Color Connections, Multijets First International Workshop on Multiple Partonic Interactions at the LHC Color Connections, Multijets The Underlying Event and Minimum-Bias Infrared Headaches Perugia Tunes Probes Drell-Yan: Primordial kT and impact of UE Top Future Directions  PYTHIA 8 + VINCIA Thanks to N. Moggi, L. Tomkins, R. Field See also talk by H. Hoeth

Multiparticle Production Disclaimer: no “theory” of UE. Models attempt to capture “most significant” physics aspects  fully exclusive events Starting point: matrix element + parton shower hard parton-parton scattering (normally 22 in MC) + bremsstrahlung associated with it  2n in LL DGLAP approximation QF 22 ISR ISR FSR FSR … 22 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 perturbative, divergent ISR ISR QF Note: Can take QF >> ΛQCD e.g. 44, 3 3, 32 Color Connections, Multijets - 2

Additional Sources of Particle Production QF >> ΛQCD ME+ISR/FSR + perturbative MPI + Stuff at QF ~ ΛQCD 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 ? Interactions with “background” vacuum, remnants, or active medium? QF 22 ISR ISR FSR FSR … 22 FSR FSR ISR ISR QF Need-to-know issues for IR sensitive quantities (e.g., Nch) Color Connections, Multijets - 3

Now Hadronize This ? hadronization pbar beam remnant p beam remnant bbar from tbar decay b from t decay qbar from W q from W hadronization ? q from W Anti-Triplet Triplet Simulation from D. B. Leinweber, hep-lat/0004025 gluon action density: 2.4 x 2.4 x 3.6 fm Color Connections, Multijets - 4

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 Color Connections, Multijets - 5

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 Color Connections, Multijets - 6

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 Color Connections, Multijets - 7

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: 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 Color Connections, Multijets - 8

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 (see talk) All models ~ ok on track multiplicity Data from CDF, Phys. Rev. D 65 (2002) 072005 Color Connections, Multijets - 9

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 (see talk) All models ~ ok on track multiplicity LHC <Nch> = 80 – 100 (generator-level) Data from CDF, Phys. Rev. D 65 (2002) 072005 Color Connections, Multijets - 10

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 Color Connections, Multijets - 11

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 Color Connections, Multijets - 12

Color Connections, Multijets - 13 Additional Probes Forward-Backward Correlations Trace long-distance correlations (level and falloff sensitive to modeling & mass distribution) ET, Nch, … -ηF ηF η 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? Color Connections, Multijets - 13

Color Connections, Multijets - 14 Additional Probes Forward-Backward Correlations Trace long-distance correlations (level and falloff sensitive to modeling & mass distribution) ET, Nch, … -ηF ηF η 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? Color Connections, Multijets - 14

Tracer of soft beam-remnant component Additional Probes Studied in Run I by N. Moggi and others, are there PID results from Run II ? 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” Particle ID at LHC (talks by F. Sikler, U. Marconi), even Ω- ! Tracer of soft beam-remnant component Simulation from D. B. Leinweber, hep-lat/0004025 LHC: Omega Old models: B number  beam pipe New beam remnant fragm  detector Color Connections, Multijets - 15

Different FSR-off-ISR kinematics! 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 μR Appears dangerously prone to overfitting. Small confidence in extrapolations Data from CDF, Phys Rev Lett 84 (2000) 845 + Run 2 ? Color Connections, Multijets - 16

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 Color Connections, Multijets - 17

Z + jets at LHC Impact of UE uncertainties on LHC Jet Rates See also talk by A. Tricoli (ATLAS) 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) Color Connections, Multijets - 18

Future Directions Monte Carlo problem So we just need … Then … 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 Color Connections, Multijets - 19

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 Color Connections, Multijets - 20

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 Color Connections, Multijets - 21

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 Color Connections, Multijets - 22

Summary Perugia Tunes Drell-Yan and Top 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) Color Connections, Multijets - 23

Color Reconnections Searched for at LEP 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? Normal W Reconnected W Colour Reconnection (example) Soft Vacuum Fields? String interactions? Size of effect < 1 GeV? 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 Color Connections, Multijets - 24

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. M. Heinz, nucl-ex/0606020; nucl-ex/0607033 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? Color Connections, Multijets - 25

Color Connections, Multijets - 26 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? Color Connections, Multijets - 26

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 Color Connections, Multijets - 27

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 Color Connections, Multijets - 28

Naming Conventions Many nomenclatures being used. … … … Underlying 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 … 22 ISR ISR FSR FSR … 22 FSR FSR ISR ISR … Qcut Underlying Event Beam Remnants Primary Interaction (~ trigger) Note: each is colored  Not possible to separate clearly at hadron level Inelastic, non-diffractive Color Connections, Multijets - 29