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 Experimentally needed to explain which partons do strings form? the rise of � p ⊥ � with multiplicity Before colour reconnection 7000 GeV pp Soft QCD q [GeV] 7.4M events 1 q Average p vs N (N > 2, p > 0.1 GeV/c) T ch ch T 〉 ATLAS T 0.9 p Pythia 8 (Def) ≥ 〈 Rivet 1.8.3, Pythia 8 (no CR) 0.8 P P 0.7 0.6 q 0.5 q mcplots.cern.ch 0.4 After colour reconnection? ATLAS_2010_S8918562 0.3 Pythia 8.176 q 0 50 100 150 200 q N ch Ratio to ATLAS 1.5 P P 1 q 0.5 q 0 50 100 150 200 JRC (Lund) Colour Reconnection July 23, EPS 2 / 10

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 Introduction - Why now? S versus transverse momentum at √ s = 7 TeV Observed Λ enhancement at Λ /K 0 S ) N ( Λ ) / N ( K 0 LHC 1 . 2 Data Pythia 1 ◮ Explained by introducing 0 . 8 “junction” structures in CR 0 . 6 ◮ Other models also explain 0 . 4 0 . 2 data, e.g. EPOS and DIPSY 0 1 . 4 MC/Data 1 . 2 To understand p ⊥ -spectra of 1 0 . 8 0 . 6 identified hadrons (not 0 2 4 6 8 10 p T [GeV/ c ] addressed further in this talk) JRC (Lund) Colour Reconnection July 23, EPS 3 / 10

The new CR model The new CR model reshuffles the Colour epsilon tensor colours just prior to hadronization corresponds to a junction based on three main principles: structure q Use the SU(3) colour rules to determine if two strings are J q i q j q k ǫ ijk q colour compatible q Use a simplistic space-time picture to tell if the two strings coexist New type of reconnection Minimize λ string-length measure to find which colour configurations Nature prefers JRC (Lund) Colour Reconnection July 23, EPS 4 / 10

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 Tests - Λ / K s and Ξ / Λ S versus transverse momentum at √ s = 7 TeV Ξ − / Λ versus transverse momentum at √ s = 7 TeV Λ /K 0 S ) N ( Ξ − ) / N ( Λ ) N ( Λ ) / N ( K 0 0 . 3 1 . 2 Data Data Pythia Pythia 0 . 25 1 Pythia new CR Pythia new CR 0 . 8 0 . 2 0 . 15 0 . 6 0 . 4 0 . 1 0 . 2 0 . 05 0 0 1 . 4 1 . 4 MC/Data 1 . 2 MC/Data 1 . 2 1 1 0 . 8 0 . 8 0 . 6 0 . 6 0 2 4 6 8 10 0 1 2 3 4 5 6 p T [GeV/ c ] p T [GeV/ c ] Λ / K S 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 Enhancement of hadronic flavor ratios 0.28 0.007 D IPSY Rope P YTHIA 8 New 0.006 N ( K ) /N ( π ) D IPSY P YTHIA 8 Def. N ( φ ) /N ( K ) 0.25 0.005 0.22 0.004 0.19 0.003 0.075 0.14 0.065 0.12 N (Λ) /N ( K ) N ( p ) /N ( π ) 0.055 0.10 0.045 0.08 0.035 0.06 0.12 0.08 0.11 0.06 N (Ξ) /N (Λ) N (Ω) /N (Ξ) 0.10 0.04 0.09 0.02 0.08 0.00 0 20 40 60 80 100 0 20 40 60 80 100 N fwd (0 . 15 GeV < p ⊥ , 2 < | η | < 5) ch 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 Λ / K 0 . 3 10 < n ch < 20 25 < n ch < 40 0 . 25 Repeat typical HI observable: 45 < n ch < 65 70 < n ch 0 . 2 Λ / K as function of p ⊥ separated 0 . 15 into different multiplicity 0 . 1 intervals (or centrality) 0 . 05 Qualitative similar effect seen in 0 2 4 6 8 10 12 14 p ⊥ [GeV] the model as in HI collisions JRC (Lund) Colour Reconnection July 23, EPS 7 / 10

CR in e + e − → W + W − Clean environment to test CR effects � δ m W � (MeV) Model CR established at 2.8 σ 170 GeV 240 GeV 350 GeV SK-I +18 +95 +72 Turn table around and use SK-II -14 +29 +18 precision studies to constrain CR SK-II’ -6 +25 +16 GM-I -41 -74 -50 (e.g. W mass measurement, see GM-II +49 +400 +369 table) GM-III +2 +104 +60 CS +7 +9 +4 Dedicated angular studies in fully hadronic WW Table : Systematic W mass shifts at three different center-of-mass energies. Multiplicity comparisons between semi-leptonic and fully hadronic WW JRC (Lund) Colour Reconnection July 23, EPS 8 / 10

ut ut ut ut ut ut CR in H → W + W − Need to include CR as an uncertainty Deviation from CP-even Higgs without CR 25 χ 2 /NDF no CR CS Example: Higgs Parity SK-I GM-I 20 SK-II GM-II measurement in WW → q ¯ qq ¯ q SK-II’ GM-III 15 ◮ Select events with four jets ◮ Compare interjet angles using 10 a simple χ 2 test 5 ◮ Compare between different CR 0 models and different amount 0 0.01 0.02 0.03 0.04 0.05 parity fraction of CP-oddness in the Higgs ◮ The analysis contains room for improvements JRC (Lund) Colour Reconnection July 23, EPS 9 / 10

Conclusion I presented a new CR model able to describe the Λ / K s 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

bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc b bc bc bc bc bc b bc b b b b b b b bc bc bc bc bc bc bc bc bc bc bc bc b bc bc bc bc bc bc bc bc b bc bc bc bc bc bc bc bc bc bc bc bc b bc 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 bc bc bc bc b Tests - p ⊥ boosts Mean p T vs mass in INEL pp collisions at √ s = 7 TeV in | y | < 0.5. Mean p ⊥ vs particle mass � p ⊥ � [GeV] � p T � (GeV/ c ) 1 . 4 Data Data Pythia Pythia 1 . 2 Pythia new CR Pythia new CR 1 1 0 . 8 0 . 6 0 . 4 K 0 ¯ Ξ + ¯ p ¯ Λ 0 . 2 π − K − ρ 0 K ∗ 0 Ξ − Λ ( Ω + ¯ φ Λ Σ Ω ) 0 1 . 4 1 . 4 MC/Data 1 . 2 MC/Data 1 . 2 1 1 0 . 8 0 . 8 0 . 6 0 . 6 0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 m (GeV/ c 2 ) mass [GeV] 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