S o f t P h e n o m e n o l o g y P. S k a n d s ( C E R N ) H a - PowerPoint PPT Presentation

S o f t P h e n o m e n o l o g y P. S k a n d s ( C E R N ) H a d r o n C o l l i d e r P h y s i c s S y m p o s i u m , N o v e m b e r 2 0 1 2 , P a r i s

Soft Physics Final-State Interactions? Double Single Diffraction Diffraction Hard Trigger Events Flow? High-Multiplicity Tail Minimum- Bias Inelastic, Non-Diffractive Zero Bias Multiple Parton Interactions (MPI) … DPI Quarkonium Elastic Minijets … Beam Remnants (BR) Strange Low High Multiplicity Multiplicity P . Skands - Soft Phenomenology 2

Terminology σ tot ≈ EXPERIMENT THEORY MODELS pp → pp ( * QED = ∞ ) ELASTIC QED+QCD ~ SINGLE DIFFRACTION pp → p+gap+X ≠ Small gaps suppressed but not zero Gap = observable pp → X+gap+X ≠ Small gaps suppressed but not zero DOUBLE DIFFRACTION Gap = observable pp → X (no gap) ≠ Large gaps suppressed but not zero INELASTIC NON-DIFFRACTIVE Gap = observable (+ multi-gap diffraction) P . Skands - Soft Phenomenology 3

Terminology σ tot ≈ EXPERIMENT THEORY MODELS pp → pp ( * QED = ∞ ) ELASTIC QED+QCD ~ SINGLE DIFFRACTION pp → p+gap+X ≠ Small gaps suppressed but not zero Gap = observable pp → X+gap+X ≠ Small gaps suppressed but not zero DOUBLE DIFFRACTION Gap = observable pp → X (no gap) ≠ Large gaps suppressed but not zero INELASTIC NON-DIFFRACTIVE Gap = observable (+ multi-gap diffraction) MB hit Min-Bias, Zero Bias, Single-Gap, etc. = Experimental trigger conditions (hardware-dependent) Corrected to hardware-independent reference conditions “Theory” for Min-Bias? Really = Model for ALL INELASTIC incl diffraction (model-dependent) Impose model-independent reference conditions to suppress or enhance diffractive components … in minimum-bias, we typically do not have a hard scale, wherefore all observables depend significantly on IR physics … PS, “Tuning MC Generators: the Perugia tunes”, PRD82(2010)074018 P . Skands - Soft Phenomenology 3

A Factorized View 1. Where is the energy going? Note: only linearized Sphericity is IR safe IR Safe Sum(pT) densities, event shapes, mini-jet rates, ctrl&fwd energy flow, energy correlations… ≈ sensitive to pQCD + pMPI 2. How many tracks is it divided onto? N tracks , dN tracks /dp T, Associated track densities, track correlations… ≈ sensitive to hadronization + soft MPI IR Sensitive 3. Are there gaps in it? Created by diffraction (and color reconnections?). Destroyed by UE. More IR 4. What kind of tracks? Sensitive Strangeness per track, baryons per track, baryon asymmetry, … hadron-hadron correlations ≈ sensitive to details of hadronization + collective effects (+Quarkonium sensitive to color reconnections?) P . Skands - Soft Phenomenology 4

Organized Tuning Can we be more general than this- tune-does-this, that-tune-does-that? Yes Schulz & PS , Eur.Phys.J. C71 (2011) 1644 The new automated tuning tools can be used to generate unbiased optimizations for different observable regions Same parameters → consistent model (not just “best tune”) Critical for this task (take home message): Need “comparable” observable sets for each region Example: use different collider energies as “regions” → test energy scaling Other complementary data sets could be used to test other model aspects P . Skands 5

QCD Models A A) Start from pQCD. Extend towards Infrared. HERWIG/JIMMY, PYTHIA, SHERPA Elastic & Diffractive Unitarity Quarks, Gluons Color Screening Treated as separate class Multiple 2 → 2 pQCD Regularization of pQCD No predictivity (MPI) 2 → 2 (Rutherford) PYTHIA uses string fragmentation , HERWIG & SHERPA use cluster fragmentation Elastic Min-Bias Dijets ∞ 0 Λ QCD 5 GeV B B) Start from Optical Theorem. Extend towards Ultraviolet. PHOJET, DPMJET Hadrons Pomerons: Diffraction Optical Theorem Hard Pomeron? Cut Pomerons: Non-diffractive (soft) pp → pp Note: PHOJET & DPMJET use string fragmentation (from PYTHIA) → some overlap P . Skands - Soft Phenomenology 6

Multi-Parton Interactions A) Start from pQCD. Extend towards Infrared. HERWIG/JIMMY, PYTHIA, SHERPA pQCD Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph] 2 → 2 = Sum of Dijet Cross Section vs p T cutoff ≈ Rutherford (t-channel gluon) !"#$%&'()*+,'*,- ./.,)&0.% ")&,'(12/)% P . Skands - Soft Phenomenology 7

Multi-Parton Interactions A) Start from pQCD. Extend towards Infrared. HERWIG/JIMMY, PYTHIA, SHERPA Becomes larger pQCD than total pp Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph] cross section? 2 → 2 At p ⊥ ≈ 5 GeV = Sum of f f o t ) n u Dijet Cross Section o C s i r r e a w p vs p T cutoff m o h o S c n r o o f t ( r a P ≈ Rutherford (t-channel gluon) !"#$%&'()*+,'*,- ./.,)&0.% ")&,'(12/)% P . Skands - Soft Phenomenology 7

Multi-Parton Interactions A) Start from pQCD. Extend towards Infrared. HERWIG/JIMMY, PYTHIA, SHERPA Becomes larger pQCD than total pp Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph] cross section? 2 → 2 At p ⊥ ≈ 5 GeV = Sum of f f o t ) n Lesson from u Dijet Cross Section o C s i r r e bremsstrahlung in a w p vs p T cutoff m o h o pQCD: divergences S c n r o o f t → fixed-order ( r a P unreliable, but pQCD still ok ≈ Rutherford if resummed (t-channel gluon) (unitarity) !"#$%&'()*+,'*,- → Resum dijets? ./.,)&0.% ")&,'(12/)% Yes → MPI! P . Skands - Soft Phenomenology 7

Color Space

Questions Colour Connections Each MPI (or cut Pomeron) exchanges color between the beams ! The colour flow determines the hadronizing string topology • Each MPI, even when soft, is a color spark Different models • Final distributions crucially depend on color space make different ansätze 1 2 # of 3 strings 4 2 9

Questions Colour Connections Each MPI (or cut Pomeron) exchanges color between the beams ! The colour flow determines the hadronizing string topology • Each MPI, even when soft, is a color spark Different models • Final distributions crucially depend on color space make different ansätze 1 2 # of 3 strings 5 3 10

Models 1. Most naive E.g., PYTHIA 6 with PARP(85)=0.0 & JIMMY/Herwig++ Each MPI ~ independent → separate singlets? Physically inconsistent with exchanged objects being gluons Corresponds to the exchange of singlets (uncut Pomerons) → All the MPI are diffractive! This is just wrong. 11 P. Skands

Models 1. Most naive E.g., PYTHIA 6 with PARP(85)=0.0 & JIMMY/Herwig++ Each MPI ~ independent → separate singlets? Physically inconsistent with exchanged objects being gluons Corresponds to the exchange of singlets (uncut Pomerons) → All the MPI are diffractive! This is just wrong. 2. Valence quarks plus t-channel gluons? Arrange original proton as (qq)-(q) system, arrange MPI gluons as (qq)-g-g-g-(q) In which order? Some options: A) Random (Perugia 2010 & 2011) or B) According to rapidity of hard 2 → 2 systems (Perugia 0) C) By hand, according to rapidity of each outgoing gluon (Tune A, DW, Q20, … + HIJING?) May be more physical … But both A & B fail on, e.g., the observed rise of <pT>(Nch) (and C “cheats” by looking at final-state gluons) This must still be wrong (though less obvious) 11 P. Skands

Color Reconnections? N C → ∞ Rapidity Multiplicity ∝ N MPI P . Skands - Soft Phenomenology 12

Color Reconnections? Do the systems really hadronize independently? Can Gaps be Created? Rapidity < Multiplicity ∝ N MPI P . Skands - Soft Phenomenology 13

Color Reconnections? In reality: The color wavefunction is N C = 3 when it collapses One parton “far away” from others will only see the sum of their colours → coherence in string formation On top of this, the systems may merge/fuse/interact with genuine dynamics (e.g., string area law) And they may continue to do so even after hadronization Elastically: hydrodynamics? Collective flow? New: basic hadron 2 → 2 re- interaction model in PYTHIA 8.157 Inelastically: re-interactions? This may not be wrong. But it sounds difficult! → Color Reconnections (in PYTHIA) , Color Disruption (in HERWIG) 14 P. Skands

B r i e f N ew s f ro m P Y T H I A 8 PYTHIA 8.157 released Nov 11

Diffraction 100 Diffractive Cross Section Formulæ: Pythia 8.130 d t d M 16 π M Pythia 6.414 SD d σ sd( AX ) ( s ) g 3I 1 P 16 π β 2 = M 2 exp( B sd( AX ) t ) F sd , 10 P β B I Phojet 1.12 P A I d t d M 2 g 2 d σ dd ( s ) 1 1 3I P = exp( B dd t ) F dd . 16 π β A I P β B I P 1 d t d M 2 1 d M 2 M 2 M 2 2 1 2 0.1 Partonic Substructure in Pomeron: p i � p i 0.01 LRG x Follows the Ingelman- 0.001 Schlein approach of x g Pompyt X 0.0001 p j 0 2 4 6 8 10 pT (GeV) � M X ≤ 10 GeV : original longitudinal string description used � M X > 10 GeV : new perturbative description used (incl full MPI+showers for system) PYTHIA 8 to I Pp ha 4) Choice between 5 Pomeron PDFs. n showers Four parameterisations of the pomeron flux available Free parameter σ I Pp needed to fix � n interactions � = σ jet / σ I Pp . 4) Choice between 5 Pomeron PDFs. Free parameter needed to fix 5) Framework needs testing and tuning, e.g. of . 5) Framework needs testing and tuning, e.g. of σ I Pp . Navin, arXiv:1005.3894 P . Skands - Soft Phenomenology 16