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From the Tevatron to LHC Min-Bias and the Underlying Event

Dive into the physics of minimum-bias and underlying events, from Tevatron to LHC, understanding particle production mechanisms, color flow, and models like Pythia 8. Discover the importance of studying UE/Min-Bias for reliable corrections and future physics directions.

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From the Tevatron to LHC Min-Bias and the Underlying Event

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

  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-pT 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 From the Tevatron to LHC - 2

  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): • It is not possible to ‘tune’ anything better than the underlying physics model allows • Failure of a physically motivated model usually points to more physics (interesting) • Failure of a fit not as interesting From the Tevatron to LHC - 3

  4. Traditional Event Generators • Basic aim: improve lowest order perturbation theory by including leading corrections  exclusive event samples • sequential resonance decays • bremsstrahlung • underlying event • hadronization • 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! From the Tevatron to LHC - 4

  5. Particle Production QF FSR FSR 22 22 ISR ISR ISR • 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 ISR FSR … FSR • But hadrons are not elementary • + QCD diverges at low pT  multiple perturbative parton-parton collisions • Normally omitted in ME/PS expansions ( ~ higher twists / powers / low-x) But still perturbative, divergent QF Note: Can take QF >> ΛQCD e.g. 44, 3 3, 32 From the Tevatron to LHC - 5

  6. Additional Sources of Particle Production QF FSR FSR 22 22 ISR ISR ISR • 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? QF >> ΛQCD ME+ISR/FSR + perturbative MPI + Stuff at QF ~ ΛQCD ISR FSR … FSR QF Need-to-know issues for IR sensitive quantities (e.g., Nch) From the Tevatron to LHC - 6

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

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

  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 From the Tevatron to LHC - 9

  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 From the Tevatron to LHC - 10

  11. MPI Models in Pythia 6.4 • Old Model: Pythia 6.2 and Pythia 6.4 • “Hard Interaction” + virtuality-ordered ISR + FSR • pT-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” + pT-ordered ISR + FSR • pT-ordered MPI + pT-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 From the Tevatron to LHC - 11

  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 modelof (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: Preconnect = 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’ ~ (pi . pj) • 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 From the Tevatron to LHC - 12

  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 From the Tevatron to LHC - 13

  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 <Nch> = 80 – 100 (generator-level) Data from CDF, Phys. Rev. D 65 (2002) 072005 From the Tevatron to LHC - 14

  15. 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 Tevatron Run II Pythia 6.2 Min-bias <pT>(Nch) Tune A Not only more (charged particles), but each one is harder Diffractive? old default Non-perturbative <pT> component in string fragmentation (LEP value) Poor statistics due to rapid drop of P(Nch) Peripheral Small UE Central Large UE Data from CDF, N. Moggi et al., 2008 From the Tevatron to LHC - 15

  16. 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 Not only more (charged particles), but each one is harder Data from CDF, N. Moggi et al., 2008 From the Tevatron to LHC - 16

  17. 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? Additional Probes ET, Nch, … -ηF ηF η 0 From the Tevatron to LHC - 17

  18. 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? Additional Probes ET, Nch, … -ηF ηF η 0 From the Tevatron to LHC - 18

  19. Additional Probes Studied in Run I by N. Moggi and others. Results from Run II very desirable • Baryon Transport and Strangeness Production • Baryon number traces component coming from beam breakup (soft) • Strangeness probes fragmentation field, same as LEP? S: K0, K*, … B+S: Λ0, Ξ-, Ω- Large differences depending on degree of “beam breakup” Tracer of soft beam-remnant component Simulation from D. B. Leinweber, hep-lat/0004025 Old models: B number  beam pipe New beam remnant fragm  detector From the Tevatron to LHC - 19

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

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

  22. Z + jets at LHC • Impact of UE uncertainties on LHC Jet Rates • Z + 2 jets at LHC, as function of 2nd jet pT • DW = DWT at Tevatron, only UE scaling is different (idem for S0,S0A) Low jet pT (~ 30) : rate uncertainty due to UE scaling = factor of 2 ET2 = 70 GeV High jet pT (> 100) : rate independent of UE scaling ET2 = 30 GeV Medium jet pT (50-70): rate uncertainty due to UE scaling ~ 30-50% Plot from Lauren Tomkins (student of B. Heinemann) From the Tevatron to LHC - 22

  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 Anno 2018 The Tevatron and LHC data will be all the energy frontier data we’ll have for a long while From the Tevatron to LHC - 23

  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”, dΦn+1/dΦn ) • The renormalization scheme (argument of αs) • The infrared cutoff contour (hadronization cutoff) Variations  Comprehensive uncertainty estimates (showers with uncertainty bands) From the Tevatron to LHC - 24

  25. 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 αs (evolution scale, pT, mother antenna mass, 2-loop, …) 3 different phase space maps Ariadne or Kosower “antenna” recoils, or Emitter + longitudinal Recoiler VINCIA VIRTUAL NUMERICAL COLLIDER WITH INTERLEAVED ANTENNAE 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 Dipoles (=Antennae, not CS) – a dual description of QCD a Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007 r b From the Tevatron to LHC - 25

  26. VINCIA Plug-in to Pythia 8 : towards  NLO multileg + NLL + uncertainties • 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 Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007 From the Tevatron to LHC - 26

  27. VINCIA Plug-in to Pythia 8 : towards  NLO multileg + NLL + uncertainties • 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 Goal: highly precise MC – with comprehensive uncertainties for fixed orders, showers, and matching Giele, Kosower, PS : PRD78(2008)014026 + Les Houches ‘NLM’ 2007 From the Tevatron to LHC - 27

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

  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 projectaims 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) From the Tevatron to LHC - 29

  30. Backup Slides

  31. (Why Perturbative MPI?) = color-screening cutoff (Ecm-dependent, but large uncert) Saturation? Current models need MPI IR cutoff >PS IR cutoff • 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 Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph] • (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 From the Tevatron to LHC - 31

  32. Color Reconnections W W Normal W W Reconnected Colour Reconnection (example) Soft Vacuum Fields? String interactions? Size of effect < 1 GeV? Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more … OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062 • Searched for at LEP • Major source of W mass uncertainty • Most aggressive scenarios excluded • But effect still largely uncertain Preconnect ~ 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? • 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 From the Tevatron to LHC - 32

  33. 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 <pT> 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? Underlying Event and Colour M. Heinz, nucl-ex/0606020; nucl-ex/0607033 From the Tevatron to LHC - 33

  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? From the Tevatron to LHC - 34

  35. More Questions • Correlations in the initial state • Underlying event: small pT, 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 See also Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more … OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062 From the Tevatron to LHC - 35

  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. angle between 2 ‘best-balancing’ pairs (Run I) CDF, PRD 56 (1997) 3811 From the Tevatron to LHC - 36

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