Performance of a GridPix detector based on the Timepix3 chip

A GridPix readout for a TPC based on the Timepix3 chip is developed for future applications at a linear collider. The GridPix detector consists of a gaseous drift volume read out by a single Timepix3 chip with an integrated amplification grid. Its performance is studied in a test beam with 2.5 GeV electrons. The GridPix detector detects single ionization electrons with high efficiency. The Timepix3 chip allowed for high sample rates and time walk corrections. Diffusion is found to be the dominating error on the track position measurement both in the pixel plane and in the drift direction, and systematic distortions in the pixel plane are below 10 $\mu$m. Using a truncated sum, an energy loss (dE/dx) resolution of 4.1% is found for an effective track length of 1 m.Comment: To be published in Nuclear Instruments and Methods in Physics Research Section

The detection of single electrons by means of a Micromegas-covered MediPix2 pixel CMOS readout circuit

A small drift chamber was read out by means of a MediPix2 readout chip as direct anode. A Micromegas foil was placed 50 $\mu$m above the chip, and electron multiplication occurred in the gap. With a He/Isobutane 80/20 mixture, gas multiplication factors up to tens of thousands were achieved, resulting in an efficiency for detecting single electrons of better than 90% . We recorded many frames containing 2D images with tracks from cosmic muons. Along these tracks, electron clusters were observed, as well as delta-rays.Comment: 15 pages, 9 included postscript figures, 5 separate jpeg figures, submitted to Nucl. Instr. and Meth. A. A complete postscript version with high resolution figures 1, 3, 11, 12 and 14 can be found at http://www.nikhef.nl/~i06/RandD/final/letter4.p

Search for charged Higgs bosons: combined results using LEP data

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for pair-produced charged Higgs bosons in the framework of Two Higgs Doublet Models (2HDMs). The data of the four experiments have been statistically combined. The results are interpreted within the 2HDM for Type I and Type II benchmark scenarios. No statistically significant excess has been observed when compared to the Standard Model background prediction, and the combined LEP data exclude large regions of the model parameter space. Charged Higgs bosons with mass below 80 GeV/c2 (Type II scenario) or 72.5 GeV/c2 (Type I scenario, for pseudo-scalar masses above 12 GeV/c2 ) are excluded at the 95 % confidence level

Determination of the b quark mass at the M_Z scale with the DELPHI detector at LEP

An experimental study of the normalized three-jet rate of b quark events with respect to light quarks events (light= \ell \equiv u,d,s) has been performed using the CAMBRIDGE and DURHAM jet algorithms. The data used were collected by the DELPHI experiment at LEP on the Z peak from 1994 to 2000. The results are found to agree with theoretical predictions treating mass corrections at next-to-leading order. Measurements of the b quark mass have also been performed for both the b pole mass: M_b and the b running mass: m_b(M_Z). Data are found to be better described when using the running mass. The measurement yields: m_b(M_Z) = 2.85 +/- 0.18 (stat) +/- 0.13 (exp) +/- 0.19 (had) +/- 0.12 (theo) GeV/c^2 for the CAMBRIDGE algorithm. This result is the most precise measurement of the b mass derived from a high energy process. When compared to other b mass determinations by experiments at lower energy scales, this value agrees with the prediction of Quantum Chromodynamics for the energy evolution of the running mass. The mass measurement is equivalent to a test of the flavour independence of the strong coupling constant with an accuracy of 7 permil.Comment: 24 pages, 10 figures, Accepted by Eur. Phys. J.