Measurement of the cross section for W -boson production in - PowerPoint PPT Presentation

Measurement of the cross section for W -boson production in association with jets in p collisions at √ s = 1.96 TeV p ¯ Alessandra Baas Universit¨ at Heidelberg, Kirchhoff-Institut f¨ ur Physik Journal Club 29.10.2012

Fermilab and Tevatron ◮ Fermi National Accelerator Laboratory (Fermilab) is a US national laboratory. ◮ Specialized on high energy physics (Tevatron) and neutrino physics (MiniBooNE, SciBooNE, MINOS). ◮ Tevatron was a p ¯ p collider, with beam energies of ∼ 1 TeV. ◮ Its circumference was 6.28 km. ◮ It was run from 1983 to 2011. ◮ The two main experiments were CDF and D Ø . ◮ One of the main discoveries was the top quark discovery in 1995. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 2 / 23

C ollider D etector at F ermilab II Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 3 / 23

Motivation for: W → e ν + ≥ n -jet ( n = 1 , . . . , 4) of the n th leading jet? as function of the E jet T Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 4 / 23

Motivation Why measure the W+jet(s) production cross section? ◮ Many physics signals have also vector boson (W,Z) plus jet(s) signals, such as t ¯ t , single top or Higgs boson production. ◮ V+jet(s) produced via QCD is large background to these processes and hence good knowledge is crucial. ◮ Stringent test of the power of perturbative QCD predictions. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 5 / 23

W +jet Production Some leading order Feynam diagrams for W +jet production. q,q g q,q W q,q q,q q,q g q,q W g W q,q q,q q,q More jets in the final state due to initial and final state radiation. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 6 / 23

W +2jets Production Does this diagrams also count for W +2jets production? (From single top production last week.) q,q q,q q,q q,q W q,q W W q,q q,q q,q q,q q,q W q,q g W q,q W q,q q,q q,q Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 7 / 23

W Selection and Jet Reconstruction W → e ν Selection: Jet Reconstruction: ◮ Online: EM calo cluster with ◮ Jets are reconstructed via E T > 18 GeV. JETCLU (cone algorithm) with ◮ Offline: Standard electron φ 2 + η 2 = 0 . 4. � R = identification cuts plus E ele ◮ Apply JES correction. > 20 GeV. T ◮ Neutrino enriched sample: ◮ E jet > 20 GeV. T E miss > 30 GeV. ◮ | η | < 2.0 . T ◮ Reduce background ◮ Jets are rejected if the lie contamination: m W T > 20 GeV. within ∆ R = 0 . 52 of the ◮ Z boson veto: reject Z → ee electron. events. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 8 / 23

W Background? Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 9 / 23

W Background ◮ Leptonic: ◮ Contains real electrons and/or neutrinos including decays of W → τν , Z → ee , WW , W γ and t ¯ t . ◮ Monte Carlo event sample with the W selection applied but without E miss requirement. T ◮ Multi-jet: ◮ One or more jets are incorrectly reconstructed as electrons and have mis-measured energy, leading to large E miss . T ◮ Modeled by an event sample from the same dataset by requiring that at least two electron criteria fail, but leave kinematic cuts unchanged. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 10 / 23

Background Estimation ◮ Background enriched samples for each W + ≥ n -jet sample by removing E miss > 30 GeV. T ◮ E miss spectrum for leptonic, multi-jet and signal histograms T are fitted. ◮ From these distributions determine the fraction for normalization of E T distribution. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 11 / 23

E miss Distributions T FIG.1: The results of fitting the signal and background E miss template T distributions to the data in the W + ≥ 1-jet sample before the final E miss cut is T applied. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 12 / 23

E miss Distributions T Systematical Uncertainty on Background: ◮ Low E jet T : 15 % independent on jet multiplicity. ◮ High E jet T : 50% in 1-jet sample, 20% in 4-jet sample. ◮ Low jet multiplicities are limited due to statistics. ◮ high jet multiplicities are dominated by 12% uncertainty on top pair production cross section. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 13 / 23

Event yield correction? Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 14 / 23

Event Yield Correction Corrections for: ◮ W → e ν selection efficiency, about (60 ± 3)%. ◮ Online trigger efficiency to accept high E T electrons, about 96.2 ± 0.6%. ◮ Calorimeter jet energy resolution on the measured x-section (to form particle level W → e ν + ≥ n -jet x-section). Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 15 / 23

Total Systematic Uncertainty on the Cross Section ◮ Increases with increasing E T (5%-20%). ◮ Dominated by 3% uncertainty on the JES correction. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 16 / 23

Measured Differential Cross Sections ◮ Statistical Uncertainty: on event yield of each bin. ◮ Systematical Uncertainty: contributions added in quadrature (on bkg. estimation, acceptance correction and jet energy measurement). ◮ Additionally 5.8% uncertainty on the integrated luminosity. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 17 / 23

Measured Total Cross Sections Total cross section σ n = σ ( W → e ν + ≥ n -jet; E nth − jet > 25 GeV) T ◮ σ 1 = ( 53 . 5 ± 0 . 6 ± 4 . 6 ± 3 . 1) pb. ◮ σ 2 = ( 6 . 8 ± 0 . 2 ± 1 . 0 ± 0 . 4 )b. ◮ σ 3 = ( 0 . 84 ± 0 . 10 ± 0 . 21 ± 0 . 05) pb. ◮ σ 4 = ( 0 . 074 ± 0 . 039 ± 0 . 035 ± 0 . 004) pb. ◮ p ¯ p → W × B ( W → e ν ) (restricted phase space): σ 0 = ( 798 ± 2 ± 40 ± 46) pb. σ n = ( value ) ± ( stat . ) ± ( syst . ) ± ( lum . ) pb. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 18 / 23

Theoretical Predictions ◮ Leading order (LO) plus modeling initial and final state partons and hadronization (through perturbative parton shower): MLM and SMPR . ◮ Next-to-leading order predictions for W + 1 and W + 2 jets: MCFM . Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 19 / 23

Cross Section Ratios I ◮ Differences between the measured cross sections and SMPR/MLM reflect the LO nature of these determinations (upper plot). ◮ Good agreement in the lower plot. Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 20 / 23

Cross Section Ratios II W + 1 jet: W + 2 jets: ◮ SMPR prediction is better than MLM prediction. ⇒ probably due to missing underlying event model in MLM. ◮ Overall good agreement between data and MCFM predictions Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 21 / 23

Cross Section Ratios III W + 3 jets: Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 22 / 23

Bibliography Talk based on a paper of the same name by CDF: arXiv: 0711.4044v2 [hep-ex] Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 23 / 23