NonPerturbative QCD Effects and the Top Mass

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Non-Perturbative QCD Effects and the Top Mass
U of C, Chicago, November 2006
Peter Skands Theoretical Physics Dept
Fermi National Accelerator Laboratory
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The (QCD) Landscape
In reality, this all happens on top of each
other. (only possible exception long-lived
colour singlet)
D. B. Leinweber, hep-lat/0004025
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Overview

• Introduction
• QCD at colliders
• Strings, colour, and the Underlying Event
• A complete model of hadron collisions
• Correllations energy vs particle flow
• A toy model for string reconnections.
• Colour Annealing
• Effects on a Top Mass Estimator

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QuantumChromoDynamics

• Known gauge group and Lagrangian
• Rich variety of dynamical phenomena

PDG J.Phys.G33(2006)1
Hadron Decays
Non-Perturbative hadronisation, colour
reconnections, beam remnants, non-perturbative
fragmentation functions, pion/proton, kaon/pion,
...
Soft Jets Jet Structure Multiple collinear/soft
emissions (initial and final state brems
radiation), Underlying Event (multiple
perturbative 2?2 interactions ?), semi-hard
separate brems jets
Exclusive
Widths
Resonance Masses
Hard Jet Tail High-pT wide-angle jets
Inclusive
s

• UNPHYSICAL SCALES
• QF , QR Factorisation Renormalisation

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QCD at High pT

• The signal
• Large cross sections for coloured BSM resonances
• E.g. monojet signature for ED relies on hard QCD
  radiation
• Cascade decays ? Many-body final states
• Backgrounds
• Also large cross sections for top, nZ/W jets,
  other resonances (?),
• Theory
• Fixed-order perturbation theory
• Asymptotic freedom ? improved convergence at high
  pT
• Phase space increases

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QCD at intermediate pT

• Extra Jets
• In signal
• extra noise / combinatorics
• In backgrounds
• Irreducible backgrounds
• Some fraction ? fakes!
• Heavy flavour
• Jet energy scale
• Jet broadening
• Underlying activity
• Theory
• Fixed Order with explicit jets
• Parton Showers / Resummation
• Models of Underlying Event

Minijets Jet Structure Semi-hard separate
brems jets (esp. ISR), jet broadening (FSR),
g?cc/bb, multiple perturbative 2?2 interactions
(underlying event), ?
LHC - sps1a - m600 GeV
Plehn, Rainwater, PS (2005)
Plehn, Rainwater, PS hep-ph/0510144
hep-ph/0511306
Problem 1 Need to get both soft and hard
emissions right Problem 2 Underlying Event not
well understood
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Collider physics at Low pT

• Measurements at LEP ?
• Fragmentation models tuned
• Strangeness and baryon production rates well
  measured
• Colour reconnections ruled out in WW (to 10)
• Measurements at hadron colliders
• Different vacuum, colour in initial state ?
  colour promiscuity?
• Underlying Event and Beam Remnants
• Intrinsic kT
• Lots of min-bias. Fragmentation tails ? fakes!

E.g. background to X e(20 GeV) X jet(15
GeV)
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The Bottom Line

• The S matrix is expressible as a series in gi,
  ln(Q1/Q2), ln(x), m-1, fp-1 ,
• To do precision physics
• Identify, compute, and control all large terms
• Solve more of QCD
• over all of phase space fixed order
  resummations
• Control it
• answer should include reliable estimate of
  uncertainties
• and be systematically improvable
• Non-perturbative effects
• dont care whether you know how to calculate them

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Monte Carlo Event Generators
Exclusive
Inclusive
Large-dimensional final state phase spaces ?
Monte Carlo integration Markov Chain
formulation of fragmentation Parton showers
iterative application of universal and
pertubatively calculable kernels for n ? n1
partons ( resummation of soft/collinear Sudakov
logarithms) Hadronization iteration of X ? X
hadron, at present according to phenomenological
models based on known properties of
nonperturbative QCD, lattice studies, and fits to
data.

• Main virtues
• Error is stochastic O(N-1/2) and independent of
  dimension
• Fully exclusive final states (for better or worse
  cf. the name Pythia )
• Only need to redo part of calculation for each
  different observable.
• Have proven essential for detailed experimental
  studies can compute detector response event by
  event

IR Sensitive
IR Safe
Underlying Event
illustration from Z. Nagy
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Overview

• Introduction
• QCD at colliders
• Strings, colour, and the Underlying Event
• A complete model of hadron collisions
• Correllations energy vs particle flow
• A toy model for string reconnections.
• Colour Annealing
• Effects on a Top Mass Estimator

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Additional Sources of Particle Production

• 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) phenomena?
• Bose-Einstein Correlations
• Non-perturbative gluon exchanges
• String-string interactions
• Interactions with background vacuum

PS Im not talking about pile-up here
e.g. 4?4, 3? 3, 3?2
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A Complete Model of Hadron Collisions
(Talked about details last time, so quick
overview only)
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Underlying Event The Basics
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Balancing Minijets
angle between 2 best-balancing pairs

• 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.

example (old Run I)
CDF, PRD 56 (1997) 3811
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Interleaved Evolution
Fixed order matrix elements
pT-ordered PS matched to ME for W/Z/H/G jet

• Underlying Event
• (note interactions correllated in colour
  hadronization not independent)

multiparton PDFs derived from sum rules
perturbative intertwining?
Beam remnants Fermi motion / primordial kT
Sjöstrand PS JHEP03(2004)053, EPJC39(2005)129
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(Remnant PDFs)
Used to select flavor and x for each
parton-parton interaction, and for the
interleaved evolution of the initial-state
shower. Dynamically evaluated.
Sjöstrand PS JHEP03(2004)053
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So what. Does it work?

• MC4LHC summer 2003
• Complete model
• Still old showers
• not yet interleaved
• One ambiguity
• Comparisons with data were encouraging
• Except for one distribution

Tune A data
By the failure of even the ?? ordering of the
colour lines in the initial state to describe the
ltpTgt(Nch) distribution, it appears that the
colour flow in physical events cannot be
correctly described by merely arranging the
colour lines present in the initial state.
Sjöstrand PS JHEP03(2004)053
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Motivation

• Min-bias collisions at the Tevatron
• Well described by Rick Fields Tune A of PYTHIA
• Theoretical framework is from 1987. Now we made
  some improvements
• Wanted to use Tune A as initial reference
  target
• But it kept on being different

Multiplicity distribution, pTZ, etc all come out
fine, but ltpTgt(Nch) never came out right ?
something must be wrong or missing?
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Underlying Event and Color

• Fragmentation
• Nch log(mstring)
• More strings ? more hadrons, but average pT stays
  same
• Flat ltpTgt(Nch) spectrum uncorrellated
  underlying event
• Space-time Area of string system is large
  potential energy

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Underlying Event and Color

• But if MPI interactions correlated in colour
• each scattering does not produce an independent
  string,
• average pT ? not flat
• Multiplicity vs pT correllation probes color
  correllations!
• Whats so special about Tune A anyway?
• It (and all other realistic tunes made) need to
  go to the very most extreme end of the parameter
  range, with 100 color correlation in final
  state.

Sjöstrand v Zijl Phys.Rev.D362019,1987 ?
Old Pythia model
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Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 Z.
Phys.C62(1994)281 more
Color Reconnections

• Searched for at LEP
• Major source of W mass uncertainty
• Most aggressive scenarios excluded
• But effect still largely uncertain 10
• Prompted by CDF data and Rick Fields studies to
  reconsider. What do we know?
• Non-trivial initial QCD vacuum
• A lot more colour flowing around, not least in
  the UE
• More prominent in hadron-hadron collisions?
• What is ltpTgt(Nch) telling us?
• Top mass?
• Implications for LHC?
• Problem existing models only for ee- ?WW

OPAL, Phys.Lett.B453(1999)153 OPAL,
hep-ex0508062
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Color Annealing

• Toy model of (non-perturbative) color
  reconnections, applicable to any final state
• At hadronisation time, each string piece has a
  probability to interact with the vacuum / other
  strings
• String formation for interacting string pieces
  determined by annealing-like minimization of
  Lambda measure (string lengthlog(m)N)
• Similar to area law for fundamental strings
• ? good enough for order-of-magnitude

cf. e.g. J. Rathsman, PLB452(1999)364
Sandhoff PS, in Les Houches 05 SMH
Proceedings, hep-ph/0604120
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First Results

• Improved Description of Min-Bias
• Effect Still largely uncertain
• Worthwhile to look at top etc

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First Results
Have all been made available via new subroutine
PYTUNE in Pythia v.6.408, see update
notes. Preferred tune of new model S0 (300)

• Improved Description of Min-Bias
• Effect Still largely uncertain
• Worthwhile to look at top etc

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Top Mass Estimator
D. Wicke (DØ)

• Event Generation Selection
• For each model 100k inclusive events were
  generated
• Jets are reconstructed using both
• Cone (?R 0.5, pT gt 15 GeV)
• kT (dcut 150 GeV2)
• Exactly 4 reconstructed Jets
• Technical simplifications
• Generator semileptonic events.
• Unique assignment to MC truth by ?R possible.
• Reconstruct mass on correct assignment only
• m2 (pbjet pqjet pqbarjet)2

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Top Mass Estimator
D. Wicke (DØ)
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Top Mass Estimator
D. Wicke (DØ)
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Top Mass Estimator
D. Wicke (DØ)
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D. Wicke (DØ)