A measurement of the energy loss spectrum of 150 GeV muons in iron

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 ${\rm {d}}P/{\rm {d}}v$ per radiation lengthof a fractional energy loss $v = \Delta E_{\mu} / E_{\mu}$ has been measured in the range $v$ = 0.01 $\div$ 0.95; it is compared with the theoretical predictions for energy losses due to bremsstrahlungand production of electron-positron pairs or of energetic knock-on electrons. The integrated probability $\int_{0.01}^{0.95} ({\rm {d}}P/{\rm {d}}v) {\rm {d}}v$ is $(1.610\pm0.015_{{\rm {stat}}}\pm0.105_{{\rm {syst}}})\cdot10^{-3}$ in agreement with the theoretical predictions $1.556\cdot10^{-3}$ and $1.619\cdot10^{-3}$. Agreement with theory is also found in two intervals of $v$ where production of electron-positron pairs and knock-on electrons dominates. In the region of bremsstrahlungdominance ($v$ = 0.12 $\div$~0.95) the measured integrated probability $(1.160\pm0.040_{{\rm {stat}}}\pm0.075_{{\rm {syst}}})\cdot 10^{-4}$ is in agreement with the theoretical value of $1.185 \cdot 10^{-4} $, obtained using the Petrukhin and Shestakov description of the bremsstrahlungprocess. 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\cdot 10^{-4}$, obtained using Tsai's description of bremsstrahlung.