1 / 22

Probing the Nucleus with Ultra-Peripheral Collisions

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 + r 0 r 0 production with nuclear excitation

lamond
Télécharger la présentation

Probing the Nucleus with Ultra-Peripheral Collisions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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)

  12. 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

  13. 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-)

  14. ‘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)

  15. 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?

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

More Related