VATLY, INST, Hanoi, Vietnam COSMIC MUONS IN HANOI PN Diep, PN Dinh, NH Duong, PTT Nhung P Darriulat, NT Thao, DQ Thieu and VV Thuan VATLY, INST, Hanoi, Vietnam

The VATLY group on the roof of the laboratory VATLY (means “physics” in Vietnamese) is a cosmic ray laboratory installed in the premises of the Institute for Nuclear Science and Technology in Hanoi. It is equipped with scintillator hodoscopes and water Cherenkov counters (one is an exact copy of an Auger surface detector counter).

 &  from cosmic ray proton or nucleus strong interaction with atmosphere + - pairs negligible weak decays real photons  &  electrons only muons negligible mesons & hyperons electromagnetic decays p1 proton 1 atmosphere 1 SK 2 Earth (oscillates over 13000km) 2 proton 2 Measuring the muon flux is of interest to atmospheric neutrino oscillation experiments such as SuperKamiokande and SNO that need a good knowledge of the neutrino flux in order to analyse their data. To the extent that electromagnetic creation of muon pairs can be neglected, each muon produced in air showers initiated by a primary cosmic nucleus (mostly proton) is accompanied by a muon neutrino.

At sea level (Hanoi) muon momenta have a broad spectrum and a mean value of 4.8GeV/c. Muon attenuation and decay are easily accounted for when comparing muons and neutrinos: the stopping power of the whole atmosphere is only 2GeV and the decay length of a 4GeV/c muon is 26km.

Longitude Latitude The rigidity cutoff that describes the effect of the earth magnetic field on primary cosmic rays is maximal in the Vietnam region: 17GV compared to 11GV in Japan and typically 4GV in most European and north American laboratories where muon fluxes have been measured. This makes the present data of particular interest.

Iron Scintillator Lucite 1.9m 80cm 40cm axis b) a) Two different scintillator arrangements have been used, a fixed hodoscope for the measurement of the vertical flux and an orientable telescope. Here we concentrate on the latter (a). It is made of six scintillator plates, each 40x80x3cm3, arranged in three coaxial sets of two plates (b): the first (upper) set is separated from the other two (lower) by 2m and the two lower sets are close by but separated by a 2cm iron plate used as a radiator for electron tagging. The acceptance is 2.22 10-2m2sr and the detector momentum cutoff is 120MeV/c.

Pulse height 1 to 4 Time of flight arb. scale The area and timing of each scintillator pulse are recorded for each trigger (a fourfold coincidence between the upper set and the pair of the lower set located in front of the iron radiator). Accidental rate is negligible. At large zenith angles a small background due to cosmic rays entering the telescope from the lower set and subsequently splashing onto the upper set is easily removed using a time of flight cut.

,  are assumed to be angle-independent Pulse height 5 and 6 Cut arb. scale e2=e1 Data have been taken at vertical incidence with 10cm of lead ( 18 radiation lengths) between the upper and the lower sets (only muons survive). The tag pulse height distribution is used to separate muons from electrons. Define N1 = 1 + e1 below cut and N2 = 1 + e1 above cut ,  are assumed to be angle-independent At vertical incidence: Lead data ( no electrons):  = N2 / N1 No lead data ( corrected for attenuation):  = Δ N2 / Δ N1 At all angles:  = (1+  )( N1 - N2 )/(-) e = (1+ )(N2 -  N1 )/(-) e/ ~ 8% with little angle dependence.

Typical corrections are at the few percent level: the cut efficiency is 91%, the electron subtraction 8%, the hadron contamination 3%. A global uncertainty of 2.2% applies to all data (from the vertical lead data). In addition point-to-point uncertainties apply to  >0. Global uncertainties Point–to-point uncertainties Background 1.2% Electron subtraction 0.7% Statistics 0.2% Backward electrons (60o) 0.4% Acceptance 1.5% Hadron contamination 0.5% Stopping muons 0.9% Angle setting accuracy Auger Cherenkov Roof Upper detector Lower detector Small Cherenkov Various checks have been made of the validity of the method of analysis including data recorded beyond iron absorbers of various thicknesses (4 to 20mm), threefold coincidence triggers, delayed coincidence rates, etc. Particularly useful is the comparison with the data taken with the fixed vertical geometry telescope that are affected by very different systematic errors. Excellent agreement is observed.

ZENITH ANGLE DISTRIBUTION For convenience we define J= /{(1-.108 sin2) cos2} where  is the measured flux measured toward the north (open circles) or averaged between east and west (full circles). Good agreement is found with Honda model’s prediction (dash and full lines respectively).

The east-west asymmetry expected from the effect of the earth magnetic field on cosmic primaries (positively charged) has been measured as a function of zenith angle and the azimuthal distribution of the muon flux has been measured at zenith angles of 500 and 650 in the region of maximal asymmetry. The data are consistent with the prediction of the Honda model but give evidence for slightly smaller amplitude of the azimuthal oscillation (by 16  4%). Both a one-dimensional and a three-dimensional version of Honda model have been compared with the data. The less steep zenith angle distribution predicted by the former is in slightly better agreement with the data.  (degrees)  (degrees)

Pierre Auger Observatory

Track length Pulse height VATLY members helped with the installation of the Auger array in the Argentinean pampa. In order to become familiar with the physics and the techniques of the Pierre Auger Observatory, the VATLY team performs measurements using a water Cherenkov counter that it has constructed and installed on the roof of the laboratory. They include efficiency measurements, muon lifetime measurements, etc. In parallel with this we analyze real Auger data that are available on the web and learn how to reconstruct them and how to understand them using toy simulations of our own whenever necessary.

Summary Measurements of the muon flux have been made in Hanoi, where the rigidity cutoff takes the very high value of 17GV, as a function of zenith angle and azimuth with a typical 2% accuracy. The results are in good agreement with the predictions of the Honda model (both one-dimensional and three-dimensional versions). The future of the VATLY Laboratory is with the Pierre Auger Observatory, this is the aim of most of our present activities. We are deeply indebted to CERN, RIKEN, the French CNRS, the Rencontres du Vietnam, the Natural Science Council of Vietnam and the Pierre Auger Collaboration for invaluable support.