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The energy loss spectrum of 150 GeV muons has been measured with a prototype of the ATLAS hadron calorimeter in the H8 beam of the CERN SPS. The differential probability dP/d upsilon per radiation length of a fractional energy loss upsilon = Delta E(mu)E(upsilon) has been measured in the range upsilon = 0.01 divided by 0.95; it is compared with the theoretical predictions for energy losses due to bremsstrahlung and production of electron-positron pairs or of energetic knock-on electrons. The integrated probability integral(0.01)(0.95)(dP/d upsilon)d upsilon is (1.610 +/- 0.015(stat) +/- 0.105(syst)). 10(-3) in agreement with the theoretical predictions 1.556 . 10(-3) and 1.619 . 10(-3). Agreement with theory is also found in two intervals of upsilon where production of electron-positron pairs and knock-on electrons dominates. In the region of bremsstrahlung dominance (upsilon = 0.12 divided by 0.95) the measured integrated probability (1.160 +/- 0.040(stat) +/- 0.075(syst)). 10(-4) is in agreement with the theoretical value of 1.185 . 10(-4), obtained using the Petrukhin and Shestakov description of the bremsstrahlung process. The same result is about 3.6 standard deviations (defined as the quadratic sum of statistical and systematic errors) lower than the theoretical prediction of 1.472 . 10(-4) obtained using Tsai's description of bremsstrahlung

The first combined test of an electromagnetic liquid argon accordion calorimeter and a hadronic scintillating-tile calorimeter was carried out at the CERN SPS. These devices are prototypes of the barrel calorimeter of the future ATLAS experiment at the LHC. The energy resolution of pions in the energy range from 20 to 300 GeV at an incident angle of about 11 is well-described by the expression =E = 46:5 6:0 = p E + 1 : 2 0 : 3 3:2 0:4 GeV=E. Shower profiles, shower leakage, and the angular resolution of hadronic showers were also studied.

A new combined test of an electromagnetic liquid argon accordion calorimeter and a hadronic scintillating-tile calorimeter was carried out at the CERN SPS. These devices are prototypes of the barrel calorimeter of the future ATLAS experiment at the LHC. The energy resolution of pions in the energy range from 10 to 300 GeV at an incident S. Akhmadaliev et al. / Nuclear Instruments and Methods in Physics Research A 449 (2000) 461}477 463In the collider reference system, which has been adopted here, the z-axis indicates the LHC beam line, the x-and y-axis the horizontal and the vertical directions, while and are the azimuthal and polar angle, respectively. The pseudorapidity is de"ned as "!ln(tan( /2)).angle of about 123 is well described by the expression /E"((41.9$1.6)%/(E#(1.8$0.1)%)(1.8$0.1)/E, where E is in GeV. The response to electrons and muons was evaluated. Shower pro"les, shower leakage and the angular resolution of hadronic showers were also studied. Results are compared with those from the previous beam test.