LHCb Flavor Physics WG Meeting 19 Nov 2008

LHCb Flavor Physics WG Meeting, 19 Nov 2008 Probing the Underlying Event with Strangeness and Baryons ►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

Monte Carlo Generators ► Basic aim: improve lowest order perturbation theory by including leading corrections “exclusive” event samples 1. sequential resonance decays 2. bremsstrahlung ► Physics Feedback 3. underlying event • Reliable correction 4. hadronization procedures 5. hadron (and τ) decays • Without reliable models, reliable extrapolations are hard to hope for Peter Skands Probing the UE with S and B

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

Classic Example: Number of tracks UA 5 @ 540 Ge. V, 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): 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 Peter Skands Probing the UE with S and B

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 QF ISR FSR FSR ►But hadrons are not elementary ►+ QCD diverges at low p. T ISR … 2 2 FSR ISR QF multiple perturbative parton-parton collisions ►Normally omitted in ME/PS expansions ( ~ higher twists / powers / low-x) Can take e. g. 4 4, 3 3, 3 2 Peter Skands Note: Probing the UE with S and B QF >> ΛQCD

Additional Sources of Particle Production QF >> ΛQCD + ME+ISR/FSR Stuff at + perturbative MPI QF ~ ΛQCD ► Hadronization ► Remnants from the incoming beams ► Additional (non-perturbative / collective) phenomena? QF ISR FSR 2 2 FSR ISR … • Bose-Einstein Correlations • Non-perturbative gluon exchanges / FSR ISR • QF Need-to-know issues for IR sensitive quantities (e. g. , Nch) Peter Skands • • 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

Naming Conventions ► 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” Many nomenclatures being used. vs “soft” models • Not without ambiguity. I use: Qcut … ISR FSR … 2 2 FSR ISR Qcut Primary Interaction (~ trigger) … Multiple Parton Interactions (MPI) Note: each is colored Not possible to separate clearly at hadron level Inelastic, non-diffractive Peter Skands Beam Remnants Probing the UE with S and B

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

Now Hadronize This n ay o ati bba pba c tbar de m o r f r niz o dr a h r be a m r emn ec rom t d am rem nan t W m fro q b f p be qbar from W ay ant ? Triplet Anti-Triplet Simulation from D. B. Leinweber, hep-lat/0004025 gluon action density: 2. 4 x 3. 6 fm Peter Skands Probing the UE with S and B

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

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

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 MPI create kinks on existing strings, rather than new strings 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” Peter Skands Probing the UE with S and B Hard System + MPI allowed to undergo color reconnections

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 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

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

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 Ge. V 630 Ge. V Track Multiplicity: All models ~ fine Peter Skands Data from CDF, Phys. Rev. D 65 (2002) 072005 Probing the UE with S and B

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

Strangeness ► Tunneling suppression due to quark mass P(mq, p. T) ~ exp ( -[mq 2 + p. T 2]/κ ) • strangeness probes fragmentation field in a unique way • Consistent with LEP? Consistent with RHIC / Tevatron? CDF Run 1 Correction factors CDF, Phys. Rev. D 72: 052001, 2005. CDF sees the hard tail, not the peak Generator p. T spectrum Less sensitive to mass effect Need experiment with good low-p. T tracking 17

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

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 Simulation from D. B. Leinweber, hep-lat/0004025 Sjöstrand & PS : Nucl. Phys. B 659(2003)243, JHEP 03(2004)053 19

This is hard (if you’re not LHCb) ► Baryon number transport: get as close to the beam as possible! CDF coverage Few percent effect Old models: B locked in remnant New Models: B carried by string junctions (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 20

Extrapolations to the LHC ► Lambdas 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 (Note that these models are by no means extreme, effect could well be larger) 21

Extrapolations to the LHC ► Cascades 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 (Note that these models are by no means extreme, effect could well be larger) 22

Extrapolations to the LHC ► Omega 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 (Note that these models are by no means extreme, effect could well be larger) 23

Summary ► Perugia Tunes (see also talks by A. Moraes and H. Hoeth last week in Perugia) • 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 Peter Skands Probing the UE with S and B

Backup Slides Peter Skands Theoretical Physics Dept. - Fermilab

(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 Bahr, Butterworth, Seymour: ar. Xiv: 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 = color-screening cutoff (Ecm-dependent, but large uncert) Saturation? Current models need MPI IR cutoff > PS IR cutoff Peter Skands Probing the UE with S and B

Color Reconnections Sjöstrand, Khoze, Phys. Rev. Lett. 72(1994)28 & Z. Phys. C 62(1994)281 + more … OPAL, Phys. Lett. B 453(1999)153 & OPAL, hep-ex 0508062 ► Searched for at LEP • Major source of W mass uncertainty • Most aggressive scenarios excluded • But effect still largely uncertain Preconnect ~ 10% W W Normal ► 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 EPJC 52(2007)133 ; Preliminary finding Delta(mtop) ~ 0. 5 Ge. V Now being studied by Tevatron top mass groups Peter Skands Probing the UE with S and B W W Reconnected Colour Reconnection (example) Soft Vacuum Fields? String interactions? Size of effect < 1 Ge. V?

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 • • • Peter Skands particle spectra. The same is seen at RHIC: For ‘Tune A’ etc, Rick noted that <p. T> 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? Probing the UE with S and B M. Heinz, nucl-ex/0606020; nucl-ex/0607033

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 k. T” • (~ 2 Ge. V of p. T 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

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 See also Peter Skands Sjöstrand, Khoze, Phys. Rev. Lett. 72(1994)28 & Z. Phys. C 62(1994)281 + more … OPAL, Phys. Lett. B 453(1999)153 & OPAL, hep-ex 0508062 Probing the UE with S and B

Multiple Interactions Balancing Minijets ► Look for additional balancing jet pairs “under” the hard interaction. angle between 2 ‘best-balancing’ pairs ► Several studies performed, most recently by Rick Field at CDF ‘lumpiness’ in the underlying event. (Run I) CDF, PRD 56 (1997) 3811 Peter Skands Probing the UE with S and B