# Presentation: 946366

## MC models of Underlying-Event / Minimum-Bias Physics
 Infrared Headaches
 Tunes
 Sensitive Probes
Special on Strangeness and Baryons 
Future Directions

- Peter Skands
- Theoretical Physics Dept. - Fermilab

- MC models of Underlying-Event / Minimum-Bias Physics
-  Infrared Headaches
-  Tunes
-  Sensitive Probes
- Special on Strangeness and Baryons
- Future Directions

- LHCb Flavor Physics WG Meeting, 19 Nov 2008

- Thanks to N. Moggi, L. Tomkins, R. Field, H. Hoeth

- Probing the Underlying Event with Strangeness and Baryons

## Monte Carlo Generators

- Probing the UE with S and B

- Peter Skands

- Basic aim: improve lowest order perturbation theory by including leading corrections  “exclusive” event samples
- sequential resonance decays
- bremsstrahlung
- underlying event
- hadronization
- hadron (and τ) decays

- Physics Feedback
- Reliable correction procedures
- Without reliable models, reliable extrapolations are hard to hope for

## The Tip of an Iceberg?

- Probing the UE with S and B

- Peter Skands

- Even the most sophisticated calculations currently only scratch the first few orders of
- Couplings
- Logs
- 1/Nc
- m
- Γ
- Powers
- “Twists”
- Spin correlations
- ...

-  “tuning” needed.
- Extreme tuning may indicate model breakdown. INTERESTING!

## Classic Example: Number of tracks

- Probing the UE with S and B

- Peter Skands

- UA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias events

- Simple physics models ~ Poisson

- More Physics:

- Moral (will return to the models later):
- It is not possible to ‘tune’ anything better than the underlying physics model allows
- Failure of a physically motivated model usually points to more physics (interesting)
- Failure of a fit not as interesting

- Can ‘tune’ to get _average_ right, but much too small _fluctuations_
-   inadequate physics model

- Multiple interactions + impact-parameter dependence

## Particle Production

- Probing the UE with S and B

- Peter Skands

- Starting point: matrix element + parton shower
- hard parton-parton scattering
- (normally 22 in MC)
- + bremsstrahlung associated with it
-  2n in (improved) LL 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)

- e.g. 44, 3 3, 32

- Note:
- Can take
- QF >> ΛQCD

- QF

- ...

## Additional Sources of Particle Production

- Probing the UE with S and B

- Peter Skands

- Need-to-know issues for IR
- sensitive quantities (e.g., Nch)

- +
- Stuff at
- QF ~ ΛQCD

- QF >> ΛQCD
- ME+ISR/FSR
- + perturbative MPI

- QF

- ...

- 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 ?
- “Plasma” effects?
- Interactions with “background” vacuum, remnants, or active medium?

## Naming Conventions

- Probing the UE with S and B

- Peter Skands

- Many nomenclatures being used.
- Not without ambiguity. I use:

- Qcut

- Qcut

- Primary
- Interaction
- (~ trigger)

- Multiple Parton Interactions (MPI)

- Beam 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

## Where Did This Pion Come From?

- Probing the UE with S and B

- Peter Skands

- Beam Remnants

- Multiple Parton Interactions

- ...

- ...

- ...

- Less MPI
- More BR

- More MPI
- Less BR

- OR

- Soft or Hard?

## Now Hadronize This

- Probing the UE with S and B

- Peter Skands

## The Underlying Event and Color

- Probing the UE with S and B

- Peter Skands

- 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

## The Underlying Event and Color

- Probing the UE with S and B

- Peter Skands

- 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

- Baryon Number acts as a Tracer!

## MPI Models in Pythia 6.4

- Probing the UE with S and B

- Peter Skands

- 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

## Color Annealing

- Probing the UE with S and B

- Peter Skands

- 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 hadronization 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 (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 string area law: Lambda ~ potential energy ~ string length ~ log(m) ~ N
-  good enough for order-of-magnitude exploration

## Probing the UE

- Peter Skands
- Theoretical Physics Dept. - Fermilab

- 1: A bunch of models that all give fair descriptions of Tevatron data
- 2: Strangeness and Baryon Number

## Pythia 6.4: PYTUNE

- Probing the UE with S and B

- Peter Skands

- Track Multiplicity:
- All models ~ fine

- 1800 GeV

- 630 GeV

- PYTUNE (MSTP(5)) kept up to date with newest tunes (see update notes)
- Most recent tunes for Perugia workshop (+ min/max versions)  6.4.20
- + new LEP tuned fragmentation pars from Professor (H. Hoeth, A. Buckley)

## Extrapolations to LHC

- Probing the UE with S and B

- Peter Skands

- Generator-Level
- LHC
- <Nch> = 80 – 100
- But that only gives us the size of the glass, not the contents of the cocktail

- First thing to measure: track multiplicity

## Strangeness

- Tunneling suppression due to quark mass
-  strangeness probes fragmentation field in a unique way
- Consistent with LEP? Consistent with RHIC / Tevatron?

*P(m**q**,p**T**) ~ exp ( -[m**q**2** + p**T**2**]/**κ** )*

- CDF Run 1
- Correction factors

- Generator  pT spectrum

-  CDF sees the hard tail, not the peak

-  Less sensitive to mass effect

-  Need experiment with good low-pT tracking

## Strangeness Distributions

- Models that agree on total amount of strangeness ...

- Disagree on where it is ...

- Need measurements at both low and high eta
- (Note: probably better to measure strangeness fraction, divide out total mult)

- (correlated with total mult production)

## Baryons

- Comes back to the color flow issues mentioned earlier
- Is the baryon number liberated from the beam?
- How far does it get?
- Any observed B excess in detector
-  important constraints (lower bounds) on beam remnant fragmentation

- Sjöstrand & PS  : Nucl.Phys.B659(2003)243, JHEP03(2004)053

- String junctions

- Pythia 6.4: new models of beam remnant fragmentation available

## This is hard (if you’re not LHCb)

- Baryon number transport: get as close to the beam as possible!

- CDF coverage

- Old models: B locked in remnant

- New Models:  B carried by string junctions

- Few percent effect

- (NOTE also: CDF only sees the high-pT tail. The one from the beam is most likely soft)
-  Need measurements at high eta and low pT

## Extrapolations to the LHC

- Lambdas

- (Note that these models are by no means extreme, effect could well be larger)

- 1 percent in ATLAS/CMS
- 
- 5 percent in LHCb
- (depends on pT cuts)

- + Could be possible to enhance effect by looking at spectra, correlations

## Extrapolations to the LHC

- Cascades

- (Note that these models are by no means extreme, effect could well be larger)

- 1 percent in ATLAS/CMS
- 
- 5 percent in LHCb
- (depends on pT cuts)

- + Could be possible to enhance effect by looking at spectra, correlations

## Extrapolations to the LHC

- Omega

- (Note that these models are by no means extreme, effect could well be larger)

- 1 percent in ATLAS/CMS
- 
- 5 percent in LHCb
- (depends on pT cuts)

- + Could be possible to enhance effect by looking at spectra, correlations

## Summary

- Probing the UE with S and B

- Peter Skands

- Perugia Tunes
- First set of tunes of new models including both Tevatron and LEP
- + First attempt at systematic “+” and “-” variations
- Data-driven, constraints  better tunes BUT ALSO better models
- Strangeness and Baryon Number
- Strangeness may be used to probe the fragmentation field
- Are strangeness production rates & spectra consistent with LEP? With Tevatron?
- Baryon Number migration traces Beam Remnant Fragmentation
- Important ingredient in constraining Monte Carlo UE models
- Fundamental probe of non-trivial string topology carrying baryon number (junctions)
- Important implications for precision on underlying event

- (see also talks by A. Moraes and H. Hoeth last week in Perugia)

## Backup Slides

- Peter Skands
- Theoretical Physics Dept. - Fermilab

## (Why Perturbative MPI?)

- Probing the UE with S and B

- Peter Skands

- Analogue: Resummation of multiple bremsstrahlung emissions
- Divergent σ for one emission (X + jet, fixed-order)
- Finite σ for divergent number of jets (X + jets, infinite-order)
- N(jets) rendered finite by finite perturbative resolution = parton shower cutoff

- (Resummation of) Multiple Perturbative Interactions
- Divergent σ for one interaction (fixed-order)
- Finite σ for divergent number of interactions (infinite-order)
-  N(jets) rendered finite by finite perturbative resolution

## 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?

- Probing the UE with S and B

- Peter 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?

- Color Reconnections

## Underlying Event and Colour

- Probing the UE with S and B

- Peter Skands

- 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?

## Questions

- Probing the UE with S and B

- Peter Skands

- 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?

## More Questions

- Probing the UE with S and B

- Peter Skands

- 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

- See also

## Multiple Interactions  Balancing Minijets

- Probing the UE with S and B

- Peter 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