W boson helicity measurement in top pair events in Dø Bertrand Martin Workshop on Top Physics : from the Tevatron to the LHC 10/20/07

Outline Motivations Analysis guideline Event selection lepton + jets , dilepton likelihood discriminant Analysis technique cos(θ*) template Results with 1 fb-1 or _

Motivations Standard Model : ν θ* t b W+ l+ Due to the observed parity violation, charged current (W boson) only couples left handed particles V-A structure of the E.W. current in the S.M. Lagrangian W boson helicity : sensitive to a possible V+A component : → Measured through the cos(θ*) distribution θ* : angle between the top quark flight direction and the charged lepton momenta in the W rest frame : t b W+ l+ ν θ* S.M. Fraction : Left handed f- ~ 0.30 Longitudinal f0 ~ 0.70 Right handed f+ ~ 1.4 10-3 A non zero f+ could sign new physics… C.-R.Chen, F. Larios, and C.-P. Yuan, Phys. Lett. B 631, 126 (2005)

Analysis guideline 2 final states : Measured dilepton lepton+jets Fixed to the SM value Select a data sample enriched in ttbar candidate events (estimate physics and instrumental background contamination) Reconstruct the event kinematics : top quark & W boson leptonic decay compute cos(θ*) Generate MC ttbar samples with - fixed f0 (0.70) - different f+ values Estimate the f+ value that best reproduces the observed cos(θ*) distribution in data

Multivariate selection (likelihood discriminant) Event selection Dilepton ( ee , eμ , μμ ) 2 high pT leptons > 15 GeV ≥ 2 high pT jets > 20 GeV Main backgrounds : - Drell-Yan : Z/γ* + jets - Diboson (WW,WZ,ZZ) - Fake lepton Lepton (e, μ) + jets Only 1 high pT lepton > 20 GeV ≥ 4 high pT jets > 20 GeV Missing ET > 20 GeV & MET not aligned with the lepton Main backgrounds : W + jets QCD multijet production Preselection Multivariate selection (likelihood discriminant) See next slide

Likelihood discriminant (l+jets) The variables used to discriminate signal and background must : be well modeled in the MC (K.S. proba > 5%) have different shapes between S and B Good MC modeling 11 “potentially good” variables ( topology, kinematics, b-tagging… ) 211 - 1 discriminants can be built out of 11 variables… Discriminating ? Not aware that TMVA exists …?

Likelihood discriminant (cont’d) Best Lt e + jets Fit ln(s/b) Likelihood optimization : ensemble tests Np observed data events after the preselection → Create a “mock data set” of the same size , picking up MC (ttbar , W+jets) and QCD events according to preselection efficiencies → Fit the ttbar fraction. Repeat 1000 times. Best Lt : gives the smallest error on the ttbar fraction Optimal cut : highest discrimination S/√(S+B) Lt is used to estimate the data sample composition, and enrich in ttbar ( Lt cut )

cos(θ*) templates Reconstruction of the W leptonic decay(s) l+jets : 1 neutrino – kinematically constrained : 1 solution , 1 unknown = pz(v) Kinematic fit used for the (lepton – b jet) pairing, assuming Mtop=172.5 GeV dilepton : 2 neutrinos unknown momenta – underconstrained Mtop assumption, algebraic resolution & average over the possible (lepton,jet) pairings Success in solving the equations : 70 % (nominal rate) 95 % if lepton and jet energies are smeared After the likelihood cut, the cos(θ*) distribution is built for : Data ttbar, for different f+ (V-A / V+A) Background Goal : find which f+ value best reproduces the data distribution

What it looked like some time ago… Analyses results l+jets (1 fb-1) Dilepton (1 fb-1) Now… What it looked like some time ago… l+jets 230 pb-1 Dilepton 370 pb-1 l+jets Later…

f+ measurement Likelihood maximization ( w.r.t. ns and nb,i ) : Gaussian term for the background normalization How well does this f+ hypothesis match with the cosθ* data distribution Observed data Predicted average nb,i and σb,i obtained after Lt cut Poissonian probability to observe in the bin j dj data events with a predicted average of

Extracting f+ l+jets Dilepton l+jets and dilepton results are marginally consistent (2.4σ stat) → Assumed it originates from statistical fluctuation → Proceed to combination Combination

Results with 1 fb-1 Combined 1 fb-1 The likelihood maximization does not guarantee f+ > 0 ! Bayesian approach : use a prior probability density flat for f+  [0 , 0.30] and null in the non-physical region Confidence interval [f+ min , f+ max] for a given Confidence Level

Summary With 1fb-1 of analyzed data, the combined lepton+jets and dilepton measurements of the right handed W fraction f+ is : Published result for 370 pb-1 : PHYSICAL REVIEW D 75, 031102(R) (2007) Compatible with the predicted Standard Model value : f+ = 1.36 x 10-3 The analyses will be updated with an integrated luminosity of ~ 2 fb-1 ( with a more optimized multivariate selection ) The best CDF preliminary results for lepton + jets with ~ 1.7fb-1 , using cos(θ*) :

Backup slides

The D0 experiment Results on the W helicity are shown for ~1fb -1 of analyzed data, while D0 has ~ 2.8 fb -1 on tape

QCD estimation The “Matrix Method” 2 samples used to estimate the QCD background preselection sample “loose” sample : all preselection cuts except lepton quality likelihood for electrons isolation for muons

Systematic uncertainties Varying parameters can affect both : the data sample composition (different selection efficiency of the likelihood discriminant) the shape of cos(θ*) distributions Effect on the fitted f+ : studied with pseudo-experiments (varying the parameters in the peudo-dataset)