Benchmark analysis

e+ e- -> tau+ tau- @ 500 GeV

Authors: Keita Yumino, Daniel Jeans

Introduction
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tau decays give acces to their polarisation (spin orientation).

this gives handle not available in other 2f production except in t-tbar.

challenge to detector and reconstruction algorithms: fully reconstruct tau decays.
high boost -> narrow jet containing a few particles: 1/3/5 charged hadrons/leptons, 1 or 2 neutrinos, often pi0s (etas) -> photons, occasionally kaons.
measureable impact parameter due to tau lifetime. for 250 GeV tau. typical tau flight distance ~ few mm (check).

ILC has well defined interaction region (smaller than d0 resolution) in plane transverse to beam directions.

event selection
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simple selection for high invariant mass tau-pair events.

tau-tau events ~back-to-back in transverse plane (assuming collinear ISR).
High mass events have little ISR -> also back-to-back in 3d.

look at PFO collection with pre-made pi0 candidates.

loose preselection on #PFOs, total energy, thrust.

1. highest pT charged track -> first tau seed.
2. then look in other phi hemisphere -> highest pT on other side -> second seed.
3. then look in cone around seeds to collect other tau decay products.
4. assume rest is from underlying event.

5. requirements on 
cone mass  -> hadronic BGs
impact paramters(?) -> leptonic, light hadronic
lepton ID -> ee, mumu
out-of-cone energy (4f, 6f)
pt of tau-tau system.

performance of this algorith: efficiency, backgrounds. (large/small/MC level).

tau decay mode identifitcation
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based on tracks and neutral PFOs in cone. number, invariant masses.

performance. (MC truth, cheat pi0, cheat tracking, small, large)
effect of material interactions.

tau momentum reconstruction (hadronic decays)
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assume beamline. scan along z. use impact parameter (multi-track vertices), neutral hadronic momentum.
impose tau inv mass. look for solution with smallest tau-tau pT, pZ, consistent tau decay lengths.

polarimeter measurement
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polarimeter = optimal estimator of spin direction.
use TauSpinner (interface to main tauola code) to get polairmeter from reconstructed tau decay products. (pi nu, rho nu, a1 (1 prong), a1 (3 prong).

physical observable
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3d distribution: angle between polarimeter and tau momentum (for tau+ and tau-), and polar angle of tau- momentum direction.

conclusion/summary
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