Top quark mass at ATLAS and CMS

Transcription

1 Top quark mass at ATLAS and CMS (Introduction) Direct measurements Template/Ideogram based Indirect measurements Unfolded shapes/cross sections Conclusions & Outlook Andreas Jung for the ATLAS & CMS collaboration Rencontres de Moriond QCD and high energy interactions March 18th, 2018

2 The top quark Top is the heaviest fundamental particle discovered so far [arxiv: ] mt = ± 0.76 GeV Unique quark: Production dominated by gg fusion: Decay channels: dilepton Observe bare quark properties Large Yukawa coupling to Higgs boson λt ~ 1 only mt is natural mass Special role in EW symmetry breaking? No fine-tuning if top quark partner exists BR, bg increase lepton+jets All hadronic 2

3 Direct vs. Indirect methods [arxiv: ] Degrassi et al. Self-consistency of SM & stability of the EW vacuum rely on the pole mass Indirect extraction from e.g. cross section, end point, J/psi method top quark pole mass Direct methods e.g. template, matrix element, likelihood, ideogram MC mass, close to pole mass Efforts to calibrate the MC mass to pole mass Estimates: GeV difference to pole mass PRL 117, (2016) 3

4 Challenges/Perspectives Direct methods: Reconstruct top candidates (kinematic fits) Determine m(t) or m(t) & Jet energy SF, b-jet energy SF, bg fraction parameters Likelihood based on templates (ATLAS+CMS) or Ideogram (CMS) Most precise results, no single large uncertainty left Alternative methods: larger uncertainties but perpendicular Indirect methods: Relies on theoretical predictions (various choices) Fully corrected data, more complex Larger uncertainties, pole mass interpretation Relative b-jet correction: ± Employ different decay channels (different systematics, in-situ jet energy scale) Use direct (classical), direct (alternative), and indirect (based on σ, dx/dσ) 4

5 Underlying event CMS-PAS-TOP CMS-PAS-TOP CMS used 13 TeV data to measure multiple jet structure variables: particle multiplicity, width, pt, E correlations Unfolded to stable particle level, used for MC tuning Sensitivity to Color Reconnection: Potential to reduce further from current 340 MeV ( QCD based and Gluon move ) Sensitivity to α(finite State Radiation): Potential to reduce systematic uncertainty further from current 240 MeV 5

6 Direct methods: l+jets CMS measurement at 13 TeV, e/μ+jets decay channel: At least 4 jets, exactly 2 jets b-tagged Kinematic fit to constrain the W mass and using decay of 2 same mass heavy particles Keep all permutations Follow same strategy as 8 TeV result: 2D fit of m(top) vs. Jet energy scale factor Mtop = ± 0.08 (stat+jsf) ± 0.62 (syst.) GeV δmt/mt = 0.36% 1st at 13 TeV PAS-TOP Dominant systematics: Flavor (0.41 GeV) Model (0.41 GeV) JSF (0.19 GeV) Note: CR treatment changed from 8 to 13 TeV 6

7 Direct methods: l+jets ATLAS measurement at 8 TeV, e/μ+jets decay channel: At least 4 jets, exactly 2 jets b-tagged Kinematic fit to constrain the W mass and using decay of 2 same mass heavy particles BDT rejects events w worse resolution Template fit to three distributions to determine: m(t), b-jsf, and JSF Mtop = ± 0.39 (stat+jsf) ± 0.82 (syst.) GeV δmt/mt = 0.53% ATLAS-CONF

8 Direct methods: dilepton ATLAS measurement at 8 TeV, dilepton decay channel: Cleanest sample At least 2 jets, at least 1 jet b-tagged Reconstruct invariant mass of b-tagged jet and lepton (templates) and use lowest average mlb Fit to signal and background templates m(t) and bg fraction free parameters Mtop = ± 0.41 (stat) ± 0.74 (syst.) GeV δmt/mt = 0.49% PLB 761(2016)350 Dominant systematics: JES (0.54 GeV) Model (0.35 GeV) b-jes (0.30 GeV) 8

9 Direct methods: dilepton CMS measurement at 8 TeV, dilepton decay channel: Cleanest sample At least 2 jets b-tagged Reconstruct invariant mass of b-tagged jet and lepton (templates) bb mlb, mt2, mlbν in 7x5 m(t) vs. JSF 1D, 2D, or hybrid fit 80% (1D) and 20% (2D) lowest uncertainty Mtop = ± 0.18 (stat) ± 0.91 (syst.) GeV δmt/mt = 0.54% PRD 96(2017) Dominant systematics: Model (0.64 GeV) JES (0.45 GeV) b-frag (0.40 GeV) 9

10 Combinations / All results Using BLUE: Latest CMS combination δmt/mt = 0.28% Latest ATLAS combination δm /m = 0.29% t t mtop = ± 0.50 GeV Anti-correlations of systematic uncertainties used, provides a non-trivial improvement ATLAS-CONF

11 Combinations / All results Using BLUE: Latest CMS combination CMS-PAS-TOP δmt/mt = 0.28% Alternative methods: CMS combination δmt/mt = 0.43% mtop = ± 0.75 GeV Orthogonal systematic uncertainties, limited (for now) by statistical uncertainties ATLAS-CONF

12 Indirect methods: l+jets CMS measurement at 13 TeV, l+jets At least 1 jets Reconstruct n(jet): N(b-tags) & N(jets) Use min(mlb) to measure cross section pb Extract pole mass from cross section: pole mtop = ± 2.7 (tot) ± 1.01 (syst.) GeV δmt/mt = 1.7% JHEP 09(2017)051 Dominant systematics: Experimental (2.4 GeV) PDF (1.0 GeV) Extrapolation (0.9 GeV) 12

13 Indirect methods: dilepton ATLAS & CMS both measure tt+1 jet dσ/dx l+jets channel for ATLAS, 7 TeV dilepton channel for CMS, 8 TeV Reconstruct JHEP 10(2015)121 pole mtop = ± 0.8 (stat) ± 1.4 (syst.) ± 0.8 (theory.) GeV pole mtop = ± 1.1 (stat) ± 2.8 (syst.) ± 2.6 (theory.) GeV CMS PAS TOP ATLAS measurement at 8 TeV, dilepton decay channel: clean sample Exactly 1 or 2 b-tagged jets 8 distributions used in a combined fit Systematics: PDF constrained by rapidities, scale by opening angle, others for m(t) ATLAS-CONF pole mtop = ± 0.8 (stat) ± 0.8 (syst.) ± 1.2 (theory.) GeV δmt/mt = 1.0% 13

14 Indirect methods: dilepton Use differential distributions to extract the well defined top quark pole mass Fixed αs and PDF set D0 conference note 6473 Translate into uncertainties ATLAS: Extract from lepton distributions D0: Extract from ptt and mttbar ATLAS-CONF

15 Indirect methods CMS [arxiv: ] CMS-TOP Extraction from production cross section not (yet) competitive with direct measurements but getting closer ATLAS: 0.9%; CMS precision at 1% D0 precision (best at Tevatron): ~ 1.5% D With ~5% theory uncertainty and ~2% exp can reach 0.5% on pole mass Phys. Rev. D (2016) ATLAS-CONF

16 Conclusions Direct methods provide high precision mtop = GeV with δmt/mt = 0.28% Alternative methods are getting more precise, huge data samples help mtop = GeV with δmt/mt = 0.43% Indirect methods are improving in precision and providing pole mass pole mtop = GeV with δmt/mt = 0.95% Only small limited selection of results shown, more information: CMS Top Physics Results ATLAS Top Physics Results Thank you! 16

17 Outlook Run II gets another 50/fb We will get about 80 million tt events Allows for multi-dimensional & simultaneous measurements of σ, αs, PDFs and properties as well ultra precision results via parton level & fiducial particle level A lot more to come... By 2019: 200/fb TODAY 17

18 Backup 18

19 Top quark production Strong interaction: Top pairs dilepton LHC (7/8 TeV): qq: ~15/13% gg: ~85/87% (~10%, 13 TeV) (~90%, 13 TeV) lepton+jets Decay channels: gg fusion BR, bg increase BR, bg decrease All hadronic Theory (NNLO+NNLL): LHC 13 TeV 19

20 Particle flow Particle flow Combines detector information to ID particles Jets and missing ET Gamma & Z-jet balance Pile-up subtraction Isolated Leptons Dilepton resonances (Z, upsilon, J/psi) CMS-JME Relative b-jet correction: ± b-tagging of jets Several techniques, dominated by silicon tracker information Top quark physics requires precise b- and c-physics (oh, well: uds-physics) 20

21 Challenges: Uncertainties Typical theoretical uncertainties are about 2-3% (scale + PDF) Typical experimental: ~3.7% Czakon et al. [arxiv: ] 21

22 Alternative methods 22

23 Alternative methods Using BLUE: Latest CMS combination mtop = ± 0.75 GeV δmt/mt = 0.43% 23

24 World combination [arxiv: ] 24

25 Direct methods: hadronic ATLAS measurement at 8 TeV, dilepton decay channel: fully resolved but large bg At least 2 jets, at least 1 jet b-tagged Reconstruct R3/2 (templates): N(b-tags) & DeltaPhi Fit to signal and background templates m(t) and bg fraction free parameters Mtop = ± 0.55 (stat) ± 1.01 (syst.) GeV Dominant systematics: Model, JES, b-jes CMS: same strategy as other 8 TeV results Mtop = ± 0.25 (stat+jsf) ± 0.59 (syst.) GeV Dominant systematics: Flavor, Model, JES 25

26 Global fits pole kin means extracted from production cross sections means direct measurements, e.g. matrix element method 26

27 EW vacuum stability Very subjective but illustrative, combined latest results from LHC & Tevatron Assumes SM to be true Most recent combinatons Meta vacu stable S M um stabl e vacc SM um yt ~ SD 1 SD Tevatron CMS [arxiv: ] ATLAS point shifted Modified from original by Degrassi et al. Caveat: Direct methods e.g. template, matrix element, likelihood, ideogram measure the MC mass, lots of effort to calibrate the MC mass PRL 117, (2016) Estimates: O(0.5 GeV) difference to pole mass 27

28 EW vacuum stability With the Higgs discovery the SM can be extrapolated to Planck scale energies Test the stability of the electroweak vacuum, under assumption of no new physics: meta-stable, life time > O(1080) tuniverse but new physics can change that dramatically SM Higgs potential dim 6 & 8 BSM modifications E. Branchina et al. 28

29 Pole mass vs. MC mass More on EW stability: K. Mukaida Self-consistency test of the SM & stability of the EW vacuum both rely/use arxiv[ ] pole mass what we measure depends on the method Indirect extraction from e.g. cross section, end point, J/psi method top quark pole mass, but more complex unfolding, larger uncertainties Direct methods e.g. template, matrix element, likelihood, ideogram MC mass, close to pole mass, very precise, large uncertainties 29

30 LHC operations Eur. Phys. J. C (2016) 76:581 30