Probing the Nucleus with Ultra-Peripheral Collisions

1. Probing the Nucleus with Ultra-Peripheral Collisions Spencer Klein, LBNL (for the STAR Collaboration) Ultra-peripheral Collisions: What and Why Photoproduction as a nuclear probe STAR Results at 130 GeV/nucleon: Au + Au --> Au + Au + r0 r0 production with nuclear excitation Direct p+p- production & interference A peek at 200 GeV/nucleon & beyond Conclusions

2. Coherent Interactions Au • b > 2RA; • no hadronic interactions • <b> ~ 25-50 fermi at RHIC • Ions are sources of fields • photons • Z2 • Pomerons or mesons (mostly f0) • A 2 (bulk)A 4/3 (surface) • Fields couple coherently to ions • Photon/Pomeron wavelength l = h/p> RA • amplitudes add with same phase • P < h/RA, ~30 MeV/c for heavy ions • P|| < gh/RA ~ 3 GeV/c at RHIC • Strong couplings --> large cross sections 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 • Electromagnetic particle production gg -- > leptons,mesons • Strong Field (nonperturbative?) QED • Za ~ 0.6 • meson spectroscopy Ggg • Ggg ~ charge content of scalar/tensor mesons • Ggg is small for glueballs 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 • qq pair emerges as a vector meson • s(r) ~ 590 mb ~ 8 % of sAuAu at 200 GeV/nucleon • 120 Hz production rate at RHIC design luminosity • r, w, f, r* rates at RHIC all > 5 Hz • J/y , Y’, f*, w*, copiously produced, U a challenge r0

5. RHIC - Au HERA data + Glauber Klein & Nystrand, 1999 HERA param. Elastic Scattering with Soft Pomerons • Glauber Calculation • parameterized HERA data • Pomeron + meson exchange • all nucleons are the same • s ~ A2 (weak scatter limit) • All nucleons participate • J/y • s ~ A 4/3 (strong scatter limit) • Surface nucleons participate • Interior cancels (interferes) out • s ~ A 5/3 (r0) • depends on s(Vp) • sensitive to shadowing? Y = 1/2 ln(2k/MV)

6. Elastic Scattering with Hard Pomerons RHIC - Au • Valid for cc or bb • ds/dy & s depend on gluon distributions • shadowing reduces mid-rapidity ds/dy • Effect grows with energy • s reduced ~ 50% at the LHC • colored glass condensates may have even bigger effect No shadowing HERA param. ds/dy Leading Twist Calculation Frankfurt, Strikman & Zhalov, 2001 Shadowed Y = 1/2 ln(2k/MV)

7. Au* Au g g(1+) r0 P Au Au* Nuclear Excitation • Nuclear excitation ‘tag’s small b • Multiple photon exchange • Mutual excitation • Au* decay via neutron emission • simple, unbiased trigger • Multiple Interactions probable • P(r0, b=2R) ~ 1% at RHIC • P(2EXC, b=2R) ~ 30% • Non-factorizable diagrams are small for AA

8. r0 with Gold @ RHIC P(b) b [fm] Interaction Probabilities & ds/dy r0 with gold @ RHIC • Excitation + r0 changes b distribution • alters photon spectrum • low <b> --> high <k> ds/dy y Exclusive - solid X10 for XnXn - dashed X100 for 1n1n - dotted Baltz, Klein & Nystrand (2002)

9. Photoproduction of Open Quarks QQ--> open charm g • gA --> ccX, bbX • sensitive to gluon structure function. • Higher order corrections problematic • Ratio s(gA)/s(gp) --> shadowing • removes most QCD uncertainties • Experimentally feasible (?) • high rates • known isolation techniques • Physics backgrounds are gg--> cc, gg --> cc • gg cross section is small • gg background appears controllableby requiring a rapidity gap g Production occurs in one ion

10. 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)

11. 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)

14. r0 Analysis • 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 • Backgrounds: • incoherent photonuclear interactions • grazing nuclear collisions • beam gas interactions

15. Exclusive r0 Signal region: pT<0.15 GeV • (prototype) trigger on 2 roughly back-to-back tracks • 30,000 events in ~ 9 hours • 2 tracks in interaction region • reject cosmic rays • peak for pT < 150 MeV/c • p+p+ andp-p- give background shape • p+p- pairs from higher multiplicity events have similar shape • scaled up by 2.1 • high pTr0 ? • asymmetric Mpp peak Preliminary r0 PT pT<0.15 GeV M(p+p-)

16. ‘Minimum Bias’ Dataset • Trigger on neutron signals in both ZDCs • ~800,000 triggers • Event selection same as peripheral • p+p+ andp-p- model background • neutron spectrum has single (1n) and multiple (Xn) neutron components • Coulomb excitation • Xn may include hadronic interactions? • Measure s(1n1n) & s(XnXn) Preliminary r0 PT ZDC Energy (arbitrary units)

17. p- p- r0 p+ p+ g g gA -- > p+p- A gA -- > r0A -- > p+p- A Direct p+p- production • The two processes interfere • 1800 phase shift at M(r0) • changes p+p- lineshape • good data with gp (HERA + fixed target) • p+p- : r0 ratio should depend on s(pA):s(rA) • decrease as A rises?

18. r0 lineshape ZEUS gp --> (r0 + p+p- )p STAR gAu --> (r0 + p+p- )Au* ds/dMpp (mb/GeV) ds/dMpp (mb/GeV) Preliminary Mpp Mpp Fit to r0 Breit-Wigner + p+p- Interference is significant p+p- fraction is comparable to ZEUS e+e- and hadronic backgrounds

19. Nucl.Breakup dN/dy for r0(XnXn) Soft Pomeron, no-shadowing, XnXn • r ds/dy are different with and without breakup • XnXn data matches simulation • Extrapolate to insensitive region After detector simulation

20. Cross Section Comparison Baltz, Klein & Nystrand (2002) Preliminary • Normalized to 7.2 b hadronic cross section • Systematic uncertainties: luminosity, overlapping events, vertex & tracking simulations, single neutron selection, etc. • Exclusive r0 bootstrapped from XnXn • Good agreement • factorization works

21. A peek at the 2001 data • 200 GeV/nucleon • higher ss • higher luminosity • ‘Production’ triggers • Minimum Bias data: • 10X statistics • Topology Data • 50X statistics • Physics • precision r0s and pT spectra • s(e+e-) and theory comparison • 4-prong events (r*(1450/1700)???) r0 spectra - 25% of the min-bias data

22. Conclusions • RHIC is a high luminosity gg and gA collider • Coherent events have distinctive kinematics • Photonuclear Interactions probe the nucleus • s(AA --> AAV) is sensitive to s(VA) • probes gluon density (shadowing) • STAR has observed three peripheral collisions processes • Au + Au -- > Au + Au + r0 • Au + Au -- > Au* + Au* + r0 • The r0:direct p+p- is similar to gA nteractions • The r0 cross sections agree with theoretical expectations