The L3+C experiment combines the high-precision spectrometer of the L3 detector at LEP, CERN, with a small air shower array. The momenta of cosmic ray induced muons can be measured from 20 to 2000 GeV/c. During the 1999 data taking period 5 billion muon events were recorded in the spectrometer. From April until mid Summer 2000 an additional 3 billion muon events have been recorded as well as 25 million air shower events. Here the first results on the muon momentum spectrum and charge ratio will be presented

Aars, Jon

Assmy, Philipp

Bagøien, Espen

Bogstad, Bjarte

Chierici, Melissa

Dalpadado, Padmini

Danielsen, Ida Kristin

Descamps, Sebastien

Eriksen, Elena

Fransson, Agneta

Frie, Anne Kirstine Højholt

Gerland, Sebastian

Green, Norman Whitaker

Grøsvik, Bjørn Einar

Hallfredsson, Elvar H.

Helgason, Lisa Bjørnsdatter

Hop, Haakon

Höffle, Hannes

Høines, Åge Sigurd

Ingvaldsen, Randi Brunvær

Itkin, Mikhail

Jelmert, Anders

Jensen, Andre Frantzen

Jensen, Henning

Johansson, Josefina

Johnsen, Hanne

Jørgensen, Lis Lindal

Kovacs, Kit M.

Leiknes, Øystein

Lien, Vidar Surén

Lind, Sigrid

Lorentsen, Svein Håkon

Lydersen, Christian

McBride, Margaret

Ottersen, Geir

Pavlova, Olga

Routti, Heli Anna Irmeli

Sandø, Anne Britt

Skaret, Georg

Skjerdal, Hilde Kristin

Skotte, Gunnar

Stene, Kristine O.

Stenevik, Erling Kåre

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Petersen, B.

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IL NUOVO CIMENTO Vol. 24 C, N. 4-5 Luglio-Ottobre 2001First results from the L3+C experiment at CERN(∗)B. Petersen for the L3 CollaborationUniversity of Nijmegen and NIKHEF - NL-6525 ED Nijmegen, The Netherlands(ricevuto il 16 Ottobre 2000; approvato il 12 Febbraio 2001)Summary. — The L3+C experiment combines the high-precision spectrometer ofthe L3 detector at LEP, CERN, with a small air shower array. The momenta ofcosmic ray induced muons can be measured from 20 to 2000 GeV/c. During the1999 data taking period 5 billion muon events were recorded in the spectrometer.From April until mid Summer 2000 an additional 3 billion muon events have beenrecorded as well as 25 million air shower events. Here the first results on the muonmomentum spectrum and charge ratio will be presented.PACS 95.85.Ry – Neutrino, muon, pion, and other elementary particles; cosmic rays.PACS 96.40.Tv – Neutrinos and muons.PACS 01.30.Cc – Conference proceedings.1. – The L3+C experimentThe L3 detector [1] at CERN’s LEP accelerator is located near Geneva (6.02◦ E,46.25◦ N) at an altitude of 450 m, and is installed underground below a 30 m layer ofmolasse. The L3 detector has been employed very successfully since 1989 in the study ofe+e− interactions for which the center-of-mass energy has ranged from 90 to 208 GeV.The muon spectrometer is located inside a half tesla solenoidal magnetic volume ex-ceeding 1000 m3; its modular structure consists of two octagonally shaped rings each witheight “octants”, see fig. 1. Each octant has 3 layers of precise multi-wire drift chambers.The outer, middle and inner layers have 16, 24 and 16 sensitive wires, respectively. Thesemeasure coordinates in the bending plane of the magnet. In addition 4 layers of driftchambers measure the coordinate along the magnetic field.Two hundreds and two square meters of scintillators were installed outside the mag-netic yoke on the face of the three upper octants. They provide the arrival time of themuons, which is needed to determine the drift times inside the chambers. An indepen-dent readout and data acquisition system has been installed, which enables data takingat all times.(∗) Paper presented at the Chacaltaya Meeting on Cosmic Ray Physics, La Paz, Bolivia,July 23-27, 2000.c© Societa` Italiana di Fisica 751752 B. PETERSEN for the L3 COLLABORATIONe-e +Muon Detector Silicon DetectorVertex DetectorHadron CalorimeterDoorCrownBarrel YokeMain CoilBGO Calorimeter  Scintillator TilesFig. 1. – Schematic view of the L3 detector. Outside the yoke the additional L3+C scintillatorscan be seen.In Spring 2000 a small air shower array was installed on the flat roof of the largehall at the surface above L3. It consists of 50 scintillator modules each with a surface of0.5 m2, which are distributed over an area of 30 × 54 m2. The independent air showertrigger has an energy threshold of about 10 TeV and is fully efficient above a showerenergy of 100 TeV. Trigger signals are exchanged between the two independent DAQsystems, which enables an offline merger of the two data streams.The L3 muon spectrometer together with the scintillators on the magnet and the airshower array constitutes the L3+C experiment [2].2. – Performance and data takingThe molasse layer provides a shield against the electro-magnetic and hadronic com-ponents of the air shower. It also imposes a muon momentum threshold of 15 GeV/cand limits the angular resolution to 0.2◦ at 100 GeV/c.A double momentum measurement (one in the top half and one in the bottom halfof the detector) enables the determination of the resolution, which at 100 GeV/c wasfound to be 7.4%. In addition, the momentum measurement, resolution and detectorefficiency have been checked using the yearly LEP calibration runs, where a few hundreds+e− → Z0/γ → µ+µ− events were recorded. For these events the muon momentum isknown to be 45.6 GeV/c.FIRST RESULTS FROM THE L3+C EXPERIMENT AT CERN 753Hayman 1962Nottingham 1984Nottingham 1971Durgapur 1972Durham 1975CAPRICE 1994+1997othersL3+C preliminarypµ / GeVRµonly data points with error smaller than 10% are shown11.21.41.61.8210 102 103P3  D µ(P) (cm -2  s -1  sr-1  /GeV2)Nottingham 1967Kiel 1971AMH 1971MARS 1975AMH 1979Nottingham 1983MASS 1993CAPRICE 1994CAPRICE 1997L3+C preliminary100.10.20.30.40.50.6102 103a)b)Fig. 2. – The very first results from the L3+C experiment. a) The flux of atmospheric muonsweighted with p3 as a function of momentum p. b) The muon charge ratio as a function ofmomentum.754 B. PETERSEN for the L3 COLLABORATIONThe energy resolution of the air shower array is expected to be 30% for events withthe core inside the array. The zenith angle can be determined using the arrival times ofparticles in the individual scintillator modules. An accuracy of 1◦–2◦ is expected.The dedicated data taking with the muon spectrometer started in 1999. From Mayto November a total of five billion events were recorded. The total number of eventsis expected to be more than 10 billion by the end of 2000. The air shower array hasbeen fully operational since April 2000, and by early July a total of 25 million air showerevents have been recorded. About 30% of these showers are accompanied by muon(s) inthe spectrometer.Data taking of the L3+C experiment will stop with end of the LEP program, whichis scheduled for November 2000.3. – Very first resultsData from September to November of 1999 were used to determine a first muonmomentum spectrum from 50 to 500 GeV/c. The total lifetime used was slightly morethan 30 days. Strict quality cuts have been applied, among others, to achieve a goodmomentum measurement in both upper and lower octant. The zenith angle has beenrestricted to a range from 0◦ to 10◦ to measure the vertical flux. The measured fluxweighted by p3 can be seen in fig. 2a together with the results of other experiments [3].The systematic error of 9% dominates the total error plotted in fig. 2a. In the future wehope to reduce the total error to 2.5%.The charge ratio was calculated from the same event sample. In fig. 2b the preliminaryresult can be seen along with the results of other published measurements [3]. With thesmall sample used for the analysis up to now the statistical error is dominant at highmomenta.4. – ConclusionsA new type of cosmic ray detector, L3+C, combines air shower data with precisemuon measurements. First preliminary results on the muon spectrum and charge ratioin the range from 50 to 500 GeV/c have been presented. A substantial reduction in thestatistical and systematic errors is expected in the future.∗ ∗ ∗We acknowledge with appreciation the effort of the engineers, technicians and supportstaff who have participated in the construction and maintenance of this experiment.REFERENCES[1] L3 Collaboration (Adeva B. et al.), Nucl. Instrum. Methods A, 289 (1990) 35.[2] http://l3www.cern.ch/l3 cosmics/[3] Hebbeker T. and Timmermans C., A Compilation of High Energy Atmospheric Muondata at Sea Level, to be published.

First results from the L3+C experiment at CERN

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