Multiparameter Fits in tt Threshold Scan

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

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 is
  sensitive to mt, as , Gt and lt (Kuehn, Jezabek,
  Teubner et al.)

Dmt 100 MeV
3
Introduction. The Top Threshold Scan (2)

• Two additional observables have been used AFB(s)
  and the peak of the top momentum distribution
  (Fermi motion)

Peak of top momentum distr.
Forward Backward asymmetry
DGt 200 MeV
DGt 200 MeV
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 0.41 syst 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

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Introduction. The Top Threshold Scan (4)
The Experimental Data
Cross section
Peak of top momentum distr.
AFB
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

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The Top Width(1)

• So far, only 1D or 2D fits used to study top
  width and Yukawa coupling.
• However, sizable correlations with mt and 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.

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
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-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 mt, so
  that the scan around 2 mt missed some energy
  points sensitive to Gt. With the 1S mass the peak
  is at 2 mt and the scan is centered
  factor 1.8
• Putting all factors together results in a factor
  8.5 good agreement with old result

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The Top Width(4)
No ISR No beam effects
No ISR No beam effects
DGt 200 MeV
DGt 200 MeV
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
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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)

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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.

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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)