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1st Future Hadron Collider Workshop, May 2014, CERN Ev en t s t ru c t u re a n d s m a l l - x i s s ue s a t 1 0 0 Te V Peter Skands (CERN TH) What does the average collision look like? How many of them are there? ( pileup ) How much

  1. 1st Future Hadron Collider Workshop, May 2014, CERN Ev en t s t ru c t u re a n d s m a l l - x i s s ue s a t 1 0 0 Te V Peter Skands (CERN TH) What does the average collision look like? How many of them are there? ( σ pileup ) How much energy in the Underlying Event? (UE) Image Credits: blepfo (deviantart.com)

  2. Event Structure at PP Colliders Dominated by QCD More than just a perturbative expansion in α s Emergent phenomena: Jets (the QCD fractal) ⟷ amplitude structures ⟷ fundamental quantum field theory. Precision jet (structure) studies. Strings (strong gluon fields) ⟷ quantum-classical correspondence. String physics. Dynamics of hadronization phase transition. Hadrons ⟷ Spectroscopy (incl excited and exotic states) , lattice QCD, (rare) decays, mixing, light nuclei. Hadron beams → MPI, diffraction, … See eg TASI lectures, e-Print: arXiv:1207.2389 2 P. S k a n d s

  3. Modeling Hadronic Final States Reality is more complicated Calculate Everything ≈ solve QCD → requires compromise! Monte Carlo Event Generators: Explicit Dynamical Modeling → complete events (can evaluate any observable you want) Factorization → Split the problem into many (nested) pieces + Quantum mechanics → Probabilities → Random Numbers (MC) Soft Physics P event = P hard ⊗ P dec ⊗ P ISR ⊗ P FSR ⊗ P MPI ⊗ P Had ⊗ . . . Matrix Elements Shower Kernels Multiple Parton Interactions Hadronization, Decays, Soft (+ Sudakov Corrections) (+ ME corrections) Hard + Soft → INEL & UE Diffraction, Beam Remnants 3 P. S k a n d s

  4. Soft Physics : Theory Models 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) → 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 (Donnachie-Landshoff + Parametrizations) + “Mixed” E.g., PYTHIA, E.g., PHOJET, EPOS, E.g., QGSJET, SIBYLL HERWIG, SHERPA SHERPA-KMR 4 P. S k a n d s

  5. Parton-Based Models : MPI B Consider the inclusive di-jet cross section in QCD pp 0.2 TeV pp 100 TeV 4 10 5 Integrated cross section [mb] 10 Integrated cross section [mb] (p p ) vs p σ ≥ (p p ) vs p σ ≥ 2 2 → T Tmin Tmin 2 → 2 T Tmin Tmin 3 TOTEM (fit) σ 10 TOTEM σ INEL INEL 4 =0.130 NNPDF2.3LO α 10 E CM = 200 GeV E CM = 100 TeV α =0.130 NNPDF2.3LO s s =0.135 CTEQ6L1 α α =0.135 CTEQ6L1 s s 2 10 3 10 hadron-hadron 10 hadron-hadron p a r 2 10 t o 1 n parton-parton - p a r single parton interaction single parton interaction t V I N C I A R O O T V I N C I A R O O T 10 o -1 10 n = good approximation = bad approximation Pythia 8.183 Pythia 8.183 -2 1 10 1.5 1.5 Ratio Ratio 1 1 0.5 0.5 0 0 0 5 10 15 20 0 5 10 15 20 p p Tmin Tmin σ 2 → 2 > σ pp interpreted as consequence of each pp containing several 2 → 2 interactions: MPI 5 P. S k a n d s

  6. Soft MPI Extrapolation to soft scales delicate. Central Jets/EWK/top/ Main applications: Impressive successes with MPI-based Higgs/New Physics 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 (and/or x ) dependence of p T0 regulator ⊥ and p 4 Modeling of the diffractive component ⊥ finite x Proton transverse mass distribution Colour Reconnections, Collective Effects Poor Man’s Saturation 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 6 P. S k a n d s

  7. Low- x Issues (in MC/PDF context) Low x : parton carries tiny fraction of beam energy. 7 TeV: x ~ 10 -5 - 10 -4 x Λ = 2 Λ QCD x ⊥ 0 = 2 p ⊥ 0 E.g.: 100 TeV: x ~ 10 -6 - 10 -4 E CM E CM 2 2 xg(x,Q = 2 GeV ) Higher x : momenta > Λ QCD 20 → pQCD ~ OK NNPDF2.3QED LO, = 0.119 α s NNPDF2.3QED NLO, = 0.119 α s Smaller x : strong non-perturbative / 15 NNPDF2.3QED NNLO, = 0.119 α s colour-screening / saturation effects expected 10 What does a PDF even mean? Highly relevant for MPI (& ISR) 5 PDF must be a probability density → can only use LO PDFs 0 arXiv:1404.5630 (+ Constraints below x ~ 10 -4 essentially just momentum conservation + flavour sum rules) -6 -5 -3 -4 -2 -1 10 10 10 10 10 10 1 x 7 P. S k a n d s

  8. MPI models and Low x Gluon PDF at low Q 2 drives MPI EXAMPLE: PYTHIA 8 Range of x values probed by 2 2 2 2 different MPI tunes x g ( x, Q x g ( x, Q = 2 GeV = 2 GeV ) ) 2 10 pp 7000 GeV N NNPDF2.3LO =0.130 α N S 10 P (x) D NNPDF2.3LO =0.119 Log (x) : including MPI α F S 2 10 . 10 3 1/n dn/dLog CTEQ6L PY8 (Monash 13) 1 MRST07lomod LO* (MRST) PY8 (4C) CTEQ6L1 CT09MC2 PY8 (2C) 10 -1 10 CT09MCS tune with CT09MC2 NNPDF2.3 tunes with -2 10 CTEQ6L1 -3 10 arXiv:1404.5630 arXiv:1404.5630 1 Gluon PDF at low Q 2 Expect consequences for event V I N C I A R O O T -4 10 Comparison between CTEQ, NNPDF, and MRST structure, especialy in FWD region Pythia 8.185 -5 10 -6 -5 -3 -4 -2 -1 10 10 10 10 10 10 -8 -6 -4 -2 0 x log 10 ( x ) Log (x) Controlling these issues will require an improved understanding of the interplay between low- x PDFs, saturation / screening, and MPI in MC context. (+ Clean model-independent experimental constraints!) Not automatic: Communities don’t speak same language (+ low visibility) 8 P. S k a n d s

  9. R ecen t P YT H I A M o d e l s / Tu n e s & Extrapolations to Event Structure at 100 TeV PYTHIA 8.1 LEP tuning undocumented (from 2009) Current Default = 4C (from 2010) LHC tuning only used very early data based on CTEQ6L1 Tunes 2C & 4C: e-Print: arXiv:1011.1759 Aims for the Monash 2013 Tune Set M13 Tune: Tune:ee = 7 in PYTHIA 8 Tune:pp = 14 Monash 2013 Tune: e-Print: arXiv:1404.5630 Revise (and document) constraints from e + e - measurements In particular in light of possible interplays with LHC measurements ! Test drive the new NNPDF 2.3 LO PDF set (with α s (m Z ) = 0.13 ) for pp & ppbar Update min-bias and UE tuning + energy scaling → 2013 Follow “Perugia” tunes for PYTHIA 6: use same α s for ISR and FSR Use the PDF value of α s for both hard processes and MPI PYTHIA 6.4 ( warning: no longer actively developed) Perugia Tunes : e-Print: arXiv:1005.3457 (+ 2011 & 2012 updates added as appendices) Default: still rather old Q 2 -ordered tune ~ Tevatron Tune A Most recent: Perugia 2012 set of p T -ordered tunes (370 - 382) + Innsbruck (IBK) Tunes (G. Rudolph) Note: I will focus on default / author tunes here (Important complementary efforts undertaken by LHC experiments)

  10. Tuni ng means di fferent thi ngs to di fferent peopl e 10% agreement is great for (N)LO + LL MB/UE/Soft: larger uncertainties since driven by non-factorizable and non-perturbative physics Complicated dynamics: If a model is simple, it is wrong (T. Sjöstrand)

  11. Cross Sections & Energy Scaling Pileup rate ∝ σ tot ( s ) = σ el ( s ) + σ inel ( s ) ∝ s 0 . 08 or ln 2 ( s ) ? Donnachie-Landshoff (or 0.096?) Froissart-Martin Bound INELASTIC PP CROSS SECTIONS AUGER (INEL = SD+DD+CD+ND) TOTEM, PRL 111 (2013) 1, 012001 TOTEM (+COMPETE) TOTEM σ inel (8 TeV) = 74 . 7 ± 1 . 7 mb σ inel (13 TeV) ∼ 80 ± 3 . 5 mb AUGER PYTHIA (DL 0.08 ) 100 TeV TOTEM ALICE 30 TeV σ inel ( 8 TeV): 73 mb ALICE σ inel ( 13 TeV): 78 mb 13 TeV PYTHIA σ inel ( 30 TeV): 89 mb ATL CMS σ inel (100 TeV): 107 mb total 8 TeV OK inelastic 7 TeV ELASTIC PHOJET elastic is too large σ el (8 TeV) = 27 . 1 ± 1 . 4 mb PYTHIA elastic TOTEM PYTHIA: 20 mb is too low PYTHIA elastic LOW (PYTHIA versions: 6.4.28 & 8.1.80) 11 P. S k a n d s

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