The Infrared Confinement Hadronization Underlying Event & - PowerPoint PPT Presentation

TASI 2012 The Infrared Confinement Hadronization Underlying Event & Soft QCD interactions Disclaimer Focus on ideas, make you think about the physics P . Skands (CERN)

From Partons to Pions Here’s a fast parton It ends up It showers It starts at a high at an effective (bremsstrahlung) factorization scale factorization scale Q = Q F = Q hard Q ~ m ρ ~ 1 GeV Q Q hard 1 GeV QCD Lecture V P . Skands 2

From Partons to Pions Here’s a fast parton It ends up It showers It starts at a high at an effective (bremsstrahlung) factorization scale factorization scale Q = Q F = Q hard Q ~ m ρ ~ 1 GeV Q Q hard 1 GeV How about I just call it a hadron? QCD Lecture V P . Skands 3

From Partons to Pions Here’s a fast parton It ends up It showers It starts at a high at an effective (bremsstrahlung) factorization scale factorization scale Q = Q F = Q hard Q ~ m ρ ~ 1 GeV Q Q hard 1 GeV How about I just call it a hadron? QCD → “Local Parton-Hadron Duality” Lecture V P . Skands 3

Parton → Hadrons? Parton Hadron Duality Universal fragmentation of a parton into hadrons π π q π QCD Lecture V *LPHD = Local Parton Hadron Duality P . Skands 4

Parton → Hadrons? Parton Hadron Duality Universal fragmentation of a parton into hadrons π π q π But … The point of confinement is that partons are colored Hadronization = the process of color neutralization I.e, the one question NOT addressed by LPHD or I.F. → Unphysical to think about independent fragmentation of individual partons QCD Lecture V *LPHD = Local Parton Hadron Duality P . Skands 4

Color Neutralization A physical hadronization model Should involve at least TWO partons, with opposite color charges (e.g., R and anti-R ) non-perturbative Late times (non-perturbative) anti-R moving along right lightcone R m o v i n g a l o n g l e f t l i g h pQCD Time t c o n e Early times (perturbative) Space Strong “confining” field emerges between the two charges QCD when their separation > ~ 1fm Lecture V P . Skands 5

Linear Confinement Lattice QCD: Potential between a quark and an antiquark as function of distance, R “Quenched” Lattice QCD QCD Lecture V P . Skands 6

Linear Confinement Lattice QCD: Potential between a quark and an antiquark as function of distance, R “Quenched” Lattice QCD Short Distances ~ pQCD Partons QCD Lecture V P . Skands 6

Linear Confinement Lattice QCD: Potential between a quark and an antiquark as function of distance, R “Quenched” Lattice QCD Short Distances ~ pQCD Partons QCD Lecture V P . Skands 6

Linear Confinement Long Distances ~ Linear Lattice QCD: Potential between a quark Confinement and an antiquark as function of distance, R “Quenched” Lattice QCD Hadrons Short Distances ~ pQCD Partons QCD Lecture V P . Skands 6

Linear Confinement Long Distances ~ Linear Lattice QCD: Potential between a quark Confinement and an antiquark as function of distance, R “Quenched” Lattice QCD Hadrons Short Distances ~ pQCD Partons QCD Lecture V P . Skands 6

Linear Confinement Long Distances ~ Linear Lattice QCD: Potential between a quark Confinement and an antiquark as function of distance, R “Quenched” Lattice QCD Hadrons Question: Short Distances ~ pQCD What physical system has a linear potential? Partons QCD Lecture V P . Skands 6

From Partons to Strings Motivates a model: Let color field collapse into a (infinitely) narrow flux tube of uniform energy density κ ~ 1 GeV / fm → Relativistic 1+1 dimensional worldsheet – string QCD Lecture V P . Skands 7

String Breaks In “unquenched” QCD g → qq → The strings would break QCD Lecture V Illustrations by T. Sjöstrand P . Skands 8

H a d ro n i z a t i o n M o d e l s The problem: • Given a set of partons resolved at a scale of ~ 1 GeV (the perturbative cutoff), need a “mapping” from this set onto a set of on-shell colour-singlet (i.e., confined) hadronic states. MC models do this in three steps 1. Map partons onto continuum of excited hadronic states (called ‘strings’ or ‘clusters’) 2. Iteratively map strings/clusters onto discrete set of primary hadrons (string breaks / cluster splittings / cluster decays) 3. Sequential decays into secondary hadrons (e.g., ρ > π π , Λ 0 > n π 0 , π 0 > γγ , ...) Distance Scales ~ 10 -15 m = 1 fermi

Color Space

Color Flow Between which partons do confining potentials arise? Set of simple rules for color flow, based on large-N limit QCD Illustrations from: P .Nason & P .S., Lecture (Never Twice Same Color: true up to O(1/N C2 )) V PDG Review on MC Event Generators , 2012 P . Skands 11

Color Flow Between which partons do confining potentials arise? Set of simple rules for color flow, based on large-N limit q → qg QCD Illustrations from: P .Nason & P .S., Lecture (Never Twice Same Color: true up to O(1/N C2 )) V PDG Review on MC Event Generators , 2012 P . Skands 11

Color Flow Between which partons do confining potentials arise? Set of simple rules for color flow, based on large-N limit g → q ¯ q → qg q QCD Illustrations from: P .Nason & P .S., Lecture (Never Twice Same Color: true up to O(1/N C2 )) V PDG Review on MC Event Generators , 2012 P . Skands 11

Color Flow Between which partons do confining potentials arise? Set of simple rules for color flow, based on large-N limit g → q ¯ q → qg q g → gg QCD Illustrations from: P .Nason & P .S., Lecture (Never Twice Same Color: true up to O(1/N C2 )) V PDG Review on MC Event Generators , 2012 P . Skands 11

From Partons to Strings Illustrations by T. Sjöstrand Map: • Quarks → String Endpoints • Gluons → Transverse Excitations (kinks) Strings stretched from q endpoint, via any number of gluons, to qbar endpoint QCD Lecture V P . Skands 12

Color Flow Example: Z 0 → qq 1 1 3 2 5 4 7 1 1 4 5 5 3 3 4 7 6 2 2 String #1 String #2 String #3 Coherence of pQCD cascades → not much “overlap” between strings → planar approx pretty good ( LEP measurements in WW confirm this (at least to order 10% ~ 1/N c2 ) ) QCD Note: (much) more color getting kicked around in hadron collisions → color reconnections important there? … Lecture V P . Skands 13

Color Flow For an entire Cascade Example: Z 0 → qq 1 1 3 2 5 4 7 1 1 4 5 5 3 3 4 7 6 2 2 String #1 String #2 String #3 Coherence of pQCD cascades → not much “overlap” between strings → planar approx pretty good ( LEP measurements in WW confirm this (at least to order 10% ~ 1/N c2 ) ) QCD Note: (much) more color getting kicked around in hadron collisions → color reconnections important there? … Lecture V P . Skands 13

String Breaks QCD Lecture V P . Skands 14

String Breaks String Breaks Modeled by tunneling QCD Lecture V P . Skands 15

String Breaks String Breaks Modeled by tunneling Also depends on: Spins, hadron multiplets, hadronic wave functions, phase space, … → (much) more complicated → many parameters → Not calulable, must be constrained by data → ‘tuning’ QCD Lecture V P . Skands 15

Fragmentation Function Spacetime Picture leftover string, further string breaks M time t The meson M takes a fraction z of the quark momentum, q z How big that fraction is, z ∈ [0,1], spatial is determined by the separation fragmentation function , f(z,Q 02 ) QCD Lecture V P . Skands 16

Fragmentation Function Spacetime Picture leftover string, further string breaks M Spacelike Separation time t The meson M takes a fraction z of the quark momentum, q z How big that fraction is, z ∈ [0,1], spatial is determined by the separation fragmentation function , f(z,Q 02 ) QCD Lecture V P . Skands 16

Left-Right Symmetry Causality → Left-Right Symmetry z → Constrains form of fragmentation function! → Lund Symmetric Fragmentation Function q − b ( m 2 h + p 2 ✓ ◆ f ( z ) ∝ 1 ? h ) z (1 − z ) a exp z Small a Small b → “high-z tail” → “low-z a=0.9 enhancement” a=0.1 b=2 2.0 b=0.5 1.5 1.5 1.0 1.0 0.5 b=1, m T =1 0.5 a=0.5, m T =1 QCD 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 Lecture V P . Skands 17

Iterative String Breaks Causality → May iterate from outside-in u ( � p ⊥ 0 , p + ) shower Q UV � + ( � p ⊥ 1 , z 1 p + ) Q IR p ⊥ 0 − � · · · d ¯ d K 0 ( � p ⊥ 2 , z 2 (1 − z 1 ) p + ) p ⊥ 1 − � s ¯ s ... QCD Lecture Illustration by T. Sjöstrand V P . Skands 18

Alternative: The Cluster Model “Preconfinement” Force g → qq splittings at Q 0 → high-mass qq “clusters” Isotropic 2-body decays to hadrons according to PS ≈ (2s 1 +1)(2s 2 +1)(p * /m) in coherent shower evolution 0 Z + − e e QCD Lecture V P . Skands 19

Alternative: The Cluster Model G Cluster Model “Preconfinement” Force g → qq splittings at Q 0 → high-mass qq “clusters” Isotropic 2-body decays to hadrons Universal according to PS ≈ (2s 1 +1)(2s 2 +1)(p * /m) spectra! in coherent shower evolution 0 Z + − e e QCD Lecture V P . Skands 19