1. Benchmark analysis of tau-pairs
Taikan Suehara (ICEPP, The Univ. of Tokyo)

2. Contents Overview Cuts selecting tau-pairs Cross section / AFB result
Cuts selecting t->pn
Polarization with t->pn events
Cuts selecting t->rn
Polarization with t->rn events
Summary

3. Charge of tau-pair analysis
Description of LOI benchmark, from ILC-MEMO
Themes
Tracking and calorimetry of concentrated particles
Gamma factor is around 140 for 250 GeV ts
p0 to 2g decay reconstruction
2gs are very closed, difficult to separate
Observables
Cross section, AFB, P(t)
Event selection and p0 reconstruction performance (efficiency and purity)

4. Events (Signal & Background)
Signal cross sections: 2.6 pb (eL), 2.0 pb (eR)
500 fb-1 is available with ILD_00
Background
Full SM sample of ILD_00 is used. (samples downloaded to KEK by Feb. 3. Some of gg->tt missing)
Event weighting is performed.
Major backgrounds:
Bhabha (35000 pb in SLAC sample)
Preselected 2 fb-1 is prepared by Akiya. Opening angle > 165deg, |cos(q)| < 0.92
gg -> tt (3700 pb in SLAC sample)
WW -> lnln (~1 pb)

5. BG suppression cuts Number of track <= 6
Veto hadronic events
Specialized jet clustering (TaJet)
Customized to taus (several particles within narrow angle)
1 positive & 1 negative jets required for further analyses
Opening angle > 178deg
Suppress WW to lnln background
40 < Evis < 450 GeV
gg->tt and Bhabha rejection
2-electron and 2-muon veto
For bhabha and ee->mm veto
E-ID by Ecal/total deposit, m-ID by hit/track energy
|cosq| < 0.9 for both jets
Bhabha is completely suppressed by this cut

6. Selections
Cos(theta) of p(tau)
Opening angle[deg]
Evis [GeV]

7. Result of SM suppression cut
event
tautau
SM gg
SM 6f
SM 4f
SM 2f
SM other
Ntrack, jet cut
572650
7.42E+08
2.35E+06
630123
opening angle
152752
6.46E+06
157423
thetacut
129176
5.03E+06
133404
ee,mumucut
118540
784486
eviscut
114811
1982.1
ILD_00 geometry
Bhabha/ part of gg->tt are not included.
Bhabha was checked in LDC’, not critical.
Preselected sample is ready, soon included.
All background is almost negligible.

8. Cross section and AFB Cross section precision: 0.3% (count based).
SM calculation
(Red: left, Blue: right)
Cross section precision: 0.3% (count based).
AFB precision: 50.5 ± 0.25%. (statistical error of signal only).

9. Status on Polarization analysis (ILD_00 is ongoing…)

10. Decay modes in Apol analysis
t -> enn
Branching ratio: 17.8%
3 body decay; pol. info is smeared
t -> mnn
Branching ratio: 17.4%
3 body decay; same as enn mode
t -> pn
Branching ratio: 10.9%
Pol. can be directly observed by p distribution
t -> rn, r -> pp
Branching ratio: 25.2%
Pol. of r can also be obtained by p distribution in r-rest frame (pol. of r is connected to pol. of t)
t -> a1n, a1 -> ppp
Branching ratio: 9.3%
Currently not used because statistics is low

11. Analysis flow Tau candidate for AFB Apol analysis highlights:
Mode selection
Invariant masses of r and p0
Apol calculation by angular distribution of ps
1 prong cut
Lepton veto
Energy cut
Rejected
Passed
Neutral particle?
Rejected
r reconstruction
Yes No
Passed
Apol
(t->pn)
p0 reconstruction
Passed
Apol
(t->rn)

12. t -> pn selection cuts
1 prong cut Jets with >2 charged particle rejected.
Lepton veto Events containing e/ms are rejected. (criteria is the same as AFB lepton-pair veto)
Energy cut Jets with energy < 10 GeV rejected. (e/m/p separation is inefficient in low energy)
Events with > 1 GeV neutral particles are rejected. In “tight cut” event with any neutrals are rejected.

13. t -> rn selection cuts
1 prong cut
Lepton veto
Energy cut (jet energy must be > 10 GeV)
Above are same as t->pn cuts
Events with > 10 GeV from neutrals (in total) are selected.
Mass of r is reconstructed, must be within 200 MeV from actual mass (770 MeV).
Mass of p0 is reconstructed with neutral particles. If # of neutrals >=3, nearest (in angle) two are combined until 2 particles are left. Application of this cut is discussed later.

14. Selection results (mode BG only)
all tau
pinu
rhonu N e p
No cut
250141 1
0.109
578308
0.252
afb cut
670700
79118
186002
1 prong
564110
78931
174357
energy cut
556681
77964
174325
mu,e veto
332444
75656
no gamma
53794
49443
ng (tight)
43173
39870
339776
167575
any gamma
258777
159036
rhomass200
93217
81949
pi0mass
67300
61000
pi0masst
50825
48975
Efficiency and purity are changed from LDC’
There might be effects of change of PandoraPFA?
Cut criteria needs to be reconsidered.

15. r and p0 reconstruction
ILD_00
ILD_00

16. Apol (pn mode) PRELIMINARY!
ILD_00 (eL)
ILD_00 (eR)
Bias on cut?
Values obtained by signal-only events!

17. ILD is all for now. Following is the old result.

18. t -> rn, r->pp distribution (1) no p0 cut
Clear difference between eL and eR observed.
Distribution is degraded due to the cut effects.
Ppol vs dist. calc

19. t -> rn, r->pp distribution (2) tight p0 cut
Number of signal is about a half.
Difference between geometry enhanced.
J4LDC is not realistic with this cut?
Background is quite low, negligible level.

20. Obtaining P(t) value
Physics Letters B, 235 (1990) 198
Combined information of t -> rn and r -> pp decay can be used in this method.

21. Apol calculation (rn mode)
Values obtained by signal-only events!

22. Summary SM separation cuts were developed. Polarization
Most of the SM process can be suppressed by the cuts efficiently.
We need to check bhabha/ (gg->tt?) for confirmation.
Cross section and AFB are obtained.
Polarization
ILD_00 analysis is ongoing.

23. Thank you for your attention.

24. Backup