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Probing the Underlying Event with Strangeness and Baryons

Explore event probing, Monte Carlo models, sensitive probes, future directions in LHCb flavor physics. Enhance perturbation theory, exclusive event samples, resonance decays, and more. Discover MPI models in Pythia 6.4 and Pythia 8.

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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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