APRIL 27-29, 2009, FERMILAB

1st Joint Workshop on1st Joint Workshop onEnergy Scaling of Hadron Collisions:Energy Scaling of Hadron Collisions:

Theory / RHIC / Tevatron / LHC Theory / RHIC / Tevatron / LHC

Welcome & ExhortationPeter Skands (Fermilab)

WelcomeWelcome

• If confused about practical matters– Ask Olivia, 3rd Floor, opposite side

• If confused about physics– Stay for discussion sessions

• If you need coffee– Nespresso machine outside (uses pods)– Or cafeteria downstairs

PlanPlan

• Day 1: Overviews and General Discussion

• Day 2am: RHIC/Tevatron Comparisons & Discussions

• Day 2pm: Underlying Event (UE)– including brand new D0 analysis blessed for this meeting!

• Day 3am: Non-perturbative / Collective Phenomena – ?

• Day 3pm: Extrapolating to the LHC

Hadron CollisionsHadron 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 ThisNow Hadronize This

Simulation fromD. B. Leinweber, hep-lat/0004025

gluon action density: 2.4 x 2.4 x 3.6 fm

Anti-Triplet

Triplet

pbar beam remnant

p beam remnantbbar

from

tbar

deca

y

b from

t d

ecay

qbar fro

m W

q from W

hadroniza

tion

?

q from W

- 6Peter Skands

What is in a Name?What is in a Name?► An important part of this workshop: language

• Not without ambiguity. I use:

Qcut

Qcut

22

ISR

ISR

FSR

FSR

22

ISR

ISR

FSR

FSR

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

ExhortationExhortation

• 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 ComplementarityRHIC/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 ExampleUA5 @ 540 GeV

Minimum-Bias

Number of Tracks

““Tuning”Tuning”

Simple physics models ~ Poisson

More Physics:

Multiple interactions +

impact-parameter

dependenceMoral:

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”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, pT spectra, etc, reasonably well)

Measurements Constraints

Zero-Bias, Minimum-Bias, and the Zero-Bias, Minimum-Bias, and the Underlying EventUnderlying 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 MeasurementsPrecision 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-perturbativehadronisation, colour reconnections, beam remnants, non-perturbative fragmentation functions, pion/proton ratio, kaon/pion ratio, ...

Soft Jets and Jet StructureSoft/collinear radiation (brems), underlying event (multiple perturbative 22 interactions + … ?), semi-hard brems jets, …

Resonance Masses…

Tail of Hard JetsHigh-pT jets at large angles

& W

idths

sInclusive

Exclusive

Hadron Decays

Collider Energy ScalesCollider Energy Scales

+ Un-Physical Scales:+ Un-Physical Scales:

• QF , QR : Factorization(s) & Renormalization(s)

• QE : Evolution(s)

Standard AssumptionsStandard 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 ParticlesIdentified Particles

• Some possible tracers– Production of strange quarks in fragmentation field

• Suppressed by ms / 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?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 LHCExtrapolations 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 EndThe End

• Let’s begin!

APRIL 27-29, 2009, FERMILAB

1st Joint Workshop on1st Joint Workshop onEnergy Scaling of Hadron Collisions:Energy Scaling of Hadron Collisions:

Theory / RHIC / Tevatron / LHC Theory / RHIC / Tevatron / LHC