color connections, multijets

Peter SkandsFermilabPeter SkandsFermilab

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

• Infrared HeadachesInfrared Headaches

• Perugia TunesPerugia Tunes

• ProbesProbes

► Drell-Yan: Primordial kT and impact of UEDrell-Yan: Primordial kT and impact of UE

►TopTop

►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: Primordial kT and impact of UEDrell-Yan: Primordial kT and impact of UE

►TopTop

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

First International Workshop on Multiple Partonic Interactions at the LHC

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

See also talk by H. Hoeth

Color Connections, MultijetsColor Connections, Multijets

Color Connections, Multijets - 2Peter Skands

Multiparticle Production

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

► Starting point: matrix element + parton shower

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

• + bremsstrahlung associated with it 2n in LL DGLAP 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 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 ?

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


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


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 (see talk)

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 (see talk)

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

Particle ID at LHC (talks by F. Sikler, U. Marconi),

even Ω- !

S: K0, K*, …

B+S: Λ0, Ξ-, Ω-

Old models: B number beam pipe

New beam remnant fragm detector

LHC: Omega

Studied in Run I by N. Moggi and others, are there

PID results from Run II ?

Color Connections, Multijets - 16Peter 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%

See also talk by A. Tricoli (ATLAS)


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


► 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

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

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)


► 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


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

color connections, multijets

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