Abstract
In the Standard Model, the Higgs boson is a CP even state with CP conserving couplings; any deviations from this would be a sign of new physics. These CP properties can be probed by measuring Higgs decays to tau lepton pairs: the transverse correlation between the tau spins depends on CP. This paper develops such an analysis, using full simulation of signal and background events in the International Large Detector concept for the International Linear Collider. We consider Higgs-strahlung events e+ e- -> Z H) in which the Z boson decays to electrons, muons, or hadrons, and the Higgs boson decays to $\tau$ leptons, which then decay either to (pi+ nu) or (pi+ pi0 nu). Assuming 2 ab^{-1} of integrated luminosity at a center-of-mass energy of 250~GeV, the mixing angle psi_CP between even and odd CP components of the tau pair from Higgs boson decays can be measured to a precision of 75~mrad (4.3 deg).
Highlights
Several electron-positron collider designs are being studied, whose major aim is to measure the Higgs sector with high precision, thereby searching for effects of new physics beyond the Standard Model (SM) [1,2,3,4]
The most mature of these designs, the International Linear Collider (ILC) [1,5], is a linear electron-positron collider with polarized beams, which will initially operate at a center-of-mass energy of 250 GeV, with possible later energy upgrades to 500 GeV and potentially 1 TeV
This paper demonstrates how the CP properties of τ lepton pairs produced in Higgs boson decays can be measured, and estimates the precision that can be achieved by the ILC operating at 250 GeV
Summary
Several electron-positron collider designs are being studied, whose major aim is to measure the Higgs sector with high precision, thereby searching for effects of new physics beyond the Standard Model (SM) [1,2,3,4]. This paper demonstrates how the CP properties of τ lepton pairs produced in Higgs boson decays can be measured, and estimates the precision that can be achieved by the ILC operating at 250 GeV. The use of τ lepton spin correlations to probe the CP nature of the Higgs boson has been investigated in several signal-only, generator or fast simulation studies, see for example [10,11,12,13,14,15,16,17,18]. A nonzero value of ψ CP in τ pairs from Higgs boson decay could arise from either or both of the above mechanisms These scenarios can be distinguished by considering other observables; for example a mixed CP Higgs mass eigenstate would affect the total Higgs-strahlung cross section, which would not be the case if non-SM CP effects occur only in the coupling to fermions. Photon PFOs close to charged particles were considered as being induced by FSR or bremsstrahlung
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