o Impact of different Geant4 effects and Genfit MSC models on - - PowerPoint PPT Presentation

• Impact of different Geant4 effects and Genfit

MSC models on Si-tracking

Moritz Nadler

Institute of High Energy Physics Austrian Academy of Sciences

March 2012

Moritz Nadler 1 HEPHY Wien & BELLE Collaboration

Overview

I will mainly present 2 studies: simulation only study investigating scattering angles of Pions tracking study about the effect of different scattering models in Geant4 and Genfit for Pions and Muons

Moritz Nadler 2 HEPHY Wien & BELLE Collaboration

Other stuff

the Genfit RKTrackRep needs modification to deal with arbitrary curling tracks: I hope to discuss this with Johannes Rauch my Genfit hit sorting for curling tracks made problems with root I/O so it had to go, but will be reintroduced soon the segmentation faults in the vertexing module were fixed by Patrick but no further development and testing was done

Moritz Nadler 3 HEPHY Wien & BELLE Collaboration

Note on multiple scattering models for track fit

MSC formula implemented in Genfit: σ2

Genfit = 225 · 10−6

p2β2 X Z Z + 1 ln(159Z−1/3) ln(287Z−1/2) Highland formula (from PDG book, H. Ozaki proposed to try it): σ2

HL = 0.01362

p2β2 X (1 + 0.038 ln(X)) both describe electro-magnetic multiple scattering the ratio σHL/σGf is 0.92, 0.75, 0.69 for X = 1, 320 µm Si, 75 µm Si Genfit formula should give a best σ for the whole scattering angle distribution; Highland a best σ for the core theoretically σHL ≈ σGf at X = 1

Moritz Nadler 4 HEPHY Wien & BELLE Collaboration

Aim and method of scattering angles study

Aim: determine strength of electro-magnetic and other effects on scattering at different energies estimate number of Pion tracks lost for reconstruction because of material interaction in sensors Method: simulated 100000 tracks with different effect and momentum combinations: all, only MSC, and no MSC calculated ∆φ and ∆θ of entry and exit points of the active sensor materials (used info from TrueHits) looked at standard deviation of whole samples, of a Gaussian fit of whole sample, and of Gaussian fit after cuts determined by ±5σ of first fit. calculated ratio of sample size before and after cuts

Moritz Nadler 5 HEPHY Wien & BELLE Collaboration

pGun parameters for simulation only studies

If not stated otherwise: p fixed at 1 GeV or 100 MeV, Φ ∈ [0◦360◦] uniformly distributed, θ = 90◦ π+, π− uniformly distributed no other material/detectors than the SVD and PXD detectors no magnetic field after 15 cm

• nly tracks with exactly one hit in every layer

100000 tracks simulated ≈ 66000 tracks used

Moritz Nadler 6 HEPHY Wien & BELLE Collaboration

Results: Details I

An example: whole ∆φ sample of layer 4 with all Pion (1 GeV) effects on:

• 0.3
• 0.2
• 0.1

0.1 500 1000 1500 2000 2500

histTitle

hist

Entries 66906 Mean

• 1.106e-05

RMS 0.002334 / ndf

2

χ 444.2 / 190 Constant 11.9 ± 2420 Mean 2.255e-06 ± 1.328e-06 Sigma 0.0000017 ± 0.0005813

histTitle

x in rad

Moritz Nadler 7 HEPHY Wien & BELLE Collaboration

Results: Details II

same as last slide but a cut of 5σ applied to the sample

• 0.002
• 0.001

0.001 0.002 50 100 150 200 250 300

histTitle

hist

Entries 66791 Mean 9.132e-07 RMS 0.0005962 / ndf

2

χ 934.2 / 828 Constant 1.3 ± 266.4 Mean 2.239e-06 ± 2.186e-06 Sigma 0.0000017 ± 0.0005734

histTitle

x in rad

Moritz Nadler 8 HEPHY Wien & BELLE Collaboration

Results: Details III

σ of ∆φ in layer 4 (all σ in mrad) 1 GeV 1 GeV 1 GeV 100 MeV 100 MeV 100 MeV all

• nly msc

no msc all

• nly msc

no msc whole 2.3 0.62 2.3 12 12 8.4 fit 0.58 0.57 × 9.9 9.9 ×

• a. cut

0.57 0.57 × 9.8 9.8 × ratio−1 1.7‰ 1.5‰ × 1.4‰ 1.2‰ × comparison with track fit MSC formulas: Highland: 0.63, 11 (1 GeV, 100 MeV) Genfit: 0.84, 14 (1 GeV, 100 MeV)

Moritz Nadler 9 HEPHY Wien & BELLE Collaboration

Overview of results and conclusions

the outlier ratio is between 1.2‰ and 2.4‰ per layer = ⇒ ≈ 2‰ of Pion tracks might be lost per Si sensor layer even with 66000 tracks hadronic scattering does not form a recognizable distribution at 1 GeV hadronic effects dominate the sample σ and produce most outliers at 100 MeV EM-MSC dominates and hadronic effects become relatively unimportant the Highland formula seems to produce a σ closer to the σ

• f Geant4’s EM-MSC than Genfit

Moritz Nadler 10 HEPHY Wien & BELLE Collaboration

Aim and method of tracking study

Aim: determine the consequences on track fit quality of different scattering effects and their description in Genfit Method: 8 different samples of 10000 tracks were fitted with 2 different MSC formulas in Genfit (Genfit and Highland) = ⇒ 16 different fitted track samples Pions (normal and only EM effects) and Muons (with and without Wentzel model) at 1 GeV and 100 MeV afterwards the p-value distributions (from total χ2), the pulls

• f p

p p and x x x of the track extrapolated to the origin with respect to the true values and other distributions were investigated

Moritz Nadler 11 HEPHY Wien & BELLE Collaboration

pGun parameters for tracking studies

If not stated otherwise: p fixed at 1 GeV or 100 MeV, Φ ∈ [0◦, 360◦] uniformly distributed, θ = 90◦ either µ+, µ− uniformly distributed or π+, π− uniformly distributed no other material/detectors than the SVD and PXD detectors no magnetic field after 15 cm truth information used as starting values for track fit and “pattern recognition”

• ther condition also optimal, e.g. only Gaussian smearing,

detector resolution the same in digitizer and Genfit 10000 tracks simulated

Moritz Nadler 12 HEPHY Wien & BELLE Collaboration

Result I: p-value/pulls oddity 1

when using the Genfit MSC model with 1 GeV EM effects

• nly Pions p values are very good but pulls are to small

(reason for H. Ozaki trying Highland) when using Highland formula pulls are good but p-values get worse

htemp Entries 10000 Mean 0.5036 RMS 0.2938 pValue_bu.data 0.2 0.4 0.6 0.8 1 20 40 60 80 100 120 140 160 htemp Entries 10000 Mean 0.5036 RMS 0.2938

pValue_bu.data

σ of pulls: 0.8 – 0.91

htemp Entries 10000 Mean 0.4684 RMS 0.2962 pValue_bu.data 0.2 0.4 0.6 0.8 1 50 100 150 200 250 htemp Entries 10000 Mean 0.4684 RMS 0.2962

pValue_bu.data

σ of pulls: 1.01 – 1.02

Moritz Nadler 13 HEPHY Wien & BELLE Collaboration

Result I: p-value/pulls oddity 2

not caused by extrapolation to origin, already present in layer wise tests cause seems to be very subtle and connected to the off diagonal elements of the Genfit covariance matrices exactly the same effect is present when looking at Muons without Wentzel when fitting with both MSC models in Genfit

Moritz Nadler 14 HEPHY Wien & BELLE Collaboration

Result II: Genfit overestimates the MSC σ at low p

at low energies the Genfit formula for MSC clearly

• verestimates the MSC σ

below a comparison of p values of 100 MeV Pions fitted with both models

htemp Entries 9851 Mean 0.7171 RMS 0.2739 pValue_bu.data 0.2 0.4 0.6 0.8 1 100 200 300 400 500 htemp Entries 9851 Mean 0.7171 RMS 0.2739

pValue_bu.data

σ of pulls: 0.7 – 0.86

htemp Entries 9899 Mean 0.4689 RMS 0.3187 pValue_bu.data 0.2 0.4 0.6 0.8 1 100 200 300 400 500 600 700 htemp Entries 9899 Mean 0.4689 RMS 0.3187

pValue_bu.data

σ of pulls: 0.94 – 1.12

Moritz Nadler 15 HEPHY Wien & BELLE Collaboration

Result III: p dependency of different outlier producing effects

at 100 MeV it has no effect on tracking results if Pions uses

• nly EM effects or all effects

= ⇒ hadronic effects become irrelevant at low p the first study already suggested this but Wentzel model’s impact grows at low p example: Muons at 100 MeV w/wo Wentzel, Highland for fitting

htemp Entries 9984 Mean 0.4485 RMS 0.3113 pValue_bu.data 0.2 0.4 0.6 0.8 1 100 200 300 400 500 600 700 htemp Entries 9984 Mean 0.4485 RMS 0.3113

pValue_bu.data

htemp Entries 9982 Mean 0.3463 RMS 0.3124 pValue_bu.data 0.2 0.4 0.6 0.8 1 200 400 600 800 1000 1200 1400 htemp Entries 9982 Mean 0.3463 RMS 0.3124

pValue_bu.data

Moritz Nadler 16 HEPHY Wien & BELLE Collaboration

Conclusions and things to do

the current Genfit MSC implementation should be modified investigate the p-value/pull oddity at low momenta in combination with thin sensor modules standard Geant4 MSC model (Urban) produces some outlier tracks with very strong scattering angles with further tracking studies I will more clearly determine the quantity of “kinky” tracks for different particles and p ranges we have several ideas how to deal with them (e.g. importing Gaussian sum filter like features) the effort put into “outlier fitting” strongly depends on the number of “kinky” tracks surviving the pattern recognition again: get into contact with Geant4 developers about Wentzel effect (also for Pions?)

Moritz Nadler 17 HEPHY Wien & BELLE Collaboration

The End

Any advice, suggestions or comments are very welcome

Moritz Nadler 18 HEPHY Wien & BELLE Collaboration