tt+X hadroproduction at NLO+SMC Zoltn Trcsnyi University of - PowerPoint PPT Presentation

− tt+X hadroproduction at NLO+SMC Zoltán Trócsányi University of Debrecen and Institute of Nuclear Research in collaboration with A. Kardos, M.V. Garzelli and HELAC group based on arXiv:1101.2672, 1108.0387 and unpublished Thursday, September 22, 2011

Outline ‣ Motivation ‣ Method ‣ Predictions ‣ Conclusions and Plans Thursday, September 22, 2011

Motivation Thursday, September 22, 2011

The importance of being top 1. The higher collider energy, the larger weight in total cross section Thursday, September 22, 2011

The importance of being top 1. The higher collider energy, the larger weight in total cross section 2. The t-quark is heavy, Yukawa coupling ∼ 1 m t = 172 . 0 ± 0 . 9 ± 1 . 3 (PDG) 173 . 3 ± 1 . 1 (TeVatron) ⇒ plays important role in Higgs physics Thursday, September 22, 2011

The importance of being top 1. The higher collider energy, the larger weight 1. The higher collider energy, the larger weight in total cross section in total cross section 2. The t-quark is heavy, Yukawa coupling ∼ 1 3. The t-quark decays before hadronization ⇒ quantum numbers more accessible than in case of other quarks | V tb | 2 ≫ | V ts | 2 , | V td | 2 Thursday, September 22, 2011

Top at the LHC 1. Present: precision measurement of production cross section, mass ATLAS: arXiv:1109.3912 Thursday, September 22, 2011

Top at the LHC 1. Present: precision measurement of production cross section, mass CMS: arXiv:1108.3773 Thursday, September 22, 2011

Top at the LHC 1. Present: precision measurement of σ tot , m t quantum numbers, decay rates 2. Future: measurement of couplings Baur et al, hep-ph/0412021, 0512262 Thursday, September 22, 2011

Top at the LHC 1. Present: precision measurement of σ tot , m t quantum numbers, decay rates 2. Future: plenty of radiation in association with t-pair σ NLO ( pp → t¯ t) = 806 pb t+jet; p j σ NLO ( pp → t¯ ⊥ > 50 GeV) = 376 pb 3. Important backgrounds to coupling measurements, Higgs searches: − − − − pp → ttj, ttbb, ttjj These require precise predictions for distributions at hadron level (with decays, top is not detected) Thursday, September 22, 2011

Method Thursday, September 22, 2011

NLO subtractions ‣ Idea: exact calculation in the first two orders of pQCD ‣ Subtraction method (FKS in POWHEG-Box) d σ NLO = [ B ( Φ n ) + V ( Φ n ) + R ( Φ n +1 )d Φ rad ] d Φ n = [ B ( Φ n ) + V ( Φ n ) + ( R ( Φ n +1 ) − A ( Φ n +1 )) d Φ rad ] d Φ n � � d σ LO , B ( Φ n ) = V ( Φ n ) = V ( Φ n )+ d Φ rad A ( Φ n +1 ) d Φ rad ∝ d t d z d φ d Φ n +1 = d Φ n d Φ rad , 2 π Thursday, September 22, 2011

From NLO to NLO+PS Idea: use NLO calculation as hard process as input for the SMC Bottleneck: how to avoid double counting of first radiation w.r.to Born process (present both in R and in the PS) Solutions: - MCatNLO [Frixione, Webber hep- ph/0204244] - POWHEG [Nason hep-ph/ 0409146, Frixione, Nason, Oleari arXiv:0709.2092] Result: PS events giving distributions exact to NLO in pQCD Nason, Ridolfi hep-ph/0606275 Thursday, September 22, 2011

From standard SMC to POWHEG MC SMC idea: use probabilistic picture of parton splitting in the collinear approximation, iterate splitting to high orders ‣ Standard MC first emission: � � ∆ SMC ( t 0 ) + ∆ SMC ( t ) α s ( t ) 1 Θ ( t − t 0 ) d Φ SMC d σ SMC = B ( Φ n )d Φ n t P ( z ) rad 2 π � �� � = lim k ⊥ → 0 R ( Φ n +1 ) /B ( Φ n ) ‣ POWHEG MC first emission: � � ⊥ ) + ∆ ( Φ n , k ⊥ ) R ( Φ n +1 ) d σ = ¯ ∆ ( Φ n , p min B ( Φ n ) Θ ( k ⊥ − p min B ( Φ n )d Φ n ⊥ ) d Φ rad � � � ¯ B ( Φ n ) = B ( Φ n ) + V ( Φ n ) + R ( Φ n +1 ) − A ( Φ n +1 ) d Φ rad Thursday, September 22, 2011

From standard SMC to POWHEG MC ‣ SMC Sudakov (probability of no emission with virtuality above t) � � α s ( t ′ ) 1 � d Φ ′ t ′ P ( z ′ ) ∆ SMC ( t ) = exp − rad 2 π t ‣ PMC Sudakov (probability of no emission with transverse momentum above p ⊥ ) � � R ( Φ n , Φ ′ rad ) � d Φ ′ Θ ( k ⊥ ( Φ n , Φ ′ ∆ ( Φ n , p ⊥ ) = exp rad ) − p ⊥ ) − rad B ( Φ n ) Thursday, September 22, 2011

Accuracy of POWHEG MC ‣ The cross section is: � � ¯ ∆ ( Φ n , p min � d σ = B d Φ n ⊥ ) d Φ rad ∆ ( Φ n , k ⊥ ) R ( Φ n +1 ) � � B ( Φ n ) Θ ( k ⊥ − p min + ⊥ ) ‣ PMC Sudakov (probability of no emission with transverse momentum above p ⊥ ) � � R ( Φ n , Φ ′ rad ) � d Φ ′ Θ ( k ⊥ ( Φ n , Φ ′ ∆ ( Φ n , p ⊥ ) = exp rad ) − p ⊥ ) − rad B ( Φ n ) Thursday, September 22, 2011

Accuracy of POWHEG MC ‣ The cross section is: � � � ¯ ∆ ( Φ n , p min d σ = B d Φ n ⊥ )   � d Φ rad ∆ ( Φ n , k ⊥ ) R ( Φ n +1 )  B ( Φ n ) Θ ( k ⊥ − p min + ⊥ )    � �� � 1 − ∆ ( Φ n ,p min ) ⊥ � � d Φ n ¯ d σ = B = σ NLO ‣ We obtained the NLO cross section This can be shown for observables as well, see Frixione, Nason, Oleari arXiv:0709.2092 Thursday, September 22, 2011

Three frameworks ‣ POWHEG-BOX [Alioli et al, 1002.2581] is used to perform the related calculations to generate equal weight events for further showering (black box) ‣ HELAC-NLO [Bevilacqua et al 1007.4918] codes are used to provide squared matrix elements ‣ Standard Shower Monte Carlo [Sjostrand et al, hep-ph/0603175, Corcella et al hep-ph/0210213] (SMC) is used to shower the events RESULT of PowHel (=POWHEG-BOX+HELAC-NLO): Les Houches file of Born and Born+1st radiation events (LHE) ready for processing with SMC followed by almost arbitrary experimental analysis Thursday, September 22, 2011

http://grid.kfki.hu/twiki/bin/view/DbTheory/ WebHome#Events_with_NLO_accuracy_for_par Thursday, September 22, 2011

SMC’s with veto ‣ In POWHEG-Box the first emission is the hardest one measured by transverse momentum ‣ If the ordering variable in the shower is different from the transverse momentum of the parton splitting, such as the angular ordering in HERWIG, then the hardest emission is not necessarily the first one ‣ In such cases the HERWIG discards shower evolutions (vetoed shower) with larger transverse momentum in all splittings occurring after the first emission ‣ In principle, a truncated shower simulating wide-angle soft emission before the first emission is also needed ‣ There is no implementation of truncated shower in HERWIG using external LHE event files, the effect of the truncated showers is absent from our predictions Thursday, September 22, 2011

Input to POWHEG-BOX ‣ Flavour structures, Born phase space ‣ From Helac-OneLoop (in the process of automatization): - Tree-level helicity amplitudes for the Born and real radiation processes (crossed into physical channels from all incoming kinematics) - One-loop corrections to the helicity amplitudes of Born processes (unitarity based numerical evaluation of one- loop amplitudes) - Use polarization vectors to project tree-level helicity- correlated matrix elements to Lorentz basis to get the spin-correlated squared matrix elements ‣ From HELAC-Dipoles: two subroutines for colour- correlated squared matrix elements of the Born processes Thursday, September 22, 2011

Checks ✓ Check (implementation of) real emission squared matrix elements in POWHEG-BOX to those from HELAC-PHEGAS in randomly chosen phase space points ✓ Check (implementation of) virtual correction in POWHEG- BOX to those from HELAC-OneLoop in randomly chosen phase space points ✓ Check the ratio of soft and collinear limits to real emission matrix elements tends to 1 in randomly chosen kinematically degenerate phase space points Thursday, September 22, 2011

Comparison to NLO ✓ Compare LO and NLO cross sections to published predictions ➡ ttZ: Lazopoulos et al, arXiv:0709.4044 ➡ tt γ : Melnikov et al, arXiv:1102.1967 ➡ ttH: Beenakker et al, hep-ph/0107081, 0211352 Reina et al, hep-ph/0107101, 0109066, 0305087 ➡ ttjet: Dittmaier et al, hep-ph/0703120, 0810.0452 ➡ ttbb: Bredenstein et al, arXiv: 0905.0110, 1001.4006 Bevilacqua et al, arXiv:0907.4723 Thursday, September 22, 2011

PowHel-NLO vs. NLO 200 350 √ s = 14TeV 180 (a) POWHEL-NLO NLO √ s = 14TeV 160 POWHEL-LO 300 LO LMMP-NLO MadEvent 140 LMMP-LO 250 d y Z [fb] 120 σ [fb] 200 100 d σ 80 150 m t = 170 . 9GeV MRST2001nlo 60 m t = 170 . 9GeV m Z = 91 . 19GeV 100 40 m Z = 91 . 18GeV µ = m t + m Z / 2 50 µ = m t + m Z / 2 MRST2001 20 0 0 1.5 K-fact K-fact 1.75 1.4 1.3 1.5 1.2 1.25 1.1 1.0 1.0 1.0 -3 -2 -1 0 1 2 3 0 20 40 60 80 100 120 140 160 180 200 y Z p ⊥ , Z [GeV] Transverse momentum & rapidity distributions of the Z 0 -boson − in pp → tt Z at the LHC Thursday, September 22, 2011

PowHel-NLO vs. NLO 1000 2 900 PowHel-NLO PowHel-NLO 10 800 NLO NLO d p ⊥ , t [fb / GeV] 700 5 d y t [fb] 600 2 500 √ s = 14TeV d σ 1 400 √ s = 14TeV d σ 300 m t = 172GeV m t = 172GeV 5 µ = m t 200 µ = m t 2 CTEQ6.6M CTEQ6.6M 100 10 -1 0 1.05 1.05 Ratio Ratio 1.0 1.0 0.95 0.95 -3 -2 -1 0 1 2 3 0 100 200 300 400 500 600 y t p ⊥ , t [GeV] Transverse momentum and rapidity distributions of the t-quark − in pp → tt γ at the LHC (with Frixione-isolation) Thursday, September 22, 2011