Winter Workshop on Recent QCD Advances at the LHC, Les Houches, F PYTHIA (6 & 8) versus pp data at the LHC Peter Skands (CERN)
QCD in PYTHIA Multiple Parton Interactions (MPI) Regularise cross section with p ⊥ 0 as free parameter ∝ α 2 s ( p 2 → α 2 s ( p 2 ⊥ 0 + p 2 ⊥ ) ⊥ ) dˆ σ IR Regularization d p 2 p 4 ( p 2 ⊥ 0 + p 2 ⊥ ) 2 ⊥ ⊥ with energy dependence � � � E CM p ⊥ 0 ( E CM ) = p ref ⊥ 0 × Energy Scaling E ref CM Matter profile in impact-parameter space See, e.g., new MCnet Review: “General-purpose event generators for LHC physics” , arXiv:1101.2599 From Tevatron to LHC E.g., Rick Field Tevatron tunes appear to be “low” on LHC data Problem for “global” tunes. Poor man’s short-term solution: dedicated LHC tunes 2 P. Skands
Tunes of PYTHIA 8 Tuning PYTHIA 8 and 4C, see: Corke, Sjöstrand, arXiv:1011.1759 Hadron Collisions: can not use P YTHIA 6 tunes (e.g., not “Perugia”, Z1, etc) . Need P YTHIA 8 ones. Tension between Tevatron and LHC? 900 GeV 1960 GeV 7000 GeV (Plots from mcplots.cern.ch) 3 P. Skands
Tuning vs Testing Models Evolution of PARP(83) with √ s Pythia 6 TEST models PARP(83) 7 TeV PARP(83) 2 Gauss Tune parameters in several Perugia 0 900 GeV 1800 & complementary regions 1960 GeV 1.5 630 GeV Consistent model → same 1 Exponential parameters Transverse Mass 0.5 Distribution Model breakdown → non- 0 universal parameters 10 3 √ s / GeV √ Evolution of PARP(82) with √ s Evolution of PARP(78) with √ s Pythia 6 Pythia 6 3 PARP(82) PARP(78) PARP(82) PARP(78) 0.5 Perugia 0 Exp=0.25 2.5 7 TeV 630 GeV 1800 & 1960 GeV dˆ p 4 ∝ σ 0.4 ⊥ � d p 2 2 � 900 GeV ⊥ Perugia 0 E CM � 630 GeV pendence E ref 0.3 ⊥ 0 × p ⊥ 0 ( E CM ) = p ref 900 GeV CM 1.5 1800 & 7 TeV 1960 GeV erlap which determines level of in impact-parameter space 0.2 1 Color Reconnection IR Regularization ariants 0.1 Strength 0.5 PDF 0 0 10 3 10 3 √ s / GeV √ s / GeV √ √ “Energy Scaling of MB Tunes”, H. Schulz + PS, in preparation 4 P. Skands
Nota Bene Crucial Task for run at 2.8 TeV Make systematic studies to map/ resolve Tevatron/LHC tension Measure regions that interpolate between Tevatron and LHC E.g., start from same phase-space region as CDF | η | < 1.0 pT > 0.4 GeV 5 P. Skands
Diffraction 100 Diffractive Cross Section Formulæ: Pythia 8.130 d t d M 16 π M Pythia 6.414 SD d σ sd( AX ) ( s ) g 3I 1 P 16 π β 2 = M 2 exp( B sd( AX ) t ) F sd , 10 P β B I Phojet 1.12 P A I d t d M 2 g 2 d σ dd ( s ) 1 1 3I P = exp( B dd t ) F dd . 16 π β A I P β B I P 1 d t d M 2 1 d M 2 M 2 M 2 2 1 2 0.1 Partonic Substructure in Pomeron: p i � p i 0.01 Follows the LRG x Ingelman-Schlein 0.001 approach of x g Pompyt X 0.0001 p j 0 2 4 6 8 10 pT (GeV) � M X ≤ 10 GeV : original longitudinal string description used � M X > 10 GeV : new perturbative description used PYTHIA 8 to I Pp ha (incl full MPI+showers for system) 4) Choice between 5 Pomeron PDFs. n showers Four parameterisations of the pomeron flux available Free parameter σ I Pp needed to fix � n interactions � = σ jet / σ I Pp . 4) Choice between 5 Pomeron PDFs. Free parameter needed to fix 5) Framework needs testing and tuning, e.g. of . 5) Framework needs testing and tuning, e.g. of σ I Pp . Navin, arXiv:1005.3894 6 P. Skands
Diffraction Framework needs testing and tuning E.g., interplay between non-diffractive and diffractive components + LEP tuning used directly for diffractive modeling Hadronization preceded by shower at LEP, but not in diffraction → dedicated diffraction tuning of fragmentation pars? Study this hump + Room for new models, e.g., KMR (SHERPA) Others? 7 P. Skands
Event Shapes PS,K.Wraight, arXiv:1101.5215 CMS: Transverse Thrust CMS-QCD-10-013, arXiv:1102.0068 CMS-QCD-10-013, arXiv:1102.0068 Matched codes exhibit interesting features away from the data. Inconsistent Matching? Inconsistent to tune without matching? Highlights need to better understand interplay of tuning and matching 8 P. Skands
Jet Shapes Jet shapes ~ shower shapes “Perugia 2010” : used (approximate) CDF jet shape measurements 1- (0.3/R) Vs Jet P � T ATLAS, arXiv:1101.0070 (0.3/R) CDF data (PRD 71, 112002) 0.3 Pythia 6.4 Tune A Pythia 6.4 Tune S0 � 1 - 0.25 Pythia 6.4 Tune PerugiaS0 Pythia 6.4 Tune Perugia10 0.2 0.15 0.1 0.05 0 50 100 150 200 250 300 350 jet P (GeV/c) T 9 P. Skands
Underlying Event Compromise between Tevatron and LHC? “Perugia 2010” : Larger UE at Tevatron → better at LHC PYTHIA 6 Recommended: Perugia 2010 (or dedicated LHC tunes AMBT1, Z1) For more on tuning PYTHIA 6, see PS, arXiv:1005.3457 PYTHIA 6 @ 1.8 TeV PYTHIA 6 @ 7 TeV (next iteration: fusion between Perugia 2010 and AMBT1, Z1?) (Plots from mcplots.cern.ch) 10 P. Skands
Underlying Event PYTHIA 6 Recommended: Perugia 2010 (or dedicated LHC tunes AMBT1, Z1) For more on tuning PYTHIA 6, see PS, arXiv:1005.3457 PYTHIA 6 @ 1.8 TeV PYTHIA 6 @ 7 TeV PYTHIA 8 Recommended: Tune 4C (probably default from next version) (Also has damped diffraction following ATLAS-CONF-2010-048) PYTHIA 8 @ 1.8 TeV PYTHIA 8 @ 7 TeV For more on tuning PYTHIA 8, see Corke, Sjostrand, arXiv:1011.1759 (Plots from mcplots.cern.ch) 11 P. Skands
New Developments in PYTHIA 8 Can choose 2 nd MPI scattering • TwoJets (with TwoBJets as subsample) • PhotonAndJet, TwoPhotons • Charmonium, Bottomonium (colour octet framework) • SingleGmZ, SingleW, GmZAndJet, WAndJet • TopPair, SingleTop See the PYTHIA 8 online documentation, under “A Second Hard Process” Rescattering . . . but Often assume should that also MPI = include Same order in α s , ∼ same propagators, but An explicit model available in PYTHIA 8 • one PDF weight less ⇒ smaller σ one jet less QCD radiation background Corke, Sjöstrand, JHEP 01(2010)035 12 P. Skands
X-Dependent Proton Size Default in PYTHIA (and all other MC * ) * : except DIPSY Factorization of longitudinal and transverse degrees of freedom f(x,b) = f(x) × g(b) OK for inclusive measurements, but: Physics: Shape = delta function at 0 for x → 1 Can also be seen in lattice studies at high x Gribov theory: high s ↔ low x ⇒ Growth of total cross section ↔ size grows ∝ ln(1/x) BFKL “intuition”: “random walk” in x from few high-x partons at small b diffuse to larger b at smaller x (More formal: Balitsky/JIMWLK and Color Glass Condensates) A Model for Phenomenological Studies Corke, Sjöstrand, arXiv:1101.5953 Basic assumption: Mass distribution = Gaussian. Make width x-dependent � 1 + a 1 ln 1 � − r 2 1 � � a ( x ) = a 0 ρ ( r, x ) ∝ a 3 ( x ) exp a 2 ( x ) x Constrain by requiring a 1 responsible for growth of cross section 13 P. Skands
Summary PYTHIA6 is winding down Recommended for PYTHIA 6: Global: “Perugia 2010” (MSTP(5)=327) Supported but not developed + LHC MB: “AMBT1” (MSTP(5)=340) Still main option for current run (sigh) + LHC UE “Z1” (MSTP(5)=341) But not after long shutdown 2013! PYTHIA8 is the natural successor Already several improvements over PYTHIA6 on soft physics (including modern range of PDFs (CTEQ6, LO*, etc) in standalone version) Though still a few things not yet carried over (such as ep , some SUSY, etc) If you want new features (e.g., x-dependent proton size, rescattering, ψ ’, VINCIA interfaces, …) then be prepared to use PYTHIA8 MadGraph-5 and Provide Feedback, both what works and what does not Do your own tunes to data and tell outcome Recommended for PYTHIA 8: There is no way back! “Tune 4C” (Tune:pp = 5) 14 P. Skands
Additional Slides Diffraction, Identified Particles, Baryon Transport, Tunes
The Pedestal Effect and Multiple Parton-Parton Interactions M I N I M U M ¡ B I A S σ parton-parton > σ hadron-hadron 5 Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph] 2 σ parton-parton σ hadron-hadron 1 p T P . Skands 16
The Pedestal Effect and Multiple Parton-Parton Interactions M I N I M U M ¡ B I A S σ parton-parton > σ hadron-hadron 5 2 + P E R I P H E R A L C E N T R A L < M P I > ¡ = ¡ 1 < M P I > ¡ = ¡ 3 5 1 2 1 p T < M P I > ¡ = ¡ 6 ¡ / ¡ 4 ¡ = ¡ 1 . 5 P . Skands 17
The Pedestal Effect and Multiple Parton-Parton Interactions J E T ¡ > ¡ 5 ¡ G e V 5 Statistically biases the selection towards 2 more central events + P E R I P H E R A L C E N T R A L with more MPI < M P I > ¡ = ¡ 1 < M P I > ¡ = ¡ 3 The assumed shape of the proton affects the rise and 5 1 <UE>/<MB> 2 1 < M P I > ¡ = ¡ 4 ¡ / ¡ 2 ¡ = ¡ 2 P . Skands 18
Tuning of PYTHIA 8 Tuning to e+e- closely related to p ⊥ -ordered PYTHIA 6.4. A few iterations already. First tuning by Professor (Hoeth) → FSR ok? Out-of- plane C Parameter pT (Plots from mcplots.cern.ch) 19 P. Skands
(Identified Particles) Interesting discrepancies in strange sector + problems with Λ /K and s spectra also at LEP? Grows worse (?) for multi-strange baryons Flood of LHC data now coming in! Interesting to do systematic LHC vs LEP studies 20 P. Skands
PYTHIA 8 Tune Parameters 21 P. Skands
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