The standard method used for tagging b-hadrons in the DELPHI experiment at the CERN LEP Collider is discussed in detail. The main ingredient of b-tagging is the impact parameters of tracks, which relies mostly on the vertex detector. Additional information, such as the mass of particles associated to a secondary vertex, significantly improves the selection efficiency and the background suppression. The paper describes various discriminating variables used for the tagging and the procedure of their combination. In addition, applications of b-tagging to some physics analyses, which depend crucially on the performance and reliability of b-tagging, are described briefly.

/pdf/b-tagging-in-delphi-at-lep-i6qwxt3avi.pdf

b-tagging in DELPHI at LEP

A hybrid active pixel sensor (HAPS) consists of an array of sensing elements which is connected to an electronic read-out unit. The most used way to connect these two different devices is bump bonding. This interconnection technique is very suitable for these systems because it allows a very fine pitch and a high number of I/Os. However, there are other interconnection techniques available such as direct bonding. This paper, as a continuation of a review [M. Lozano, E. Cabruja, A. Collado, J. Santander, M. Ullan, Nucl. Instr. and Meth. A 473 (1–2) (2001) 95–101] published in 2001, presents an update of the different advanced bonding techniques available for manufacturing a hybrid active pixel detector.

Technical reviews Bonding techniques for hybrid active pixel sensors (HAPS)

The DELPHI Vertex Detector has been upgraded for LEP200 with 2 layers of pixel detectors and 2 layers of ministrips as endcaps, thus covering the polar angular range between 10° and 21° and allowing for standalone pattern recognition. During the 1995/96 shutdown the first 95 modules were installed and the installation was completed in May 97 with all 152 modules. Production yields and failures during the assembly will be presented. The detector operated at a threshold of around 9000 electrons resulting in a number of noisy pixels at the 10 −3 level. After masking these hot pixels in the readout, the remaining number of random hits per event is at the 10 −6 level. The observed resolution of about 100 μm is close to the expectation for binary readout. The track reconstruction efficiency increased by more than 100% in the central part of the forward tracker.

Commissioning of the DELPHI Pixel Detector

The events with two photons and missing (transverse) energy collected by the DELPHI detector at centre-of-mass energies between 130 GeV and 183 GeV have been studied to search for processes of the type e+e- → YY with the subsequent decay Y → X7, where X is an undetectable neutral particle. Reactions of this kind are expected in supersymmetric models, where the Y particle can be either the lightest neutralino, decaying to a photon and a gravitino, or the next-to-lightest neutralino, decaying to a photon and the lightest neutralino. To study the case of long-lived Y particles, a search for single-photon events with the reconstructed photon axis pointing far from the beam interaction region has also been performed. No evidence for a deviation from Standard Model expectations has been observed in the data and upper limits have been derived on the signal cross-section as a function of the the X and Y masses and of the Y mean decay path.

/pdf/search-for-pair-produced-neutralinos-in-events-with-photons-3a466oclk4.pdf

Search for pair produced neutralinos in events with photons and missing energy from $e^{+} e^{-}$ collisions at $S^{(1/2)}$ = 130-GeV to 183-GeV

The design of a new silicon tracker detector for the forward region in the DELPHI experiment is presented. It consists of two layers of macropixel and two layers of ministrip detectors in both the forward directions. The motivations and the requirements for this detector will be shown together with test beam results.

The DELPHI very forward tracker for LEP200

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The DELPHI very forward tracker for LEP200