Showing papers by "Achim Geiser published in 1999"

Forward jet production in deep inelastic scattering at HERA

Abstract: The production of forward jets has been measured in deep inelastic ep collisions at HERA. The results are presented in terms of single differential cross sections as a function of the Bjorken scaling variable (x_{Bj}) and as triple differential cross sections d^3 \sigma / dx_{Bj} dQ^2 dp_{t,jet}^2, where Q^2 is the four momentum transfer squared and p_{t,jet}^2 is the squared transverse momentum of the forward jet. Also cross sections for events with a di-jet system in addition to the forward jet are measured as a function of the rapidity separation between the forward jet and the two additional jets. The measurements are compared with next-to-leading order QCD calculations and with the predictions of various QCD-based models.

Distinction of atmospheric $\nu_\mu$ – $\nu_\tau$ and $\nu_\mu$ – $\nu_{\rm s}$ oscillations using short or intermediate baseline experiments

Abstract: The current case for atmospheric $ u_\mu$ oscillations in active or sterile neutrinos is reviewed. It is argued that neither the study of neutral current events at Super-Kamiokande, nor the information obtained from future long baseline experiments might be sufficient to unambiguously decide between these two scenarios. However, a combination of these results with the results from future short or intermediate baseline $\tau$ appearance experiments would clearly resolve most of the remaining ambiguities. This conclusion does not strongly depend on whether the results from the LSND experiment will be confirmed or not. In the case that LSND would be confirmed, a negative result in such a short or intermediate baseline experiment would also unambiguously exclude the interpretation of LSND as indirect $ u_\mu$ – $ u_\tau$ – $ u_{\rm e}$ oscillations.

Pseudo-Dirac neutrinos as a potential complete solution to the neutrino oscillation puzzle

Abstract: A solution for the neutrino mass and mixing pattern is proposed which is compatible with all available experimental data on neutrino oscillations. This solution involves Majorana neutrinos of the pseudo-Dirac type, i.e. m_Majorana << m_Dirac. The solar and atmospheric neutrino observations are mainly explained as nu_e - nu_e^S and nu_mu - nu_mu^S oscillations, where S indicates the sterile (``righthanded'') partner of each neutrino generation, while the LSND result is interpreted in terms of standard nu_mu - nu_e oscillations. The resulting constraints on nu_mu - nu_tau and nu_tau - nu_tau^S oscillations are also discussed. This solution leaves room for a hierarchical mass and mixing scheme with a nu_tau mass in the few eV range, as favoured by some dark matter scenarios. The apparent conflict with standard Big Bang nucleosynthesis is addressed and the implications for current and future experiments are discussed. It is argued that both short and long baseline accelerator neutrino experiments are needed in order to decide between this solution and other oscillation scenarios.