Precise Vertex Research Articles

The physics performance of LHCb

The LHCb experiment is based on a single arm forward spectrometer optimized to study CP violation in the B system at the LHC. The experiment includes a dedicated trigger system, particle identification and a precise vertex determination; recent developments in these areas will be summarized. The detector will provide precise measurements of all angles of the unitarity triangle, the length of one of the sides (via B s 0− B ̄ s 0 mixing) and the height of the triangle. The physics performance for selected channels will be reviewed.

A new robust fitting algorithm for vertex reconstruction in the CERES experiment

In this paper we report on the application of a robust approach based on a reweighted least squares method for the vertex reconstruction of the CERES/NA45 experiment at the CERN SPS for the case of 160 GeV/u Pb+Au collisions and compare its performance with the presently used algorithm based on the minimization of summed Gaussian weights. In the CERES/NA45 experiment a doublet of cylindrical silicon drift detectors (SDD) is used for vertex reconstruction and tracking. The vertex reconstruction algorithm has to cope with a background-contaminated environment. Furthermore, in the case of ultrarelativistic heavy-ion collisions where high event multiplicities occur ( dn ch dη ⋍ 500 ), the probability of confusing hits from close or intersecting tracks is high. The algorithm needs to provide the precise vertex location even at high occupancies, at a reasonable speed. Our findings are that the robust iterative procedure presented here is about an order of magnitude faster than approaches using standard minimization packages, at a comparable or slightly better accuracy.

LHC-B: overview and recent developments

The proposed LHC-B experiment is a single arm forward spectrometer optimised to study CP violation in the B system at the LHC. The essential features of the experiment are described, and those attributes emphasised which are of particular importance for B physics. These include a dedicated trigger system, particle identification and precise vertex, proper time and mass resolution. Recent progress is summarised in the areas of detector design and in triggering. The physics performance is presented for selected channels.

Influence of the full vertex and vacuum polarization on the fermion propagator in (2+1)-dimensional QED.

We investigate the influence of the full vacuum polarization and vertex function on the fermion propagator, using the coupled Dyson-Schwinger equations for the photon and fermion propagator. We show that, within a range of vertex functions, the general behavior of the fermion propagator does not depend on the exact details of the vertex, both in the massless and in the massive phase. Independent of the precise vertex function, there is a critical number of fermion flavors for dynamical mass generation in (2 + 1)-dimensional QED. A consistent treatment of the vacuum polarization is essential for these results.

Measurement of the tau lifetime at SLD.

A measurement of the lifetime of the tau lepton has been made using a sample of 1671 $Z^0\rightarrow\tau^+\tau^-$ decays collected by the SLD detector at SLC. The measurement benefits from the small and stable collision region at SLC and the precision vertex detector of SLD. Three analysis techniques have been used: decay length, impact parameter, and impact parameter difference methods. The combined result is $\tau_\tau = 297\pm 9(stat.)\pm 5(syst.)$ fs.

Vertex detection at LHC

Physics motivations and experimental conditions for high precision vertex detection at future hadron colliders, such as LHC and SSC, are given. A conceptual design of the ATLAS vertex detector is presented including results from Monte Carlo simulations. The status of technology and some results of R&D prototyping are described.

A precise measurement of the τ lepton lifetime

Three different techniques are used to measure the mean decay length of the τ lepton with a high precision vertex detector in a sample of 11 800 τ pairs coming from Z decays, collected in1991 by ALEPH at LEP. Events in which both τ's decay into one charged track are analyzed using two largely independent methods. Displaced vertices in three-prong decays yield another independent measurement. The derived lifetime is 295.5 ± 5.9 ± 3.1 fs, using m τ = 1777.1 ± 0.5 MeV/ c 2. Including previous (1989–1990) ALEPH measurements, the combined τ lifetime is 294.7 ± 5.4 ± 3.0 fs.

An observation of exclusive Λ +c decays into Σ + and mesons

Using data from the NA32 experiment at CERN we have studied the Λ + c decays containing a Σ + among the decay products. The interactions of 230 GeV π − with a Cu target were analysed using a precise vertex telescope (charge-coupled devices and silicon microstrip detectors) and the ACCMOR spectrometer. We have found eleven Λ + c→Σ +π +π −, one Λ + c→Σ +K +K −, two Λ + c→Σ +K +π − and one Λ + c→Σ +π +π −π +π − decays practically without any backgroun d. We have measured the branching ratios with respect to the Λ + c→pK −π + channel.

Measurement of the bb-bar fraction in hadronic Z0 decays with precision vertex detectors.

We have measured the fraction of bb¯ events in hadronic Z0 decays, Rbb¯, using the vertex detector system of the Mark II detector at the SLAC Linear Collider. We tag bb¯ events by requiring the coincidence of three or more tracks with significant impact parameters. This tag is 50% efficient and results in a sample of 85% purity. We find Rbb¯=0.251±0.049±0.030, in good agreement with other measurements and the standard model prediction.Received 5 August 1991DOI:https://doi.org/10.1103/PhysRevLett.67.3347©1991 American Physical Society

The DELPHI experiment

The DELPHI experiment is designed to study in detail the final state particles produced in electron-positron collisions at LEP. The detector has 4π acceptance with emphasis on three-dimensional information, particle identification, high granularity and precise vertex determination. The detector and its performance during its first year of operation is described.

The DELPHI detector at LEP

DELPHI is a 4π detector with emphasis on particle identification, three-dimensional information, high granularity and precise vertex determination. The design criteria, the construction of the detector and the performance during the first year of operation at the large electron positron collider (LEP) at CERN are described.

The OPAL vertex drift chamber

A high precision vertex drift chamber has been installed in the OPAL experiment at LEP. The design of the chamber and the associated readout electronics is described. The performance of the system has been studied using cosmic ray muons and the results of these studies are presented. A space resolution of 50 μm in the drift direction is obtained using the OPAL central detector gas mixture at 4 bar.

The performance of “Virtual Phase” CCDs as detectors of minimum-ionizing particles

The Texas Instruments “Virtual Phase” CCD has been the basis of an ambitious design for a precision vertex detector to be used at the Stanford Linear Collider. The performance of this chip shows promise for future use in electron linear colliders. Experimental results are reported in addition to description of the electronic readout and preliminary mechanical design.

Determination of the lifetime of bottom hadrons.

We have measured the mean lifetime of the hadrons containing b quarks that are produced in e+e− annihilation at √s =29 GeV. We use the full sample of data collected by the MAC detector at the SLAC storage ring, PEP, including those recently acquired with a precision vertex detector. The result is τb=[1.29±0.20(stat) ±0.07(syst) ps]×(1.00±0.15), where the last factor is the systematic uncertainty in the scale.Received 27 October 1986DOI:https://doi.org/10.1103/PhysRevLett.58.640©1987 American Physical Society

The use of a nitrogen laser for observation of space charge effects in drift chambers and development of high precision vertex chambers

A two-stage nitrogen laser has been built and is presently being used to investigate properties of drift chambers. In this paper we present the measured parameters of this laser, the results of a study of the double track separation in a drift chamber of jet design and a measurement of the saturation effects in this chamber as a function of total gain. An understanding of the saturation effects is important for drift chambers with multiple hit capability. The laser technique reported here represents a major improvement in the simplicity of investigation of the double track separation and saturation effects. In addition, we have demonstrated the usefulness of this kind of laser in the development of high precision drift chambers working in a resolution range of ∼ 20–30 μm.

A time projection chamber for precise vertex determination

A time projection chamber, to be used for improving the vertex reconstruction accuracy of the Omega spectrometer at CERN, has been tested on a 7 GeV/ c hadron beam. Results on efficiency, space accuracy, multitrack handling and vertex reconstruction precision are presented together with a description of the read-out electronics and details of construction.