TPC Momentum Resolution - a full simulation

TPC Momentum Resolution- a full simulationD. Karlen / U. Victoria & TRIUMFLC TPC mini workshop LAL-Orsay, Paris Jan 12, 2005

TPC Momentum Resolution

IntroductionSeveral groups (including Victoria) are using small prototype TPCs to characterize the performance for a LC TPCOur data taking run with cosmics and laser tracks in the DESY magnet last summer was very successfulpresentation at Durham LCWS showed preliminary resultsfull analysis is getting underway hope to have more complete results for the SLAC LCWS

This talk presents a simulation study of momentum resolution for a large GEM/MM TPCextrapolation of small prototypes to a full size TPC


GoalsCheck that the track fit can achieve desired momentum resolutionTest resolution dependence onpad sizeschannel to channel gain variationselectronics noisethresholdsother systematic effects


Momentum resolution studyFull simulation scheme:GEANT3 simulation of muon propagation and energy loss in a 140 cm length of TDR gas in 4.0 T fieldenergy losses in 1 mm segments are saved to flat filesjtpc simulation program:reads energy losses and converts to electron/ion pairselectrons drift, diffuse, pass through GEM holes, are amplified, diffuse, get collected on pads, pad signals generated, digitized, and signals stored in data filesjtpc data analysis program:read data files, signals converted to charge estimateslikelihood track fit performed to estimate track parametersmomentum resolution determined


Comparison of prototype and simulationagreement is reasonably goodtransverse resolution (mm)drift time (50 ns bins)300 mm30 mmB = 0, Dt = 700 mm/cmB = 0.9 T, Dt = 170 mm/cmB = 1.5 T, Dt = 110 mm/cmtarget = 0.3 mm


jtpc tracking detailsPad response functionin a GEM (or micromegas) TPC this can be parameterized analytically, using a simple modelFour track parameters: x0 (x at y=0) f0 (azimuthal angle at y=0) s (transverse s.d. of cloud) 1/r (radius of curvature)


Traditional trackingThe traditional approach:examine data from each row separately: define a point along the track


Traditional trackingThe traditional approach:examine data from each row separately: define a point along the trackfind best track that goes through points


Traditional trackingProblem with the traditional approach:information in one row is not sufficient to define a point along the track:charge sharing depends onx coordinatelocal azimuthal anglewidth of charge cloudeffect is largest when few pads hit per rowdependence is non-linearlinear centroid finding degrades resolution


Whole track approachThe whole track approach:fit information from all rows to determine the track parameters at once

Benefits of the whole track approach:no empirical parametersless calibrationreasonable estimates for error matrixbetter resolution


TPC simulationTDR gas at 4 T assumed:


TPC simulationcont


SimulationOther parameters:


Example 10 GeV muon eventPad size: 2 7 mm2 Drift distance: 2.5 mwith track fit overlaidEvent #1: true pt: 10.0013 GeV/c fit pt: 10.0017 GeV/c (Too good!)


Momentum resolutionrepeated events (same track parameters)


Resolution vs ptTESLA TDR Goal


Wider padsCharge sharing is less effective for wider pads:2 7 mm24 8 mm24 14 mm2


Wider pads4 mm appears to be too wide


Other pad sizesLonger or narrower2 7 mm22 14 mm21.5 10 mm2


Other pad sizes2mm x 7 mm seems reasonable


Influence of noiseNumber of electrons collected by a pad is estimated by integrating the pulse over 7 time bins (peak +/- 3).Prior to fitting, Gaussian noise is addedNoise on each channel assumed to be independent (no common mode included)


Influence of noiseSignificant noise can be tolerated in this caseEffective GEM gain: 5000


Comparison to standard parameterizationsAgreement with point resolution ~ 300 mm


SummaryInitial results encouraging:likelihood track fit appears to work reasonably wellresolution roughly agrees with parameterizationTDR resolution goal nearly attained with TDR gaswith large diffusion: relatively insensitive to noise at 5-10k e level

To do:repeat analysis with lower diffusion gas: eg. P10move tracks aroundlook at inclined trackslook at other systematics?


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TPC Momentum Resolution - a full simulation