The L3+C detector, a unique tool-set to study cosmic rays

doi:10.1016/S0168-9002(02)00479-5

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 488, Issues 1–2, 1 August 2002, Pages 209-225

https://doi.org/10.1016/S0168-9002(02)00479-5 Get rights and content

Abstract

The L3 detector at the CERN electron–positron collider, LEP, has been employed for the study of cosmic ray muons. The muon spectrometer of L3 consists of a set of high-precision drift chambers installed inside a magnet with a volume of about $1000 m^{3}$ and a field of $0.5 T$ . Muon momenta are measured with a resolution of a few percent at $50 GeV$ . The detector is located under $30 m$ of overburden. A scintillator air shower array of $54 m$ by $30 m$ is installed on the roof of the surface hall above L3 in order to estimate the energy and the core position of the shower associated with a sample of detected muons. Thanks to the unique properties of the L3+C detector, muon research topics relevant to various current problems in cosmic ray and particle astrophysics can be studied.

Introduction

The L3+C experiment (Fig. 1, Fig. 2), installed at the Large Electron Positron collider (LEP) at CERN, Geneva, consists of two major parts: firstly, below ground, the L3 muon spectrometer [1], which is comprised of a large $0.5 T$ magnet with a volume of $1000 m^{3}$ , three layers of high-precision drift chambers installed in two wheels of eight octants, and $200 m^{2}$ of timing scintillators on top of the magnet; secondly, a scintillator array is mounted on the roof of the surface hall $60 m$ above the beam axis of the spectrometer.

L3+C is a unique detector in several respects. It has a large acceptance of some $200 m^{2} sr$ and is covered by only $30 m$ of overburden, in contrast to other underground muon experiments. The energy threshold is therefore accordingly low ( $15 GeV$ ), and the multiple scattering of muon tracks remains below $4 mrad$ above $100 GeV$ energy. The precision drift chambers allow the measurement of the muon momenta between $20 GeV$ and $2 TeV$ , with one of the best resolutions obtained so far. Thanks to the possibility of running simultaneously with LEP, Z-decay muons can be used to check the momentum measurement, to determine the resolution and to estimate the detector efficiency. The surface scintillator array is used to estimate the energy range and to determine the core axis of the air showers associated with the muons measured underground.

The main measurements to be performed are the cosmic ray muon momentum spectrum between 20 and $2000 GeV$ , the muon charge ratio, and the angular dependence of both. The results of these data give information on the primary composition of cosmic rays, the inclusive hadron cross-sections at high momenta and set the normalization of the atmospheric muon neutrino spectrum above $10 GeV$ . The observed multiple muon events with momentum determination and the simultaneous measurement of the associated air shower parameters are sensitive to the nuclear composition of the primary spectrum in the knee region and to the physics of the very forward particle production. Other topics related to astroparticle physics and cosmic rays can be studied thanks to the particular properties of the L3+C experiment.

Measuring precisely the muon spectrum over two orders of magnitude in momentum is a very challenging task. At an early stage of this project, cosmic ray muon data, which were taken for the purpose of calibrating the L3 electromagnetic calorimeter in 1991, confirmed the feasibility of this experiment [2], [3].

Section snippets

The L3+C components

The L3 detector [1] was designed to measure very accurately γ's, e's, μ's and hadrons produced in e⁺e⁻ collisions. The properties of this device are particularly well suited to also be beneficial for studies of cosmic ray muons. In this respect the addition of an air shower scintillator array at the surface opens new research possibilities. The individual parts of the detector are described in 3 L3 detector, 4 Air shower scintillator array.

The center of the L3 detector is located $44.8 m$ below

L3 detector

The L3 detector [1] (Fig. 4) has a huge solenoidal magnet of $12 m$ diameter, $12 m$ length, which produces a field of $0.5 T$ . Inside this magnetic cave of $1000 m^{3}$ , a high-precision muon detector is installed. The central part of the detector (not used for the cosmic ray studies) consists of a sampling Uranium hadron calorimeter, a barrel-shaped trigger scintillator system, a BGO crystal electromagnetic calorimeter, and tracking and vertex detectors.

Array geometry

The layout for the 50 detectors is shown schematically in Fig. 17. It shows that 47 of the 50 detectors have been arranged in six rows, each with eight detectors (except the 3rd row which has only seven detectors due to the folding type of the roof of the hall just above the main shaft). The average separation between these detectors is about $7 m$ . The remaining three detectors are located on the roofs of the adjacent halls which are a few meters lower. The compact layout for the 47 detectors

Performance of the L3+C detector system

After test-runs in 1998, data acquisition started on the 5th of May 1999 and stopped on the 8th of November 1999 for a first time for the LEP winter shutdown. The trigger rate was around $500 Hz$ , the live time during runs amounted to 98±1%. The trigger efficiency was extracted from a trigger simulation, and found to be (99.2±0.3)%. Running continued in 2000 from the 28th of March to the 13th of November with additional data from the scintillator air shower array. The trigger classes were

Conclusions

The described L3+C detector has finished its data taking on the 13th of November 2000, 11 days after the end of LEP running. It has accumulated some 1.2×10¹⁰ events in a total of 312 days. The quality of the data is excellent. The analysis is in progress and the physics results will prove that the L3+C muon spectrometer, together with its surface array, is indeed a unique tool for the study of cosmic ray muon physics.

Acknowledgements

The L3+C group would like to express their thanks to Edgar Bugaev, John Ellis, Anatoly Erlykin, Tomas Gaisser, Vadim Naumov, Leonidas Resvanis, and Todor Stanev for helpful discussions and their continuous support.

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²: Supported by the National Natural Science Foundation of China, CNSF.

¹: Permanent address. Osaka City University, Osaka, Japan.

^✠: Deceased.

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