A measurement of the material in the ATLAS inner detector using secondary hadronic interactions

TL;DR

This study directly maps the material in the ATLAS inner detector by reconstructing secondary vertices from hadronic interactions in 7 TeV pp collisions, and verifies the detector's material description by comparing data to a detailed Geant4-based simulation. The analysis employs an inclusive two-track vertex finder with iterative merging to isolate nuclear-interaction vertices, achieving sub-millimeter spatial resolution and precise localization of the material. The data/MC comparisons show agreement at the ~7% level, after accounting for systematic uncertainties, and reveal small but important discrepancies such as beam-pipe offset and pixel-cooling-fluid modeling that informed MC refinements. These results enhance confidence in track, lepton, jet, and MET reconstructions and demonstrate the value of hadronic-vertex radiography for detector material studies.

Abstract

The ATLAS inner detector is used to reconstruct secondary vertices due to hadronic interactions of primary collision products, so probing the location and amount of material in the inner region of ATLAS. Data collected in 7 TeV pp collisions at the LHC, with a minimum bias trigger, are used for comparisons with simulated events. The reconstructed secondary vertices have spatial resolutions ranging from ~200 microns to 1 mm. The overall material description in the simulation is validated to within an experimental uncertainty of about 7%. This will lead to a better understanding of the reconstruction of various objects such as tracks, leptons, jets, and missing transverse momentum.

Figures (12)

• Figure 1: Plan view of a quadrant of the inner detector showing each of the major detector elements with their active dimensions and envelopes. The lower part shows a zoom of the pixel region.
• Figure 2: Momentum spectrum of primary tracks intersecting the beam pipe at $|z| < 300$ mm in data (points) and MC (filled histogram). The two spectra are normalized to unity.
• Figure 3: Non-diffractive MC: (a) z and (b) R resolutions for vertices reconstructed in the beam pipe.
• Figure 4: Mass of reconstructed vertices in data. All secondary vertices with $|z|<700$ mm have been used.
• Figure 5: The R vs. z distribution of secondary vertices reconstructed in data. The bin width is 7 mm in z and 1 mm in R. To aid the eye, only bins with 5 or more entries have been displayed.