Measurement of EW production! of Z+2j at the LHC

Transcription

1 Measurement of EW production! of Z+j at the LHC Frontiers of Fundamental Physics, Marseille Kiran Joshi On behalf the ATLAS and CMS collaborations /7/

2 Overview Introduction and motivation ATLAS measurement at 8 TeV Phase space definition Cross section extraction & techniue validation Systematic uncertainties & results Unfolded distributions CMS measurement at 8 TeV Event selection Background modelling Signal extraction & results Measurements of hadronic activity Summary /7/

3 Motivation Electroweak production of a Z boson plus two jets (Z+j) is a relatively rare process allowed by the Standard Model.! Includes the vector boson fusion (VBF) production of Z+j. Similar to VBF production of a Higgs boson plus two jets (H+j). Important to study electroweak Z+j as a background to H+j.! Gain experience with useful distributions and analysis techniues. Very large background from Z+j produced by strong interactions. Precisely understanding this background can assist in understanding QCD and constrain uncertainties associated with QCD modelling of Z+j. /7/

4 Introduction (I) Electroweak production of Z+j can proceed through various mechanisms: µ,e Z W + Z µ,e µ +,e + Z µ,e W µ +,e + W ± Z µ +,e + Vector boson fusion Z-boson bremsstrahlung non-resonant Background contributions predominantly from strong Z+j and diboson-initiated Z+j production. g µ,e Z µ,e W ± Z µ +,e + µ +,e + W ± g /7/

5 Introduction (II) Electroweak Z+j event topology: Outgoing uarks recoiling against W-bosons in VBF case leads to tagging jets produced widely separated in rapidity, with large transverse momenta and high invariant mass. Normalised to unity - ATLAS EW Zjj Background Normalised to unity ATLAS EW Zjj Background y - [GeV] m jj Lack of colour flow across diagram leads to little radiation produced in the rapidity interval between the two tagging jets. W + Z µ,e Normalised to unity.8.6 ATLAS EW Zjj Background W µ +,e gap N jet /7/

6 Measurement of the electroweak production of dijets in association with a Z-boson and distributions sensitive to vector boson fusion at s = 8 TeV arxiv:.76 /7/ JHEP() 6

7 Electroweak Z+j phase space definition Define a search region of phase space in which the electroweak contribution is enhanced: Z-boson candidate from high-pt isolated electrons, muons: > GeV, pj T /7/ > GeV, mjj > GeV pj T Z-boson candidate well-balanced against dijet system: pbalance = T > GeV p`` T High-pT jets with large invariant mass 8 mll GeV, p~t` + p~t` + p~tj + p~tj p~t` + p~t` + p~tj + p~tj <. Jet veto: no additional jets with pt > GeV in the rapidity interval bounded by the two leading jets. 7

8 Perform a fit to the invariant mass distribution in the search region, using signal and constrained background templates from MC. Signal template from Sherpa electroweak Z+j Background template from Sherpa QCD Z+j (+diboson +top-antitop) Nobs / GeV Electroweak Z+j extraction ATLAS L dt =. fb- s = 8 TeV control region! () Shape of background template crucial for fit. Shape constrained through the use of a control region. /7/ Orthogonal to search region, reuire at least one jet in between tagging jets. Compare mjj distribution in data to MC in control region. Fit the data/mc ratio and use the polynomial to reweight the background template in the search region. 8 Background Background + EW Zjj MC. p + p mjj + p mjj p + p mjj. mjj [GeV]

9 Fit performed using log-likelihood maximisation. Normalisation of templates allowed to float. Number of electroweak events, NEW extracted and converted to fiducial cross-section. NEW R = EW! L dt CEW Correction factor to account for detector acceptance & resolutions. 9 ATLAS L dt =. fb- s = 8 TeV search region () Background. Background + EW Zjj.. /7/ BKG BKG + EW Nobs / GeV Electroweak Z+j fit results mjj [GeV] constrained unconstrained mjj [GeV]

10 Electroweak Z+j control region validation Many cross-checks performed to validate use of the control region to constrain the background mjj template. Choice of polynomial used to fit data/mc ratio (cf. slide 8) First order vs. second order polynomial produces ~% change in fitted NEW.! Choice of control region studied by splitting into several sub-regions. New background reweighting function obtained in each sub-region and fit for NEW repeated. Spread between functions large at high mjj, but impact of background template very small in this region. Background dominates at mjj~ TeV MC ATLAS control region s = 8 TeV default y.8 y >.8 Njet = Njet p > 8 GeV T < p 8 GeV T Consistent NEW results obtained with each reweighting function.. φ(j,j)/π <.9, jj p > GeV T mjj [GeV] /7/

11 Electroweak Z+j uncertainties Dominant experimental systematic uncertainty from jet energy scale -.6% uncertainty. Uncertainty on signal template by comparing systematically-shifted distribution to the nominal and repeating fit for NEW. Background template constrained by data, therefore only effect on extrapolation between control and search regions is relevant.! /7/ Uncertainty in modelling of signal and background assessed using dedicated MC samples with scale/parameter variations. Renormalisation and factorisation scale, CKKW matching parameter, PDF uncertainty, amount of activity from multiple parton interactions (MPI). Signal modelling uncertainty = 8.9% Background modelling uncertainty = 7.%

12 Electroweak Z+j results Measured fiducial cross-section: EW In good agreement with NLO prediction from POWHEG: Powheg EW /7/ =.7 ±.6 (stat) (syst) ±. (lumi) fb. = 6. ±. (stat) +.. (scale) ±.8 (PDF) ±. (model) fb Background-only hypothesis rejected at >σ.

13 Inclusive Z+j fiducial cross-sections baseline: Z-candidate + jet pt> GeV, pt>gev high-mass: baseline + mjj > TeV high-pt: Z-candidate + jet pt>8 GeV, pt>7gev σzjj [pb] ATLAS L dt =. fb- s = 8 TeV - σdata σtheory Powheg (Zjj) + Sherpa (VZ) baseline /7/.. high p T search. control.. high mass All measurements in good agreement with predictions from theory.

14 Inclusive Z+j differential cross-sections Iterative Bayesian unfolding techniue used to correct measured distributions for detector effects. dσ σ d y dσ σ dm jj () - Sherpa Zjj (QCD + EW) Sherpa Zjj (QCD) ATLAS - L dt =. fb s = 8 TeV baseline region - Powheg Zjj (QCD + EW) - Powheg Zjj (QCD) - - () Sherpa Zjj (QCD + EW) ATLAS - L dt =. fb s = 8 TeV baseline region -6 Sherpa Zjj (QCD) - Powheg Zjj (QCD + EW) Powheg Zjj (QCD). Sherpa Sherpa -7 /7/ Powheg Powheg..8 mjj [GeV] y Better description of mjj and Δy distributions by POWHEG.

15 gap Njet Jet veto efficiency Inclusive Z+j differential cross-sections ATLAS - L dt =. fb s = 8 TeV baseline region.9 Sherpa Zjj (QCD) Powheg Zjj (QCD + EW).6.8 Powheg Zjj (QCD).. ().6. Sherpa Zjj (QCD + EW) Powheg Zjj (QCD + EW) Sherpa Powheg Zjj (QCD). ATLAS - L dt =. fb s = 8 TeV baseline region. Sherpa Zjj (QCD). Sherpa Sherpa Zjj (QCD + EW) Powheg Powheg () y /7/ y Better description of additional jet activity by Sherpa. POWHEG predicts too few additional jets.

16 Measurement of pure electroweak production of a Z boson in association with forward/backward jets at s = 8 TeV CMS PAS FSQ-- Previous measurement of electroweak Z+j at 7 TeV. JHEP () Background-only hypothesis excluded at.6σ /7/ 6

17 Event selection Event pre-selection cuts: Two high-pt, isolated electrons or muons: pt > GeV, ηe., ημ., MZ - Mll < GeV Two jets with pt > GeV and η.7! Signal cross-section extracted using two methods, template fits to data using a Boosted Decision Tree and Fisher discriminant. Confidence in background modelling very important before signal can be extracted. Two methods explored to estimate uncertainty on background template: Simulation-based driven /7/ 7

18 Simulation-based background modelling Dominant background from QCD Z+j. Default modelling performed using MadGraph+Pythia (leading order). Estimate effect of higher-order corrections by comparing to MCFM (NLO). Reweighting factor determined in bins of mjj and Z-boson rapidity in rest frame of dijet system (y*). Post-reweighting mjj and y* distributions, fair agreement between data and MC: Events CMS preliminary, - s=8 TeV, L=9.7 fb µµ events EWK Zjj QCD Zjj Top VV data Events CMS preliminary, - s=8 TeV, L=9.7 fb QCD Zjj Top VV EWK Zjj data µµ events BG (-BG)/ - - Dijet invariant mass [GeV] EWK Zjj BG /Σ y* = y - (y +y )/ Z j j..8 ratio ratio± JES /7/ 8

19 -driven background uncertainty QCD production of γ+j is expected to resemble that of QCD Z+j. Select a sample of γ+j events and reweight pt(γ) to pt(z). γ+j and Z+j events with high invariant mass are uite compatible: Insufficient statistics to define correction factors. Compatibility test results used to assign an uncertainty on the final shape.! Final shapes of QCD Z+j obtained after subtracting residual contamination from electroweak production of γ+j. Estimated using MadGraph. Events (a.u.) Ratio CMS simulation, [M >7] jj s=8 TeV QCD Z jj QCD γ jj stat unc. QCD Z jj/qcd γ jj loose-tight pure-tight 6 8 Dijet pseudo-rapidity distance ( η) /7/ 9

20 Signal discrimination Two techniues used to extract signal: Boosted Decision Tree using variables such as: mjj, Δy, Δφ, Δy(j,Z), yz, y* χ fit to inclusive data (μμ channel only). Linear Fisher discriminant based on dijet kinematics - Δηjj, mjj, ΔpT rel. Profile likelihood ratio method (μμ and ee channels). Events CMS preliminary, - s=8 TeV, L=9.7 fb EWK Zjj QCD Zjj Top VV µµ events Events CMS preliminary, VV QCD Zjj(data) data - L=9.7 fb s=8 TeV, Top EWK Zjj -Σ Bckg ee/µµ events, M >7 GeV jj Fisher discriminant BG (-BG)/ Boosted decision tree discriminator EWK Zjj Bckg /Σ Fisher discriminant..8 /7/

21 Systematic uncertainties Various sources of systematic uncertainty considered. Luminosity:.6% Trigger and selection efficiencies: - % Jet energy scale and resolution estimated by scaling jet energy: -% Pileup modelling uncertainty by reweighting: 6% Theory uncertainties PDF uncertainty Renormalisation and factorisation scale variations! Total ~8%. /7/

22 Results Measured cross section using Boosted Decision Tree: (EWK `` + jj) I = 9 ± 9 stat. ± 9 syst. fb Measured cross section using Fisher discriminant: (EWK `` + jj) II = ± 9 stat. ± 7 syst. fb Combined result: (EWK `` + jj) = 6 ± 6 stat. ± syst. fb In good agreement with theory prediction: NLO = 9 fb Background-only hypothesis excluded at >σ /7/

23 Study of hadronic activity Hadronic activity studied in a region sensitive to electroweak production (mjj > GeV). Events CMS preliminary, s=8 TeV, - L=9.7 fb VV Top QCD Zjj(data) EWK Zjj data Bckg -Σ ee/µµ events, M > GeV jj jets = jets = jets = jets = jets Central jet count Gap fraction.8.6 CMS preliminary, - L=9.7 fb s=8 TeV, ee/µµ events, M > GeV jj. Total expected (MC). Total expectations (data) data = jets = jets = jets = jets jets Central jet count Third jet p T [GeV] Bckg /Σ..8 Bckg /Σ..8 Clear that electroweak signal is reuired to describe distributions sensitive to additional jet activity. /7/

24 Summary ATLAS and CMS produced measurements of fiducial cross sections for electroweak Z+j production. -driven techniues used to constrain background modelling - QCD Z+j. Both electroweak cross sections measurements compatible with predictions from theory and exclude background-only hypotheses at >σ.! ATLAS also measured fiducial cross section for inclusive Z+j production in several phase regions with varying sensitivity to the electroweak component. All consistent with theoretical predictions.! /7/ Both collaborations measured distributions of hadronic jet activity. Interesting to study variables used in Higgs studies. Can probe and constrain the QCD modelling of the Z+j background.

25 Thank you /7/