Ultra-Peripheral Collisions at RHIC What are ultra-peripheral collisions? Physics Interest STAR Results at 130 GeV/nucleon: Au + Au --> Au + Au +  0  0 production with nuclear excitation e + e - pair production A peek at 200 GeV/nucleon & beyond Conclusions Spencer Klein, LBNL (for the STAR Collaboration)

Coherent Interactions n b > 2R A; u no hadronic interactions u ~ fermi at RHIC 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  Photon/Pomeron wavelength = h/p> R A F amplitudes add with same phase F P  < h/R A, ~30 MeV/c for heavy ions  P || <  h/R A ~ 3 GeV/c at RHIC n Strong couplings --> large cross sections Au Coupling ~ nuclear form factor , P, or meson

Unique Features of Ultra- peripheral collisions n Very strong electromagnetic fields   --> e + e - and  --> qq u Multiple production u Coherent Decays n Unique Geometry u 2-source interferometer n Nuclear Environment u Particle Production with capture  Large  for e -

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 & Decay n Electromagnetic particle production   leptons,mesons u Strong Field (nonperturbative?) QED  Z  ~ 0.6  meson spectroscopy      ~ charge content of scalar/tensor mesons    is small for glueballs 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 u Weizsacker-Williams flux n The photon fluctuates to a qq pair n The pair scatters elastically from the other nucleus n qq pair emerges as a vector meson  ~ 590 mb; 8 % of  AuAu (had.) at 200 GeV/nucleon u 120 Hz production rate at RHIC design luminosity  RHIC , , ,  * rates all > 5 Hz  J/  ’,  *,  *, copiously produced,  a challenge n The LHC is a vector meson factory   ~ 5.2 b; 65% of  PbPb (had.)  230 kHz  0 production rate with calcium Au 00  qq

Elastic Scattering with Soft Pomerons n Glauber Calculation u parameterized HERA data F Pomeron + meson exchange F all nucleons are the same   ~ A 2 (weak scatter limit) F All nucleons participate  J/    ~ A 4/3 (strong scatter limit) F Surface nucleons participate F Interior cancels (interferes) out   ~ A 5/3 (  0 ) depends on  (Vp) u sensitive to shadowing? Y = 1/2 ln(2k/M V ) RHIC - Au HERA data + Glauber Klein & Nystrand, 1999 HERA param.

Shadowing & Hard Pomerons n If pomerons are 2-gluon ladders u P shadowing ~ (gluon shadowing) 2 n Valid for cc or bb d  /dy &  depend on gluon distributions reduces mid-rapidity d  /dy u Suppression grows with energy   reduced ~ 50% at the LHC n colored glass condensates may have even bigger effect u high density phase of gluonic matter Y = 1/2 ln(2k/M V ) RHIC - Au Leading Twist Calculation Frankfurt, Strikman & Zhalov, 2001 No shadowing Shadowed HERA param. d  /dy

Nuclear Excitation n Nuclear excitation ‘tag’s small b n Multiple photon exchange u Mutual excitation n Au* decay via neutron emission u simple, unbiased trigger n Multiple Interactions probable  P(  0, b=2R) ~ 1% at RHIC u P(2EXC, b=2R) ~ 30% n Non-factorizable diagrams are small for AA Au  P Au*  (1+) 00

Interaction Probabilities & d  /dy Excitation +  0 changes b distribution u alters photon spectrum F low --> high Baltz, Klein & Nystrand (2002) b [fm]  0 with RHIC P(b) Exclusive - solid X10 for XnXn - dashed X100 for 1n1n - dotted  0 with RHIC d  /dy y

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

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) ++

Interference and Nuclear Excitation n Smaller --> interference at higher  0 prod at RHICJ/  prod at the LHC Solid - no breakup criteria Dashed lines 1n1n and XnXn breakup

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 Enhancement (ala HBT) for production from same ion (away from y=0) u Vector meson superradiance F toward a vector meson laser   p < h/R A u Like production coherence u Large fraction of pairs n Stimulated decays? P(b) b (fermi) Production with gold at RHIC

Electron Production w/ Capture   e + e - u Electron is bound to nucleus u Probe of atomic physics u non-perturbative   uncertain, ~ barns n Focused +78 Au beam n RHIC Rate ~ 10,000 particles/sec u beam ~ mW n LHC rate ~ 1M particles/sec u beam ~ W u can quench superconducting magnets F limits LHC luminosity w/ Pb n Could extract as external beam Z  ~ 0.6; is N  > 1? ss e-e- e+e+

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

 0 Analysis 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 Backgrounds: u incoherent photonuclear interactions u grazing nuclear collisions u beam gas interactions

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

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 9 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 tracks in interaction region u vertex in diamond u reject cosmic rays  non-coplanar;  < 3 rad n peak for p T < 150 MeV/c     and      give background shape      pairs from higher multiplicity events have similar shape u scaled up by 2.1  high p T  0 ? asymmetric M  peak M(     ) p T <0.15 GeV  0 P T Signal region: p T <0.15 GeV

‘Minimum Bias’ Dataset n Trigger on neutron signals in both ZDCs n ~800,000 triggers n Event selection same as peripheral     and      model background n single (1n) and multiple (Xn) neutron production u Coulomb excitation F Giant Dipole Resonance u Xn may include hadronic interactions?  Measure  (1n1n) &  (XnXn)  0 P T ZDC Energy (arbitrary units) Signal region: p T <0.15 GeV

Direct  +  - production n The two processes interfere  phase shift at M(  0 )  changes  +  - lineshape good data with  p (HERA + fixed target)  +  - :   0 ratio should depend on  (  ):  (  A) u decrease as A rises? --  ++ --  A -- >  0 A -- >  +  - A  ++  A -- >  +  - A 00

 0 lineshape Fit to  0 Breit-Wigner +  +  - Interference is significant  +  - fraction is comparable to ZEUS ZEUS  p --> (  0 +  +  - )p e + e - and hadronic backgrounds STAR  Au --> (  0 +  +  - )Au* Preliminary M  d  /dM   mb/GeV  d  /dM   b/GeV  M 

dN/dy for  0 (XnXn)  d  /dy are different with and without breakup n XnXn data matches simulation n Extrapolate to insensitive region Nucl.Breakup Soft Pomeron, no-shadowing, XnXn After detector simulation

Cross Section Comparison n Normalized to 7.2 b hadronic cross section n Systematic uncertainties: luminosity, overlapping events, vertex & tracking simulations, single neutron selection, etc. Exclusive  0 bootstrapped from XnXn u limited by statistics for XnXn in topology trigger n Good agreement u factorization works Baltz, Klein & Nystrand (2002)

P (GeV/c) A look 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

A peek at the 2001 data n 200 GeV/nucleon  higher  s u higher luminosity n ‘Production’ triggers n Minimum Bias data: u 10X statistics n Topology Data u ~20X statistics n Physics  precision  0  and p T spectra   --> f 2 (1270)   (e + e - ) and theory comparison  4-prong events (  *(1450/1700)?)  0 spectra - 25% of the min-bias data

Physics Recap RHIC is a high luminosity  and  A collider n The strong fields and 2-source geometry allow many unique studies 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 Tests of strong field QED 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 - The cross sections for  0 production are in agreement with theoretical models Interference between  0 and direct     is seen n Next year: higher energy, more luminosity, more data, more detector subsystems,... n Ultra-peripheral collisions is in it’s infancy  STAR  0 paper out in ~ 1 week u It works!!! u come back next year!