The Belle detector

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Abstract

The Belle detector was designed and constructed to carry out quantitative studies of rare B-meson decay modes with very small branching fractions using an asymmetric e+e collider operating at the ϒ(4S) resonance, the KEK-B-factory. Such studies require data samples containing ∼107 B-meson decays. The Belle detector is configured around a 1.5T superconducting solenoid and iron structure surrounding the KEK-B beams at the Tsukuba interaction region. B-meson decay vertices are measured by a silicon vertex detector situated just outside of a cylindrical beryllium beam pipe. Charged particle tracking is performed by a wire drift chamber (CDC). Particle identification is provided by dE/dx measurements in CDC, aerogel threshold Cherenkov counter and time-of-flight counter placed radially outside of CDC. Electromagnetic showers are detected in an array of CsI(Tl) crystals located inside the solenoid coil. Muons and KL mesons are identified by arrays of resistive plate counters interspersed in the iron yoke. The detector covers the θ region extending from 17° to 150°. The part of the uncovered small-angle region is instrumented with a pair of BGO crystal arrays placed on the surfaces of the QCS cryostats in the forward and backward directions. Details of the design and development works of the detector subsystems, which include trigger, data acquisition and computer systems, are described. Results of performance of the detector subsystems are also presented.

Introduction

The Belle collaboration evolved from the B-factory task force that was organized to study the physics potential of a high luminosity, asymmetric e+e collider operating at the ϒ(4S) resonance. In particular, the task force addressed the possibilities for experiments that tested the Kobayashi–Maskawa mechanism for CP-violation [1]. It was demonstrated that such tests could be done with a data of ∼107 B-meson decays, corresponding to integrated luminosities at the ϒ(4S) of order 100/fb, accumulated with a 4π detector with state-of-the-art capabilities [2].

The scientific goals of the Belle collaboration were discussed in a Letter of Intent [3] submitted to the March 1994 TPAC (TRISTAN Program Advisory Committee) meeting. The LoI describes the implications of these goals for the detector and provides a reference detector design based on the R&D program initiated by the task force. With the approval of the LoI the Technical Design Report was written by the Belle collaboration [4].

Fig. 1 shows the configuration of the Belle detector. The detector is configured around a 1.5T superconducting solenoid and iron structure surrounding the KEKB beams at the Tsukuba interaction region [5]. The beam crossing angle is ±11mr. B-meson decay vertices are measured by a silicon vertex detector (SVD) situated just outside of a cylindrical beryllium beam pipe. Charged particle tracking is provided by a wire drift chamber (CDC). Particle identification is provided by dE/dx measurements in CDC, and aerogel Cherenkov counters (ACC) and time-of-flight counters (TOF) situated radially outside of CDC. Electromagnetic showers are detected in an array of CsI(Tl) crystals located inside the solenoid coil. Muons and KL mesons are identified by arrays of resistive plate counters interspersed in the iron yoke. The detector covers the θ region extending from 17° to 150°. A part of the otherwise uncovered small-angle region is instrumented with a pair of BGO crystal arrays (EFC) placed on the surfaces of the QCS cryostats in the forward and backward directions. The expected (or achieved) performance of the detector is summarized in Table 1.

At the time of writing of TDR the detector technologies for particle identification and extreme forward calorimeters were not finalized, and R&D works were continued. All the other detector components entered the full construction stage. After extensive studies and tests of a few options for particle identification techniques the ACC system was chosen as the particle identification system. The extreme forward calorimeter system with BGO crystal arrays was also chosen as EFC over the option of a silicon–tungsten sandwich calorimeter. Confronted with various technical difficulties the design of SVD was changed to the present design following the recommendation made by the SVD review committee of June 1997.

Along with development and construction works of readout electronics for all the detector components, the trigger, data acquisition, and computing systems are also developed.

The present report summarizes the results of works by the Belle collaboration during the design, construction, testing, and commissioning stages of the Belle detector.

Section snippets

Beam crossing angle

The layout of the interaction region is shown in Fig. 2 [4]. The beam crossing angle of ±11mr allows us to fill all RF buckets with the beam and still avoid parasitic collisions, thus permitting higher luminosity. Another important merit of the large crossing-angle scheme is that it eliminates the need for the separation-bend magnets, significantly reducing beam-related backgrounds in the detector. The risk associated with this choice of a non-zero crossing angle is the possibility of

Extreme forward calorimeter, EFC

In order to improve the experimental sensitivity to some physics processes such as B→τν, the extreme forward calorimeter, EFC, is needed to further extend the polar angle coverage by ECL, 17°<θ<150° [4]. EFC covers the angular range from 6.4° to 11.5° in the forward direction and 163.3° to 171.2° in the backward direction. The EFC detector is attached to the front faces of the cryostats of the compensation solenoid magnets of the KEKB accelerator, surrounding the beam pipe [6], [7]. EFC is also

Silicon vertex detector (SVD)

A primary goal of the Belle experiment is to observe time-dependent CP asymmetries in the decays of B mesons. Doing so requires the measurement of the difference in z-vertex positions for B meson pairs with a precision of ∼100μm. In addition, the vertex detector is useful for identifying and measuring the decay vertices of D and τ particles. It also contributes to the tracking.

Since most particles of interest in Belle have momenta of 1GeV/c or less, the vertex resolution is dominated by the

Central tracking chamber, CDC

The efficient reconstruction of charged particle tracks and precise determination of their momenta is an essential ingredient to virtually all of the measurements planned for the Belle experiment. Specifically, the physics goals of the experiment require a momentum resolution of σpt/pt∼0.5%1+pt2 (pt in GeV/c) for all charged particles with pt⩾100MeV/c in the polar angle region of 17°⩽θ⩽150°. In addition, the charged particle tracking system is expected to provide important information for the

Aerogel Cherenkov counter system, ACC

Particle identification, specifically the ability to distinguish π± from K±, plays a key role in the elucidation of CP violation in the B system. An array of silica aerogel threshold Cherenkov counters has been selected as part of the Belle particle identification system to extend the momentum coverage beyond the reach of dE/dx measurements by CDC and time-of-flight measurements by TOF [4], [6].

Time-of-flight counters (TOF)

A time-of-flight (TOF) detector system using plastic scintillation counters is very powerful for particle identification in e+e collider detectors. For a 1.2m flight path, the TOF system with 100ps time resolution is effective for particle momenta below about 1.2GeV/c, which encompasses 90% of the particles produced in ϒ(4S) decays. It can provide clean and efficient b-flavor tagging.

In addition to particle identification, the TOF counters provide fast timing signals for the trigger system to

Electromagnetic calorimetry, ECL

The main purpose of the electromagnetic calorimeter is the detection of photons from B-meson decays with high efficiency and good resolutions in energy and position. Since most of these photons are end products of cascade decays, they have relatively low energies and, thus, good performance below 500MeV is especially important. Important two-body decay modes such as BKγ and B0→π0π0 produce photons energies up to 4GeV and good high resolution is needed to reduce backgrounds for these modes.

KL and muon detection system, KLM

The KLM detection system was designed to identify KL's and muons with high efficiency over a broad momentum range greater than 600MeV/c. The barrel-shaped region around the interaction point covers an angular range from 45° to 125° in the polar angle and the end-caps in the forward and backward directions extend this range to 20° and 155° [4], [68].

Solenoid magnet

A superconducting solenoid provides a magnetic field of 1.5T in a cylindrical volume of 3.4m in diameter and 4.4m in length [77], [4], [6]. The coil is surrounded by a multilayer structure consisting of iron plates and calorimeters, which is integrated into a magnetic return circuit. The main coil parameters are summarized in Table 11. The overall structure of the cryostat and the schematic drawing of the coil cross-section are shown in Fig. 111.

Iron yoke

The iron structure of the Belle detector serves

Trigger

The total cross-sections and trigger rates at the goal luminosity of 1034/cm2/s for various physical processes of interest are listed in Table 12. We need to accumulate samples of Bhabha and γγ events to measure the luminosity and to calibrate the detector responses, but, since their rates are very large, these trigger rates must be prescaled by a factor ∼100. Because of their distinct signatures, this should not be difficult. Although the cross-section for physics events of interest is

Data acquisition

In order to satisfy the data acquisition requirements so that it works at 500Hz with a deadtime fraction of less than 10%, the distributed-parallel system has been devised. The global scheme of the system is shown in Fig. 138. The entire system is segmented into 7 subsystems running in parallel, each handling the data from a sub-detector. Data from each subsystem are combined into a single event record by an event builder, which converts “detector-by-detector” parallel data streams to an

Offline computing system

The computing and software system is of great importance to the Belle experiment as very complex data analysis techniques using a large amount of data are required for physics discoveries. A traditional HEP computing model has been adopted by the Belle collaboration. Namely, the Belle collaboration has chosen to use tape library systems with the sequential access method for the input and output of experimental data as the mass storage system.

Acknowledgements

We extend deep thanks to the staffs of KEK and collaborating institutions for their contributions to the present work. We acknowledge support from the Ministry of Education, Science, Sports and Culture of Japan and the Japan Society for the Promotion of Science; the Australian Research Council and the Australian Department of Industry, Science and Resources; the Department of Science and Technology of India; the BK21 program of the Ministry of Education of Korea and the Basic Science program of

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    1

    Permanent address. Newman Laboratory, Cornell University, Ithaca, NY 14853-5001, USA.

    2

    Present address. ICRR, Tokyo University, Tanashi, Tokyo 188-8502, Japan.

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    Present address. University of Toronto, Toronto, Ont., Canada M5S 1A7.

    4

    Present address. IPST, University of Maryland, College Park, MD 20742, USA.

    Deceased.

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    Present address. Iwate University, Morioka 020-8551, Japan.

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    Permanent address. LANL, Los Alamos, NM 87545, USA.

    7

    On leave from Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia.

    8

    Present address. JAERI, Nakameguro, Meguro-ku, Tokyo 153-0061, Japan.

    9

    Present address. NIFS, Toki, Gifu 509-5292, Japan.

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