Perspectives for a measurement of τ polarization in Z τ τ with CMS

Transcription

1 Perspectives for a measurement of τ polarization in Z τ τ with CMS Vladimir Cherepanov for the CMS Collaboration III.Phys.Institut B, RWTH Aachen Tau216,

2 Introduction The parity violation in the weak neutral current introduces the polarization asymmetry of τ leptons produced in Z τ τ decay. Knowledge of τ polarization provides: Measurement of the ratio of vector to axialvector neutral couplings for τ leptons Measurement of effective weak mixing angle sin 2 θ eff Techniques to analyze the spin of τ leptons can be used to measure CP properties of Higgs boson in the decay H ττ First step towards precision measurements at LHC with τ leptons A first look at τ polarization at LHC in the decay Z ττ is performed using τ ρν and τ a 1 ν decays 2/24

3 Asymmetry in the process q q Z ττ q R τ + L Coupling: g R Z = (g τ V θτ τr q L g τ ).463 A q R τ+ R Coupling g L Z = (g τ V θτ τ L q L + g τ ).537 A τ are preferably with helicity 1 Polarization asymmetry: A pol = 1 σ [σ(h τ = +1) σ(h τ = 1)] At the Zpole A pol 2 gτ V g τ A 2 8 sin 2 θ W τ helicity state has to be accessed 3/24

4 CMS detector and default τ reconstruction 4/24

5 The CMS detector 5/24

6 Tau Reconstruction and identification (216 JINST 11 p119) The main challenge in tau reconstruction is to discriminate between τ had and QCD jets. The experimental signatures of hadronicallydecaying taus: collimated jet low multiplicity (up to three charged hadrons and up to two π s) decay products are isolated (require low detector activity around taujet direction) A good performance in terms of efficiency&fakerate is achieved by analyzing jet constituents and building individual decay modes: π ± ν τ,π ± π ν τ, (πππ) ± ν τ For more details see talk by Olivier Davignon on TauID at CMS. 6/24

7 τ polarization observables in the decay τ ρν π ± π ν 7/24

8 The chargeneutral energy asymmetry in the decay τ ρν π ± π ν In the tau decay, τ ρν π ± π ν, the energy asymmetry between π ± and π is a spinsensitive variable ρ ψ* π cos ψ [E(π ± ) E(π )]/[E(π ± ) + E(π )] The chargeneutral energy asymmetry is used to measure τ polarziation in the decay Z τ µ τ ρ π cosψ* = mρ 2 m ρ 4m 2 π E π P π E π + P π 8/24

9 Energy asymmetry distribution The chargeneutral energy asymmetry in Monte Carlo simulation Arbitrary Unit CMS Simulation Preliminary Z ττ (lefthanded) Z ττ (righthanded) Arbitrary Unit CMS Simulation Preliminary 12 Z ττ (lefthanded) Z ττ (righthanded) chargeneutral energy asymmetry chargeneutral energy asymmetry Generator level After reconstruction Different τ ρ helicity states are well separated! 9/24

10 Polarization fit using Z τ µ τ ρ decay The τ polarization is measured using Z ττ events selected from 2.3 fb 1 collected by CMS detector at 13 TeV. The control sample selection: µ τ h trigger Isolation of τ candidates τ h decay mode Missing transverse mass, M T(µ, E miss T ) Events CMS Preliminary , 2.3 fb Observed tt Electroweak QCD multijet Z ττ (lefthanded) Z ττ (13TeV) (righthanded) ML fitting with right and lefthanded templates to observed distribution in data. Ratio chargeneutral energy asymmetry Extracted value (dependent on the acceptance efficiency) : < P τ > = ( 33.6 ± 3.7(stat. only))% 1/24

11 τ polarization observables in the decay τ a 1 ν 3πν 11/24

12 Spin observables for the decay τ ± a ± 1 ν 3π± ν Spin configurations for the decay τ a 1 ντ in the τ rest frame: ν τ ν τ τ τ a1 θ * a1 θ * The combined distribution: 1 dγ Γ dcosθ 1 + α a1 P τcosθ ; 12/24

13 Spin observables II Spin analyzers for a 1 3π decay in a 1 rest frame: γ describes the relative pions orientation within its plane β is the angle between laboratory and the 3π plane 13/24

14 Optimal observable (M. Davier et al., Phys. Lett. B 36 (1993) ) The measured decay distribution depends linearly on the weighting of two helicity states, P τ For any tau decay: ξ i = (cos θ, γ, β...) 1 Γ i d n Γ i d n ξ i = f i ( ξ i ) + P τg i ( ξ i ).12 ττ µ3h(π ) ω ω + One dimensional variable: ω = M +( ξ) 2 M ( ξ) 2 M + ( ξ) 2 + M ( ξ) 2 = g( ξ) f( ξ) ω a1 pre selection and reconstruction All polarization sensitive variables ξ can be converted into onedimensional ω without loss of sensitivity reconstruction of ξ and hence ω requires the rest frame of tau! 14/24

15 Reconstruction of Z ττ µνν,3πν The measurement relies on the ability of CMS detector to measure flight direction of τ a1 Events / CMS Preliminary cut fb Observed ττ µ3h(π ) Electroweak WJets QCD (8 TeV) MC uncertainty Obs/Exp σ PVSV The distance between primary vertex and point of τ decay in units of uncertainties. 15/24

16 Reconstruction of Z τ τ µνν, 3πν (Event Fit) Reconsctruction of τ a1 direction Calculation of τ a1 momentum Assume τ leptons from Z decay and apply constraints: µ ν ν Invariant mass of two taus is equal to M Z (PDG) Transverse momentum balance of τ pair τ 2 Constraints on τ leptons direction using muon helix and measured vertices +1 overconstrained fit allows to fully reconstruct system with kinematic of both τ leptons ν π π SV a 1 π τ 1 PV The procedure is applicable to any decay ττ X,3πν 16/24

17 τ τ µ3h( π ) τ τ µ3h( π ) Reconstruction of spin sensitive angles The knowledge of τ lepton kinematics allows to reconstruct the spin sensistive angles CMS Simulation Preliminary CMS Simulation Preliminary gen reco cosθ cosθ Resolution of angle θ τ ν τ gen cos2γ cos2γ reco Resolution of angle γ a1 θ * 17/24

18 τ τ µ3h(π ) τ τ µ3h( π ) Reconstruction of spin sensitive angles CMS Simulation Preliminary CMS SImulation Preliminary gen reco cosβ cosβ Resolution of angle β gen reco gen (ω a1 ω a1 )/ω a1 resolution of the combined observable ω a1 18/24

19 Optimal observable distribution The distribution of ω a1 in Monte Carlo simulation CMS Simulation Preliminary CMS Simulation Preliminary a.u τ τ µ3h(π ) h τ = +1 h τ = 1 a.u..1.8 h = +1 τ τ = 1.12 τ µ3h(π ) h τ ω a ω a1 Generator level After reconstruction Sizeable separation of helicity states is achieved in τ a 1 ν decay. 19/24

20 Z τ µ τ 3π control sample Selection of Z τ µτ 3π using 19.7 fb 1 collected at 8 TeV µ τ h trigger Isolation of τ candidates τ h decay mode Flight length of τ a1 Missing transverse mass, M T (µ, E miss T ) Main background contribution from QCD multijet and W+Jets events are estimated from data Mass of visible decay products Events / Observed Obs/Exp CMS Preliminary fb ττ µ3h(π ) Electroweak WJets QCD (8 TeV) MC uncertainty M a1µ, GeV 2/24

21 Sum Polarization fit using Z τ µ τ a1 decay The τ polarization is measured using Z ττ events selected from 19.7 fb 1 collected by CMS detector at 8 TeV. Strategy similar to τ ρν: Events 9 Observed 8 Fit h τ = +1 7 h τ = 1 6 Bkg 5 CMS Preliminary fb (8 TeV) Fitting the observed distribution of ω a1 by left and right handed templates with their relative fraction as a free parameter. Fit/Obs ω ω 1 a1 Extracted value (dependent on the acceptance efficiency) : < P τ > = ( 35.5 ± 6.4(stat. only))% 21/24

22 Sum Summary First measurements of τ polarization at LHC have started. Events CMS Preliminary , 2.3 fb (13TeV) Observed tt Electroweak QCD multijet Z ττ (lefthanded) Z ττ (righthanded) Analysis of τ ρν and τ a 1 ν indicates the feasibility of this measurement. The obtained results in both channels are consistent with expected values. Systematic uncertainties and bias correction require a bit more efforts. Ratio Events CMS Preliminary chargeneutral energy asymmetry 9 Observed Fit h τ h τ Bkg fb (8 TeV) = +1 = 1 1 The precision will grow including more data and other τ decay channels. Fit/Obs ω 1 a1 ω 22/24

23 Backup 23/24

24 Hadron Plus Strips Algorithm (HPS) 1 Cluster photons within the jet into strips accounting for possible broadening due to photon conversions 2 Combine charged particles in the jet with strips and reconstruct individual τ h decay mode: π ± ν τ,π ± π ν τ, (πππ) ± ν τ 3 Highest pt decay hypothesis with compatible visible mass is given preference 24/24