Magnetic Field Issues for Simulation and Reconstruction N. Amapane, N. Neumeister Workshop on LHC Physics with High-p T Muons in CMS Bologna, April 9-12, 2003

Nicola AmapaneBologna, April 10, Outline Simulation and reconstruction tasks require knowledge of different details of the detector geometry Connection of geometry and magnetic field How/why are we going to (re-)implement geometry representations? Status, plans, current work

Nicola AmapaneBologna, April 10, Geometry: Requirements Simulation Geometry: must be detailed –Used for tracking –Includes both active and passive material –Currently described in XML To be checked/updated by detector experts Not only chambers but also return yoke! –GEANT volumes are constructed out of it Reconstruction Geometry : must be lightweight –To allow fast traversal (navigation) –Currently, includes only chambers (active material) Extracted from same XML description; ORCA implementation No material at all (just volumes and positions) Recent optimisation  major speed-up –But material description is needed for global reconstruction… To extrapolate track parameters through the yoke (propagation) ORCA 6: we instantiate simulation geometry anyhow (GEANE)

Nicola AmapaneBologna, April 10, Muon Reconstruction Requires ability to extrapolate track parameters: Propagation –Non const, discontinuous field (Runge-Kutta Integration) –large amount of material  Multiple scattering, energy loss –non trivial! Ingredients: 1.Parameter estimation (5 track parameters) 2.Error propagation Currently: GEANE –based on GEANT3 (FORTRAN) –does both 1. and 2. –Uses simulation geometry: too many details  too slow! –Phased out in 2003 GEANT4 provides only 1. –Not (yet?) 2.

Nicola AmapaneBologna, April 10, “Geometries” in ORCA 6 Simulation Geometry (Material) Reconstruction Geometry (Detector shapes/positions) Tracking Propagation Navigation Simulation Reconstruction Magnetic Field Map Magnetic Geometry Magnetic field map: one more geometry! Track Fit decomposition: –Navigation (where to look for hits) –Propagation (extrapolate there) Including energy loss, multiple scattering –Parameter update (math)

Nicola AmapaneBologna, April 10, Magnetic Field Used both by simulation and reconstruction Must be known with good precision –Proved impact on quality of reconstruction Current implementation: –From an independent magnetic geometry Differences wrt. simulation geometry!!! Values on a grid in global coordinates Requirements: –Current system must be replaced In 2003 ZEBRA will be phased out –Fast! Performance issue for simulation and reconstruction Now: linear search in a ZEBRA bank + interpolation –Consistent with other geometries –Ability to introduce misalignment 3D TOSCA calculation TOSCA geometry (magnetic materials) ZEBRA bank Field values in global coordinates Field Interface: FieldValue(GlobalPoint)

Nicola AmapaneBologna, April 10, Magnetic Field Geometry V. Klyukhin

Nicola AmapaneBologna, April 10, Field Map V. Andreev

Nicola AmapaneBologna, April 10, Misalignment Misaligned chambers go inside the yoke’s field! Nominal Misaligned Field value sits in global space

Nicola AmapaneBologna, April 10, Misalignment (2) Magnetic field should be “attached” to each volume Nominal Misaligned

Nicola AmapaneBologna, April 10, A Different Approach (Teddy) Propagation problem decomposed in: –Geometry of non-sensitive materials with optimised navigation Fast location of homogeneous volumes Each volume knows: –the magnetic field inside it (in local coordinates) –The properties of its material –its neighbors (= navigation solved) –Propagation in a single volume through homogeneous material and continuous (non-constant) magnetic field Parameter estimation –In vacuum: Runge-Kutta integration –Addition of material effects (energy loss, multiple scattering) Error estimation –Propagation of error matrix Muon responsibility From the tracker community

Nicola AmapaneBologna, April 10, “Geometries” in ORCA 7 Simulation Geometry (Material) Reconstruction Geometry (Detectors) Tracking Propagation Navigation Simulation Reconstruction Magnetic Field Magnetic/Material Geometry Find volume (Navigation) Material props XML description

Nicola AmapaneBologna, April 10, More on Runge-Kutta Integration Numerical tools available/being developed Convergence of solution depends on the form of the field and its derivatives Linear interpolation in a 3D grid is not optimal! –Discontinuous derivatives –Treatment of borders (discontinuities) Inside one homogeneous volume a parametrisation can be used: –Field is “smooth”, well-behaved –Derivatives are known analytically –Match Runge-Kutta order to order of polynomials

Nicola AmapaneBologna, April 10, Ongoing Work Field Parametrisation V. Andreev Field values per volume S.Valuev Field values in XML Navigable Geometry Nicola, Teddy Material properties […] Volume Propagator Teddy, Are, Wolfgang 3D TOSCA calculation TOSCA geometry (magnetic materials) Field values in global coordinates XML mapping V. Andreev Field in Volume And: Integration, testing, prototyping

Nicola AmapaneBologna, April 10, Navigable Geometry Build a seamless set of inter-connected volumes –Each volume knows its surfaces –Each surface knows its neighbor volumes Navigation solved; optimized by design –In fact, problem moved to: how to build the volumes and their connections? –Hard in the generic case –Feasible if the geometry is “simple”, e.g. onion-like Design and base classes: advanced status (Teddy) Volume instantiation: started (NA)

Nicola AmapaneBologna, April 10, Summary GEANT3/ZEBRA will be abandoned this year –Must provide replacements for magnetic field map and propagation –Performance and flexibility are a REAL problem (eg. in trigger!) Good occasion to improve things A set of dedicated, optimised components is being developed Does not come for free –The contribution of the muon community is essential