NLO WG 2002 (KEK) shigeru.odaka@kek.jp - PowerPoint PPT Presentation

NLO WG 報告 2002 年 尾高 茂 (KEK) shigeru.odaka@kek.jp 年 月 日 アトラス日本 物理分科会 1 2002 12 24 WS,

GR@PPA generators (LO) S. Tsuno (U. of Tsukuba) • GR@PPA_4b – Official release in Apr. 2002 (version 1.06) http://atlas.kek.jp/physics/nlo-wg/grappa.html – A paper was submitted to CPC (hep-ph/0204222; KEK Preprint 2002-7). • GR@PPA_all – Many other generators are added. + − ( ) , , , ( ) , , , , , (6 body) W → ff bb Wbb Zbb W → ff cc Wcc Zcc τ τ bb Wbbbb tt – Not well tested yet, especially at the LHC condition. • Others – Generators for W + jets (up to 4 jets) are being developed. 年 月 日 アトラス日本 物理分科会 2 2002 12 24 WS,

GR@PPA_4b vs PYTHIA for QCD 4- b production PYTHIA: solid: GR@PPA_4b, dashed: PYTHIA 10 3 10 3 QCD 2-jet (MSEL = 1) number of events number of events 1st b jet 1st b jet Detector: 10 2 10 2 Active detector in | η | ≤ 4.5 b -quark tag in | η | ≤ 2.5 10 10 Event selection: 1 0 50 100 150 200 -4 -2 0 2 4 E t (GeV/c) η 4 b -jets with E T ≥ 15 GeV ∆ R ≥ 0.4 for all b -jet pairs 10 3 number of events number of events 4th b jet 4th b jet 10 3 E T ≥ 50 GeV for leading 2 b -jets 10 2 Result: 10 2 GR@PPA_4b: (348 ± 3) pb 10 10 PYTHIA: (447 ± 12) pb 1 1 PYTHIA ≈ 1.3 × GR@PPA_4b 0 20 40 60 80 100 -4 -2 0 2 4 E t (GeV/c) η 年 月 日 アトラス日本 物理分科会 3 2002 12 24 WS,

Origin of PYTHIA events number of events non-leading two b-jets 2250 2000 gg → gg ~ 50% 1750 ~ 35% gb → gb 1500 Additional b ’s from the final-state PS. 1250 PYTHIA 1000 ~ 10% ( ) → + gg qq bb bb → bb 750 GR@PPA_4b Additional b ’s from the initial-state PS. 500 in PYTHIA gg → gg Consistent with the result in: 250 0 Richter-Was and Froidevaux, Z. Phys. C 76, 665 (1997) 0 0.5 1 1.5 2 2.5 3 ∆φ and our Feb-2002 result. Comparable results under a reasonable detection condition, if the QCD 2-jet subset (MSEL = 1) is chosen in PYTHIA. PYTHIA events are dominated by gg → gg and gb → gb interactions. The contribution of b -pair producing processes is small (~10%). Note: PYTHIA ≈ 70 × GR@PPA_4b in CPU-time. 年 月 日 アトラス日本 物理分科会 4 2002 12 24 WS,

NLL-jet Monte Carlo implementation (parton shower) of the next-to-leading logarithmic order QCD evolution in the initial state LLA NLL 1 Q^2=10^3 GeV^2 • NLL-PS ’OUTG01.DAT’ – All branching functions are ready (Sugiura 0.1 and Tanaka at Rikkyo U.). dN/dx1 – A preliminary coding with a temporary 0.01 u quark distribution at Q 2 = 10 3 GeV 2 kinematics implementation has been tested Input: GRV98 at Q 02 = 1 GeV 2 (Tanaka). 0.001 Histograms: GRV98 at Q 2 = 10 3 GeV 2 • X -deterministic forward evolution PS 0.0001 0.001 0.01 0.1 1 – A new method of the initial-state PS with a x1 forward Q 2 evolution (Y. Kurihara, hep- CTEQ5L : 100 GeV Q = m ⇒ Q = 0 b ph/0207214). u u – The final x is fixed before determining x values at intermediate branches. – A good efficiency can be achieved. – The idea has been tested for LL evolution. 年 月 日 アトラス日本 物理分科会 5 2002 12 24 WS,

NLO event generator • One-loop corrections (only 3-points yet) are already in the CHANEL library. • Modified LL-subtraction method to solve Gluon- p T in the DY process the double counting problem in ME and PS. 40 GeV, 1 GeV s − ≥ k T ≥ + µ µ – Numerical subtraction of LL terms + − : ( µ µ ) PS ∗ qq → ⊗ from ME + − hist : ( from PS rejected) qq → µ µ g g [ ] + [ ] + − ˜ – A maximum p T of the radiation (= o : ( ) PS PS qq → µ µ ⊗ σ ⊗ LL − sub factorisation scale) has to be introduced to reproduce the actual implementation of PS and PDF. – Y. Kurihara, hep-ph/0207214. • Modified BASES/SPRING to handle negative-weight events. – The weight of generated events = ± 1. – The fraction of negative-weight events is very small after the LL subtraction.. 年 月 日 アトラス日本 物理分科会 6 2002 12 24 WS,

NLO Drell-Yan QED only • Test and demonstration of the ideas Solid: non-rad. (mod. LL included) ⊗ PS Dashed: rad. (mod. LL subtracted) ⊗ PS – Kurihara et al., hep/ph- Star: NLO (= non-rad. ⊗ PS + rad. ⊗ PS) Dot-dashed: rad. ⊗ PDF (naive) 0212216 − ≥ 10 GeV – NLO calculation and event s µ µ + generation in the framework of GRACE – Numerical modified LL subtraction – X-deterministic forward evolution LL-PS • Smooth transition from a PS- dominated small E T region to a ME-dominated large E T region 年 月 日 アトラス日本 物理分科会 7 2002 12 24 WS,

ME-PS matching double-counting problem q 2 = Q 2 = 0 • A serious problem when ... – Colored parton(s) in the final-state of ME, and q 2 = q 02 > q 12 > ... > q n2 Q 2 = Q 02 < Q 12 < ... < Q n2 – E T (jet) < µ F ME • For example: PS/PDF – NLO W – W/Z + jet(s) ... – t -pair + jet(s) – H + jet(s) " W " ⊗ PS • Being seriously discussed at Tevatron " W + jet" ⊗ PS – “MC Tuning WS” – Next meeting on Jan. 14 at " W + jet": Q ∼ µ F Durham (England) • Various possible solutions have been proposed. 0 p T ( W ) µ F 年 月 日 アトラス日本 物理分科会 8 2002 12 24 WS,

Possible solutions • (Modified) Log-term subtraction – Kurihara, hep-ph/0207214 – Subtract Log terms from ME, and add non-radiative process + PS, instead – Theoretically well defined, but hard to implement in multi-jet cases – Perhaps, a similar method in MC@NLO (Frixione and Webber, hep/ph-0204244) • Hard-PS rejection – Mangano at the last “MC Tuning WS” on Nov. 16 @ FNAL – Require a matching between ME partons and observed jets -> reject mismatched events – Easy to implement, but theoretical basis is not clear • Extended CKKW method – Mrenna at the last “MC Tuning WS” on Nov. 16 @ FNAL – CKKW (Catani, Kraus, Kuhn and Webber, hep/ph-0109231) for multi-jet Z decays – Generate events of a large jet multiplicity, applying PS with a small µ F -> merge the jets and correct the event weight using the Sudakov suppression factor – Theoretically well defined, but need to generate multi-jet events • (Simple) phase-space slicing – Simple and well defined, but hard to avoid discontinuity 年 月 日 アトラス日本 物理分科会 9 2002 12 24 WS,

2003 年カレンダー 月 日 日： • 2 17 - 21 ATLAS Week (CERN) 月 日 日： アテネ • 5 21 - 25 ATLAS Physics WS ( ) 月 日 月 日： Physics at TeV Colliders WS • 5 27 - 6 6 (Les Houches, France) – http://wwwlapp.in2p3.fr/conferences/LesHouches/Houches2003/ 月 日 日： • 6 23 - 27 ATLAS Week (CERN) 月 日 月 日： • 7 7 - 8 2 LHC Monte Carlo WS (CERN) – http://lhc-monte-carlo.web.cern.ch/lhc-monte-carlo/workshop.html 月 日 日： プラハ • 9 13 - 19 ATLAS Week ( ) 年 月 日 アトラス日本 物理分科会 10 2002 12 24 WS,