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POLfit status report

POLfit presentation of the new version 'polfit5' with a focus on p0 mass constraint and improving fit results for charge patterns in high-energy physics experiments. The method compares observed and expected charge patterns using an algorithm to find the best match.

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POLfit status report

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  1. POLfit status report Maximilien Fechner (Saclay) • POLfit presentation • New version : ‘polfit5’ • p0 mass constraint

  2. NEW POLfit (pattern of light fit) reduce p0 BG in FC 1R-e for ne search (ex. JHF-LOI) • Input:(vertex,1stg direction, total visible energy) from the standard fit • POLfit also works in 2 stages : p0fit and e- fit • Fit two g rings by changing the vertex, the 2ndg direction & the energy fraction(note: only the 1st dir. is fixed) • Method : compare the observed with the expected(direct+scatter) charge pattern until the best match is found using a simplex algorithm (6 free parameters) • e- fit : maximize the likelihood for a single e- track, by changing the vertex (3 free par.) • Output: likelihood ratio (p0likelihood/ e- likelihood), energy fraction & direction of the 2ndg direction, reconstructed vertices from both fits Used for the 2km detector MC at the last T2K meeting (‘polfit4’) g-g plane(q=0) highest charge dir -f E2 E1 vertex f = E2/ Evis g2 1st dir g1

  3. Problems • Vertex fit necessary (There is a systematic bias along the ring direction in the reconstruction of the vertex @ 1KT ( crp analysis) : ‘polfit4’ (last august T2K coll. meeting) uses a 3D vertex fit too slow, 1D fit along the track direction is enough • Changing the vertex is expensive : the vertex should not be a simplex parameter (ie we should maximize the likelihood at a given vertex, allow the vertex to change, then repeat the procedure …) speed improved by a factor 3~4 • Use Dave Casper’s new expected charge calculation library in order to take reflections into account polfit5

  4. POLfit5 (pattern of light fit) • Input:(vertex,1stg direction, total visible energy) from the standard fit • POLfit also works in 2 stages : p0fit and e- fit • Fit two g rings by changing the vertex, the 2ndg direction & the energy fraction(note: only the 1st dir. is fixed) • Method : compare the observed with the expected(direct+scatter) charge pattern until the best match is found using a simplex algorithm (4 free parameters, special method) • e- fit : maximize the likelihood for a single e- track, by changing the vertex (1 free par.) • Output: likelihood ratio (p0likelihood/ e- likelihood), energy fraction & direction of the 2ndg direction, reconstructed vertices from both fits g-g plane(q=0) highest charge dir -f E2 E1 vertex f = E2/ Evis g2 1st dir g1

  5. New expq calculation with reflections Tested at the 1KT, using 1000 e- @ 500 MeV/c Better agreement between obs and exp

  6. Vertex fit results Red e- Blue pi0 cm Polfit 4 polfit5 polfit4 Projection of (V(reconst)-V(true)) along the input track Bias difference comes from the absence/presence of reflections 1KT MC @ 500 MeV/c

  7. e- fit cm polfit4 polfit5 Same resolution as before

  8. Comparison with std fit cm cm Distance between reconstructed and true vertex cm

  9. Polfit5 performance (preliminary) @1KT (50t FV), 500 MeV/c

  10. Polfit5 with mass constraint • Enforce the pi0 mass constraint during the fit (by adding a function that is zero when the mass constraint is respected, grows exponentially when leaving the physical region) • Why : for e- polfit usually reconstructs the second g track on top of the electron track. The pi0 mass constraint prevents this behaviour, making the pi0 pattern disagree with the observed charge for e- events. • Advantage : removes a free parameter, removes a discriminating variable ( pi0mass) making the POLfit cut easier to tune

  11. Reconstructed second g Polfit5_mconst degrees degrees Angle between reconst 2nd gamma And true 2nd gamma Angle between 2nd gamma and e- track polfit5

  12. Check with 2ring e-like pi0 events polfit5 Polfit5_mconst

  13. Preliminary results @1KT, 500 MeV/c Only 1 cut on the Dlikelihood

  14. performance • Same performance for vertex fit • Roughly same performance for e/pi0 separation at 500 MeV/c @1KT, more tests going on at different momenta • Estimation of the systematic error from the input vertex shift is also underway

  15. Efficiencies 1Re-like cuts && POLfit cuts @ 1KT POLfit cuts DL < 25 DL < 150 && Mp0 < 100 MeV/c² DL < 80 && Mp0 < 100 MeV/c² Checks at 200 MeV/c and 800 MeV/c @ 1KT underway

  16. Conclusions • ‘polfit5’ soon to be put in the repository • Better performance (expq++) and less sensitive to bias on the input vertex • Optional version with pi0 mass constraint • Tests with SK MC (monoenergetic and T2K beam) underway, more stats and momenta @ 1KT available next week • TODO : apply on 2km MC when G4 is tuned • Still slow despite optimizations to the vertex fit and expq++ library

  17. Backup slides

  18. New p0 vertex fit likelihood function 1. Standard method : Using a 4D simplex 2. New method : polfit5 Using a 1D simplex fit to find x At each x, a 3D simplex fit is used Instead of ~60 vertex changes (method #1), the vertex is changed ~15 times with no performance loss on the vertex & track fit

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