Probing the Nucleus with UltraPeripheral Collisions Spencer Klein

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 Ge. V/nucleon: Au + Au --> Au + r 0 production with nuclear excitation Direct + - production & interference A peek at 200 Ge. V/nucleon & beyond Conclusions

n b > 2 RA; Coherent Interactions no hadronic interactions u <b> ~ 25 -50 fermi at RHIC u n Ions are sources of fields u Au Z 2 Pomerons or mesons (mostly f 0) F u g, P, or meson photons F u Au A 2 (bulk) A 4/3 (surface) Coupling ~ nuclear form factor Fields couple coherently to ions Photon/Pomeron wavelength l = h/p> RA F amplitudes add with same phase F P < h/RA, ~30 Me. V/c for heavy ions F P|| < gh/RA ~ 3 Ge. V/c at RHIC F n Strong couplings --> large cross sections

Specific Channels n Vector meson production g. A -- > ’ r 0, w, f, J/y, … A Production cross sections --> s(VN) u Vector meson spectroscopy (r*, w*, f*, …) u Wave function collapse g u n VM Production occurs in/near one ion Electromagnetic particle production gg u > leptons, mesons Strong Field (nonperturbative? ) QED F u -Za ~ 0. 6 meson spectroscopy Ggg ~ charge content of scalar/tensor mesons F Ggg is small for glueballs F gs e+e-, qq, . . . Za ~ 0. 6; is Ng > 1?

Exclusive r 0 Production Au n n n One nucleus emits a photon r 0 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 s. Au at 200 Ge. V/nucleon u n n g qq 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 Au

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

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

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

Interaction Probabilities & ds/dy Excitation + r 0 changes b distribution u alters photon spectrum F r 0 with gold @ RHIC low <b> --> high <k> P(b) r 0 with Gold @ RHIC ds/dy n y b [fm] Exclusive - solid X 10 for Xn. Xn - dashed X 100 for 1 n 1 n - dotted Baltz, Klein & Nystrand (2002)

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

Interference n n n n 2 indistinguishable possibilities u Interference!! Similar to pp bremsstrahlung u no dipole moment, so u no dipole radiation 2 -source interferometer u separation b r, w, f, J/y are JPC = 1 - Amplitudes have opposite signs s ~ |A 1 - A 2 eip·b|2 b is unknown u For p. T << 1/<b> F No Interference y=0 destructive interference r 0 --> + - p. T (Ge. V/c)

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

0 r n Exclusive Channels u r 0 and nothing else F F n n Analysis 2 charged particles net charge 0 Coherent Coupling u Sp. T < 2 h/RA ~100 Me. V/c u back to back in transverse plane Backgrounds: u incoherent photonuclear interactions u grazing nuclear collisions u beam gas interactions

Exclusive n n (prototype) trigger on 2 roughly back-to -back tracks u 30, 000 events in ~ 9 hours 2 tracks in interaction region u reject cosmic rays peak for p. T < 150 Me. V/c + + and - - give background shape u + - pairs from higher multiplicity events have similar shape u scaled up by 2. 1 F n 0 r high p. T r 0 ? Signal region: p. T<0. 15 Ge. V Pre r 0 P T lim ina ry p. T<0. 15 Ge. V asymmetric M peak M( + -)

‘Minimum Bias’ Dataset n n n Trigger on neutron signals in both ZDCs ~800, 000 triggers Event selection same as peripheral + + and - - model background neutron spectrum has single (1 n) and multiple (Xn) neutron components u u u Coulomb excitation Xn may include hadronic interactions? Measure s(1 n 1 n) & s(Xn. Xn) r 0 Pre PT lim ina ry ZDC Energy (arbitrary units)

Direct p+ p- production + g g. A -- > + - A n + g. A -- > r 0 A -- > + - A phase shift at M(r 0) F changes n g - The two processes interfere u 1800 n r 0 + - lineshape good data with gp (HERA + fixed target) + - : r 0 ratio should depend on s( A): s(r. A) u decrease as A rises?

r 0 lineshape STAR g. Au --> (r 0 + + - )Au* ds/d. M (mb/Ge. V) ZEUS gp --> (r 0 + + - )p M e+e- and hadronic backgrounds Prel imin ar y M Fit to r 0 Breit-Wigner + + Interference is significant + - fraction is comparable to ZEUS

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

Cross Section Comparison imin n n ary Baltz, Klein & Nystrand (2002) Prel Normalized to 7. 2 b hadronic cross section Systematic uncertainties: luminosity, overlapping events, vertex & tracking simulations, single neutron selection, etc. Exclusive r 0 bootstrapped from Xn. Xn Good agreement u factorization works

A peek at the 2001 data n 200 Ge. V/nucleon u n n n higher luminosity ‘Production’ triggers Minimum Bias data: u n 10 X statistics Topology Data u n higher ss 50 X statistics Physics precision r 0 s and p. T spectra u s(e+e-) and theory comparison u 4 -prong events (r*(1450/1700)? ? ? ) u r 0 spectra - 25% of the min-bias data

Conclusions n n n RHIC is a high luminosity gg and g. A collider Coherent events have distinctive kinematics Photonuclear Interactions probe the nucleus u s(AA --> AAV) is sensitive to s(VA) F n STAR has observed three peripheral collisions processes u Au + Au -- > Au + r 0 u Au + Au -- > Au* + r 0 F n probes gluon density (shadowing) The r 0: direct + - is similar to g. A nteractions The r 0 cross sections agree with theoretical expectations