Aspen Winter Conference, January 2006 Peter Skands Matching (who’s doing it, how, and where?)...
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Transcript of Aspen Winter Conference, January 2006 Peter Skands Matching (who’s doing it, how, and where?)...
Aspen Winter Conference, January 2006
Peter Skands
• Matching (who’s doing it, how, and where?)Matching (who’s doing it, how, and where?)
• New ideas and C++New ideas and C++
Event Generator Status
Peter Skands Event Generator Status 2
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
Traditional Event Generators
► Basic aim: improve lowest order perturbation theory by including leading corrections exclusive event samples1. sequential resonance decays
2. parton showers
3. underlying event
4. hadronization
5. hadron and tau decays
Helper: I’ll try to avoid acronyms, but this list will be on all slides and may help when I fail. It is ordered alphabetically.
Peter Skands Event Generator Status 3
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
Modern Event Generators► Specialized tools for calculating higher fixed orders (and
BSM processes) plus matching techniques
hard subprocess (and to some extent resonance decays) increasingly handled by separate codes (LO … NnLO)
Need universal interfaces and standards [e.g. the Les Houches Accords (Les Houches 2007: Jun 11-29, France) ]
► Beyond fixed order
MC4LHC `06: “A standard format for Les Houches Event Files” - hep-ph/0609017
Better understanding of PS uncertainties – À LA ERROR PDF’S?
Improved PS formulations – MORE CONSISTENT, MATCHING TO NnLO, RESUMMATION OF HIGHER LOGS & SMALL-X EFFECTS (BFKL), …
Better understanding of the underlying event and non-perturbative effects - ESPECIALLY IN THE BUSY ENVIRONMENT OFFERED BY LHC
Entering era of precision event generators for hadron colliders
Peter Skands Event Generator Status 4
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
Matching► Matching of up to one hard additional jet
• PYTHIA-style (reweight shower)
• HERWIG-style (add separate events from ME: weight = ME-PS)
• MC@NLO-style (ME-PS subtraction similar to HERWIG, but NLO)
► Matching of generic (multijet) topologies:
• 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 (Nason)
• CKKW-style at NLO (Nagy, Soper)
• SCET approach (based on SCET – Bauer, Schwarz, SEE BAUER’S TALK ON FRIDAY!)
• VINCIA (based on QCD antennae – Giele, Kosower, PS, THIS TALK)
Evolution
Peter Skands Event Generator Status 5
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
ALPGEN► “MLM” matching (Mangano)
• Simpler but similar in spirit to CKKW
► First generate events the “stupid” way:
1. [Wn]ME + showering
2. [Wn+1]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.
► Virtue: can be done without knowledge of the internal workings of the generator. Only the fully showered final events are needed
n inclusiven inclusive
n+1 inclusiven+1 inclusive
n+2 inclusiven+2 inclusive
n exclusiven exclusive
n+1 exclusiven+1 exclusive
n+2 inclusiven+2 inclusive
Peter Skands Event Generator Status 6
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063SHERPA, ARIADNE, PATRIOT
► The CKKW algorithm
• Slices phase space :• ME for pT > pTcut
• PS for pT < pTcut
1. [Wn]ME |pT>pTcut * Wveto(pTcut) + showeringpT<pTcut
2. [Wn+1]ME|pT>pTcut * Wveto(pTcut) + showeringpT<pTcut
3. … [Wnmax] ME|pT>pTcut + showering
• Wveto < 1 is the probability that no parton shower emission happened above pTcut. Computed using clustered ‘parton shower histories’ on the ME final states. SHERPA uses an approximate analytical formula
• Lönnblad’s ARIADNE-style uses a ‘trial’ or ‘pseudo’ shower, vetoing those events which branch above pTcut
► Gets rid of double counting: those events that would have caused it are precisely those which do branch above pTcut
► A final improvement by Mrenna and Richardson was to require physical flavour and colour flows in the ‘preclustering’ step PATRIOT database (and HERWIG++?)
L. L¨onnblad, JHEP05 (2002) 046S. Mrenna, P. Richardson, JHEP 0405(2004)040
Peter Skands Event Generator Status 7
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
MC@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
Peter Skands Event Generator Status 8
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
VINCIA
► VINCIA Dipole shower
• C++ code for gluon showers – running
• Can evolve in either of 2 different shower evolution variables:
• pT-ordering (~ ARIADNE)
• Virtuality-ordering (~ Pythia 6.2 & SHERPA)
• + not hard to generalize to arbitrary IR safe
• For each evolution variable: • an infinite family of radiation functions
implemented, all with correct collinear and soft behaviour (= “antenna functions”)
► First parton shower with systematic possibility of variation of shower variable and shower functon control uncertainties
• (Future plans include also variation of kinematic map)
► To any fixed order, these variations can be absorbed by a new type of matching to matrix elements
Dipoles – a dual description of QCD
1
3
2
virtual numerical collider with interlinked antennae
Giele, Kosower, PS : in progress
Peter Skands Event Generator Status 9
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
► Subtraction method: subtract showering off n-parton state from n+1-parton Matrix Element subtracted (IR finite) matrix elements.
1. Generate parton-level configurations, with weights given by the subtracted Matrix Elements
2. Shower them using the subtraction function from step 1.
► Similar to HERWIG and MC@NLO approaches (with antenna subtraction instead of Catani-Seymour) matching at NLO
► Can simultaneously include arbitrarily many tree-level ME’s multijet matching
• Combines virtue of CKKW (multijet matching) with that of MC@NLO (NLO matching)
• Avoids the vices of CKKW (dependence on unphysical clustering and cuts, LO) and MC@NLO (complicated, not applicable beyond one jet)
VINCIA-style matchingGiele, Kosower, PS : in progress
Peter Skands Event Generator Status 10
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
Hgg: Example
VINCIA 0.008
Unmatched
“soft” |A|2
VINCIA 0.008
Unmatched
“hard” |A|2
First Branching ~ first order in perturbation theoryVINCIA 0.008
Matched
“soft” |A|2
VINCIA 0.008
Matched
“hard” |A|2
Antenna Function
IR singularities plus arbitrary finite terms
|A(sa,sb;s)|2
(a.k.a. Radiation Function, a.k.a. Splitting Kernel)
Systematically improved by matching
Expect public Expect public code and long code and long writeup before writeup before
summersummer
Giele, Kosower, PS : in progress
y12
y23
y23
y23
y23
Later: plug-in for Later: plug-in for Pythia 8 ?Pythia 8 ?
Next: Quarks Next: Quarks and ISR and ISR
hadron collider hadron collider event generatorevent generator
y12
Peter Skands Event Generator Status 11
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
C++ Players► HERWIG++: complete reimplementation
• Improved PS 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
• F. Krauss; T. Fischer, T. Gleisberg, S. Hoeche, T. Laubrich, A. Schaelicke, S. Schumann, C. Semmling, J. Winter
► PYTHIA8: selective reimplementation• Improved PS and UE, limited number of hard subprocesses
• Many obsolete features not carried over simpler, less parameters
• T. Sjöstrand, S. Mrenna, P. Skands
Peter Skands Event Generator Status 12
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
PYTHIA 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 pythia8070.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 13
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event D. B. Leinweber, hep-lat/0004025
Anti-Triplet
Triplet
pbar beam remnant
p beam remnant
bbar
from
tbar
dec
ay
b fro
m t
deca
yq
bar fro
m W
q from W
hadronization
?
q from W
In reality, this all happens on top of each other.
(only possible exception: long-lived colour singlet)
The (QCD) Landscape
Peter Skands Event Generator Status 14
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
New physics in the Underlying Event?
►Given the lack of analytical solutions impacts on hadronic precision observables poorly known
►Example: sensitivity of Tevatron top mass analysis (simplified!) to variations of:
• Colour reconnections • string-string interactions?
• string-vacuum (re)interactions? “colour annealing” model large strength ( >10% ) required
for acceptable fits to min-bias
• Parton showers• Pythia: mass-ordered vs pT-ordered
• Underlying-event parameters• E.g. Tune A vs Tune DW, etc
ΔPS ~ 0.75 GeV
<Δ>UE/CR ~ 0.4 GeV
PS, D. Wicke : preliminary
Δmtop
Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120
Peter Skands Event Generator Status 15
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
Beyond the Standard Model► SUSY Les Houches Accord being extended to RPV,
NMFV, CPV, NMSSM, … SLHA2
• Apologies for slow progress. A skeleton exists, but so far only small number of practical implementations to test viability
• General CPV and NMFV: MSSM implementation for PYTHIA8
• NMSSM:
► Les Houches BSM Tools Repository:
► Recent years more focus on non-SUSY
• 2nd MC4BSM: Princeton, March 21-24 2007 (in conjunction with LHC Olympics)
• Teams of “writers” + teams of “testers”: sign up for the fun!
G. Bozzi, B. Fuks, M. Klasen, PS : in progress
Ellwanger, Hugonie, Moretti, Pukhov, … : in progress
http://www.ippp.dur.ac.uk/montecarlo/BSM
http://www.phys.ufl.edu/~matchev/MC4BSM/
B. Allanach et al, in hep-ph/0602198
Summary of 1st MC4BSM: J. Hubisz, PS, FERMILAB-CONF-05-082-T
PS et al, in hep-ph/0602198
lots of material, e.g. for warped ED: R. Contino et al, hep-ph/0612180
Peter Skands Event Generator Status 16
BR: Beam Remnant
CR: Colour Reconnection
FSR: Final-State Radiation
ISR: Initial-State Radiation
Matching: Combining PS & ME consistently (e.g. CKKW, MLM)
ME: Matrix Element
MI: Multiple parton-partonInteractions(not pile-up)
PS: Parton Shower
PT: Perturbation Theory
Tune: A set of generator parameters
UE: Underlying Event
The Generator Outlook► Generators in state of continuous development:
► Better & more user-friendly general-purpose ME calculators+integrators
► New libraries of physics processes, also to NLO
► Improved parton showers
► Better matching of matrix elements to showers
► Improved models for underlying events / minimum bias
► Upgrades of hadronization and decays
► Moving to C++
more precise, more reliable theoretical predictions
Ultimately, the interesting talk is the experimental one, how good is a good calculation? Compare to LEP, Tevatron, and RHIC data constraints. Absolutely vital for high precision + often uncovers defects, and even hints of new phenomena…