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

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 model New colour reconnection model Conclusion Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 2 / 15

b b b b b b b b b b b Motivation S versus rapidity at √ s = 7 TeV Λ /K 0 We want to introduce more S ) 0 . 7 N ( Λ ) / N ( K 0 0 . 6 of the SU(3) structure from 0 . 5 QCD into the description 0 . 4 0 . 3 Data 0 . 2 Monash Provide a better description 0 . 1 of especially Λ production at 0 1 . 4 MC/Data 1 . 2 hadron colliders. 1 0 . 8 0 . 6 0 0 . 5 1 1 . 5 2 Top mass measurement - see | y | T. Sj¨ ostrand’s talk (arXiv:1102.4282) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 3 / 15

New beam remnant model Beam Remnant 1 The beam remnant model comes after the perturbative machinery Overall idea of the model: MPI 1 MPI 2 ◮ A game of conservation laws ◮ Add the minimal required ... amount of extra particles Beam Remnant 2 - Example of two scattered gluons from a proton: Flavour conservation Energy/momentum Add two up and one down quark conservation Choose x according to modified Baryon number conservation PDFs and rescale to match overall momentum conservation Turn two quarks into a diquark Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 4 / 15

New beam remnant model - colour conservation Possible colour states for the two gluons: 8 ⊗ 8 = 27 ⊕ 10 ⊕ 10 ⊕ 8 ⊕ 8 ⊕ 1 27 8 10 1 10 2 C & 2 AC 1 C & 1 AC 3 C & 0 AC 0 C & 0 AC 0 C & 3 AC + 1 gluon + 1 gluon + 0 gluon + 0 gluon + 0 gluon (junction) (not allowed) (junction) Examples of the 27 and the 8 configurations: Beam Remnant 1 Beam Remnant 1 MPI 1 MPI 2 MPI 1 MPI 2 ... ... Beam Remnant 2 Beam Remnant 2 Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 5 / 15

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

b b b b b b b b b b b b b b b b b b b Comparisons to data Charged particle | η | at 7 TeV, track p ⊥ > 40 MeV, for N ch ≥ 1 dN / d η 4 . 5 Relative large x and small 4 3 . 5 p ⊥ ⇒ forward physics 3 2 . 5 Data Comparison to forward 2 Max saturation 1 . 5 No saturation Monash TOTEM measurements. 1 0 . 5 0 10 % difference between no 1 . 4 MC/Data 1 . 2 and maximal saturation 1 0 . 8 0 . 6 The old model is similar to 5 . 4 5 . 6 5 . 8 6 . 2 6 . 4 6 | η | maximal saturation (arXiv:1205.4105) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 7 / 15

Baryon production 0.8 d|y| dN Monash (all particles) events 0.7 The new models allow for 1 Maximum saturation (all) N No saturation (all) 0.6 additional production of Monash (Baryons) 0.5 Maximum saturation (Baryons) junction structures No saturation (Baryons) 0.4 Comparison between 0.3 maximal saturation and no 0.2 saturation as a function 0.1 rapidity. 0 1 2 3 4 5 6 7 8 9 10 All particles 1.2 max saturation Baryons no saturation 1.1 Only directly produced 1 0.9 particles 0.8 (HadronLevel:decay = off) 0.7 0 1 2 3 4 5 6 7 8 9 10 |y| Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 8 / 15

New colour reconnection model Before colour reconnection q q Colour reconnection allows us to reshuffle the colours P P before hadronization q New model relies on two q main principles After colour reconnection ◮ SU(3) colour rules from q q QCD - tells us which reconnections are allowed ◮ minimize λ measure - tells P P us which reconnections are preferred q q Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 9 / 15

Possible reconnections Ordinary string reconnection Double junction reconnection (qq: 1/9, gg: 1/8, model: 1/9) (qq: 1/3, gg: 10/64, model: 2/9) Triple junction reconnection Zipping reconnection (qq: 1/27, gg: 5/256, model: 2/81) (Depends on number of gluons) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 10 / 15

The λ measure Generalization of λ -measure ( s ≫ m 2 0 ) s λ = log(1 + 0 ) ⇒ The λ -measure is the 2 m 2 √ √ 2 E 1 2 E 2 λ = log( m 0 ) + log( m 0 ) rapidity span of a string (dipole restframe) s i λ ≈ � dipoles log(1 + 0 ) 2 m 2 Interpret as contributions from each dipole end, similar for junctions except Add free parameter for for three legs: minimum gain for junction √ √ √ 2 E 1 2 E 2 2 E 3 λ = log( m 0 ) + log( m 0 ) + log( m 0 ) structures (allow negative for enhancement) To handle ( s ∼ m 2 0 ): √ √ 2 E 1 2 E 1 log( m 0 ) → log(1 + m 0 ) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 11 / 15

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 Comparison to LHC data S versus rapidity at √ s = 7 TeV S versus transverse momentum at √ s = 7 TeV Λ /K 0 Λ /K 0 S ) 0 . 7 S ) N ( Λ ) / N ( K 0 N ( Λ ) / N ( K 0 1 . 4 Data 0 . 6 New model 1 . 2 Monash 0 . 5 1 0 . 4 0 . 8 0 . 3 0 . 6 Data 0 . 2 New model 0 . 4 Monash 0 . 1 0 . 2 0 0 1 . 6 1 . 4 1 . 4 MC/Data 1 . 2 MC/Data 1 . 2 1 1 0 . 8 0 . 8 0 . 6 0 . 6 0 0 . 5 1 1 . 5 2 0 2 4 6 8 10 | y | p T [GeV/ c ] (arXiv:1102.4282) (arXiv:1102.4282) Can describe Λ / K s ratios (tuned) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 12 / 15

Distinguish new model from old model Observables to distinguish junction baryons from diquark baryons 45 > Baryons Best observable found so far All (new model) 40 <N Junctions (new model) can be seen on the right 35 diquark (new model) (again hadron decays are All (Monash) 30 turned off) 25 20 Still looking for more 15 observables 10 The difference between 5 0 Monash and the diquark 0 50 100 150 200 250 300 Multiplicity curve can be understood by looking at the masses of the strings Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 13 / 15

Distinguish new model from old model Observables to distinguish junction baryons from diquark baryons dLog(M) 3 dN Best observable found so far All strings Junctions evt can be seen on the right 1 N 2.5 Ordinary strings (again hadron decays are Monash 2 turned off) 1.5 Still looking for more 1 observables 0.5 The difference between 0 Monash and the diquark -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 Log (M [GeV]) string curve can be understood by looking at the masses of the strings Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 14 / 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

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

Baryon production 0.8 dN | η d| events Monash (all particles) 0.7 The new models allow for 1 Maximum saturation (all) N No saturation (all) 0.6 additional production of Monash (Baryons) 0.5 Maximum saturation (Baryons) junction structures No saturation (Baryons) 0.4 Comparison between 0.3 maximal saturation and no 0.2 saturation as a function 0.1 rapidity. 0 1 2 3 4 5 6 7 8 9 10 All particles 1.2 max saturation Baryons no saturation 1.1 Only directly produced 1 0.9 particles 0.8 (HadronLevel:decay = off) 0.7 0 1 2 3 4 5 6 7 8 9 10 η | | Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 15 / 15

b b b b b b b b b b b Tuning LHC tuning par Monash new LEP tuning p ref 2.28 2.15 ⊥ 0 m 0 - 2.8 par Monash new MinGainJun - -0.65 0.335 0.305 σ p ⊥ aLund 0.68 0.38 Mean p ⊥ vs charged hadron multiplicity, | η | < 2.4, √ s = 7 TeV bLund 0.98 0.64 0 . 9 � p ⊥ � [GeV] StoUD 0.217 0.19 0 . 8 Data New model 0 . 7 Monash 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 0 . 6 0 . 5 0 . 4 0 . 3 0 . 2 0 . 1 0 1 . 4 First tune iteration, still MC/Data 1 . 2 1 needs several additional 0 . 8 0 . 6 iterations 20 40 60 80 100 120 140 160 180 n (arXiv:1011.5531) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 15 / 15

Additional details Gluon splitting Only local minimization Ignore dipoles with invariant mass below m 0 No annihilation of junctions Double junction - Start with ordinary reconnection The hadronization can not handle junction connected Multi junction with other junctions - need to split them up (see examples) Jesper Roy Christiansen (Lund) Non pertubative colours November 3, MPI@LHC 15 / 15