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Carleton U, May 2007. Pythia Mini-Tutorial 2 Advanced Topics. Peter Skands Fermilab / Particle Physics Division / Theoretical Physics. Sj ö strand, PS : NPB659(2003)243, JHEP03(2004)053, EPJC39(2005)129. Overview. Matching When needed? MLM, CKKW, MC@NLO, and all that The Underlying Event - PowerPoint PPT Presentation
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Carleton U, May 2007
Pythia Mini-Tutorial 2Advanced Topics
Peter Skands
Fermilab / Particle Physics Division / Theoretical Physics
Sjöstrand, PS : NPB659(2003)243, JHEP03(2004)053, EPJC39(2005)129
Peter Skands Event Generator Status 2
OverviewOverview► Matching
• When needed?
• MLM, CKKW, MC@NLO, and all that
► The Underlying Event
• Models• The ‘Old’ PYTHIA model (e.g. Tune A): impact parameter dependence• The ‘Intermediate’ PYTHIA model: fancy beam remnants• The ‘New’ PYTHIA model: interleaved evolution• Not an expert on HERWIG/JIMMY, but I can still tell you as much as I know• What is still not there?
• Tuning
• Early constraints from LHC
► The Event Generator Outlook
• The move to C++
Matching
Fixed Order Matrix Elements and Parton Shower Resummations
Peter Skands Event Generator Status 4
A ProblemA Problem►The best of both worlds? We want:
• A description which accurately predicts hard additional jets
• + jet structure and the effects of multiple soft emissions
►How to do it? • Compute emission rates by parton showering (PS)?
• Misses relevant terms for hard jets, rates only correct for strongly ordered emissions pT1 >> pT2 >> pT3 ...
• (common misconception that showers are soft, but that need not be the case. They can err on either side of the right answer.)
• Unknown contributions from higher logarithmic orders
• Compute emission rates with matrix elements (ME)?• Misses relevant terms for soft/collinear emissions, rates only correct for
well-separated individual partons• Quickly becomes intractable beyond one loop and a handfull of legs• Unknown contributions from higher fixed orders
Peter Skands Event Generator Status 5
Double CountingDouble Counting► Combine different multiplicites inclusive sample?
► In practice – Combine
1. [X]ME + showering
2. [X + 1 jet]ME + showering
3. …
► Double Counting:
• [X]ME + showering produces some X + jet configurations• The result is X + jet in the shower approximation
• If we now add the complete [X + jet]ME as well• the total rate of X+jet is now approximate + exact ~ double !!
• some configurations are generated twice.
• and the total inclusive cross section is also not well defined
► When going to X, X+j, X+2j, X+3j, etc, this problem gets worse
X inclusiveX inclusive
X+1 inclusiveX+1 inclusive
X+2 inclusiveX+2 inclusive ≠X exclusiveX exclusive
X+1 exclusiveX+1 exclusive
X+2 inclusiveX+2 inclusive
Peter Skands Event Generator Status 6
Evolution
MatchingMatching► Matching of up to one hard additional jet, for specific processes
• PYTHIA-style (reweight shower: MEX+jet = w*PS)
• HERWIG-style (add separate X+jet events: w = MEX+jet-PS)
• MC@NLO-style (ME-PS subtraction similar to HERWIG, but NLO)
► Matching of generic (multijet) topologies (at tree level)
• ALPGEN-style (MLM)
• SHERPA-style (CKKW)
• ARIADNE-style (Lönnblad-CKKW)
• PATRIOT-style (Mrenna & Richardson)
► Brand new approaches (still in the oven)
• Refinements of MC@NLO (Frixione, Nason, Oleari)
• CKKW-style at NLO + “Quantum Monte Carlo” (Nagy, Soper)
• SCET approach (based on SCET – Bauer, Tackmann; Alwall, Mrenna, Schwarz)
• VINCIA (based on QCD antennae – Giele, Kosower, PS)
ME: Matrix ElementPS: Parton Shower
Peter Skands Event Generator Status 7
MC@NLOMC@NLO
Nason’s approach:
Generate 1st shower emission separately easier matching
Avoid negative weights + explicit study of ZZ production
Frixione, Nason, Webber, JHEP 0206(2002)029 and 0308(2003)007
JHEP 0411(2004)040
JHEP 0608(2006)077
► MC@NLO in comparison• Superior precision for total cross section• Equivalent to tree-level matching for event shapes (differences higher order)• Inferior to multi-jet matching for multijet topologies• So far has been using HERWIG parton shower complicated subtractions
Peter Skands Event Generator Status 8
S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063
SHERPA and ARIADNESHERPA and ARIADNE► The CKKW algorithm
• Slices phase space – two regions:• uses matrix elements to describe the distribution of particles with a
phase-space separation pT > pTcut • uses parton showers to describe particles with a smaller separation
1. [W]ME |pT>pTcut * Wveto(pTcut) + showeringpT<pTcut
2. [W + j]ME|pT>pTcut * Wveto(pTcut) + showeringpT<pTcut
3. …
• Where Wveto < 1 is there to get rid of the double counting
• Wveto is : the probability that no parton shower emission happened above pTcut (this is called the Sudakov factor, the no-emission probability)
• SHERPA uses an approximate analytical formula
• Lönnblad’s ARIADNE-style is to run a ‘trial’ or ‘pseudo’ shower, vetoing those events which branch above pTcut
► This gets rid of double counting since those events that would have caused it are precisely those which do branch above pTcut
L. L¨onnblad, JHEP05 (2002) 046
Peter Skands Event Generator Status 9
Matched Mix of W+0,1,2,3,4 jetsMatched Mix of W+0,1,2,3,4 jets
S. Mrenna, P. Richardson, JHEP0405 (2004) 040
► Matching can also be done with PYTHIA, HERWIG, but so far not automated
Peter Skands Event Generator Status 10
ALPGENALPGEN► “MLM” matching (Mangano)
• Simpler but similar in spirit to CKKW
► First generate events the “stupid” way:
1. [W]ME + showering
2. [W + jet]ME + showering
3. …
► a set of fully showered events, with double counting. To get rid of the excess, accept/reject each event based on:
• (cone-)cluster showered event njets
• match partons from the ME to the clustered jets
• If all partons are matched, keep event. Else discard it.
► Roughly equivalent to the pseudoshower approach above
• Virtue: can be done without knowledge of the internal workings of the generator. Only the fully showered final events are needed
Peter Skands Event Generator Status 11
•MC@NLO: •Used to think it was impossible! •But complicated much work needed for each process •“Only” gets first jet right (rest is PS) •Hardwired to HERWIG
•CKKW & MLM: •Best approach when multiple hard jets important.•Relatively straightforward (but still very time-consuming)•Retains LO normalization •Dependence on matching scale
• All constructed to use existing showers (HW or PY) hard to trace analytically•Not easy to control theoretical uncertainty on exponentiated part
MC@
NLO
MLM
CKKW
New Approaches – Why Bother?New Approaches – Why Bother?
For more, see theory seminar tomorrow
The Underlying Event
Towards a complete picture of hadron collisions
Peter Skands Event Generator Status 13
► Domain of fixed order and parton shower calculations: hard partonic scattering, and bremsstrahlung associated with it.
► But hadrons are not elementary
► + QCD diverges at low pT
► multiple perturbative parton-parton collisions should occur
► Normally omitted in explicit perturbative expansions
► + Remnants from the incoming beams
► + additional (non-perturbative / collective) phenomena?• Bose-Einstein Correlations• Non-perturbative gluon exchanges / colour reconnections ?• String-string interactions / collective multi-string effects ?• Interactions with “background” vacuum / with remnants / with active
medium?
e.g. 44, 3 3, 32
Additional Sources of Particle ProductionAdditional Sources of Particle Production
Peter Skands Event Generator Status 14
Classic Example: Number of tracksClassic Example: Number of tracksUA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias
events
Simple physics models ~ Poisson
Can ‘tune’ to get average right, but
much too small fluctuations
inadequate physics model
More Physics:
Multiple interactions +
impact-parameter
dependenceMorale (will return to the models later):
1) It is not possible to ‘tune’ anything better than the underlying physics model allows
2) Failure of a physically motivated model usually points to more physics
Peter Skands Event Generator Status 15
Multiple Interactions Multiple Interactions Balancing Minijets Balancing Minijets
► Look for additional balancing jet pairs “under” the hard interaction.
► Several studies performed, most recently by Rick Field at CDF ‘lumpiness’ in the underlying event.
(Run I)
angle between 2 ‘best-balancing’ pairs
CDF, PRD 56 (1997) 3811
Peter Skands Event Generator Status 16
Basic PhysicsBasic Physics► Sjöstrand and van Zijl (1987):
• First serious model for the underlying event
• Based on resummation of perturbative QCD 22 scatterings at successively smaller scales multiple parton-parton interactions
• Dependence on impact parameter crucial to explain Nch distributions.
• Peripheral collisions little matter overlap few interactions. Central collisions many
• Nch Poissonian for each impact parameter convolution with impact parameter profile wider than Poissonian!
• Colour correlations also essential• Determine between which partons
hadronizing strings form (each string log(mstring) hadrons)
• Important ambiguity: what determines how strings form between the different interactions?
UA5
Nch
540 GeV
Peter Skands Event Generator Status 17
Underlying Event and ColourUnderlying Event and Colour► In PYTHIA (up to 6.2), some “theoretically sensible” default values for the
colour correlation parameters had been chosen
• Rick Field (CDF) noted that the default model produced too soft charged-particle spectra.
• (The same is seen at RHIC:)
• For ‘Tune A’ etc, Rick noted that <pT> increased when he increased the colour correlation parameters
• Virtually all ‘tunes’ now used by the Tevatron and LHC experiments employ these more ‘extreme’ correlations
• Tune A, and hence its more extreme colour correlations are now the default in PYTHIA
M. Heinz (STAR), nucl-ex/0606020; nucl-ex/0607033
STAR
pp @ 200GeV
(will return to this tomorrow…)
Peter Skands Event Generator Status 18
The ‘Intermediate’ ModelThe ‘Intermediate’ Model► Meanwhile in Lund: Sjöstrand and PS (2003):
• Further developments on the multiple-interactions idea
• First serious attempt at constructing multi-parton densitities• If sea quark kicked out, “companion” antiquark introduced in remnant (distribution
derived from gluon PDF and gluon splitting kernel)
• If valence quark kicked out, remaining valence content reduced
• Introduction of “string junctions” to represent beam baryon number• Detailed hadronization model for junction fragmentation can address baryon
number flow separately from valence quarks
Sjöstrand & PS : Nucl.Phys.B659(2003)243, JHEP03(2004)053
Peter Skands Event Generator Status 19
The ‘New’ ModelThe ‘New’ Model► Sjöstrand and PS (2005):
• ‘Interleaved’ evolution of multiple interactions and parton showers
Sjöstrand & PS : JHEP03(2004)053, EPJC39(2005)129
multipartonPDFs derivedfrom sum rules
Beam remnantsFermi motion / primordial kT
Fixed ordermatrix elements
pT-orderedparton shower(matched to MEfor W/Z/H/G + jet)
perturbative “intertwining”?
NB: Tune A still default since more thoroughly tested. To use new models, see e.g. PYTUNE (Pythia6.408+)
Peter Skands Event Generator Status 20
A First StudyA First Study► Using Tevatron min-bias as constraint
• Those were the distributions that started it all
• High-multiplicity tail should be somewhat similar to top less extrapolation required
• Why not use LEP? Again, since the extrapolation might not be valid.• No UE in ee, no beam remnants, less strings, no ‘bags’ in initial state.
• The comparison would still be interesting and should be included in a future study
► As a baseline, all models were tuned to describe Nch and <pT>(Nch)
No CR
Field’s Tunes &
new models
► Improved Description of Min-Bias
► Effect Still largely uncertain
► Worthwhile to look at top etc
Tevatron Run II min-bias
PYTHIA 6.408 PYTHIA 6.408
Peter Skands Event Generator Status 21
(Beam Remnants and Multiple Interactions)(Beam Remnants and Multiple Interactions)
The Generator Outlook
The C++ Monte Carlos
Peter Skands Event Generator Status 23
C++ PlayersC++ Players► HERWIG++: complete reimplementation
• Improved parton shower and decay algorithms
• Eventually to include CKKW-style matching (?)
• B.R. Webber; S. Gieseke, D. Grellscheid, A. Ribon, P. Richardson, M. Seymour, P. Stephens, . . .
► SHERPA: complete implementation, has CKKW• ME generator + wrappers to / adaptations of PYTHIA, HERWIG parton
showers, underlying event, hadronization
• F. Krauss; T. Fischer, T. Gleisberg, S. Hoeche, T. Laubrich, A. Schaelicke, S. Schumann, C. Semmling, J. Winter
► PYTHIA8: selective reimplementation• Improved parton shower and underlying event, limited number of hard
subprocesses
• Many obsolete features not carried over simpler, less parameters
• T. Sjöstrand, S. Mrenna, P. Skands
► (+ various more specialized packages)
Peter Skands Event Generator Status 24
PYTHIA 8PYTHIA 8
Basic generator already there
Includes a few processes (+ full Pythia6 library), new pT-ordered showers, new UE, Les Houches interfaces, and more
You are invited to try it out
Click /future/ on the Pythia homepage, download pythia8080.tgz, follow instructions in readme (./configure, ./make, and have fun)
Still not advised for production runs
If you have suggestions, now is the time!
Timeline:
Spring 2007: QED showers, LHAPDF, interleaved FSR, beam remnants, colour reconnections useful
Fall-Winter 2007: resonance decays, GUI, official release?
Peter Skands Event Generator Status 25
The Generator OutlookThe Generator Outlook► Generators in state of continuous development:
► Better & more user-friendly general-purpose matrix element calculators+integrators
► Improved parton showers and improved matching to matrix elements
► Improved models for underlying events / minimum bias
► Upgrades of hadronization and decays
► Moving to C++
► Data needed to constrain models & rule out crazy ideas• New methods could QCD become a precision science?
► Important for virtually all other measurements + can shed light on fundamental & interesting aspects of QCD (e.g. string interactions)