Millepede package

Spiros found the Millepede program for Linear Least Squares Fits with a Large Number of Parameters. The Millepede's idea is to define two sets of parameters:

local ones which are associated with the given event. In our case these are the position of primary vertex and direction and momentum of primary tracks)
global ones which are associated with detector alignment and which are common for all events.

and fit local parameters for each event with fixed global ones, substruct result from resudials and then fit global parameters. The procedure is iterative because it supposes to refit local parameters with the corrected global ones and tigher cuts.

The test case provided with the package contains alignment of 10 planes with one measurement calibrated with straught tracks.

Plane test

To check performance of MIllepede for our case

two measurement per plane and
6 alignment parameters

I create a plane test example. The events contain 5 tracks coming from the same vertex with pT exponentially distributed from 0.2 GeV/c

10K events
100K events

Notation: (u,v,w) are (x,y,z) is the plane local coordinated system, (α,β,γ) are rotation along (u,v,w). Histogram is simulated misalignment, points are reconstructed ones.
Correlations

Layer alignment

Yuri Fisyak

Last modified: Wed Feb 1 14:49:09 EST 2006