A new Colour Reconnection model within Pythia Jesper Roy - - PowerPoint PPT Presentation

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A new Colour Reconnection model within Pythia Jesper Roy - - PowerPoint PPT Presentation

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A new Colour Reconnection model within Pythia Jesper Roy Christiansen Lund University November 3, 2014 MPI@LHC Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 1 / 15 Talk overview Motivation New beam remnant

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SLIDE 1

A new Colour Reconnection model within Pythia

Jesper Roy Christiansen

Lund University

November 3, 2014 MPI@LHC

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 1 / 15

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SLIDE 2

Talk overview

Motivation New beam remnant model New colour reconnection model Conclusion

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 2 / 15

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SLIDE 3

Motivation

We want to introduce more

  • f the SU(3) structure from

QCD into the description Provide a better description

  • f especially Λ production at

hadron colliders. Top mass measurement - see

  • T. Sj¨
  • strand’s talk

b b b b b b b b b b

Data

b

Monash 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Λ/K0

S versus rapidity at √s = 7 TeV

N(Λ) / N(K0

S)

0.5 1 1.5 2 0.6 0.8 1 1.2 1.4 |y| MC/Data

(arXiv:1102.4282) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 3 / 15

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SLIDE 4

New beam remnant model

The beam remnant model comes after the perturbative machinery Overall idea of the model:

◮ A game of conservation laws ◮ Add the minimal required

amount of extra particles

MPI 1 MPI 2 Beam Remnant 2

...

Beam Remnant 1

  • Example of two scattered gluons from a proton:

Flavour conservation

Add two up and one down quark

Baryon number conservation

Turn two quarks into a diquark

Energy/momentum conservation

Choose x according to modified PDFs and rescale to match

  • verall momentum conservation

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 4 / 15

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SLIDE 5

New beam remnant model - colour conservation

Possible colour states for the two gluons: 8 ⊗ 8 = 27 ⊕ 10 ⊕ 10 ⊕ 8 ⊕ 8 ⊕ 1

27

2 C & 2 AC + 1 gluon

10

0 C & 3 AC + 0 gluon (junction)

10

3 C & 0 AC + 1 gluon (junction)

8

1 C & 1 AC + 0 gluon

1

0 C & 0 AC + 0 gluon (not allowed)

Examples of the 27 and the 8 configurations:

MPI 1 MPI 2 Beam Remnant 2

...

Beam Remnant 1 MPI 1 MPI 2 Beam Remnant 2

...

Beam Remnant 1

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 5 / 15

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SLIDE 6

Saturation

Are the partons uncorrelated? Included as a simple suppression: exp (−M/k), where M is the multiplet size and k is a free parameter

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 6 / 15

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SLIDE 7

Comparisons to data

Relative large x and small p⊥ ⇒ forward physics Comparison to forward TOTEM measurements. 10 % difference between no and maximal saturation The old model is similar to maximal saturation

b b b b b b b b b b b b b b b b b b

Data

b

Max saturation No saturation Monash 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Charged particle |η| at 7 TeV, track p⊥ > 40 MeV, for Nch ≥ 1 dN/dη 5.4 5.6 5.8 6 6.2 6.4 0.6 0.8 1 1.2 1.4 |η| MC/Data

(arXiv:1205.4105) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 7 / 15

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SLIDE 8

Baryon production

The new models allow for additional production of junction structures Comparison between maximal saturation and no saturation as a function rapidity. Only directly produced particles (HadronLevel:decay = off)

1 2 3 4 5 6 7 8 9 10

d|y| dN

events

N 1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Monash (all particles) Maximum saturation (all) No saturation (all) Monash (Baryons) Maximum saturation (Baryons) No saturation (Baryons)

|y|

1 2 3 4 5 6 7 8 9 10

max saturation no saturation

0.7 0.8 0.9 1 1.1 1.2

All particles Baryons

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 8 / 15

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SLIDE 9

New colour reconnection model

Colour reconnection allows us to reshuffle the colours before hadronization New model relies on two main principles

◮ SU(3) colour rules from

QCD - tells us which reconnections are allowed

◮ minimize λ measure - tells

us which reconnections are preferred

Before colour reconnection

P P q q q q

After colour reconnection

P P q q q q

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 9 / 15

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SLIDE 10

Possible reconnections

Ordinary string reconnection

(qq: 1/9, gg: 1/8, model: 1/9)

Triple junction reconnection

(qq: 1/27, gg: 5/256, model: 2/81)

Double junction reconnection

(qq: 1/3, gg: 10/64, model: 2/9)

Zipping reconnection

(Depends on number of gluons)

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 10 / 15

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SLIDE 11

The λ measure

The λ-measure is the rapidity span of a string λ ≈

dipoles log(1 + si 2m2

0 )

Add free parameter for minimum gain for junction structures (allow negative for enhancement)

Generalization of λ-measure (s ≫ m2

0)

λ = log(1 +

s 2m2

0 ) ⇒

λ = log(

√ 2E1 m0 ) + log( √ 2E2 m0 )

(dipole restframe) Interpret as contributions from each dipole end, similar for junctions except for three legs: λ = log(

√ 2E1 m0 ) + log( √ 2E2 m0 ) + log( √ 2E3 m0 )

To handle (s ∼ m2

0):

log(

√ 2E1 m0 ) → log(1 + √ 2E1 m0 )

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 11 / 15

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SLIDE 12

Comparison to LHC data

b b b b b b b b b b

Data

b

New model Monash 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Λ/K0

S versus rapidity at √s = 7 TeV

N(Λ) / N(K0

S)

0.5 1 1.5 2 0.6 0.8 1 1.2 1.4 |y| MC/Data

(arXiv:1102.4282)

b b b b b b b b b b b b b b b b b b b b b b b b

Data

b

New model Monash 0.2 0.4 0.6 0.8 1 1.2 1.4 Λ/K0

S versus transverse momentum at √s = 7 TeV

N(Λ) / N(K0

S)

2 4 6 8 10 0.6 0.8 1 1.2 1.4 1.6 pT [GeV/c] MC/Data

(arXiv:1102.4282)

Can describe Λ/Ks ratios (tuned)

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 12 / 15

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SLIDE 13

Distinguish new model from old model

Observables to distinguish junction baryons from diquark baryons Best observable found so far can be seen on the right (again hadron decays are turned off) Still looking for more

  • bservables

The difference between Monash and the diquark curve can be understood by looking at the masses of the strings

Multiplicity 50 100 150 200 250 300 >

Baryons

<N 5 10 15 20 25 30 35 40 45

All (new model) Junctions (new model) diquark (new model) All (Monash)

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 13 / 15

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SLIDE 14

Distinguish new model from old model

Observables to distinguish junction baryons from diquark baryons Best observable found so far can be seen on the right (again hadron decays are turned off) Still looking for more

  • bservables

The difference between Monash and the diquark curve can be understood by looking at the masses of the strings

[GeV])

string

Log (M

  • 1
  • 0.5

0.5 1 1.5 2 2.5 3 3.5 4 dLog(M) dN

evt

N 1 0.5 1 1.5 2 2.5 3

All strings Junctions Ordinary strings Monash

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 14 / 15

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SLIDE 15

Conclusion

Only possible to distinguish new beam remnant model from old model in very forward regions The new colour reconnection model can be used to describe the Λ-production Both models are released along with Pythia 8.2 Future plan:

◮ Identify more observables that can distinguish junction baryons from

diquark baryons

◮ Apply model to the top mass measurement Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 15 / 15

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SLIDE 16

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 15 / 15

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SLIDE 17

Baryon production

The new models allow for additional production of junction structures Comparison between maximal saturation and no saturation as a function rapidity. Only directly produced particles (HadronLevel:decay = off)

1 2 3 4 5 6 7 8 9 10

| η d| dN

events

N 1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Monash (all particles) Maximum saturation (all) No saturation (all) Monash (Baryons) Maximum saturation (Baryons) No saturation (Baryons)

| η |

1 2 3 4 5 6 7 8 9 10

max saturation no saturation

0.7 0.8 0.9 1 1.1 1.2

All particles Baryons

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 15 / 15

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SLIDE 18

Tuning

LEP tuning

par Monash new σp⊥ 0.335 0.305 aLund 0.68 0.38 bLund 0.98 0.64 StoUD 0.217 0.19

First tune iteration, still needs several additional iterations

LHC tuning

par Monash new pref

⊥0

2.28 2.15 m0

  • 2.8

MinGainJun

  • 0.65
b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

Data

b

New model Monash 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Mean p⊥ vs charged hadron multiplicity, |η| < 2.4, √s = 7 TeV p⊥ [GeV] 20 40 60 80 100 120 140 160 180 0.6 0.8 1 1.2 1.4 n MC/Data

(arXiv:1011.5531) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 15 / 15

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SLIDE 19

Additional details

Only local minimization Ignore dipoles with invariant mass below m0 No annihilation of junctions

  • Start with ordinary

reconnection The hadronization can not handle junction connected with other junctions - need to split them up (see examples)

Gluon splitting Double junction Multi junction

Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 15 / 15