STAR Stv tracker & Forward detectors

STAR

STAR Stv tracker & Forward detectors

V. PerevoztchikovBrookhaven National Laboratory,USA

STAR Victor Perevoztchikov, BNLSTAR Upgrade Review

Why new tracker?For more than 6 years ITTF (Sti) has been used as our

tracker. It works well but there are some important limitations• Geometry is too limited and cannot be enough accurate. As a result energy

loss and multiple scattering are not accounted with the needed precision;

•Geometry ordering , essential for Sti, does not fit well even for TPC and practically impossible for the new detectors;

•Forward tracking was not foreseen and is not possible with Sti Geometry. Only TPC like detectors are allowed. Sensitive planes must be oriented strongly along Z axis.

•Introducing of new detectors demands rather complicated job with a additional coding of Sti geometry, hits loader etc… ;

•Magnetic field must be constant and along Z.

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STAR Victor Perevoztchikov, BNL STAR Upgrade Review

The main features, related to this Workshop are:

• Flexibility to add and modify new detectors. Practically only Geometry description is needed.

• Automatic recognition of detector orientation. Orientation could be arbitrary, with plane or cylindrical surfaces;

• The relationship between hits and detector elements. I.e. Geant volume id and blueprint relationship is created automatically;

• Arbitrary magnetic field orientation;

• Multiple Seed Finders

• Track extension to other , not tracking detectors;

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Stv tracker. Main features.

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• ROOT TGeo package is chosen as the Geometry description.

• TGeo could be created from many sources. In our case from STARSIM & AgML;

• TGeo allows detailed description of the detector. Thus energy loss and multiple scattering would be accurate;

• The same Geometry could be used for simulation and reconstruction;• Geant tracking engine, based on TGeo used in reconstruction.

• TGeo Stv extension• Each TGeo volume has a proxy object which contains additional Stv

information. Most of information is automatically created during initialization stage. Some could be defined by user;

• Description of sensitive detector element: orientation, type (plane, cylindrical, …), hit error functors, activity status etc…

• Hits containers with hits associated with this detector element;• Etc…

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ROOT TGeo and extensions

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• Geant VMC was selected as a tracking engine. • The standard Geant machinery is used to do tracking

thru the complicated Geometry, accounting for magnetic field, energy loss and multiple scattering;

• When track is entering into a sensitive volume, then the according hit error functor, hit container, activity flag etc… are accessible.

• Relationship of hits with detector elements also provided by Stv using Geant.

• User does not need to define or change his Geant <==> Blueprint lookup tables, after each design modification;

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Geant tracking engine

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What should be done to add new detector?Make Geometry description of the detector;Make StHit container in StEvent. If, by any reason, it is

inconvenient, the special adaptor could be made;Make hit errors definition. It could be done via StHit or

as a functor inherited from StvHitErrCalculator;Any detectors providing 3D hits can be easilly added

today with no additional work;

Any detectors having other hits can be added with minimal work , adding specific seed finder.

Job is done. But tuning still needed.

How to add or modify New detector

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We use Geant as a tracking engine. Geant allows tracking in arbitrary magnetic field. But tracking only is not enough. To do fitting the error propagation is also needed. Right now error propagation is implemented for Z magnetic field only. But Stv design allows another implementation with arbitrary magnetic field.

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Arbitrary magnetic field orientation

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By design Stv uses several seed finders. The first one is the default seed finder and others specialized ones.

Stv allows multiple seed finders to be executed sequentially;

Seed finders may be called repetitively;Detector specific seed finders may be defined, otherwise

default is used;At the end of the tracking phase, the default seed finder is

called repetitively until no additional tracks are found.

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Stv Seed Finders

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Non standard seed finders are needed in two cases:

1.Increase performance for concrete detector(like CA for TPC now);

2.Non standard detector, for which default seed finder does not work;

The typical example of point 2. is Forward GEM Tracker (FGT) FGT hits are elongated. Some hits define only Z and Rxy, others Z and Phi. In addition, there are amplitudes which could be used as well.

Right now we have:•Default seed finder, could work with any detector ;

•CA seed finder oriented for TPC only;

•2D hit generic seed finder in development.(To be discussed later)

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Stv Seed Finders continue

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2D hit Seed finder.

2D seed finder is a generic one for all 2D hit detectors.It is based on the following ideas:

All 2d hits are joined into 3D hits, with a many fake ones. Amplitudes could be used to decrease amount of fake hits. 1st hit selected. Its coordinates considered as zero.

All hits in neighborhood projected onto 2d plane, using their φ,λ angles. Real track on this plane represented by spot. Using well known “K Neighbor” method we find the spot.If there is no spot, the first hit is fake. Select the next.When spot is found, the seed is created from all related hits

This is not yet ready, only part of code is written.

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ETR Example

We tested our approach on ETR. It is forward detector with non TPC-like orientation. In reality it has 2D hits, but in this case we used 3D hits from fast simulator. For ETR, the point is that the naive approach already showsassociations but a tailored approach will be better. Especially,we have seen split (we create two tracks per each ETR seed /segment) and this needs to be studied. At that stage, importantto note that nothing comes straight out of the box when oneintegrates information from multiple detectors to achieveintegrated tracking.

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Reconstructed ETR event in X,Y

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Reconstructed ETR event in Z,Y

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Reconstructed ETR event in X,Z

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This spot is a groups of

hits


Amplified spot

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Benefits obtained from Stv:

• ROOT TGeo as the Geometry description: accurate account of energy loss and multiple scattering;

• TGeo Stv extension: automation of adding new detectors with arbitrary orientation of sensitive surfaces;

• Geant tracking engine:• passing thru multiple detectors with arbitrary magnetic field;• providing automatic hits <==> detector elements relationship;• track extension to other , not-tracking detectors;

• Seed Finder list: adding non standard seed finders;

Summary.

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STAR

Conclusion

• First try of integrating ETR produced results (Ming's talk). Framework demonstrated inclusion of new detectors is possible and simple enough

• Work ongoing to tune hit errors for best tracking (TPC focused for now)

• Stv review is planned for after QM.

Victor Perevoztchikov, BNL STAR Upgrade Review17

STAR Stv tracker & Forward detectors

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