Top Quark Physics at Terascale - pku.edu.cnfaculty.pku.edu.cn/_tsf/00/0B/fyIRJ3byymaa.pdf · Top-quark as a link to new physics top-quark Effective Operators Effective Couplings Weakly

Top Quark Physicsat Terascale

（TeV能区的顶夸克物理）

曹庆宏（Qing-Hong Cao)

北京大学物理学院

Emphasize on contributions from Chinese community

曹庆宏 (Qing-Hong Cao) 第十⼀一届全国粒子物理学术会议

• Large mass：173 GeV ( yt ~O(1) )Top-quark: king of the SM

t W b

5⇥ 10�27 spQCD WEAK

• “bare” quark: spin info well kept among its decay products

u

d̄ s̄

c

hadronization

2

mt ' mW +mZ

• Short lifetime: (Coincidence ?)


Top-quark as a link to new physics

top-quark

Effective Operators EffectiveCouplings

Weaklyinteracting

models

StronglyInteracting

models

ExtraDimension

Theorymotivated

DataDriven

3

SU(3)⇥ SU(2)⇥ U(1)Y U(1)EM


Top-quark as a link to new physics

top-quark

Effective Operators EffectiveCouplings

Weaklyinteracting

models

StronglyInteracting

models

ExtraDimension

Theorymotivated

DataDriven

163,000 top-quark pair events76,000 single top quark events

At the LHC ( 7TeV, 1fb-1 )

3

SU(3)⇥ SU(2)⇥ U(1)Y U(1)EM


Top-quark as a probe of new physics

Gluino

Exoticcoloredstates

4th Gen

Vector-likeQuark

Z’ W’

tFCNC

G’

ChargedHiggs

AFB

Extra gauge bosons

New heavyquarks

CP

4

Heavy quark production via pQCD

Top-quark productionin the SM


Top pair production in the SM

6

q

qTevatron: 90% 10%LHC (7TeV): 20% 80%LHC (14TeV): 10% 90%

t

t

g

g

t

t

NLO + threshold res. (NLL): Moch Uwer, Cacciari et al; Kidonakis, VogtNNLL extensions at threshold:

Czakon et al; Beneke et al; Ahrens, L. L. Yang, et alPartial results at NNLL QCD: Czakon; Bonciani et alttbar + jet at NLO: Dittmaier et al; Melikov, Schulzettbar + bb: Bredenstein et al, Bevilacqua et alttbar + jet with top decay at NLO: Melnikov, Schulze; with weak interference corr. Bernreuther, Zong-Guo Sittbar spin correlations: Mahlon, Parke; Bernreuther, Zong-Guo Si


Top pair production cross section

[pb]tt

50 100 150 200 250 300 350

ATLAS Preliminary

Data 2011

Channel & Lumi.

New measurements

19 March 2012Theory (approx. NNLO)

= 172.5 GeVtfor m

stat. uncertaintytotal uncertainty

(lumi)±(syst) ±(stat) ± tt

Single lepton -10.70 fb 7 pb± 9 ± 4 ±179

Dilepton -10.70 fb pb- 7+ 8 - 11

+ 14 6 ±173

All hadronic-11.02 fb

6 pb± 78 ± 18 ±167

Combination 7 pb± - 7+ 8 3 ±177

+ jetshad-11.67 fb 43 pb± 19 ±200

All hadronic-14.7 fb

6 pb± - 57+ 60 12 ±168

) (pb)t(tσ0 50 100 150 200 250 300

-0.5

8.8

Appr

ox. N

NLO

QCD

NLO

QCD

+jetsµCMS e/ 7± 2936 ± 14 ±173

arXiv:1106.0902 (L=36/pb) lum)± syst. ± stat. ±(val

)µ,eµµCMS dilepton (ee, 7± 1414 ± 18 ±168


+jets+btagµCMS e/ 6± 1717 ± 9 ±150


CMS 2010 combination 6± 1717 ±154 arXiv:1108.3773 (L=36/pb) lum.)± tot. ±(val

)τµCMS dilepton ( 9± 2626 ± 24 ±149

TOP-11-006 (L=1.09/fb) lum)± syst. ± stat. ±(val

CMS all-hadronic 8± 4040 ± 20 ±136


)µ,eµµCMS dilepton (ee, 8± 1616 ± 4 ±170


+jets+btagµCMS e/ 7± 1212 ± 3 ±164

TOP-11-003 (L=0.8-1.09/pb) lum)± syst. ± stat. ±(val

=7 TeVsCMS Preliminary,

Theory: Langenfeld, Moch, Uwer, Phys. Rev. D80 (2009) 054009 PDF(90% C.L.) uncertainty⊗MSTW2008(N)NLO PDF, scale

7

Exp uncertainties ~10-20%


Single top production in the SM

8

b

g

t

WtW

u

d t

b

s-channel

Wu d

b tt-channel

W

u

d t

bW’ u u

u/c tZ’

b

g

t

W

b’

FCNC Excitedquark

H+S6

Newresonance

Q2W > 0 Q2W < 0 Q

2W = m

2W


Single top production at the NLO

9

b

g

t

WtW

u

d t

b

s-channel

W

u d

b tt-channel

W


Single top production at the NLO

9

b

g

t

WtW

u

d t

b

s-channel

W

u d

b tt-channel

W

Qing-Hong Cao, C.-P. Yuan, PRD71 (2005) 054002

Shou-Hua Zhu, PLB524 (2002) 283

Harris, et al, PRD66 (2002) 054024

Harris, et al, PRD66 (2002) 054024

Campbell, et al, PRD70 (2004) 094012

Campbell, et al, PRD70 (2004) 094012

Qing-Hong Cao, et al, PRD72 (2005)094027Frixione, et al, JHEP 0807 (2008) 029

Campbell, et al, PRL102 (2009) 182003

Zhu, C. S. Li, Wang, Zhang, JHEP 1102 (2011) 099

Wang, C. S. Li, Zhu, Zhang, 1010.4509

Qing-Hong Cao, 0801.0539

Heim, Qing-Hong Cao, et al, PRD81 (2010) 034005

Reinhdard, Yuan, Mueller, Qing-Hong Cao, PRD83 (2011) 034019


Single top measurements at LHC

10

9

The two measurements are compatibile, taking into account correlated and uncorrelated un-certainties. The latter category includes the uncertainties on the W+heavy flavours extraction,on the QCD extraction procedure, on the lepton reconstruction and trigger efficiencies, and onthe hadronic part of the trigger.

The combination of the muon and electron measurement gives:

st�ch. = 70.2 ± 5.2(stat.) ± 10.4(syst.)± 3.4(lumi.) pb (combined)

Figure 5 shows the comparison of the current measurement with the Standard Model expecta-tion.

[TeV]s0 2 4 6 8 10

[pb]

σ

1

10

210

-1CMS preliminary, 1.14/1.51 fb

-1D0, 5.4 fb

-1CDF, 3.2 fb

NLO QCD (5 flavour scheme) PDF)⊕theory uncertainty (scale

Campbell, Frederix, Maltoni, Tramontano, JHEP 10 (2009) 042

NLO+NNLL QCD PDF)⊕theory uncertainty (scale

Kidonakis, Phys.Rev.D 83 (2011) 091503

t-channel single top quark production

Figure 5: Single top cross section in the t-channel versus centre-of-mass energy, comparing ourmeasurement with the dedicated t-channel cross section measurements at Tevatron [21, 22] andwith the QCD expectations computed at NLO with MCFM in the 5-flavour scheme [23] and atNLO+NNLL [1]. The error band (width of the curve) is obtained by varying the top masswithin its current uncertainty [24], estimating the PDF uncertainty according to the HEPDATArecommendations [25], and varying the factorization and renormalization scales coherently bya factor two up and down.

The absolute value of the CKM element |Vtb| is determined (similar to [2]) assuming that |Vtd|and |Vts| are much smaller than |Vtb|, resulting in:

|Vtb| =s

st�ch.stht�ch.

= 1.04 ± 0.09 (exp.) ± 0.02 (th.) , (1)

where stht�ch. is the SM prediction assuming |Vtb| = 1.

7 Conclusion

We measured the cross section of t-channel single-top quark production in pp collisions using2011 data in semi leptonic top decay mode with improved precision compared to our ear-lier measurement. The characteristic pseudorapidity distribution of the light quark recoiling

9





[TeV]s0 2 4 6 8 10

[pb]

σ

1

10

210


-1D0, 5.4 fb

-1CDF, 3.2 fb








|Vtb| =s

st�ch.stht�ch.

= 1.04 ± 0.09 (exp.) ± 0.02 (th.) , (1)


7 Conclusion


9





[TeV]s0 2 4 6 8 10

[pb]

σ

1

10

210


-1D0, 5.4 fb

-1CDF, 3.2 fb








|Vtb| =s

st�ch.stht�ch.

= 1.04 ± 0.09 (exp.) ± 0.02 (th.) , (1)


7 Conclusion


LHC is powerful

Top-quark Forward-Backward Asymmetries


Top-quark F-B asymmetry in the SM• A charge asymmetry arises at NLO

VOLUME 81, NUMBER 1 P HY S I CA L REV I EW LE T T ER S 6 JULY 1998

Charge Asymmetry in Hadroproduction of Heavy Quarks

J. H. Kühn and G. RodrigoInstitut für Theoretische Teilchenphysik, Universität Karlsruhe, D-76128 Karlsruhe, Germany

(Received 12 February 1998; revised manuscript received 17 April 1998)A sizable difference in the differential production cross section of top and antitop quarks, respectively,

is predicted for hadronically produced heavy quarks. It is of order as

and arises from the interferencebetween charge odd and even amplitudes, respectively. For the Fermilab Tevatron it amounts to upto 15% for the differential distribution in suitable chosen kinematical regions. The resulting integratedforward-backward asymmetry of 4% 5% could be measured in the next round of experiments. Atthe CERN Large Hadron Collider the asymmetry can be studied by selecting appropriately chosenkinematical regions. [S0031-9007(98)06481-3]

PACS numbers: 12.38.Qk, 12.38.Bx, 13.87.Ce, 14.65.Ha

Top quark production at hadron colliders has becomeone of the central issues of theoretical [1] and experimen-tal [2] research. The investigation and understanding ofthe production mechanism is crucial for the determina-tion of the top quark couplings, its mass, and the searchfor new physics involving the top system. A lot of efforthas been invested in the prediction of the total cross sec-tion and, more recently, of inclusive transverse momen-tum distributions [1].In this Letter we will point to a different aspect of the

hadronic production process, which can be studied witha fairly modest sample of quarks. Top quarks producedthrough light quark-antiquark annihilation will exhibita sizable charge asymmetry—an excess of top versusantitop quarks in specific kinematic regions—inducedthrough the interference of the final state with initial-state radiation [Figs. 1(a) and 1(b)] and the interferenceof the box with the lowest-order diagram [Figs. 1(c)and 1(d)]. The asymmetry is thus of order a

s

relativeto the dominant production process. In suitable chosenkinematical regions it reaches up to 15%, the integratedforward-backward asymmetry amounts to 4%–5%. Topquarks are tagged through their decay t ! b W1 and canthus be distinguished experimentally from antitop quarksthrough the sign of the lepton in the semileptonic modeand eventually also through the b tag. A sample of 100to 200 tagged top quarks should, in fact, be sufficient fora first indication of the effect.Top production at the Fermilab Tevatron is dominated

by quark-antiquark annihilation, hence the charge asym-metry will be reflected not only in the partonic rest framebut also in the center of mass system of proton and an-tiproton. The situation is more intricate for proton-protoncollisions at the CERN Large Hadron Collider (LHC),where no preferred direction is at hand in the laboratoryframe. Nevertheless, it is also in this case possible topick kinematical configurations which allow the study ofthe charge asymmetry.The charge asymmetry has also been investigated in

[3] for a top mass of 45 GeV. There, however, only

the contribution from real gluon emission was consideredrequiring the introduction of a physical cutoff on thegluon energy and rapidity to avoid infrared and collinearsingularities. Experimentally, however, only inclusivetop-antitop production has been studied to date, and theseparation of an additional soft gluon will in general bedifficult. In this Letter, we will therefore include virtualcorrections and consider inclusive distributions only. Wewill see below that the sign of the asymmetry for inclusiveproduction is opposite to the one given for the t¯tg processin [3]. The charge asymmetry of heavy flavor productionin quark-antiquark annihilation to bottom quarks was alsodiscussed in [4–6] where its contribution to the forward-backward asymmetry in proton-antiproton collisions wasshown to be very small. In addition, there is also a slightdifference between the distribution of top and antitopquarks in the reaction gq ! t¯tq. At the Tevatron itscontribution is below 1024. (This effect should not beconfused with the large asymmetry in the top quarks’angular or rapidity distribution in this reaction which is atrivial consequence of the asymmetric partonic initial stateand vanishes after summing over the incoming partonbeams.)

(c) (d)

(b)(a)

q

q

Q

Q

FIG. 1. Origin of the QCD charge asymmetry in hadroproduc-tion of heavy quarks: interference of final-state (a) with initial-state (b) gluon bremsstrahlung plus interference of the box (c)with the Born diagram (d).

0031-9007y98y81(1)y49(4)$15.00 © 1998 The American Physical Society 49

t̄

t

P (q) P̄ (q̄)

Top quarks are produced along the direction of the incoming quark

tttt

tt

ttpp

yyyyNyNyNyNA

yNyNyNyN

A

−=Δ=Δ

Δ=

=>+>

>−>=

)9(078.0)0()0()0()0(

)6(051.0)0()0()0()0(

12

ForwardBackward


Top-quark AFB at the Tevatron

13

CDF new data (8.7fb-1):

AinclusiveFB = 0.162± 0.041± 0.022

ANLO+EWFB = 0.066

20

)2 (GeV/ctt

Parton Level M350 400 450 500 550 600 650 700 750

(pb)

y)Δ)d

(tt

d(M

σ2 d

0

0.5

1

1.5

2

2.5 y > 0Δl+Jets Data,

y < 0Δl+Jets Data,

-1CDF Run II Preliminary L = 8.7 fb

FIG. 21: Parton level Mtt̄ distributions for events with positive and negative �y.

)2 (GeV/cttParton Level M350 400 450 500 550 600 650 700 750

FBA

0

0.1

0.2

0.3

0.4

0.5

0.6


l+Jets Data

tNLO (QCD+EW) t


FBA

0

0.1

0.2

0.3

0.4

0.5

0.6


l+Jets Data-410× 5.0)± = (15.6

ttMα(Correlated Uncertainties)

tNLO (QCD+EW) t-410× = 3.3

ttMα

FIG. 22: Parton level AFB as a function of Mtt̄ (left) and the same distribution with a best-fit line superimposed (right).

level results from this analysis with the same divisions into two bins in order to directly compare to the previousanalysis. The change in central values across the two bins has been reduced somewhat compared to the previousanalysis, but the trend of growth of the asymmetry with mass and |�y| remains.

VIII. CONCLUSIONS

We have studied the forward-backward asymmetry AFB in top quark pair production in the full CDF dataset.In the full dataset, we observe a raw asymmetry of 0.066 ± 0.020, and an approximately linear dependence onboth |�y| and M

tt̄

. After subtracting o↵ the predicted background contribution, we determine the significance ofthe rapidity and mass dependence by comparing the best fit slopes in the data to the standard model powhegprediction, finding a p-value of 0.00892 for AFB as a function of |�y| and a p-value of 0.00646 for AFB as a functionof M

tt̄

. Finally, we correct our results to the parton level to find the di↵erential cross-section in �y and allow

20

)2 (GeV/ctt

Parton Level M350 400 450 500 550 600 650 700 750

(pb)

y)Δ)d

(tt

d(M

σ2 d

0

0.5

1

1.5

2

2.5 y > 0Δl+Jets Data,

y < 0Δl+Jets Data,


FIG. 21: Parton level Mtt̄ distributions for events with positive and negative �y.


FBA

0

0.1

0.2

0.3

0.4

0.5

0.6


l+Jets Data

tNLO (QCD+EW) t


FBA

0

0.1

0.2

0.3

0.4

0.5

0.6


l+Jets Data-410× 5.0)± = (15.6

ttMα(Correlated Uncertainties)

tNLO (QCD+EW) t-410× = 3.3

ttMα

FIG. 22: Parton level AFB as a function of Mtt̄ (left) and the same distribution with a best-fit line superimposed (right).

level results from this analysis with the same divisions into two bins in order to directly compare to the previousanalysis. The change in central values across the two bins has been reduced somewhat compared to the previousanalysis, but the trend of growth of the asymmetry with mass and |�y| remains.

VIII. CONCLUSIONS

We have studied the forward-backward asymmetry AFB in top quark pair production in the full CDF dataset.In the full dataset, we observe a raw asymmetry of 0.066 ± 0.020, and an approximately linear dependence onboth |�y| and M

tt̄

. After subtracting o↵ the predicted background contribution, we determine the significance ofthe rapidity and mass dependence by comparing the best fit slopes in the data to the standard model powhegprediction, finding a p-value of 0.00892 for AFB as a function of |�y| and a p-value of 0.00646 for AFB as a functionof M

tt̄

. Finally, we correct our results to the parton level to find the di↵erential cross-section in �y and allow

1101.0034 (cited > 240)


Top-quark AFB at the Tevatron

13

CDF new data (8.7fb-1):

AinclusiveFB = 0.162± 0.041± 0.022

ANLO+EWFB = 0.066

19

VII. DIFFERENTIAL CROSS-SECTION AND PARTON LEVEL RESULTS

Applying our correction procedure to the data yields the di↵erential cross-section shown in Figure 19 comparedto the standard model powheg prediction. We find an inclusive asymmetry of 0.162 ± 0.041 ± 0.022. The |�y|dependence of this distribution is shown in Figure 20, with the di↵erential asymmetry values being summarized inTable XVI. Performing a linear fit to the parton level results, we find a slope ↵�y = (30.6± 8.6)⇥ 10�2, comparedto an expected slope of 10.3⇥ 10�2. In performing this fit in the data, we utilize the full covariance matrix for thecorrected AFB values when minimizing �2 in order to account for the correlations between bins in the parton leveldistribution. The systematic uncertainties on AFB in each bin are added to the diagonals of the covariance matrix.

yΔParton Level 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

FBA

0

0.2

0.4

0.6


l+Jets Data

tNLO (QCD+EW) t

yΔParton Level 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

FBA

0

0.2

0.4

0.6


l+Jets Data-210× 8.6)± = (30.6 yΔα

(Correlated Uncertainties)tNLO (QCD+EW) t

-210× = 10.3yΔα

FIG. 20: Parton level AFB as a function of |�y| (left) and the same distribution with a best-fit line superimposed (right).

CDF Run II Preliminary L = 8.7 fb�1

Parton Level Data NLO (QCD+EW) tt̄|�y| AFB (± stat. ± syst.) AFB

Inclusive 0.162 ± 0.041 ± 0.022 0.066< 0.5 0.037 ± 0.035 ± 0.020 0.023

0.5� 1.0 0.163 ± 0.058 ± 0.036 0.0721.0� 1.5 0.384 ± 0.084 ± 0.041 0.119� 1.5 0.547 ± 0.140 ± 0.085 0.185< 1.0 0.088 ± 0.042 ± 0.022 0.043� 1.0 0.433 ± 0.097 ± 0.050 0.139

Data NLO (QCD+EW) tt̄Slope ↵�y of Best-Fit Line (30.6 ± 8.6)⇥ 10�2 10.3⇥ 10�2

TABLE XVI: Measured and predicted parton level asymmetries as a function of |�y|.

We also can determine the parton level mass dependence of AFB by correcting the �y and Mtt̄

distributionssimultaneously. Doing so yields the M

tt̄

distributions for forward and backward events shown in Figure 21. Thesedistributions can then be combined to determine the di↵erential AFB as a function of M

tt̄

shown in Figure 22and summarized in Table XVII. The best fit line to the parton level data has a slope ↵

Mtt̄= (15.6 ± 5.0)⇥ 10�4,

compared to the powheg prediction of 3.3⇥ 10�4.

A. Comparison to Previous Results

In the 5.3 fb�1 version of this analysis, parton level di↵erential asymmetries were considered in two bins of |�y|(above and below 1.0) and two bins of M

tt̄

(above and below 450 GeV/c2). In Table XVIII, we provide the parton

19

VII. DIFFERENTIAL CROSS-SECTION AND PARTON LEVEL RESULTS

Applying our correction procedure to the data yields the di↵erential cross-section shown in Figure 19 comparedto the standard model powheg prediction. We find an inclusive asymmetry of 0.162 ± 0.041 ± 0.022. The |�y|dependence of this distribution is shown in Figure 20, with the di↵erential asymmetry values being summarized inTable XVI. Performing a linear fit to the parton level results, we find a slope ↵�y = (30.6± 8.6)⇥ 10�2, comparedto an expected slope of 10.3⇥ 10�2. In performing this fit in the data, we utilize the full covariance matrix for thecorrected AFB values when minimizing �2 in order to account for the correlations between bins in the parton leveldistribution. The systematic uncertainties on AFB in each bin are added to the diagonals of the covariance matrix.

yΔParton Level 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

FBA

0

0.2

0.4

0.6


l+Jets Data

tNLO (QCD+EW) t

yΔParton Level 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

FBA

0

0.2

0.4

0.6


l+Jets Data-210× 8.6)± = (30.6 yΔα

(Correlated Uncertainties)tNLO (QCD+EW) t

-210× = 10.3yΔα

FIG. 20: Parton level AFB as a function of |�y| (left) and the same distribution with a best-fit line superimposed (right).

CDF Run II Preliminary L = 8.7 fb�1

Parton Level Data NLO (QCD+EW) tt̄|�y| AFB (± stat. ± syst.) AFB

Inclusive 0.162 ± 0.041 ± 0.022 0.066< 0.5 0.037 ± 0.035 ± 0.020 0.023

0.5� 1.0 0.163 ± 0.058 ± 0.036 0.0721.0� 1.5 0.384 ± 0.084 ± 0.041 0.119� 1.5 0.547 ± 0.140 ± 0.085 0.185< 1.0 0.088 ± 0.042 ± 0.022 0.043� 1.0 0.433 ± 0.097 ± 0.050 0.139

Data NLO (QCD+EW) tt̄Slope ↵�y of Best-Fit Line (30.6 ± 8.6)⇥ 10�2 10.3⇥ 10�2

TABLE XVI: Measured and predicted parton level asymmetries as a function of |�y|.

We also can determine the parton level mass dependence of AFB by correcting the �y and Mtt̄

distributionssimultaneously. Doing so yields the M

tt̄

distributions for forward and backward events shown in Figure 21. Thesedistributions can then be combined to determine the di↵erential AFB as a function of M

tt̄

shown in Figure 22and summarized in Table XVII. The best fit line to the parton level data has a slope ↵

Mtt̄= (15.6 ± 5.0)⇥ 10�4,

compared to the powheg prediction of 3.3⇥ 10�4.

A. Comparison to Previous Results

In the 5.3 fb�1 version of this analysis, parton level di↵erential asymmetries were considered in two bins of |�y|(above and below 1.0) and two bins of M

tt̄

(above and below 450 GeV/c2). In Table XVIII, we provide the parton

1101.0034 (cited > 240)


Top-quark AFB and NP models

14

CDF, Phys.Rev.Lett. 102 (2009) 222003

It provides upper bounds on NP resonance. The large bin (800GeV-1400GeV) is the most sensitive to a heavy resonance


Top-quark AFB and NP models

15

s-channel t-channel (Flavor changing)

Effective theory

Cheung, Yuan, 1101.1445

Cao, Wang, Wu, Yang, 1101.4456

Bai, et al, 1101.5203

Shu, Wang, Zhu, 1104.0083

Cui, Han, Schwartz, 1106.3086

Wang, Wang, Xiao, Xu, Zhu, 1104.1917

Chen, Law, Li, 1104.1497

Shao, C. S. Li, Wang, Gao, Zhang, Zhu, 1107.4012

Wang, Wang, Xiao, Zhu, 1107.5769

Liu, Tang, Wu, 1108.5012

Yan, Wang, Shao, Li, 1110.6684

Cao, Hikasa, Wang, Wu, Yang, 1109.6543

Zhu, C. S. Li, Shao, Wang, Yuan, 1201.0672

Duffty, Sullivan, Zhang, 1203.4489

Wang, Wu, Yang, 1111.4771

Han, Liu, Wu, Yang, 1203.2321

Frampton, Shu, Wang, 0911.2955

Shu, Tait, Wang, 0911.3237

Cheung, Keung, Yuan, 0908.2589

Cao, Heng, Wu, Yang, 0912.1447

Qing-Hong Cao, McKeen, et al, 1003.3461



q

q

t

t

(7 operators)

q

q

q

q

t

t

t

t

Z 0,W 0,�

�6,�3̄

q

q

t

t

q

q

t

t

g

G0

SM


S-channel: axigluon• Axigluon

additional gauge group• KK-Gluon

new space-time structure

16

Boosted top “jet”(jet substructure)

q

q

t

t

G0

Dijet constraints requires SMALL couplings to light flavor quarks.

Large AFB demands LARGE couplings to top quarks.

3

First we define the NP cross section, σNP, as the dif-ference between tt̄ cross section in a model with a newmassive COVB, and the SM:

σNP = σSM+NP − σSM. (10)

The SM cross sections, including both the qq̄- and gg-channel, are calculated with the program MCFM [32].

(GeV)Gm1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000

(pb)

σ

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

Fixed Scale LOFixed Scale NLONaive K-factor

(GeV)Gm1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000

Dynamical Scale LODynamical Scale NLONaive K-factor

FIG. 1. σNP, defined in Eq. (10), as functions of MG fortwo different benchmark schemes. The black (red) bands arethe LO (NLO) uncertainties, estimated by varying the scalesaround their default values by a factor of two within eachscheme. The blue dashed lines are the naive estimates of theNLO effects by simple rescaling of the LO results with theSM K-factor.

In Fig. 1, we plot the NP contribution to the crosssection at the LHC with

√s = 7 TeV, as a function of

MG. The bands reflect the scale uncertainties estimatedby varying the renormalization and factorization scalesaround their default values by a factor of two. We presentthe results in two benchmark schemes, namely the fixedscale scheme, where the scales are fixed to be mt, and thedynamical scale scheme, where the scales are set to be theinvariant mass of the top quark pair mtt̄. We find thatthe NLO QCD effects are much larger (by about 50%)in the fixed scale scheme than the dynamical scheme forour choice of parameters, and the naive estimate of theNLO effects by simple rescaling of the LO results withthe SM NLO K-factor is not appropriate. It is also clearthat the inclusion of NLO QCD effects strongly reducesthe theoretical uncertainty of the NLO cross section ineither scheme, comparing with the LO ones, as expected.Fig. 2 gives the LO and NLO invariant mass distribu-

tion of the top quark pair at the LHC with√s = 7 TeV,

including contribution from the SM LO and NLO topquark pair production. It can be seen that in the fixedscale scheme, NLO QCD corrections significantly changethe shape of the LO curve, leading to the reduction of

0 200 400 600 800 1000 1200 1400 1600 1800 2000

(fb/G

eV)

tt/d

mσd

-210

-110

1

10

210

310

t=mfµ=rµ

tt=mfµ=rµSM QCD NLO + NP LOSM QCD NLO + NP NLO

(GeV) ttm0 200 400 600 800 1000 1200 1400 1600 1800 2000

ratio

-3-2-10123

DS)/LODS-LOFS(LO

DS)/NLODS-NLOFS(NLO

FIG. 2. LO (dashed lines) and NLO (solid lines) invariantmass distribution of the top quark pair at the LHC with

√

s =7 TeV. Also shown are the differences between two differentschemes at the LO and NLO.

the width of the resonance, which is important for accu-rate extraction of the mass and width of resonance fromtt̄ invariant mass distribution experimentally. The NLOwidth can be expressed analytically as

ΓNLO(µ)

ΓLO(µ)=

[

1 +αs(µ)

π

(

167

12− π2 −

15

4ln

M2Gµ2

)]

.

(11)From Eq. (11), it’s obvious that the width of COVB isreduced at the NLO when µ = mt, and the large log-arithmic contribution can be canceled by a dynamicalscale choice µ = mtt̄. This is confirmed in Fig. 2, wherethe NLO results for the dynamical scale scheme has rela-tively small corrections comparing with the LO one. Thepredictions in the two scheme at the NLO level are closeto each other, while at the LO level they show large differ-ence, as shown in the lower panel of Fig. 2. The differencebetween the two schemes reflects the uncertainty of thetheoretical prediction. Hence, the NLO result leads to asmaller theoretical uncertainty in mtt̄ distribution, whichcould improve the accuracy of extracting the theory pa-rameters of NP models from comparing to experimentaldata. We also note that similar conclusion holds in othercases, e.g., KK gluon in RS model.Fig. 3 shows the LO and NLO contribution to the AFB

as a function of the mass of the COVB at the Tevatronwith

√s = 1.96 TeV in the center of mass frame of the tt̄

pair. The results are given for both the total asymmetryand the asymmetry in the large invariant mass region,mtt̄ > 450 GeV, respectively. As we know, the NLOQCD corrections in the SM enhance the AFB comparingwith the LO results [7], but the NP contributions at theNLO level only reduce the AFB by 1 − 2%. Similar be-haviors are also found for vector-like coupling (not shown

Zhu, C. S. Li, Shao, Wang, Yuan, 1201.0672(NLO QCD corrections)

q

q

t

t

g


Minimal FCNC Z’ is disfavored

17

J. Cao, Wang, Wu, Yang, 1101.4456J. Cao et al, hep-ph/0703308, hep-ph/0409334

Z’

Berger, Qing-Hong Cao, Chen, C. S. Li, Zhang, PRL 106 (2011) 201801,

u t

u t

L = gū�µ(fLPL + fRPR)tZ 0µ + h.c.

Left-handed coupling is highlyconstrained by - mixing.Bd Bd

200 400 600 800 1000mZ’ (GeV)

0

1

2

3

4

5

6

7

8

f R (b)

5000 eve

nts

1000 even

ts

53

100 events

200 400 600 800 1000mZ’ (GeV)

0

1

2

3

4

5

6

7

8

f R (b)

5000 eve

nts

1000 even

ts

53

100 events

Other studies on same-sign top pair :

AFB prefers a LARGE fR.


Minimal FCNC Z’ is disfavored

17

J. Cao, Wang, Wu, Yang, 1101.4456J. Cao et al, hep-ph/0703308, hep-ph/0409334

Z’

Berger, Qing-Hong Cao, Chen, C. S. Li, Zhang, PRL 106 (2011) 201801,

u t

u t

L = gū�µ(fLPL + fRPR)tZ 0µ + h.c.

Left-handed coupling is highlyconstrained by - mixing.Bd Bd

m(Z') GeV200 400 600 800 1000 1200 1400 1600 1800 2000

Rf

00.5

11.5

22.5

33.5

44.5

5

, Berger et al.FB consistent with Aσ1 , Berger et al.FB consistent with Aσ2Combined Observed Limit tt + ttj

-1 = 4.7 fbint = 7 TeV, LsCMS Preliminary,

Excluded

m(Z') GeV200 400 600 800 1000 1200 1400 1600 1800 2000

Rf

00.5

11.5

22.5

33.5

44.5

5

, Berger et al.FB consistent with Aσ1 , Berger et al.FB consistent with Aσ2Combined Observed Limit tt + ttj

-1 = 4.7 fbint = 7 TeV, LsCMS Preliminary,

Excluded

Other studies on same-sign top pair :

AFB prefers a LARGE fR.


• AC definition

18

Att̄C =�(�y > 0)� �(�y < 0)�(�y > 0) + �(�y < 0)

23rd Recontres de Blois - May/June 2011 Fabio Maltoni

PROGRESS IN SM TOP PREDICTIONS:

EXAMPLE 3: COLOR CHARGE ASYMM.

�yTEV = yt � yt̄ �yLHC = |yt|�| yt̄|

Att̄CC =�(�y > 0)� �(�y < 0)�(�y > 0) + �(�y < 0)

Other definitions are used: lab frame at Tevatron, central charge [Antunano, et al,] and one-side asymmetries [Wang et al. 2010] at the LHC which depend on a cut. ACC at the LHC has been introduced by CMS (in terms of pseudo-rapidity). LHCB does not need any special definition [Kagan et al.]

Monday 30 May 2011

�yTev = yt � yt̄ �yLHC = |yt|� |yt̄|

• One side asymmetry

2

via gg fusion efficiently. In this paper, we propose anew definition of FB asymmetry, namely the one-side FB asymmetry AOFB, to conquer this diffi-culty. AOFB can be large and arises from the sameO(α3s) contributions which induce the observed FBasymmetry at Tevatron. This quantity can be ex-amined at the LHC and cross-checked to the cor-responding measurements at the Tevatron.

At the LHC, up to next-to-leading order (NLO)QCD, top pair events can be generated through thechannels qq̄ → tt̄, qq̄ → tt̄g, qg → tt̄q or q̄g → tt̄q̄and gg → tt̄ at the partonic level. Being a proton-proton collider, LHC does not have the preferreddirections in the laboratory rest frame. Howeverexcept the symmetric gluons, the incoming par-tons do have preferred direction. Usually the va-lence quark momentum is larger than that of thesea quark. For example, for the process uū → tt̄(taking the momentum of the u quark as the posi-tive z direction), momentum of u is most probablylarger than that of ū. On average, this will inducethe z direction tt̄ total momentum in lab frameP ztt̄ > 0. So even for the pp collider, uū → tt̄ cancontribute an asymmetric tt̄ distribution. How-ever, this asymmetry is completely canceled withthe opposite direction ūu → tt̄ events. If we ob-serve only one-side tt̄ events, i.e. P ztt̄ > 0, suchasymmetry will be kept. To maintain the partonicasymmetry and suppress the symmetric events, werequire a cut |P ztt̄| > P

zcut on the z direction top

pair momentum of the final tt̄ pair in the pp restframe. One may argue that determination of themomentum in beam line direction may be problem-atic, especially when one neutrino is missing whenusing the associated charged lepton to trigger thetop/antitop event. This issue can be solved by re-quiring invariant mass of the neutrino and chargedlepton just equal to that of the W boson, which isassumed to be the decay product of the top quark.Thus P ztt̄ is still a measurable quantity [13].

The new one-side forward-backward asymmetryAOFB can be defined in the pp rest frame as

AOFB =σ(∆Y >0)−σ(∆Y 0)+σ(∆Y P zcut,Mtt̄>Mcut

= σ(∆Y 0)σ(∆Y 0) |P ztt̄Mcut(4)

or

AOFB =F− +B−F+ +B+

≡σA

σ, (5)

with

F± = (σ(∆Y > 0)± σ(∆Y < 0))|P ztt̄>P z

cut,Mtt̄>Mcut

(6)

B± = (σ(∆Y < 0)± σ(∆Y > 0))|P ztt̄Mcut

(7)The asymmetry defined in Eq.(5) is the same

as that in Eq.(4) except the statistics are dou-bled. We will adopt the asymmetry definition inEq.(5) in the following evaluation. The goal toapply constraint on P ztt̄ and Mtt̄ is to exclude thesymmetric gg → tt̄ events. In Eq.(5), the asym-metric cross section in the numerator arises fromO(α3s) in QCD, and the denominator is the to-tal cross section. Although some high order ef-fects in tt̄ production have been considered, suchas soft gluon resummation [14, 15] and the ex-clusive next-to-leading order cross section of tt̄ +jet production[16–18], the exact inclusive next-to-leading order asymmetric cross section which in-volves the two-loop contributions is still unknown.For consistency, we choose the lowest order resultof total cross section at O(α2s) as a rough estima-tion.

The typical Feynman diagrams of O(α2s) for thedenominator in Eq. 4 are drawn in Fig. 1.

u

u

t

tg

g

g

t

tg

g

g

t

t

t

FIG. 1: Typical Feynman diagrams for tt̄ pair produc-tion at LHC at O(α2s).

In the SM, the leading asymmetric cross sectionarises at O(α3s). The related Feynman diagramsat partonic level can be classified into three cate-gories: (1) the interference among virtual box inFig. 2 and the leading diagrams for the processqq̄ → tt̄ in Fig. 1; (2) the interference among ini-tial and final gluon radiation diagrams of qq̄ → tt̄gin Fig. 3; and (3) contributions from diagrams ofqg → tt̄q and q̄g → tt̄q̄ in Fig. 4.

The asymmetric cross section at the parton levelwas given analytically in Ref. [7]. However, wecarry out independent calculations [19] with thehelp of FeynCalc [20], FormCalc [21] and QCD-Loop [22].

The asymmetric cross section σA contributedfrom each of the above three categories is UV and

You-Kai Wang, Bo Xiao, Shou-Hua Zhu, 1008.2685

Top-quark AFB at the LHC

q

t

q

cms rest frame

t

q q

q

t

q

LAB frame

q

t

q

Quarks carry more momenta than antiquarks

LHC is symmetric (no F or B)


• AC definition

19

Att̄C =�(�y > 0)� �(�y < 0)�(�y > 0) + �(�y < 0)

• One side asymmetry

2

via gg fusion efficiently. In this paper, we propose anew definition of FB asymmetry, namely the one-side FB asymmetry AOFB, to conquer this diffi-culty. AOFB can be large and arises from the sameO(α3s) contributions which induce the observed FBasymmetry at Tevatron. This quantity can be ex-amined at the LHC and cross-checked to the cor-responding measurements at the Tevatron.

At the LHC, up to next-to-leading order (NLO)QCD, top pair events can be generated through thechannels qq̄ → tt̄, qq̄ → tt̄g, qg → tt̄q or q̄g → tt̄q̄and gg → tt̄ at the partonic level. Being a proton-proton collider, LHC does not have the preferreddirections in the laboratory rest frame. Howeverexcept the symmetric gluons, the incoming par-tons do have preferred direction. Usually the va-lence quark momentum is larger than that of thesea quark. For example, for the process uū → tt̄(taking the momentum of the u quark as the posi-tive z direction), momentum of u is most probablylarger than that of ū. On average, this will inducethe z direction tt̄ total momentum in lab frameP ztt̄ > 0. So even for the pp collider, uū → tt̄ cancontribute an asymmetric tt̄ distribution. How-ever, this asymmetry is completely canceled withthe opposite direction ūu → tt̄ events. If we ob-serve only one-side tt̄ events, i.e. P ztt̄ > 0, suchasymmetry will be kept. To maintain the partonicasymmetry and suppress the symmetric events, werequire a cut |P ztt̄| > P

zcut on the z direction top

pair momentum of the final tt̄ pair in the pp restframe. One may argue that determination of themomentum in beam line direction may be problem-atic, especially when one neutrino is missing whenusing the associated charged lepton to trigger thetop/antitop event. This issue can be solved by re-quiring invariant mass of the neutrino and chargedlepton just equal to that of the W boson, which isassumed to be the decay product of the top quark.Thus P ztt̄ is still a measurable quantity [13].

The new one-side forward-backward asymmetryAOFB can be defined in the pp rest frame as

AOFB =σ(∆Y >0)−σ(∆Y 0)+σ(∆Y P zcut,Mtt̄>Mcut

= σ(∆Y 0)σ(∆Y 0) |P ztt̄Mcut(4)

or

AOFB =F− +B−F+ +B+

≡σA

σ, (5)

with

F± = (σ(∆Y > 0)± σ(∆Y < 0))|P ztt̄>P z

cut,Mtt̄>Mcut

(6)

B± = (σ(∆Y < 0)± σ(∆Y > 0))|P ztt̄Mcut

(7)The asymmetry defined in Eq.(5) is the same

as that in Eq.(4) except the statistics are dou-bled. We will adopt the asymmetry definition inEq.(5) in the following evaluation. The goal toapply constraint on P ztt̄ and Mtt̄ is to exclude thesymmetric gg → tt̄ events. In Eq.(5), the asym-metric cross section in the numerator arises fromO(α3s) in QCD, and the denominator is the to-tal cross section. Although some high order ef-fects in tt̄ production have been considered, suchas soft gluon resummation [14, 15] and the ex-clusive next-to-leading order cross section of tt̄ +jet production[16–18], the exact inclusive next-to-leading order asymmetric cross section which in-volves the two-loop contributions is still unknown.For consistency, we choose the lowest order resultof total cross section at O(α2s) as a rough estima-tion.

The typical Feynman diagrams of O(α2s) for thedenominator in Eq. 4 are drawn in Fig. 1.

u

u

t

tg

g

g

t

tg

g

g

t

t

t

FIG. 1: Typical Feynman diagrams for tt̄ pair produc-tion at LHC at O(α2s).

In the SM, the leading asymmetric cross sectionarises at O(α3s). The related Feynman diagramsat partonic level can be classified into three cate-gories: (1) the interference among virtual box inFig. 2 and the leading diagrams for the processqq̄ → tt̄ in Fig. 1; (2) the interference among ini-tial and final gluon radiation diagrams of qq̄ → tt̄gin Fig. 3; and (3) contributions from diagrams ofqg → tt̄q and q̄g → tt̄q̄ in Fig. 4.

The asymmetric cross section at the parton levelwas given analytically in Ref. [7]. However, wecarry out independent calculations [19] with thehelp of FeynCalc [20], FormCalc [21] and QCD-Loop [22].

The asymmetric cross section σA contributedfrom each of the above three categories is UV and

You-Kai Wang, Bo Xiao, Shou-Hua Zhu, 1008.2685

• Difficulty: gg fusion is dominant and symmetric

Top-quark AFB at the LHC

AC ⇠�qq̄

�qq̄ + �gg⇥AtFB ⇥ ✏

⇠ 20%⇥ 5%⇥ 50% ⇠ 0.005

It is hard to measurein hadron collision.

Separate qq and gg initial state

23rd Recontres de Blois - May/June 2011 Fabio Maltoni

PROGRESS IN SM TOP PREDICTIONS:

EXAMPLE 3: COLOR CHARGE ASYMM.

�yTEV = yt � yt̄ �yLHC = |yt|�| yt̄|

Att̄CC =�(�y > 0)� �(�y < 0)�(�y > 0) + �(�y < 0)

Other definitions are used: lab frame at Tevatron, central charge [Antunano, et al,] and one-side asymmetries [Wang et al. 2010] at the LHC which depend on a cut. ACC at the LHC has been introduced by CMS (in terms of pseudo-rapidity). LHCB does not need any special definition [Kagan et al.]

Monday 30 May 2011

�yTev = yt � yt̄ �yLHC = |yt|� |yt̄|


• Charged lepton is maximally correlated with top-spin.

20

q

q

t

bn

l+

D0: AtFB = 0.196± 0.065

A`FB = 0.152± 0.040

SM:A`FB = 0.021± 0.001AtFB = 0.051± 0.001

A`FBAtFB

��SM

⇠ 12

A`FBAtFB

��D0

⇠ 34

Bernreuther, Zong-Guo Si, NPB837 (2010) 90

CDF:(8.7fb-1)

t

A`FBAtFB

��>450

⇠ 35

A`FBAtFB

��inc

⇠ 34

A`FB = 0.066± 0.025AtFB = 0.085± 0.025

(Before unfolding)

versus AtFBA`FB


• and is connected by the top-quark and charged lepton spin correlation. AtFB A`FB

versus AtFBA`FB

Berger, Qing-Hong Cao, Chen, Yu, Zhang, PRL 108 (2012) 072002

21

A`FB ⇡ ⇢tLAtLFB ⇥

�2RtLF � 1

�+⇢tRA

tRFB ⇥

�2RtRF � 1

�

(%)tFBA10 12 14 16 18 20 22 24 26 28 30

(%)

l FBA

0

2

4

6

8

10

12

14

16

18

20

(200GeV,400GeV)

(400GeV,600GeV)

(600GeV,800GeV)

(800GeV,1000GeV)

(b)

(%)tFBA10 12 14 16 18 20 22 24 26 28 30

(%)

l FBA

0

2

4

6

8

10

12

14

16

18

20

(800GeV,1000GeV)

(1000GeV,1200GeV)

(1200GeV,1400GeV)

(1400GeV,1600GeV)

(1600GeV,2500GeV)

(a) G0

A`FB ' 0.47⇥AtFB + 0.25% A`FB ' 0.75⇥AtFB � 2.1%

W 0

Search for heavy resonances(tt, tt, tb, tt+VV, direct-t)


Measuring W’-t-b and Z’-t-t couplings

23

★ Top polarization can probe the handness of W’-t-b coupling.

Gopalakrishna, Han, Lewis, Si, Zhou,PRD82 (2010) 115020

W ′+t

b̄

u

d̄

-1 -0.5 0 0.5 1cos θ

la

00.10.20.30.40.50.60.70.80.9

dσ/d

cos θla (p

b)

W’RW’L

(a)

-1 -0.5 0 0.5 1cos θ

la

0

50

100

150

dσ/d

cos θla (f

b)

W’RW’L

(b)

Figure 10: The angular distribution of the charged lepton in pp → W ′ → tb̄ → bb̄ !+ν! productionat the LHC for MW ′ = 1 TeV in the top quark rest-frame with respect to a spin quantizationdirection â taken to be the top direction in the c.m. frame, for (a) without smearing or cuts, and(b) with energy smearing and cuts in Eqs. (16),(18),(19),(23), and tagging the softest b-jet.

Table 4: Forward-backward asymmetry of the charged lepton in pp → tb̄ → bb̄!+ν! for ! = e+ orµ+ at the LHC for MW ′ = 1TeV with and without the SM W contribution.

A W +W ′L W +W′R W

′L W

′R

No Cuts or smearing −0.42 0.17 −0.48 0.48No Cuts −0.42 0.15 −0.49 0.45Cuts Eqs.(16) −0.48 0.24 −0.51 0.37+Eq.(19) −0.49 0.39 −0.49 0.40+Eq.(18) −0.53 0.36 −0.53 0.37+Eq. (23) & tagging 1 b-jet −0.48 0.40 −0.48 0.40

Using the reconstructed events we can also determine the asymmetric observable A. The results

for A are given in Table 4 for the signal of W ′L and W′R with and without including the SM

W contribution. To demonstrate the realistic kinematical effects, we give the asymmetries with

consecutive cuts in the table. Once all the cuts have been applied we still obtain a very good

determination of the chirality of the W ′.

4.3 W ′ chiral couplings from transverse momentum distributions

As discussed already in Sec. 3.2, the pT distributions also convey information on the W ′ chirality

as shown in Fig. 3 due to their spin correlations. The charged lepton pT in the case of W ′R is

harder than that in W ′L. This can be understood from angular-momentum conservation; for the

23

t

-1 -0.5 0 0.5 1cosθ

µ+

0

0.1

1/σ

dσ

/dco

sθµ+

no cutwith cuts

-1 -0.5 0 0.5 1cosθ

µ+

0

0.1

1/σ

dσ

/dco

sθµ+

no cutwith cuts

-1 -0.5 0 0.5 1cosθ

µ+

0

0.1

1/σ

dσ

/dco

sθµ+

no cutwith cuts

(a) SM (b) purely ZR, (c) purely ZL

,

(1+cosθ)/2 (1-cosθ)/2

FIG. 6: Distributions in cos θ! of the lepton from the decay of top quarks produced in tt̄ events

before and after cuts: (a) SM, (b) right-handed polarized top quarks in Z ′ decay; (c) left-handed

polarized top quarks in Z ′ decay. The distributions for W ′ decay are similar to those for Z ′ decay.

models. We adopt a general linear least squares fit in this study to estimate how well the

degree of top quark polarization can be determined.

An observed angular distribution O(y) after the SM background is subtracted can be

expressed as

O(y) = !L FL(y) + !R FR(y), (12)

where !L (!R) is the fraction of left-handed (right-handed) top quarks. The values of !L and

!R are chosen as the best parameters that minimize χ2, defined as

χ2 =N∑

i=1

[

O(yi)− !LFL(yi)− !RFR(yi)σi

]2

, (13)

where N is the number of bins, and σi =√

O(yi) is the statistical error (standard deviation)

of the ith data point. The minimum of Eq. 13 occurs where the derivative of χ2 with respect

to both !L and !R vanishes, yielding the normal equations of a least-squares problem:

0 =N∑

i=1

1

σ2i[O(yi)− !LFL(yi)− !RFR(yi)]Fl(yi), where l = L(R). (14)

Interchanging the order of summations, one can write the above equations as matrix equa-

tions,

αLL!L + αLR!R = βL, αRL!L + αRR!R = βR, (15)

15

★ Top polarization can probe the handness of Z’-t-t coupling.

Berger, Qing-Hong Cao, Chen, Zhang, PRD83 (2011) 114026


Exotic color scalars• Same-sign top-quark pair

24

• Four top-quarks or two top-quarks plus jets

Mohapatra, Okada, Hai-Bo Yu, 0709.1486Berger, Qing-Hong Cao, Chen, Shaughnessy, Zhang, PRL 105 (2010) 181802

Chen, Klemm, Rentala, Wang, 0811.2105

t

tt

t

t

ut

u

3⌦ 3 = 6� 3̄

u

u

tL

tL

(tR)

(tR)


Motivation for heavy quark• Natural NP models always have non-trial couplings

between tops and new physics:

Higgsless, Little Higgs, RS, SUSY, TC, ...

• New heavy quark loops stabilize EWSB...

25

~ Λ2 λt2 + λT

2 − $λT2( ) = Λ2 0( )

The Little Higgs Models


Heavy quark production and decay

26

• Pair production via QCD- Major discovery channel for small MQ- Sensitive to decay BRs, but not the couplings

q

q̄

Q̄

Q

q

b

q′

T

q

q̄′ T

b̄(B̄)

q

q̄′ E(e)

ν̄(N̄) q

q̄ N

N̄ q

q̄ E−

E+

Z Z/γ

(a) (b) (c)

(d) (e) (f )

W

g WW

• Single production via EW - Determine the weak coupling strength of heavy quark - Probe the mixing of SM quarks and heavy quarks - Depend on quark flavors

• Heavy quark decay - through Yukawa mixing with SM quarks - via CKM mixing ! Zt

! htT 0

! W+beg.

q

q̄

Q̄

Q

q

b

q′

T

q

q̄′ T

b̄(B̄)

q

q̄′ E(e)

ν̄(N̄) q

q̄ N

N̄ q

q̄ E−

E+

Z Z/γ

(a) (b) (c)

(d) (e) (f )

W

g WW


Direct top-quark production• Anomalous g-q-t FCNC coupling

27

arX

iv:1

104.

4945

v3 [

hep-

ph]

23 O

ct 2

011

ZU-TH 09/11

Search for anomalous top quark production at the early LHC

Jun Gao,1 Chong Sheng Li,1, ∗ Li Lin Yang,2 and Hao Zhang1

1Department of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China2Institute for Theoretical Physics, University of Zürich, CH-8057 Zürich, Switzerland

We present a detailed study of the anomalous top quark production with subsequent decay at theLHC induced by model-independent flavor-changing neutral-current couplings, incorporating thecomplete next-to-leading order QCD effects. Our results show that, taking into account the currentlimits from the Tevatron, the LHC with

√s = 7 TeV may discover the anomalous coupling at 5σ

level for a very low integrated luminosity of 61 pb−1. The discovery potentials for the anomalouscouplings at the LHC are examined in detail. We also discuss the possibility of using the chargeratio to distinguish the tug and tcg couplings.

The CERN Large Hadron Collider (LHC) is currentlyoperating at a center-of-mass (c.m.) energy of 7TeV.Even with a relatively low integrated luminosity (∼40 pb−1), it has already delivered many interesting re-sults, including new tests on the standard model (SM) inboth electroweak and strong interacting sectors, as wellas constraints on new physics models beyond the SM. Inparticular, both the ATLAS and the CMS collaborationshave measured the cross section for top quark pair pro-duction with a precision of around 10%. With more databeing collected, it can be expected that the top quarkproperties will be well measured in the near future, andexotic physics in the top quark sector can also be poten-tially probed.In the SM, flavor-changing neutral-currents (FCNC)

in the top quark sector are strongly suppressed. How-ever, many new physics models can induce large FCNCcouplings of the top quark with a light up-type quarkand a gluon, which can be possibly detected at theLHC [1, 2]. Such couplings can be incorporated into themodel-independent effective Lagrangian [3, 4]

L = gs∑

q=u,c

κtqgΛ

t̄σµνT a(fLq PL + fRq PR)qG

aµν + h.c. ,

where κtqg/Λ are real numbers representing the strengthof the couplings, and fL,Rq are chiral parameters normal-ized to |fLq |2 + |fRq |2 = 2. Both the CDF and D0 col-laborations at the Tevatron have searched for processesinduced by these operators and provided constraints onthe anomalous couplings [5, 6]. The most stringent one-dimensional exclusion limit (assuming only one couplingis non-zero) is given by [6]

κtug/Λ < 0.013TeV−1 , κtcg/Λ < 0.057TeV

−1 , (1)

at the 95% confidence level (C.L.). The above anomalouscouplings can induce various rare processes at hadron col-liders. Among them, the most interesting one is directtop quark production, where a single top quark is pro-duced without any additional particle. The signature ofthis process is different from the single top productionin the SM (where the top quark is always accompaniedby other particles). Given the couplings allowed by the

Tevatron search, the production rate for this process canstill be large at the LHC, which makes it a promisingchannel to search for new physics in the flavor sector.Any observation of this characteristic process definitelyindicates the existence of the tqg anomalous couplings,and the underlying new physics.

There have been several analyses in the literature ofdirect top quark production at the LHC at the leadingorder (LO) [2, 4, 7]. The next-to-leading order (NLO)QCD correction to the total cross section of this pro-cess has also been calculated in [8]. However, there is nodetailed phenomenological study based on the NLO re-sult. Also, the previous LO studies [2, 4] only focusedon the LHC with

√s = 14TeV. Moreover, they did

not include the SM single top quark production in thebackground processes, and therefore underestimated thebackground rate. The SM single top quark productioncan mimic the signal process if the additional jet is notreconstructed. At the NLO, the signal process can alsoemit an additional jet which makes the single top back-ground more prominent. With these considerations, itis therefore very important to perform an analysis tak-ing into account the NLO QCD effects and all the SMbackgrounds for the early LHC search of the anomalouscouplings with

√s = 7TeV. Besides, in order to pro-

vide a more complete NLO prediction, the QCD effectsin the top quark decay process should also be included.While the QCD correction to the decay process does notalter the inclusive rate, it may change the signal accep-tance significantly when kinematic cuts are applied. Inthis Letter, we present a detailed study of the direct topquark production with subsequent decay at the LHC, in-cluding NLO QCD corrections for both the productionpart and decay part. The SM backgrounds and the LHCdiscovery potential of the anomalous couplings are alsoexamined in detail.

The NLO QCD corrections to the direct top quark pro-duction with subsequent decay can be factorized into twoindependent gauge invariant parts, i.e., the top quarkproduction at NLO with subsequent decay at LO, andproduction at LO with subsequent decay at NLO, by us-ing the modified narrow width approximation incorpo-

★ NLO KF ~1.2

Zhang, C. S. Li, Gao, Zhang, Li,PRL 102 (2009) 072001

t u/c

gGao, C. S. Li, Yang, Zhang, PRL 107 (2011) 092002

★ NLO KF ~1.3-1.5 ★ promising at the LHC

g

u/ct


More ...• CP violation in single-top production

and top-quark pair productions

28

• Top-quark effective theory‣ Wtb coupling (W Helicity)‣ Top-quark chromo-dipole, etc.

• Top-quark spin correlations• Top-quark rare decay• Top-quark Yukawa couplings• ...


Summary

29

Questions Measurements

What is the Higgsboson mass?

Do we understand heavy flavor production in QCD?

Are there more than three fermion generations?

Are there new massive particles?

Does top quark have the expected couplings?

mtt̄ distribution

Top quark mass

Charge asymmetry of top pair

Single top production

Constraints on Wtb coupling

H+ ! tb̄ t! H+b̄Searches for or

W boson helicity

Search for t-prime quark

Search for FCNC top interaction

Top quark pair production cross section

感谢兄弟院校的大力支持！

SUSY国际会议

Pre-SUSY暑期学校

Kingman Cheung, Keith Dienes, Yuanning Gao, Rohoni Godbole, Hong-Jian He, Gondon Kane, Rocky Kolb, Shih-Chang Lee, Kirill Melnikov, Takeo Moroi, Hitoshi Murayama, Pran Nath, Sanjay Padhi, Pavel Fileviez Perez, Adrian Perieanu, Pascal Pralavorio, Fernando Quevedo, Nathan Seiberg, Guido Tonelli, Michael Turner, Henry Tye, Liantao Wang, Yi-Fang Wang, Yueliang Wu, Zhi-Zhong Xing, C.-P. Yuan, Dieter Zeppenfeld

• Standard Model (Jens Erler)

• SUSY theories (X. Tata)

• SUSY phenomenology (Mihoko Nojiri)

• Higgs physics

• BSM (C. Chacko)

• Monte Carlo tools (Johan Alwall)

• Collider phenomenology (Michael Spannowsky)

• Dark matter (Jason Kumar)

(plenary speaker)

ThankYou!

Documents

Top Quark Physics at Terascale - pku.edu.cnfaculty.pku.edu.cn/_tsf/00/0B/fyIRJ3byymaa.pdf · Top-quark as a link to new physics top-quark Effective Operators Effective Couplings Weakly