1 / 29

W + n jet production at NLO

W + n jet production at NLO. Lance Dixon (SLAC) representing the BlackHat Collaboration C. Berger, Z. Bern, L.D., F. Febres Cordero, D. Forde, T. Gleisberg, H. Ita, D. Kosower, D. Ma î tre 0803.4180 [hep-ph], 0808.0941 [hep-ph], 0902.2760 [hep-ph] US ATLAS Hadronic Final State Forum

toril
Download Presentation

W + n jet production at NLO

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. W + n jet production at NLO Lance Dixon (SLAC) representing the BlackHat Collaboration C. Berger, Z. Bern, L.D., F. Febres Cordero, D. Forde, T. Gleisberg, H. Ita, D. Kosower, D. Maître 0803.4180 [hep-ph], 0808.0941 [hep-ph], 0902.2760 [hep-ph] US ATLAS Hadronic Final State Forum Joint Theory/Experiment Open Session April 9, 2009

  2. (2007) by NNLO, a precision observable Why NLO? Because leading order (LO) predictions are only qualitative in normalization, due to poor convergence of expansion in LO NLO NNLO Example: Z production at Tevatron Distribution in rapidity Y has ADMP (2004) still ~50% corrections, LO  NLO Atlas Forum April 9, 2009

  3. Why W (or Z) + n jets? Gianotti & Mangano, hep-ph/0504221 Mangano et al. (2002) • Supersymmetric cascade: • gluinos  squarks  neutralinos • (dark matter, escapes detector) • Signal: missing energy + jets • SM background from Z + jets, • when Z neutrinos Early studies using parton shower Monte Carlo PYTHIA overly optimistic • ALPGEN based on tree amplitudes, • much better than PYTHIA at • modeling hard jets • But even normalization of ALPGEN • quite uncertain still beyond state-of-art Atlas Forum April 9, 2009

  4. Reduction in Uncertainty In going from LO to NLO for “total cross section” for W + n jets at the Tevatron: As we’ll see, “shapes” (distributions within the total event sample) can be affected as well as normalizations Atlas Forum April 9, 2009

  5. Background for current SM searches at the Tevatron: • Single top (with a b tag) • WWor WZ with one W/Z jets • (warm-up for WH with H  bb _ W + 2 jets D0, 0811.3873 4.4s evidence ALPGEN for shape CDF, 0903.0814 “just missed” “shape” – slope in dijet invariant mass very important Atlas Forum April 9, 2009

  6. Strong growth in difficulty at one loop (NLO) with number of final-state objects # of jets # 1-loop Feynman diagrams (gluons only) 3 4 5 6 Atlas Forum April 9, 2009

  7. Instead of Feynman diagrams, we use Analyticity “One of the most remarkable discoveries in elementary particle physics has been the existence of the complex plane” -J. Schwinger Atlas Forum April 9, 2009

  8. Analyticity Includes Factorization How amplitudes “fall apart” into simpler ones in special limits Atlas Forum April 9, 2009

  9. Green (Efficient/Fast) Computation Reduce the number of objects – use on-shell amplitudes, not Feynman diagrams Reuse building blocks over & over Recycle lower-point (1-loop) & lower-loop (tree) on-shell amplitudes Atlas Forum April 9, 2009

  10. Lots of related recent one-loop work by other groups, for example: Ossola, Papadopolous, Pittau, 0711.3596 [CutTools] Ossola, Papadopolous, Pittau,0802.1876 Mastrolia, Ossola, Papadopolous, Pittau, 0803.3964 Binoth, Ossola, Papadopolous, Pittau, 0804.0350 Draggiotis, Garzelli, Papadopoulos, Pittau, 0903.0356 van Hameren, Papadopoulos, Pittau, 0903.4665 Giele, Kunszt, Melnikov, 0801.2237 Giele, Zanderighi, 0805.2152 [ROCKET] Ellis, Giele, Kunszt, Melnikov, 0806.3467 Ellis, Giele, Kunszt, Melnikov, Zanderighi, 0810.2763 Ellis, Melnikov, Zanderighi, 0901.4101 Atlas Forum April 9, 2009

  11. Simple Example: Z production convolute with pdfs apply cuts LO tree dim. reg. first, cancel infrared divergences ( ) between virtual & real NLO 1 loop tree + 1 parton BlackHat Catani-Seymour dipole subtraction, automated by Gleisberg, Krauss, 0709.2881, incorporated in SHERPA Adding real radiation BlackHat is a “black box” for computing one-loop (virtual) corrections. We also need to add the real radiative corrections Atlas Forum April 9, 2009

  12. TevatronW + n jets Data n = 1 NLO (MCFM) CDF, 0711.4044 [hep-ex] LO with different matching schemes n = 2 n = 3: only LO was available until this year Atlas Forum April 9, 2009

  13. First reproduce W + 2 jets [MCFM] Campbell, Ellis, hep-ph/0202176 Cuts: Central Scale: Range Atlas Forum April 9, 2009

  14. Note on Cone Algorithms • CDF data used JETCLU with R = 0.4. • JETCLU is IR unsafe, cannot be used at NLO. • We used SISCone, an IR safe, seedless and fast cone algorithm, also with R = 0.4. Salam, Soyez, 0704.0292 • MCFM used midpoint for W2j. • For jet ET andh distributions the seeding may not matter much, but if you happen to “look inside the jets” (jet masses, etc.), effects can be ~ 30%. • Please adopt an IR safe jet algorithm! Atlas Forum April 9, 2009

  15. A closer look at W + 2 jets • “LC NLO” is same as NLO, but it drops subleading-color terms • in the finite parts of the one-loop amplitudes. • For SU(3)  SU(Nc), these terms are suppressed parametrically • by 1/Nc2 or nf/Nc,and in fact are only a few percent, for W1j or W2j • Yet they are much more expensive to compute. • So we will drop them (for now) for W3j. Atlas Forum April 9, 2009

  16. W + 3 jets at Tevatron 0902.2760 [hep-ph] same cuts, cone algorithms, and scale variation as for W + 2 jets Atlas Forum April 9, 2009

  17. Distribution in Traditional SUSY search variable Atlas Forum April 9, 2009

  18. On to the LHC SISCone Cuts: Atlas Forum April 9, 2009

  19. LHC total cross section Common scale m for every event Preliminary Atlas Forum April 9, 2009

  20. Tevatron to LHC • K factor atm = MW drops from 1 to ½. • Much of this can probably be attributed • to a shift from quark to gluon initial states • as the average x drops: • in going from LO to NLO • for 0.01 < x < 0.1 • g(x) decreases while q(x) increases Atlas Forum April 9, 2009

  21. Pseudorapidity of first jet Preliminary ~ no change in shape from LO to NLO Atlas Forum April 9, 2009

  22. Distance between first and second jet • Significant • change in shape • from LO to NLO. • More chance for • hardest jets to be • closer if extra • radiation. • Large h separation • more costly in terms of parton luminosity if extra radiation. Preliminary Atlas Forum April 9, 2009

  23. Look for radiation in “gap” Campbell, Huston, Stirling, hep-ph/0611148 Need this fraction to remain small for non-color-singlet exchange at large Dh, in order for central jet veto to be effective in studies of Higgs production via vector boson fusion. vs. LO theory still Atlas Forum April 9, 2009

  24. Distribution in Tail much more suppressed, from LO to NLO, compared with Tevatron Preliminary Atlas Forum April 9, 2009

  25. Conclusions • New and efficient computational approaches to one-loop QCD amplitudes exploit analyticity and build loop amplitudes out of on-shell ingredients. • They are (finally) producing useful new NLO results for the LHC, in particular for W + 3 jets. Z + 3 jets is also on the way. • W/Z + 4 jets is probably also feasible (without waiting another seven years). • Shapes as well as normalizations can be affected by NLO corrections, even for complex final states • Because the present NLO results are at parton level, and not yet matched to a parton shower (which may take quite a while), some thought should be given to how to make them most useful to you • best distributions to study • best ratios of different processes to take: W Z, etc. • We are looking forward to a fruitful dialog! Atlas Forum April 9, 2009

  26. Extra Slides Atlas Forum April 9, 2009

  27. Tevatron W + n jet cross sections Atlas Forum April 9, 2009

  28. Turning trees into loops BlackHat: C++ program Berger, Bern, LD, Febres Cordero, Forde, H. Ita, D. Kosower, D. Maître, 0803.4180[ph] Atlas Forum April 9, 2009

  29. Inside BlackHat Atlas Forum April 9, 2009

More Related