47 research outputs found
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
Vector-Meson Electroproduction from Generalized Vector Dominance
Including destructively interfering off-diagonal transitions of
diffraction-dissociation type, we arrive at a formulation of GVD for exclusive
vector-meson production in terms of a continuous spectral representation of
dipole form. The transverse cross-section sigma_T for gamma* p -> V p behaves
asymptotically as 1/Q^4, while R_V = sigma_L/sigma_T becomes asymptotically
constant. Contributions violating s-channel helicity conservation stay at the
10-15% level established in low-energy photoproduction and diffractive
hadron--hadron interactions. The data for phi- and rho-meson production for 0
\lsim Q^2 \lsim 20 GeV^2 from HERA are found to be in agreement with these
predictions.Comment: 16 pages, latex, 3 figure
Triple GEM Detectors for the Forward Tracker in STAR
Future measurements of the flavor-separated spin structure of the proton via
parity-violating W boson production at RHIC require an upgrade of the forward
tracking system of the STAR detector. This upgrade will allow the
reconstruction of the charge sign of electrons and positrons produced from
decaying W bosons. A design based on six large area triple GEM disks using GEM
foils produced by Tech-Etch Inc. has emerged as a cost-effective solution to
provide the necessary tracking precision. We report first results from a beam
test of three test detectors using Tech-Etch produced GEM foils and a laser
etched two dimensional strip readout. The detectors show good operational
stability, high efficiency and a spacial resolution of around 70 um or better,
exceeding the requirements for the forward tracking upgrade. The influence of
the angle of incidence of the particles on the spatial resolution of the
detectors has also been studied in detail.Comment: 5 pages, 8 figures, presented at the IEEE Nuclear Science Symposium
in Honolulu, HI, USA, October 27 - November 3, 200
Recent STAR results in high-energy polarized proton-proton collisions at RHIC
The STAR experiment at the Relativistic Heavy-Ion Collider at Brookhaven
National Laboratory is carrying out a spin physics program in high-energy
polarized collisions at 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
extraction of the polarized gluon distribution function based on measurements
of gluon initiated processes, such as hadron and jet production. The STAR
detector is well suited for the reconstruction of various final states
involving jets, , , e and , which allows to
measure several different processes. Recent results will be shown on the
measurement of jet production and hadron production at GeV. The
RHIC spin physics program has recently completed the first data taking period
in 2009 of polarized collisions at GeV. This
opens a new era in the study of the spin-flavor structure of the proton based
on the production of bosons. Recent STAR results on the first
measurement of boson production in polarized collisions
will be shown.Comment: 10 pages, 9 figures, Talk presented at the 26th Winter Workshop on
Nuclear Dynamics, Ocho Rios, Jamaica, January 2-9, 2010 to be published in
Journal of Physics: Conference Series (JPCS) The author may be contacted via:
[email protected]
Low-x scaling in total cross sections
We show that the experimental data for the total virtual-photon proton cross
section, \sigma_{\gamma^*p} (W^2, Q^2), for x_{bj} \lsim 0.1 lie on a universal
curve, when plotted against \eta = (Q^2 + m^2_0)/ \Lambda^2(W^2), where
\Lambda^2 (W^2) = C_1 (W^2+W^2_0)^{C_2} is determined by the parameters m^2_0,
C_1, C_2 and W^2_0. The observed scaling law follows from the
generalized-vector-dominance/color-dipole picture of low-x deep inelastic
scattering.Comment: LaTeX, 14 pages, 4 figures, Title changed, final version published in
Phys.Let
Small-x behavior of the structure function F_2 and its slope partial ln(F_2)/partial ln(1/x) for "frozen" and analytic strong-coupling constants
Using the leading-twist approximation of the Wilson operator product
expansion with "frozen" and analytic versions of the strong-coupling constant,
we show that the Bessel-inspired behavior of the structure function F_2 and its
slope\break partial ln(F_2)/partial ln(1/x) at small values of x, obtained for
a flat initial condition in the DGLAP evolution equations, leads to good
agreement with experimental data of deep-inelastic scattering at DESY HERA.Comment: new curves added to Figs. 1 and 2, minor changes to the text,
accepted for publication in Phys. Lett.
The RHIC SPIN Program: Achievements and Future Opportunities
Time and again, spin has been a key element in the exploration of fundamental
physics. Spin-dependent observables have often revealed deficits in the assumed
theoretical framework and have led to novel developments and concepts. Spin is
exploited in many parity-violating experiments searching for physics beyond the
Standard Model or studying the nature of nucleon-nucleon forces. The RHIC spin
program plays a special role in this grand scheme: it uses spin to study how a
complex many-body system such as the proton arises from the dynamics of QCD.
Many exciting results from RHIC spin have emerged to date, most of them from
RHIC running after the 2007 Long Range Plan. In this document we present
highlights from the RHIC program to date and lay out the roadmap for the
significant advances that are possible with future RHIC running