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Understanding Hadronisation at PP Colliders P e t e r S k a n d s ( M o n a s h U n i v e r s i t y ) Fermilab LPC - Topic of the Week August 2016 Monte Carlos and Fragmentation Monte Carlo generators aim to give fully exclusive

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SLIDE 1

Understanding Hadronisation at PP Colliders

P e t e r S k a n d s ( M o n a s h U n i v e r s i t y ) Fermilab LPC - Topic of the Week August 2016

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P e t e r S k a n d s

Monte Carlos and Fragmentation

2

๏Monte Carlo generators aim to give fully exclusive descriptions of

collider final states - within and beyond the Standard Model

  • Including effects of initial- and final-state radiation (ISR & FSR showers)
  • + (Sequential) Resonance decays (top quarks, Z/W/H bosons, & BSM)
  • + Soft physics: Underlying Event, Hadronisation, Decays, Beam Remnants
๏Explicit modelling of QCD dynamics ⟷ comparison to measurements ๏E.g., MC models were crucial to establish “string effect” in early 80s ๏Extensively used to design/optimise analyses (& planning future ones)
  • Study observables, sensitivities, effects of cuts, detector efficiencies, derive

correction factors, extract fundamental parameters, cross sections, …

๏Lund String Model has probably been the most successful hadronisation

model over the last 30 years.

  • This talk: it is beginning to show some interesting failures at LHC
  • Impact on hadronisation corrections for high-pT analyses?

M o n a s h U n i v e r s i t y

See, e.g., MCnet review arXiv:1101.2599, or TASI lectures arXiv:1207.2389

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P e t e r S k a n d s

QCD is more than a (fixed-order) expansion in αs

3

Jets (perturbative QCD, initial- and final-state radiation) ⟷ QFT amplitude structures, factorisation & unitarity ⟷ Precision jet (structure) studies, calibrations. Strings (strong gluon fields) ⟷ quantum-classical

  • correspondence. String physics. String breaks. Dynamics of

hadronisation phase transition. Hadronisation corrections. Hadrons ⟷ Spectroscopy (incl excited and exotic states), lattice QCD, (rare) decays, mixing, light nuclei. Hadron beams → multiparton interactions, diffraction, …

๏Challenges Beyond Fixed Order: “Emergent Phenomena”
  • Fractal Structures: scale Invariance of massless Lagrangian →

jets-within-jets-within-jets (& loops-within-loops-within-loops)

  • Confinement (win $1,000,000 if you can prove)

M o n a s h U n i v e r s i t y

most of my research

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P e t e r S k a n d s

We strongly suspect there is more to (particle) physics … but are still looking for deviations from the Standard Model Accurate modelling of QCD → improve searches & precision

Ulterior Motives for Studying QCD

4

M o n a s h U n i v e r s i t y

The Standard Model

encouraging start

There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy

Shakespeare, Hamlet.

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SLIDE 5

P e t e r S k a n d s

time

The Phenomenology Pipeline

5

M o n a s h U n i v e r s i t y

qi qj ga (−igsta

ijγµ)

THEORY EXPERIMENT

example:

“QCD” “Jets” PHENOMENOLOGY INTERPRETATION

Drawing by

  • T. Sjöstrand

Model Calculations Observables Analyses Planning Design R&D Hardware Triggers … Measurements Corrections Systematics Exclusions Hints Evidence Discoveries Surprises Statistical Tests Validate/Falsify Models Constrain Free Parameters

The Pipeline looks something like this:

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P e t e r S k a n d s

Monte Carlo Event Generators

6

๏Factorization → Split the problem into many (nested) pieces

M o n a s h U n i v e r s i t y

Pevent = Phard ⊗ Pdec ⊗ PISR ⊗ PFSR ⊗ PMPI ⊗ PHad ⊗ . . .

Hard Process & Decays:

Use process-specific (N)LO matrix elements → Sets “hard” resolution scale for process: QMAX

ISR & FSR (Initial & Final-State Radiation):

Universal DGLAP equations → differential evolution, dP/dQ2, as function of resolution scale; run from QMAX to QConfinement ~ 1 GeV

MPI (Multi-Parton Interactions)

Additional (soft) parton-parton interactions: LO matrix elements → Additional (soft) “Underlying-Event” activity

Hadronization

Non-perturbative model of color-singlet parton systems → hadrons

+ Quantum mechanics → Probabilities → Random Numbers

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P e t e r S k a n d s

Hadronisation − What do we know?

7

M o n a s h U n i v e r s i t y

๏Quark-Antiquark Potential
  • As function of separation distance

46 STATIC QUARK-ANTIQUARK

POTENTIAL:

  • SCALING. . .

2641

Scaling plot

2GeV-

1 GeV—

2

I

  • 2

k,

t

0.5

1.

5

1 fm

2.5

l~

RK

B= 6.0, L=16 B= 6.0, L=32 B= 6.2, L=24 B= 6.4, L-24

B = 6.4, L=32

3.

5

~ 'V ~ ~ I ~ A I

4 2'

  • FIG. 4. All potential

data of the five lattices have been scaled to a universal curve by subtracting

Vo and measuring

energies and distances

in appropriate units of &E. The dashed curve correspond

to V(R)=R —

~/12R. Physical units are calculated

by exploit- ing the relation &cr =420 MeV.

AM~a=46. 1A~ &235(2)(13) MeV .

Needless

to say, this value does not necessarily

apply to full QCD.

In addition

to the long-range

behavior of the confining potential it is of considerable interest to investigate its ul- traviolet

structure. As we proceed into the weak cou-

pling regime lattice simulations

are expected to meet per-

turbative results. Although

we are aware that our lattice

resolution is not yet really

suScient,

we might

dare to

previe~ the

continuum behavior

  • f the

Coulomb-like term from our results.

In Fig. 6(a) [6(b)] we visualize the

confidence regions

in the K-e plane from fits to various

  • n- and off-axis potentials
  • n the 32

lattices at P=6.0

[6.4]. We observe that the impact of lattice discretization

  • n e decreases by a factor 2, as we step up from P=6.0 to

150 140

Barkai '84

  • MTC

'90

Our results:---

130-

120-

110-

100-

80—

5.6 5.8

6.2 6.4

  • FIG. 5. The on-axis string tension

[in units of the quantity

c =&E /(a AL )] as a function of P. Our results are combined

with pre- vious values obtained by the MTc collaboration

[10]and Barkai, Moriarty,

and Rebbi [11].

~ Force required to lift a 16-ton truck

LATTICE QCD SIMULATION. Bali and Schilling Phys Rev D46 (1992) 2636

What physical! system has a ! linear potential?

Short Distances ~ “Coulomb”

“Free” Partons

Long Distances ~ Linear Potential

“Confined” Partons (a.k.a. Hadrons)

(in “quenched” approximation)

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P e t e r S k a n d s

A Brief History of Vortex Lines

8

๏1911: Discover of superconductivity (K. Onnes) ๏1933: Discovery of flux expulsion (Meissner & Ochsenfeld)
  • Penetration depth : λ (distance over which field decays by 1/e)
๏1957: Vortex Lines (Abrikosov) (in Type II SC)
  • Swirling supercurrents produce a non-SC “core”
  • Core size : ξ (aka “coherence length”; exp decay outside core)
  • Flux Quantisation: each core carries a single unit of flux
  • Type II if core size small (otherwise Type I)
๏1960s - 1970s: “Dual models” for strong force
  • Regge Theory: massless endpoints on rotating relativistic strings
  • Nielsen-Olesen: Higgs-type Lagrangians → vortex lines ⟷ Nambu strings
  • Advent of SM (QCD) → string models refocus on gravity (& EW cosmic strings)
  • 1974: Artru & Mennessier, “String model and multiproduction”
  • Ca 1980: Andersson, Gustafson, Sjöstrand, et al: the Lund String Model

M o n a s h U n i v e r s i t y

ξ < √ 2λ

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P e t e r S k a n d s

Which Charges? Colour Flow

9

๏After the parton shower finishes, there can be lots of partons,

𝒫(10-100). The main question is therefore:

๏Between which partons do confining potentials arise?
  • MC generators use a simple set of rules for colour flow, based
  • n large-NC limit (valid to ~ 1/NC2 ~ 10%)

M o n a s h U n i v e r s i t y

Illustrations from: Nason & Skands, PDG Review on MC Event Generators, 2014

q → qg g → q¯ q g → gg

  • G. ’t Hooft, Nucl.Phys. B72 (1974) 461.
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P e t e r S k a n d s

Colour Flow

10

๏For an entire Cascade

M o n a s h U n i v e r s i t y

Example: Z0 → qq

String #1 String #2 String #3

Coherence of pQCD cascades (angular ordering or boosted dipoles/antennae) → not much “overlap” between strings → Leading-colour approximation pretty good

1 1 1 1 2 2 2 4 4 4 3 3 3 5 5 5 6 7 7

For a single fragmenting system: (The trouble at LHC: MPI & ISR → many such systems; overlapping)

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P e t e r S k a n d s

The (Lund) String Model

11

M o n a s h U n i v e r s i t y

Map:

  • Quarks → String

Endpoints

  • Gluons → Transverse

Excitations (kinks)

  • Physics then in terms of

string worldsheet evolving in spacetime

  • Probability of string

break (by quantum tunneling) constant per unit area → AREA LAW

Simple space-time picture

Details of string breaks more complicated (e.g., baryons, spin multiplets)

→ STRING EFFECT

Pedagogical Review: B. Andersson, The Lund model. Camb. Monogr. Part. Phys. Nucl. Phys. Cosmol., 1997.

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P e t e r S k a n d s

Differences Between Quark and Gluon Jets

12

M o n a s h U n i v e r s i t y

[GeV]

T

Jet p 500 1000 1500 〉

charged

n 〈 20 ATLAS

= 8 TeV s = 20.3

int

L

> 0.5 GeV

track T

p

Quark Jets (Data) Gluon Jets (Data) Quark Jets (Pythia 8 AU2) Gluon Jets (Pythia 8 AU2) LO pQCD

3

Quark Jets N LO pQCD

3

Gluon Jets N

quark antiquark gluon string motion in the event plane (without breakups)

Gluon connected to two string pieces Each quark connected to one string piece → expect factor 2 ~ CA/CF larger particle multiplicity in gluon jets vs quark jets Can be important for discriminating new-physics signals (decays to quarks vs decays to gluons, vs composition of background and bremsstrahlung combinatorics ) Example of Recent Studies

ATLAS, Eur.Phys.J. C76 (2016) no.6, 322 See also Larkoski et al., JHEP 1411 (2014) 129 Thaler et al., Les Houches, arXiv:1605.04692

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SLIDE 13

P e t e r S k a n d s

The Effects of Hadronisation

13

๏Generally, expect few-hundred MeV shifts by hadronisation
  • Corrections to IR safe observables are “power corrections”
  • Corrections for jets
  • of radius

M o n a s h U n i v e r s i t y

  • 0.6
  • 0.5
  • 0.4
  • 0.3
  • 0.2
  • 0.1

0.1 200 500 100 1000 pp, 7 T eV, no UE Δpthadr × R CF/C [GeV] pt (parton) [GeV] hadronisation pt shift (scaled by R CF/C) Herwig 6 (AUET2) Pythia 8 (Monash 13) R=0.2, quarks R=0.4, quarks R=0.2, gluons R=0.4, gluons Monte Carlo tune jet radius, flavour simple analytical estimate

Simple analytical estimate → ~ 0.5 GeV / R correction from hadronisation (scaled by colour factor)

Dasgupta, Dreyer, Salam, Soyez, JHEP 1606 (2016) 057

R = ∆η × ∆φ ∝ Λ2

QCD/Q2 OBS

∝ 1/R

Significant differences between codes/tunes → important to pin down with precise QCD hadronisation measurements at LHC

See Korchemsky, Sterman, NPB 437 (1995) 415 Seymour, NPB 513 (1998) 269 Dasgupta, Magnea, Salam, JHEP 0802 (2008) 055

LES HOUCHES STUDY (ARXIV:1605.04692): Q/G CAN BE HIGHLY AFFECTED BY COLOUR RECONNECTIONS

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SLIDE 14

P e t e r S k a n d s

Colour Confusion ?

14

M o n a s h U n i v e r s i t y

LC

CR

Proton-Proton (LHC)

A lot more colour kicked around (& also colour in initial state) Include “Beam Remnants” Still might look relatively simple, to begin with

With several parton-parton interactions (MPI → UE):

How to make sense of the colour structure?

  • (+baryon beam remnants → “string junctions”)

String-fragmentation of junctions: Sjöstrand & Skands NPB 659 (2003) 243; CR with junctions: Christiansen & Skands JHEP 1508 (2015) 003

๏Next-to-simplest: 2 string systems
  • Several studies at LEP2 (ee → WW → 4 jets)
๏CR implied a non-perturbative uncertainty on the

W mass measurement, ΔMW ~ 40 MeV

  • CR strength best fit ~ 10% ~ 1/NC

2

  • But in WW, overlaps are expected to be suppressed

by kinematics, and there are “only” two strings;

  • In pp, MPI can create (many) more … ?

Overviews of recent models: arXiv:1507.02091 , arXiv:1603.05298

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P e t e r S k a n d s

Colour: What’s the Problem?

15

M o n a s h U n i v e r s i t y

Beam Direction

MPI

Without Colour Reconnections Each MPI hadronizes independently of all others

Outgoing parton

(including MPI: Multiple Parton-Parton Interactions ~ the “underlying event”)

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SLIDE 16

P e t e r S k a n d s

Colour: What’s the Problem?

16

M o n a s h U n i v e r s i t y

Beam Direction

MPI

Without Colour Reconnections Each MPI hadronizes independently of all others

Outgoing parton String Piece

(including MPI: Multiple Parton-Parton Interactions ~ the “underlying event”) So many strings in so little space If true → Very high energy densities QGP-like “core” with hydro? → Thermal? E.g., EPOS

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SLIDE 17

P e t e r S k a n d s

Colour Reconnections

17

M o n a s h U n i v e r s i t y

Beam Direction

MPI

With Colour Reconnections MPI hadronize collectively

Outgoing parton String Piece

See also Ortiz et al., Phys.Rev.Lett. 111 (2013) 4, 042001

comoving hadrons

Highly interesting theory questions now. Is there collective flow in pp? If yes, what is its origin? Is it stringy, or hydrodynamic ? (or …?) Or Thermal? Or Higher String Tension?

E.g., EPOS E.g., DIPSY rope

(including MPI: Multiple Parton-Parton Interactions ~ the “underlying event”) String-Length Minimisation E.g., PYTHIA, HERWIG E.g., do most patches of event look the same (thermalised?) or do they look more independent?

See e.g., Skands & Wraight: arXiv:1101.5215

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P e t e r S k a n d s

What do we see?

18

M o n a s h U n i v e r s i t y

submicron particles dispersed in superfluid 4He

“Direct

  • bservation of

Kelvin waves excited by quantized vortex reconnection” Visualisation by: Fonda, Meichle, Ouellette, Hormoz, Lathrop, PNAS 111(2014)4707 http://www.pnas.org/content/suppl/2014/03/20/1312536110.DCSupplemental

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SLIDE 19

P e t e r S k a n d s

What do we see in pp collisions?

19

M o n a s h U n i v e r s i t y

Average pT increases with particle multiplicity and (faster than predicted) with particle mass without CR w i t h ( t u n e d ) C R <pT> vs Number of Particles <pT> vs Particle Mass

Note: from RHIC (200 GeV)

Plots from mcplots.cern.ch ‘New Look’

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P e t e r S k a n d s

The “CMS Ridge”

20

M o n a s h U n i v e r s i t y

φ ∆

  • 1

1 2 3 4 ) φ ∆ Y( 7 7.05 7.1 7.15 7.2 7.25 7.3

) φ ∆ Y( ) φ ∆ (

periph

G + FY ) φ ∆ (

templ

Y (0)

periph

G + FY (0)

periph

+FY

ridge

Y

ATLAS =13 TeV s <5.0 GeV

a,b T

0.5<p |<5.0 η ∆ 2.0<| 120 ≥

rec ch

N

e’ [CMS PRL 116(2016)172302][ATLAS PRL 116(2016)172301]

η ∆

  • 4
  • 3
  • 2
  • 1

1 2 3 4

(radians) φ ∆

  • 1

1 2 3 4

φ ∆ d η ∆ d

pair

N

2

d

trig

N 1

1.6 1.65 1.7

105 ≥

  • ffline

trk

= 13 TeV, N s CMS pp < 3 GeV/c

T

1 < p (b)

Reminiscent of the (much stronger) ridge seen in HI collisions. Surprisingly strong also in proton-Lead High-Multiplicity pp collisions

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SLIDE 21

P e t e r S k a n d s

Strangeness

21

M o n a s h U n i v e r s i t y

>

ch

<n <n>

  • 3

10

  • 2

10

  • 1

10 1 10 Meson Fractions

Pythia 8.183 Data from PDG/HEPDATA

LEP + SLD PY8 (Monash) PY8 (Default) PY8 (Fischer)

bins

/N

2 5%

χ 0.0 ± 0.6 0.0 ± 1.2 0.0 ± 1.2

V I N C I A R O O T

±

π π

±

K η ' η

±

ρ ρ

± *

K ω φ

K*/K-

R

/K* φ

R

/K- φ

R

  • π

/ φ

R

Theory/Data 0.6 0.8 1 1.2 1.4

>

ch

<n <n>

  • 4

10

  • 3

10

  • 2

10

  • 1

10 1 Baryon Fractions

Pythia 8.183 Data from PDG/HEPDATA

LEP PY8 (Monash) PY8 (Default) PY8 (Fischer)

bins

/N

2 5%

χ 0.1 ± 1.1 0.0 ± 2.2 0.0 ± 2.2

V I N C I A R O O T

p Λ

/p Λ

R

/K Λ

R

±

Σ Σ

++

∆

*

Σ

±

Ξ

*0

Ξ Ω

Theory/Data 0.6 0.8 1 1.2 1.4

Z Decays

/dy>

K

<dn

NSD

1/n

0.2 0.4 0.6 0.8 )/d|y|> Rapidity (NSD)

S

<dn(K

Pythia 8.185 Data from JHEP 1105 (2011) 064

CMS PY8 (Monash 13) PY8 (4C) PY8 (2C)

bins

/N

2 5%

χ 0.0 ± 0.1 0.0 ± 0.9 0.0 ± 9.6

V I N C I A R O O T

pp

7000 GeV

y

0.5 1 1.5 2

Theory/Data 0.6 0.8 1 1.2 1.4

CMS

Kaon Rate ~ OK

(within uncertainty allowed by ee data) /dy>

Λ

<dn

NSD

1/N

0.1 0.2 0.3 0.4 )/d|y|> (NSD) Λ <dn(

Pythia 8.185 Data from JHEP 1105 (2011) 064

CMS PY8 (Monash 13) PY8 (4C) PY8 (2C)

bins

/N

2 5%

χ 0.0 ± 6.4 0.0 ± 7.8 0.1 ± 14.7

V I N C I A R O O T

pp

7000 GeV

y

0.5 1 1.5 2

Theory/Data 0.6 0.8 1 1.2 1.4 Lambda Rate ~ 2/3 of data

(not compatible with uncertainty in ee data) /dy>

Ξ

<dn

NSD

1/N

0.01 0.02 0.03 0.04 )/d|y|> (NSD) Ξ <dn(

Pythia 8.185 Data from JHEP 1105 (2011) 064

CMS PY8 (Monash 13) PY8 (4C) PY8 (2C)

bins

/N

2 5%

χ 0.1 ± 8.9 0.1 ± 12.9 0.1 ± 19.2

V I N C I A R O O T

pp

7000 GeV

y

0.5 1 1.5 2

Theory/Data 0.5 1 1.5 2

Xi Rate ~ 1/2 of data

(not compatible with uncertainty in ee data) (note: old tunes may be low on everything)

This is the data used to tune the models

Plots from the Monash tune paper Eur.Phys.J. C74 (2014) no.8, 3024

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SLIDE 22

/dln(x)

K

> dn

K

1/<n

  • 3

10

  • 2

10

  • 1

10 1 10 ) (Combined)

±

x(K

Pythia 8.183 Data from ZPC66(1995)355, ZPC63(1994)181, EPJC5(1998)585

LEP (A+D+O) PY8 (Monash) PY8 (Default) PY8 (Fischer)

bins

/N

2 5%

χ 0.1 ± 1.6 0.0 ± 1.4 0.1 ± 1.9

V I N C I A R O O T

)

p

ln(x

  • 4
  • 2

Theory/Data 0.6 0.8 1 1.2 1.4

2

T

/dp

K

dn

K

1/n

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 1 10 (|y|<2.0, NSD)

T

p

S

K

Pythia 8.181 Data from JHEP 1105 (2011) 064

CMS PY8 (Monash 13) PY8 (4C) PY8 (2C)

bins

/N

2 5%

χ 0.1 ± 7.1 0.0 ± 3.3 0.1 ± 2.2

V I N C I A R O O T

7000 GeV

pp

[GeV]

T

p

2 4 6 8 10

Theory/Data 0.6 0.8 1 1.2 1.4

P e t e r S k a n d s

Strangeness Spectra

22

M o n a s h U n i v e r s i t y

Kaon spectrum at LEP Kaon spectrum at LHC Note: rates normalised to unity now (+ Several measurements by ALICE, LHCb)

Plots from the Monash tune paper Eur.Phys.J. C74 (2014) no.8, 3024

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SLIDE 23

ξ /d

Λ

> dn

Λ

1/<n

0.2 0.4 0.6 )]| Λ |Ln[x(

Pythia 8.183 Data from EPJ C16 (2000) 613

ALEPH PY8 (Monash) PY8 (Default) PY8 (Fischer)

bins

/N

2 5%

χ 0.1 ± 0.8 0.1 ± 1.5 0.1 ± 1.2

V I N C I A R O O T

p

ξ

1 2 3 4 5

Theory/Data 0.6 0.8 1 1.2 1.4

T

/dp

Λ

dn

Λ

1/n

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 1 10 (|y|<2.0, NSD)

T

p Λ

Pythia 8.181 Data from JHEP 1105 (2011) 064

CMS PY8 (Monash 13) PY8 (4C) PY8 (2C)

bins

/N

2 5%

χ 0.1 ± 5.8 0.3 ± 6.7 0.5 ± 10.3

V I N C I A R O O T

7000 GeV

pp

[GeV]

T

p

2 4 6 8 10

Theory/Data 0.6 0.8 1 1.2 1.4

P e t e r S k a n d s

Strangeness Spectra

23

M o n a s h U n i v e r s i t y

Lambda spectrum at LEP Lambda spectrum at LHC Note: rates normalised to unity now (+ Several measurements by ALICE, LHCb)

Plots from the Monash tune paper Eur.Phys.J. C74 (2014) no.8, 3024

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SLIDE 24

P e t e r S k a n d s

CMS: Strangeness in the Underlying Event

24

๏Effect also present in UE (note: effect enhanced by pT cuts, cf spectra)

M o n a s h U n i v e r s i t y

Kaons Lambdas

Do MC jets have the right particle content and spectra? Implications for particle-flow modeling, JES calibrations, Q/G discrimination? Further measurements? (in jets, along jet rapidity axis, …)

Plots from mcplots.cern.ch

Protons more numerous than Lambda; but probably have to ask ALICE?

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SLIDE 25

P e t e r S k a n d s

→ Extensions of CMS UE Study?

25

M o n a s h U n i v e r s i t y

Probing Collective Effects in Hadronisation with the Extremes of the Underlying Event

  • T. Martin, P. Skands, S. Farrington, Eur.Phys.J. C76 (2016) no.5, 299

5 10 15 20 25 30 35

> [Trans.]

Inc.

< N 5 10 15 20 25 30 35 40 45 50

Pythia 8.210 Monash Pythia 8.210 Monash + New CR EPOS 1.3 LHC DIPSY NoSwing DIPSY Rope

= 13 TeV s

[GeV]

T

Leading Track-Jet p 5 10 15 20 25 30 35 Monash MC 0.6 0.8 1 1.2 1.4

) > [Trans.]

S

)/N(K Λ Λ < N( 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7

Pythia 8.210 Monash Pythia 8.210 Monash + New CR EPOS 1.3 LHC DIPSY NoSwing DIPSY Rope

= 13 TeV s

T

R

1 −

10 × 2 1 2 3 4 Monash MC 1 1.5 ) > [Trans.] Λ Λ )/N(

+

Ξ

  • Ξ

< N( 0.08 0.1 0.12 0.14 0.16 0.18

Pythia 8.210 Monash Pythia 8.210 Monash + New CR EPOS 1.3 LHC DIPSY NoSwing DIPSY Rope

= 13 TeV s

T

R

1 −

10 × 2 1 2 3 4 Monash MC 1 1.5 2 2.5

Lambda/K Xi/Lambda

Lead pT <N> Transverse to Lead pT

RT < 1 RT > 1 RT > 2 RT > 3

From T. Martin, ICHEP 2016

“Extreme UE”

slide-26
SLIDE 26

P e t e r S k a n d s

Recent news from ALICE (ICHEP 2016)

26

๏A clear enhancement of strangeness

with (pp) event multiplicity is

  • bserved
  • Especially for multi-strange baryons
  • No corresponding enhancement for

protons → this really must be a strangeness effect

  • Cross-check measurements of the

phi meson are now underway

๏Jet universality: jets at LHC modelled

the same as jets at LEP

  • Flat line ! (cf PYTHIA)
  • DIPSY includes “colour ropes”
  • EPOS includes hydrodynamic “core”

M o n a s h U n i v e r s i t y D.D. Chinellato – 38th International Conference on High

|< 0.5 η |

〉 η /d

ch

N d 〈

10

2

10

3

10

)

+

π +

−

π Ratio of yields to (

3 −

10

2 −

10

1 −

10

16) × (

+

Ω +

−

Ω 6) × (

+

Ξ +

−

Ξ 2) × ( Λ + Λ

S

2K ALICE = 7 TeV s pp, = 5.02 TeV

NN

s p-Pb, = 2.76 TeV

NN

s Pb-Pb,

PYTHIA8 DIPSY EPOS LHC ALICE, arXiv:1606.07424

S

2K 2) × ( Λ + Λ 6) × (

+

Ξ +

−

Ξ 16) × (

+

Ω +

−

Ω [1] [2] [3]

D.D. Chinellato – 38th International Conference on High Energy Physics

slide-27
SLIDE 27

P e t e r S k a n d s

The Plot Thickens

27

๏Looks like the effect, whatever it

is, continues smoothly into p-Pb

M o n a s h U n i v e r s i t y D.D. Chinellato – 38th International Conference on High

|< 0.5 η |

〉 η /d

ch

N d 〈

10

2

10

3

10

)

+

π +

−

π Ratio of yields to (

3 −

10

2 −

10

1 −

10

16) × (

+

Ω +

−

Ω 6) × (

+

Ξ +

−

Ξ 2) × ( Λ + Λ

S

2K ALICE = 7 TeV s pp, = 5.02 TeV

NN

s p-Pb, = 2.76 TeV

NN

s Pb-Pb,

PYTHIA8 DIPSY EPOS LHC ALICE, arXiv:1606.07424

S

2K 2) × ( Λ + Λ 6) × (

+

Ξ +

−

Ξ 16) × (

+

Ω +

−

Ω [1] [2] [3]

D.D. Chinellato – 38th International Conference on High Energy Physics

slide-28
SLIDE 28

P e t e r S k a n d s

The Plot Thickens

28

M o n a s h U n i v e r s i t y D.D. Chinellato – 38th International Conference on High

|< 0.5 η |

〉 η /d

ch

N d 〈

10

2

10

3

10

)

+

π +

−

π Ratio of yields to (

3 −

10

2 −

10

1 −

10

16) × (

+

Ω +

−

Ω 6) × (

+

Ξ +

−

Ξ 2) × ( Λ + Λ

S

2K ALICE = 7 TeV s pp, = 5.02 TeV

NN

s p-Pb, = 2.76 TeV

NN

s Pb-Pb,

PYTHIA8 DIPSY EPOS LHC ALICE, arXiv:1606.07424 [1] [2] [3]

๏Looks like the effect, whatever it

is, continues smoothly into p-Pb

  • … and into Pb-Pb !
  • Unexpected.
๏Looks like jet universality and

hadronisation in pp is up for revision.

  • Is it thermal? Stringy? Both?
  • Collective? Flowy? …
๏Physics must explain smooth

transition to heavy ions. No abrupt “phase transition” seen in these observables

D.D. Chinellato – 38th International Conference on High Energy Physics

slide-29
SLIDE 29

P e t e r S k a n d s

Summary

29

๏Higgs-type Lagrangians → Vortex Lines → String Models
  • Remain our best bet at modelling hadronisation in QCD
  • High-multiplicity & high-pT triggered events: large amounts of colour

kicked around: soft event structure appears to require (at least) going beyond Leading Colour → Colour Reconnections (CR)

  • Beyond CR, it now appears that the effective QCD scale is increasing
  • What are the dynamics of pp / multi-string environments?
  • Phenomenology: Modern revisions of the Lund string model
  • What measurements can be performed to shed more light?
  • Possible to get more information from lattice? Multi-string systems?
๏By the way (advertisement):
  • Did you know you can get automated shower-uncertainty weights ?
๏Automated Parton-Shower Uncertainties in PYTHIA 8 ๏Similar capabilities in HERWIG++, SHERPA, VINCIA

M o n a s h U n i v e r s i t y

Mrenna & Skands, arXiv:1605.08352 Giele, Kosower, Skands PRD84 (2011) 054003 Bellm, Plätzer, Richardson, Siodmok, Webster 1605.08256 Bothmann, Schönherr, Schumann 1606.08753

slide-30
SLIDE 30
slide-31
SLIDE 31

New research at Monash

PRECISION LHC PHENOMENOLOGY PYTHIA & VINCIA NLO EVENT GENERATORS QCD STRINGS, HADRONISATION SUPPORT LHC EXPERIMENTS, ASTRO-PARTICLE COMMUNITY, AND FUTURE ACCELERATORS +OUTREACH AND CITIZEN SCIENCE

+ Partnerships: Warwick Alliance, MCnet, CoEPP

New joint research program with Warwick ATLAS, on developing and testing advanced colllider-QCD

  • models. Opportunities for PhD students based at

Monash + exchange to UK/CERN.

p p

See: arXiv:1603.05298

MCnet is an EU Marie Curie Training Network (ITN)

  • n MC generators for LHC (Herwig, Pythia,

Sherpa). Funded for Horizon 2020! Starting in 2017 with Monash an associate partner

slide-32
SLIDE 32

P e t e r S k a n d s

No Enhancement for Protons

32

M o n a s h U n i v e r s i t y

|< 0.5 η |

〉 η /d

ch

N d 〈

10

Baryon to meson ratio

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

PYTHIA8 DIPSY EPOS LHC ALICE = 7 TeV s pp, = 5.02 TeV

NN

s p-Pb, 2) × ( π p/

S

/K Λ

D.D. Chinellato – 38th International Conference on High

|< 0.5 η |

〉 η /d

ch

N d 〈

10

2

10

3

10

)

+

π +

−

π Ratio of yields to (

3 −

10

2 −

10

1 −

10

16) × (

+

Ω +

−

Ω 6) × (

+

Ξ +

−

Ξ 2) × ( Λ + Λ

S

2K ALICE = 7 TeV s pp, = 5.02 TeV

NN

s p-Pb, = 2.76 TeV

NN

s Pb-Pb,

PYTHIA8 DIPSY EPOS LHC ALICE, arXiv:1606.07424

S

2K 2) × ( Λ + Λ 6) × (

+

Ξ +

−

Ξ 16) × (

+

Ω +

−

Ω [1] [2] [3]

D.D. Chinellato – 38th International Conference on High Energy Physics

E

(sss)

F

(dss)

G

(uds)

H

(uud)

slide-33
SLIDE 33

f

P e t e r S k a n d s

All on the same plot

33

๏Including K* and protons

M o n a s h U n i v e r s i t y

slide-34
SLIDE 34

P e t e r S k a n d s

pT Dependence

34

M o n a s h U n i v e r s i t y 0.1 1 10

Ratio to INEL>0

2 3 4 5 6 7 8 9 10

(I)

=7 TeV s ALICE Preliminary, pp at V0M Multiplicity Classes

  • π

+

+

π

  • +K

+

K p p+ Λ + Λ

+

Ξ +

  • Ξ

* K K*+

) c (GeV/

T

p

0.1 1 10 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

(IX)

ALI−PREL−109358

t

1 10

)

  • π

+

+

π ) / ( p (p +

0.2 0.4 0.6 0.8 1

= 7 TeV s ALICE Preliminary pp

= 21.3 〉 η /d

ch

dN 〈 V0M Class I, = 2.3 〉 η /d

ch

dN 〈 V0M Class X, (V0M Multiplicity Classes)

0.2 0.4 0.6 0.8 1

ALI-PREL-110279

pp

low-pT mid-pT high-pT

6-.//60 :;<=>2 ≈ A. 2

Spectra become harder at high multiplicities

More pronounced for baryons than mesons

<dN/dη>=21.3 <dN/dη>~3?

slide-35
SLIDE 35

1980: string (colour coherence) effect

quark antiquark gluon string motion in the event plane (without breakups)

Predicted unique event structure; inside & between jets. Confirmed first by JADE 1980.

Generator crucial to sell physics!

(today: PS, M&M, MPI, . . . )

Torbj¨

  • rn Sj¨
  • strand

Status and Developments of Event Generators slide 5/28

slide-36
SLIDE 36

1980: string (colour coherence) effect

quark antiquark gluon string motion in the event plane (without breakups)

Predicted unique event structure; inside & between jets. Confirmed first by JADE 1980.

Generator crucial to sell physics!

(today: PS, M&M, MPI, . . . )

Torbj¨

  • rn Sj¨
  • strand

Status and Developments of Event Generators slide 5/28

slide-37
SLIDE 37

P e t e r S k a n d s

  • Breakup vertices causally disconnected → order is irrelevant → iterative algorithm

String Breaks

37

M o n a s h U n i v e r s i t y

Pedagogical Review: B. Andersson, The Lund model. Camb. Monogr. Part. Phys. Nucl. Phys. Cosmol., 1997.

Schwinger Effect + ÷ Non-perturbative creation

  • f e+e- pairs in a strong

external Electric field

~ E

e- e+

P ∝ exp ✓−m2 − p2

⊥

κ/π ◆

Probability from Tunneling Factor

(κ is the string tension equivalent)

๏In “unquenched” QCD
  • g→qq → The strings will break

→ Gaussian pT spectrum Heavier quarks suppressed. Prob(q=d,u,s,c) ≈ 1 : 1 : 0.2 : 10-11

String Breaks by Tunneling (Schwinger Type)

slide-38
SLIDE 38

P e t e r S k a n d s

String Breaks

38

M o n a s h U n i v e r s i t y

๏In QCD, strings can (and do) break!
  • (In superconductors, would require magnetic monopoles)
  • In QCD, the roles of electric and magnetic are reversed
  • Quarks (and antiquarks) are “chromoelectric monopoles”
  • There are at least two possible analogies ~ tunneling:

Schwinger Effect + ÷ Non-perturbative creation

  • f e+e- pairs in a strong

external Electric field

~ E

e- e+

P ∝ exp ✓−m2 − p2

⊥

κ/π ◆

Probability from Tunneling Factor

(κ is the string tension equivalent)

CANONICAL Hawking Radiation M

~ g

Non-perturbative creation

  • f radiation quanta in a

strong gravitational field

HORIZON HORIZON

Thermal (Boltzmann) Factor

P ∝ exp ✓ −E kBTH ◆

Linear Energy Exponent

ALTERNATIVE? 1) 2)

slide-39
SLIDE 39

P e t e r S k a n d s

What are “Colour Reconnections”?

39

๏Simple example:
  • Intensely studied at LEP2.
๏CR implied a non-perturbative uncertainty on the W mass

measurement, ΔMW ~ 40 MeV

  • CR constrained to ~ 10% ~ 1/NC2
  • Simple two-string system. What about pp?
๏Several modelling attempts
  • Based on “just” minimising the string action
๏String interactions (Khoze, Sjostrand) ๏Generalized Area Law (Rathsman et al.) ๏Colour Annealing (Skands et al.) ๏Gluon Move Model (Sjostrand et al.)
  • More recently: SU(3)C group multiplet weights
๏Dipole Swing (Lonnblad et al.); Colour Ropes (Bierlich et al.) ๏String Formation Beyond Leading Colour (Skands et al.)

M o n a s h U n i v e r s i t y

A B

e+e− → W +W − → hadrons

See Christiansen & Skands and references therein, JHEP 1508 (2015) 003

3 ⊗ ¯ 3 = 8 ⊕ 1 3 ⊗ 3 = 6 ⊕ ¯ 3 3 ⊗ 8 = 15 ⊕ 6 ⊕ 3 8 ⊗ 8 = 27 ⊕ 10 ⊕ 10 ⊕ 8 ⊕ 8 ⊕ 1