Forward-Backward Asymmetries in Top-Antitop Quark Pair Production at - - PowerPoint PPT Presentation

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Forward-Backward Asymmetries in Top-Antitop Quark Pair Production at the Fermilab Tevatron

Ziqing Hong, for the CDF and D0 Collaborations

DPF 2017 July 31, 2017

Top AFB Tevatron combination 1/13

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Forward–backward asymmetry

◮ p¯

p collision at Tevatron with √s = 1.96TeV

◮ Top quarks primarily pair produced (p¯

p → t¯ t)

◮ Heavy and short-lived, decay quickly ◮ AFB measurements are simply answering:

Does the top quark prefer the proton direction or the

• pposite?

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Complementarity between the Asymmetry at the Tevatron and the LHC

(a) (b)

◮ p¯

p collision at Tevatron instead of pp collision at LHC

◮ Asymmetry from q¯

q annihilation (a)

◮ Tevatron t¯

t production dominated by q¯ q annihilation (85%, a)

◮ LHC dominated by gluon fusion (90%, b)

◮ Sizeable effect at Tevatron, very small

asymmetry (central vs. outer) at LHC

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Top AFB: Theoretical predictions

◮ No net asymmetry at leading order

◮ Asymmetry only from higher order effects

◮ Slight asymmetry starting from

next-to-leading order (NLO) effects

◮ Interference among diagrams ◮ Positive from born+box, negative from ISR/FSR

q ¯ q t ¯ t

◮ Substential EW correction and

higher order QCD corrections complicate the calculation

◮ Precision probe of SM

predictions of top quark physics

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Top AFB: Why interesting?

◮ First set of measurements

showed larger-than-SM values which leaves room for various beyond-SM models

◮ s-channel axigluon, t-channel W ’,

Z’, etc.

◮ SM predictions improved by

going to higher order and adding more effects

◮ Experimental results improved

with more data as well as better simulation/unfolding techniques

◮ The tension between measurements and SM predictions

has been reduced with time

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Top AFB at Tevatron ∆y ¯ p (¯ q, g) p (q, g) t ¯ t

y = 1 2 ln E + pz E − pz

◮ Quantify the asymmetry with the

rapidity difference (∆y) between top and anti-top

◮ At¯

t FB = N(∆y > 0) − N(∆y < 0)

N(∆y > 0) + N(∆y < 0)

◮ Also can measure AFB using one or

two leptons from top decays

Aℓ

FB = N(qℓηℓ > 0) − N(qℓηℓ < 0)

N(qℓηℓ > 0) + N(qℓηℓ < 0) Aℓℓ

FB = N(∆η > 0) − N(∆η < 0)

N(∆η > 0) + N(∆η < 0) ∆η = ηℓ+ − ηℓ−

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Top AFB at Tevatron

◮ Measurements done in multiple channels at CDF

& D0

CDF ℓ+jets CDF dilepton D0 ℓ+jets D0 dilepton At¯

t FB

PRD 87, 092002 PRD 93, 112005 PRD 90, 072011 PRD 92, 052007 (2013) (2016) (2014) (2015) Aℓ

FB

PRD 88, 072003 PRL 113, 042001 PRD 90, 072001 PRD 88, 112002 (2013) (2014) (2014) (2013) Aℓℓ

FB

PRL 113, 042001 PRD 88, 112002 (2014) (2013) ◮ Results for both inclusive asymmetries and

differential ones as functions of ∆y or mt¯

t

◮ Combining all of them for a final word from

Tevatron

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Inclusive AFB combinations

◮ BLUE combination, with

correlations between results accounted

◮ All results limited by

statistical uncertainty

◮ No evidence for deviation

from SM

◮ At¯

t FB 1.3σ from NNLO QCD

+ NLO EW prediction

◮ Aℓ

FB 1.6σ from NLO QCD +

NLO EW prediction

◮ Aℓℓ

FB 1.3σ from NLO QCD +

NLO EW prediction

◮ FERMILAB-CONF-16-386-PPD

Tevatron preliminary

−15 15 30 Asymmetry [%] t¯ t ∆y Asymmetry

• At¯

t FB

• CDF ℓ+jets
• 9.4 fb−1

PRD 87, 092002 (2013)

16.4 ± 4.7 CDF ℓℓ

• 9.1 fb−1

PRD 93, 112005 (2016)

12 ± 13 D0 ℓ+jets

• 9.7 fb−1

PRD 90, 072011 (2014)

10.6 ± 3.0 D0 ℓℓ

• 9.7 fb−1

PRD 92, 052007 (2015)

17.5 ± 6.3 Tevatron combination 12.8 ± 2.5 Lepton qη Asymmetry

• Aℓ

FB

• CDF ℓ+jets
• 9.4 fb−1

PRD 88, 072003 (2013)

10.5 ±

3.2 2.9

CDF ℓℓ

• 9.1 fb−1

PRL 113, 042001 (2014)

7.2 ± 6.0 D0 ℓ+jets

• 9.7 fb−1

PRD 90, 072001 (2014)

5.0 ±

3.4 3.7

D0 ℓℓ

• 9.7 fb−1

PRD 88, 112002 (2013)

4.4 ± 3.9 Tevatron combination 7.3 ± 2.0 Lepton ∆η Asymmetry

• Aℓℓ

FB

• CDF ℓℓ
• 9.1 fb−1

PRL 113, 042001 (2014)

7.6 ± 8.2 D0 ℓℓ

• 9.7 fb−1

PRD 88, 112002 (2013)

12.3 ± 5.6 Tevatron combination 10.8 ± 4.6 NNLO QCD + NLO EW [Czakon et al.] NLO SM [Bernreuther and Si] Top AFB Tevatron combination 8/13

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At¯

t FB vs. mt¯ t

◮ At¯

t FB vs. mt¯ t measured in

CDF ℓ+jets and D0 ℓ+jets

◮ With same binning ◮ Bin-by-bin BLUE

combination, with all correlations taken into account

◮ Agreement with NNLO

QCD + NLO EW at 1.3σ

◮ Czakon et al. arXiv:1705.04105

◮ New results!

Manuscript to be submitted soon.

Tevatron preliminary

350 450 550 650 mt¯

t

• GeV/c2

−0.4 −0.2 0.0 0.2 0.4 0.6 At¯

t FB

D0 CDF Tevatron combination αmt¯

t = (9.71±3.28)×10−4 GeV−1

NNLO QCD + NLO EW [Czakon et al.]

Tevatron preliminary

4 6 8 10 12 14 Slope αmt¯

t

• 10−4/
• GeV/c2

0.05 0.10 0.15 0.20 AFB at mt¯

t = 450 GeV/c2

AFB (mt¯

t) = αmt¯

t

• mt¯

t −450 GeV/c2

+AFB (450) Tevatron combination NNLO QCD + NLO EW [Czakon et al.]

Top AFB Tevatron combination 9/13

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At¯

t FB vs. mt¯ t - correlation

◮ CDF and D0 results appear to

have different trend in high mass

◮ Combination result in 3rd mass

bin lower than both inputs

◮ Different correlations between

last two bins

◮ Positive for CDF and negative for

D0

◮ Caused by different unfolding

methods

◮ Combination shows good

agreement between CDF and D0

◮ χ2/d.o.f = 5.19/4

Tevatron preliminary

0.05 0.10 0.15 0.20 0.25 0.30 0.35 At¯

t FB

• 450 < mt¯

t < 550 GeV/c2

−0.05 0.00 0.05 0.10 0.15 At¯

t FB

• mt¯

t < 450 GeV/c2

(a) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 At¯

t FB

• 550 < mt¯

t < 650 GeV/c2

−0.1 0.0 0.1 0.2 0.3 0.4 At¯

t FB

• 450 < mt¯

t < 550 GeV/c2

(b) −0.6 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 At¯

t FB

• 650 GeV/c2 < mt¯

t

• −0.2

0.0 0.2 0.4 0.6 At¯

t FB

• 550 < mt¯

t < 650 GeV/c2

(c) D0 CDF Tevatron combination NNLO QCD + NLO EW [Czakon et al.] Top AFB Tevatron combination 10/13

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At¯

t FB vs. ∆y

◮ At¯

t FB vs. ∆y measured in

CDF ℓ+jets, CDF dilepton, and D0 ℓ+jets

◮ Binning choices are

different, cannot combine bin-by-bin

◮ Doing a simultaneous linear

fit to all data points

◮ Measured slope

0.187 ± 0.038

◮ Cf. 0.129+0.006

−0.012

NNLO QCD + NLO EW,

Czakon et al. arXiv:1705.04105

◮ Agreement at 1.5σ

Tevatron preliminary

0.0 0.4 0.8 1.2 1.6 2.0 |∆yt¯

t|

−0.2 0.0 0.2 0.4 0.6 0.8 At¯

t FB

Tevatron combination: α∆yt¯

t = 0.187±0.038

NNLO QCD + NLO EW [Czakon et al.] D0 ℓ+jets CDF ℓ+jets CDF ℓℓ

◮ New results!

Manuscript to be submitted soon.

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Differential Aℓ

FB and Aℓℓ FB

◮ Putting all data on the

same figure for good visualization

◮ Theory curves are tanh

functions scaled to NLO QCD + NLO EW predictions

Tevatron preliminary

0.0 0.4 0.8 1.2 1.6 |qℓηℓ| −0.3 −0.2 −0.1 0.0 0.1 0.2 Aℓ

FB

D0 ℓ+jets, 9.7fb−1 CDF ℓ+jets, 9.4fb−1 D0 ℓℓ, 9.7fb−1 CDF ℓℓ, 9.1fb−1 NLO SM [Bernreuther and Si; Hong et al.] 0.0 0.4 0.8 1.2 1.6 |∆η| −0.6 −0.4 −0.2 0.0 0.2 0.4 Aℓℓ

FB

D0 ℓℓ, 9.7fb−1 CDF ℓℓ, 9.1fb−1 NLO SM [Bernreuther and Si; Hong et al.]

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Conclusions

◮ The AFB of top-pairs at the Tevatron has been a

hot topic for years

◮ Measurements of At¯

t FB, Aℓ FB and Aℓℓ FB probe the

production and decay of t¯ t

◮ Results from CDF and D0 combined for a final

word as Tevatron legacy

◮ Manuscript in preparation, to be submitted soon

◮ No clear sign of new physics ◮ Agreement between data and latest SM

predictions at 1.3-1.6σ

◮ The interaction of experiment and theory has

been healthy

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