color connections, multijets
DESCRIPTION
First International Workshop on Multiple Partonic Interactions at the LHC. Color Connections, Multijets. The Underlying Event and Minimum-Bias Infrared Headaches Perugia Tunes Probes Drell-Yan: Primordial kT and impact of UE Top Future Directions PYTHIA 8 + VINCIA. - PowerPoint PPT PresentationTRANSCRIPT
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Peter SkandsFermilabPeter SkandsFermilab
► The Underlying Event and Minimum-BiasThe Underlying Event and Minimum-Bias
• Infrared HeadachesInfrared Headaches
• Perugia TunesPerugia Tunes
• ProbesProbes
► Drell-Yan: Primordial kT and impact of UEDrell-Yan: Primordial kT and impact of UE
►TopTop
►Future Directions Future Directions PYTHIA 8 + VINCIA PYTHIA 8 + VINCIA
► The Underlying Event and Minimum-BiasThe Underlying Event and Minimum-Bias
• Infrared HeadachesInfrared Headaches
• Perugia TunesPerugia Tunes
• ProbesProbes
► Drell-Yan: Primordial kT and impact of UEDrell-Yan: Primordial kT and impact of UE
►TopTop
►Future Directions Future Directions PYTHIA 8 + VINCIA PYTHIA 8 + VINCIA
First International Workshop on Multiple Partonic Interactions at the LHC
Thanks to N. Moggi, L. Tomkins, R. Field
See also talk by H. Hoeth
Color Connections, MultijetsColor Connections, Multijets
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Color Connections, Multijets - 2Peter Skands
Multiparticle Production
Disclaimer: no “theory” of UE. Models attempt to capture “most significant” physics aspects fully exclusive events
► Starting point: matrix element + parton shower
• hard parton-parton scattering (normally 22 in MC)
• + bremsstrahlung associated with it 2n in LL DGLAP approximation
►But hadrons are not elementary
►+ QCD diverges at low pT
multiple perturbative parton-parton collisions
►Normally omitted in ME/PS expansions ( ~ higher twists / powers / low-x), but perturbative, divergente.g. 44, 3 3, 32
Note:
Can take
QF >> ΛQCD
QF
QF
…22
ISR
ISR
FSR
FSR
22
ISR
ISR
FSR
FSR
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Color Connections, Multijets - 3Peter Skands
Additional Sources of Particle Production
Need-to-know issues for IRsensitive quantities (e.g., Nch)
+
Stuff at
QF ~ ΛQCD
QF >> ΛQCD
ME+ISR/FSR
+ perturbative MPI
QF
QF
…22
ISR
ISR
FSR
FSR
22
ISR
ISR
FSR
FSR
► Hadronization
► Remnants from the incoming beams
► Additional (non-perturbative / collective) phenomena?• Bose-Einstein Correlations
• Non-perturbative gluon exchanges / color reconnections ?
• String-string interactions / collective multi-string effects ?
• Interactions with “background” vacuum, remnants, or active medium?
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Color Connections, Multijets - 4Peter Skands
Now Hadronize This
Simulation fromD. B. Leinweber, hep-lat/0004025
gluon action density: 2.4 x 2.4 x 3.6 fm
Anti-Triplet
Triplet
pbar beam remnant
p beam remnantbbar
from
tbar
deca
y
b from
t d
ecay
qbar fro
m W
q from W
hadroniza
tion
?
q from W
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Color Connections, Multijets - 5Peter Skands
The Underlying Event and Color► The colour flow determines the hadronizing string topology
• Each MPI, even when soft, is a color spark
• Final distributions crucially depend on color space
Note: this just color connections, then there may be color reconnections too
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Color Connections, Multijets - 6Peter Skands
The Underlying Event and Color► The colour flow determines the hadronizing string topology
• Each MPI, even when soft, is a color spark
• Final distributions crucially depend on color space
Note: this just color connections, then there may be color reconnections too
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Color Connections, Multijets - 7Peter Skands
MPI Models in Pythia 6.4► Old Model: Pythia 6.2 and Pythia 6.4
• “Hard Interaction” + virtuality-ordered ISR + FSR
• pT-ordered MPI: no ISR/FSR
• Momentum and color explicitly conserved
• Color connections: PARP(85:86) 1 in Rick Field’s Tunes
• No explicit color reconnections
► New Model: Pythia 6.4 and Pythia 8
• “Hard Interaction” + pT-ordered ISR + FSR
• pT-ordered MPI + pT-ordered ISR + FSR ISR and FSR have dipole kinematics “Interleaved” with evolution of hard interaction in one common sequence
• Momentum, color, and flavor explicitly conserverd
• Color connections: random or ordered
• Toy Model of Color reconnections: “color annealing”
MPI create kinks on existing
strings, rather than new strings
Hard System + MPI allowed to undergo color reconnections
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Color Connections, Multijets - 8Peter Skands
Color AnnealingSandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120
► Prompted by CDF data and Rick Field’s studies to reconsider. What do we know? Implications for precision measurements?
► Toy model of (non-perturbative) color reconnections, applicable to any final state
• At hadronisation time, each string piece gets a probability to interact with the vacuum / other strings:
Preconnect = 1 – (1-χ)n
χ = strength parameter: fundamental reconnection probability (free parameter: PARP(78))
n = # of multiple interactions in current event ( ~ counts # of possible interactions)
► For the interacting string pieces:
• New string topology determined by annealing-like minimization of ‘Lambda measure’ ~ (pi . pj)
Inspired by area law for fundamental strings: Lambda ~ potential energy ~ string length ~ log(m) ~ N
► good enough for order-of-magnitude exploration
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Color Connections, Multijets - 9Peter Skands
Perugia Tunes► Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data
• + min/max variations
• + LEP tuned fragmentation pars from Professor, courtesy H. Hoeth (see talk)
All models ~ ok on track multiplicity
Data from CDF, Phys. Rev. D 65 (2002) 072005
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Color Connections, Multijets - 10Peter Skands
Perugia Tunes► Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data
• + min/max variations
• + LEP tuned fragmentation pars from Professor, courtesy H. Hoeth (see talk)
All models ~ ok on track multiplicity
LHC
<Nch> = 80 – 100(generator-level)
Data from CDF, Phys. Rev. D 65 (2002) 072005
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Color Connections, Multijets - 11Peter Skands
Tevatron Run IIPythia 6.2 Min-bias <pT>(Nch)
Tune
A
old default
CentralLarge UE
Peripheral Small UE
Non-perturbative <pT> component in string fragmentation (LEP value)
Not only more (charged particles), but each one is harder
Diff
ract
ive?
Perugia Tunes► Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data
• Average track pT as a function of multiplicity: sensitive probe of CR?
• Used to fix CR strength parameter in tunes
Poor statistics due to rapid drop of P(Nch)
Data from CDF, N. Moggi et al., 2008
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Color Connections, Multijets - 12Peter Skands
Not only more (charged particles), but each one is harder
Perugia Tunes► Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data
• Average track pT as a function of multiplicity: sensitive probe of CR?
• Used to fix CR strength parameter in tunes
Data from CDF, N. Moggi et al., 2008
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Color Connections, Multijets - 13Peter Skands
Additional Probes► Forward-Backward Correlations
• Trace long-distance correlations (level and falloff sensitive to modeling & mass distribution)
• If F fluctuates up …
• How likely is it that B fluctuates up too?
• I.e., how correlated are they?
• How quickly does it die out?
0
ηF-ηF
η
ET, Nch, …
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Color Connections, Multijets - 14Peter Skands
Additional Probes► Forward-Backward Correlations
• Trace long-distance correlations (level and falloff sensitive to modeling & mass distribution)
• If F fluctuates up …
• How likely is it that B fluctuates up too?
• I.e., how correlated are they?
• How quickly does it die out?
0
ηF-ηF
η
ET, Nch, …
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Color Connections, Multijets - 15Peter Skands
Simulation fromD. B. Leinweber, hep-lat/0004025
Additional Probes► Baryon Transport and Strangeness Production
• Baryon number traces component coming from beam breakup (soft)
• Strangeness probes fragmentation field, same as LEP?
Large differences depending on degree of “beam breakup”
Tracer of soft beam-remnant component
Particle ID at LHC (talks by F. Sikler, U. Marconi),
even Ω- !
S: K0, K*, …
B+S: Λ0, Ξ-, Ω-
Old models: B number beam pipe
New beam remnant fragm detector
LHC: Omega
Studied in Run I by N. Moggi and others, are there
PID results from Run II ?
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Color Connections, Multijets - 16Peter SkandsData from CDF, Phys Rev Lett 84 (2000) 845 + Run 2 ?
Drell-Yan► DY is the benchmark for ISR
• Its low-pT peak seems to require ~ 2 GeV of “primordial kT”
Different FSR-off-ISR kinematics!
150 MeV !
Exchanged for lower ISR cutoff and smaller μR
Appears dangerously prone to overfitting. Small confidence in extrapolations
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Color Connections, Multijets - 17Peter Skands
► DW, S0, etc all roughly agree for Drell-Yan (except for Tune A)
• What about top?
Top
Tevatron LHC
Models that were equal for DY are no longer equal for top not enough to tune to DY
Note: matching will not change this much
(mentioned in talks by R. Chierici, A. Tricoli)
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Color Connections, Multijets - 18Peter Skands
Z + jets at LHC► Impact of UE uncertainties on LHC Jet Rates
• Z + 2 jets at LHC, as function of 2nd jet pT
• DW = DWT at Tevatron, only UE scaling is different (idem for S0,S0A)
ET2 = 70 GeV
ET2 = 30 GeV
High jet pT (> 100) : rate
independent of UE scaling
Low jet pT (~ 30) : rate
uncertainty due to UE scaling = factor of 2
Plot from Lauren Tomkins (student of B. Heinemann)
Medium jet pT (50-70): rate
uncertainty due to UE scaling ~
30-50%
See also talk by A. Tricoli (ATLAS)
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Color Connections, Multijets - 19Peter Skands
Future Directions► Monte Carlo problem
• Uncertainty on fixed orders and logs obscures clear view on hadronization and the underlying event
► So we just need …
• An NNLO + NLO multileg + NLL Monte Carlo (incl small-x logs), with uncertainty bands, please
► Then …
• We could see hadronization and UE clearly solid constraints
Energy Frontier
Inte
nsity
Fro
ntierThe Astro G
uys
Precision Frontier
The Tevatron and LHC data will be all the energy frontier data we’ll have for a long while
Anno 2018
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Color Connections, Multijets - 20Peter Skands
► Can vary • evolution variable, kinematics maps,
radiation functions, renormalization choice, matching strategy (here just showing radiation functions)
► At Pure LL, • can definitely see a non-perturbative
correction, but hard to precisely constrain it
VINCIAPlug-in to Pythia 8 : towards Plug-in to Pythia 8 : towards NLO multileg + NLL + uncertainties NLO multileg + NLL + uncertainties
Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007
Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching
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Color Connections, Multijets - 21Peter Skands
► Can vary • evolution variable, kinematics maps,
radiation functions, renormalization choice, matching strategy (here just showing radiation functions)
► At Pure LL, • can definitely see a non-perturbative
correction, but hard to precisely constrain it
VINCIA
Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007
Plug-in to Pythia 8 : towards Plug-in to Pythia 8 : towards NLO multileg + NLL + uncertainties NLO multileg + NLL + uncertainties
Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching
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Color Connections, Multijets - 22Peter Skands
► Can vary • evolution variable, kinematics maps,
radiation functions, renormalization choice, matching strategy (here just showing radiation functions)
► After 2nd order matching Non-pert part can be precisely
constrained.(will need 2nd order logs as well for full variation)
VINCIA
Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007
Plug-in to Pythia 8 : towards Plug-in to Pythia 8 : towards NLO multileg + NLL + uncertainties NLO multileg + NLL + uncertainties
Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching
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Color Connections, Multijets - 23Peter Skands
Summary► Perugia Tunes
• First set of tunes of new models including both Tevatron and LEP New LEP parameters can ~ be “slapped onto” existing tunes without invalidating their
fits to UE/MB/DY data S0-H, APT-H, etc… (useful for new ATLAS tune too?)
• + First attempt at systematic “+” and “-” variations
• Data-driven, constraints better tunes BUT ALSO better models
► Drell-Yan and Top
• “Primordial kT”: perturbative uncertainties very large in low-pT region Without better perturbative showers, cannot isolate genuine non-pert component
• Much remains to be learned, even for these “benchmark” processes
► Future (still at conceptual stage):
• VINCIA project aims to reduce perturbative sources of uncertainty
cleaner look at UE and non-perturbative phenomena
(see also talks by A. Moraes and H. Hoeth)
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Color Connections, Multijets - 24Peter Skands
► Searched for at LEP • Major source of W mass uncertainty
• Most aggressive scenarios excluded
• But effect still largely uncertain Preconnect ~ 10%
► Prompted by CDF data and Rick Field’s studies to reconsider. What do we know?
• Non-trivial initial QCD vacuum
• A lot more colour flowing around, not least in the UE
• String-string interactions? String coalescence?
• Collective hadronization effects?
• More prominent in hadron-hadron collisions?
• What (else) is RHIC, Tevatron telling us?
• Implications for precision measurements:Top mass? LHC?
Normal
W W
Reconnected
W W
OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062
Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more …
Colour Reconnection
(example)
Soft Vacuum Fields?String interactions?
Size of effect < 1 GeV?
Color Reconnections
Existing models only for WW a new toy model for all final states: colour annealingAttempts to minimize total area of strings in space-time (similar to Uppsala GAL)
• Improves description of minimum-bias collisionsPS, Wicke EPJC52(2007)133 ;
Preliminary finding Delta(mtop) ~ 0.5 GeVNow being studied by Tevatron top mass groups
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Color Connections, Multijets - 25Peter Skands
Underlying Event and Colour► Not much was known about the colour correlations, so some “theoretically
sensible” default values were 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
• But needed ~ 100% correlation. So far not explained
• Virtually all ‘tunes’ now used by the Tevatron and LHC experiments employ these more ‘extreme’ correlations
• What is their origin? Why are they needed?
M. Heinz, nucl-ex/0606020; nucl-ex/0607033
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Color Connections, Multijets - 26Peter Skands
Questions► Transverse hadron structure
• How important is the assumption f(x,b) = f(x) g(b)
• What observables could be used to improve transverse structure?
► How important are flavour correlations?
• Companion quarks, etc. Does it really matter?
• Experimental constraints on multi-parton pdfs?
• What are the analytical properties of interleaved evolution?
• Factorization?
► “Primordial kT”
• (~ 2 GeV of pT needed at start of DGLAP to reproduce Drell-Yan)
• Is it just a fudge parameter?
• Is this a low-x issue? Is it perturbative? Non-perturbative?
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Color Connections, Multijets - 27Peter Skands
More Questions► Correlations in the initial state
• Underlying event: small pT, small x ( although x/X can be large )
• Infrared regulation of MPI (+ISR) evolution connected to saturation?
• Additional low-x / saturation physics required to describe final state?
• Diffractive topologies?
► Colour correlations in the final state
• MPI color sparks naïvely lots of strings spanning central region
• What does this colour field do?
• Collapse to string configuration dominated by colour flow from the “perturbative era”? or by “optimal” string configuration?
• Are (area-law-minimizing) string interactions important?
• Is this relevant to model (part of) diffractive topologies?
• What about baryon number transport? Connections to heavy-ion programme
OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062
Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more …See also
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Color Connections, Multijets - 28Peter Skands
Multiple Interactions 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
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Color Connections, Multijets - 29Peter Skands
Naming Conventions► Many nomenclatures being used.
• Not without ambiguity. I use:
Qcut
Qcut
22
ISR
ISR
FSR
FSR
22
ISR
ISR
FSR
FSR
Primary
Interaction
(~ trigger)
Underlying
EventBeam
Remnants
Note: each is colored Not possible to separate clearly at hadron level
Some freedom in how much particle production is ascribed to each:
“hard” vs “soft” models
…
…
…
See also Tevatron-for-LHC Report of the QCD Working Group, hep-ph/0610012
Inelastic, non-diffractive