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Collider Phenomenology & Jets at the UW

Big Picture:. Collider Phenomenology & Jets at the UW. For the next decade the focus of particle physics phenomenology will be on the LHC. The LHC will be both very exciting and very challenging - addressing a wealth of essential scientific questions

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Collider Phenomenology & Jets at the UW

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  1. Big Picture: Collider Phenomenology & Jets at the UW For the next decade the focus of particle physics phenomenology will be on the LHC. The LHC will be both very exciting and very challenging - • addressing a wealth of essential scientific questions • with new (not understood) detectors • operating at high energy and high luminosity. • The recent HEPAP Subpanel Report says – progress will come only from the coordinated efforts of large numbers of theorists and experimentalists • We are working to create the necessary collaborative environment in the NW. We have an outstanding history of such collaboration at the UW. Stephen D. Ellis UW 11/05/07

  2. Outline • Why jets? • UW Phenomenology Assets • Jets at Colliders • Review of cone jets • Review of kT jets • Looking to the LHC, event-by-event QCD likelihood • From jet masses • From internal kT jet structure S. D. Ellis - DoE Site Visit 2007

  3. Essentially all LHC events involve an important hadronic component, only avoids this constraintThe primary tool for hadronic analysis is the study of jets, to map long distance degrees of freedom (i.e., detected) onto short distance dof (in the Lagrangian)UW has a long history of expertise with jetsJets used at the Tevatron to test the SM, will be used at the LHC to test for non-SM-ness Not so Big Picture: Why jets ? S. D. Ellis - DoE Site Visit 2007

  4. Local Assets • Theory/Pheno Faculty: Steve Ellis, Ann Nelson • Theory/Pheno Graduate Students: Jon Walsh (LHC Fellow), Chris Vermilion, Jacob Miner (2006 REU student), Rob Schabinger; also AndrewLarkoski (UnderGrad now at Stanford) Other Theory (the environment matters): BSM: A. Nelson, D. Kaplan (INT) String Theory: A. Karch AdS/CFT – QCD L. Yaffe, D. Son (INT), A. Karch Experiment: D0, ATLAS – T. Burnett, H. Lubatti, A. Goussiou, G. Watts Collider QCD: W.-K. Tung (Affiliate Faculty) – pdf & heavy flavor expert S. D. Ellis - DoE Site Visit 2007

  5. Not so Local Context: Proposed Northwest Center for Terascale Physics:Joint with the University of Oregon PIs - A. Nelson, A. Goussiou, D. Soper and E. Torrence Theorists: N. Deshpande, S. Hsu, G. Kribs, D. Soper Experimentalists: J. Brau, R. Frey, D. Strom, E. Torrence The opportunities for synergy and collaboration are enormous, sharing ideas, Post Docs, and students. Physical proximity and e-conferencing technology should make the collaboration easily doable with only a small enhancement in resources – a sound investment. S. D. Ellis - DoE Site Visit 2007

  6. Even Larger Context: CDF, ATLAS – J. Huston (working with Ellis on jets) ATLAS, CMS, LHCb – Ellis has just written review/preview paper “Jet in Hadron-Hadron Collisions” with J. Huston, K. Hatakeyama, P. Loch for Prog. Part. Nucl. Phys. TeV4LHC Workshop (hep-ph/0610012): Ellis wrote the Jet Section – What jet algorithm to use at the LHC? West Coast LHC Theory Network - participating S. D. Ellis - DoE Site Visit 2007

  7. Recent Change : • Matt Strassler recently left UW to join the new Phenomenology Axis of Rutgers/Princeton/IASMatt remains active in the current LHC phenomenology work at UWArguably Matt’s credentials as a Phenomenologist were established by his activities/collaborations (including what he learned from the people) here at the UWMatt’s work on Hidden Valley physics (and the resulting long-lived particles) in collaboration with Lubatti, et al., has become a important new physics initiative within ATLAS driving the decisions about triggers S. D. Ellis - DoE Site Visit 2007

  8. Jet Physics: The Basis of QCD Collider Phenomenology Long distance physics = complicated (all orders showering of colored objects, nonperturbative hadronization = organization into color singlets) Measure this in the detector pdf Short distance physics = simple (perturbative) Fragmentation fct Want to talk about this Correlated by Underlying Event (UE) color correlations Stuck with this, small? More long distance physics, but measured in pdfs S. D. Ellis - DoE Site Visit 2007

  9. Since large momentum transfer is rare (small running coupling) and physics is dominated by collinear (or soft) emissions (gauge theory), spreading is limited in QCD  Jet Structure. Clear in data! real simulated S. D. Ellis - DoE Site Visit 2007

  10. Use this jet structure* • Reconstruct the short-distance kinematics, masses, etc. • Map the observed (hadronic) final states onto the (short-distance) partons by summing up all the approximately collinear stuff, ideally on an event-by-event basis. • Need rules for summing  jet algorithm Start with list of particles/towers End with list of jets (and stuff not in jets) • E.g., • Cone Algorithms, based on fixed geometry – focus on core of jetWell suited to hadron colliders with UEs (Snowmass, Ellis, et al., 1990; NLO, Ellis, Kunszt & Soper, 1989) • kT Algorithm, based on pairwise merging, lowest pT first – undo shower Tends to “vacuum up” soft particles, well suited to e+e- colliders (Hadron Collider, Ellis & Soper, 1993) S. D. Ellis - DoE Site Visit 2007 *Beginning of jets in hadron collisions, Ellis & Kislinger, 1974

  11. The good news about jet algorithms: • Render PertThy IR & Collinear Safe, potential singularities cancel • Simple, in principle, to apply to data and to theory • Relatively insensitive to perturbative showering and hadronization The bad news about jet algorithms: • The mapping of color singlet hadrons on to colored partons can never be 1 to 1, event-by-event! • There is no unique, perfect algorithm; all have systematic issues • Different experiments use different algorithms • The detailed result depends on the algorithm S. D. Ellis - DoE Site Visit 2007

  12. Different algorithms  different jets (same CDF event) S. D. Ellis - DoE Site Visit 2007 EM, Hadronic

  13. Must choose Figure of Merit for Jet Algorithms • At the Tevatron jet studies have been driven by “testing” QCD, comparing data and PertThy for inclusive jet cross section – [Cone, DØ] Ratio data/NLO theory Inclusive Jet cross section Range ~ 108 Uncertainty > 10% S. D. Ellis - DoE Site Visit 2007

  14. Similar situation for kT jets [kT, CDF] S. D. Ellis - DoE Site Visit 2007

  15. Generally - 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 • 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 (theory and experiment, different experiments) S. D. Ellis - DoE Site Visit 2007

  16. Cone Jet at the Tevatron – Systematics are the issue • Cone Algorithm – focus on the core of jet • Jet = “stable cone”  4-vector of cone contents || cone direction • Well studied – several issues • Cone Algorithm – particles, calorimeter towers, partons in cone of size R, defined in angular space, e.g., (y,), • CONE center - • CONE i C iff • Cone Contents  4-vector • 4-vector direction • Jet = stable coneFind by iteration, i.e., put next trial cone at S. D. Ellis - DoE Site Visit 2007

  17. Example Lego & Flow S. D. Ellis - DoE Site Visit 2007

  18. Cone Systematic Issues: (The devil we know) • Stable Cones can and do overlap, need to define rules for merging and splitting, more parameters (but CDF and D0 choose different parameters, no analogue in NLO PertThy) • Seeds – experiments only look for jets near very active regions (save computer time)  problem for theory, IR sensitive (Unsafe?) at NNLO Don’t find “possible” central jet between two well separated proto-jets (partons) • Dark Towers - Energy in secondary showers may not be clustered in any jet No seed Seed NLO NNLO • Expected stable cone not stable due to smearing from showering/hadronization (compared to PertThy) • Under-estimate ET (~ 5% effect for jet cross section) S. D. Ellis - DoE Site Visit 2007

  19. Cone Fixes - • All experiments use the same split/merge parametersStill not true at the Tevatron • Use seedless cone algorithm (e.g., SIScone) or correct data for seed effects Small effect (1-2 %) in data, big issue in pert Thy • No good solution yet to Dark towers except to look for 2nd pass jets after removing the 1st pass jets from the analysis.  The Cone Algorithm, with all its problems, is well understood and will be used at the LHC, but ….. What else can we do? S. D. Ellis - DoE Site Visit 2007

  20. kT Algorithm – focus on undoing the shower pairwise kT algorithm  Merge partons, particles or towers pairwise based on “closeness” defined by minimum value ofIf kT,(ij)2is the minimum, merge pair and redo list;IfkT,i2is the minimum →i is a jet!(no more merging for i), 1 parameterD (NLO, equals cone for D = R, Rsep = 1) • Jet identification is unique – no merge/split stage • Everything in a jet, no Dark Towers • Naively computer intensive (time grows like N3) but faster versions (N2 and even N ln N) now exist • Resulting jets are more amorphous, energy calibration difficult (subtraction for UE?), Impact of UE and pile-up not well understood, especially at LHC Still not perfect – can we get past counting jets? Measure QCDness? S. D. Ellis - DoE Site Visit 2007

  21. Recall: Goals at LHC Different - • Find Physics Beyond the Standard Model • Event structure likely different from QCD, more jets? Different structure within jets? Must be able to reconstruct masses from multi-jets • Highly boosted SM particles – W, Z, top  single jet instead of 2 or 3 jets, focus on substructure in jets • Want to select events by non-QCD-ness • Much work to be done, and we are doing (some of) it!! S. D. Ellis - DoE Site Visit 2007

  22. Jet Masses in QCD: To compare to non-QCD Dimensions Phase space from dpfs, f ~ 1 • In NLO PertThy Jet Size, R ~ , determined by jet algorithm Note Useful QCD “Rule-of-Thumb” S. D. Ellis - DoE Site Visit 2007

  23. Compare to (simulated) QCD LHC data: Various algorithms applied to simulated LHC data (diamond, square, circle) NLO Cone Theory, various Rsep values (lines, triangles) } } Rsep = 2, SnowmassRsep =1.3, EKSRsep= 2.0, kT Theory Brackets data S. D. Ellis - DoE Site Visit 2007

  24. At the LHC – new TeV-scale physics  boosted W/Z/t decay into (just) 1 jet Look for Z as single jet in Simulated LHC Data From Jon Walsh UW data analyzed with 2 kinds of jets Z bump above QCDbackground S. D. Ellis - DoE Site Visit 2007

  25. Another Approach - kT Algorithm describes event in terms of (ordered) list of –pairwisebranchings, each with scale kT,(ij) and opening angle Rij, pT of branches in between • Suggests displaying as a tree – a Baobab tree Radius of branching = kT,(ij)2thickness ~ pT, color of branch ~ m/pTangle ~ detector angle S. D. Ellis - DoE Site Visit 2007

  26. Samples – Jon will say more – QCD trees  non-QCD trees Note: the outer branches are always the same QCD, Fewer energetic jets ttbar, More energetic jets S. D. Ellis - DoE Site Visit 2007

  27. Characterize the QCD tree  QCD Likelihood The QCD Tree, like the shower in a Monte Carlo, is well described in the Double Leading Log Approximation (keeping only the soft & collinear physics) Branching (of branch with UV scale pJ) probability/unit kT Probability of not branching between scale kT and IR scale Q1 (Sudakov) Rij S. D. Ellis - DoE Site Visit 2007

  28. UV scale can vary with branch • Likelihood function (small means QCD-like) • Jon will provide more details kT Resolution parameter Product over (selected) branchings S. D. Ellis - DoE Site Visit 2007

  29. Sample Histograms of Likelihood from Jon QCD Jets only – QCD-like frequency ttbar – not so QCD-like frequency -log[likelihood] -log[likelihood] S. D. Ellis - DoE Site Visit 2007

  30. Future for LHC jets Better understanding and use of jets & algorithms • LHC will employ “standard” Cone and kTalgorithms to count jets, but must – • Use same split/merge parameters • Not use seeds • Address dark towers • New ways to use jets via QCD & non-QCD Likelihoods for events • Use jet masses • Use internal structure of kT jets • Look for inspiration from conformal field theory and string theory for better jet/event description – see Andreas  Find New Physics at the LHC using jets S. D. Ellis - DoE Site Visit 2007

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