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Multiparameter Fits in tt Threshold Scan

Multiparameter Fits in tt Threshold Scan. Manel Martinez, IFAE (Barcelona) Ramon Miquel, LBNL (Berkeley) Introduction. The top threshold scan m t and a s The top width The top Yukawa coupling Conclusions. Introduction. The Top Threshold Scan (1).

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Multiparameter Fits in tt Threshold Scan

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  1. Multiparameter Fits in tt Threshold Scan Manel Martinez, IFAE (Barcelona) Ramon Miquel, LBNL (Berkeley) • Introduction. The top threshold scan • mt and as • The top width • The top Yukawa coupling • Conclusions Ramon Miquel, LBNL

  2. Introduction. The Top Threshold Scan (1) Cross section • Top threshold scan studied in great detail for many years in the contexts of both NLC/JLC and TESLA • The cross section in the threshold region issensitive to mt, as , Gt and lt(Kuehn, Jezabek, Teubner et al.) Dmt = 100 MeV Ramon Miquel, LBNL

  3. Introduction. The Top Threshold Scan (2) Peak of top momentum distr. Forward Backward asymmetry DGt = 200 MeV DGt = 200 MeV • Two additional observables have been used: AFB(s) and the peak of the top momentum distribution (Fermi motion) Ramon Miquel, LBNL

  4. Introduction. The Top Threshold Scan (3) • Experimental study based on work of “Barcelona” group: Martinez, R.M., Comas, Juste, Merino, Orteu • Outcome of experimental simulations: s:e = 0.41 syst = 3% bkgd = 0.0085 pb AFB:e = 0.11 syst = negligible P:e = 0.41syst = 4% • Assume: • Luminosity = 300 fb-1, TESLA beam spectrum. 9 point scan plus one point well below threshold for bkgd. determination • mt = 175 GeV, as(MZ)=0.120, MH = 120 GeV, Gt and lt as in the Standard Model Ramon Miquel, LBNL

  5. Introduction. The Top Threshold Scan (4) The “Experimental” Data Cross section Peak of top momentum distr. AFB Ramon Miquel, LBNL

  6. mt and as • Previous studies focused on mt determination: • Large correlation with as • Broken with new definitions of mt: “potential subtracted” mass and “1S” mass (Beneke, Huang, Teubner et al.) • Results (experimental errors only), using 1S mass: Dmt = 16 MeV Das = 0.0011 r = 0.34 (Including theoretical errors: Dmt = 100 MeV) • Using only cross section: • Dmt = 24 MeV Das = 0.0017 r = 0.74 • Improvement due to peak of momentum distribution • Now it is possible to look for sensitivity to top width and Yukawa coupling Ramon Miquel, LBNL

  7. The Top Width(1) • So far, only 1D or 2D fits used to study top width and Yukawa coupling. • However, sizable correlations with mtand as exist Multiparameter fits needed • Impractical with original TOPPIK(Kuehn, Jezabek, Teubner et al.) program. Used 4D interpolation routine instead. • 24 hours on PIII 400MHz to build 4D grid. Then fits take a few seconds. Ramon Miquel, LBNL

  8. The Top Width(2) Cross section • Sizable sensitivity in all observables • Leave the top width free in fit • Fix the Yukawa coupling to its SM value • Results: Dmt = 18 MeV Das = 0.0015 DGt = 32 MeV All correlations below 50% DGt = 200 MeV Ramon Miquel, LBNL

  9. The Top Width (3) • Previous result (Comas et al. 1996) had an 18% uncertainty while for new result it is about 2%. Why? • Higher luminosity:300 fb-1instead of50 fb-1factor 2.4 • Higher selection efficiency: 41% instead of 25% factor 1.3 • Sharper TESLA beam spectrum factor 1.5 • Previously, the pole mass was used and the peak of the cross section was 2 GeV below 2 mt, so that the scan around 2 mtmissed some energy points sensitive to Gt. With the 1S mass the peak is at 2 mtand the scan is centered factor 1.8 • Putting all factors together results in a factor 8.5: good agreement with old result Ramon Miquel, LBNL

  10. The Top Width(4) No ISR No beam effects No ISR No beam effects DGt = 200 MeV DGt = 200 MeV Ramon Miquel, LBNL

  11. The Yukawa Coupling (1) Cross section • Sensitivity very small in all observables • To start with, fix all other parameters (unrealistic) • Leave the Yukawa free • Result: Dlt/lt = +0.17 -0.24 • Assume now a 1% syst. in cross section (realistic) Dlt/lt = +0.12 -0.17 Dlt/lt= 0.25 Ramon Miquel, LBNL

  12. The Yukawa Coupling (2) • Next step: leave mt and as free. Fix Gt to SM value. Add external constraint to as: 0.001. Systematic in cross section lowered to 1%. Results: Dmt = 25 MeV Das = 0.001 (constraint) DGt = fixed Dlt/lt= +0.31 -0.49 (correlations up to 0.8) • Finally, one could try to leave also Gt free: four parameter fit with 0.001 constraint on as. Results: Dmt = 30 MeV Das = 0.001 (constraint) DGt= 35 MeV Dlt/lt= +0.33 -0.57 (correlations up to 0.85) Ramon Miquel, LBNL

  13. Conclusions(1) • First 4-parameter fits to all three threshold observables have been carried out. • Correlations important: only multiparameter fits make sense. • Experimental uncertainty in top mass around 30 MeV. • Experimental uncertainty in top width at the level of 2%. Ramon Miquel, LBNL

  14. Conclusions(2) • Measuring the top Yukawa coupling with a top threshold scan looks difficult • Experimental error around 30% • More difficult for Higgs masses above 120 GeV • Current theoretical error could be larger for all parameters (but it is being improved) Ramon Miquel, LBNL

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