K. Winter

Measurement of charm production in neutrino charged-current interactions

The nuclear emulsion target of the CHORUS detector was exposed to the wide-band neutrino beam of the CERN SPS of 27 GeV average neutrino energy from 1994 to 1997. In total, about 100 000 charged-current (CC) neutrino interactions with at least one identified muon were located in the emulsion target and fully reconstructed, using newly developed automated scanning systems. Charmed particles were searched for by a program recognizing particle decays. The observation of the decay in nuclear emulsion makes it possible to select a sample with very low background and minimal kinematical bias. In all, 2013 CC interactions with a charmed hadron candidate in the final state were selected and confirmed through visual inspection. The charm production rate induced by neutrinos relative to the CC cross-section is measured to be σ(νμN→μ−CX)/σ(CC)=(5.75 ± 0.32(stat)±0.30(syst))%. The charm production cross-section as a function of neutrino energy is also obtained. The results are in good agreement with previous measurements. The charm-quark hadronization produces the following charmed hadrons with relative fractions (in %): fD0=43.7±4.5, fΛc+=19.2±4.2, fD+=25.3±4.2 and fDs+=11.8±4.7.

Search for νμ → ντ oscillation

Abstract The fine granularity of the CHARM-II detector has been exploited to search, in the CERN-SPS wide band neutrino beam, for quasi-elastic ντ interactions followed by the decay τ → πντ. Since the sampling thickness of the target calorimeter corresponds to ∼ 1 9 of an interaction length, these events appear in the detector as a single track followed by a hadronic shower. The study of the “single pion” events is used to set limits on the νμ → ντ oscillation parameters. The maximum sensitivity to the mixing angle θ is reached for Δm2 = 50 eV2 allowing to exclude values of sin22θ greater than 6.4 × 10−3 at 90% CL.

The CHORUS scintillating fiber tracker and opto-electronic readout system

Abstract A scintillating fiber tracker system consisting of more than one million fibers has been successfully constructed and made operational for the CERN WA95/CHORUS experiment. The design and construction of the tracker system as well as its opto-electronic readout are described. The performance of the detector with high-energy muons is presented.

High-resolution tracking using large capillary bundles filled with liquid scintillator

Abstract We have developed large high-resolution tracking detectors based on glass capillaries filled with organic liquid scintillator of high refractive index. These liquid-core scintillating optical fibres act simultaneously as detectors of charged particles and as image guides. Track images projected onto the readout end of a capillary bundle are visualized by an optoelectronic chain consisting of a set of image-intensifier tubes followed by a photosensitive CCD or by an EBCCD camera. Two prototype detectors, each composed of ≈10 6 capillaries with 20– 25 μm diameter and 0.9–1.8 m length, have been tested, and a spatial resolution of the order of 20– 40 μm has been attained. A high scintillation efficiency and a large light-attenuation length, in excess of 3 m, was achieved through special purification of the liquid scintillator. Along the tracks of minimum-ionizing particles, the hit densities obtained were ∼8 hits / mm at the readout window, and ∼3 hits / mm at ∼1 m away. The level of radiation resistance of the prototype detectors is at least an order of magnitude higher than that of other tracking devices of comparable performance.

A multichannel single-photon sensitive detector for high-energy physics: the megapixel EBCCD

Abstract We present a study of the characteristics of a hybrid image intensifier tube based on a thinned backside Electron Bombarded CCD (EBCCD) with better performance in spatial resolution, single photoelectron detection and gain stability than the conventional intensified systems based on a Micro Channel Plate followed by a CCD. Single photon detection sensitivity has been studied and a procedure to correct the gain non-uniformity of the EBCCD has been developed. The EBCCD, operating at 14 kV, has an average gain of ∼3000 electrons/photoelectron and a noise of the order of 100 electron/pixel. These characteristics make the EBCCD a very attractive device for many applications in high-energy physics, astrophysics and biomedicine. A possible application in a neutrino oscillation experiment is presented.

Charged-particle multiplicities in charged-current neutrino– and anti-neutrino–nucleus interactions

The CHORUS experiment, designed to search for νμ→ντ oscillations, consists of a nuclear emulsion target and electronic detectors. In this paper, results on the production of charged particles in a small sample of charged-current neutrino– and anti-neutrino–nucleus interactions at high energy are presented. For each event, the emission angle and the ionization features of the charged particles produced in the interaction are recorded, while the standard kinematic variables are reconstructed using the electronic detectors. The average multiplicities for charged tracks, the pseudo-rapidity distributions, the dispersion in the multiplicity of charged particles and the KNO scaling are studied in different kinematical regions. A study of quasi-elastic topologies performed for the first time in nuclear emulsions is also reported. The results are presented in a form suitable for use in the validation of Monte Carlo generators of neutrino–nucleus interactions.

Reconstruction of neutrino interactions observed in a liquid-core fibre detector

Abstract A large-volume prototype of a liquid-core fibre detector has been built and installed in the CERN neutrino beam. For the first time high-quality images of neutrino interactions were recorded with such a detector. The analysis presented in this paper is based on a sample of 151 recorded interactions. It aims at evaluating the precision of the neutrino vertex reconstruction, to determine the capability of the technique for the detection of short-lived particles. To gather experience, a small sample of 31 events has been analysed on an event-by-event basis using an interactive track finding program. A non-interactive algorithm has been applied to the complete sample. A reconstruction efficiency of 84% and a vertex resolution of 30 μm (transverse to the beam direction) were achieved. The instrumental impact parameter of the measured tracks relative to the vertex is., on average, 43 μm for interactions in the sensitive part of the detector and 66 μm for events occurring in upstream materials (where the extrapolation distance to the vertex is longer).

A single-photon multichannel detector: the megapixel EBCCD

The CERN RD46 Collaboration is developing a new high resolution tracking detector based on glass capillaries filled with liquid scintillator. In this framework we have built a read-out chain based on a newly developed device: the Megapixel Electron Bombarded CCD (EBCCD). This device is a hybrid image-intensifier tube, with a 1024×1024 pixels CCD chip in place of the phosphor screen. The tube has a 40 mm diameter photocathode, is gateable and zoomable from 0.62 to 1.3. The EBCCD tube is able to detect single photoelectrons, with a signal to noise ratio better than 10. This Megapixel multichannel photon counting device is attractive for many applications in high-energy physics, astrophysics, biomedical diagnostics, and very low-light imaging.

High resolution tracking devices based on capillaries filled with liquid scintillator

The aim of this project is to develop high resolution tracking devices based on thin glass capillary arrays filled with liquid scintillator. This technique provides high hit densities and a position resolution better than 20 μm. The radiation hardness of more than 100 Mrad makes capillary arrays superior to other types of tracking devices with comparable performance. The technique is attractive for inner tracking in collider experiments, micro-vertex devices, or active targets for short-lived particle detection in high-energy physics experiments. During 1996 and 1997, two prototype capillary bundles (1.8 m and 0.9 m in length) were put into operation. One of them was equipped with a conventional opto-electronic read-out chain and the other, for the first time, with a new device, the Electron-Bombarded CCD (EBCCD) imaging tube. The development of planar capillary layers is another aspect of the R&D activity. Details of the design and the performance are given.