From ZZ 4l to Vector Boson Scattering at LHC

Transcription

1 From ZZ 4l to Vector Boson Scattering at LHC Linda Finco Supervisor: Nicola Amapane Second Year Seminar

2 Outline LHC and the CMS Experiment VBS: theoretic introduction and motivations ZZ+2jets Channel ZZ 4l inclusive cross section measurement Unfolding differential cross sections VBS ZZ+2jets: signal definition and search region Conclusions and Perspectives L. Finco Second Year Seminar 2

3 LHC and CMS CMS DETECTOR Total weight: tonnes Overall diameter: 15.0 m Overall length: 28.7 m Magnetic field: 3.8 T STEEL RETURN YOKE SILICON TRACKERS SUPERCONDUCTING SOLENOID MUON CHAMBERS PRESHOWER FORWARD CALORIMETER ELECTOMAGNETIC CALORIMETER HADRON CALORIMETER L. Finco Second Year Seminar 3

4 Precise SM Measurements with CMS Good understanding of the detector + accurate theory predictions Precise measurements of the Standard Model processes over many orders of magnitude L. Finco Second Year Seminar 4

5 Still, a lot to be done In July 2012 a new particle was discovered: the Higgs boson Open issues: understand the nature of this Higgs boson and if there is new physics beyond the SM How? keep searching for new particles (more Higgs bosons, sparticles, something new ) measure with high precision its properties to the %level measure the Higgs other important role: unitarization of the VV scattering L. Finco Second Year Seminar 5

6 The SM Higgs Boson The Higgs mechanism explains how the elementary particles get mass The W and Z acquire the longitudinal degree of freedom (W L, Z L ) Without the Higgs, V L V L V L V L would break unitarity (for s > 1.2 TeV) σ VL V L V L V L s t + s2 s m H 2 + t2 t m H 2 L. Finco Second Year Seminar 6

7 The Vector Boson Scattering High energy vector boson scattering plays a central role: test of the nature of the Higgs boson main experimental grounds to the understanding of which alternative theory is at work. L. Finco Second Year Seminar 7

8 qq ZZ+2jets Channel Z Z Main features: Energetic jets in the forward and backward directions (p T > 30 GeV) Large rapidity separation Large invariant mass of the two tagging jets ZZ decay products between tagging jets Little gluon radiation in the central-rapidity region, due to colorless W/Z exchange (no extra jets between tagging jets) L. Finco Second Year Seminar 8

9 Before L. Finco Second Year Seminar 9

10 ZZ 4l Cross Section σ ZZ 4l = N sig L A ε Main contributions: Z 1 l + Z 1 l + l l l + l + Z 2 Z 2 l l l = μ or e L. Finco Second Year Seminar 10

11 Signal and Backgrounds σ ZZ 4l = N sig L A ε Taken from data N sig = N sel N bkg Irreducible background: events which contain 4 prompt leptons from nonsignal processes (ttz, WWZ, ttww), very small estimated from MC sample Reducible background: processes which contain one or more non-prompt leptons in the four-lepton final state (DY, tt, WZ, WWW) not well represented by MC samples low statistics estimated from a data driven method based on the lepton-to-jet fake rate L. Finco Second Year Seminar 11

12 Signal Region (Initial State) L. Finco Second Year Seminar 12

13 Signal Region (Final State) L. Finco Second Year Seminar 13

14 Acceptance and Luminosity σ ZZ 4l = L = 19.7 fb 1 Final State A ε ZZ 4µ ZZ 4e ZZ 2e2µ N sig L A ε Defined as A ε = N reco MC N sig where: N reco = # events passing reconstruction N MC sig = # events passing signal definition Evaluated using the PDF4LHC prescription to take into account PDF uncertainties L. Finco Second Year Seminar 14

15 Results σ ZZ 4l = N sig L A ε Final State σ [pb] Stat. Unc. Syst. Unc. ZZ 4µ 7.3 ± 0.8 ± 0.4 ZZ 4e 6.8 ± 1.0 ± 0.4 ZZ 2e2µ 8.0 ± 0.7 ± 0.5 ZZ 4l 7.5 ± 0.5 ± 0.3 Theoretical value 7.7 ± 0.6 pb L. Finco Second Year Seminar 15

16 Next L. Finco Second Year Seminar 16

17 ZZ 4l Differential Cross Section Δη N jets m ZZ m jj L. Finco Second Year Seminar 17

18 ZZ 4l Differential Cross Section Δη N jets m ZZ m jj L. Finco Second Year Seminar 18

19 Unfolding Procedure MC Truth Matrix Inversion Response Matrix Reco distribution (data) Reco distribution (MC) Unfolded distribution (data) Data at particle level Distribution correction for experimental effects L. Finco Second Year Seminar 19

20 ZZ 4l Differential Cross Section Δη N jets m ZZ m jj L. Finco Second Year Seminar 20

21 ZZ 4l Differential Cross Section Δη N jets m ZZ m jj L. Finco Second Year Seminar 21

22 NgenCentralJets Response Matrix Bayesan algorithm: the most robust 2 iterations: lowest χ 2 with respect to the true distribution Response matrix: qq ZZ + gg ZZ NCentralJets L. Finco Second Year Seminar 22

23 ZZ 4µ MC Unfolding (Example) Bayes Algorithm 2 Iterations N jets True Measured Unfolded ± 5.40 ± 0.19 ± ± 2.85 ± 0.11 ± ± 1.49 ± 0.06 ± ± 0.72 ± 0.03 ± ± 0.29 ± 0.01 ± L. Finco Second Year Seminar 23

24 Systematic Uncertainties Unfolding procedure errors from the covariance matrix given by the unfolding (RooUnfold) Response matrix errors from the response matrix (RooUnfold) σqq/σgg ratio (0.4% - 1.5%) MC unfolded using new matrices obtained by varying σqq and σgg of ± dσ MC generator Data/MC ratio JES, JER Comparing unfolding results from Madgraph+MCFM and Powheg+MCFM samples on data MC unfolded using matrices weighted by the Data/MC ratio MC unfolded using new matrices obtained by varying JES and JER w.r.t. their uncertainties L. Finco Second Year Seminar 24 ON GOING

25 Finally, back to the Vector Boson Scattering L. Finco Second Year Seminar 25

26 VBS: ZZ+2jets Channel Z Z Main features: Energetic jets in the forward and backward directions (p T > 30 GeV) Large rapidity separation Large invariant mass of the two tagging jets ZZ decay products between tagging jets Little gluon radiation in the central-rapidity region, due to colorless W/Z exchange (no extra jets between tagging jets) L. Finco Second Year Seminar 26

27 Signal Definition Z Z At generator level required: ZZ signal definition At least 2 (tag) quarks with E > 20 GeV and η < 5 No additional leptons at matrix element level No additional bosons (Z or W) L. Finco Second Year Seminar 27

28 Search Region Definition Challenge: background 20 times the signal cross section ( 0.1 fb) Search Region starting point ZZ baseline selection Two very energetic jets in the forward-backward region Variables Δη between the two tag jets Δy between the two tag jets Tag jets energy Invariant mass of the 6 fermions (4 leptons + 2 jets) Invariant mass of the ZZ system Invariant mass of the di-jet system Missing Transverse Energy L. Finco Second Year Seminar 28

29 Variable Distributions: Δη and m jj Δη m jj L. Finco Second Year Seminar 29

30 Potential Search Region Reconstuction level cuts: p jet1 T > 100 GeV and p jet2 T > 70 GeV MET < 60 GeV L. Finco Second Year Seminar 30

31 Potential Search Region Reconstuction level cuts: p T jet1 > 100 GeV and p T jet2 > 70 GeV MET < 60 GeV Δη > 2 Signal Tot Bkg m jj > 500 GeV L. Finco Second Year Seminar 31

32 Conclusions and Perspectives What I have done measured ZZ 4l inclusive cross section started unfolding variables (njets) to the differential cross section measurement studied signal definition and search region for the VBF ZZ+2jets channel What I still have to do unfold all the interesting variables to the measurement of the differential cross section (Δη, m ZZ, m jj ) finalize the study of the VBS ZZ+2jets channel look at the data L. Finco Second Year Seminar 32

33 Precise SM Measurements with CMS σ W σ z L. Finco Second Year Seminar 33

34 Precise SM Measurements with CMS σ W+njets σ z+njets L. Finco Second Year Seminar 34

35 Precise SM Measurements with CMS σ zz L. Finco Second Year Seminar 35

36 Precise SM Measurements with CMS σ zz+njets??? L. Finco Second Year Seminar 36

37 Publications The CMS Collaboration, Measurement of the properties of a Higgs boson in the fourlepton final state, arxiv: , Phys. Rev. D 89, The CMS Collaboration, Constraints on the Higgs boson width from off-shell production and decay to Z-boson pairs, arxiv: , Phys.Lett. B 736 (2014) 64. Proceedings: L. Finco, "Properties of the Higgs-like boson in the H ZZ 4l decay channel at CMS", Published in Nuovo Cim. C037 (2014) 04 L. Finco, "Recent Results on the Higgs Boson Properties in the H ZZ 4l decay channel at CMS", e-print: arxiv: L. Finco, "Higgs boson couplings and properties with CMS", Published in PoS DIS2014 (2014) 108 L. Finco Second Year Seminar 37

38 Conferences and Posters June 4, 2014: Large Hadron Collider Physics, New York (United States): "Recent results on the Higgs boson properties in the H ZZ 4l decay channel at CMS (Poster). April 30, 2014: XXII International Workshop on Deep-Inelastic Scattering and Related Subjects, Warsaw (Poland): "Higgs boson couplings and properties with CMS". April 10, 2014: Incontri sulla Fisica delle Alte Energie, L Aquila (Italy): "Recent results properties of the Higgs boson in the H ZZ 4l decay channel at CMS March 5, 2014: Posters@LHCC, CERN (Switzerland): "Properties of the Higgs-like boson in the H ZZ 4l decay channel at CMS" (Poster). September 23, 2013: XCIX Congresso Società Italiana di Fisica, Trieste (Italy): "Properties of the Higgs-like boson in the decay H ZZ 4l in pp collision at 7 and 8 TeV in the CMS Experiment". L. Finco Second Year Seminar 38

39 Schools and Courses January 8-12, 2013: CMS Data Analysis School, DESY, Hamburg (Germany). October 22-25, 2013: XXIII Giornate di Studio sui Rivelatori, University of Torino, Torino (Italy). June 18 - July 1, 2014: European School of High-Energy Physics, Garderen (Netherlands). October 28-31, 2014: XXIV Giornate di Studio sui Rivelatori, University of Torino, Torino (Italy). L. Finco Second Year Seminar 39

40 Backup L. Finco Second Year Seminar 40

41 Event Selection Same particle selection of H ZZ 4l analysis Leptons Electrons: BDT multivariate technique η e < 2.5 p T e > 7 GeV Muons: PF selection η μ < 2.4 p T μ > 5 GeV R iso < 0.4 SIP= IP/σ IP < 4 Jets p T jet > 30 GeV η jet < 4.7 PF jet, loose ID+PU veto Baseline selection requiring events with: 60 < m Z1 < 120 GeV 60 < m Z2 < 120 GeV (If # Z2 >1 the pair of leptons with highest scalar sum of pt is chosen) m ZZ > 100 GeV m llcrossed < 4 GeV At least two leptons with p T > 10 GeV and one with p T > 20 GeV L. Finco Second Year Seminar 44

42 Systematic Uncertainties Source Systematic Uncertainty Trigger 1.5% Lepton ID and Isolation 1-2% Luminosity 2.6% Reducible Background 33-66% Irreducible Background 13-19% Acceptance for PDF 5% Trigger/lepton ID and ISO/acceptance uncertainties taken from ZZ inclusive analysis (for now) L. Finco Second Year Seminar 45

43 ZZ 4e MC Unfolding Bayes Algorithm 2 Iterations N jets True Measured Unfolded ± 4.49 ± 0.15 ± ± 2.42 ± 0.10 ± ± 1.23 ± 0.05 ± ± 0.55 ± 0.02 ± ± 0.25 ± 0.01 ± L. Finco Second Year Seminar 46

44 ZZ 2e2µ MC Unfolding Bayes Algorithm 2 Iterations N jets True Measured Unfolded ± 7.03 ± 0.24 ± ± 3.77 ± 0.15 ± ± 1.91 ± 0.08 ± ± 0.88 ± 0.03 ± ± 0.34 ± 0.01 ± L. Finco Second Year Seminar 47

45 Figure of Merit L. Finco Second Year Seminar 48

46 pp ZZ+2jets Channel tagging jet η l + l l l + tagging jet L. Finco Second Year Seminar 49

47 Δη Signal Region Reconstuction level cuts: p T1 > 100 GeV and p T2 > 70 GeV m jj > 500 GeV Signal Tot Bkg MET < 60 GeV L. Finco Second Year Seminar 50

48 m jj Signal Region Reconstuction level cuts: p T1 > 100 GeV and p T2 > 70 GeV Δη > 2 Signal Tot Bkg MET < 60 GeV L. Finco Second Year Seminar 51