46 research outputs found
Measurement of the Cross Section Asymmetry of the Reaction gp-->pi0p in the Resonance Energy Region Eg = 0.5 - 1.1 GeV
The cross section asymmetry Sigma has been measured for the photoproduction
of pi0-mesons off protons, using polarized photons in the energy range Eg = 0.5
- 1.1 GeV. The CM angular coverage is Theta = 85 - 125 deg with energy and
angle steps of 25 MeV and 5 deg, respectively. The obtained Sigma data, which
cover the second and third resonance regions, are compared with existing
experimental data and recent phenomenological analyses. The influence of these
measurements on such analyses is also considered
Measurement of Angular Distributions and R= sigma_L/sigma_T in Diffractive Electroproduction of rho^0 Mesons
Production and decay angular distributions were extracted from measurements
of exclusive electroproduction of the rho^0(770) meson over a range in the
virtual photon negative four-momentum squared 0.5< Q^2 <4 GeV^2 and the
photon-nucleon invariant mass range 3.8< W <6.5 GeV. The experiment was
performed with the HERMES spectrometer, using a longitudinally polarized
positron beam and a ^3He gas target internal to the HERA e^{+-} storage ring.
The event sample combines rho^0 mesons produced incoherently off individual
nucleons and coherently off the nucleus as a whole. The distributions in one
production angle and two angles describing the rho^0 -> pi+ pi- decay yielded
measurements of eight elements of the spin-density matrix, including one that
had not been measured before. The results are consistent with the dominance of
helicity-conserving amplitudes and natural parity exchange. The improved
precision achieved at 47 GeV,
reveals evidence for an energy dependence in the ratio R of the longitudinal to
transverse cross sections at constant Q^2.Comment: 15 pages, 15 embedded figures, LaTeX for SVJour(epj) document class
Revision: Fig. 15 corrected, recent data added to Figs. 10,12,14,15; minor
changes to tex
Observation of a Coherence Length Effect in Exclusive Rho^0 Electroproduction
Exclusive incoherent electroproduction of the rho^0(770) meson from 1H, 2H,
3He, and 14N targets has been studied by the HERMES experiment at squared
four-momentum transfer Q**2>0.4 GeV**2 and positron energy loss nu from 9 to 20
GeV. The ratio of the 14N to 1H cross sections per nucleon, known as the
nuclear transparency, was found to decrease with increasing coherence length of
quark-antiquark fluctuations of the virtual photon. The data provide clear
evidence of the interaction of the quark- antiquark fluctuations with the
nuclear medium.Comment: RevTeX, 5 pages, 3 figure
Determination of the Deep Inelastic Contribution to the Generalised Gerasimov-Drell-Hearn Integral for the Proton and Neutron
The virtual photon absorption cross section differences [sigma_1/2-sigma_3/2]
for the proton and neutron have been determined from measurements of polarised
cross section asymmetries in deep inelastic scattering of 27.5 GeV
longitudinally polarised positrons from polarised 1H and 3He internal gas
targets. The data were collected in the region above the nucleon resonances in
the kinematic range nu < 23.5 GeV and 0.8 GeV**2 < Q**2 < 12 GeV**2. For the
proton the contribution to the generalised Gerasimov-Drell-Hearn integral was
found to be substantial and must be included for an accurate determination of
the full integral. Furthermore the data are consistent with a QCD
next-to-leading order fit based on previous deep inelastic scattering data.
Therefore higher twist effects do not appear significant.Comment: 6 pages, 3 figures, 1 table, revte
B decays
We review the prospects for B decay studies at the LHC. Contributing authors: J. Baines, S.P. Baranov, P. Bartalini, M. Beneke, E. Bouhova, G. Buchalla, I. Caprini, F. Charles, J. Charles, Y. Coadou, P. Colangelo, P. Colrain, J. Damet, F. De Fazio, A. Dighe, H. Dijkstra, P. Eerola, N. Ellis, B. Epp, S. Gadomski, P. Galumian, I. Gavrilenko, S. George, V.M. Ghete, V. Gibson, L. Guy, Y. Hasegawa, P. Iengo, A. Jacholkowska, R. Jones, A. Khodjamirian, E. Kneringer, P. Koppenburg, H. Korsmo, N. Labanca, L. Lellouch, M. Lehto, Y. Lemoigne, J. Libby, J. Matias, S. Mele, M. Misiak, A.M. Nairz, T. Nakada, A. Nikitenko, N. Nikitin, A. Nisati, F. Palla, E. Polycarpo, J. Rademacker, F. Rizatdinova, S. Robins, D. Rousseau, W. Ruckstuhl, M.A. Sanchis, O. Schneider, M. Shapiro, C. Shepherd-Themistocleous, P. Sherwood, L. Smirnova, M. Smizanska, A. Starodumov, N. Stepanov, Z. Xie, N. Zaitse
Measurement of the Neutron Spin Structure Function with a Polarized ^3He Target
Results are reported from the HERMES experiment at HERA on a measurement of
the neutron spin structure function in deep inelastic scattering
using 27.5 GeV longitudinally polarized positrons incident on a polarized
He internal gas target. The data cover the kinematic range
and . The integral evaluated at a fixed of is . Assuming Regge behavior at low , the first
moment is .Comment: 4 pages TEX, text available at
http://www.krl.caltech.edu/preprints/OAP.htm
Flavor Decomposition of the Polarized Quark Distributions in the Nucleon from Inclusive and Semi-inclusive Deep-inelastic Scattering
Spin asymmetries of semi-inclusive cross sections for the production of
positively and negatively charged hadrons have been measured in deep-inelastic
scattering of polarized positrons on polarized hydrogen and 3He targets, in the
kinematic range 0.023<x<0.6 and 1 GeV^2<Q^2<10 GeV^2. Polarized quark
distributions are extracted as a function of x for up $(u+u_bar) and down
(d+d_bar) flavors. The up quark polarization is positive and the down quark
polarization is negative in the measured range. The polarization of the sea is
compatible with zero. The first moments of the polarized quark distributions
are presented. The isospin non-singlet combination Delta_q_3 is consistent with
the prediction based on the Bjorken sum rule. The moments of the polarized
quark distributions are compared to predictions based on SU(3)_f flavor
symmetry and to a prediction from lattice QCD.Comment: 14 pages, 6 figures (eps format), 10 tables in Latex New version
contains tables of asymmetries and correlation matri
The NOMAD Experiment at the CERN SPS
The NOMAD experiment is a short base-line search for ν<sub>μ</sub> − ν<sub>τ</sub> oscillations in the CERN neutrino beam. The ν<sub>τ</sub>'s are searched for through their charged current interactions followed by the observation of the resulting τ− through its electronic, muonic or hadronic decays. These decays are recognized using kinematical criteria necessitating the use of a light target which enables the reconstruction of individual particles produced in the neutrino interactions. This paper describes the various components of the NOMAD detector: the target and muon drift chambers, the electromagnetic and hadronic calorimeters, the preshower and transition radiation detectors and the veto and trigger scintillation counters. The beam and data acquisition system are also described. The quality of the reconstruction and individual particles is demonstrated through the ability of NOMAD to observe K<sub>s</sub><sup>0</sup>'s, Λ<sup>0</sup>'s and π<sup>0</sup>'s. Finally, the observation of τ− through its electronic decay being one of the most promising channels in the search, the identification of electrons in NOMAD is discussed