1. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Top Charge Measurement using Jet Charge Templates Per Hansson Christophe Clement David Milstead Royal Institute of Technology (KTH) and Stockholm University Sweden  Analysis strategy  Jet charge algorithm: selections and why it works  Jet charge performance in data  Top charge

2. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Analysis strategy Goal: discriminate between |Q top | = 2e/3 and |Q ”top” |=4e/3 The top charge is given by the sum of the charge of its decay products The analysis proceeds: Select lepton+4≥jets events with 2 SVT tagged jets (S/B~14) Compute the jet charge of the 2 SVT-tagged jets Associate the b-jets to correct W boson (charged lepton) using HITFIT Combine the 2 jet charges and the lepton charge information to derive the two top charges Compare the observed top charge distributions with the expected 2e/3 and 4e/3 expectations.

3. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Jet Charge Algorithm Compute jet charge only for SVT-tagged jets Taggable jets are:  Standard jet ID cuts, p T >15 GeV, |  |<2.5  Matched to a track-jet (DR=0.5),  At least 2 tracks in DR=0.5, Dz<2cm  Each track has at least 1 SMT hit in barrel or F-disks, p T >0.5 GeV  At least 1 track with pT>1 GeV  DCA<0.2cm and zDCA<0.4cm SVT tagged jets have an explicitely reconstructed secondary vertex with:  At least 2 tracks,  Transverse decay length significance of 7,  Chi2<10,... Jet charge= weighted sum the charges of the tracks above 0.5GeV within DR=0.5 of the jet axis Weighted by the p T of the tracks (a=0.6) Jet Charge only on SVT tagged jets DERIVE the performance on DATA with minimal input from Monte Carlo Algorithm:

4. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Why does it work? The charge of the quark is correlated with the charge of the highest pT hadron resulting of the hadronization Simple study carried out with Pythia: Generated 2  2 process, pT>15GeV And look at hight pT u or b quarks produced in the process. Usually large number of hadrons Produced, most quite modest pT Thís is then smeared by detector effects... Charge of b-quark Charge of highest pT hadron

5. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Jet Charge Performance in Data Use a tag and probe method in data: Select ”pure” bbbar events in data (”triple tag sample”), require: ”Simple minded”: the charge of the muon gives the charge of the opposite b- quark In reality: Is it pure bbbar? (ccbar, flavor excitation, gluon splitting?) The muon can come from a D-meson, cascade decay, B-mixed meson, Charge misidentification ”Tag and probe method”  >3.0

6. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Data Calibration Corrections True performance in data ”blurred” by processes affecting the sign of the tagging muon, the muon: comes from a ccbar event (background) comes from the decay of a B meson after mixing is a ”cascade” muon (eg. B  D  , J/    +  -, etc,.. ) has misidentified charge The true jet charge distribution for b and anti-b jets can be obtained by correcting the Triple Tag data sample for these processes Discriminant Power Tag and probe method

7. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Is the triple tag sample pure ”bbbar”? The fraction of c-jets in the triple tag sample is determined by pTrel fit of the order of a few percents, Flavor excitation/ splitting?  >3.0 2 b jets back 2 back dominates Phys. Rev. D 65, 094006 (2002)

8. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Correction for ccbar in triple tag Triple Tag sample is not pure b-jets: x c ~3% ccbar, calculated from p Trel template fit Makes the discriminating power better than it really is. Associated systematic errors: Error on the fitted correction Vary c-fraction within its error c-corrected data Jet charge from triple tag data b- jet charge from MC b- and c-jet charges from MC mixed with x c

9. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Correction for Cascade, B-mixing,... Correcting for processes that ”swaps” the sign of the charge of the tagging muon ”Cascade” muons Muons from ”B-mixed” mesons Charge misreconstruction Associated systematic errors Amount of cascade decay in Monte Carlo/data Stat error on the correction Charge misidentification of the tagging muon different in data and MC c-corrected data Cascade/mix/c -corrected data True b-jet charge- from MC b-jet charge from tag/probe method in MC 26-30%

10. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Final b-jet charge templates Discriminating power dependent on the  of the SVT-tagged jet: Different in  top and triple tag data Reweight  distr. of the triple tag sample to have same  spectrum as top events Carry out the entire procedure with updated corrections as function of p Trel cut on the tagging muon

11. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 SM Top Charge Observables We need an observable and an expectation for the ”2/3” and ”4/3” scenarios Consider only lepton+jets channel (e/µ + 4 jets) double-tagged events Two top quarks in the event  measure the charge ”twice” Use HitFit (kinematic fit) to assign b-jets to correct W-bosons in ttbar MC q b and q B are taken from the data derived jet charge templates q b = b lept. side q B = b hadr. side qBqB qbqb qBqB qbqb

12. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Exotic Top Charge Observables q b and q B are taken from the data derived jet charge templates The exotic scenario is obtained by permuting the charge of the SVT tagged jets q b = b lept. side q B = b hadr. side qBqB qBqB qbqb qbqb

13. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Concerning HITFIT convergence In this analysis the fraction of events in which HITFIT converges is ~81% Other analyses give a fraction of converged Hitfit of 99% Where does the difference come from? ~99% is how often at least of the many Hitfit hypothesis converges, Cross check  we obtain also 99% in this case. In our analysis we use only the combinations for which both Hitfit and the SVT tagged ”agree” on what jets are the b-jets from top. (avoid combinatorial backgrounds) For eg. in events with 4 jets it means there are 12 combinations but only 2 for which Hitfit agree with the two SVT-tagged jets. Hitfit converges for at least one of these 2 combinations in 81% of the events. 53% of the events in which HITFIT fails to converge for these two combinations are due to events where one of the tagged jets is a c-jet.

14. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Result on ~370pb -1 data 21 Lepton + jets channel, double tagged events only 17 were HitFit converges  34 measured top charges Perform likelihood ratio test 34 q i s

15. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Result on ~370pb -1 data Expected confidence level is obtained by ensemble tests Exclution of exotic scenario from data: Consistency with Standard Model: Confidence computed with 2 independent codes Expected CL if 100% of correct combinations without HITFIT and assuming discriminating power from MC CL= 99.999%

16. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Jet charge specific systematics Error on c-fraction in triple-tag sample. Uncertainty in Monte Carlo concerning cascade decay and B-mixing We derive a 4% systematic due to pT spectrum difference between data and Monte Carlo Tagging muon charge misidentification 2% systematic due to discrepancy between Data and Monte Carlo Uncertainty due to limited Monte Carlo statistics in data calibration General systematics (affecting kinematical fit) Top Mass (default is 175 GeV) Jet Energy Scale Jet Energy Resolution JetReco  ID efficiency Fraction of background events is let to Poisson fluctuate Systematic summary

17. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Conclusions We have already performed many cross checks, the result seems to be stable The background composition in l+jets cross section analysis has changed a little  estimate the effect on the final result. Would like to include the effect of high pT electron charge misID (0.991+/- 0.001) We have also estimated the effect of using ttbar+1jet ALPGEN sample, the effect seem to be small.

18. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Backup slides

19. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Jet eta spectrum different for top and bbar data where I derive jet charge templates Reweight the data derived jet charges to ttbar MC according to the ratio of the eta distributions Each jet charge is reweighted according to the jet eta when building Standard Model and Exotic template Jet Charge eta dependence

20. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 I only use C and B templates and neglect light contribution C fraction in triple tag selection

21. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Only affecting the matching since jet charge is independent of jet pT (?) Jet Energy Scale Fraction Lowest  2 permutation correct 0.79  0.02-1  0.79  0.02+1  0.79  0.02 Cen

22. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Used samples with varied Jet Energy Resolution to build the templates Jet Energy Resolution Fraction Lowest  2 permutation correct 0.79  0.02-1  0.81  0.02+1  0.79  0.02 Cen

23. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Used samples with varied Jetreco*ID scale factor Jet reco * ID efficiency Fraction Lowest  2 permutation correct 0.79  0.02-1  0.80  0.02+1  0.79  0.02 Cen

24. Section of experimental particle physics Per Hansson Top Charge @ D0 Summer Physics Workshop June 22- 05 Binned top charge: predicted and observed