Recent developments. Monte-Carlo Event Generators

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1 Recent developments in Monte-Carlo Event Generators Marek Schönherr Universität Zürich ISMD 205 Wildbad Kreuth Marek Schönherr /28

2 Contents Parton shower developments Higher precision in parton showers Electroweak corrections in parton showers 2 Matching fixed order calculations to parton showers Matching calculations to parton showers Matching N calculations to parton showers 3 Multijet merging at higher accuracy Merging multiple matched calculations of successive jet multiplicity Examples Marek Schönherr 2/28

3 Parton shower developments Matching fixed order calculations to parton showers Multijet merging at higher accuracy Introduction Monte-Carlo event generators are the workhorses for collider physics analyses provide fully exclusive stable final state: e, µ, γ, π ±, K ±, K L, p ±, n,... factorise dynamics into event stages at different energy regimes and characteristic scales, roughly: primary hard scattering and parton evolution secondary soft scatterings hadronisation hadron decays Construction principle behind HERWIG++, PYTHIA8 and SHERPA Marek Schönherr 3/28

4 Introduction Monte-Carlo event generators are the workhorses for collider physics analyses allows to be used as data stand-in calculate realistic expectation values for measurable observables extrapolate into unmeasurable regions calculate signal/background composition... necessitates highest precision focus in recent years has been on improving the perturbative accuracy of the hard scattering very little to no development in non-perturbative regimes Marek Schönherr 4/28

5 State of the art a few years ago PS MC@, POWHEG one process/signature described at matched to PS for further emissions MEPS CKKW, MLM simultaneously description of processes/signatures of successive jet multies at LO, e.g. pp W +, 2, 3,... jets embeds them in parton shower evolution work well, but ideally combine approaches Prediction / Data ATLAS Inclusive 0 s = 7 TeV; Data uncertainty PYTHIA6-AMBT POWHEG+PYTHIA6 MC@+HERWIG ALPGEN+HERWIG SHERPA ATLAS arxiv: L dt = 4.7 fb 2 0 pz [GeV] T Marek Schönherr 5/28

6 T Parton shower developments Matching fixed order calculations to parton showers Multijet merging at higher accuracy State of the art a few years ago PS MC@, POWHEG one process/signature described at matched to PS for further emissions [/GeV] jet (/σ Z/γ * l + l -) dσ/dp ATLAS arxiv: ATLAS Z/γ*( l l )+ jet (l=e,µ) - L dt = 4.6 fb Data 20 ( s = 7 TeV) anti-k t jets, R = 0.4 jet jet p > 30 GeV, y < 4.4 T ALPGEN SHERPA MC@ BLACKHAT + SHERPA MEPS CKKW, MLM simultaneously description of processes/signatures of successive jet multies at LO, e.g. pp W +, 2, 3,... jets.2 embeds them in parton 0.8 shower evolution 0.6 work well, but ideally combine approaches / Data MC / Data MC / Data SHERPA BLACKHAT ALPGEN SHERPA jet p (leading jet) [GeV] T Marek Schönherr 5/28

7 Contents Parton shower developments Higher precision in parton showers Electroweak corrections in parton showers 2 Matching fixed order calculations to parton showers Matching calculations to parton showers Matching N calculations to parton showers 3 Multijet merging at higher accuracy Merging multiple matched calculations of successive jet multiplicity Examples Marek Schönherr 6/28

8 Higher precision in parton showers Higher precision in parton showers N dn/dτ Thrust, τ = T full shower strict large-nc Parton showers operate in leading colour spin-avaraged approximation Improved colour treatment full-colour first emission Höche, Krauss, MS, Siegert arxiv:.220 diagonalised full colour evolution LC+ approximation Plätzer, Sjödahl arxiv: Nagy, Soper arxiv: small effects for most observables x/full DipoleShower + ColorFull τ Marek Schönherr 7/28

9 Parton shower developments Matching fixed order calculations to parton showers Multijet merging at higher accuracy Higher precision in parton showers Improved analytical understanding Höche, Prestel arxiv: Premise: build parton shower with controlled resummation behaviour dipole shower for controlled collinear and soft limits fulfills all momentum and flavour sum rules correct anomalous dimensions data description also good DIRE dipole resummation /σ dσ/dt MC/Data Thrust (E CMS = 9.2 GeV) ALEPH data Eur.Phys.J. C35 (2004) 457 Dire T Sherpa MC Marek Schönherr 8/28

10 Parton shower developments Matching fixed order calculations to parton showers Multijet merging at higher accuracy Electroweak corrections in parton showers Parton shower developments Electroweak corrections in parton showers add splitting functions for W ±, Z, problem: highly spin dependent ME to correct first emission Christiansen, Sjöstrand arxiv: E V m V limit with prefactors to account for spin composition Krauss, Petrov, MS, Spannowsky arxiv:402.xxxx interleaved evolution can lead to same signatures from different starting points, e.g. pp Wjj from pp W and 2 QCD emissions or pp jj and W emission No generic solution in spin-avaraged parton shower σ(z e + e + Njet) [pb] MC/Data dσ/dm23 [pb/2 GeV] 0 2 σ( Njet), Z e + e, p (jet) > 30 GeV, yjet < ATLAS data Weak path QCD path Combined > 750 GeV ptj m 23 [GeV] f = 0.0 f =.0 f =. f = Njet 00 Marek Schönherr 9/28

11 Contents Parton shower developments Higher precision in parton showers Electroweak corrections in parton showers 2 Matching fixed order calculations to parton showers Matching calculations to parton showers Matching N calculations to parton showers 3 Multijet merging at higher accuracy Merging multiple matched calculations of successive jet multiplicity Examples Marek Schönherr 0/28

12 Matching calculations to parton showers PS matching Two traditional schemes Frixione, Webber hep-ph/ POWHEG Nason hep-ph/040946, Frixione, Nason, Oleari arxiv: two sides of the same medal Höche, Krauss, MS, Siegert arxiv:.220 [ ] dσ = B 0 (Φ 0 ) PS 0 (t max ) + R 0 PS () 0 (Φ ) PS (t ) correction smeared through resummation phase space A differing scheme UPS [ dσ = B 0 (Φ 0 ) PS 0 (µ 2 Q) + + [ B 0 B 0 ] (Φ0 ) R 0 PS () 0 Lönnblad, Prestel arxiv: ] (Φ ) PS (t ) correction localised in Born phase space, not showered same formal accuracy, differ at O(α 2 s ) Marek Schönherr /28

13 Matching calculations to parton showers and POWHEG pb GeV (t h ) dσ fid dp 2 0 T 0 ATLAS arxiv: ATLAS Data s = 7 TeV, 4.6 fb stat. syst. uncert. POWHEG+PYTHIA POWHEG+HERWIG POWHEG(HERAPDF)+PYTHIA MC@+HERWIG σ dσ(ww + 0 jets)/dp l T,max CMS arxiv: CMS 9.4 fb (8 TeV) Data Madgraph MC@ Powheg Expected/Data p (t h ) [GeV] T Theory / Data Theory / Data Theory / Data Madgraph+Pythia normalized to σ N MC@+Herwig normalized to σ N Powheg+Pythia normalized to σ N p l T,max (GeV) Marek Schönherr 2/28

14 Matching calculations to parton showers CKKW/Mi m h dσ/dp H [pb/gev] T Hamilton, Nason, Zanderighi arxiv: H PWG HJ MI HJ RUN HJ FXD Example pp h + j: α 3 s (µ R ) = α 3 s (m h ) Ratio p H T [GeV] Sudakov resums p (j) in pp hj wrt. pp h local scale choice resums logs associated with emission embeds first emission in evolution off pp h process necessary for MEPS merging, improves PS with jet in Φ 0 Marek Schönherr 3/28

15 Matching calculations to parton showers CKKW/Mi p (p, m h ) m h dσ/dp H [pb/gev] T Hamilton, Nason, Zanderighi arxiv: H PWG HJ MI HJ RUN HJ FXD Example pp h + j: α 3 s ( µ R ) = α 2 s (m h )α s (p ) Ratio p H T [GeV] Sudakov resums p (j) in pp hj wrt. pp h local scale choice resums logs associated with emission embeds first emission in evolution off pp h process necessary for MEPS merging, improves PS with jet in Φ 0 Marek Schönherr 3/28

16 Matching calculations to parton showers Mi p (p, m h ) m h Example pp h + j: α 3 s ( µ R ) = α 2 s (m h )α s (p ) inclusion of Sudakov of appropriate logarithmic accuracy leads to finite result as p (j) 0 inclusion of process dependent coefficients in Sudakov reaches accuracy fo pp h as p (j) 0 simultaneous accurate description of pp h and pp h + j Marek Schönherr 4/28

17 Matching calculations to parton showers Mi p (p, m h ) m h Example pp h + j: α 3 s ( µ R ) = α 2 s (m h )α s (p ) dσ/dp H [pb/gev] T ratio Hamilton, Nason, Oleari, Zanderighi arxiv: HJ+Pythia H+Pythia [GeV] inclusion of Sudakov of appropriate logarithmic accuracy leads to finite result as p (j) 0 inclusion of process dependent coefficients in Sudakov reaches accuracy fo pp h as p (j) 0 simultaneous accurate description of pp h and pp h + j p H T Marek Schönherr 4/28

18 Matching N calculations to parton showers NPS Two approaches POWHEG/Mi + reweighting pp h pp l + l Karlberg, Re, Zanderighi arxiv: Hamilton, Nason, Re, Zanderighi arxiv: S-MC@ + UPS + q -slicing pp l + l Höche, Li, Prestel arxiv: pp h Höche, Li, Prestel arxiv: both are not proper NPS matchings but rather exploit the simplified structure of logarithms and N correction in singlet production Marek Schönherr 5/28

19 Matching N calculations to parton showers NPS with POWHEG/Mi + reweighting Hamilton, Nason, Re, Zanderighi arxiv: , Karlberg, Re, Zanderighi arxiv: A simulation that is accurate to in both Φ 0 and Φ can be reweighted differentially in Φ 0 to N. dσ/dp T,Z [pb/gev] LHC 4 TeV DYN Zj-Mi NPS dσ/dp H [pb/gev] T Ratio [GeV] p H T NPS HQT p T,Z [GeV] Marek Schönherr 6/28

20 Matching N calculations to parton showers NPS with S-MC@ + UPS + q -slicing Höche, Li, Prestel arxiv: , arxiv: Φ 0 is localised at q = 0. For q > q,cut (q,cut < t c ) a S-MC@ calculation in Φ can be used to complement an exclusive N calculation at zero q. Overlaps of the Sudakov form factor and the N calculation have to be subtracted, e.g. UPS-style. [pb/gev] - T,e 0 2 s = 7 TeV 60 GeV<m <20 GeV ll UN 2 LOPS MC@ N [pb/gev] T,H s = 4 TeV UN 2 LOPS HqT N MC@ Sherpa MC dσ/dp 0 dσ/dp 0 - m ll /2< µ <2m R/F ll m ll /2< µ <2m Q ll Sherpa+BlackHat 0-2 m H /2< µ <2m R/F H m H /4< µ <m Q H m H /4< µ <m H HqT Q Ratio to N p [GeV] 00 - T,e Ratio to HqT p [GeV] T,H Marek Schönherr 7/28

21 Contents Parton shower developments Higher precision in parton showers Electroweak corrections in parton showers 2 Matching fixed order calculations to parton showers Matching calculations to parton showers Matching N calculations to parton showers 3 Multijet merging at higher accuracy Merging multiple matched calculations of successive jet multiplicity Examples Marek Schönherr 8/28

22 Merging multiple matched calculations of successive jet multiplicity Multijet merging at LO PS LO LO LO Parton showers resummation of (soft-)collinear limit intrajet evolution matrix elements (ME) and parton showers (PS) are approximations in different regions of phase space MEPS combines multiple LOPS keeping either accuracy multiplicities are defined using resolution criterion merging cut Marek Schönherr 9/28

23 Merging multiple matched calculations of successive jet multiplicity Multijet merging at LO ME LO LO LO LO Matrix elements fixed-order in α s hard wide-angle emissions interference terms matrix elements (ME) and parton showers (PS) are approximations in different regions of phase space MEPS combines multiple LOPS keeping either accuracy multiplicities are defined using resolution criterion merging cut Marek Schönherr 9/28

24 Merging multiple matched calculations of successive jet multiplicity Multijet merging at LO ME PS LO LO LO LO LO LO LO MEPS (CKKW,MLM) Catani, Krauss, Kuhn, Webber JHEP(200)063 Lönnblad JHEP05(2002)046 Höche, Krauss, Schumann, Siegert JHEP05(2009)053 matrix elements (ME) and parton showers (PS) are approximations in different regions of phase space MEPS combines multiple LOPS keeping either accuracy multiplicities are defined using resolution criterion merging cut Marek Schönherr 9/28

25 Merging multiple matched calculations of successive jet multiplicity Multijet merging at PS PS Frixione, Webber JHEP06(2002)029 Nason JHEP(2004)040, Frixione et.al. JHEP(2007)070 Höche, Krauss, MS, Siegert JHEP09(202)049 PS elevate LOPS to accuracy MEPS supplements core PS with higher multiplicities LOPS combines multiple PS keeping either accuracy all methods need resolution criterion separating n and n + jet calculations Marek Schönherr 20/28

26 Merging multiple matched calculations of successive jet multiplicity Multijet merging at ME PS LO LO LO LO MEPS Hamilton, Nason JHEP06(200)039 Höche, Krauss, MS, Siegert JHEP08(20)23 Gehrmann, Höche, Krauss, MS, Siegert JHEP0(203)44 PS elevate LOPS to accuracy MEPS supplements core PS with higher multiplicities LOPS combines multiple PS keeping either accuracy all methods need resolution criterion separating n and n + jet calculations Marek Schönherr 20/28

27 Merging multiple matched calculations of successive jet multiplicity Multijet merging at ME PS PS elevate LOPS to accuracy Lavesson, Lönnblad JHEP2(2008)070 Höche, Krauss, MS, Siegert JHEP04(203)027 Gehrmann, Höche, Krauss, MS, Siegert JHEP0(203)44 Lönnblad, Prestel JHEP03(203)66 Plätzer JHEP08(203)4 MEPS supplements core PS with higher multiplicities LOPS combines multiple PS keeping either accuracy all methods need resolution criterion separating n and n + jet calculations Marek Schönherr 20/28

28 Merging multiple matched calculations of successive jet multiplicity Multijet merging at ME PS PS elevate LOPS to accuracy Lavesson, Lönnblad JHEP2(2008)070 Höche, Krauss, MS, Siegert JHEP04(203)027 Gehrmann, Höche, Krauss, MS, Siegert JHEP0(203)44 Lönnblad, Prestel JHEP03(203)66 Plätzer JHEP08(203)4 MEPS supplements core PS with higher multiplicities LOPS combines multiple PS keeping either accuracy all methods need resolution criterion separating n and n + jet calculations Marek Schönherr 20/28

29 Merging multiple matched calculations of successive jet multiplicity Multijet merging at Höche, Krauss, MS, Siegert arxiv: Gehrmann, Höche, Krauss, MS, Siegert arxiv: generalisation of CKKW to accuracy, proof along the same lines merges PS instead of LOPS for each multiplicity FxFx Frederix, Frixione arxiv: generalisation of MLM to accuracy, as at LO proof not provided yet sss UPS Lavesson, Lönnblad arxiv: Lönnblad, Prestel arxiv: generalisation of CKKW-L to accuracy, proof along same lines merges PS (UPS) for each multiplicity Marek Schönherr 2/28

30 Examples pp t t+jets Light jet transverse momenta dσ/dpt [pb/gev] MEPS@.65 MEPS@LO S-MC@ Sherpa+OpenLoops 3rd jet st jet 2nd jet Ratio to MEPS@ Ratio to MEPS@ Ratio to MEPS@ st jet 2nd jet 3rd jet pt (light jet) [GeV] Shapes are generally stable Uncertainties are much smaller p T (light jet) [GeV] Marek Schönherr 22/28

31 Examples FxFx Frederix, Frixione arxiv: pp t t + jets pp h + jets Marek Schönherr 23/28

32 Examples UPS Lönnblad, Prestel arxiv: Pythia8 POWHEG W+jet, cc UPS t MS=5 GeV, cc UPS t MS=30 GeV, cc UPS t MS=45 GeV, cc Pythia8 UPS (no K-factor) t MS=5 GeV, cc UPS (no K-factor) t MS=30 GeV, cc UPS (no K-factor) t MS=45 GeV, cc d /d p [mb/gev] d /d p [mb/gev] Deviation [%] POWHEG W+jet t MS= GeV, ll t MS= GeV, cc t MS= GeV, hh Deviation [%] POWHEG H+jet t MS= GeV, ll t MS= GeV, cc t MS= GeV, hh p [GeV] pp W + jets p [GeV] pp h + jets Marek Schönherr 24/28

33 Examples Use of state-of-the-art generators ATLAS arxiv: σ / σ tot 0 ATLAS pp H data, tot. unc. syst. unc. s NPS+PY8 + XH MG5_aMC@+PY8 + XH SHERPA XH XH = VBF + VH - = 8 TeV, 20.3 fb anti-k t R jet = 0.4, p T + tth + bbh > 30 GeV [/GeV] H T /σ dσ / dp 2 0 s HRES + XH NPS+PY8 + XH MG5_aMC@+PY8 + XH SHERPA XH XH = VBF + VH data, tot. unc. - = 8 TeV, 20.3 fb ATLAS pp H + tth + bbh syst. unc. 3 0 Ratio to NPS = 0 = = 2 3 N jets Ratio to HRes ph T [GeV] Marek Schönherr 25/28

34 Examples Conclusion there has been tremendous progress in increasing the theoretical precision in the perturbative description increasingly precise calculation of observables senstive to large momentum transfers (high p, m inv, etc) theoretical precision in describing observables sensitive to small scales remains basically unchanged: many described by phenomenolgical models with unclear uncertainties central value nonetheless gives good description well-tuned models with enough parameters Marek Schönherr 26/28

35 Examples Stress testing the new prediction Approaches aim at describing all observables of one process class with one calculation. As the uncertainty bands collaps with the recent developments, the limits of the inherent approximations need to be tested. measure observables that are sensitive to different characteristic scales simultaneously examples: select events with 3 comparably hard jets in Mercedes star configuration in transverse plane, where does a much softer fourth jet go? select event with l + l -pair and 3 jets of comparable energy in tetrahedral configuration, where is the additional hadronic activity concentrated select events with l + l -pair and 2 hard back-to-back jets, measure R between jet and l + l -pair also new challenges for experiments Marek Schönherr 27/28

36 Examples Thank you for your attention! Marek Schönherr 28/28

arxiv: v1 [hep-ph] 12 Dec 2015

arxiv: v1 [hep-ph] 12 Dec 2015 EPJ Web of Conferences will be set by the publisher DOI: will be set by the publisher c Owned by the authors, published by EDP Sciences, 208 arxiv:52.03974v [hep-ph] 2 Dec 205 Recent developments in Monte-Carlo