1. 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!

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

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

4. S.D. Ellis: West Coast LHC Theory Network 2/3/06 4 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)

5. S.D. Ellis: West Coast LHC Theory Network 2/3/06 5 Resources HERA – LHC Workshop http://www.desy.de/~heralhc/ TeV4LHC Workshop http://conferences.fnal.gov/tev4lhc/ Computer Tools http://www.cedar.ac.uk/ e.g., http://hepforge.cedar.ac.uk/ktjet/

6. S.D. Ellis: West Coast LHC Theory Network 2/3/06 6 Long Distance QCD Primer What’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 proton after emitting gluon, plus gluons in proton Short Distance = 1/Hard Scattering Scale And Long Distance   Bummer!!

7. S.D. Ellis: West Coast LHC Theory Network 2/3/06 7 When in doubt, Sum those logs!!  Renormalized, evolving quark distribution in proton: where x = momentum fraction (of quark in proton), 1/  is the Factorization Scale 1/Q is Short Distance (the resolving scale) 1/  is long distance scale DIS picture

8. S.D. Ellis: West Coast LHC Theory Network 2/3/06 8 Renormalized PDF Logs 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

9. S.D. Ellis: West Coast LHC Theory Network 2/3/06 9  Parton Distribution Fcts Internal (showering/singular) structure of partons in hadrons, determined by Perturbative evolution – LO, NLO, (NNLO) Initial conditions  (Global) fits to data e.g., CTEQ – See Soper & Tung MRST Alekhin – NNLO & DIS  New tools for propagating errors in data to uncertainties in cross sections calculated with PDFs

10. S.D. Ellis: West Coast LHC Theory Network 2/3/06 10 Current Status Measures of parton luminosity uncertainties CTEQ Where is the total hadronic energy, and is the total partonic, hard scattering energy  Uncertainties > 10% except for large x gluons (just where we need them!)

11. S.D. Ellis: West Coast LHC Theory Network 2/3/06 11 Current PDF issues More precision for the Gluons Flavor, charge asymmetries, e.g., Heavy flavors (c,b) experimental determination include mass effects, defining thresholds role of nonperturbative effects (i.e., besides perturbative gluon splitting) Do we need NNLO fits? (global data probably not that good yet)

12. S.D. Ellis: West Coast LHC Theory Network 2/3/06 12 (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!

13. S.D. Ellis: West Coast LHC Theory Network 2/3/06 13 Naïve picture with short distance partons, long distance hadrons (incoming F and outgoing D), plus UE Outgoing Long Distance  QCD Collinear singularities (real and virtual) bite us again, plus issue of hadronization

14. S.D. Ellis: West Coast LHC Theory Network 2/3/06 14 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

15. S.D. Ellis: West Coast LHC Theory Network 2/3/06 15 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) k T Algorithm, based on pairwise merging, nearest in p-space, lowest p T 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!

16. S.D. Ellis: West Coast LHC Theory Network 2/3/06 16 Think of the algorithm as a “microscope” for seeing the (colorful) underlying structure -

17. S.D. Ellis: West Coast LHC Theory Network 2/3/06 17 ATLAS 2-Jet Event

18. S.D. Ellis: West Coast LHC Theory Network 2/3/06 18 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!

19. S.D. Ellis: West Coast LHC Theory Network 2/3/06 19 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 = p T 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 (  R SEP ) Cone Jets  k T jets NLONNLO

20. S.D. Ellis: West Coast LHC Theory Network 2/3/06 20 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., R SEP ) 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)

21. S.D. Ellis: West Coast LHC Theory Network 2/3/06 21 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)!!

22. S.D. Ellis: West Coast LHC Theory Network 2/3/06 22 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

23. S.D. Ellis: West Coast LHC Theory Network 2/3/06 23 Cone Algorithms in Run II DØ using the MidPoint Cone Algorithm with f merge = 0.50, a la the Run II agreement CDF using the smaller “search cone” fix (for Dark Towers) with f merge = 0.75  2 Experiments still different!!

24. S.D. Ellis: West Coast LHC Theory Network 2/3/06 24 The 2 algorithms are different in the data (but not in the pert theory)!  6% difference at large p T – search cone collects about 1% more p T  At low p T all the details matter!!  From Wobisch

25. S.D. Ellis: West Coast LHC Theory Network 2/3/06 25 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!

26. S.D. Ellis: West Coast LHC Theory Network 2/3/06 26 Comment for 4-vectors: Jet Masses useful at LHC with large boosts Original BB light jet mass distribution, P T > 25 GeV top W/Z? boost

27. S.D. Ellis: West Coast LHC Theory Network 2/3/06 27 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

28. S.D. Ellis: West Coast LHC Theory Network 2/3/06 28 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

29. S.D. Ellis: West Coast LHC Theory Network 2/3/06 29 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?

30. S.D. Ellis: West Coast LHC Theory Network 2/3/06 30 Other - Energy Flow/Correlation Analyses Sum 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?

31. S.D. Ellis: West Coast LHC Theory Network 2/3/06 31 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)!

32. S.D. Ellis: West Coast LHC Theory Network 2/3/06 32  “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 p T 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?