The structure of the photon and its interactions

The OPAL experiment at LEP has performed a variety of measurements of photon-photon and electron-photon scattering at the electron-positron collider LEP to gain a deeper insight into the structure of the photon and its interactions. This review presents a summary of these results.Comment: 18 pages with 20 eps files. Invited talk given at the Workshop on Photon Interactions and the Photon Structure, Lund, Sweden, September 10-13, 1998 to be published in the proceeding

Recent results of high-energy spin phenomena of gluons and sea-quarks in polarized proton-proton collisions at RHIC at BNL

The STAR experiment at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory is carrying out a spin physics program in high-energy polarized proton collisions at $\sqrt{s}=200\,$GeV and $\sqrt{s}=500\,$GeV to gain a deeper insight into the spin structure and dynamics of the proton. One of the main objectives of the spin physics program at RHIC is the precise determination of the polarized gluon distribution function. The STAR detector is well suited for the reconstruction of various final states involving jets, $\pi^{0}$, $\pi^{\pm}$, e$^{\pm}$ and $\gamma$, which allows to measure several different processes. Recent results suggest a gluon spin contribution to the proton spin at the same level as the quark spin contribution itself. The production of $W$ bosons in polarized p+p collisions at $\sqrt{s}=500\,$GeV opens a new era in the study of the spin-flavor structure of the proton. $W^{-(+)}$ bosons are produced in $\bar{u}+d\;(\bar{d}+u)$ collisions and can be detected through their leptonic decays, $e^{-}+\bar{\nu}_{e}\;(e^{+}+\nu_{e})$, where only the respective charged lepton is measured. Results of $W^{-(+)}$ production suggest a large asymmetry between the polarization of anti-$u$ and anti-$d$ quarks.Comment: 8 pages, 8 figures, Invited plenary talk at QCD Evolution Workshop (QCD2013), May 6-10, 2013, Thomas Jefferson National Accelerator Facility, Newport News, VA, US

Spin asymmetry at large x_F and k_T

We suggest that the large single spin asymmetries observed at high momentum fractions x_F and transverse momenta k_T of the pion in p^\uparrow p -> \pi(x_F,k_T)+X arise from the coherence of the soft interactions with the hard parton scattering process. Such coherence can be maintained if x_F -> 1 as k_T -> \infty, while k_T^2(1-x_F) ~ \Lambda_{QCD}^2 stays fixed. Analogous coherence effects have been seen experimentally in the Drell-Yan process at high x_F. We find that the p^\uparrow p -> \pi X production amplitudes have large dynamic phases and that helicity flip contributions are unsuppressed in this limit, giving rise to potentially large single spin asymmetries.Comment: 3 pages, 1 figure. Talk given at EPS HEP 2007, Mancheste

Research and Development of Commercially Manufactured Large GEM Foils

The recently completed Forward GEM Tracker (FGT) of the STAR experiment at RHIC took advantage of commercially produced GEM foils based on double-mask chemical etching techniques. With future experiments proposing detectors that utilize very large-area GEM foils, there is a need for commercially available GEM foils. Double-mask etching techniques pose a clear limitation in the maximum size. In contrast, single-mask techniques developed at CERN would allow one to overcome those limitations. We report on results obtained using 10 $\times$ 10 cm$^2$ and 40$\times$40 cm$^2$ GEM foils produced by Tech-Etch Inc. of Plymouth, MA, USA using single-mask techniques and thus the beginning for large GEM foil production on a commercial basis. A quality assurance procedure has been established through electrical and optical analyses via leakage current measurements and an automated high-resolution CCD scanner. The Tech-Etch foils show excellent electrical properties with leakage currents typically measured below 1 nA. The geometrical properties of the Tech-Etch single-mask foils were found to be consistent with one another, and were in line with geometrical specifications from previously measured double-mask foils. The single-mask foils displayed good inner and outer hole diameter uniformities over the entire active area.Comment: Presented at the 2014 IEEE conference in Seattle, W