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 A FB Tevatron combination 1/13
Forward–backward asymmetry p collision at Tevatron with √ s = 1 . 96TeV ◮ p ¯ p → t ¯ ◮ Top quarks primarily pair produced ( p ¯ t ) ◮ Heavy and short-lived, decay quickly ◮ A FB measurements are simply answering: Does the top quark prefer the proton direction or the opposite? Top A FB Tevatron combination 2/13
Complementarity between the Asymmetry at the Tevatron and the LHC ◮ p ¯ p collision at Tevatron instead of pp collision at LHC ◮ Asymmetry from q ¯ q annihilation (a) (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 (b) asymmetry (central vs. outer) at LHC
Top A FB : Theoretical predictions ◮ No net asymmetry at leading order ¯ q ¯ t ◮ Asymmetry only from higher order effects ◮ Slight asymmetry starting from next-to-leading order (NLO) effects q t ◮ Interference among diagrams ◮ Positive from born+box, negative from ISR/FSR ◮ Substential EW correction and higher order QCD corrections complicate the calculation ◮ Precision probe of SM predictions of top quark physics Top A FB Tevatron combination 4/13
Top A FB : 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 Top A FB Tevatron combination 5/13
Top A FB at Tevatron t ◮ Quantify the asymmetry with the rapidity difference (∆ y ) between top p ( q , g ) p (¯ ¯ q , g ) and anti-top FB = N (∆ y > 0) − N (∆ y < 0) ◮ A t ¯ ¯ t t N (∆ y > 0) + N (∆ y < 0) ∆ y ◮ Also can measure A FB using one or two leptons from top decays y = 1 2 ln E + p z FB = N ( q ℓ η ℓ > 0) − N ( q ℓ η ℓ < 0) E − p z A ℓ N ( q ℓ η ℓ > 0) + N ( q ℓ η ℓ < 0) FB = N (∆ η > 0) − N (∆ η < 0) A ℓℓ N (∆ η > 0) + N (∆ η < 0) ∆ η = η ℓ + − η ℓ − Top A FB Tevatron combination 6/13
Top A FB at Tevatron ◮ Measurements done in multiple channels at CDF & D0 CDF ℓ +jets CDF dilepton D0 ℓ +jets D0 dilepton A t ¯ t PRD 87, 092002 PRD 93, 112005 PRD 90, 072011 PRD 92, 052007 FB (2013) (2016) (2014) (2015) A ℓ PRD 88, 072003 PRL 113, 042001 PRD 90, 072001 PRD 88, 112002 FB (2013) (2014) (2014) (2013) A ℓℓ PRL 113, 042001 PRD 88, 112002 FB (2014) (2013) ◮ Results for both inclusive asymmetries and differential ones as functions of ∆ y or m t ¯ t ◮ Combining all of them for a final word from Tevatron Top A FB Tevatron combination 7/13
Inclusive A FB combinations Tevatron preliminary ◮ BLUE combination, with � A t ¯ t � t ¯ t ∆ y Asymmetry FB 9 . 4 fb − 1 � CDF ℓ + jets � 16.4 ± 4.7 correlations between results PRD 87, 092002 (2013) � 9 . 1 fb − 1 � ± 13 CDF ℓℓ 12 PRD 93, 112005 (2016) accounted 9 . 7 fb − 1 � D0 ℓ + jets � 10.6 ± 3.0 PRD 90, 072011 (2014) 9 . 7 fb − 1 � D0 ℓℓ � 17.5 ± 6.3 ◮ All results limited by PRD 92, 052007 (2015) 12.8 ± Tevatron combination 2.5 statistical uncertainty A ℓ Lepton q η Asymmetry � � FB 9 . 4 fb − 1 � � 10.5 ± CDF ℓ + jets 3 . 2 ◮ No evidence for deviation 2 . 9 PRD 88, 072003 (2013) � 9 . 1 fb − 1 � CDF ℓℓ 7.2 ± 6.0 from SM PRL 113, 042001 (2014) 9 . 7 fb − 1 � D0 ℓ + jets � 5.0 ± 3 . 4 3 . 7 PRD 90, 072001 (2014) ◮ A t ¯ 9 . 7 fb − 1 � t � 4.4 ± FB 1.3 σ from NNLO QCD D0 ℓℓ 3.9 PRD 88, 112002 (2013) Tevatron combination 7.3 ± 2.0 + NLO EW prediction A ℓℓ Lepton ∆ η Asymmetry � � ◮ A ℓ FB FB 1.6 σ from NLO QCD + � 9 . 1 fb − 1 � 7.6 ± CDF ℓℓ 8.2 PRL 113, 042001 (2014) NLO EW prediction � 9 . 7 fb − 1 � D0 ℓℓ 12.3 ± 5.6 PRD 88, 112002 (2013) ◮ A ℓℓ FB 1.3 σ from NLO QCD + 10.8 ± Tevatron combination 4.6 NNLO QCD + NLO EW [Czakon et al. ] NLO EW prediction NLO SM [Bernreuther and Si] − 15 0 15 30 ◮ FERMILAB-CONF-16-386-PPD Asymmetry [%] Top A FB Tevatron combination 8/13
A t ¯ Tevatron preliminary FB vs. m t ¯ t t 0 . 6 ◮ A t ¯ 0 . 4 t FB vs. m t ¯ t measured in 0 . 2 CDF ℓ +jets and D0 ℓ +jets FB A t ¯ t − 0 . 4 − 0 . 2 0 . 0 D0 CDF ◮ With same binning Tevatron combination t = ( 9 . 71 ± 3 . 28 ) × 10 − 4 GeV − 1 ◮ Bin-by-bin BLUE α m t ¯ NNLO QCD + NLO EW [Czakon et al. ] combination, with all 350 450 550 650 � GeV / c 2 � m t ¯ t correlations taken into Tevatron preliminary account 0 . 20 � t − 450 GeV / c 2 � A FB ( m t ¯ t ) = α m t ¯ + A FB ( 450 ) m t ¯ ◮ Agreement with NNLO t t = 450 GeV / c 2 0 . 15 QCD + NLO EW at 1 . 3 σ ◮ Czakon et al. arXiv:1705.04105 A FB at m t ¯ 0 . 10 ◮ New results! Tevatron combination Manuscript to be submitted soon. 0 . 05 NNLO QCD + NLO EW [Czakon et al. ] 4 6 8 10 12 14 � 10 − 4 / � GeV / c 2 �� Slope α m t ¯ t Top A FB Tevatron combination 9/13
Tevatron preliminary A t ¯ FB vs. m t ¯ t t - correlation (a) 0 . 15 t < 450 GeV / c 2 � 0 . 10 ◮ CDF and D0 results appear to 0 . 05 m t ¯ have different trend in high mass − 0 . 05 0 . 00 � FB A t ¯ t ◮ Combination result in 3rd mass 0 . 05 0 . 10 0 . 15 0 . 20 0 . 25 0 . 30 0 . 35 A t ¯ t t < 550 GeV / c 2 � � 450 < m t ¯ bin lower than both inputs FB 0 . 4 t < 550 GeV / c 2 � (b) ◮ Different correlations between 0 . 3 0 . 2 last two bins 450 < m t ¯ 0 . 1 ◮ Positive for CDF and negative for 0 . 0 � D0 FB − 0 . 1 A t ¯ t 0 . 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 ◮ Caused by different unfolding A t ¯ t � 550 < m t ¯ t < 650 GeV / c 2 � FB methods t < 650 GeV / c 2 � (c) 0 . 6 ◮ Combination shows good 0 . 4 agreement between CDF and D0 0 . 2 550 < m t ¯ ◮ χ 2 / d.o.f = 5 . 19 / 4 0 . 0 � D0 CDF Tevatron combination − 0 . 2 FB A t ¯ t NNLO QCD + NLO EW [Czakon et al. ] − 0 . 6 − 0 . 4 − 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 A t ¯ 650 GeV / c 2 < m t ¯ t � � t FB Top A FB Tevatron combination 10/13
A t ¯ FB vs. ∆ y t ◮ A t ¯ t FB vs. ∆ y measured in CDF ℓ +jets, CDF dilepton, Tevatron preliminary 0 . 8 Tevatron combination: α ∆ y t ¯ t = 0 . 187 ± 0 . 038 and D0 ℓ +jets NNLO QCD + NLO EW [Czakon et al. ] 0 . 6 ◮ Binning choices are D0 ℓ + jets CDF ℓ + jets CDF ℓℓ 0 . 4 different, cannot combine FB A t ¯ t 0 . 2 bin-by-bin 0 . 0 ◮ Doing a simultaneous linear − 0 . 2 fit to all data points 0 . 0 0 . 4 0 . 8 1 . 2 1 . 6 2 . 0 ◮ Measured slope | ∆ y t ¯ t | ◮ New results! 0 . 187 ± 0 . 038 ◮ Cf. 0 . 129 +0 . 006 Manuscript to be − 0 . 012 NNLO QCD + NLO EW, submitted soon. Czakon et al. arXiv:1705.04105 ◮ Agreement at 1 . 5 σ Top A FB Tevatron combination 11/13
Differential A ℓ FB and A ℓℓ Tevatron preliminary FB 0 . 2 0 . 1 − 0 . 3 − 0 . 2 − 0 . 1 0 . 0 FB D0 ℓ + jets, 9 . 7fb − 1 A ℓ ◮ Putting all data on the CDF ℓ + jets, 9 . 4fb − 1 D0 ℓℓ , 9 . 7fb − 1 same figure for good CDF ℓℓ , 9 . 1fb − 1 NLO SM [Bernreuther and Si; Hong et al. ] visualization 0 . 0 0 . 4 0 . 8 1 . 2 1 . 6 | q ℓ η ℓ | ◮ Theory curves are tanh 0 . 4 functions scaled to NLO 0 . 2 QCD + NLO EW 0 . 0 FB predictions A ℓℓ − 0 . 6 − 0 . 4 − 0 . 2 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 | Top A FB Tevatron combination 12/13
Conclusions ◮ The A FB of top-pairs at the Tevatron has been a hot topic for years ◮ Measurements of A t ¯ FB , A ℓ FB and A ℓℓ t 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 . 6 σ ◮ The interaction of experiment and theory has been healthy Top A FB Tevatron combination 13/13
Backup Slides Backup slides Top A FB Tevatron combination 14/13
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