peter skands theoretical physics dept., fermilab ► the underlying event and minimum-bias infrared...

Peter SkandsTheoretical Physics Dept., FermilabPeter SkandsTheoretical Physics Dept., Fermilab

► The Underlying Event and Minimum-BiasThe Underlying Event and Minimum-Bias

• Infrared HeadachesInfrared Headaches

• Perugia TunesPerugia Tunes

• ProbesProbes

► Drell-Yan and TopDrell-Yan and Top

►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 and TopDrell-Yan and Top

►Future Directions Future Directions PYTHIA 8 + VINCIA PYTHIA 8 + VINCIA

ATLAS MC Meeting, 5 Nov 2008

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

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

From the Tevatron to LHC - 2Peter Skands

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 existsExperimental input vital to guide its construction


Classic Example: Number of tracksUA5 @ 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

dependenceMoral (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


Traditional Event Generators

► Basic aim: improve lowest order perturbation theory by including leading corrections exclusive event samples1. sequential resonance decays

2. bremsstrahlung

3. underlying event

4. hadronization

5. 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!


Particle 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)

But still perturbative, divergente.g. 44, 3 3, 32

Note:

Can take

QF >> ΛQCD

QF

QF

…22

ISR

ISR

FSR

FSR

22

ISR

ISR

FSR

FSR


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 ?

• “Plasma” effects?

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


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


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


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


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


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

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


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



• + 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


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?


• 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


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


• 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


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, …


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

S: K0, K*, …

B+S: Λ0, 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 - 20Peter 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


► 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)


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%


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


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)


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

VINCIA

► 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 ANTENNAEVIRTUAL NUMERICAL COLLIDER WITH INTERLEAVED ANTENNAE

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


► 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


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




► At Pure LL, • can definitely see a non-perturbative

correction, but hard to precisely constrain it

VINCIA


Plug-in to Pythia 8 : towards Plug-in to Pythia 8 : towards NLO multileg + NLL + uncertainties NLO multileg + NLL + uncertainties





► After 2nd order matching Non-pert part can be precisely

constrained.(will need 2nd order logs as well for full variation)

VINCIA


Plug-in to Pythia 8 : towards Plug-in to Pythia 8 : towards NLO multileg + NLL + uncertainties NLO multileg + NLL + uncertainties



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)

Peter SkandsTheoretical Physics Dept., FermilabPeter SkandsTheoretical Physics Dept., Fermilab

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

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]


► 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


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


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?


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


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

peter skands theoretical physics dept., fermilab ► the underlying event and minimum-bias infrared...

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