Soft QCD: Theory P e t e r S k a n d s ( C E R N T h e o r e t i c a l P h y s i c s D e p t ) B o s t o n J e t s W o r k s h o p M I T, J a n u a r y 2 1 - 2 3 2 0 1 4
Questions Pileup How much? In central & fwd acceptance? Structure: averages + fluctuations, particle composition, lumpiness, … Scaling to 13 TeV and beyond Underlying Event ~ “A handful of pileup” ? Hadronizes with Main Event → “Color reconnections” Additional “minijets” from multiple parton interactions Hadronization Models from the 80ies, mainly constrained in 90ies Meanwhile, perturbative models have evolved Dipole/Antenna showers, ME matching, NLO corrections, … Precision → re-examine non-perturbative models and constraints New clean constraints from LHC (& future colliders)? Hadronization models ⥂ analytical NP corrections? Uses and Limits of “Tuning” 2 P. S k a n d s
From Hard to Soft Factorization and IR safety ” e g d i R “ S M C Main tools for jet calculations s e i t i c i l p i t l u m k c Corrections suppressed by powers a r T p T spectra of Λ QCD /Q Hard s e l c t i r a P d e fi i t n e d I HADRONIZATION Soft QCD / Pileup Baryon Transport NO HARD SCALE C o r r e l a t i o n s Typical Q scales ~ Λ QCD C e n t r a l v s F o r w a r d Extremely sensitive to IR effects Collective Effects? → Excellent LAB for studying IR effects C o l o r ~ ∞ statistics for min-bias C o r r e l a t i o n s Rapidity Gaps → Access tails, limits Universality: Recycling PU ⬌ MB ⬌ UE 3 P. S k a n d s
What is Pileup / Min-Bias? We use Minimum-Bias (MB) data to test soft-QCD models Pileup = “Zero-bias” “Minimum-Bias” typically suppresses diffraction by requiring two-armed coincidence, and/or ≥ n particle(s) in central region Hit Hit Hit MB SD Veto → NSD → Pileup contains more diffraction than Min-Bias Total diffractive cross section ~ 1/3 σ inel Most diffraction is low-mass → no contribution in central regions High-mass tails could be relevant in FWD region → direct constraints on diffractive components ( → later) 4 P. S k a n d s
The Total Cross Section Pileup rate ∝ σ tot ( s ) = σ el ( s ) + σ inel ( s ) ∝ s 0 . 08 or ln 2 ( s ) ? Donnachie-Landshoff Froissart-Martin Bound σ tot (13 TeV) ∼ 110 ± 6 mb PP CROSS SECTIONS AUGER PYTHIA: 100 mb TOTEM, PRL 111 (2013) 1, 012001 σ inel (13 TeV) ∼ 80 ± 3 . 5 mb TOTEM PYTHIA: 78 mb AUGER TOTEM ALICE (2 . 9%) σ tot (8 TeV) = 101 ± 2 . 9 mb ALICE 13 TeV PYTHIA PYTHIA: 93 mb ATL CMS total 8 TeV (2 . 3%) inelastic σ inel (8 TeV) = 74 . 7 ± 1 . 7 mb 7 TeV PYTHIA: 73 mb PYTHIA elastic (5 . 1%) TOTEM is too low PYTHIA σ el (8 TeV) = 27 . 1 ± 1 . 4 mb elastic PYTHIA: 20 mb (PYTHIA versions: 6.4.28 & 8.1.80) 5 P. S k a n d s
The Inelastic Cross Section First try: decompose σ inel = σ sd + σ dd + σ cd + σ nd + Parametrizations of diffractive components: dM 2 /M 2 d σ sd( AX ) ( s ) g 3I 1 P 16 π β 2 = M 2 exp( B sd( AX ) t ) F sd , P β B I P A I + Integrate and d t d M 2 PYTHIA: g 2 solve for σ nd d σ dd ( s ) 1 1 3I P = exp( B dd t ) F dd . 16 π β A I P β B I P d t d M 2 1 d M 2 M 2 M 2 2 1 2 What Cross Section? σ INEL @ 100 TeV: σ INEL @ 30 TeV: Total Inelastic ~ 108 mb Fraction with one charged particle in | η |<1 ~ 90 mb Ambiguous Theory Definition Ambiguous Theory Definition Ambiguous Theory Definition σ INEL @ 13 TeV ~ 80 mb σ inel (13 TeV) ∼ 80 ± 3 . 5 mb Observed fraction corrected to total ALICE def : SD has MX<200 Note problem of σ SD : a few mb larger than at 7 TeV principle: Q.M. σ DD ~ just over 10 mb requires distinguishable final states log 10 ( √ s/ GeV) 6 P. S k a n d s
Models of Soft QCD - Disclaimer May not always reflect “best” TH understanding Not just a matter of cranking perturbative orders Harder due to requirement of fully differential dynamical modeling (event generators), not just cross section formulae May not always reflect “best” EXP constraints Not just a matter of “tuning” ( + tunnel vision: exp comparisons for searches or EW measurements rarely formulated as QCD constraints) Modeling: identify “new” physics + build and constrain models (beyond perturbative leading-twist) Few people working on soft QCD models → long cycles 7 P. S k a n d s
Dynamical Models of Soft QCD See e.g. Reviews by MCnet [arXiv:1101.2599] and KMR [arXiv:1102.2844] Regge Theory Parton Based A B d σ 2 → 2 / dp 2 ⊗ PDFs ⊥ p 4 ⊥ Optical Theorem + Eikonal multi-Pomeron exchanges + Unitarity & Saturation σ tot,inel ∝ log 2 (s) Froissart-Martin Bound → Multi-parton interactions (MPI) + Parton Showers & Hadronization Cut Pomerons → Flux Tubes (strings) Regulate d σ at low p T0 ~ few GeV Uncut Pomerons → Elastic (& eikonalization) Screening/Saturation → energy-dependent p T0 Cuts unify treatment of all soft processes EL, SD, DD, … , ND Total cross sections from Regge Theory (Perturbative contributions added above Q 0 ) (e.g., Donnachie-Landshoff + Parametrizations) + “Mixed” E.g., PYTHIA, PYTHIA, E.g., PHOJET, EPOS, E.g., QGSJET, SIBYLL HERWIG, SHERPA SHERPA-KMR 8 P. S k a n d s
Parton-Based Models Central Jets/EWK/top/ Extrapolation to soft scales delicate. Main applications: Higgs/New Physics Impressive successes with MPI-based models but still far from a solved problem Saturation Form of PDFs at small x and Q 2 High Q 2 d σ 2 → 2 / dp 2 ⊗ PDFs Form and E cm dependence of p T0 regulator ⊥ and p 4 Modeling of the diffractive component ⊥ finite x Proton transverse mass distribution Colour Reconnections, Collective Effects See talk on UE Poor Man’s Saturation by W. Waalewijn 7 p T0 scale vs CM energy 6 Range for Pythia 6 p T0 [GeV] Perugia 2012 tunes 5 100 TeV 4 Gluon PDF 30 TeV 3 x*f(x) 7 TeV 2 Q 2 = 1 GeV 2 E CM [GeV] Warning: 0.9 TeV NLO PDFs < 0 1 5000 1 ¥ 10 4 5 ¥ 10 4 1 ¥ 10 5 100 500 1000 See also Connecting hard to soft: KMR, EPJ C71 (2011) 1617 + PYTHIA “Perugia Tunes”: PS, PRD82 (2010) 074018 + arXiv:1308.2813 9 P. S k a n d s
Minimum-Bias: Averages Discovery at LHC Min-Bias & UE are 10-20% larger than we thought Scale a bit faster with energy → Be sure to use up-to-date (LHC) tunes A SENSITIVE E-SCALING PROBE: Central Charged-Track Multiplicity Relative increase in the central charged-track 7000 GeV pp Soft QCD (mb,diff,fwd) multiplicity from 0.9 to 2.36 and 7 TeV η 9 4.2M events dN/d Charged Particle Distribution (N > 0, | | < 1.0, all p ) η η INEL>0 | η |<1 ch T Representative plot. 8 ALICE Several MB/UE Pythia 6 (350:P2011) ≥ Rivet 1.8.2, PHOJET Pythia 6 (370:P2012) models/tunes and Pythia 6 (320:P0) 7 observables show Pythia 6 (327:P2010) same behavior. PY 6 DW 6 PY 6 Perugia 0 Post-LHC Min/Max 5 PY 6 Perugia 2012 Pre-LHC Range Tevatron tunes were ~ 10-20% low PY8 4C (def) 4 on MB and UE mcplots.cern.ch PY8 Monash 2013 3 ALICE_2010_S8625980 Pythia 6.427 0% 10% 20% 30% 40% 50% 60% 70% -1 -0.5 0 0.5 1 Data from ALICE EPJ C68 (2010) 345, Plot from arXiv:1308.2813 η See also energy-scaling tuning study, Schulz & PS, EPJ C71 (2011) 1644 10 P. S k a n d s
Sum(E T ) Central Forward | η |<0.8 4<| η |<4.8 post-LHC PY8 doing better than PY6 pre-LHC Plots from mcplots.cern.ch 11 P. S k a n d s
The Forward Region More sensitive to low x & diffraction pp pp 7000 GeV 7000 GeV 500 6 Forward Energy Flow > > Charged Multiplicity η MB Fwd E Flow (n 1 in both 3.23<| |<4.65) η <dn /d > (n 1, p >0.04, 5.3<| |<6.5) ≥ η η ≥ η <dE/d /d Ch ch ch T Ch CMS TOTEM 2 2 /N /N χ χ <dn 5 bins bins 5% 5% 400 PY8 (Monash 13) PY8 (Monash 13) 0.2 0.0 0.2 0.0 ± ± Totem PY8 (4C) PY8 (4C) 0.4 0.0 2.6 0.0 ± ± 4 PY8 (2C) PY8 (2C) 2.2 0.0 6.1 0.0 ± ± 1/n 300 3 200 2 100 V I N C I A R O O T V I N C I A R O O T 1 Data from JHEP 11 (2011) 148 Data from Europhys.Lett. 98 (2012) 31002 Pythia 8.181 Pythia 8.181 0 0 1.4 1.4 Theory/Data Theory/Data 1.2 1.2 1 1 0.8 0.8 0.6 0.6 3 3.5 4 4.5 5 5.5 6 6.5 η η 2C : an older Tevatron tune 4C : the current LHC tune (Default in Pythia 8.1) Monash 2013 : a new LEP + LHC tune (Default from Pythia 8.2?) 12 P. S k a n d s
H adroni z ati on c o lo r f lo w, c o l o r r ec o n n e ct i o ns, par ti cl e spect ra
Color Connections Leading N C : each parton-parton interaction scatters ‘new’ colors → incoherent addition of colors 1 or 2 strings per MPI Rapidity Quite clean, factorized picture WRONG! Multiplicity ∝ N MPI 14 P. S k a n d s
Color Reconnections? E.g., Generalized Area Law (Rathsman: Phys. Lett. B452 (1999) 364) N C =3: Colors add coherently Color Annealing (P.S., Wicke: Eur. Phys. J. C52 (2007) 133) … + collective effects? Hydro? Coherence Better theory models needed Coherence Study: coherence and/or finite-N C effects Rapidity String formation at finite N C In context of multi-parton interactions LEP constraints? Additional collectivity? (a la HI? BE?) < Multiplicity ∝ N MPI 15 P. S k a n d s
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