Determination of alpha_S using hadronic event shape distributions of data taken with the OPAL detector

The measurement of the strong coupling alpha_S using hadronic event shape distributions measured with the OPAL detector at center-of-mass energies between 91 and 209 GeV is summarized. For this measurement hadronic event shape distributions are compared to theoretical predictions based on next-to-next-to-leading-calculations (NNLO) and NNLO combined with resummed next-to-leading-logarithm calculations (NLLA). The combined result using NNLO calculations is alpha_S(MZ)=0.1201+-0.0008(stat.)+-0.0013(exp.)+-0.0010(had.)+-0.0024(theo.) and the result using NLLO and NLLA calculations is alpha_S(MZ)=0.1189+-0.0008(stat.)+-0.0016(exp.)=-0.0010(had.)+-0.0036(theo.), with both measurements being in agreement with the world average.Comment: 4 pages, 2 figures, contribution to the Proceedings of Rencontres de Moriond, QCD Session, March 20th - 27th, 2011 in La Thuile, Italy; based on arXiv:1101.147

Measurement of alpha_s in radiative hadronic events at OPAL

Hadronic final states with a hard isolated photon are studied using data taken at centre-of-mass energies around \Mz with the OPAL detector at LEP. The strong coupling \as is extracted by fitting event shape variables for the reduced centre-of-mass energies ranging from 20 GeV to 80 GeV, and the energy dependence of \as is studied. Combining all the values using different event shape variables and energies gives: \asmz=0.1176\pm 0.0012(stat.)^{+0.0093}_{-0.0085}(syst.).Comment: 4 pages, 1 figure, contribution to the QCD '03 conferenc

DEPFET detectors for direct detection of MeV Dark Matter particles

The existence of dark matter is undisputed, while the nature of it is still unknown. Explaining dark matter with the existence of a new unobserved particle is among the most promising possible solutions. Recently dark matter candidates in the MeV mass region received more and more interest. In comparison to the mass region between a few GeV to several TeV, this region is experimentally largely unexplored. We discuss the application of a RNDR DEPFET semiconductor detector for direct searches for dark matter in the MeV mass region. We present the working principle of the RNDR DEPFET devices and review the performance obtained by previously performed prototype measurements. The future potential of the technology as dark matter detector is discussed and the sensitivity for MeV dark matter detection with RNDR DEPFET sensors is presented. Under the assumption of three background events in the region of interest and an exposure of one kg$\cdot$y a sensitivity of $\bar{\sigma}_{\bar{e}} = 10^{-41}$ cm$^{2}$ for dark matter particles with a mass of 10 MeV can be reached.Comment: submitted to EPJ

A neural network z-vertex trigger for Belle II

We present the concept of a track trigger for the Belle II experiment, based on a neural network approach, that is able to reconstruct the z (longitudinal) position of the event vertex within the latency of the first level trigger. The trigger will thus be able to suppress a large fraction of the dominating background from events outside of the interaction region. The trigger uses the drift time information of the hits from the Central Drift Chamber (CDC) of Belle II within narrow cones in polar and azimuthal angle as well as in transverse momentum (sectors), and estimates the z-vertex without explicit track reconstruction. The preprocessing for the track trigger is based on the track information provided by the standard CDC trigger. It takes input from the 2D ($r - \varphi$) track finder, adds information from the stereo wires of the CDC, and finds the appropriate sectors in the CDC for each track in a given event. Within each sector, the z-vertex of the associated track is estimated by a specialized neural network, with a continuous output corresponding to the scaled z-vertex. The input values for the neural network are calculated from the wire hits of the CDC.Comment: Proceedings of the 16th International workshop on Advanced Computing and Analysis Techniques in physics research (ACAT), Preprint, reviewed version (only minor corrections

Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at s√ = 8 TeV with the ATLAS detector

The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1 fb−1 of proton-proton collision data at s√ = 8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via t¯ →tχ¯01 or t¯ →bχ¯±1 →bW(∗)χ¯01 , where χ¯01 (χ¯±1) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of t¯ →tχ¯01 . For a branching fraction of 100%, top squark masses in the range 270–645 GeV are excluded for χ¯01 masses below 30 GeV. For a branching fraction of 50% to either t¯ →tχ¯01 or t¯ →bχ¯±1 , and assuming the χ¯±1 mass to be twice the χ¯01 mass, top squark masses in the range 250–550 GeV are excluded for χ¯01 masses below 60 GeV