Measuring the ATLAS ITk pixel detector material via multiple scattering of positrons at the CERN PS

The ITk is a new silicon tracker for the ATLAS experiment designed to increase detector resolution, readout capacity, and radiation hardness, in preparation for the larger number of simultaneous proton-proton interactions at the High Luminosity LHC. This paper presents the first direct measurement of the material budget of an ATLAS ITk pixel module, performed at a testbeam at the CERN Proton Synchrotron via the multiple scattering of low energy positrons within the module volume. Using a four plane telescope of thin monolithic pixel detectors from the MALTA Collaboration, scattering datasets were recorded at a beam energy of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>1.2</mml:mn> <mml:mspace></mml:mspace> <mml:mtext>GeV</mml:mtext></mml:mrow> </mml:math> . Kink angle distributions were extracted from tracks derived with and without information from the ITk pixel module, and were fit to extract the RMS scattering angle, which was converted to a fractional radiation length <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>x</mml:mi> <mml:mo>/</mml:mo> <mml:msub><mml:mi>X</mml:mi> <mml:mn>0</mml:mn></mml:msub> </mml:mrow> </mml:math> . The average <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>x</mml:mi> <mml:mo>/</mml:mo> <mml:msub><mml:mi>X</mml:mi> <mml:mn>0</mml:mn></mml:msub> </mml:mrow> </mml:math> across the module was measured as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>[</mml:mo> <mml:mn>0.89</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.01</mml:mn> <mml:mspace></mml:mspace> <mml:mtext>(resolution)</mml:mtext> <mml:mo>±</mml:mo> <mml:mn>0.01</mml:mn> <mml:mspace></mml:mspace> <mml:mtext>(subtraction)</mml:mtext> <mml:mspace></mml:mspace> <mml:mspace></mml:mspace> <mml:mo>±</mml:mo> <mml:mspace></mml:mspace> <mml:mspace></mml:mspace> <mml:mn>0.08</mml:mn> <mml:mspace></mml:mspace> <mml:mtext>(beam momentum band)</mml:mtext> <mml:mo>]</mml:mo> <mml:mo>%</mml:mo></mml:mrow> </mml:math> , which agrees within uncertainties with an estimate of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>0.88</mml:mn> <mml:mo>%</mml:mo></mml:mrow> </mml:math> derived from material component expectations.