Standard but not boring Regina Demina University of Rochester Wine and cheese FNAL, Batavia, Il 02/19/16

Regina Demina2Outline Top quark production at LHC Top quark mass – a bit of history Associated production of top and bottom quark pairs ttbar differential cross section –full reco Top Yukawa coupling global variables Asymmetries in ttbar production Spin correlations in ttbar production Diboson production Conclusions

Total cross section (13TeV) TOP accepted by PRL  tt¯)= 772 ± 60 (stat) ± 62 (syst) ± 93 (lumi) pb, in agreement with the expectations from the standard model The cross section is (very) large The signal is clean, yet… 02/19/16Regina Demina3

02/19/16Regina Demina4 Challenges of ttbar reconstruction Lepton+Jets Channel Observed Final state  Combinatorics:  Solution for neutrino (more later) 12 possible jet-parton assignments 6 with 1 b-tag (b-tag helps) 2 with 2 b-tags  Jet energy scale and resolution Note that two jets come from a decay of a particle with well measured mass – W-boson – built-in thermometer for jet energies Initial and final state gluon radiation and multiple interactions and pileup –Can lead to jet misassignment and –gluon radiation changes kinematics of the final state partons –Pile up is actually not a big issue, ISR is Non-ttbar backgrounds are very small

02/19/16Regina Demina5 Then and now: top mass Run 1 CDF ’ s evidence PRD 50,2966(1994): 7 events l+4jets (at least 1 b-tag) “ Under the assumption that the excess yield over background is due to ttbar, constrained fitting on a subset of the events yields a mass of 174± GeV/c 2 for the top quark. ” 9.4% precision Run 1 CMS 2015 arXiv: v1 28,295 events l+4jets (at least 2 b-tag) ±0.16 ± 0.48GeV/c 2 Ideograms method considers 4 highest p T jets in forming permutations, b-jets are required to be b-tagged. Jets are hypothized to originate from W-boson are constrained to W- mass. Only permutations with P>0.2 are considered. 2D fit with two parameters – top mass and common multiplicative factor for jet energy scale, JSF 1D fit – top mass. JES is determined from  (Z)+jet and dijet-balancing techniques Hybrid fit uses a prior for JES. CDF 1994

Top mass Run 1 CMS 2015 arXiv: v1 Combination of 7TeVand 8 TeV results in all-jet, lepton+jets and dilepton channels yields: M t = ±0.13 ± 0.47GeV/c 2 0.3% precision gt=0.9913±0.0007± Why (expected) top Yukawa coupling is so large? Why is it so close to 1? Does top play a special role in EWSB? Does it point to deeper underlying physics of EWSB? Can we measure top coupling to Higgs directly? 02/19/16Regina Demina6

ttbb in the lepton+jets final state at 8 TeV (top13-016) On the way to observe ttH production, H  bb Background from continuum ttbb production σ(tt¯bb¯)= ± (stat) ± 32.2 (syst) ± 7.0 (lumi) fb, σ(tt¯jj)=23.1 ± 2.3 (stat) ± 2.9 (syst) ± 0.6 (lumi) pb σ(tt¯bb¯) / σ(tt¯jj)= ± (stat) ± (syst) 02/19/16Regina Demina7

Top Yukawa 02/19/16Regina Demina8 P. Uwer et al. [arxiv: , Phys. Rev. D 91, (Feb 2015)] Top Top Top quark interaction with Higgs affects the ttbar differential cross section Problem: systematics at low Mtt – mostly due to parton shower modeling Eur. Phys. J. C 75 (2015) 542 Eur. Phys. J. C 75 (2015) 542

02/19/16Regina Demina9 Generator level particles and partons Partons (quarks produced as a result of hard collision) realize themselves as jets seen by detectors –Due to strong interaction partons turn into parton jets –Each quark hardonizes into particles ( ,K) –Energy of these particles is absorbed by calorimeter –Clustered into calorimeter jet Jet energy is not exactly equal to parton energy –Particles can get out of cone –Some energy due to underlying event (and detector noise) can get added –Detector response has its resolution –Depending on parton shower events can drift in and out of the acceptance Gen-level Particles Gen-level Partons Reco level

Differential cross section of ttbar production TOP Normalised differential cross sections for ttbar production with respect to event-level observables This makes the data directly suitable for comparisons with particle level predictions and the tuning of simulations l+4 jets sample, 2 b-tags –Small bg contributions are subtracted based on simulation prediction, QCD is estimated from control samples. Observed distributions are unfolded to particle level observables, e. g. –H T – scalar sum of all jets p T –S T =H T +E T mis +p T lepton 02/19/16Regina Demina10

Neutrino reconstruction in ttbar decay B. Betchart, R. Demina, A. Harel,NIM A 736 (2014) 169. Typically, in l+jets channel p T is taken to be equal to E t miss – the variable that has the worst experimental resolution. W-mass constraint is used to solve for p z – quadratic equation – two or zero solutions. In this approach the top and W mass constraints are imposed simultaneously and a full set of possible p solutions (an ellipse in 3D) is found. The solution closest to measured E t miss in the transverse plane is chosen Unique solution for p z Improved resolution for p T The distance from the ellipse of solutions to E t miss is a useful discriminating variable helpful to choose the correct jet-to- parton assignment 02/19/16Regina Demina11

ttbar asymmetry – template method Top Accepted by PRD Distribution  Y tt ) is broken down into symmetric [   ] and antisymmetric [   ] components at reco level The asymmetry is extracted from the fit of the data distribution in  tt to the sum of MC derived templates with one free parameter  02/19/16Regina Demina12

ttbar asym 02/19/16Regina Demina13 A y c = [ 0.33 ±0.26 (stat) ± 0.33 (syst) ] %

ttbar spin correlations in dilepton channel Lifetime: ~ sec Hadronization timescale: ~ sec Spin decorrelation timescale: ~ sec Full reconstruction and unfolding to parton level of ttbar system: –  (leptons) in ttbar rest frame; –Cos  *(l) wrt top direction  top polarization; –Cos  *(l + ) Cos  *(l - )  spin correlation Define asymmetries: D=-2A(  – spin correlation coefficient (Bernreuther &Si Phys. Lett. B 725 (2013) 115) 02/19/16Regina Demina14

ttbar spin correlations in dilepton channel Define asymmetries: –C hel =-4A c1c2  – spin correlation coefficient C (B&S ref) –Top(antitop) polarization P +(-) =2A P –SM fraction 0.87±0.17±0.21±0.04 –Using MEM in l+jets find 0.72±0.09± /19/16Regina Demina15

ZZ production PLB 740 (2015) 250 pb 02/19/16Regina Demina16 SMP ZZ→lll′l′, where l=e,μ l′=e,μ,τ The invariant mass distribution of the four-lepton system is used to set limits on anomalous ZZZ and ZZ  couplings at the 95% CL

WW production SMP Submitted to Eur. Phys. J. C  (W W)=60.1 ± 0.9 (stat) ± 3.2 (exp) ± 3.1 (theo) ± 1.6 (lumi) pb = 60.1 ± 4.8 pb Theory prediction NNLO pb (arXiv: ) 02/19/16Regina Demina17

02/19/16Regina Demina18Conclusions CMS performed a thorough test of the SM at 7 and 8 TeV and is producing results at 13TeV The main factors in this success are –LHC remarkable performance and large data sample –Excellent CMS performance –Development of advanced data analysis methods A lot of emphasis is put on careful evaluation of systematic uncertainties

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