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Probing the Underlying Event with Strangeness and Baryons

Probing the Underlying Event with Strangeness and Baryons. LHCb Flavor Physics WG Meeting, 19 Nov 2008. MC models of Underlying-Event / Minimum-Bias Physics Infrared Headaches Tunes Sensitive Probes Special on Strangeness and Baryons Future Directions. Peter Skands

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Probing the Underlying Event with Strangeness and Baryons

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  1. Probing the Underlying Event with Strangeness and Baryons LHCb Flavor Physics WG Meeting, 19 Nov 2008 • MC models of Underlying-Event / Minimum-Bias Physics • Infrared Headaches • Tunes • Sensitive Probes • Special on Strangeness and Baryons • Future Directions Peter Skands Theoretical Physics Dept. - Fermilab Thanks to N. Moggi, L. Tomkins, R. Field, H. Hoeth

  2. Monte Carlo 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 • Physics Feedback • Reliable correction procedures • Without reliable models, reliable extrapolations are hard to hope for Peter Skands Probing the UE with S and B

  3. The Tip of an Iceberg? • Even the most sophisticated calculations currently only scratch the first few orders of • Couplings • Logs • 1/Nc • m • Γ • Powers • “Twists” • Spin correlations • ...  “tuning” needed. Extreme tuning may indicate model breakdown. INTERESTING! Peter Skands Probing the UE with S and B 3

  4. Classic Example: Number of tracks UA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias events Simple physics models ~ Poisson More Physics: Can ‘tune’ to get average right, but much too small fluctuations  inadequate physics model 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 Peter Skands Probing the UE with S and B

  5. Particle Production QF 22 22 ISR ISR ISR ISR FSR FSR FSR FSR • 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 pT •  multiple perturbative parton-parton collisions • Normally omitted in ME/PS expansions • ( ~ higher twists / powers / low-x) QF Note: Can take QF >> ΛQCD e.g. 44, 3 3, 32 Peter Skands Probing the UE with S and B

  6. Additional Sources of Particle Production QF 22 22 ISR ISR ISR ISR FSR FSR FSR FSR • 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 … QF Need-to-know issues for IR sensitive quantities (e.g., Nch) Peter Skands Probing the UE with S and B

  7. Naming Conventions 22 22 ISR ISR ISR FSR ISR FSR FSR FSR 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 … … … Qcut Multiple Parton Interactions (MPI) Beam Remnants Primary Interaction (~ trigger) Note: each is colored  Not possible to separate clearly at hadron level Inelastic, non-diffractive Peter Skands Probing the UE with S and B

  8. Where Did This Pion Come From? 2 I I F F Soft or Hard? More MPI Less BR OR Beam Remnants Multiple Parton Interactions … Less MPI More BR … … Peter Skands Probing the UE with S and B

  9. Simulation from D. B. Leinweber, hep-lat/0004025 gluon action density: 2.4 x 2.4 x 3.6 fm Anti-Triplet Now Hadronize This pbar beam remnant qbar from W Triplet bbar from tbar decay q from W q from W p beam remnant b from t decay hadronization ? Peter Skands Probing the UE with S and B

  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 Peter Skands Probing the UE with S and B

  11. 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 Baryon Number acts as a Tracer! Note: this just color connections, then there may be color reconnections too Peter Skands Probing the UE with S and B

  12. 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 Peter Skands Probing the UE with S and B

  13. Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120 Color Annealing • Prompted by CDF data and Rick Field’s studies to reconsider. What do we know? Implications for precision measurements? • Toy modelof (non-perturbative) color reconnectionsapplicable to any final state • At hadronization 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 (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 string area law: Lambda ~ potential energy ~ string length ~ log(m) ~ N •  good enough for order-of-magnitude exploration Peter Skands Probing the UE with S and B

  14. Probing the UE • 1: A bunch of models that all give fair descriptions of Tevatron data • 2: Strangeness and Baryon Number Peter Skands Theoretical Physics Dept. - Fermilab

  15. Data from CDF, Phys. Rev. D 65 (2002) 072005 Pythia 6.4: PYTUNE • PYTUNE (MSTP(5)) kept up to date with newest tunes (see update notes) • Most recent tunes for Perugia workshop (+ min/max versions)  6.4.20 • + new LEP tuned fragmentation pars from Professor (H. Hoeth, A. Buckley) 1800 GeV 630 GeV Track Multiplicity: All models ~ fine Peter Skands Probing the UE with S and B 15

  16. Extrapolations to LHC First thing to measure: track multiplicity Generator-Level LHC <Nch> = 80 – 100 But that only gives us the size of the glass, not the contents of the cocktail Peter Skands Probing the UE with S and B

  17. Strangeness CDF, Phys.Rev.D72:052001,2005. • Tunneling suppression due to quark mass •  strangeness probes fragmentation field in a unique way • Consistent with LEP? Consistent with RHIC / Tevatron? P(mq,pT) ~ exp ( -[mq2 + pT2]/κ ) CDF Run 1 Correction factors Generator pT spectrum  CDF sees the hard tail, not the peak  Less sensitive to mass effect  Need experiment with good low-pT tracking

  18. Models that agree on total amount of strangeness … Disagree on where it is … Strangeness Distributions (correlated with total mult production) Need measurements at both low and high eta (Note: probably better to measure strangeness fraction, divide out total mult)

  19. Simulation from D. B. Leinweber, hep-lat/0004025 Baryons • Comes back to the color flow issues mentioned earlier • Is the baryon number liberated from the beam? • How far does it get? • Any observed B excess in detector •  important constraints (lower bounds) on beam remnant fragmentation Pythia 6.4: new models of beam remnant fragmentation available String junctions Sjöstrand & PS : Nucl.Phys.B659(2003)243, JHEP03(2004)053

  20. This is hard (if you’re not LHCb) • Baryon number transport: get as close to the beam as possible! Few percent effect CDF coverage Old models: B locked in remnant New Models: B carried by string junctions (NOTE also: CDF only sees the high-pT tail. The one from the beam is most likely soft)  Need measurements at high eta and low pT

  21. Extrapolations to the LHC • Lambdas 1 percent in ATLAS/CMS  5 percent in LHCb (depends on pT cuts) + Could be possible to enhance effect by looking at spectra, correlations (Note that these models are by no means extreme, effect could well be larger)

  22. Extrapolations to the LHC • Cascades 1 percent in ATLAS/CMS  5 percent in LHCb (depends on pT cuts) + Could be possible to enhance effect by looking at spectra, correlations (Note that these models are by no means extreme, effect could well be larger)

  23. Extrapolations to the LHC • Omega 1 percent in ATLAS/CMS  5 percent in LHCb (depends on pT cuts) + Could be possible to enhance effect by looking at spectra, correlations (Note that these models are by no means extreme, effect could well be larger)

  24. Summary • Perugia Tunes • First set of tunes of new models including both Tevatron and LEP • + First attempt at systematic “+” and “-” variations • Data-driven, constraints  better tunes BUT ALSO better models • Strangeness and Baryon Number • Strangeness may be used to probe the fragmentation field • Are strangeness production rates & spectra consistent with LEP? With Tevatron? • Baryon Number migration traces Beam Remnant Fragmentation • Important ingredient in constraining Monte Carlo UE models • Fundamental probe of non-trivial string topology carrying baryon number (junctions) • Important implications for precision on underlying event (see also talks by A. Moraes and H. Hoeth last week in Perugia) Peter Skands Probing the UE with S and B

  25. Backup Slides Peter Skands Theoretical Physics Dept. - Fermilab

  26. Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph] = color-screening cutoff (Ecm-dependent, but large uncert) (Why Perturbative MPI?) 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 • (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 Peter Skands Probing the UE with S and B

  27. 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 OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062 Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more … Color Reconnections W W W W Colour Reconnection (example) Reconnected Normal Soft Vacuum Fields? String interactions? Size of effect < 1 GeV? • 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? Peter Skands Probing the UE with S and B

  28. M. Heinz, nucl-ex/0606020; nucl-ex/0607033 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 <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? Peter Skands Probing the UE with S and B

  29. 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? Peter Skands Probing the UE with S and B

  30. OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062 Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more … 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 Peter Skands Probing the UE with S and B

  31. 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 Peter Skands Probing the UE with S and B

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