Top production at the Tevatron

Transcription

1 Top production at the Tevatron Bob Hirosky University of Virginia for the CDF and D0 Collaborations Moriond Moriond QCD

2 Top production at the Tevatron Top mass from differential cross section Polarization tt asymmetry (Tevatron combination) s Tevatron Run II data taking ( ): p-p collisions at s = 1.96 TeV ~12 fb-1 delivered per experiment ~10 fb-1 for analysis 2

3 Top quark production ~85% ~15% At the Tevatron top mostly produced in pairs via qq annihilation a unique data set for the top quark studies LHC: 80-90% gluon fusion Cross-section (NNLO +NNLL QCD for mt=172.5 GeV): Czakon,Fiedler and Mitov, PRL 110 (2013) Czakon and Mitov, Comput. Phys. Commun. 185 (2014)

4 Top pair production signatures Well known decay signatures Effective event ID using: high mass (kinematic selections) heavy flavor jet tagging isolated high pt lepton(s), missing pt 4

5 Signal and Background tt simulation: ALPGEN + PYTHIA (parton showering + hadronization) Systematic uncertainty studies: MC@NLO+HERWIG, different PYTHIA versions Acceptance 10-20% ( 4 6 reconstructed objects in the final state) Lepton + Jets Dominant backgrounds W+jets (Wbbj, Wccj, Wjjj) Multijet events with misidentified leptons Dibosons (WW, WZ, ZZ) +jets Dileptons Z μμ, ee, ττ + jets (D-Y) Dibosons (WW, WZ, ZZ) + jets W+jets and multijet events with misidentified leptons 5

6 Signal and Background Example S/B performance (from D0 inclusive tt cross section analysis) Lepton + Jets (MET) Good signal purity, high statistics, 1 neutrino in reconstruction Dileptons (b-tag MVA) Very good signal purity, more limited statistics, 2 neutrinos Phys. Rev. D 94, (2016) 6

7 Result 1 Top pole mass from differential cross sections D0CONF 6473 Kinematic distributions: Top quark momentum, tt invariant mass, etc. are sensitive to the top quark mass Expect improved sensitivity vs extraction from total cross-section eg. Phys. Rev. D 94, (2016) Use the D0 lepton+jets measurement: Phys. Rev. D 90, (2014) Compare differential distributions to NNLO QCD calculation of TOP++ using the pole mass: Czakon, Fiedler, Heymes and Mitov, JHEP 1605, 034 (2016) 7

8 Ratios of NNLO calculation D0CONF 6473 to data Sensitivity Good NNLO QCD scale uncertainties < 5% for pttop 10% maximum for m(tt) Better 8

9 Top pole mass from differential cross sections D0CONF 6473 Unfolded cross sections Use χ2 fit to measure the mass Include full 2-D correlation matrix in m(tt), pttop Precision: Precision: 1.5% 1.5% ~ ~ 25% 25% improvement improvement over over using using inclusive inclusive XS XS 9

10 Result 2 Top quark polarization SM top quark production at Tevatron is nearly unpolarized Any top quark polarization at production is undiluted by hadronization (small τtop) BSM models predict enhanced polarization (eg Z, Axigluon) Polarization Pn^ measured from angular distribution (θ) of decay products wrt quantization axis n^ Beam basis: use proton beam direction boosted in tt rest frame as quantization axis Helicity basis: use tt axis in tt rest frame Transverse basis: cross product (beam axis, top direction) in tt rest frame 10

11 Top quark polarization Phys. Rev. D 95, (R) (2017) Beam and transverse polarizations expected to be larger in pp vs pp Sensitive to BSM models with nonzero polarization use angular distribution of lepton from W decay => best analyzing power kinematic discriminant to separate top signal and dominant W+jets bkg l+3jets, l+4jets samples analyzed lepton angles from kinematic reconstruction of tt event cosθl,nn distributions beam basis helicity basis transverse basis 11

12 l+jets channel, full statistics 9.7 fb-1 Polarization Results Measure polarization by fit to reconstructed cosθi,nndistribution using tt templates of Pnn = ±1 and background templates normalized to the expected event yield Phys. Rev. D 95, (R) (2017) Longitudinal polarizations World s 1st measurement wrt transverse axis Combined w/ dilepton result wrt beam axis Most precise polarization measured in pp Results consistent w/ SM and 0 polarization Comparison w/ various non-sm scenarios 12

13 Result 3 tt production asymmetry Forward-backward production asymmetries have been discussed extensively since Initial MC predictions suggested very small asymmetry, increasing significantly from higher order corrections. CDF and D0 have completed extensive studies and recently provided combined Tevatron results. 13

14 tt production asymmetry => forward-backward asymmetry in pp collisions SM predicts asymmetry in tt production at NLO from events with qq initial states (gg is symmetric) Czakon, Fiedler and Mitov, PRL 115, 5, (2015) SM asymmetry is small, ~9.5% (NNLO) => sensitive to test new physics contributions Contrast with pp collisions (charge asym. ~ 1%) 14

15 Asymmetry measurement l Measured either wrt : reconstructed (anti)top direction location of lepton in ll, l+jets events (correlated with direction of top quark) Reconstruction of the Δy distribution Background subtraction Atop: unfolding Alep: correction affected by the top polarization Alep~ Atop/2 at 1.96 TeV W ν b t Early History 15

16 Asymmetry measurement l Measured either wrt : reconstructed (anti)top direction location of lepton in ll, l+jets events (correlated with direction of top quark) W ν b t Early History 16

17 Asymmetry:Tevatron Combination Preliminary Use BLUE to combine measurements Standardize and combine systematic uncertainties. All results are limited by the statistical uncertainty Consistency: tt asymmetry vs NNLO prediction: 1.3 SD Lepton qη asymmetry vs NLO prediction: 1.6 SD Lepton Δη asymmetry vs NLO prediction: 1.3 SD Note: the three asymmetry measurements are correlated! FERMILAB-CONF PPD 17

18 Differential asymmetry combinations Afbtt(mtt) Afbtt(Δy) Slope of Afbtt(Δy) Compared w/ SM NNLO predictions Work in progress on slopes Determine combination bins Account for bin-to-bin correlations Combine systematic uncertainties 18

19 Conclusion Twenty years after discovery, Tevatron data still providing valuable insight into top quark physics Precise measurements, complementary to LHC s pp initial state (production asym., transverse polarization, s-chan single top, m top,...) AFBtt in agreement with Standard Model Final measurements of the top-quark production asymmetry and more refined predictions are in better agreement (~1.5σ) top quark polarization consistent with expectations New, more precise method of top pole mass using differential cross sections demonstrated Latest measurements and combinations to finalize Tevatron legacy 19

20 Additional slides 20

21 Typical Event Selections Dileptons Lepton + Jets Trigger: single lepton and lepton+jets Isolated electron or muon pt > 20 GeV η(μ) < 2.0, η(e) < 1.1 At least 3 jets pt>20 GeV Missing transverse momentum > 20 GeV Additional selections: Use of the b-jet identification Event kinematic selections Trigger: dileptons Two isolated leptons (electrons and/or muons) pt > 15 GeV η(μ) < 2.0 η(e) < 1.1 and 1.5< η(e) <2.5 At least 2 jets pt>20 GeV Missing transverse momentum > 20 GeV Additional selections: Use of the b-jet identification Event kinematic selections Moriond QCD,

22 Top pole mass from inclusive XS Direct measures shown based on mtmc!=mtpole (but close Δ<~1GeV) Can not be used directly for precise NLO / NNLO theoretical predictions mtpole can be extracted from inclusive cross-section measurement Well defined mass parameter in QFT Use l+jets and dilepton channel data MVA method, simultaneous template fit for XS 22

23 Top pole mass from inclusive XS mtpole determined using experimental dependence of σtt on mt. Most probable mt and uncertainty calculated via joint likelihood of data, NNLO pqcd, and related uncertainties In agreement with world average MC mass of ±0.76 GeV Most precise determination of mtpole at the Tevatron. 23

24 Helicity bases lab frame top rest frame tt rest frame s Beam basis Transverse basis Helic i t y ba s is s 24

25 tt production asymmetry top and tbar show a forward-backward asymmetry in pp collisions NLO 2 2 (interference between Born and box diagrams), LO 2 3: expect (5 10)% asymmetry (higher order corrections are small) NLO 2 3 (ISR/FSR interference): has the negative asymmetry and reduce expected asymmetry significantly strong dependence from phase space region gg initiated processes are symmetric SM asymmetry is small, ~9.5% (NNLO) => sensitive to test new physics contributions Czakon, Fiedler and Mitov, PRL 115, 5, (2015) 25

26 time Asymmetry measurement 26

27 Forward Backward Asymmetry at D0 tt asymmetry in l+jets channel Phys. Rev. D 90, (2014) Leptonic asymmetry in l+jets chan. Phys. Rev. D 90, (2014) tt asymmetry in diepton channel Phys. Rev. D 92, (2015) Leptonic asymmetry in l+jets chan. Phys. Rev. D 88, (2013) 27