1 / 31

Associated Top-Pair Production with a Heavy Boson at the LHC

This presentation discusses the accuracy of associated top-pair production with a heavy boson at the LHC, focusing on NNLL+NLO accuracy. It explores the theoretical status of TTbar+Higgs and TTbar+W/Z production, highlighting their importance in understanding the Higgs sector and searching for deviations from the standard model. Resummation techniques are also discussed for extending the accuracy of perturbative predictions.

iank
Download Presentation

Associated Top-Pair Production with a Heavy Boson at the LHC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Associated Top-pair Production with a heavy boson through NNLL+NLO accuracy at the LHC Anna Kulesza University of Münster In collaboration with L. Motyka, D. Schwartländer, T. Stebel and V. Theeuwes • Rencontres de Moriond QCD & High Energy Interactions 23-30 March 2019

  2. Ttbar+Heavy Boson production A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  3. Associated Higgs Production with Top Quarks • Direct probe of the strength of the top-Yukawa coupling • An example of a 2⟶3 process for which full NNLO results are not available yet • Crucial for understanding the Higgs sector and in searches for deviations from the SM • Far-reaching consequences → stability of our Universe ATLAS, Run1 + Run2: 6.3σ (5.1σ exp.) CMS, Run1 + Run2: 5.2σ (4.9σ exp.) [Buttazzo et al.’13] A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  4. TTbar+W,Z • Probes of top-quark couplings • Sensitive to BSM contributions • Dominant backgrounds to searches and SM precision measurements (ttH included) • Signal strength (CMS): ttW ttZ CMS, 1711.02547 ATLAS, 1609.01599 A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  5. Theory Status for TTbarH Fixed order perturbation theory • NLO QCD available since (almost) 20 years [Beenakker, Dittmaier, Krämer, Plumper, Spira, Zerwas ’01-’02][Reina, Dawson’01][Reina, Dawson, Wackeroth’02][Dawson,Orr,Reina,Wackeroth’03] [Dawson, Jackson, Orr, Reina, Wackeroth’03] • NLO QCD matched with parton showers • POWHEG-Box [Garzelli, Kardos,Papadopoulos,Trocsanyi’11] [Hartanto, Jäger,Reina, Wackeroth’15] • aMC@NLO[Frederix, Frixione, Hirschi, Maltoni, Pittau, Torrielli’11] • SHERPA+RECOLA [Biedermann et al.’17] • QCD and EW@NLO [Frixione, Hirschi, Pagani,Shao,Zaro’14-’15][Zhang, Ma, Zhang, Chen, Guo’14][Biedermann, Bräuer, Denner, Pellen, Schumann, Thompson’17] • NLO QCD [Denner, Feger’15] and EW [Denner,Lang, Pellen, Uccirati’16] with tops off-shell • Top decays into b quarks and leptons: pp➝ ttH ➝ W+W-bbH➝e+νeμ-νμbb H • All resonant, non-resonant, interference and off-shell effects included - A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  6. Theory Status For TTbar+W,Z Fixed order perturbation theory • NLO QCD [Lazopoulos, Melnikov, Petriello’07] [Lazopoulos, McElmurry, Melnikov, Petriello’08] [Kardos, Trocsanyi, Papadopoulos’12 ] withdecays at NLO [Campbell, Ellis’12][Roentsch, Schulze’14-’15] • NLO interfaced to parton showers inaMC@NLO[Alwall, Frederix, Frixione, Hirschi, Maltoni, Mattalaer, Shao Stelzer, Torrieli, Zaro,’14] [Maltoni, Mangano, Tsinikos, Zaro,’14] [Maltoni, Pagani, Tsinikos’15]and POWHEG-Box [Garzelli, Kardos,Papadopoulos,Trocsanyi’12] • NLO EW (+QCD) corrections [Frixione, Hirschi, Pagani,Shao,Zaro’14-15][Frederix, Pagani, Zaro’18] A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  7. Why Resummation? • Extending accuracy of the perturbative prediction beyond fixed-order by adding a systematic treatment of logarithmic contributions due to soft gluon emission is a common standard in precision phenomenology • better description of physics: additional corrections to cross sections… especially interesting when NNLO calculations out of reach • improvement or, in some cases, restoration of predictive power of perturbation theory • reduction of the theory error due to scale variation sums up LL: αsn log n+1 (N) NLL: αsn log n (N) NNLL: αsn log n-1 (N) Factorization at threshold: space of Mellin moments N, taken wrt. M2/S (absolute threshold) or Q2/S (invariant mass threshold) A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  8. Beyond NLO QCD: Resummation 2 • ttH: NLL+NLO resummation in the absolute threshold limit, obtained using direct QCD approach • ttH: “Approximated” NNLO based on the SCET approach to resummation in the invariant mass limit • ttH: NLL+NLO in the invariant mass limit for production with pseudoscalar Yukawa couplings • ttW: NNLL+NLO resummation in the invariant mass limit, SCET [Li, Li, Li’14] • ttH, ttW, ttZ: NNLL+NLO resummation in the invariant mass limit, hybrid SCET/direct QCD method[Broggio, Ferroglia, Ossola, Pecjak’16] [Broggio, Ferroglia, Pecjak, Yang’16][Broggio, Ferroglia, Ossola, Pecjak, Samoshima’17] • ttH, ttW, ttZ: NNLL+NLO resummation in the invariant mass limit, direct QCD method [AK, Motyka, Stebel, Theeuwes’16], [AK, Motyka, Stebel, Theeuwes’17], [AK, Motyka, Schwartländer, Stebel, Theeuwes’18] [AK, Motyka, Stebel, Theeuwes’15] [Broggio, Ferroglia, Pecjak, Signer, Yang’15] [Broggio, Ferroglia, Fiolhais, Onofre’17] A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  9. Resummation forTTbar+B • Thresholdlimit in thetriple invariant masskinematics: • Direct QCD → Nspace: • Factorization ⟷ resummation Hard off-shell Collinear Soft wide-angle ✓ Universal, known Process-dependent Hard and soft functions are now matrices in colour space A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  10. NNLL forTtbar+B = • Soft functionfromthesolutionofthe RG equation • Soft anomalous dimensions known at two loops for any number of legs [Mert-Aybat, Dixon, Sterman’06] [Becher, Neubert’09][Mitov, Sterman, Sung’09-’10] [Ferroglia, Neubert, Pecjak, Yang’09] [Beneke, Falgari, Schwinn’09], [Czakon, Mitov, Sterman’09] [Kidonakis’10]✓ • Tocalculate, eliminatepath-orderedexponentials: • usebasisthatdiagonalize→ enoughfor NLL accuracy • For NNLL, treatmatricesU perturbatively[Buras‘80][Buchalla, Buras, Lautenbacher’96] ] [Ahrens, Ferroglia, Neubert, Pecjak, Yang’10] A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  11. NNLL forTtbar+B = • Soft functionfromthesolutionofthe RGE equation • Soft anomalous dimensions known at two loops for any number of legs [Mert-Aybat, Dixon, Sterman’06] [Becher, Neubert’09][Mitov, Sterman, Sung’09-’10] [Ferroglia, Neubert, Pecjak, Yang’09] [Beneke, Falgari, Schwinn’09], [Czakon, Mitov, Sterman’09] [Kidonakis’10]✓ • Tocalculate, eliminatepath-orderedexponentials: • usebasisthatdiagonalize→ enoughfor NLL accuracy • For NNLL, treatmatricesU perturbatively[Buras‘80][Buchalla, Buras, Lautenbacher’96] ] [Ahrens, Ferroglia, Neubert, Pecjak, Yang’10] • Hard function • H(1)extracted from aMC@NLO[Frederix, Frixione, Hirschi, Maltoni, Pittau, Torrielli’11] (ttH, ttW, ttZ) andPowHel (HELAC-NLO [Bevilacqua et al.’11] +POWHEG-Box) package [Garzelli, Kardos, Papadopoulos,Trocksanyi’11] (ttH) A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  12. Invariant Mass Distribution for TTbarH [AK, Motyka, Stebel, Theeuwes’17] • NNLL+NLO distributions for the two considered scale choices very close, NLO results differ visibly → KNNLL = 𝜎NLO+NNLL / 𝜎NLOfactors also different • NNLL+NLO error band slightly narrower than NLO (7-point method) A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  13. Invariant Mass Distribution for TTbarH [AK, Motyka, Stebel, Theeuwes’17] • NNLL+NLO distributions for the two considered scale choices very close, NLO results differ visibly → KNNLL = 𝜎NLO+NNLL / 𝜎NLO factors also different • NNLL+NLO error band slightly narrower than NLO (7-point method) A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  14. Total Cross Section for TTbarH [AK, Motyka, Stebel, Theeuwes’17] KNNLL= 𝜎NLO+NNLL / 𝜎NLO A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  15. Total Cross Section for TTbarH [AK, Motyka, Stebel, Theeuwes’17] • Compared to NLO, remarkable stability of NLO+NNLL • Stability improves with increasing accuracy of resummation • Reduction of the theory scale error • “Best” NNLL+NLO prediction in agreement with NLO at μ0 = M/2 A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  16. Total Cross Section for TTbar+W,Z [AK, Motyka, Schwartläder, Stebel, Theeuwes’18] • Significant reduction in the scale dependence of the ttZ total cross section • Comparison with SCET results, wherever possible [Li et al.’14], [Broggio et al.’16-’17] ✓ A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  17. Ttbar+W,Z at NLO+NNLL [AK, Motyka, Schwartländer, Stebel, Theeuwes’18] • State of the art: QCD NLO+NNLL, combined with (additively) NLO EW [Frixione et al. ’14-15] • Central values of the theoretical predictions brought closer to the measured cross section; theory errors reduced (by half for ttZ) A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  18. Summary • ttH production @ the LHC is one of the most promising windows onto new physics • ttW and ttZ also very important for BSM searches as well as tests of the SM Precise theoretical prediction are essential ➝ a lot of recent progress! • Resummed calculations for reach NNLL+NLO accuracy -- first application of resummation to the class of 2->3 processes • Remarkable stability of the NNLL+NLO differential and total cross sections w.r.t. scale variation; improving stability with growing accuracy • Reduction of the theory error due to scale variation A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  19. BACKUP A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  20. Invariant Mass Kinematics Problem: hard and soft functions are now (and in general) matrices in colour space Diagonalization procedure leads to, at NLL: [Kidonakis, Oderda, Sterman’98] where λ’s are eigenvalues of the one-loop soft-anomalous dimension matrix A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  21. Invariant Mass Kinematics Problem: hard and soft functions are now (and in general) matrices in colour space Diagonalization procedure leads to, at NLL: [Kidonakis, Oderda, Sterman’98] where λ’s are eigenvalues of the one-loop soft-anomalous dimension matrix A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  22. Invariant Mass Kinematics Ctnd. • Extending resummation in this kinematics to NNLL requires: • Knowledge of the two-loop soft anomalous dimension • Amended treatment of the path-ordered exponential to account for it • Knowledge of the one-loop hard-matching coefficient A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  23. Invariant Mass Kinematics Ctnd. • Extending resummation in this kinematics to NNLL requires: • Knowledge of the two-loop soft anomalous dimension ✔Soft anomalous dimensions known at two loops for any number of legs[Mert-Aybat, Dixon, Sterman’06] [Becher, Neubert’09][Mitov, Sterman, Sung’09-’10] [Ferroglia, Neubert, Pecjak, Yang’09] [Beneke, Falgari, Schwinn’09], [Czakon, Mitov, Sterman’09] [Kidonakis’10] A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  24. Invariant Mass Kinematics Ctnd. • Extending resummation in this kinematics to NNLL requires: • Knowledge of the two-loop soft anomalous dimension • Amended treatment of the path-ordered exponential to account for it ✔Perturbative expansion [Buchalla, Buras, Lautenbacher’96] [Ahrens, Neubert, Pecjak, Yang’10] eigenvalues of Γ(1) A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  25. Invariant Mass Kinematics Ctnd. • Extending resummation in this kinematics to NNLL requires: • Knowledge of the two-loop soft anomalous dimension • Amended treatment of the path-ordered exponential to account for it • Knowledge of the one-loop hard function HIJ • needs access to colour structure of virtual corrections • ✔extracted from the results provided by the PowHel (HELAC-NLO [Bevilacqua et al.’11] +POWHEG-Box) package [Garzelli, Kardos, Papadopoulos,Trocksanyi’11] • translation between the colour flow and singlet-octet basis • implementation checked against colour-averaged virtual corrections obtained from public POWHEG [Hartanto, Jäger,Reina,Wackeroth’15] and aMC@NLO implementations [Frederix, Frixione, Hirschi, Maltoni, Pittau, Torrielli’11] A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  26. Scale dependence of the total cross section [AK, Motyka, Stebel, Theeuwes’17] μF = μ0 = Q μR = μ0 = Q • Apparent cancellations between μFand μRscale dependence • No significant change in dependence on μRwhile increasing the accuracy: αs running effect • μFdependence modified by the hard-matching coefficient 7-point method A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  27. Absolute Threshold Resummation for QQB @NLO+NLL Colour space basis in which ΓIJ is diagonal in the threshold limit incoming jet factors, known soft-wide angle emission hard function Hab →kl B,I At NLL accuracy Cab →kl B,I = 1 A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  28. One-Loop Soft Anomalous Dimension singlet-octet colour basis Reduces to the 2->2 case in the limit pB→0, mB→0 Absolute threshold limit: non-diagonal terms vanish Coefficients D(1)ab →kl B,I governing soft emission same as for the QQbar process: soft emission at absolute threshold driven only by the color structure A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  29. The Case of Associated Heavy Boson-Heavy quark Pair Production • Absolute threshold limit but: • LO cross section suppressed in the limit β→0 as β4 due to massive 3-particle phase-space • Absolute threshold scale M away from the region contributing the most nevertheless: • Well defined class of corrections which can be resummed [AK, Motyka, Stebel, Theeuwes’15] - PDF4LHC15_100 pdfs, NLO from aMC@NLO [Alwall, Frederix, Frixione, Hirschi, Maltoni, Mattelaer, Shao, Stelzer, Torrielli, Zaro] A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  30. Associated Higgs Production with Top Quarks, Resummation in the absolute threshold limit [ AK, Motyka, Stebel, Theeuwes’15] • Shows also a strong impact of the hard-matching coefficient C on the predictions → contributions away from the absolute threshold matter! • Part of these contributions can be accounted for if instead of resummation for total cross section, resummation for invariant mass of the ttbarH system is considered A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

  31. Associated Higgs Production with Top Quarks, Resummation in the absolute threshold limit AK, Motyka, Stebel, Theeuwes’15] • Threshold logarithms under scrutiny: • NLL accuracy requires knowledge of NLO soft anomalous dimension with 2⟶3 kinematics and NLO cross section at threshold split into colour channels MMHT2014NLO NLO obtained with aMC@NLO A. Kulesza, Associated top-pair & heavy boson production through NNLL+NLO accuracy at the LHC

More Related