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

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

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

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

Probing the UE with S and B Peter Skands 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) e.g. 4  4, 3  3, 3  2 Note: Can take Q F >> Λ QCD QFQF QFQF … 2222 ISR FSR 2222 ISR FSR

Probing the UE with S and B Peter Skands 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 ISR FSR 2222 ISR 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?

Probing the UE with S and B Peter Skands Naming Conventions ►Many nomenclatures being used. Not without ambiguity. I use: Q cut 2222 ISR FSR 2222 ISR FSR Primary Interaction (~ trigger) Multiple Parton Interactions (MPI) 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/ Inelastic, non-diffractive

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

Probing the UE with S and B Peter Skands Now Hadronize This Simulation from D. B. Leinweber, hep-lat/ 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

Probing the UE with S and B Peter Skands 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

Probing the UE with S and B Peter Skands 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 Baryon Number acts as a Tracer!

Probing the UE with S and B Peter Skands 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

Probing the UE with S and B Peter Skands Color Annealing Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/ ►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 hadronization 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 (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 string area law: Lambda ~ potential energy ~ string length ~ log(m) ~ N ►  good enough for order-of-magnitude exploration

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

Probing the UE with S and B 15 Peter Skands Pythia 6.4: PYTUNE Track Multiplicity: All models ~ fine Data from CDF, Phys. Rev. D 65 (2002) GeV630 GeV ►PYTUNE (MSTP(5)) kept up to date with newest tunes (see update notes) Most recent tunes for Perugia workshop (+ min/max versions)  new LEP tuned fragmentation pars from Professor (H. Hoeth, A. Buckley)

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

17 Strangeness ►Tunneling suppression due to quark mass  strangeness probes fragmentation field in a unique way Consistent with LEP? Consistent with RHIC / Tevatron? P(m q,p T ) ~ exp ( -[m q 2 + p T 2 ]/κ ) CDF Run 1 Correction factors CDF, Phys.Rev.D72:052001,2005. Generator p T spectrum  CDF sees the hard tail, not the peak  Less sensitive to mass effect  Need experiment with good low-p T tracking

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

19 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 Simulation from D. B. Leinweber, hep-lat/ Sjöstrand & PS : Nucl.Phys.B659(2003)243, JHEP03(2004)053 String junctions Pythia 6.4: new models of beam remnant fragmentation available

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

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

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

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

Probing the UE with S and B Peter Skands 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 Theoretical Physics Dept. - Fermilab Peter Skands Theoretical Physics Dept. - Fermilab Backup Slides

Probing the UE with S and B Peter Skands (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: [hep-ph]

Probing the UE with S and B Peter Skands ►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-ex 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

Probing the UE with S and B Peter Skands 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/ ; nucl-ex/

Probing the UE with S and B Peter Skands 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?

Probing the UE with S and B Peter Skands 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-ex Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more … See also

Probing the UE with S and B Peter Skands 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