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IPPP Open Presentation on Strong Interactions

This presentation covers recent developments in QCD theory and phenomenology at the LHC, including topics like parton distributions, Higgs cross sections, and challenges in multiparticle events. It also discusses collaborations and future goals in exploring new physics.

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IPPP Open Presentation on Strong Interactions

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  1. IPPP Open Presentation on Strong Interactions 11:30OverviewNigel Glover 12:00Towards LHC Phenomenology beyond the Leading Order Gudrun Heinrich 12:20Threshold resummation for coloured heavy particles at the LHC PietroFalgari 12:40One-loop Higgs plus four gluon amplitudes: Full analytic results Ciaran Williams

  2. Overview Nigel Glover 14 September 2009

  3. QCD related PhD Theses since 2005 http://www.ippp.dur.ac.uk/Research/theses.htm

  4. Some QCD related workshops in past 12 months

  5. Martin, Watt, +Stirling, Thorne MSTW Parton distributions MSTW 2008 partons • Long term Durham activity, still going strong • Uncertainties on alpha(S) in global PDF analyses arXiv:0905.3531 • Parton distributions for the LHC arXiv:0901.0002 http://projects.hepforge.org/mstwpdf/ Longstanding collaboration with UCL and now Cambridge reinforced by arrival of Graeme Watt in March 09

  6. MSTW Parton distributions • MSTW 2008 (LO, NLO, NNLO) PDF fits [arXiv:0901.0002] are the most comprehensive to date: supersede MRST sets. • Pre-2006 MRST NNLO PDF sets should be considered obsolete due to incomplete heavy flavor treatment. • TevatronRun II jets prefer smaller high-x gluon than Run I: impact on Higgs cross sections at Tevatron. • Improved “dynamic tolerance” controlling propagation of experimental errors through to PDF uncertainties. • Now possible to consistently calculate combined “PDF+αS” uncertainty on cross sections

  7. NNLO trend similar to NLO: smaller 2008 gluon at high x, larger 2008 gluon at low x (momentum sum rule). Higgs cross sections smaller at Tevatron with 2008 PDFs

  8. Main advantages Accurate mass measurement (irrespective of the decay mode). Quantum number filter/analyser. (0++ dominance ;CP-even) H ->bb not swamped by background H →WW*/WW ()less challenging experimentally Theoretical challenges : need for developed and well tested models for soft diffraction unusually & uncomfortably large higher-order QCD effects (e.g. Sudakov. and non-Sudakov Logs… ). no suitable conventional analytic or MC results readily available (colour singlet, Jz=0) Khoze, Martin, Stirling + de Roeck, Kaidalov, Ryskin Diffractive processes as a means to study new physics at the LHC Longstanding collaboration with Manchester, FP420 and ATLAS/CMS upgrade reinforced by arrival of Andy Pilkington in December 09

  9. hard single diffraction ‘rapidity gap’ collision events typical jet event hard double pomeron hard color singlet exchange

  10. Khoze, Martin, Ryskin, Stirling, Kaidalov; +Harland-Lang Central Exclusive Production of heavy quarkonia main aim: • to demonstrate that CEP can open a new way to study the properties of heavy quarkonia, and, in particular, to serve as a spin-parity analyser • New SUPERCHIC MCHarland-Lang, Stirling • Important test of theoretical model, particularly • Eikonalrescatt: between protons (conserves soft factorisation) • Enhanced rescatt: involving intermediate partons (violates soft factorisation) CDF data appears to agree with KMR predictions

  11. Multiparticle challenges for the LHC • LHC brings new frontiers in energy and luminosity • typical SM process is accompanied by multiple radiation to form multi-jet events • most BSM signals involve pair-production and subsequent chain decays • Big effort to provide more accurate predictions for multiparticle events Twin Goals: • Identification and study of New Physics • Precision measurements (e.g. αs, PDF’s) leading to improved theoretical predictions

  12. NLO calculations Several different approaches • Automation based on Feynman diagrams See talk by Gudrun • Automation based on unitarity methods • Analytic methods based on unitaritySee talk by Ciaran • Merging with Monte Carlos POWHEG, CKKW@NLO one-loop 2 -> 3 process looks like 3 jets in final state tree-level 2 -> 4 process looks like 3 or 4 jets in final state plus method for combining theinfrareddivergent parts - dipole subtraction

  13. Maitre+Berger, Bern, Dixon, Febres Cordero, Forde, Ita, Kosower, Gleisberg Collaboration with SLAC through exchange of Maitre and Gleisberg

  14. Maitre+Berger, Bern, Dixon, Febres Cordero, Forde, Ita, Kosower, Gleisberg NLO QCD predictions for W+3 jets BlackHat • Goal: automate computation of virtual one-loop amplitudes for QCD processes • C++ framework • Uses new progress in the unitarity techniques, spinor formalism, complex momenta Cut containing part: 4 Dim, using Forde's method Rational part: One- loop recursion (reuse of lower point results ) Rational extraction using D-dim unitarity SHERPA Provides • Efficient phase space integration • Event generation • Analysis framework • Automated dipole subtraction for the real part • (and much more) • Is written in C++ Gleisberg,Hoeche,Krauss,Schoenherr,Schumann,Siegert,Winter

  15. NLO QCD predictions for W+3 jets • Complete results 0907.1984 • Different scale choices – W transverse momentum, or “hotness” of event give quite different results • Able to start asking interesting questions about scale choices for less inclusive observables.

  16. Wealth of phenomenology opening up

  17. Smillie + Andersen, White Constructing All-Order Corrections to Multi-Jet Rates • We know amplitudes exhibit universal behaviour where rapidity range increases (t-channel exchanges dominate) • Aim: to exploit this behaviour to create simple building blocks to sum corrections to n jet rates from higher orders. Enhanced collaboration with UCL (through Jenni Smillie from October 09) and reinforced by arrival of Chris White in October 09

  18. Constructing All-Order Corrections to Multi-Jet Rates This has been done for Higgs +n jet events in the fully flexible Monte Carlo, MJEV Resumsreal and approximate virtual higher order corrections. Andersen and White (08) Building blocks now extended to include pure jets, W+jets, Z+jetsand improved H+jets. Validate by comparing to known tree-level fixed order results. Implementing now. Andersen, Smillie, 0908.2786

  19. Gehrmann, Gehrmann-De Ridder, Glover, Heinrich NNLO Jets in e+e- • Two loop matrix elements computed some time ago • Antenna subtraction scheme to isolate infrared divergences • Analytic integration of infrared poles • Analytic cancellation of infrared poles

  20. NNLO Jets in e+e- • First NNLO results for 3-jet event shapes in 2007 Gehrmann, Gehrmann-De Ridder, NG, Heinrich (07) • Problem in the two-jet region identified in two colour structures Becher, Schwartz (08); Weinzierl (08) • over-subtraction of wide angle soft emission • now fixed - minor correction in three-jet region

  21. NNLO Jets in e+e- • Moments of event shapes Gehrmann, Gehrmann-De Ridder, NG, Heinrich (09) • Agreement with independent calculation of Weinzierl (09) • NNLO corrections are moderate for all event shapes • NNLO corrections result in a substantial reduction of the theoretical uncertainty on these predictions • size of power corrections appears to be reduced

  22. NNLO Jets in e+e- • Application: extraction of αs at NNLO+NLLA • Dissertori, Gehrmann, Gehrmann-De Ridder, NG, Heinrich, Luisoni, Stenzel • Bethke, Kluth, Pahl, Schieck, the JADE Collaboration • clear improvements over NLO+NLLA • spread of αsdeterminations reduced • renormalisation scale reduced • theory error larger for NNLO+NLLA than NNLO because of mismatch in the cancellation of renormalisation scale logarithms • fit to ALEPH data for six event shape observables yields • αs(MZ) = 0.1224 ± 0.0009(stat) ± 0.0009(exp) ± 0.0012(had) ± 0.0035(theo)

  23. Glover, Pires NNLO Jets in pp • Needed for better determinations of PDF’s and αs • NLO accuracy at best ±30% • NNLO accuracy ~10% • needed to constrain PDF’s and fix strong coupling • matrix elements known for some time Anastasiou et al, Bern et al • NLO antenna subtraction terms worked out Daleo, Gehrmann, Maitre • All gluon NNLO antenna constructed and implemented in code based on QGRAF Pires, NG

  24. Gehrmann, Glover Other NNLO calculations • ep-> 2 + 1 jets • needed to constrain gluon PDF and fix strong coupling • NLO uncertainty dominates experimental error Gouzevitch • two-loop helicityamplitudes recently worked out Gehrmann, NG

  25. Glover, Ikizlerli; Heinrich Beyond NNLO • Three-loop vertex quark and gluon ingredient of NNNLO cross section • Useful for standard candle cross sections • Drell-Yan • Higgs production via gluon fusion • and infrared structure of amplitudes Relies on knowledge of master integrals Heinrich, Huber, Kosower, Smirnov Baikov, Chetyrkin, Steinhauser, Smirnov, Smirnov (09); Heinrich, Huber, Kosower, Smirnov (09) Three most difficult nine-propagator master integrals evaluated using Mellin-Barnes methods and checked between the two groups

  26. Glover, Ikizlerli; Heinrich; Vogt Beyond NNLO Results through to O(1/e) by Moch, Vermaseren, Vogt to O(1) still incomplete Complete reduction to master integrals NG, Ikizlerli Collaboration with Gehrmann, Huber to complete finite terms

  27. Beyond QCD: N=4 SYM BDS ansatz relates all-loop MHV amplitudes to the one loop amplitude Maximally supersymmetric gauge theory has similarities with QCD • Infrared structure • Coupling doesn’t run • Can we learn from the simpler theory and apply it to real QCD? Key element is the exponentiation of the amplitude But ansatz fails for six-point two-loop amplitudes!!

  28. Where does BDS ansatz break down? • Exploit Wilson-loop amplitude duality • Numerically computed Wilson loop and remainder function up to eight legs Anastasiou, Khoze, Brandhuber, Heslop, Spence, Travaglini • Examine amplitudes in the high energy limit • Studied simplest multi Regge kinematics, but found that R does not contribute • Need to study more general multi Regge Kinematics Del Duca, Duhr, NG Paul Heslop and Claude Duhr arriving in October 09

  29. IPPP Open Presentation on Strong Interactions 11:30OverviewNigel Glover 12:00Towards LHC Phenomenology beyond the Leading Order Gudrun Heinrich 12:20Threshold resummation for coloured heavy particles at the LHC PietroFalgari 12:40One-loop Higgs plus four gluon amplitudes: Full analytic results Ciaran Williams

  30. Kaidalov Ryskin Kizilersu Khoze Stirling De Roeck Bashir Martin Pennington Watt Nicotri Ball Buckley Richardson Khoze Krauss Schwinn Grellscheid Glover Dixon Gehrmann Huston Maxwell Heinrich Smillie Falgari Maitre Signer Figy

  31. Run II jet data prefer smaller gluon distribution at high x. • MSTW 2008 : minor refinements to NNLO evolution code: • added perturbative NNLO generation of quark- antiquarkasymmetry (very small). • improved definition of aS(MRST form unconventional). • evolution checked against public pegasus [Vogt ’04] and hoppet[Salam, Rojo ’08] codes for fitted input PDFs. • Complete treatment of correlated systematic errors. • Use fastnlo code [Kluge, Rabbertz, Wobisch ’06] to calculate NLO cross sections exactly during the fit.

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