Top Quark Physics at Hadron Colliders

Transcription

1 School of Physics, Shandong University 16 Oct 2014, USTC, Hefei

2 : Outline Properties of Top Quark Theoretical Framework Top Quark Physics within SM Top Quark Decay Cross Section for Hadronic t t Production Charge Asymmetry in Hadronic t t Production Top Quark Spin Effects in Hadronic t t Production Top Quark Physics BSM Top Quark Spin and Anomalous Top Quark Coupling Top Quark Spin and W Chiral Coupling Outlook

3 Properties of Top Quark

4 Properties of Top Quark Top Quark mass: m t ± 052 ± 072 Spin: 1 Color: SU(3) 2 C Triplet Electric Charge: 2 3 e The heavist elementary particle Decay Width: Γ SM t 13 GeV, PDG 2013: Γ t = 20 ± 05 GeV Lifetime τ t sec << Characteristic Hadronization time sec Unique opportunity to investigate interactions of a bare quark! Interactions are governed by short distance dynamics!! Theoretical predictions related to top quark are reliable!!

5 Top Quark Physics Dynamics of top production and decay is not known very precisely so far: Is t b decay vertex really (V-A)? New decay modes, eg, t c +? Exp analyses require precise SM predictions Excellent probe of mechanism of EWSB Higgs boson H has been found within SM, the Yukawa coupling(s) y t t th y t = m t /(246GeV) 07 Search for heavy resonances, eg heavy non-standard Higgs bosons, that couple strongly to t t Good probe for non-sm parity and/or non-sm CP violation: effects could be induced, eg, by non-standard Higgs bosons = Important tool to precisely test SM and search for new physics BSM

6 Theoretical Framework

7 Theoretical Framework

8 Theoretical Framework

9 Calculation of dσ(h 1 h 2 t tx 6f + X at NLO QCD)

10 Calculation of dσ(h 1 h 2 t tx 6f(+g) + X at NLO QCD)

11 Technical Method Ṭhe contribution from gq( q t tq( q) is small

12 Top Quark Physics within SM

13 Top Quark Decays t q + W + (q = d,s,b) Γ(t qw + ) V tq 2 Unitarity Relation: V tb 2 + V ts 2 + V td 2 = 1 V td , V ts 00407, V tb Br(t bw + ) 0998 Br(t sw + ) Br(t dw + ) Dominant Decay Channel: t bw + Top Quark Decay Width ( Γ t G F mt 3 V tb 2 8π 1 M2 W 2 mt 2 ) 2 ( M2 W m 2 t )[ ( )] 1 2α s 2π 2 3π 3 2

14 Top Quark Decays W -boson Helicity Within SM, the structure of tbw vertex is the universal (V A) charged current interaction 1 t bw + (h W = 1) Allowed: Prob 30% b t = W + 2 t bw + (h W = 0) Allowed: Prob 70% b t W + 3 t bw + (h W = +1) Forbidden for m b = 0 b t W + = non-zero m b +QCD+EW Corr Prob 01% Do et al 03

15 Top Quark Decays W -boson Helicity Information on W polarization can be obtained from W + l + ν l : 1 Γ dγ d cosψ = 3 4 F 0 sin 2 Ψ F (1 cosψ ) F + (1 + cosψ ) 2 the decay functions: F 0,± B[t bw(λ W = 0,±1)] F 0 + F + F + = 1 F 0 F + F Tevatron: CDF ± 008 D0 062 ± ± 007 LHC: ATLAS 067 ± ± ± 004

16 Top Quark Decay

17 Cross Section for Hadronic t t Production Present Status: top quark pair production at hadron colliders NLO QCD corrections to t t production Nason, Dawson, Ellis; Beenakker, et al; Mangano, Nason, Ridolfi NLO QCD+ Threshold resummation Boncani, et al; Moch, Uwer; Cacciari et al; Kidonakis, Vogt; Banfi, Lanenen Mixed weak-qcd corrections Beenakker, et al; Kao, Ladinsky, Yuan; Bernreuther, Fücker, Si; Kühn, Scharf, Uwer; Moretti, et al Mixed QED-QCD corrections P T resummation for t t production Hollik, Pagani C S Li, et al NLO QCD corrections to t t + jet production Dittmaier, Uwer, Weinzierl; Melnikov, Schulze NNLO QCD corrections to t t production M Czakon, et al

18 Top Quark Pair Production within SM: Total Cross Section Cross section(σ t t) from QCD and weak interactions µ = m t /2 µ = m t µ = 2m t Tevatron NLO QCD (pb) Weak LHC NLO QCD (pb) Weak NLO QCD and Weak corrections: Weak contributions 13% at LHC, CTEQ61M 05% at Tevatron Weak/(NLO QED) corrections smaller than the scale uncertainties of the fixed-order NLO QCD corrections Non-factorizable Contributions are small

19 M tt (GeV) M tt (GeV) Figure: Left: dσ/dm tt for LO(solid), NLO(dashed) and NLOW(dotted), Right: the ratio of dσ NLOW /dm tt and dσ NLO /dm tt at LHC Weak Corrections Weak corrections to total cross section is tiny Weak corrections to M tt distributions are negative except close to 2m t and become larger than 2% when M tt > 12TeV

20 Top Quark Pair Production within SM: Total Cross Section t t production cross section at LHC LHC SM prediction LHC measurement 7TeV 172 ± 7 173± 10 (pb) (LHC comb) 8TeV 246 ± ± 32 (ATLAS) (pb) 227 ± 15 (CMS) SM predictions agree with data quite well!

21 Charge Asymmetry in Hadronic t t production Charge Asymmetry A charge = N t(cosθ) N t(cosθ) N t (cosθ) + N t(cosθ) 0 generated from the interference of even and odd terms under t t: dσ(t, t) = dσ( t,t) t t Charge Asymmetry comes from the Interference between initial and final state gluon radiation for q q t t ṭ t Charge Asymmetry also comes from gq( q) t tq( q)

22 Charge Asymmetry in Hadronic t t production Ratio of Charge Asymmetry from mixed QED-QCD and pure QCD f q = O(α2 sα QED ) O(α 3 s) = 4αe qe t α s d 2 abc /4 = 24αe qe t 5α s O(01) = Mixed QCD-QED and QED-weak contributions are important! Within SM, Charge Asymmetry comes from: q q t t(g), gq( q) t tq( q) Present status: 1 QCD at O(α 3 s): Kühn, Rodrigo 98, 08; Bowen, Ellis, Rainwater mixed QCD-weak at O(α 2 sα) and O(α 2 ): Bernreuther, Si 10, 12; Hollik, Pagani mixed QCD-QED at O(α 2 sα): Hollik, Pagani 11; Bernreuther, Si 12

23 Top Quark Charge Asymmetry at Tevatron Tevatron: p(p) p( p) > is CP eigenstate: CP invariance N t(y t) = N t ( y t ) /σ dσ/dy y

24 Top Quark Charge Asymmetry at Tevatron charge asymmetry: A t FB = N t(y t >0) N t (y t <0) N t (y t >0)+N t (y t <0) and A t FB = A t FB pair asymmetry: A t t = N t t( y>0) N t t( y<0) N t t( y>0)+n t t( y<0) with y = y t y t i t t/n tot t t Tevatron Nt i /Nt tot (%) N O(α 3 s) uū d d qg O(α 2 ) weak uū d d O(αα 2 s) weak uū d d O(αα 2 s) QED uū d d

25 Top Quark Charge Asymmetry at Tevatron CDF 11 CDF 12 SM Prediction A t FB 0150 ± ± 0004 A t t 0158 ± ± ± 0006 A t t( y 1) 0026 ± ± A t t( y > 1) 0611 ± ± A t t(m t t 450 GeV) 0116 ± ± A t t(m t t > 450 GeV) 0475 ± ± D0 11 for A t t: 0196 ± 0065 The largest deviation between data and SM prediction < 3σ For p p t t + jet + jet X with P T > 20 GeV Dittmaier, Uwer, Weinzierl: A t FB = 0015 ± 0015 Melnikov, Schulze: A t FB 2% LO)

26 Top Quark Charge Asymmetry at Tevatron For the process p p t tx l + l + X define the charge asymmetry wrt charged leptons Bernreuther, Si 10, 12 A l (y) = N l +(y>0) N l (y>0) N l + (y>0)+n l (y>0), Al+ l (y) = N(δy>0) N(δy<0) N(δy>0)+N(δy<0) Tevatron SM data A l (%) QCD: 31 (3) D0 11: 152 ± 40 QCD + EW: 38 (3) CDF 12: 66 ± 25 A l (%) QCD: 58 (5) (m t t 450 GeV) QCD + EW: 70 (5) CDF 12: 116 ± 42 A l (%) QCD: 15 (1) (m t t < 450 GeV) QCD + EW: 18 (1) CDF 12: 37 ± 31 A ll (%) QCD: 40 (4) QCD + EW: 48 (4) D0 12: 58 ± 79 ± 29

27 Top Quark Charge Asymmetry at LHC LHC: p(p)p( p) > is Parity eigenstate in lab frame without asymmetric cuts: Parity invariance A t FB = A t FB = 0 Charge Asymmetry at LHC: no contribution from gg t t(g) due to Bose symmetry q q t t,q = u,d production dominated by q with large x q and q with small x q 1 NLO QCD t( t) emitted in the direction of q( q) with large probabilty 2 Boost to lab frame: t in the forward and backword region t in the central region differential charge asymmetry A(y) 0, though A(y)dy = 0 likewise: qg t tg with suitable cuts, charge asymmetry can be non-zero in SM

28 Top Quark Charge Asymmetry at LHC Cut-dependent charge asymmetry 1 Central Charge Asymmetry Antunano, Kühn, Rodrigo 08 A C = N( y t <y c ) N( y t <y c N( y t <y c )+N( y t <y c 2 One-side FB asymmetry Wang, Xiao, Zhu 11 A FB O = N( y>0) N( y<0) N( y>0)+n( y<0) M t t >M c P z t t >P c Cut-independent charge asymmetry 1 A y C = N( y >0) N( y <0) N( y >0)+N( y <0) with y = y t y t 2 A η C = N( η >0) N( η <0) N( η >0)+N( η <0) with η = η t η t kühn, Rordrigo 12

29 Top Quark Charge Asymmetry at LHC Bernreuther, Si 12 s=7 TeV Mt t 2m t M t t 05TeV M t t 1TeV A y C QCD (%): 107(4) 127(4) 206(5) QCD+EW (%): 123(5) 148(4) 240(6) CMS 12(%): 04 ± 10 ± 12 ATLAS 13(%): 06 ± 10 A η C QCD (%): 136(6) 139(5) 215(5) QCD+EW (%): 156(7) 164(6) 252(5) CMS 12(%) 17 ± within the large experimental error, SM predictions agree with data

30 Top Quark Charge Asymmetry at LHC Charge asymmetry wrt charged lepton for pp t tx l + l X A η l = N ll( η l > 0) N ll ( η l < 0) N ll ( η l > 0) + N ll ( η l < 0), η l = η l + η l s=7 TeV Mt t 2m t M t t 05TeV M t t 1TeV A η l C QCD (%): 041(2) 094(4) 163(2) QCD+EW (%): 049(1) 113(2) 194(1) ATLAS 12(%): 23 ± 12 ± 08 Bernreuther, Si 12

31 Top Quark Spin Effects in Hadronic t t Production Possible Spin-Effects 1 Polarization of t, t: (very) Small Normal to Production Plane(P-even, T-odd) due to QCD Absorptive Parts (Bernreuther, Uwer) Polarization in Production Plane(Parity-violation) due to Weak Interactions (Bernreuther, Fuecker, Si) 2 t t Spin Correlations: Large Effect in SM, mainly due to QCD (Mahlon, Parke; Brandenburg; Bernreuther, Brandenburg, Si, Uwer) Strength Depends on the Choice of Reference Axes t, t Spin Quantization Axes(Mahlon, Parke; Uwer)

32 Double Distribution for pp/p p t t + X l l + + X process 1 σ d 2 σ = 1 d cosθ l + cosθ l 4 [1+B 1 cosθ l + +B 2 cosθ l C cosθ l + cosθ l ] 1 B 1 and B 2 reflects top quark spin polarization pure QCD effects: component normal to scattering plane Weak int leads to a component parallel to scattering plane 2 C reflects spin-spin correlations between t and t contr from initial q q and gg induced by pure QCD effects have different sign = C can be used as a tool to determine PDF ATLAS 13:C hel = 023 ± 006 ± 010, SM prediction: C hel = 0310 ± 002 ATLAS 13: C maximal hel = 036 ± 006 ± 009 SM prediction: 044 ± 003

33 Top spin induced distributions and correlations 1/σ dσ/d(cos(θ l + )cos(θ l-)) -1 CMS, 50 fb at s = 7 TeV 18 WBernreuther & Z-GSi ( Data- bkg ) unfolded 16 (SM, µ = m t ) WBernreuther & Z-GSi (uncorrelated, µ = m t ) Syst uncertainty MC@NLO parton level cos(θ l +)cos(θ l -) SM predictions agree with LHC data quite well

34 Di-lepton azimuthal opening angle( ϕ Distribution) 1/σ dσ/d( φ l + l - ) CMS, 50 fb at s = 7 TeV WBernreuther & Z-GSi (SM, µ = m t ) WBernreuther & Z-GSi (uncorrelated, µ = m t ) ( Data- bkg ) unfolded Syst uncertainty MC@NLO parton level (radians) φ l + l - t t pairs produced at LHC are spin correlated

35 Top Quark Physics BSM

36 Top quark spin and top quark coupling Observables related to top quark spin can be used to trace top quark coupling Two examples Top Quark Spin and Anomalous Top Quark Coupling Top Quark Spin and W Chiral Coupling

37 Top quark spin and anomalous top quark coupling Consider the process Assume: pp t t + X l + l + X New physics is induced by new heavy pariticle exchanges Consider the interaction of mass dimension 5, Construct L eff wrt t tg Leff LSM (Re[ˆµt] + i Im[ˆµt]) tσ µν T a tg a µν (Re[ˆdt ] + i Im[ˆdt ]) i tσ µν γ 5 T a tg a µν + Our aim: to find suitable observables to trace the coupling Bernreuther, Si, 13

38 Tracing Re[ˆµ t ] Observable < [Ŝ t ˆk t ][Ŝ t ˆk t ] > and the distribution 1 dσ σ ( 1 dσ = do l σ do l )SM ( 1 dσ + Re[ˆµ σ do t ], O l = cosθ l + cosθ l l )NP can be used to trace Re[ˆµ t ] 1/σ dσ/d(cos(θ l + )cos(θ l-)) 18 WBernreuther & Z-GSi ( Data- bkg ) unfolded (SM, µ = m t ) Syst uncertainty 16 WBernreuther & Z-GSi (uncorrelated, µ = m t ) MC@NLO parton level CMS, 50 fb at s = 7 TeV cos(θ l +)cos(θ l -)

39 Tracing Re[ˆµ t ] cos 1 cos 2

40 Top quark spin and W chiral coupling New heavy gauge bosons in many extensions of the Standard Model,eg, W L Ψ i uγ µ g τ V τ ij P τψ j d W τ µ+ + hc τ=l,r 1 g L = 1, g R = 0 for pure left-handed theory W L 2 g L = 0, g R = 1 for pure right-handed theory W R 3 g L = 1, g R = 1 for pure left-right symmetric theory W W is observed distinguish the W chiral interaction for pp W t b b blνl, the leptonic angular distribution 1 σ dσ d cosθ = 1 2 { } 1 + Acosθ, A = σ(cosθ 0) σ(cosθ 0) σ(cosθ 0) + σ(cosθ 0) is a good diagnostic for the top quark spin

41 Top quark spin and W chiral coupling dσ/dcos θ la (pb) W L W R cos θ la 05 Clearly distinguish between the left-handed and righthanded cases

42 Top quark spin and W chiral coupling Table: Forward-backward asymmetry A at the LHC for M W = 1TeV A W + W L W + W R W L W R No Cuts or smearing No Cuts Cuts & tagging 1 b-jet Gopalakrishna, Han, Lewis, Si, Zhou, 10 1 pt l > 20GeV, η l < 25, P j T > 50GeV, η j < 30, E T > 25GeV 2 R lb > 03, R bb > 04 3 M W M t b 100GeV, mt rec m t < 20GeV 4 P j T,max > 300GeV

43 Outlook 1 Test SM predictions as precisely as possible 2 Top quark mass close to the scale of EWSB, Y t 1 Probe the Mechanism of EWSB 3 Observables wrt Top quark spin can be predicted perturbatively and measured Good Probe for non-sm Parity and/or CP Violation Study top quark couplings 4 Dynamics of Top Production and Decay is not fully explored so far New Decay Modes, eg, t Z q,? New Resonance Production? 5 Is top quark still point-like? 6 Thanks a lot for your attention!