Peter SkandsTheoretical Physics Dept. - FermilabPeter SkandsTheoretical Physics Dept. - Fermilab

►MC models of Underlying-Event / Minimum-Bias PhysicsMC models of Underlying-Event / Minimum-Bias Physics

• Infrared HeadachesInfrared Headaches

• TunesTunes

• Sensitive ProbesSensitive Probes

►Special on Strangeness and Baryons Special on Strangeness and Baryons

►Future DirectionsFuture Directions

►MC models of Underlying-Event / Minimum-Bias PhysicsMC models of Underlying-Event / Minimum-Bias Physics

• Infrared HeadachesInfrared Headaches

• TunesTunes

• Sensitive ProbesSensitive Probes

►Special on Strangeness and Baryons Special on Strangeness and Baryons

►Future DirectionsFuture Directions

LHCb Flavor Physics WG Meeting, 19 Nov 2008

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

Probing the Underlying Event with Probing the Underlying Event with Strangeness and BaryonsStrangeness and Baryons

Probing the Underlying Event with Probing the Underlying Event with Strangeness and BaryonsStrangeness and Baryons

Probing the UE with S and BPeter Skands

Monte Carlo GeneratorsMonte Carlo Generators

► Basic aim: improve lowest order perturbation theory by including leading corrections “exclusive” event samples

1. sequential resonance decays

2. bremsstrahlung

3. underlying event

4. hadronization

5. hadron (and τ) decays

► Physics Feedback

• Reliable correction procedures

• Without reliable models, reliable extrapolations are hard to hope for

3Probing the UE with S and BPeter Skands

The Tip of an Iceberg?The Tip of an Iceberg?

►Even the most sophisticated calculations currently

only scratch the first few orders of Couplings

Logs

1/Nc

m

Γ

Powers

“Twists”

Spin correlations

...

3

“tuning” needed.

Extreme tuning may indicate model

breakdown. INTERESTING!

Probing the UE with S and BPeter Skands

Classic Example: Number of tracksClassic Example: Number of tracksUA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias

events

Simple physics models ~ Poisson

More Physics:

Moral (will return to the models later):

1) It is not possible to ‘tune’ anything better than the underlying physics model allows

2) Failure of a physically motivated model usually points to more physics (interesting)

3) 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

Probing the UE with S and BPeter Skands

Particle ProductionParticle Production

► Starting point: matrix element + parton shower

• hard parton-parton scattering (normally 22 in MC)

• + bremsstrahlung associated with it 2n 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. 44, 3 3, 32

Note:

Can take

QF >> ΛQCD

QF

QF

…22

ISR

ISR

FSR

FSR

22

ISR

ISR

FSR

FSR

Probing the UE with S and BPeter Skands

Additional Sources of Particle ProductionAdditional 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 ?

• “Plasma” effects?

• Interactions with “background” vacuum, remnants, or active medium?

Probing the UE with S and BPeter Skands

Naming ConventionsNaming 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)

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

8Probing the UE with S and BPeter Skands

Where Did This Pion Come From?Where Did This Pion Come From?

Beam Remnants

2

I

I

F

F

Multiple Parton

Interactions

…

…

…

Less MPIMore BR

More MPILess BR

OR

Soft or Hard?

Probing the UE with S and BPeter Skands

Now Hadronize ThisNow 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 fr

om t d

ecay

qbar fro

m W

q from W

hadroniza

tion

?

q from W

Probing the UE with S and BPeter 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

Probing the UE with S and BPeter 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

Baryon Number acts as a Tracer!

Probing the UE with S and BPeter 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

Probing the UE with S and BPeter 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 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

Peter SkandsTheoretical Physics Dept. - FermilabPeter SkandsTheoretical Physics Dept. - Fermilab

Probing the UEProbing the UE

1: A bunch of models that all give fair descriptions of Tevatron data

2: Strangeness and Baryon Number

1: A bunch of models that all give fair descriptions of Tevatron data

2: Strangeness and Baryon Number

Probing the UE with S and B 15

Peter Skands

Pythia 6.4: PYTUNE

Track Multiplicity:

All models ~ fine

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

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)

Probing the UE with S and BPeter Skands

Extrapolations to LHCExtrapolations to LHC

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

17

Strangeness► Tunneling suppression due to quark mass

• strangeness probes fragmentation field in a unique way

• Consistent with LEP? Consistent with RHIC / Tevatron?

P(mq,pT) ~ exp ( -[mq2 + pT

2]/κ )

CDF Run 1

Correction factors

CDF, Phys.Rev.D72:052001,2005.

Generator pT spectrum CDF sees

the hard tail, not the peak

Less sensitive to mass effect

Need experiment with good low-pT tracking

18

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)

19

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

Simulation fromD. B. Leinweber, hep-lat/0004025

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

String junctions

Pythia 6.4: new models of beam remnant fragmentation available

20

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

21

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

22

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

23

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

Probing the UE with S and BPeter Skands

Summary► 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)

Peter SkandsTheoretical Physics Dept. - FermilabPeter SkandsTheoretical Physics Dept. - Fermilab

Backup Slides

Probing the UE with S and BPeter Skands

(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

Saturation? Current models need MPI IR cutoff > PS IR cutoff

= color-screening cutoff(Ecm-dependent, but large uncert)

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

Probing the UE with S and BPeter 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

Probing the UE with S and BPeter 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

Probing the UE with S and BPeter 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?

Probing the UE with S and BPeter 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

Probing the UE with S and BPeter 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