# Presentation: 056035

##  The Underlying Event and Minimum-Bias
 Infrared Headaches
 Perugia Tunes
 Probes
 Drell-Yan and Top
Future Directions  PYTHIA 8 + VINCIA

-  Infrared Headaches
-  Perugia Tunes
-  Probes
-  Drell-Yan and Top
- Future Directions  PYTHIA 8 + VINCIA

- ATLAS MC Meeting, 5 Nov 2008

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

- From the Tevatron to LHCMin-Bias and the Underlying Event

## Why study UE/Min-Bias?

- Why study Min-Bias and Underlying Event?
- Solving QCD requires compromise
- Construct and constrain models (~ sets of compromises)
-  precision knowledge + constrained pheno models
- Feedback to high-pT physics
- Reliable correction procedures
- Without reliable models, reliable extrapolations are hard to hope for

- Disclaimer: no “theory” of UE. Models attempt to capture “most significant” physics aspects  fully exclusive events

- No theory now does not mean no theory exists
- Experimental input vital to guide its construction

- From the Tevatron to LHC

## Classic Example: Number of tracks

- UA5 @ 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 dependence

- 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

- From the Tevatron to LHC

## Traditional Event Generators

- 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**

- Even the most sophisticated calculations currently only scratch the first few orders of couplings, logs, powers, twists, ...  “tuning” needed. Extreme tuning may indicate model breakdown. INTERESTING!

- From the Tevatron to LHC

## Particle Production

- 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)
- But still **perturbative, divergent**

- e.g. 44, 3 3, 32

- Note:
- Can take
- QF >> ΛQCD

- QF

- ...

- ISR

- FSR

- FSR

- From the Tevatron to LHC

## Additional Sources of Particle Production

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

- +
- Stuff at
- QF ~ ΛQCD

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

- QF

- ...

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

- From the Tevatron to LHC

## Naming Conventions

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

- Qcut

- Qcut

- ISR

- FSR

- FSR

- Primary
- Interaction
- (~ trigger)

- Underlying
- Event

- 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

- From the Tevatron to LHC

## Now Hadronize This

- Simulation from
- D. B. Leinweber, hep-lat/0004025
-  gluon action density: 2.4 x 2.4 x 3.6 fm

- From the Tevatron to LHC

## 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

- From the Tevatron to LHC

## 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

- From the Tevatron to LHC

## 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 **re**connections
**New Model: Pythia 6.4 and Pythia 8**
- “Hard Interaction” + **p****T****-ordered** ISR + FSR
- pT-ordered MPI + **p****T****-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

- From the Tevatron to LHC

## Color Annealing

- Sandhoff + 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
- But bear in mind: this is not (yet) a “precision” model

- From the Tevatron to LHC

## 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, A. Buckley

- All models ~ ok on track multiplicity

- Data from CDF, Phys. Rev. D 65 (2002) 072005

- From the Tevatron to LHC

## 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, A. Buckley

- All models ~ ok on track multiplicity

- LHC
- <Nch> = 80 – 100
- (generator-level)

- Data from CDF, Phys. Rev. D 65 (2002) 072005

- From the Tevatron to LHC

## Perugia Tunes

**Tevatron Run II**
- Pythia 6.2 Min-bias
*<p**T**>(N**ch**)*

- Tune A

- old default

- Central
- Large UE

- Peripheral
- Small UE

- Non-perturbative <pT> component in string fragmentation (LEP value)

- Not only more (charged particles), but each one is harder

*Diffractive?*

- 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

- From the Tevatron to LHC

## 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

- From the Tevatron to LHC

## 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, ...

- From the Tevatron to LHC

## 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, ...

- From the Tevatron to LHC

## Additional Probes

- Simulation from
- D. B. Leinweber, hep-lat/0004025

- 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**

*S: K**0**, K*****, ...*
*B+S: **Λ**0**, **Ξ**-**, **Ω**-*

- Old models: B number  beam pipe
- New beam remnant fragm  detector

- Studied in Run I by N. Moggi and others. Results from Run II very desirable

- From the Tevatron to LHC

## Drell-Yan

- DY is the benchmark for ISR
- Its low-pT peak seems to require ~ 2 GeV of “primordial kT”

- Appears dangerously prone to overfitting. Small confidence in extrapolations

- From the Tevatron to LHC

## DW, S0, etc all roughly agree for Drell-Yan (except for Tune A)
What about top?

- 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)

- From the Tevatron to LHC

## 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%

- From the Tevatron to LHC

## 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

- Intensity Frontier

- The Astro Guys

- Precision Frontier

- The Tevatron and LHC data will be all the
- energy frontier data we’ll have for a long while

- Anno 2018

- From the Tevatron to LHC

## Constructing LL Showers

- The final answer will depend on:
- The choice of evolution “time”
- The splitting functions (finite terms not fixed)
- The phase space map (“recoils”, dΦn+1/dΦn )
- The renormalization scheme (argument of αs)
- The infrared cutoff contour (hadronization cutoff)

- Variations 
- Comprehensive uncertainty estimates
- (showers with uncertainty bands)

- From the Tevatron to LHC

## VINCIA

- Gustafson, PLB175(1986)453; Lönnblad (ARIADNE), CPC71(1992)15.
- Azimov, Dokshitzer, Khoze, Troyan, PLB165B(1985)147
- Kosower PRD57(1998)5410; Campbell,Cullen,Glover EPJC9(1999)245

- Based on Dipole-Antennae
- Shower off color-connected pairs of partons
- Plug-in to PYTHIA 8 (C++)
- So far:
- 3 different shower evolution variables:
- pT-ordering (= ARIADNE ~ PYTHIA 8)
- Dipole-mass-ordering (~ but not = PYTHIA 6, SHERPA)
- Thrust-ordering (3-parton Thrust)
- For each: an infinite family of antenna  functions
- Laurent series in branching invariants with arbitrary finite terms
- Shower cutoff contour: independent of evolution variable
- IR factorization “universal”
- Several different choices for αs (evolution scale, pT, mother antenna mass, 2-loop, ...)
- 3 different phase space maps
- Ariadne or Kosower “antenna” recoils, or Emitter + longitudinal Recoiler

- Dipoles (=Antennae, not CS) – a dual description of QCD

- a

- b

- r

- VIRTUAL NUMERICAL COLLIDER WITH INTERLEAVED ANTENNAE

- Giele, Kosower, PS : PRD78(2008)014026  + Les Houches ‘NLM’ 2007

- From the Tevatron to LHC

## 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

- 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

- Plug-in to Pythia 8 : towards  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

- From the Tevatron to LHC

## 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

- 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  NLO multileg + NLL + uncertainties

- Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching

- From the Tevatron to LHC

## 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)

- 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  NLO multileg + NLL + uncertainties

- Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching

- From the Tevatron to LHC

## 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 last week in Perugia)

- From the Tevatron to LHC

## Backup Slides

## (Why Perturbative MPI?)

- 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

- Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph]

- From the Tevatron to LHC

## 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?

- 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?*

**OPAL**, Phys.Lett.B453(1999)153 & **OPAL**, hep-ex0508062

- Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more ...

- Color Reconnections

- Existing models only for WW  a new toy model for all final states: **colour annealing**
- Attempts to minimize total area of strings in space-time (similar to Uppsala GAL)
-  Improves description of minimum-bias collisions
- PS, Wicke EPJC52(2007)133 ;
- Preliminary finding Delta(mtop) ~ 0.5 GeV
- Now being studied by Tevatron top mass groups

- From the Tevatron to LHC

## 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

- From the Tevatron to LHC

## 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?

- From the Tevatron to LHC

## 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

- From the Tevatron to LHC

## 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

- From the Tevatron to LHC