Colour Reconnection - Models and Tests Jesper Roy Christiansen Lund - - PowerPoint PPT Presentation

Colour Reconnection - Models and Tests

Jesper Roy Christiansen

Lund University

July 23, 2015 EPS HEP, Vienna

JRC (Lund) Colour Reconnection July 23, EPS 1 / 10

Introduction - What is Colour Reconnection?

CR address the question: between which partons do strings form?

Before colour reconnection

P P q q q q

After colour reconnection?

P P q q q q

Experimentally needed to explain the rise of p⊥ with multiplicity

ch

N 50 100 150 200 [GeV] 〉

T

p 〈 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

ATLAS Pythia 8 (Def) Pythia 8 (no CR)

7000 GeV pp

Soft QCD

mcplots.cern.ch 7.4M events ≥ Rivet 1.8.3, Pythia 8.176 ATLAS_2010_S8918562 > 0.1 GeV/c)

T

> 2, p

ch

(N

ch

vs N

T

Average p

50 100 150 200 0.5 1 1.5

Ratio to ATLAS

JRC (Lund) Colour Reconnection July 23, EPS 2 / 10

Introduction - Why now?

Observed Λ enhancement at LHC

◮ Explained by introducing

“junction” structures in CR

◮ Other models also explain

data, e.g. EPOS and DIPSY

To understand p⊥-spectra of identified hadrons (not addressed further in this talk)

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

Pythia 0.2 0.4 0.6 0.8 1 1.2 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 pT [GeV/c] MC/Data

JRC (Lund) Colour Reconnection July 23, EPS 3 / 10

The new CR model

The new CR model reshuffles the colours just prior to hadronization based on three main principles: Use the SU(3) colour rules to determine if two strings are colour compatible Use a simplistic space-time picture to tell if the two strings coexist Minimize λ string-length measure to find which colour configurations Nature prefers Colour epsilon tensor corresponds to a junction structure qiqjqkǫijk q q q J New type of reconnection

JRC (Lund) Colour Reconnection July 23, EPS 4 / 10

Tests - Λ/Ks and Ξ/Λ

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

Pythia Pythia new CR 0.2 0.4 0.6 0.8 1 1.2 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 pT [GeV/c] MC/Data

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

Data

b

Pythia Pythia new CR 0.05 0.1 0.15 0.2 0.25 0.3 Ξ−/Λ versus transverse momentum at √s = 7 TeV N(Ξ−) / N(Λ) 1 2 3 4 5 6 0.6 0.8 1 1.2 1.4 pT [GeV/c] MC/Data

Λ/KS is better described by the model (overall yield is tuned) (No rate change in e+e−) Ξ/Λ is the same as old model - no strangeness enhancement

JRC (Lund) Colour Reconnection July 23, EPS 5 / 10

Multiplicity dependent particle ratios

0.19 0.22 0.25 0.28 N(K)/N(π)

DIPSY Rope DIPSY PYTHIA 8 New PYTHIA 8 Def.

0.003 0.004 0.005 0.006 0.007 N(φ)/N(K) 0.035 0.045 0.055 0.065 0.075 N(p)/N(π) 0.06 0.08 0.10 0.12 0.14 N(Λ)/N(K) 20 40 60 80 100 0.08 0.09 0.10 0.11 0.12 N(Ξ)/N(Λ) 20 40 60 80 100 0.00 0.02 0.04 0.06 0.08 N(Ω)/N(Ξ)

Nfwd

ch

(0.15 GeV < p⊥, 2 < |η| < 5) Enhancement of hadronic flavor ratios

JRC (Lund) Colour Reconnection July 23, EPS 6 / 10

CR and Flow-like effects

Flow-like effects observed in pp is potentially connected with CR Repeat typical HI observable: Λ/K as function of p⊥ separated into different multiplicity intervals (or centrality) Qualitative similar effect seen in the model as in HI collisions

10 < nch < 20 25 < nch < 40 45 < nch < 65 70 < nch 2 4 6 8 10 12 14 0.05 0.1 0.15 0.2 0.25 0.3 p⊥ [GeV] Λ/K

JRC (Lund) Colour Reconnection July 23, EPS 7 / 10

CR in e+e− → W +W −

Clean environment to test CR effects CR established at 2.8 σ Turn table around and use precision studies to constrain CR (e.g. W mass measurement, see table) Dedicated angular studies in fully hadronic WW Multiplicity comparisons between semi-leptonic and fully hadronic WW

Model δmW (MeV) 170 GeV 240 GeV 350 GeV SK-I +18 +95 +72 SK-II

• 14

+29 +18 SK-II’

• 6

+25 +16 GM-I

• 41
• 74
• 50

GM-II +49 +400 +369 GM-III +2 +104 +60 CS +7 +9 +4

Table : Systematic W mass shifts at three different center-of-mass energies.

JRC (Lund) Colour Reconnection July 23, EPS 8 / 10

CR in H → W +W −

Need to include CR as an uncertainty Example: Higgs Parity measurement in WW → q¯ qq¯ q

◮ Select events with four jets ◮ Compare interjet angles using

a simple χ2 test

◮ Compare between different CR

models and different amount

• f CP-oddness in the Higgs

◮ The analysis contains room

for improvements

ut ut ut ut ut

no CR

ut

SK-I SK-II SK-II’ CS GM-I GM-II GM-III 0.01 0.02 0.03 0.04 0.05 5 10 15 20 25 Deviation from CP-even Higgs without CR parity fraction χ2/NDF

JRC (Lund) Colour Reconnection July 23, EPS 9 / 10

Conclusion

I presented a new CR model able to describe the Λ/Ks ratio Identified particle ratios as a function of multiplicity is an excellent probe to test CR models Similarity between CR and flow-like effects in pp was presented, more studies still needed CR in e+e− collisions both provides constraints and needs to be included as an uncertainty All the CR models are publicly available in Pythia 8.210 For more details see: arXiv:1507.02091, arXiv:1506.09085, arXiv:1505.01681

JRC (Lund) Colour Reconnection July 23, EPS 10 / 10

JRC (Lund) Colour Reconnection July 23, EPS 10 / 10

Tests - p⊥ boosts

b b b b b b b b b b b b b b bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc

π− K− K0 ρ0 K∗0 ¯ p φ Λ ¯ Λ Ξ− ¯ Ξ+ Σ Λ (Ω + ¯ Ω) Data

b

Pythia

bc

Pythia new CR

bc

0.2 0.4 0.6 0.8 1 1.2 1.4 Mean p⊥ vs particle mass p⊥ [GeV]

bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0.6 0.8 1 1.2 1.4 mass [GeV] MC/Data

b b b b b b b b b bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc

Data

b

Pythia

bc

Pythia new CR

bc

1 Mean pT vs mass in INEL pp collisions at √s = 7 TeV in |y| < 0.5. pT (GeV/c)

bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0.6 0.8 1 1.2 1.4 m (GeV/c2) MC/Data

Expected larger boosts for heavier particles - no effects for new model Discrepancy largest at low CM energies

JRC (Lund) Colour Reconnection July 23, EPS 10 / 10