1. Peter Skands Theoretical Physics Dept., Fermilab ► The Underlying Event and Minimum-Bias Infrared Headaches Infrared Headaches Perugia Tunes Perugia Tunes Probes Probes ► Drell-Yan and Top ►Future Directions  PYTHIA 8 + VINCIA ► The Underlying Event and Minimum-Bias Infrared Headaches Infrared Headaches Perugia Tunes Perugia Tunes Probes Probes ► Drell-Yan and Top ►Future Directions  PYTHIA 8 + VINCIA ATLAS MC Meeting, 5 Nov 2008 Thanks to N. Moggi, L. Tomkins, R. Field, H. Hoeth From the Tevatron to LHC Min-Bias and the Underlying Event

2. Peter Skands From the Tevatron to LHC - 2 Why study UE/Min-Bias? ►Why study Min-Bias and Underlying Event? Solving QCD requires compromise Construct and constrain models (~ sets of compromises)  precision knowledge + constrained pheno models ►Feedback to high-p T physics Reliable correction procedures Without reliable models, reliable extrapolations are hard to hope for Disclaimer: no “theory” of UE. Models attempt to capture “most significant” physics aspects  fully exclusive events No theory now does not mean no theory exists Experimental input vital to guide its construction

3. Peter Skands From the Tevatron to LHC - 3 Classic Example: Number of tracks UA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias events Simple physics models ~ Poisson Can ‘tune’ to get average right, but much too small fluctuations  inadequate physics model More Physics: Multiple interactions + impact-parameter dependence Moral (will return to the models later) : 1)It is not possible to ‘tune’ anything better than the underlying physics model allows 2)Failure of a physically motivated model usually points to more physics (interesting) 3)Failure of a fit not as interesting

4. Peter Skands From the Tevatron to LHC - 4 Traditional Event Generators ►Basic aim: improve lowest order perturbation theory by including leading corrections  exclusive event samples 1. sequential resonance decays 2. bremsstrahlung 3. underlying event 4. hadronization 5. hadron (and τ ) decays Even the most sophisticated calculations currently only scratch the first few orders of couplings, logs, powers, twists, …  “tuning” needed. Extreme tuning may indicate model breakdown. INTERESTING!

5. Peter Skands From the Tevatron to LHC - 5 Particle Production ►Starting point: matrix element + parton shower hard parton-parton scattering  (normally 2  2 in MC) + bremsstrahlung associated with it   2  n in (improved) LL approximation ► But hadrons are not elementary ► + QCD diverges at low p T  multiple perturbative parton-parton collisions ► Normally omitted in ME/PS expansions ( ~ higher twists / powers / low-x) But still perturbative, divergent e.g. 4  4, 3  3, 3  2 Note: Can take Q F >> Λ QCD QFQF QFQF … 2222 IS R FS R 2222 IS R FS R

6. Peter Skands From the Tevatron to LHC - 6 Additional Sources of Particle Production Need-to-know issues for IR sensitive quantities (e.g., N ch ) + Stuff at Q F ~ Λ QCD Q F >> Λ QCD ME+ISR/FSR + perturbative MPI QFQF QFQF … 2222 IS R FS R 2222 IS R FS R ►Hadronization ►Remnants from the incoming beams ►Additional (non-perturbative / collective) phenomena? Bose-Einstein Correlations Non-perturbative gluon exchanges / color reconnections ? String-string interactions / collective multi-string effects ? “Plasma” effects? Interactions with “background” vacuum, remnants, or active medium?

7. Peter Skands From the Tevatron to LHC - 7 Naming Conventions ►Many nomenclatures being used. Not without ambiguity. I use: Q cut 2222 IS R FS R 2222 IS R FS R Primary Interaction (~ trigger) Underlying Event Beam Remnants Note: each is colored  Not possible to separate clearly at hadron level Some freedom in how much particle production is ascribed to each: “hard” vs “soft” models … … … See also Tevatron-for-LHC Report of the QCD Working Group, hep-ph/0610012 Inelastic, non-diffractive

8. Peter Skands From the Tevatron to LHC - 8 Now Hadronize This Simulation from D. B. Leinweber, hep-lat/0004025 gluon action density: 2.4 x 2.4 x 3.6 fm Anti-Triplet Triplet pbar beam remnant p beam remnant bbar from tbar decay b from t decay qbar from W q from W hadronization ? q from W

9. Peter Skands From the Tevatron to LHC - 9 The Underlying Event and Color ►The colour flow determines the hadronizing string topology Each MPI, even when soft, is a color spark Final distributions crucially depend on color space Note: this just color connections, then there may be color reconnections too

10. Peter Skands From the Tevatron to LHC - 10 The Underlying Event and Color ►The colour flow determines the hadronizing string topology Each MPI, even when soft, is a color spark Final distributions crucially depend on color space Note: this just color connections, then there may be color reconnections too

11. Peter Skands From the Tevatron to LHC - 11 MPI Models in Pythia 6.4 ►Old Model: Pythia 6.2 and Pythia 6.4 “Hard Interaction” + virtuality-ordered ISR + FSR p T -ordered MPI: no ISR/FSR Momentum and color explicitly conserved Color connections: PARP(85:86)  1 in Rick Field’s Tunes No explicit color reconnections ►New Model: Pythia 6.4 and Pythia 8 “Hard Interaction” + p T -ordered ISR + FSR p T -ordered MPI + p T -ordered ISR + FSR  ISR and FSR have dipole kinematics  “Interleaved” with evolution of hard interaction in one common sequence Momentum, color, and flavor explicitly conserverd Color connections: random or ordered Toy Model of Color reconnections: “color annealing” MPI create kinks on existing strings, rather than new strings Hard System + MPI allowed to undergo color reconnections

12. Peter Skands From the Tevatron to LHC - 12 Color Annealing Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120 ►Prompted by CDF data and Rick Field’s studies to reconsider. What do we know? Implications for precision measurements? ►Toy model of (non-perturbative) color reconnections, applicable to any final state At hadronisation time, each string piece gets a probability to interact with the vacuum / other strings: P reconnect = 1 – (1-χ) n  χ = strength parameter: fundamental reconnection probability (free parameter: PARP(78))  n = # of multiple interactions in current event ( ~ counts # of possible interactions) ►For the interacting string pieces: New string topology determined by annealing-like minimization of ‘Lambda measure’ ~ (p i. p j )  Inspired by area law for fundamental strings: Lambda ~ potential energy ~ string length ~ log(m) ~ N ►  good enough for order-of-magnitude exploration But bear in mind: this is not (yet) a “precision” model

13. Peter Skands From the Tevatron to LHC - 13 Perugia Tunes ►Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data + min/max variations + LEP tuned fragmentation pars from Professor, courtesy H. Hoeth, A. Buckley All models ~ ok on track multiplicity Data from CDF, Phys. Rev. D 65 (2002) 072005

14. Peter Skands From the Tevatron to LHC - 14 Perugia Tunes ►Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data + min/max variations + LEP tuned fragmentation pars from Professor, courtesy H. Hoeth, A. Buckley All models ~ ok on track multiplicity LHC = 80 – 100 (generator-level) Data from CDF, Phys. Rev. D 65 (2002) 072005

15. Peter Skands From the Tevatron to LHC - 15 Tevatron Run II Pythia 6.2 Min-bias (N ch ) Tune A old default Central Large UE Peripheral Small UE Non-perturbative component in string fragmentation (LEP value) Not only more (charged particles), but each one is harder Diffractive? Perugia Tunes ►Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data Average track pT as a function of multiplicity: sensitive probe of CR? Used to fix CR strength parameter in tunes Poor statistics due to rapid drop of P(N ch ) Data from CDF, N. Moggi et al., 2008

16. Peter Skands From the Tevatron to LHC - 16 Not only more (charged particles), but each one is harder Perugia Tunes ►Perugia tunes of new model, using Tevatron 630/1800/1960 GeV data Average track pT as a function of multiplicity: sensitive probe of CR? Used to fix CR strength parameter in tunes Data from CDF, N. Moggi et al., 2008

17. Peter Skands From the Tevatron to LHC - 17 Additional Probes ►Forward-Backward Correlations Trace long-distance correlations (level and falloff sensitive to modeling & mass distribution) If F fluctuates up … How likely is it that B fluctuates up too? I.e., how correlated are they? How quickly does it die out? 0 ηFηF -ηF-ηF η E T, N ch, …

18. Peter Skands From the Tevatron to LHC - 18 Additional Probes ►Forward-Backward Correlations Trace long-distance correlations (level and falloff sensitive to modeling & mass distribution) If F fluctuates up … How likely is it that B fluctuates up too? I.e., how correlated are they? How quickly does it die out? 0 ηFηF -ηF-ηF η E T, N ch, …

19. Peter Skands From the Tevatron to LHC - 19 Simulation from D. B. Leinweber, hep-lat/0004025 Additional Probes ►Baryon Transport and Strangeness Production Baryon number traces component coming from beam breakup (soft) Strangeness probes fragmentation field, same as LEP? Large differences depending on degree of “beam breakup” Tracer of soft beam- remnant component S: K 0, K *, … B+S: Λ 0, Ξ -, Ω - Old models: B number  beam pipe New beam remnant fragm  detector Studied in Run I by N. Moggi and others. Results from Run II very desirable

20. Peter Skands From the Tevatron to LHC - 20 Data from CDF, Phys Rev Lett 84 (2000) 845 + Run 2 ? Drell-Yan ►DY is the benchmark for ISR Its low-pT peak seems to require ~ 2 GeV of “primordial kT” Different FSR-off- ISR kinematics! 150 MeV ! Exchanged for lower ISR cutoff and smaller μ R Appears dangerously prone to overfitting. Small confidence in extrapolations

21. Peter Skands From the Tevatron to LHC - 21 ►DW, S0, etc all roughly agree for Drell-Yan (except for Tune A) What about top? Top TevatronLHC Models that were equal for DY are no longer equal for top  not enough to tune to DY Note: matching will not change this much (mentioned in talks by R. Chierici, A. Tricoli)

22. Peter Skands From the Tevatron to LHC - 22 Z + jets at LHC ►Impact of UE uncertainties on LHC Jet Rates Z + 2 jets at LHC, as function of 2 nd jet pT DW = DWT at Tevatron, only UE scaling is different (idem for S0,S0A) E T2 = 70 GeV E T2 = 30 GeV High jet p T (> 100) : rate independent of UE scaling Low jet p T (~ 30) : rate uncertainty due to UE scaling = factor of 2 Plot from Lauren Tomkins (student of B. Heinemann) Medium jet p T (50-70): rate uncertainty due to UE scaling ~ 30-50%

23. Peter Skands From the Tevatron to LHC - 23 Future Directions ►Monte Carlo problem Uncertainty on fixed orders and logs obscures clear view on hadronization and the underlying event ►So we just need … An NNLO + NLO multileg + NLL Monte Carlo (incl small-x logs), with uncertainty bands, please ►Then … We could see hadronization and UE clearly  solid constraints  Energy Frontier Intensity Frontier The Astro Guys Precision Frontier The Tevatron and LHC data will be all the energy frontier data we’ll have for a long while Anno 2018

24. Peter Skands From the Tevatron to LHC - 24 Constructing LL Showers ►The final answer will depend on: The choice of evolution “time” The splitting functions (finite terms not fixed) The phase space map ( “recoils”, dΦ n+1 /dΦ n ) The renormalization scheme (argument of α s ) The infrared cutoff contour (hadronization cutoff) Variations  Comprehensive uncertainty estimates (showers with uncertainty bands)

25. Peter Skands From the Tevatron to LHC - 25 Gustafson, PLB175(1986)453; Lönnblad (ARIADNE), CPC71(1992)15. Azimov, Dokshitzer, Khoze, Troyan, PLB165B(1985)147 Kosower PRD57(1998)5410; Campbell,Cullen,Glover EPJC9(1999)245 VINCIA ►Based on Dipole-Antennae  Shower off color-connected pairs of partons  Plug-in to PYTHIA 8 (C++) ►So far: 3 different shower evolution variables:  pT-ordering (= ARIADNE ~ PYTHIA 8)  Dipole-mass-ordering (~ but not = PYTHIA 6, SHERPA)  Thrust-ordering (3-parton Thrust) For each: an infinite family of antenna functions  Laurent series in branching invariants with arbitrary finite terms Shower cutoff contour: independent of evolution variable  IR factorization “universal” Several different choices for α s (evolution scale, p T, mother antenna mass, 2-loop, …) 3 different phase space maps  Ariadne or Kosower “antenna” recoils, or Emitter + longitudinal Recoiler Dipoles (=Antennae, not CS) – a dual description of QCD a b r VIRTUAL NUMERICAL COLLIDER WITH INTERLEAVED ANTENNAE Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007

26. Peter Skands From the Tevatron to LHC - 26 ►Can vary evolution variable, kinematics maps, radiation functions, renormalization choice, matching strategy (here just showing radiation functions) ►At Pure LL, can definitely see a non-perturbative correction, but hard to precisely constrain it VINCIA Plug-in to Pythia 8 : towards  NLO multileg + NLL + uncertainties Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007 Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching

27. Peter Skands From the Tevatron to LHC - 27 ►Can vary evolution variable, kinematics maps, radiation functions, renormalization choice, matching strategy (here just showing radiation functions) ►At Pure LL, can definitely see a non-perturbative correction, but hard to precisely constrain it VINCIA Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007 Plug-in to Pythia 8 : towards  NLO multileg + NLL + uncertainties Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching

28. Peter Skands From the Tevatron to LHC - 28 ►Can vary evolution variable, kinematics maps, radiation functions, renormalization choice, matching strategy (here just showing radiation functions) ►After 2 nd order matching  Non-pert part can be precisely constrained. (will need 2 nd order logs as well for full variation) VINCIA Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007 Plug-in to Pythia 8 : towards  NLO multileg + NLL + uncertainties Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching

29. Peter Skands From the Tevatron to LHC - 29 Summary ►Perugia Tunes First set of tunes of new models including both Tevatron and LEP  New LEP parameters can ~ be “slapped onto” existing tunes without invalidating their fits to UE/MB/DY data  S0-H, APT-H, etc… (useful for new ATLAS tune too?) + First attempt at systematic “+” and “-” variations Data-driven, constraints  better tunes BUT ALSO better models ►Drell-Yan and Top “Primordial kT”: perturbative uncertainties very large in low-pT region  Without better perturbative showers, cannot isolate genuine non-pert component Much remains to be learned, even for these “benchmark” processes ►Future (still at conceptual stage): VINCIA project aims to reduce perturbative sources of uncertainty  cleaner look at UE and non-perturbative phenomena (see also talks by A. Moraes and H. Hoeth last week in Perugia)

30. Peter Skands Theoretical Physics Dept., Fermilab Backup Slides

31. Peter Skands From the Tevatron to LHC - 31 (Why Perturbative MPI?) ►Analogue: Resummation of multiple bremsstrahlung emissions Divergent σ for one emission (X + jet, fixed-order)  Finite σ for divergent number of jets (X + jets, infinite-order)  N(jets) rendered finite by finite perturbative resolution = parton shower cutoff ►(Resummation of) Multiple Perturbative Interactions Divergent σ for one interaction (fixed-order)  Finite σ for divergent number of interactions (infinite-order)  N(jets) rendered finite by finite perturbative resolution Saturation? Current models need MPI IR cutoff > PS IR cutoff = color-screening cutoff (E cm -dependent, but large uncert) Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph]

32. Peter Skands From the Tevatron to LHC - 32 ►Searched for at LEP Major source of W mass uncertainty Most aggressive scenarios excluded But effect still largely uncertain P reconnect ~ 10% ►Prompted by CDF data and Rick Field’s studies to reconsider. What do we know? Non-trivial initial QCD vacuum A lot more colour flowing around, not least in the UE String-string interactions? String coalescence? Collective hadronization effects? More prominent in hadron-hadron collisions? What (else) is RHIC, Tevatron telling us? Implications for precision measurements:Top mass? LHC? Normal WW Reconnected WW OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062 Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more … Colour Reconnection (example) Soft Vacuum Fields? String interactions? Size of effect < 1 GeV? Color Reconnections Existing models only for WW  a new toy model for all final states: colour annealing Attempts to minimize total area of strings in space-time (similar to Uppsala GAL) Improves description of minimum-bias collisions PS, Wicke EPJC52(2007)133 ; Preliminary finding Delta(mtop) ~ 0.5 GeV Now being studied by Tevatron top mass groups

33. Peter Skands From the Tevatron to LHC - 33 Underlying Event and Colour ►Not much was known about the colour correlations, so some “theoretically sensible” default values were chosen Rick Field (CDF) noted that the default model produced too soft charged- particle spectra. The same is seen at RHIC: For ‘Tune A’ etc, Rick noted that increased when he increased the colour correlation parameters But needed ~ 100% correlation. So far not explained Virtually all ‘tunes’ now used by the Tevatron and LHC experiments employ these more ‘extreme’ correlations What is their origin? Why are they needed? M. Heinz, nucl-ex/0606020; nucl-ex/0607033

34. Peter Skands From the Tevatron to LHC - 34 Questions ►Transverse hadron structure How important is the assumption f(x,b) = f(x) g(b) What observables could be used to improve transverse structure? ►How important are flavour correlations? Companion quarks, etc. Does it really matter? Experimental constraints on multi-parton pdfs? What are the analytical properties of interleaved evolution? Factorization? ►“Primordial kT” (~ 2 GeV of pT needed at start of DGLAP to reproduce Drell-Yan) Is it just a fudge parameter? Is this a low-x issue? Is it perturbative? Non-perturbative?

35. Peter Skands From the Tevatron to LHC - 35 More Questions ►Correlations in the initial state Underlying event: small p T, small x ( although x/X can be large ) Infrared regulation of MPI (+ISR) evolution connected to saturation? Additional low-x / saturation physics required to describe final state? Diffractive topologies? ►Colour correlations in the final state MPI  color sparks  naïvely lots of strings spanning central region What does this colour field do? Collapse to string configuration dominated by colour flow from the “perturbative era”? or by “optimal” string configuration? Are (area-law-minimizing) string interactions important? Is this relevant to model (part of) diffractive topologies? What about baryon number transport?  Connections to heavy-ion programme OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062 Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more … See also

36. Peter Skands From the Tevatron to LHC - 36 Multiple Interactions  Balancing Minijets ►Look for additional balancing jet pairs “under” the hard interaction. ►Several studies performed, most recently by Rick Field at CDF  ‘lumpiness’ in the underlying event. (Run I) angle between 2 ‘best-balancing’ pairs CDF, PRD 56 (1997) 3811