NLO merging in t t+jets 1

Transcription

1 merging in t t+jets Institute for Particle Physics Phenomenology EPS 2013, Stockholm, 18/07/2013 arxiv: in collaboration with S. Höche, J. Huang, G. Luisoni, J. Winter merging in t t+jets 1

2 Contents 1 MEPS@ Introduction of the method to merge MC@ for p p t t, p p t t + 1j, p p t t + 2j, etc. used in SHERPA. More details in tomorrow s talk at 09:25, QCD session. 2 Results Results for top pair production at the Tevatron, with emphasis on the forward-backward asymmetry. 3 Conclusions merging in t t+jets 2

3 Method PS 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 PS keeping either accuracy PS elevate PS to accuracy MEPS supplements core PS with higher multiplicities PS merging in t t+jets 3

4 Method ME 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 PS keeping either accuracy PS elevate PS to accuracy MEPS supplements core PS with higher multiplicities PS merging in t t+jets 3

5 Method ME PS MEPS (CKKW,MLM) Catani, Krauss, Kuhn, Webber JHEP11(2001)063 Lönnblad JHEP05(2002)046 Alwall et.al. EPJC53(2008) 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 PS keeping either accuracy PS elevate PS to accuracy MEPS supplements core PS with higher multiplicities PS merging in t t+jets 3

6 Method PS PS Frixione, Webber JHEP06(2002)029 Nason JHEP11(2004)040, Frixione et.al. JHEP11(2007)070 Höche, Krauss, MS, Siegert JHEP09(2012)049 matrix elements (ME) and parton showers (PS) are approximations in different regions of phase space MEPS combines multiple PS keeping either accuracy PS elevate PS to accuracy MEPS supplements core PS with higher multiplicities PS merging in t t+jets 3

7 Method ME PS MEPS Hamilton, Nason JHEP06(2010)039 Höche, Krauss, MS, Siegert JHEP08(2011)123 Höche, Krauss, MS, Siegert JHEP01(2013)144 matrix elements (ME) and parton showers (PS) are approximations in different regions of phase space MEPS combines multiple PS keeping either accuracy PS elevate PS to accuracy MEPS supplements core PS with higher multiplicities PS merging in t t+jets 3

8 Method ME PS Lavesson, Lönnblad JHEP12(2008)070 Höche, Krauss, MS, Siegert JHEP04(2013)027 Höche, Krauss, MS, Siegert JHEP01(2013)144 matrix elements (ME) and parton showers (PS) are approximations in different regions of phase space MEPS combines multiple PS keeping either accuracy PS elevate PS to accuracy MEPS supplements core PS with higher multiplicities PS combines multiple PS keeping either accuracy merging in t t+jets 3

9 Method ME PS Lavesson, Lönnblad JHEP12(2008)070 Höche, Krauss, MS, Siegert JHEP04(2013)027 Höche, Krauss, MS, Siegert JHEP01(2013)144 matrix elements (ME) and parton showers (PS) are approximations in different regions of phase space MEPS combines multiple PS keeping either accuracy PS elevate PS to accuracy MEPS supplements core PS with higher multiplicities PS combines multiple PS keeping either accuracy merging in t t+jets 3

10 Contents 1 MEPS@ Introduction of the method to merge MC@ for p p t t, p p t t + 1j, p p t t + 2j, etc. used in SHERPA. More details in tomorrow s talk at 09:25, QCD session. 2 Results Results for top pair production at the Tevatron, with emphasis on the forward-backward asymmetry. 3 Conclusions merging in t t+jets 4

11 Results p p t t + jets A FB Parton showers and the t t-asymmetry at the Tevatron Skands, Webber, Winter JHEP07(2012)151 if colour coherence is respected, PS creates an asymmetry because of asymmetric colour flow HERWIG respects colour correlations through angular ordering (a) q _ t t _ q q t (b) _ q CSSHOWER++ (CS dipoles, 1 2 splittings, recoil to large-n c partner) respects colour correlations by choice of radiating dipoles/recoil partners it is important to respect colour-correlations _ t merging in t t+jets 5

12 Results p p t t + jets A FB Definition of forward-backward asymmetry of an observable O A FB (O) = dσ t t do y>0 dσ t t + do y>0 dσ t t do y<0 dσ t t do y<0 A FB is ratio of expectation values conventional scale variations by factor 2 will largely cancel for uncertainty on A FB use different functional forms of the scale defintion that behave differently in y > 0 and y < 0 for a realistic estimate of uncertainty merging in t t+jets 6

13 Results p p t t + jets A FB Setup: p p t t+ jets purely perturbative calculation (no hadronisation, MPI, etc.) 0,1 Q cut = 7 GeV perturbative scale variations µ R/F [ 1 2, 2] µ def µ Q [ 1 2, 2] µ core variation of merging parameter Q cut {5, 7, 10} GeV Höche, Huang, Luisoni, MS, Winter arxiv: dσ/dp,t [pb/gev] Transverse momentum of the top quark Sherpa+GoSam Meps@Nlo µcore = m t t Meps@Lo µcore = m t t p,t [GeV] scale choices: α k+n s (µ eff ) = α k s(µ) α s (t 1 ) α s (t n ) 1) µ core = m t t 2) µ core = µ QCD = 2 p ip j i, j... N c colour partners, chooses between s, t, u different behaviour for forward/backward configurations merging in t t+jets 7

14 Results p p t t + jets A FB Setup: p p t t+ jets purely perturbative calculation (no hadronisation, MPI, etc.) 0,1 Q cut = 7 GeV perturbative scale variations µ R/F [ 1 2, 2] µ def µ Q [ 1 2, 2] µ core variation of merging parameter Q cut {5, 7, 10} GeV Höche, Huang, Luisoni, MS, Winter arxiv: dσ/dp,t [pb/gev] Transverse momentum of the top quark Sherpa+GoSam Meps@Nlo µcore = µ QCD Meps@Lo µcore = µ QCD p,t [GeV] scale choices: α k+n s (µ eff ) = α k s(µ) α s (t 1 ) α s (t n ) 1) µ core = m t t 2) µ core = µ QCD = 2 p ip j i, j... N c colour partners, chooses between s, t, u different behaviour for forward/backward configurations merging in t t+jets 7

15 Results p p t t + jets A FB Setup: p p t t+ jets purely perturbative calculation (no hadronisation, MPI, etc.) 0,1 Q cut = 7 GeV perturbative scale variations µ R/F [ 1 2, 2] µ def µ Q [ 1 2, 2] µ core variation of merging parameter Q cut {5, 7, 10} GeV Höche, Huang, Luisoni, MS, Winter arxiv: dσ/dp,t [pb/gev] Transverse momentum of the top quark Sherpa+GoSam Meps@Nlo µcore = µ QCD Meps@Nlo µcore = m t t Meps@Lo µcore = µ QCD Meps@Lo µcore = m t t p,t [GeV] scale choices: α k+n s (µ eff ) = α k s(µ) α s (t 1 ) α s (t n ) 1) µ core = m t t 2) µ core = µ QCD = 2 p ip j i, j... N c colour partners, chooses between s, t, u different behaviour for forward/backward configurations merging in t t+jets 7

16 Results p p t t + jets A FB dσ/dlog(p T,t t /GeV) [pb] PS / MC@ Transverse momentum of top-quark pair Sherpa+GoSam MC@, H (A) = 0 µ = 1/ kt PS, B B µ = 1/ kt log(p T,t t /GeV) Höche, Huang, Luisoni, MS, Winter arxiv: Importance of N c = 3 colour coherence (SHERPA s MC@) vs. N c colour coherence (SHERPA s CSSHOWER++) small effect on standard (rapidity blind) observables, e.g. p,t t some destructive interference at large p,t t large effect on A F B (p,t t) subleading colour terms increase asym. radiation pattern merging in t t+jets 8

17 Results p p t t + jets A FB AFB(p T,t t ) Transverse momentum dependent forward-backward asymmetry MC@, H (A) = 0 µ = 1/ kt PS, B B µ = 1/ kt Höche, Huang, Luisoni, MS, Winter arxiv: Importance of N c = 3 colour coherence (SHERPA s MC@) vs. N c colour coherence (SHERPA s CSSHOWER++) Sherpa+GoSam p T,t t [GeV] small effect on standard (rapidity blind) observables, e.g. p,t t some destructive interference at large p,t t large effect on A F B (p,t t) subleading colour terms increase asym. radiation pattern merging in t t+jets 8

18 Results Conclusions Results p p t t + jets A FB Höche, Huang, Luisoni, MS, Winter arxiv: CDF data Phys.Rev.D87(2013) AFB(p T,t t ) Transverse momentum dependent forward-backward asymmetry Reconstructed top Sherpa+GoSam Meps@Nlo µcore = µ QCD perturbative uncertainty Meps@Nlo µcore = m t t perturbative uncertainty CDF data Phys. Rev. D87 (2013) p T,t t [GeV] p p t t+jets ) A F B (p,t t) accurate in all but the first bin tops reconstructed from decay products (jets, lepton, MET) no EW corrections merging in t t+jets 9

19 Results Conclusions Results p p t t + jets A FB Höche, Huang, Luisoni, MS, Winter arxiv: CDF data Phys.Rev.D87(2013) AFB( y,t t ) Rapidity dependent forward-backward asymmetry Parton level CDF data Phys. Rev. D87 (2013) Meps@Nlo µcore = µ QCD perturbative uncertainty Meps@Nlo µcore = m t t perturbative uncertainty AFB(m t t ) Mass dependent forward-backward asymmetry Parton level CDF data Phys. Rev. D87 (2013) Meps@Nlo µcore = µ QCD perturbative uncertainty Meps@Nlo µcore = m t t perturbative uncertainty Sherpa+GoSam y,t t parton level (exact top quarks) no EW corrections ( 20%) effected right qualitative bahviour, but consistently below data 0.1 Sherpa+GoSam m t t [GeV] merging in t t+jets 10

20 Results p p t t + jets A FB Inclusive asymmetries on parton level Höche, Huang, Luisoni, MS, Winter arxiv: CDF data Phys.Rev.D87(2013) Source A FB [%] A FB (m t t) [%] A FB (p T,t t) [%] inclusive m < 450 GeV m > 450 GeV p T < 50 GeV p T > 50 GeV CDF data 16.4 ± ± ± 6.7 MEPS@, µ = µ QCD MEPS@, µ = m t t MEPS, µ = µ QCD MEPS, µ = m t t p p t t merging in t t+jets 11

21 Conclusions MEPS merging methods have evolved to : MEPS@ taking colour correlations properly into account already produces an asymmetry in parton showers asymmetry increases when parton shower includes subleading colour terms decreasing uncertainties PS/MEPS/PS MEPS@ uncertainties on asymmetry observables should be evaluated by choosing different functional forms with different properties towards the asymmetry can be improved by adding higher order calculations (N)NLL resummation N corrections Bärnreuther, Czakon, Mitov Phys.Rev.Lett.109(2012) Czakon, Mitov JHEP01(2013)080 Czakon, Fiedler, Mitov arxiv: current release SHERPA-2.0.β 2, when fully tuned SHERPA merging in t t+jets 12

22 Thank you for your attention! merging in t t+jets 13

23 Results p p t t + jets dσ/dlog(p T,t t /GeV) [pb] Qcut / 7 GeV Transverse momentum of the top-quark pair Sherpa+GoSam Qcut = 5 GeV Qcut = 7 GeV Qcut = 10 GeV log(p T,t t /GeV) Höche, Huang, Luisoni, MS, Winter arxiv: AFB(p T,t t ) Transverse momentum dependent forward-backward asymmetry Sherpa+GoSam µcore = µ QCD µcore = m t t µcore = µ QCD µcore = m t t p T,t t [GeV] very small Q cut dependence scale variation shrinks going to (both factor and functional form) merging in t t+jets 14