Purpose of tracking detectors with stereo design

[tgessler]

Purpose of tracking detectors with "stereo design"

HERA-B used silicon microstrip detectors for their vertex detector. Behind each two sided detector with strips in x- and y direction, a second identical detector was placed, rotated by 5 degrees. LHCb uses a similar design for the phi-detector in their vertex locator.

I don't understand the purpose of this geometry. Of course an additional detector in a different position increases resolution, but why exactly 5 degrees and not, say, 45 degrees? How is that "stereo"?

I read in an LHCb doc: "This ensures that adjacent stations are able to distinguish ghost hits from true hits through the use of traditional stereo view." But this still doesn't explain the rotation angle. Couldn't this distinction be achieved without any rotation?

 

[Vanadium 50]

Fundamentally the reason for stereo is to resolve ambiguities. Suppose you had a detector that read out x and y. If you get a particle at (1,5), no problem: iot reads out x = 1 and y = 5, and you can infer what happened.

But suppose you get two particles: one at (1,5) and another at (2,3). Now you don't know if you have (1,5) and (2,3) or (1,3) and (2,5), because you get x = 1 and 2, and y = 3 and 5, but no information on what goes with what.

If you add a second measuring station tilted with respect to the first, you can resolve these ambiguities.

As far as the rotation angle, that's a design element that can vary between experiments. Sometimes its determined for performance reasons, and sometimes its determined by where there's room for the cables. And everything in between.