S.D. Ellis: West Coast LHC Theory Network 2/3/06 1 PREPARING for the Future QCD for LHC What We Need to Know/Learn: About Long Distances Go!

S.D. Ellis: West Coast LHC Theory Network 2/3/06

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PREPARING for the FuturePREPARING for the FutureQCD for LHCQCD for LHC

What We Need to Know/Learn:What We Need to Know/Learn: About Long Distances About Long Distances

Go!


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Goals for QCD

• Be able to identify short distance structure of event (presence of squarks, Higgs, etc.) from measurements of long distance experimental measurements (“objects”) – leptons, ’s, jets (especially jets with heavy flavor – tagged jets) and “”s (MET)

• Be able to reliably (< 10 % uncertainty) calculate rates of signal and background processes


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Action Items

• Learn what is already known – who are the West Coast experts? (Not me!)

• Study the essential issues that are not yet known – who is working on what?

Today’s assistants –

• Short Distance QCD – Zvi

• Long Distance QCD - Me


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Outline• Long Distance Before – Mapping Hadrons onto

Partons – Parton Distribution Functions (PDFs)

• Long Distance After – Mapping Hadrons (detectors) onto Partons – Jets

Related Special Topics –

MC@NLO for Jets

Theory for energy correlations in Events (Beyond perturbation theory)


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Resources• HERA – LHC Workshop

http://www.desy.de/~heralhc/

• TeV4LHC Workshophttp://conferences.fnal.gov/tev4lhc/

• Computer Toolshttp://www.cedar.ac.uk/

e.g., http://hepforge.cedar.ac.uk/ktjet/


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Long Distance QCD PrimerWhat’s the big deal?

QCD is gauge theory with “massless” particles has collinear (and soft) singularities (real & virtual) Logs

Quark in proton(parton model)

QCD

Quark in protonafter emitting gluon, plus gluons in proton

Long Distanceln

Short Distances

Short Distance = 1/Hard Scattering ScaleAnd Long Distance Bummer!!


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When in doubt, Sum those logs!! Renormalized, evolving quark

distribution in proton:

where x = momentum fraction (of quark in proton),1/ is the Factorization Scale1/Q is Short Distance (the resolving scale)1/ is long distance scale

,q x

DIS picture


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Renormalized PDFLogs of (Q/) in short distance QCD (at fixed order in

perturbation theory)

[Must match Logs of order by order]

Logs of (/) in PDF (to all orders in Perturbation theory)

QCD tells us how q(x,) changes with (order by order in perturbation theory - DGLAP), but, since we hid (in ), we must measure , i.e., at some scale.

But PDF is “universal” – fit to all data

0,q x ,q x


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Parton Distribution Fcts

Internal (showering/singular) structure of partons in hadrons, determined by

Perturbative evolution – LO, NLO, (NNLO)

Initial conditions (Global) fits to datae.g., CTEQ – See Soper & Tung

MRST Alekhin – NNLO & DIS New tools for propagating errors in data

to uncertainties in cross sections calculated with PDFs


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Current Status

Measures of parton luminosity uncertainties

1 2 1 2 1 2ˆ, , sL dx dx g x g x x x s

CTEQ

Where is the total hadronic energy, and is the total partonic, hard scattering energy

ss

Uncertainties > 10% except for large x gluons (just where we need them!)


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Current PDF issues• More precision for the Gluons• Flavor, charge asymmetries, e.g., • Heavy flavors (c,b)

experimental determinationinclude mass effects, defining thresholdsrole of nonperturbative effects (i.e., besides

perturbative gluon splitting)• Do we need NNLO fits? (global data probably not

that good yet)

vs ss


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(The Dreaded) Underlying Event (UE)Q: What happens to the other partons in the proton?

A: They interact (softly?) and make lots of other hadronic stuff = THE UNDERLYING EVENT, which contributes to jets

UE is approximately independent of the hard scattering, but there must be color correlations! The UE will likely be relevant at the LHC, as will the simultaneous interactions of multiple protons at high beam luminosity!


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Naïve picture with short distance partons, long distance hadrons (incoming F and outgoing D), plus UE

Outgoing Long Distance

QCD

Long Distanceln

Short Distances

Collinear singularities (real and virtual) bite us again, plus issue of hadronization


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Outgoing Long Distance• Colored objects shower (into more colored objects) –

perturbative showering (large logs)

• Colored objects (eventually) organize into color singlet hadrons – non-perturbative hadronization

“Hide” details (and large logs) with inclusive measure of final state - sum over all emitted and hadronized objects in direction of original parton so details (and logs) don’t matter

JETS


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Associate “nearby” hadrons or partons into JETS via ALGORITHMS, i.e., rules that can

be applied to data, MC and theory• Cone Algorithms, e.g., Snowmass, based on fixed geometry (well

suited to hadron colliders with UEs)

• kT Algorithm, based on pairwise merging, nearest in p-space, lowest pT first (familiar at e+e- colliders)

Render PertThy IR & Collinear Safe

But mapping of hadrons to partons can never be 1 to 1, event-by-event! colored states ≠ singlet states!

always some splash in/out!


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Think of the algorithm as a “microscope” for seeing the (colorful) underlying structure -


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ATLAS 2-Jet Event


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Events in (D0) Detector

Associate nearby “stuff”, e.g., in circle in eta/phi. This is a “cone” jet, presumed to arise from a hard scattered parton with similar kinematic properties!


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How well does this work?OK at ~10% level, but it ain’t as easy as that! There are lots of details that matter when two jets

are nearby – (happens often in a collinear singular theory):

Cone jets are defined by “stable” cones (geometric center = pT weighted center) and they can overlap – must decide how to split/merge

Experimenters only look around hot spots (seeds) and miss 2 cones that could fit in a single cone (RSEP)Cone Jets kT jets

NLO NNLO


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Goals of IDEAL ALGORITHM• Fully Specified: including defining in

detail any preclustering, merging, and splitting issues

• Theoretically Well Behaved: the algorithm should be infrared and collinear safe (and insensitive) with no ad hoc clustering parameters (e.g., RSEP)

• Detector Independence: there should be no dependence on cell type, numbers, or size

• Order Independence: The algorithms should behave equally at the parton, particle, and detector levels.

• Uniformity: everyone uses the same algorithms (at least some of the time)


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Fundamental Issue – Compare Experiments to each other & to Theory

Warning:

We should all use the same algorithm!!(as closely as humanly possible), i.e. both ATLAS & CMS (and theorists).

This is NOT the case at the Tevatron, even in Run II (nor was it in Run I)!!


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Current situation for Algorithms at the Tevatron – see talk by Markus Wobisch at

Tev4LHC, 10/2005 at Fermilab

Dark towers

Merged jets

UN Merged

jets

CDF Legacy Cone

Run II Cones


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Cone Algorithms in Run II

• DØ using the MidPoint Cone Algorithm with fmerge = 0.50, a la the Run II agreement

• CDF using the smaller “search cone” fix (for Dark Towers) with fmerge = 0.75

2 Experiments still different!!


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The 2 algorithms are different in the data (but not in the pert theory)!

6% difference at large pT – search cone collects about 1% more pT

At low pT all the details matter!! From Wobisch


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Comment on Heavy Flavor in Jets

• Do we understand how much heavy flavor (c,b) in jets arises from Long Distance, i.e., gluon showering? Either theoretically or experimentally? Impacts estimates of tagging

• Current uncertainty seems to be of order 30%!

• Needs work!


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Comment for 4-vectors:Jet Masses useful at LHC with large boosts

100 150 200 250 300l jet MassGeV

0

200

400

600

800

lstejTP52,

ate

25VeGni

B

Original BB light jet mass distribution,

PT > 25 GeV

top

W/Z?

boost


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Jet Summary

The largest initial challenge at the LHC will be understanding how the jet algorithms work

Determining the “Jet Energy Scale”, i.e., the correction factor from the observed energies of stuff in jet to energy of underlying partons (e.g., using photons, Z’s recoiling against jets, etc.)

Ensuring the experiments use the “same” algorithm


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Other - MC@NLO• JET studies clearly call for a conjoined NLO and MC tool:

(Frixione & Webber) http://www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/

• The MC includes dominant long distance QCD (showering and hadronization) contributions “associated” with participants in short distance QCD, and generates “full” events that can be processed by a detector simulation (but have used LO short distance QCD)

• “Marry” this to precise NLO description of short distance QCD to facilitate more accurate/realistic studies of final states (including jets)

• MC@NLO v. 3.2 includes production of single & double vector bosons, single (new) and pair top quarks, Higgs, Higgs with W/Z and lepton pairs


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MC@NLO• Issue is the merging of short distance with long

distance without double counting; first emission is in short distance, not long distance (must subtract)

• Techniques for matching spelled out by F & W (“similar” to matching of short distance and PDF)

• Many possibilities to double count for light jet amplitudes, everything is colored (top quarks never find out they are colored)

• Ellis & Kilgore, are working on this, but we ain’t done! Any volunteers?


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Other - Energy Flow/Correlation AnalysesSum lots of Logs

• Berger & Sterman, et al.

• Dokshitzer, Marchesini, Webber, et al.

• Dasgupta, Salam, et al.

Can these methods calculate “splash-in/out” down to non-perturbative level?

Can energy flow measures be used to isolate new physics?

Who, on the West Coast, is working on this?


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Suggests we do something very different – not an integer jet count event-by-event?

Keep more information from each event – “extreme” example –

JET ENERGY FLOW (JEF) – Berger, et al. hep-ph/0202207, Snowmass 2001 F.V. Tkachov, hep-ph/9901444

Each event yields a distribution, E versus angle, instead of a jet count!

Needs more study (or at least I need to understand it)!


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“Summary”

Long Distance QCD is understood at the ~10% level, especially where MC@NLO is available (need this for jets)

PDF – Want better knowledge of glue & heavy flavors Run II data will help, especially precision W/Z versus pT

measurements Jets – Differences between experimental implementations in

Run II are probably as important as differences between algorithms – Can we expect ATLAS & CMS to use the same algorithm? Can we determine the long distance heavy quark content?

Can we find Something New & Different, e.g., Jet Energy Flows? Can we use better the improved understanding of energy flows to better define or replace jets?

Documents

S.D. Ellis: West Coast LHC Theory Network 2/3/06 1 PREPARING for the Future QCD for LHC What We Need to Know/Learn: About Long Distances Go!