1. 01/08/2002Ramon Miquel, LBNL1 Multiparameter Fits in tt Threshold Scan Manel Martinez, IFAE (Barcelona) Ramon Miquel, LBNL (Berkeley) Introduction. The top threshold scan m t and  s The top width The top Yukawa coupling Conclusions

2. 01/08/2002Ramon Miquel, LBNL2 Introduction. The Top Threshold Scan (1) 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 is sensitive to m t,  s,  t and t (Kuehn, Jezabek, Teubner et al.)  m t = 100 MeV Cross section

3. 01/08/2002Ramon Miquel, LBNL3 Introduction. The Top Threshold Scan (2) Two additional observables have been used: A FB (s) and the peak of the top momentum distribution (Fermi motion)  t = 200 MeV Forward Backward asymmetry Peak of top momentum distr.

4. 01/08/2002Ramon Miquel, LBNL4 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:  :  = 0.41 syst = 3% bkgd = 0.0085 pb A FB :  = 0.11 syst = negligible P:  = 0.41 syst = 4% Assume: –Luminosity = 300 fb -1, TESLA beam spectrum. 9 point scan plus one point well below threshold for bkgd. determination –m t = 175 GeV,  s  (M Z )=0.120, M H = 120 GeV,  t and t as in the Standard Model

5. 01/08/2002Ramon Miquel, LBNL5 Introduction. The Top Threshold Scan (4) Cross sectionPeak of top momentum distr.A FB The “Experimental” Data

6. 01/08/2002Ramon Miquel, LBNL6 m t and  s Previous studies focused on m t determination: –Large correlation with  s –Broken with new definitions of m t : “potential subtracted” mass and “1S” mass (Beneke, Huang, Teubner et al.) −Results (experimental errors only), using 1S mass:  m t = 16 MeV  s = 0.0011  = 0.34 (Including theoretical errors:  m t = 100 MeV) −Using only cross section:  m t = 24 MeV  s = 0.0017  = 0.74  Improvement due to peak of momentum distribution Now it is possible to look for sensitivity to top width and Yukawa coupling

7. 01/08/2002Ramon Miquel, LBNL7 The Top Width(1) So far, only 1D or 2D fits used to study top width and Yukawa coupling. However, sizable correlations with m t and  s 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.

8. 01/08/2002Ramon Miquel, LBNL8 The Top Width(2) Sizable sensitivity in all observables Leave the top width free in fit Fix the Yukawa coupling to its SM value Results:  m t = 18 MeV  s = 0.0015  t = 32 MeV All correlations below 50% Cross section  t = 200 MeV

9. 01/08/2002Ramon Miquel, LBNL9 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 -1 instead of 50 fb -1 factor 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 m t, so that the scan around 2 m t missed some energy points sensitive to  t. With the 1S mass the peak is at 2 m t and the scan is centered factor 1.8 Putting all factors together results in a factor 8.5: good agreement with old result

10. 01/08/2002Ramon Miquel, LBNL10 The Top Width(4) No ISR No beam effects No ISR No beam effects  t = 200 MeV

11. 01/08/2002Ramon Miquel, LBNL11 The Yukawa Coupling (1) Sensitivity very small in all observables To start with, fix all other parameters (unrealistic) Leave the Yukawa free Result:  t / t = +0.17 -0.24 Assume now a 1% syst. in cross section (realistic)  t / t = +0.12 -0.17 Cross section  t / t 

12. 01/08/2002Ramon Miquel, LBNL12 The Yukawa Coupling (2) Next step: leave m t and  s free. Fix  t to SM value. Add external constraint to  s : 0.001. Systematic in cross section lowered to 1%. Results:  m t = 25 MeV  s = 0.001 (constraint)  t = fixed  t / t = +0.31 -0.49 (correlations up to 0.8) Finally, one could try to leave also  t free: four parameter fit with 0.001 constraint on  s. Results:  m t = 30 MeV  s = 0.001 (constraint)  t  = 35 MeV  t / t = +0.33 -0.57 (correlations up to 0.85)

13. 01/08/2002Ramon Miquel, LBNL13 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%.

14. 01/08/2002Ramon Miquel, LBNL14 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)