APRIL 27-29, 2009, FERMILAB 1st Joint Workshop on Energy Scaling of Hadron Collisions: Theory / RHIC / Tevatron / LHC Welcome & Exhortation Peter Skands (Fermilab)

Welcome If confused about practical matters –Ask Olivia, 3 rd Floor, opposite side If confused about physics –Stay for discussion sessions If you need coffee –Nespresso machine outside (uses pods) –Or cafeteria downstairs

Plan Day 1: Overviews and General Discussion Day 2 am : RHIC/Tevatron Comparisons & Discussions Day 2 pm : Underlying Event (UE) –including brand new D0 analysis blessed for this meeting! Day 3 am : Non-perturbative / Collective Phenomena – ? Day 3 pm : Extrapolating to the LHC

Hadron Collisions A complete description = complete solution –Not quite there for QCD Factorization + Infrared safety allow us to –Ignore QCD? Use leptons in final state + factorized PDFs But PDFs, higher orders, isolation, fakes  sensitivity to QCD –Address QCD partially? (N)LO QCD + (N)LL resummations Infrared safe observables  limited sensitivity to non-pert effects But scale hierachies, high precision, jet calibration  yet more QCD –Model QCD? (N)LO QCD + (N)LL resummations + (good) models Model all parts of QCD  complete (?) descriptions  complete (?) solutions

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

Peter Skands - 6 What is in a Name? ►An important part of this workshop: language Not without ambiguity. I use: Q cut 2222 IS R FS R 2222 IS R FS R Primary Interaction (~ trigger) Underlying Event 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 … … … Inelastic, non-diffractive Multiple Parton Interactions

Exhortation Some areas we need to be vigilant in checking / improving: –Beyond-fixed-order pert. phenomena: Bremsstrahlung, Jet Broadening –Minimum-Bias (MB) + Underlying Event (UE) (relation to MB, multi-parton interactions (MPI), beam remnants (BR), …?) Non-perturbative phenomena, –Hadronization (vec/psd, frag-func, baryons, strange, hvy-Q, onia, correlations, Bose-Einstein, corrections,…) + Dynamic effects (string interactions / reconnections, Cronin effect, …) –Collective effects (  extrapolation to heavy-ion) The energy scaling of these phenomena with beam energy ex- "thoroughly" + hortari "encourage, urge"

RHIC/Tevatron Complementarity Often, studies of each of these separately at each collider (sometimes even each experiment!) –Similar measurements, but different theoretical bias Strong influence of pQCD pheno at Tevatron –Less focus on non-pQCD measurements Strong influence of heavy-ion pheno at RHIC –Less focus on pQCD (?) So we are complementary  compare and learn Best of both worlds? Measure the measurable

Classic Example 540 GeV Minimum-Bias Number of Tracks “Tuning” Simple physics models ~ Poisson More Physics: Multiple interactions + impact-parameter dependence Moral: 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, interesting physics Can ‘tune’ to get average right, but much too small fluctuations  inadequate physics model Measurements  Constraints 

“Tuning” Models only as good as –Their underlying physics assumptions (if a model is simple, it is wrong) –Their parameter constraints (even the most fancy is useless without constraints) E.g., even a great Tevatron tune –May be totally off at RHIC/LHC energies if energy scaling not well modeled & constrained –May be off for quantities that were inclusively summed over (e.g., you could switch off strangeness or baryons and still describe total multiplicities, p T spectra, etc, reasonably well) Measurements  Constraints 

Zero-Bias, Minimum-Bias, and the Underlying Event Is Underlying Event ~ Minimum-Bias? –No, “jet pedestals” have been known for a long time –UE much more active than MB Alternative: MB is “soft”, UE is “a different thing” –No, min-bias goes smoothly to dijets, and so should the models Recent exp studies have put this field on a solid footing –Message got out to the general public –Increased scientific credibility (a feeling in the community that these things are not just voodoo, but can be studied (and modeled) systematically and rigorously) When we’re talking across experiments & colliders, we also need to discuss how we define “minimum”-bias, “charged particle”, etc Can we correct to a “common” “MC-friendly” benchmark definition?

Precision Measurements Infrared safe observables –Insensitive to long-distance physics up to ~ (worse if not infrared safe  leftover logs) So sets an absolute limit on precision? Not if we can model / control the long-distance physics Recent example: top mass at ~ GeV precision Required improved constraints on QCD models / tunes Another important example: high-precision jet calibration

Non-perturbative hadronisation, colour reconnections, beam remnants, non-perturbative fragmentation functions, pion/proton ratio, kaon/pion ratio,... Soft Jets and Jet Structure Soft/collinear radiation (brems), underlying event (multiple perturbative 2  2 interactions + … ?), semi-hard brems jets, … Resonance Masses… Tail of Hard Jets High-p T jets at large angles & Widths s Inclusive Exclusive Hadron Decays Collider Energy Scales + Un-Physical Scales: Q F, Q R : Factorization(s) & Renormalization(s) Q E : Evolution(s)

Standard Assumptions Jet universality –Jets fragment in the same way in pp as they did at LEP –Not unreasonable, but must be tested in situ –Cannot be expected to hold to infinite precision Look for when breakdown occurs  tracers needed … –Assumed by all models / tunes Remnant Fragmentation –Gives “Soft Component” vs “Hard” MPI component –What is the balance between the two? Need tracers …

Identified Particles Some possible tracers –Production of strange quarks in fragmentation field Suppressed by m s / string tension  Sensitive to changes in the confinement field –Remnant fragmentation should produce softer spectra, with flatter rapidity profile, but impossible to say whether a given pion came from MPI or BR Smoking gun: an excess baryon … not in a jet  use baryon stopping as tracer of remnant fragmentation?

Non-perturbative / Collective Effects? What is what? –Large difference in language used by different communities … Not clear how collective effects, rescatterings, color reconnections, interacting strings, remnant effects, etc, are connected What are the salient properties of each model? –Which salient properties are present in the data? How can we obtain sufficient observables with sufficiently unambiguous interpretations to make clear statements?  clear constraints and improvements

Extrapolations to LHC The heavy-ion community had a workshop last year where everyone had to put their predictions on the table The pp community didn’t I’m not saying we’re a bunch of chickens, but … How often does an LHC start?

The End Let’s begin! APRIL 27-29, 2009, FERMILAB 1st Joint Workshop on Energy Scaling of Hadron Collisions: Theory / RHIC / Tevatron / LHC