# Presentation: 683933

## TrkLoopFix

- A module in TrkFixup
- Gerry  Lynch
- October 1, 2005

- Gerry Lynch                   tools workshop                October 1, 2005

## What is a Looper?

- Simple definition:
- A track that goes at least a full turn in the detector.
- Alternate Definition:
- A track that is fragmented into two or more tracks, which share many layers.
- Operational Definition:
- Pairs of low momentum tracks that are 180 degrees apart.

- Gerry Lynch                   tools workshop                October 1, 2005

## Looper Azimuth difference

- Before TrkLoopFix a selection  is made that keeps  low momentum track pairs that are within 0.2 radians of 180 degrees in both azimuth and dip.  These go in the work list..

- There is a small (~7%) “background” under the peak.

- Gerry Lynch                   tools workshop                October 1, 2005

## Looper Sample

-  We see that most of the tracks that are in the looper sample are tracks that hit the outer wall of the DCH.
- The preselection for the “work” list used by TrkLoopFix has a cut at 100 cm,

- Gerry Lynch                   tools workshop                October 1, 2005

-   Looper Multipicity

- The number of tracks for each looper size is shown here.

- Most looper candidates have two tracks

- Gerry Lynch                   tools workshop                October 1, 2005

## Apogee, Parigee

- In the looper at the right,Track 1 is first.
- Tracks 1 and 2 form an apogee  pair.
- Tracks 2 and 3 form a parigee pair.
- Our goal is to keep 1 and reject 2 and 3.
- The procedure for classifying the looper candidates starts with labeling each pair as apogee. perigee, neither, or either

- In any looper pair one tracks is going out and the other going in.
-   But we fit the tracks as if they were going out.  So one is wrong.

- Gerry Lynch                   tools workshop                October 1, 2005

## Method 1

- When we plot Sum® against Sum(d0), we See a clear band of apogee pairs.
-  Very few are rejected.
- The vertical black band is a mixture of apogee and perigee pairs.

- Gerry Lynch                   tools workshop                October 1, 2005

## Method 2 

-      In this plot of the calculated number of turns from z02-z01, omega, and tanDip,we see clear apogee and (impure) perigee bands at dip angles above about 0.15.
- The number rejected is small, but larger than for method 1

- Gerry Lynch                   tools workshop                October 1, 2005

## Beam-line Chi-squared

- Pairs with both tracks primary are not loopers.
- Pairs with one track primary are probably apogee pairs.
- Note that many of the good apogee pairs have no primary track.

- Gerry Lynch                   tools workshop                October 1, 2005

## Position Chi-squared

- We get chi-squareds for the comparison of the orbits at parigee and apogee.
- The full chi-squared is not very useful, but the position component is valuable.
- This shows how these chi-squared agree with method 2 decisions.
- This position chi-squared is used to classify the pairs that were not decided before.

- Gerry Lynch                   tools workshop                October 1, 2005

## Procedures

-  The result of the classification of single pair loopers is
-      67% apogee         17% neither ( many of these are not loopers)
-      15% perigee           1%  either
- For  a single pair looper we keep the best track and discard the other
-       for apogee and perigee pairs and keep both for the others.
- For multi-pair loopers the procedure is to choose the best pair
-       by a formula that  prefers the apogee pair with the highest
-       momentum.  If that pair is apogee or parigee, the preferred track in that
-      pair is kept and all others rejected.
- For multi-pair loopers that are classified  either or neither, both
-     tracks in the highest momentum pair are kept.

- Gerry Lynch                   tools workshop                October 1, 2005

## MC Checks

- If our apogee pairs are right, the sign of curvature will agree with the Monte Carlo truth.
- The present program gets agreement with the MC 98% of the time, showing that the correct track is is being selected.
- For perigee pairs the fraction in the opposite curvature sign  is 61%.    I think that this should be larger.
- Another check is to compare fitted track parameters,    To do this one needs to pay attention to the track direction and what sheet of the helix the parameters represent.

- Gerry Lynch                   tools workshop                October 1, 2005

## MC Parameter Test

- The parameters of the tracks are compared with the Monte Carlo truth.
- There is excellent agreement.  Multi-pair loopers look almost as good.

- Gerry Lynch                   tools workshop                October 1, 2005

## Problems

- As indicated on the MC checks page, the performance for  parigee pairs is suspect.
- One is tempted to conclude from the MC parameters test that things are ok because when the chi-squared is small the track was accepted.
- But all the small chi-squareds are from tracks in  apogee pairs.  The parigee pair tracks do not have small chi-squareds, probably because they are fitted in the wrong direction,.
- One approach to this would be to fit all of the parigee candidate tracks  and work with two possible solutions when searching for best solution.
- We will have to refit the retained perigee track in any case because it is in the wrong direction.

- Gerry Lynch                   tools workshop                October 1, 2005