Tracking Corrections
Page maintainer: Gunar
TMC section:
 |
This page is considered done. It been reviewed by Sergey. There may be missing elements, but they are all flagged and the text has no errors. |
RMC and HTC sections:
 |
This page is not yet ready for use or review. It is assigned to no one. The page isn't being edited right this minute, so feel free to add any information you have about this subject. |
Intro
The magnetic holding field for the polarized target and the field of the recoil magnet deflected charged particles in the target region. For the longitudinally polarized target the field was directed along the Fehler beim Parsen (Konvertierungsfehler. Der Server („https://wikimedia.org/api/rest_“) hat berichtet: „Cannot get mml. Server problem.“): {\displaystyle z} -axis and thus had only little impact on tracks detectable by the forward spectrometer as for that the momentum components perpendicular to the field were small.
In contrast, for the transversely polarized target the field was directed along the (negative) -axis and thus always had a large component perpendicular to the particle momentum. This configuration leads to a deflection of particle in the front region close to the target, which in turn affects the track reconstruction and thus the particle's kinematics at the production/scattering vertex. The transverse magnet correction (TMC) accounts for this deflection of charged-particle tracks.
For the data with recoil detector, again a solenoid was used. However, both its effects on forward tracks through the fringe fields and its effects on recoil tracks, which experience a large magnetic field perpendicular to the track momenta, had to be taken care of in the reconstruction. For forward tracks the (into the main spectrometer) recoil magnet correction (RMC) (aka Siguang-TMC) was available but never used in productions.
A new track reconstruction based on Kalman Filter was developed by Alexander Kisselev. HTC runs on top of HRC correcting tracks taking into account all magnetic fields as well as energy loss and multiple scattering because of material in the path of the particle. HTC can in principle be used for all data-taking periods. So far (as of Oct. 2008) only the 2007 production is available but more years are planned to be reproduced with HTC.
TMC (Transverse Magnet Correction)
- For the 2002-2005 polarized data, a correction for the transverse target magnet field (TMC) is applied to every track. Results of 2 methods are available in the uDST productions:
- Method 1 (TMC1, "Witold's method") uses a sort of lookup table.
- Method 2 (TMC2, "Stan's method") uses a transfer matrix from the target region to a reference plane in the front region, where the target field is negligible.
Within the target region, TMC2 attempts to find the closes approach of a helix to the beam line, while TMC1 looks up tracks that come a priori from the beam line.
TMC might become obsolete with the introduction of HTC.
However, so far no production with HTC for data with transverse target magnet is available (as of October 2008).
All the latest productions of transverse data (currently, all d productions) have HTC included.
Siguang TMC
- In 2006 and 2007, the Recoil Detector was installed together with the recoil solenoid around the target region.
The magnet has shown to have some impact on the tracking in the front spectrometer. Siguang TMC was developed to help in the analysis of long-living particles that decayed some distance away from the scattering vertex.
HTC
A new tracking code, HTC, by Alexander Kisselev is designed to take care of any (known) magnetic field configuration in the target area. First round of 2002-2007 data productions done in the beginning of 2009 demonstrated that HTC can really improve track resolutions significantly (in particular Lambda and K-short invariant mass peaks became noticeably more narrow in respective analyses). Whether survey-based alignment used in these productions really improved systematic effects assiciated with the top-to-bottom detector misalignment, as well as a possible non-zero beam slopes, is still an open question as of this writing (July'2009).
Please see the HTC page for more details for users.
Essentials
TMC
- For information about where the results are stored and which methods are valid for which productions, please follow this link: [1]
- With the introduction of TMC version 10, the helix parametrization of the track within the target cell became available. This parametrization could be used for a better reconstruction of, e.g., decaying particles, the scattering vertex for SIDIS events (by finding the closest approach of at least two tracks to each other and maybe the beam line) etc. So far, no uDST production has made used of this feature as first TMC has to be run over the current hrc productions to fill the required tables in the TMC output files.
- In the 04c1 and 05c1 productions, beam shifts are taken into account by TMC (i.e., TMC does not try to reconstruct the track to (0,0,z), but to the specified beam position). The beam positions were estimated from the 04b and 05b productions, e.g., by means of Alexander's beam finder, which eventually led to the solution of the "2005 Phi puzzle". (it is foreseen that the correct beam position will be taken into account in all the productions, i.e., 02 to 05, as soon as the final numbers are available and a new hrc/uDST production can be started)
- The beam shifts used for the 04c1 and 05c1 are summarized below (see also here for how to use them in MC productions)
| year | x-position | y-position |
| 2004 | 0.247cm | 0.065cm |
| 2005 | 0.233cm | 0.01cm |
(note that when the transverse target is on additional shifts come on top of these as the target magnet deflected the beam)
For information on the beam position in general please see also the Beam position page.
TMC in the various uDST productions
- The 02 productions (including 02c1) only have Stan's method (method 2) correctly implemented.
- The 03 productions (including 03c1) only have Witold's method (method 1).
- The 04c1 and 05c1 productions provide both TMC methods.
Handling of tracks and events where TMC failed to correct the track parameters
From time to time it can happen that TMC fails to find an appropriate correction for a given track. One frequent problem is, e.g., a track originating from outside the target cell (and/or not from the beam line). The question then arises what to do with the track and/or the event this track belonged to. There are no general recommendations or guidelines (yet), but the following might be a useful guidance to make a decision depending on the various analyses.
In general, if one method fails one can always fall back to the other method (if available). There could be reasons for not doing so (e.g., want to be consistent and only use one method), but there is no real objection to this approach. But even then, it can happen that also the other method failed leading to a track that is uncorrected for the influence of the transverse magnet. In most cases ( inclusive and semi-inclusive analyses), one can just disregard that track and continue with the event like the track did not exist. An analogy would be a short track. Such a track is usually not considered in the analysis even though the track was clearly present in the event. This does not mean one should discard the whole event.
The situation is different, however, for exclusive reactions. Even though the track might not be useful, it still needs to be accounted for in the exclusivity requirements. Take for example DVCS: Clearly there should be no charged track besides the lepton, even if that additional charged track could not be corrected by TMC.
Also for the reconstruction of longer-living particles, like the Fehler beim Parsen (MathML mit SVG- oder PNG-Rückgriff (empfohlen für moderne Browser und Barrierefreiheitswerkzeuge): Ungültige Antwort („Math extension cannot connect to Restbase.“) von Server „https://wikimedia.org/api/rest_v1/“:): {\displaystyle \Lambda} , one does not need to discard an uncorrectable track. The decay particles of such a longer-living particle are just prime examples of tracks that don't fall under the category "coming from the beam pipe" and thus should not be handled by TMC (especially when the decay took place somewhere outside the target region or, in particular, outside the area where the holding field is sizable).
It is also customary to make a cut on the total energy reconstructed in an event (e.g., sum of all particle-momenta should be less then roughly the beam energy). For this calculation, the uncorrected track should still be considered (in fact the energy, or better the momentum, of the particle is unaffected by the transverse holding field and thus by TMC).
One last note - as most failures stem from the z-vertex being outside the target cell the track often gets rejected anyway (though not always).
Siguang TMC
Corrections to the HRC tracking for the transverse target and recoil magnetic fields were developed for the Lambda and Exotics analyses. To make it easier the abbreviation SiguangTMC is usually called sTMC.
sTMC is a general method to accurately describe relativistic charged particle transport within a 3-dimensional, non-uniform magnetic field. In our case, it uses a precisely measured 3D map of target magnetic field (it is available for the different experimental configurations between the years 2002 to 2007). To start its "magnetic swimming", sTMC only needs to know the position, the momentum and the particle ID of every track at any point downstream the target magnetic field region. It can provide then the position and the momentum at any moment of the corrected trajectories (usually parametrized by the z coordinate). In addition, sTMC can reconstruct complex decaying topologies which involves one or more concatenated decays. Basically, it calculates the point of closest approach (crossing) between any two tracks to be a decay vertex candidate and reconstruct the "mother particle" to continue the tracking back procedure. Crossings with the beam are also supported: Beam is also treated as a any other particle trajectory as long as position and slopes could be provided.
It is important to remark that sTMC was thought to be an user-end tracking correction. That means that, unlike the others tracking corrections (TMC, TMC2 and HTC), it doesn't take part in the standard main production chain, but the analyzer must use it on top of the final uDST output files.
There is a dedicated page: Siguang-TMC page
HTC
HTC is a single track parameter and track-to-track vertex (re)fitting code. It takes tracks already found by HRC and does sofisticated iterative Kalman-filter based trajectory parameter tuning in the vicinity of the original HRC track hypothesis. HTC utilizes knowledge over all the associated low-level information needed to do progressive track propagation through the HERMES spectrometer (3D geometry and material database; drift chamber alignment, resolutions and calibrations; magnetic field maps; beam line offsets and slopes, etc). It allows to give perhaps best-possible estimates on track parameters at the beam crossing and/or at the (possible) vertices with other tracks of a given event. HTC also provides covariance matrices on the found parameters, as well as verifies track and vertex "quality".
An example code which explains how to extract HTC-related information from uDST files, together with an extensive README file is available as a self-contained bundle /hermes/r26/htc/ucode-1.6.tar.gz .
Please see the HTC page for more detail from/for users.
More Info
TMC
- Internal note 07-008: [2] TMC - Vertex Reconstruction in the Presence of the HERMES Transverse Target Magnet
- Internal note 07-035: [3] Handling arbitrary beam trajectories in GMC
Siguang TMC
- 2008/02/07: [4] Alexander's mail on Recoil/spectrometer fringe field map
- 2007/06/18: [5] Siguang's talk about his Recoil Magnet Correction algorithm
- Internal note 08-004: [6] Track Reconstruction Using a 3-D Map of the Target Magnetic Field.
HTC
Talks
See the list on the HTC page