Physics of Ultra-Peripheral Nuclear Collisions

Moving highly-charged ions carry strong electromagnetic fields that act as a field of photons. In collisions at large impact parameters, hadronic interactions are not possible, and the ions interact through photon-ion and photon-photon collisions known as {\it ultra-peripheral collisions} (UPC). Hadron colliders like the Relativistic Heavy Ion Collider (RHIC), the Tevatron and the Large Hadron Collider (LHC) produce photonuclear and two-photon interactions at luminosities and energies beyond that accessible elsewhere; the LHC will reach a $\gamma p$ energy ten times that of the Hadron-Electron Ring Accelerator (HERA). Reactions as diverse as the production of anti-hydrogen, photoproduction of the $\rho^0$, transmutation of lead into bismuth and excitation of collective nuclear resonances have already been studied. At the LHC, UPCs can study many types of `new physics.'Comment: 47 pages, to appear in Annual Review of Nuclear and Particle Scienc

A consistent treatment for pion form factors in space-like and time-like regions

We write down some relevant matrix elements for the scattering and decay processes of the pion by considering a quark-meson vertex function. The pion charge and transition form factors $F_\pi$, $F_{\pi\gamma}$, and $F_{\pi\gamma^*}$ are extracted from these matrix elements using a relativistic quark model on the light-front. We found that, the form factors $F_\pi$ and $F_{\pi\gamma}$ in the space-like region agree well with experiment. Furthermore, the branching ratios of all observed decay modes of the neutral pion, that are related to the form factors $F_{\pi\gamma}$ and $F_{\pi\gamma^*}$ in the time-like region, are all consistent with the data as well. Additionally, $F_\pi$ in the time-like region, which deals with the nonvalence contribution, is also discussed.Comment: 24 pages, 6 figures, to appear in Phys. Rev.

Observation of πK-atoms with DIRAC-II

We present evidence for the first observation of electromagnetically bound π ± K  ∓ -pairs (πK-atoms) with the DIRAC-II experiment at the CERN-PS. The πK-atoms are produced by the 24 GeV/c proton beam in a thin Pt-target and the dissociated π ± and K  ∓ -mesons analyzed in a two-arm magnetic spectrometer. The observed enhancement at low relative momentum corresponds to the production of 173±54 πK-atoms. The mean life of πK-atoms is related to the s-wave πK-scattering lengths. From these first data we derive a lower limit for the mean life of 1.5 fs at 84% confidence level