Study of the acceptance for ZZ to four lepton events produced in pp collisions at s = 13TeV in the ATLAS detector at CERN.

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1 Study of the acceptance for ZZ to four lepton events produced in pp collisions at s = 13TeV in the ATLAS detector at CERN. Skorda Eleni Supervisor: Assistant Professor Konstantinos Kordas Aristotle University Thessaloniki April 8, 2017 Work done with: K. Kordas and A. Leisos Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

2 Introduction Introduction Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

3 Introduction ZZ production Standard Model ZZ production (lowest level diagrams): q q u and t channels gg initiated processes contribute 6% ZZ production is also possible through Higgs decay and non SM sources. We are interested in leptonic decays where lepton is either e or µ Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

4 Introduction ZZ production measurement N signal pp ZZ 4l = L σtot pp ZZ BR ZZ 4l A ZZ Czz L: Integrated Luminosity σ tot pp ZZ : total cross section of ZZ production BR ZZ 4l : branching ratio of the two Z bosons to decay to either electrons or muons A ZZ : Acceptance correction for the geometrical and kinematic criteria A ZZ = fiducial events total events C zz: Efficiency correction for detector ability to reconstruct these objects C ZZ = reconstructed events fiducial generated events Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

5 Introduction Fiducial and total cross section Fiducial Cross Section extrapolation Total Cross Section the measured cross section : σ fid pp ZZ 4l = N obs N bkg L C ZZ corresponds to the reduced phase-space of the actual measurement it is a fraction of the total cross section Extrapolate the fiducial to the total cross section for ZZ production σ tot pp ZZ 4l = N obs N bkg L A ZZ C ZZ *include branching ratio σ fid pp ZZ 4l = σ tot pp ZZ BR ZZ 4l A ZZ Acceptance Factor A ZZ A zz = σfid pp ZZ 4l σ tot pp ZZ 4l Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

6 Introduction Calculation of Extrapolation factor A ZZ Monte Carlo generators used: MCFM 6.8 : A ZZ nominal value,2e2µ channel only POWHEG (for the q q processes) and SHERPA (for the gg processes) : cross-check and to estimate the correction for the same flavor channels. We generate samples of ZZ events. Z mass range: GeV For the 2e2mu channel: each lepton pair must have invariant mass in the range ( 66, 116GeV ) In the 4e and 4µ channels: first we must form the correct pairs and then apply the mass cut From all possible SFOC lepton pairs we select the pairing that is minimizing m ll m Z + m l l m Z Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

7 Introduction MCFM,Monte Carlo event generator: process used: pp ZZ e e + µ µ +. 4e and 4µ decay channels not available in MCFM parton level predictions can be NLO or LO produces weighted events handy tool to study easily the pdf and scale uncertainties on the cross section efficient in generating very large statistics Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

8 Calculation of A ZZ and its systematic uncertainties Calculation of A ZZ and its systematic uncertainties Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

9 Calculation of A ZZ and its systematic uncertainties Fiducial phase space definition Fiducial phase space (subset of the total MCFM sample): All leptons must have P T > 5 GeV and η < 2.7 The three highest-p T leptons must satisfy P T > 20, 15, 10 GeV R(l i l j) > 0.2 (0.1) between any two different (same) flavour leptons i, j All possible SFOC dileptons in the final selected quadruplet have mass > 5 GeV Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

10 Calculation of A ZZ and its systematic uncertainties Study of gg contribution with MCFM *gg initiated processes are calculate in LO, gg initiated cross sections are scaled by an NLO k-factor of 1.67 ± The folowing table is obtained using MCFM and for 2e2µ channel only. configuration σ tot [fb] σ fid [fb] A ZZ with gg ± ± ± (Nominal MCFM weights) only gg ± ± ± without gg ± ± ± with gg ± ± ± Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

11 Calculation of A ZZ and its systematic uncertainties Correction from dilepton mass to Z mass N observed N bkg =L σ pp ZZ BR ZZ 2e2µ A ZZ (2e2µ) C ZZ (2e2µ) + L σ pp ZZ BR ZZ 4e Dzz A ZZ (4e) C ZZ (4e) + L σ pp ZZ BR ZZ 4µ Dzz A ZZ (4µ) C ZZ (4µ) A correction to A ZZ should be provided in the 4e and 4µ channels because of the pairing algorithm bias and interference effects. The correction factor D ZZ is calculated by : D ZZ = σtot ZZ (4e) + σ tot ZZ (4µ) σ tot ZZ (2e2µ) The ratio is found using POWHEGq q and SHERPA gg initiated samples: σ tot(2e2µ) [fb] σ tot(4e)[fb] σ tot(4µ) [fb] D ZZ POWHEG(q q)+ SHERPA(gg) ± ± ± ±0.005 Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

12 Calculation of A ZZ and its systematic uncertainties PDF uncertainty From the CT10 family set variations. A ZZ is also calculated using MSTW and NNPDF23 PDF sets and the deviations from CT10 value are calculated. PDF set 2e2µ CT ± CT10 variations , (+0.34%, 0.43%) CT10-MSTW (MSTW2008nnlo68cl) % CT10-NNPDF23 (NNPDF23 nlo as 0118) % Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

13 Calculation of A ZZ and its systematic uncertainties Renormalization and factorization scale uncertainty To find renormalization and factorization scale uncertainty µ R = µ f = 1.0 m ZZ, is chosen as the nominal value. independent samples varying µ R, µ f together and independently around the nominal value The absolute uncertainty is the difference between the nominal A ZZ value and the values computed from the other samples. The maximum positive and negative deviations are taken as the uncertainty on A ZZ The folowing table is obtained using MCFM and for 2e2µ channel only µ R /m ZZ µ f /m ZZ A ZZ (2e2µ) nominal ± ± ± ± ± ± Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

14 Calculation of A ZZ and its systematic uncertainties Generator uncertainty Uncertainty due to the choise of MC generator A ZZ is estimated using POWHEG(q q) and SHERPA(gg). The difference between the MCFM value and this one is taken as the uncertainty. A ZZ (2e2µ) POWHEG(qq)+SHERPA(gg) ± MCFM(qq+1.76gg) ± MCFM(qq+1.76gg)-POWHEG(qq)+SHERPA(gg) ± Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

15 Results Results Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

16 Results Results Nominal value of A ZZ for the 2e2µ channel and its total uncertainty. Nominal total unc. A ZZ Contributions to the total uncertainty Source Abs. Unc. Rel. Unc. Statistical CT10 error set % 0.1% +0.34% 0.43% +0.79% 0.75% QCD scale Generator ± ±0.63% Total % 1.12% A ZZ for the 4e and the 4µ channel The value estimated for the 2e2µ channel is multiplied with D ZZ before it is used for extrapolation Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

17 Results Cross sections at s = 13TeV, L = 3.2 fb 1 A ZZ 0.39 ± 0.02 C ZZ (4e) 0.55 ± 0.02 C ZZ (4µ) 0.81 ± 0.03 C ZZ (2e2mu) 0.63 ± 0.02 Observed events Channel 4e 15 4mu 30 2e2µ 18 Total 63 Events Total expected background new results compatible with last year now in the process of being approved by the SM group and be circulated in ATLAS in the next few weeks, aiming for a publication. Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

18 Results Conclusion We estimated the value of the extrapolations factor A ZZ and its systematic uncertainties for the 2e2µ channel using MCFM 6.8 For the same flavor channels we corrected the value to take into account the pairing algorithm bias The cross sections were scaled to match NLO predictions The result is used in the extrapolation of the ZZ 4l cross section in 13 TeV pp collisions with the ATLAS detector Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19

19 Results Thank you for your attention Skorda Eleni Computational Physics MSc (A.U.Th) H.E.P 2017 Ioannina April 8, / 19