Measuring the CP state of tau pairs from Higgs decay at ILC in ILD - PowerPoint PPT Presentation

Measuring the CP state of tau pairs from Higgs decay at ILC in ILD still preliminary, preparing for Santander Daniel Jeans May/June 2016 1

CP Violation needed to explain baryon asymmetry of universe Currently known sources of CPV not sufficient Higgs of the minimal SM is CP even eigenstate more complex scenarios may also have CP odd states ( H 125 being pure CP odd is ~ruled out by LHC) In the case of CP violation in the Higgs sector, H may not be an eigenstate of CP could the Higgs sector be an additional source of CPV? 2

Tree-level coupling of Higgs to fermions L = g f ( cos ψ + i γ 5 sin ψ ) f H CP conserving coupling: ψ=0 violating ψ≠0 CP violation via loops can be searched for in H → ZZ, WW 3

projection of spin on some axis: ↑, ↓ spin state of pair of spin ½ particles produced by spin-0 parent: (1/√2) ( |↑↓> + e 2iψ |↓↑> ) ψ = 0 : CP even eigenstate ψ = π/2 : CP odd eigenstate otherwise a mixture 4

distribution of spins (s) of fermions (f) from spin-zero parent spin components parallel / perpendicular to flight direction s z / s ⊥ ∼ z s z + s + ⊥ s ⊥ Γ( CP even→ f + f - ) 1 − s + ∼ z s z - s + ⊥ s ⊥ Γ( CP odd → f + f - ) 1 − s + CP state affects transverse spin correlations 5

To measure this, we need fermions: with an appreciable coupling to Higgs whose spins we can (at least partially) reconstruct neutrinos → difficult to observe electrons, muons → difficult to observe spin lighter quarks → hadronisation washes out spin correl tau lepton and top quark → potentially OK significant coupling to H distribution of decay products → spin information 6

tau decay; polarimeter vector Γ ( τ → X ) ~ ( 1 + a h (X) · s ) Polarimeter vector h couples to spin s depends on momenta of τ decay products factor a depends on decay mode: maximal for hadronic decays, smaller for leptonic decays h can be easily calculated from visible daughters for τ ± → π ± ν and τ ± → π ± π 0 ν if τ momentum is known (BR ~ 11% and 25% respectively) 7

General strategy select Higgs-strahlung events, with H → tau tau Fully reconstruct tau momenta I know how to do it for Higgs-strahlung with hadronic taus decays Reconstruct tau polarimeter vectors I know how to do it in τ ± → π ± ν and τ ± → π ± π 0 ν decay modes look at correlation between transverse components of polarimeter vectors Extract CP-violating angle ψ using full detector simulation and reconstruction 8

simulation, reconstruction whizard 2.2.8, CIRCE2 beams, ISR e+ e- → f+f-τ+τ- (τ+τ- from 125 GeV Higgs) e+ e- → f+f-τ+τ- (τ+τ- not from Higgs) f = mu, e, uds (some generator level cuts, particularly for e+e-τ+τ-) pythia v8.212 for hadronisation & FSR tauola c++ v1.1.4 1. signal decays: τ ± → π ± ν and/or τ ± → π ± π 0 ν “rho / ρ” 2. all τ ± decays include spin correlations (H SM , H CP (ψ=π/4) , non-H) Mokka simulation ILD_o1_v05 standard Marlin/ILDConfig reconstruction ilcsoft v01-17-09, standard Pandora steering (latest photon reco) background overlay 9

electron or muon charged hadron photon Leptonic Z decay channels 10

electron or muon charged hadron photon look for Z → e+e-, mu+mu- same flavour leptons (PandoraPFA PID) opposite charge associate FSR invariant mass not too far from m Z require exactly 1 Z candidate reject Z → ee with very forward electrons to reduce non-H backgrounds 11

electron or muon charged hadron photon Identify hadronic tau candidates >=2 additional charged PFOs 2 most energetic → tau seeds require oppositely charged seeds not identified as e/mu group remaining photons into pi0s if m < m tau with a tau seed use mass constrained kinematic pi0 fit add unpaired photons to nearest tau candidate if resulting mass < m tau 12

electron or muon charged hadron photon reject events with too much charged / neutral-hadron energy or pT in addition to Z and tau candidates (some from underlying event allowed) 13

electron or muon charged hadron photon select tau → pi, tau → rho decays based on number of photons/pi0 and visible mass of tau jet reconstruct tau momenta using impact parameters, measured momenta imposing tau mass, pT balance (details in NIM A 810 p51) require successful reconstruction select H → tau tau require tau-tau mass consistent with m H require Z recoil mass consistent with m H measure CP properties reconstruct tau polarimeters using measured momenta and reconstructed p tau 14 correlation of transverse components of polarimeters → CP

leptonic Z channel look for Z → μ+μ- , e+e- : use Pandora PID, and FSR recovery require exactly one good Z candidate look for two single-prong taus decays: require at least 2 additional charged PFOs assume two most energetic are from tau decays not muon-like or electron-like opposite charge treat as tau jet seeds not too much energy/pT in additional charged PFOs not too much energy/pT in neutral hadron PFOs starting with highest energy photon candidates, try to make pi0 consistent with tau mass apply mass constraint to pi0s: keep if good probability apply cuts on invariant mass and number of photons in tau jet add remaining “orphan” photons to tau jet 15 if they don't take it over the tau mass

main updates since last time: increased MC statistics include all tau decay modes more recent reconstruction no cheating of PID add Z → electron channel changes to event selection change to final likelihood fitting 16

lepton-lepton mass (after selection) showing LR polarisation, scaled to H20 @ 250 GeV: 1350 fb -1 Z → e e Z → mu mu Events / bin Events / bin (stacked histograms) lepton-lepton mass [GeV] lepton-lepton mass [GeV] SIGNAL PROCESS, SIGNAL TAU DECAY SIGNAL PROCESS, OTHER TAU DECAY 17 BACKGROUND PROCESS (non-H)

recoil mass (after selection) showing LR polarisation, scaled to H20 @ 250 GeV: 1350 fb -1 Z → e e Z → mu mu Events / bin Events / bin recoil mass [GeV] recoil mass [GeV] SIGNAL PROCESS, SIGNAL TAU DECAY SIGNAL PROCESS, OTHER TAU DECAY 18 BACKGROUND PROCESS (non-H)

tau-tau mass (after selection) showing LR polarisation, scaled to H20 @ 250 GeV: 1350 fb -1 Z → e e Z → mu mu Events / bin Events / bin tau-tau mass [GeV] tau-tau mass [GeV] SIGNAL PROCESS, SIGNAL TAU DECAY SIGNAL PROCESS, OTHER TAU DECAY 19 BACKGROUND PROCESS (non-H)

transverse polarimeter correlation (after selection) showing LR polarisation, scaled to H20 @ 250 GeV: 1350 fb -1 Z → e e Z → mu mu Events / bin Events / bin transverse polarimeter correlation [rad] I am surprised there is more BG in SIGNAL PROCESS, SIGNAL TAU DECAY muon channel: need to check SIGNAL PROCESS, OTHER TAU DECAY 20 BACKGROUND PROCESS (non-H)

e e tau tau channel *** lumi 0 ECOM= 250 GeV Lumi= 1350 fb-1 Pol (e-,e+)=(-0.80, 0.30) 1.35e+06 XSEC #EVTS EFF process 0 eett_nonH sel step 0 15.5409 20980.2 100 sel step 1 15.3813 20764.7 98.973 nonH BG sel step 2 0.642117 866.858 4.13179 sel step 3 0.00366338 4.94556 0.0235725 process 1 eett_SMHiggs_signal sel step 0 0.0874662 118.079 100 sel step 1 0.0872358 117.768 99.7366 SIGNAL sel step 2 0.0484956 65.4691 55.445 sel step 3 0.0326782 44.1155 37.3609 process 2 eett_SMHiggs_othertau sel step 0 0.589066 795.239 100 other tau sel step 1 0.582987 787.033 98.968 decays sel step 2 0.0584068 78.8492 9.91515 sel step 3 0.00905114 12.219 1.53652 21 selection step 0 = all ; 1 ~ find Z ; 2 ~ presel, find & fit tau tau ; 3 ~ full selection

mu mu tau tau channel *** lumi 0 ECOM= 250 GeV Lumi= 1350 fb-1 Pol (e-,e+)=(-0.80, 0.30) 1.35e+06 XSEC #EVTS EFF process 8 mmtt_nonH sel step 0 20.8433 28138.5 100 sel step 1 20.8201 28107.1 99.8884 nonH BG sel step 2 3.27458 4420.69 15.7105 sel step 3 0.0164947 22.2679 0.0791366 process 9 mmtt_SMHiggs_signal sel step 0 0.0837425 113.052 100 sel step 1 0.0834467 112.653 99.6467 SIGNAL sel step 2 0.0492233 66.4514 58.7793 sel step 3 0.036084 48.7134 43.0892 process 10 mmtt_SMHiggs_othertau sel step 0 0.560827 757.116 100 other tau sel step 1 0.553412 747.106 98.6778 decays sel step 2 0.0519277 70.1023 9.25912 sel step 3 0.00875604 11.8207 1.56127 22 selection step 0 = all ; 1 ~ find Z ; 2 ~ presel, find & fit tau tau ; 3 = full selection

Does the selection bias the delta-phi distribution? → looks flat → OK full selection efficiency colours = different tau decay modes (Z → electron channel) transverse polarimeter correlation (delta Phi) [rad] 23

Standard Model Higgs CP violating Events / bin full reco & selection signal only signal only transverse polarimeter correlation [rad] 24