tgeo-material

desire.physics.ucsd.edu/tgeo-material

Extracting material distributions from detector geometries as loaded into ROOT's TGeoManager — radiation and interaction length along chosen paths, rather than averaged over all directions.

• Material budget 2-point functions, CMS Phase-2
  How much material a particle crosses between two tracker layers, as a function of which two points it travels between. Ten adjacent-layer correlators from the innermost pixel layer to the barrel timing layer, in both X₀ and λ_I.
  9 September 2026 · cmsSimGeom-Run4D123
• Where the material is: composition, endcaps, a chimney hunt
  Is the budget in the layers or the gaps, and what is it made of? Plus the endcap disc-to-disc correlators away from the transition region, a negative result on an azimuthally localised service shaft, and a TGeoPcon stack overflow found along the way.
  9 September 2026 · follow-up to the above
• The barrel/endcap transition, and how to represent it
  Where surfaces and discs interleave and neither parametrisation applies. Only a quarter of the material there sits inside a measurement surface, against four fifths in the barrel — measured, with what that implies for the shape of a usable material map.
  9 September 2026 · part 3

Why this shape

The standard material-budget plot answers "how much material at this pseudorapidity", integrating outward from the interaction point. That is the right question for a photon conversion probability and the wrong one for multiple scattering between two measurements, which depends on the pair of surfaces and on the direction of travel between them. These pages compute the two-point version, and keep enough of the parameter space to show where the one-point answer is misleading.

The extraction is meant to be consumed, not just read: the same stepper produces the numbers behind these plots and can be called directly on an arbitrary path, which is usually cheaper and always more accurate than interpolating a precomputed grid.