LCD Fast MC Track Smear

Details

The track smear takes the MC truth (x,p) parameters for charged final state particles and, if they pass hard polar angle and minimum momentum cuts, will smear the parameters and construct a 'track'.

The 5-parameter tracking errors are calculated by two routines (one for cylindrical layers and the other for disks) based on the algorithm outlined by Pierre Billoir in NIM 225, p352, with one modification: Billoir's trick for avoiding repeated error matrix inversion is not used, because saving CPU cycles is not an issue for these calculations. In addition, some approximations have been made to avoid complexity:

1. The track helix has been approximated by a parabola in the r/phi projection
2. The track helix has been aproximated as a straight line in the r/z projection. Both of these become inaccurate as the angle subtended by the circular arc in the r/phi view becomes appreciable. Thus, calculated errors for tracks with momentum below 1 GeV/c or so are suspect.
   

The algorithm works from outside in, adding the measurement information and multiple scattering error to the weight and error matrices, respectively, layer by layer. For the cylindrical algorithm, material from disks can be included in the error calculation, but the algorithm is not capable of including measurements from disk layers. Likewise, for the disk algorithm, passive cylindrical layers can be included, by measurements from cylindrical layers can not. Both algorithms include the beam constraint (if requested) for the momentum, phi, and theta errors, but automatically exclude it for the r/phi and r/z impact errors.

Correlations between the tracking parameters are available from the algorithm, but have not been made available in the simulation package. The algorithm calculates tracking errors in a grid of total momentum and theta. The smearing routine in the simulation interpolates within (and exptrapolates without) this grid to estimate the track parameter errors for a given candidate track.

Properties

• 'Final state' is defined as StdHep's idhep code = 1.
• At present, no efficiences are applied inside the acceptance window.
• acceptances and the resolution parameters are input from ascii files containing
  • minimum momentum to accept
  • inner polar angle cutoff for barrel
  • outer polar angle cutoff for barrel/endcap, also used for acceptance
  • currently, if there is no endcap, the two polar angle cut parameters must be equal
  • name(s) of the file(s) containing the resolution parameter lookup tables
• no track parameter errors are computed
• tracks are cast into a helical parameter set: phi, lambda (pi/2-theta), 1/pt, z, and r_2d and smearing applied to these parameters, followed by conversion back to cartesian coordinates. If the smeared 1/pt changes sign, the 'found' charge is flipped.
• 2D lookup tables are generated, currently by Bruce Schumm, for given tracker geometries (combinations of vertex detector and central tracker). Linear interpolation is applied to the tables, with the appropriate positions in the grid found by binary search. The interpolation is extended to cover the low momenta (below 1 GeV) as well. No values of polar angle or momentum greater than the maximum in the tables are permitted.
  • the polar angle region is typically divided into 6 to 8 points while there are 13 momentum points ranging up to 1 TeV.
  • there can be separate lookup tables for barrel and endcap
    • if there are two tables, the polar angle upper bound of the barrel must equal the lower bound of the endcap.

Code Description

• Classes
  • TrackFullSmear: driver for the smear. Selects MC particles and applies acceptance cuts.
  • SmearTrack: given input MC particle, creates a 'track' after smearing the parameters. Also parses input ascii parameter file giving acceptance cuts and pointing at the lookup table file(s).
  • GetTrackLookups: parses lookup tables.
  • LookUp2d: Each lookup table contains the grid points for polar angle and momentum, plus a matrix of the resolution values. The interpolateVal member function finds an interpolated value given input polar angle and momentum, using linear interpolation. For momenta between zero and the first bin (1 GeV), the interpolation is extended outside the grid.

R.Dubois Last modified: 01/13/04 12:45