuclear breakup

10. Au* Au g g(1+) r0 P Au Au* Nuclear Excitation • Nuclear excitation ‘tag’s small b • Multiple Interactions probable • P(r0, b=2R) ~ 1% • P(2EXC, b=2R) ~ 30% • Factorization appears valid • s(2EXC +r) ~ 1/8 s(r) • useful for triggering • select small b for interference studies • Au* decay via neutron emission Preliminary

13. Experimental Studies • 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 • Nuclear breakup possible • Backgrounds: • incoherent photonuclear interactions • grazing nuclear collisions • beam gas

14. Peripheral Trigger & Data Collection • Level 0 - prototype, circa 2000 • hits in opposing CTB quadrants • Rate 20-40 /sec • Level 3 (online reconstruction) • vertex position, multiplicity • Rate 1-2 /sec • 7 hours of data ~ 30,000 events • Backgrounds • cosmic rays • beam gas • upstream interactions • out-of-time events

15. Exclusive r0 r0 PT • 2 track vertex • vertex in diamond • non-coplanar, q < 3 rad • reject cosmic rays • track dE/dx consistent with p • No neutrons in ZDC • peak for pT < 100 MeV/c • asymmetric Mpp peak • p+p+ andp-p- model background • matches pairs from higher multiplicity events • scaled up by 2.1 • flat/phase space in pT • mostly at low M(p+p-) Preliminary M(p+p-)

16. ‘Minimum Bias’ Dataset r0 PT • Trigger on >1 neutron signal in both zero degree calorimeters • ~800,000 triggers • Event selection same as peripheral • but no ZDC cuts • p+p+ andp-p- model background • phase space in pT • small Preliminary M(p+p-)

17. p- p- r0 p+ p+ g g gA -- > p+p- A gA -- > r0A -- > p+p- A Direct p+p- production • Direct p+p- is independent of energy • The two processes interfere • 1800 phase shift at M(r0) • changes p+p- lineshape • good data with gp (HERA + fixed target) • p+p- fraction should decrease as A rises

18. Fit Data r0 p+p- r0 lineshape ZEUS gp --> (r0 + p+p- )p STAR gAu --> (r0 + p+p- )Au Preliminary Fit all data to r0 + p+p- interference is significant p+p- fraction is high (background?) Set =0 for STAR

19. A peek at gg --> e+e- Blue - all particles red - e+ e- pairs p dE/dx (keV/cm) • ‘Minimum bias dataset • Select electrons by dE/dx • in region p< 140 MeV/c • Select identified pairs • pT peaked at 1/<b> e Preliminary P (GeV/c) Events Pt (GeVc)

20. 2001 • RHIC - higher luminosity, energy, longer run • larger cross sections • STAR • improved triggering • trigger in parallel with central collisions program • 10-100X ‘topological’ data • expanded solid angle (FTPC, MWC,…) • 10X minimum bias data • PHOBOS • “interest in pursuing it, but will require major trigger reconfiguration and enhancements before we can get serious data”

21. 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.

22. Physics Recap • RHIC is a high luminosity gg and gA collider • Coherent events have distinctive kinematics • Ultra-peripheral collisions allow for many studies • Measurement of s(VA) • Vector meson spectroscopy • A measurement of the r0 pT spectrum can test if particle decay triggers wave function collapse • multiple vector meson production • Strong field QED tests • Studies of charge content of glueball candidates

23. Conclusions • STAR has observed three peripheral collisions processes • Au + Au -- > Au + Au + r0 • Au + Au -- > Au* + Au* + r0 • Au + Au -- > Au* + Au* + e+e- • Interference between r0 and direct p+p- is seen • Next year: higher energy, more luminosity, more data, more experiments, more detector subsystems, ... • More channels, cross sections, pT spectra,... • Ultra-peripheral collisions is in it’s infancy • It works!!! • come back next year!