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Center of mass energy determination from Z g events

Center of mass energy determination from Z g events. Introduction/ motivation. Selection, measurement method. Results 189GeV-207GeV. Systematic Error- Focus on: fragmentation. ISR simulation. detector effects. Other LEP experiments results. Benjamin Trocmé LAPP Annecy.

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Center of mass energy determination from Z g events

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  1. Center of mass energy determination from Zg events • Introduction/ motivation. • Selection, measurement method. • Results 189GeV-207GeV. • Systematic Error- Focus on: • fragmentation. • ISR simulation. • detector effects. • Other LEP experiments results. Benjamin Trocmé LAPP Annecy

  2. Motivation to measure beam energy • Last Aleph W mass measurement sent to Moriond 2001: • Systematic correlated between all channels and experiments!!

  3. Classical determination of Ebeam • Magnetic field measurement with NMR probes; calibrated at low energy with resonant depolarisation+extrapolation. • Linear behaviour of NMR probes checked by flux loop.

  4. Classical determination of Ebeam(2) • Alternative methods to constraint high energy region: • spectrometer + Qs measurement: performed during dedicated periods. Final results not yet published. • Zg analysis.

  5. Zg analysis: principle • Z resonance still very important at LEP2 with radiative return processes: • Z resonance parameters well known  extract . • Analysis started by Eugeni Grauges: • paper published at 183GeV. • presentation in Thursday meeting 13/1/2000.

  6. Considered events • 3 types of events: • e+e-qq + g undetected • e+e-qq + g detected • e+e-m+m- +g undetected

  7. e+e-qq + g undetected

  8. e+e-qq + g detected

  9. e+e- m+m- +g undetected

  10. Extraction of x • Classical selection leading to high level of purity (~90%). • x=1-s’/s: fraction of beam energy by ISR photon deduced: • muonic channel:classical angle formulae • hadronic channel: kinematic reconstruction, taking into account jet energies. When detected,photon axis is • considered.

  11. Reweighting fit • Use of similar method as W mass measurement to extract Ecm. • Reference Monte Carlo: KK2f. • Main constraint: peak position.

  12. Result at 188.6GeV: e+e-qq + g undetected. • Parabolic • behaviour

  13. Result at 188.6GeV: e+e- m+m- + g undetected.

  14. Analysis between 189GeV and 207GeV • One per energy/ channel. • Combination of new quantity: • Statistical error: Ezg only. • Systematic error: ELEP and Ezg. • Uncorrelated between ELEP and Ezg. • 100% correlated between years.

  15. e+e-qq + g undetected e+e-qq + g detected

  16. e+e- m+m- +g undetected General combination • 2.3s

  17. Systematic error • Procedure follows as much as possible the one used for W mass.

  18. LEP simulation • Angles between beams: • 1mrad (overestimated) would lead to 4MeV bias. • Energy dispersion in a given sample • related systematic ~ 3MeV • Differences of energies between Ee+ and Ee- • additionnal boost (24MeV in 98): no impact on peak position; only on resolution.

  19. Fragmentation • Used samples: • 500K of kk2f hadronised with Jetset. • 500K of kk2f hadronised with Herwig v6.2, using Kinagain facility. • 48 pseudo experiments with 2 different reference samples.

  20. Fragmentation (2) • bias consistent with observed by Opal: • Need further investigations (DMW<15MeV).

  21. ISR simulation • KK2f is known to be the most achieved available Monte Carlo: • includes ISR/FSR interference for muons in CEEX scheme. • higher order of correction. • simulates beam energy spread.

  22. ISR simulation(2) KoralZ KK2f KK2f • Corrections (a2L)>corr.(a3L3) • CEEX at order (a2) is more accurate than EEX at order (a3).

  23. ISR simulation(3) • Procedure to estimate ISR systematic. • Reference: CEEX at order (a2). • Observe bias when degrading computation by two different ways: • CEEX at order (a1). • EEX at order (a3). • Very easy to do in KK2f (weighting procedure).

  24. ISR simulation in hadronic channel • CEEX at order (a1). • EEX at order (a3). • Impact < 20MeV: order of magnitude expected by Jadach.

  25. Detector effects:preliminary remarks • Why is Zg measurement so sensitive to detector effect?? • Very characteristic topology due to Z0 boost: • many objects with cos(Q)~0.7-0.8. • Most of events have pair of jets contained in one half space. or

  26. Another typical Zg event

  27. Detector effects:preliminary remarks(2) • (ECM) a (jets opening angle) • 2mrad • 220MeV!! • Opening angle sensitivity increases with Ecm.

  28. Jet angular bias • Estimated by comparing tracks content to photon content. • 2mrad!! • This a non signed bias estimate: add or subtract to jet angles. • Will systematically increase/decrease opening angle

  29. Jet angular bias impact(2) • 96 pseudo experiments at 189GeV • Huge effect: 147MeV(combined)!

  30. Jet angular bias (3) • Historically, previous function was chosen to estimate the largest potential effect. • was found to be important for Zg. • Why is it so small for W? • no systematic effect on opening angle: Opening angle Opening angle increased decreased

  31. Calorimeter simulation • Apply W mass procedure: perform 48 pseudo experiments with 2 different Monte Carlo: • reference. • shaken calorimeters. • Rescaling depends on polar angle and is performed before jet clustering • angle are modified.

  32. Calorimeter simulation(2) • Impact on Zg measurement between 98MeV and 128MeV depending on sample. • All samples combined:109MeV

  33. Conclusion on Aleph result • 2.3s disagreement between ELEP and Ezg: • Various checks proved result stability. • Conservative systematic error. • dominant part of detector systematic. • double counting: calorimeter syst/ jet angular bias. • Zg special topology can turn almost negligible error for W into the main systematic!! • Investigations to sign biases and correct them.

  34. Other LEP experiment results • This method was initiated by Aleph. • Presentation at April 2000 LEP workshop  other experiments were asked to look at their data.

  35. Other LEP experiment results (2) • UNOFFICIAL results presented at Lisbon (november 2000). • BEWARE:Ebeam

  36. Opal PRELIMINARY result

  37. Other LEP experiment results(3) • Disagreement seems to be confirmed by other experiments, at least in hadronic channel. • Measurements are all dominated by detector systematic • 3 Zg LEP meetings since Lisbon workshops. • Activity from mainly Opal and Delphi. Lack of manpower in L3. Very useful inputs from theorist (Jadach et al.).

  38. LEP Plans • Meeting next tuesday: • updates with Y2K result by Opal? Delphi? • following meeting with Jadach, comparisons on ISR systematic. • plans for future combination hoped for summer conf.

  39. Impact on W mass • Reminder: • If deviation is confirmed by other LEP exp., several possible origins: • Monte Carlo problem: could affect W mass. • wrong LEP energy:would affect W mass: very unlikely. • If deviation is not confirmed, one must hope apparatus effect, that does not affect W mass (it is likely, considering Zg special topology.) • ONGOING INVESTIGATIONS

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