Search

Proceedings of DIS 2007, p.1121-1124, ISBN 978-3-935702-23-2, ISSN 1435-8077

Hypothetical axion-like particles with a two-photon interaction would be produced in the sun by the Primakoff process. In a laboratory magnetic field they would be transformed into X-rays with energies of a few keV. The CAST experiment at CERN is using a decommissioned LHC magnet as an axion helioscope in order to search for these axion-like particles. The analysis of the 2003 data has shown no signal above the background, thus implying an upper limit to the axion-photon coupling < 1.16*10^{-10} GeV^{-1} at 95% CL for m_{a}

The three reactions $\overline{p}p \rightarrow \pi^0\pi^0\pi^0$ , $\pi^0\pi^0\eta$ and $\pi^0\eta\eta$ in proton-antiproton annihilation in flight at 900 MeV/c are used to search for isoscalar $0^{++}$ and $2^{++}$ mesons in the mass range 1000–2000 MeV, in particular the $f_0(1710)$ . The description of both $\pi^0\pi^0\pi^0$ and $\pi^0\eta\eta$ data sets requires an isoscalar tensor resonance decaying into $\pi^0\pi^0$ and $\eta\eta$ with a mass of $(1867\pm46)$ MeV and a width of $(385\pm58)$ MeV. The ratio of partial widths $\Gamma(\eta\eta)/\Gamma(\pi\pi)$ is $0.27 \pm 0.10$ . The analyses of both $\pi^0\pi^0\pi^0$ and $\pi^0\eta\eta$ show no signal for the $f_0(1710)$ . The $\pi^0\eta\eta$ data set shows a strong signal of the $f_2^{\prime}(1525)$ suggesting a large OZI rule violation for tensor meson production in $\overline{p}p$ annihilation. The $\pi^0\pi^0\pi^0$ data set also requires the $f_2(1565)$ . The signal for the $f_2(1810)$ reported by earlier experiments is confirmed neither in $\pi^0\pi^0\pi^0$ nor in $\pi^0\eta\eta$ . The analysis of $\pi^0\pi^0\eta$ leads to an isovector tensor state decaying into $\pi^0\eta$ with a mass of $(1698\pm44)$ MeV and a width of $(265\pm55)$ MeV.

Measurements of the individual multiplicities of pi+, pi- and pi0 produced in the deep-inelastic scattering of 27.5 GeV positrons on hydrogen are presented. The average charged pion multiplicity is the same as for neutral pions, up to approximately z= 0.7, where z is the fraction of the energy transferred in the scattering process carried by the pion. This result (below z= 0.7) is consistent with isospin invariance. The total energy fraction associated with charged and neutral pions is 0.51 +/- 0.01 (stat.) +/- 0.08 (syst.) and 0.26 +/- 0.01 (stat.) +/- 0.04 (syst.), respectively. For fixed z, the measured multiplicities depend on both the negative squared four momentum transfer Q^2 and the Bjorken variable x. The observed dependence on Q^2 agrees qualitatively with the expected behaviour based on NLO-QCD evolution, while the dependence on x is consistent with that of previous data after corrections have been made for the expected Q^2-dependence., Comment: Revised version submitted to the European Physical Journal C; 10 pages, 9 figures, 1 table

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy \nu and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio R_M^h at low \nu and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the \nu-dependence of R_M^h is found between positive and negative hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the \nu- and z-dependence of R_M^h., Comment: 10 pages, 5 figures, LaTex (RevTeX)