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D. BOUMEDIENE CEA - SACLAY

The new W mass measurement in the tn qq channel. D. BOUMEDIENE CEA - SACLAY. 1- Introduction 2- Reconstruction of the tn qq events Method Test of the reconstruction 3- Mass extraction Method and performances 4- Results at all the energies

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D. BOUMEDIENE CEA - SACLAY

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  1. The new W mass measurement in the tnqq channel D. BOUMEDIENE CEA - SACLAY • 1- Introduction • 2- Reconstruction of the tnqq events • Method • Test of the reconstruction • 3- Mass extraction • Method and performances • 4- Results at all the energies • 5- Systematic uncertainties study • 6- Conclusion ALEPH Thursday meeting

  2. INTRODUCTION 1- The problem: - The old analysis was based on a reconstruction optimized for the selection - Indications that events had bad kinematic properties and that the reconstruction was not adapted - MW resolution was far from what we get in e(µ)qq channels. Stat. Error = 286MeV/c² in latest paper,normalized to 200 pb-1 :267 MeV/c² 2-Purpose : a) to change the reconstruction b)application to event selection and finally the W mass measurement ALEPH Thursday meeting

  3. Event reconstruction ALEPH Note 2000 - 039 • 1- Events properties: • The difference between WWqqen and WW  qqtn events comes from the different  decay products • We have to start the reconstruction with a special t finding step • 2- Reconstruction of the t : • tdecays give very different final states: • a-1 particle: e(g),m,p • very small visible mass • b- N>1 particles:pp0 (r1), p2p0,ppp,...…invariant mass>0.5 GeV/c2 • Use of a jet reco. Algorithm: different optima for the ycut ALEPH Thursday meeting

  4. Event reconstruction • Start with a jet reconstruction (JADE) • ycut = (0.75/Evis)2 • tjet = jet that maximizes isolation-momentum variable:P (1-cosqiso) • qiso = minimum jet-jet angle • If no ‘good’ t,loop on ycut(increasing ycut) • good t criteria • 1- Cut on P(1- cosqiso) depending on particles identification • 2- Number of charged tracks 1-5 • 3- Mhadronic [60,105] GeV/c2 ALEPH Thursday meeting

  5. The Reconstruction scheme ALEPH Thursday meeting

  6. Test of the reconstruction (at 189 GeV) • 2 variables are defined per t jet: • r: the purity of thet jet = • Number of ‘goodt tracks’ • Number of tracks in recotjet • e :the efficiency = • Number of ‘goodt tracks’ • Total number of visiblettracks in evt • re = 1 perfect reconstruction • (the best we can do at the detector level) • re = 0 ‘bad’ reconstruction • (no track in common with the true t, cannot do worse!) ALEPH Thursday meeting

  7. Test of the reconstruction (at 189 GeV) The RMS is divided by 2 for the same events  One expects an improvement on the W mass resolution. ALEPH Thursday meeting

  8. t jet mass Clear separation between the two t decay ‘classes’. ALEPH Thursday meeting

  9. The selection • Better reconstruction  topological variables are more discriminating • Use of a nn : • - Input: Global variables + Topological variables • The selection is improved and prefers the ‘good’ events (re =1) • (+4% of re =1, -3% of re =0) ALEPH Thursday meeting

  10. Jet 1 n Jet 2 t The kinematic fit - 2q jets fit (1C fit). For the W Mass no information from the K. fit Constraint: E=ELEP/2MW, D MW (Pc²) ALEPH Thursday meeting

  11. The mass extraction • 1- Description: • ALL the selected events which pass the kinematic fit and the mass window are used for the mass extraction • Two quantities per event are used: • MW and D MW • 1st class (Pc² > 0.03): 2D fit •  2nd class (Pc² < 0.03): 1D fit using MW • ( The Pc² cut used in the paper is replaced by a combined fit of 2 event classes ) • no sensitive dependence on the ‘transition’ value (0.03) ALEPH Thursday meeting

  12. The mass extraction • The fit is performed using reweighted MC ( with CC03 matrix elements) • 1- Results : • With Old analysis (same selection, Pc² cut, So same events) BUT new reconstruction : • Expected statistical uncertainty= • 240 MeV/c² • using the new analysis: • 213 MeV/c² ALEPH Thursday meeting

  13. Results @189 GeV It was : MW=80.385 ±0.285 GeV/c² in the previous analysis ALEPH Thursday meeting

  14. Mass distribution - previous Analysis @189GeV ALEPH Thursday meeting

  15. New Mass distribution @189GeV ALEPH Thursday meeting

  16. SUMMARY - performances Expected error Exp. Err. @ 189GeV for 200pb-1 Prev. Ana. 286267 New reco. 240 225 Old selection New analysis 213 198(MeV/c²) With an improved reconstruction we improve the selection, the efficiency of the kinematic fit and we get a more sensitive mass distribution. ~30% improvement ALEPH Thursday meeting

  17. Results at all the energies MW 0bserved / Expctd error (RMS) 189 80.490 ± .209 .213 (14) 192 80.627 ± .440 --- 196 80.553 ± .284 .315 (30) 200 80.311 ± .306 .310 (31) 202 80.058 ± .535 .482 (60) 205 80.639 ± .323 .346 (40) 207 80.137 ± .290 .267 (23) Combination 189-207 MW = 80.438 ± 0.113(GeV/c²)

  18. Main Systematic uncertainties SourceD MW (MeV/c2) Charged tracking 3 Jet energy resolution 11 Calorimeter simulation 9 Jet angle 3 Fragmentation 15 LEP energy 17 Background norm. 2 ALEPH Thursday meeting

  19. CONCLUSION This new reconstruction provides better quality events; The improvement has an impact on all the steps of the mass analysis. The W mass resolution is improved by ~30% without new or large systematic effects. The reconstruction could be used also for the width analysis as well as for the TGC. ALEPH Thursday meeting

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