Perturbative QCD in hadron collisions

Transcription

1 Perturbative QCD in hadron collisions Gavin Salam CERN, LPTHE/CNRS (Paris) & Princeton University IX Latin American Symposium on High Energy Physics (SILAFAE 2012) São Paolo, Brazil, December 2012

2 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 An exciting past 24 months Higgs(-like) discovery t t asymmetry W + dijet CDF anomaly Exclusion of swathes of SUSY, etc.... This talk: examine recent collider-qcd developments and the role they re playing in some of these headline topics, as well as touch on some open problems

3 Some of what goes into collider predictions Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 τ + τ π, K, p, etc. hadronisation u H hard proc. shower proton u underlying event u proton

4 Some of what goes into collider predictions Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 τ + τ π, K, p, etc. hadronisation u α n + α n H hard proc. shower u u proton underlying event proton

5 What a perturbative series should look like Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 E.g. QCD corrections to e + e hadrons cross section: R = α s +0.14α 2 s 0.41α3 s 0.82α4 s keep in mind α s (m Z ) 0.118

6 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 dσ/dp t [pb/gev] What it looks like at hadron colliders Consider LO, NLO and their ratio K = NLO LO Adapted from Rubin, GPS & Sapeta 10 NLO/LO 1.2 LO NLO 0.01 pp, 14 TeV FastNLO, k t R= p t [GeV] Look at p t of quark or gluon (jets) proton quark gluon proton 1+2α s looks like a reasonable series

7 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 dσ/dp t,z [fb / 100 GeV] What it looks like at hadron colliders Consider LO, NLO and their ratio K = NLO LO Adapted from Rubin, GPS & Sapeta 10 NLO/LO 1.5 pp, 14 TeV 10-2 MCFM p t,z [GeV] LO NLO Look at p t of Z boson proton 1+C α s, with quite large C 5 quark Z boson proton To date, no generalised understanding of size of C when in range 5 10

8 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 dσ/dp t,j1 [fb / 100 GeV] What it looks like at hadron colliders Consider LO, NLO and their ratio K = NLO LO Adapted from Rubin, GPS & Sapeta 10 NLO/LO 5 pp, 14 TeV 10-2 MCFM p t,j1 [GeV] LO NLO Look at p t of quark (jet) proton quark Z boson 1+Cα s C = 50?!! Often driven by new topologies proton

9 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 The NLO revolution and one way it s being used

10 SUSY example: gluino pair production Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Signal q q χ 0 g ~ g ~ q ~ q ~ g ~ g g χ 0 q q

11 SUSY example: gluino pair production Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Signal jet q jet q χ 0 g ~ g ~ q E/ T E/ T ~ q ~ g ~ g g χ 0 q jet q jet

12 SUSY example: gluino pair production Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Signal Background g jet q ~ g jet q ~ q χ 0 E/ T g g q q E/ T χ 0 ~ q q jet q jet ~ g ~ g g q g Z ν ν

13 SUSY example: gluino pair production Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Signal Background g jet q ~ g jet q ~ q χ 0 E/ T g jet q g jet q E/ T χ 0 ~ q q jet q jet ~ g ~ g g q jet g Z jet ν ν E/ T

14 SUSY example: gluino pair production Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Signal Background jet q jet q g g q Z ν ν E/ T jet g jet Complexity of NLO calculation determined by final-state multiplicity: a 2 5 process.

15 NLO timeline

16 NLO timeline : NLO Drell-Yan [Altarelli, Ellis & Martinelli] 1991: NLO gg Higgs [Dawson; Djouadi, Spira & Zerwas]

17 NLO timeline : NLO high-p t photoproduction [Aurenche et al] 1988: NLO b b, t t [Nason et al] 1988: NLO dijets [Aversa et al] 1993: Vj [JETRAD, Giele, Glover & Kosower]

18 NLO timeline : NLO Wb b [MCFM: Ellis & Veseli] 2000: NLO Zb b [MCFM: Campbell & Ellis] 2001: NLO 3j [NLOJet++: Nagy] 2007: NLO t tj [Dittmaier, Uwer & Weinzierl 07]

19 NLO timeline (W/Z+3j, ttbb, ttjj,...)

20 NLO timeline (W/Z+3j, ttbb, ttjj,...) : NLO W +3j [Rocket: Ellis, Melnikov & Zanderighi] [unitarity] 2009: NLO W +3j [BlackHat+Sherpa: Berger et al] [unitarity] 2009: NLO t tb b [Bredenstein et al] [traditional] 2009: NLO t tb b [HELAC-NLO: Bevilacqua et al] [unitarity] 2009: NLO q q b bb b [Golem: Binoth et al] [traditional] 2010: NLO t tjj [HELAC-NLO: Bevilacqua et al] [unitarity] 2010: NLO Z +3j [BlackHat+Sherpa: Berger et al]... [unitarity]

21 NLO timeline (W/Z+3j, ttbb, ttjj,...) 2 5 (W+4j, Z+4j) automation 2 6 (ee 7j [LC]) : NLO W +4j [BlackHat+Sherpa: Berger et al] [unitarity] 2011/12: NLO WWjj [Rocket: Melia et al; GoSaM+MadX Greiner et al] [unitarity] 2011: NLO Z +4j [BlackHat+Sherpa: Ita et al] [unitarity] 2011/12: NLO 4j [BlackHat/NGluons+Sherpa: Bern et al; Badger et al] [unitarity] 2011 : first automation [MadNLO: Hirschi et al] [unitarity + feyn.diags] 2011 : first automation [Helac NLO: Bevilacqua et al] [unitarity] 2011 : first automation [GoSam: Cullen et al] [feyn.diags(+unitarity)] 2011: e + e 7j [Becker et al, leading colour] [numerical loops]

22 W + 0,1,2,3,4 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 jets) [pb] σ(w + N jet Ldt=36 pb -1 anti-k T jets, R=0.4 jet jet p >30 GeV, y <4.4 T W lν + jets Data 2010, s=7 TeV ALPGEN SHERPA PYTHIA BLACKHAT-SHERPA ATLAS Technical revolution has gone hand-in-hand with LHC measurements of these complex processes. Powerful validation of NLO approach. Theory/Data 1 1 So do SUSY searches now just compare data to NLO? Inclusive Jet Multiplicity, N jet

23 Two plots from a CMS SUSY analysis Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Data v. Monte Carlo backgrounds Data v. data-driven backgrounds So where are the NLO predictions being used?

24 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 The CMS search did not estimate Z+jets bkgd from NLO. Instead used dσ Z+jets dh T = ( dσ γ+jets) dh T data ( dσ Z+jets ) / dσγ+jets dh T dh T NLO Example of widely used data-driven bkgd estimates Combine best of theory knowledge with best of experimental knowledge.

25 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Merging NLO and showers and the CDF W + dijet anomaly

26 Remember the CDF W+dijet excess? Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Events/(8 GeV/c 2 ) 2 ) x x x x x x x x -1 CDF data (4.3 fb ) Gaussian 2.5% WW+WZ 4.8% W+Jets 78.0% Top 6.3% Z+jets 2.8% QCD 5.1% (c) Events/(8 GeV/c Bkg Sub Data (4.3 fb ) Gaussian WW+WZ (a) M jj 2 [GeV/c ] M jj [GeV/c ]

27 and the D0 W+dijet non-excess? Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 ) 2 Events / (10 GeV/c DØ, 4.3 fb (a) Data Diboson W+Jets Z+Jets Top Multijets Gaussian (4 pb) 2 = 145 GeV/c Dijet Mass [GeV/c ] M jj ) 2 Events / (10 GeV/c DØ, 4.3 fb (b) Data - Bkgd Bkgd ± 1 s.d. Diboson Gaussian (4 pb) 2 = 145 GeV/c Dijet Mass [GeV/c ] M jj P(χ 2 ) = CDF and DØ data are not being compared to NLO (=W+partons): They are detector-level data and can only be compared to hadron-level calculations + detector simulation. In this case hadron-level = Alpgen Pythia

28 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Perturbative expansion: for precision. Parton Showers (PS): for realism; To combine them: must remove double counting Tree-level (LO) + PS Different tree-level multiplicites (W, W+1j, W+2j, etc.) get combined MLM/CKKW: Alpgen+Pythia/Herwig, MadGraph, Sherpa,... Fully automated τ + τ π, K, p, etc. hadronisation τ + τ π, K, p, etc. hadronisation u u α n + α n H hard proc. shower H hard proc. shower u u u u proton underlying event proton proton underlying event proton

29 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 NLO + PS MC@NLO, POWHEG Greater accuracy, but harder to perform than LO+PS: NLO contains more physics than LO, so more double-counting with parton shower Less available than tree+ps: until recently, A single (low) multiplicity, e.g. W@NLO + PS Programmed manually for each process Recently: move towards automation: POWHEGBox: t t+jet, W + W + +2j,... amc@nlo (MadLoop + auto MC@NLO): W+2j, Z+2b,... + ideas for combining multiplicities, extending their applicability e.g. MENLOPS, MINLO, FxFx merging, Sherpa merging, UNLOPS,... One application of this progress has been to the W+dijet anomaly

30 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 CDF & DØ use Alpgen (scaled): tree level QCD + parton shower adapted from Frederix et al

31 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 CDF & DØ use Alpgen (scaled): tree level QCD + parton shower NLO has substantial shape differences: should we worry? adapted from Frederix et al

32 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 CDF & DØ use Alpgen (scaled): tree level QCD + parton shower NLO has substantial shape differences: should we worry? NLO + parton shower (amc@nlo) is close to Alpgen QCD under good control adapted from Frederix et al

33 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Events/(8 GeV/c 2 ) 700 CDF data (4.3 fb ) Gaussian 2.5% WW+WZ 4.8% 600 W+Jets 78.0% Top 6.3% 500 Z+jets 2.8% QCD 5.1% 400 (c) x x x x x x x x x x x x x x x x x x x x x x x x x x x x x M jj -1 2 [GeV/c ] Instead of data MC data/mc Data / MC CDF lνjj data (7.3 fb -1 ) MC = VV, V+j, ttbar, QCD MC + Gaussian M jj [GeV/c 2 ] Data extracted from CDF plots with aid of g3data

34 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Events/(8 GeV/c 2 ) 700 CDF data (4.3 fb ) Gaussian 2.5% WW+WZ 4.8% 600 W+Jets 78.0% Top 6.3% 500 Z+jets 2.8% QCD 5.1% 400 (c) x x x x x x x x x x x x x x x x x x x x x x x x x x x x x M jj -1 2 [GeV/c ] Instead of data MC data/mc Data / MC CDF lνjj data (7.3 fb -1 ) MC = VV, V+j, ttbar, QCD M jj [GeV/c 2 ] Data extracted from CDF plots with aid of g3data

35 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Events/(8 GeV/c 2 ) 700 CDF data (4.3 fb ) Gaussian 2.5% WW+WZ 4.8% 600 W+Jets 78.0% Top 6.3% 500 Z+jets 2.8% QCD 5.1% 400 (c) x x x x x x x x x x x x x x x x x x x x x x x x x x x x x M jj -1 2 [GeV/c ] Instead of data MC data/mc Data / MC CDF lνjj data (7.3 fb -1 ) MC = VV, V+j, ttbar, QCD M jj [GeV/c 2 ] Data extracted from CDF plots with aid of g3data amc@nlo uncertainties:

36 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Events/(8 GeV/c 2 ) x x x x x x x x x x x x x x x x x x x x x x x x x x x x x M jj -1 CDF data (4.3 fb ) Gaussian 2.5% WW+WZ 4.8% W+Jets 78.0% Top 6.3% Z+jets 2.8% QCD 5.1% Anomaly 500 is a 10% effect (not clear it s really a peak) 10% is clearly at limit of NLO accuracy (c) 2 [GeV/c ] Instead of data MC data/mc Data / MC CDF lνjj data (7.3 fb -1 ) MC = VV, V+j, ttbar, QCD M jj [GeV/c 2 ] Data extracted from CDF plots with aid of g3data amc@nlo uncertainties:

37 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Going beyond limitations of NLO [two of the options] High precision NNLO is crucial for key processes, but not yet always available: W, Z, Higgs, γγ, VBF, VH, (t t) V V, (t t), inclusive jets, etc. Important also to develop methods so that we re less sensitive to limits on our precision. Generally by finding ways to distinguish signals from the background more efficiently, i.e. increasing S/B.

38 NNLO: crucial for precision Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 New in 2010: NNLO VBF H New in 2011: NNLO WH (differential) 1 σ (pb) at LHC s = 7 TeV LO NLO NNLO 10-1 scale choice: Q/4 µ R,µ F 4Q LO NLO NNLO 1.08 σ(µ R,µ F )/σ NNLO (Q) m H (GeV) Bolzoni, Maltoni, Moch & Zaro Ferrera, Grazzini & Tramontano

39 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Most groundbreaking new NNLO calculation of past years: q q t t Baernreuther, Czakon and Mitov 2012 First NNLO calculation with coloured particles in the initial and final state. Its new techniques may help open the way to many other important NNLO calculations.

40

41 Baernreuther, Czakon & Mitov NNLO q q t t cross-section LO, NLO, NNLO res

42 NNLO: yet not always reassuring Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 New in 2011: NNLO γγ NNLO Some key processes see large or giant NLO/NNLO corrections. NLO Can t help but wonder if we re missing something, especially in the gg H case. LO Catani, Cieri, de Florian, Ferrera & Grazzini

43 One series that s ugly: gluon fusion Higgs cross section Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 For 8 TeV pp collisions, m H = 125 GeV: σ gg H = 6.8 pb ( 1+9.9α s +36α 2 s + ) = 6.8 pb( ) = 19.0 pb σ gg H = 5.6 pb ( α s +63α 2 s + ) for µ R = µ F = 1 2 m H, α s (µ R ) = = 5.6 pb( ) = 17.2 pb for µ R = µ F = m H, α s (µ R ) = There are explanations: threshold logarithms, π 2 terms from analytic continuation. A problem is perhaps that there are too many explanations... Baglio & Djouadi have raised the convergence issue before

44 Consequences of a bad series Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Everything else you try about (gluon-fusion) Higgs production suffers as a result of the original bad series: e.g. jet veto efficiency 1 FIXED ORDER PREDICTION Higgs production (m H = 125 GeV), NNLO 0.8 pp, 7 TeV m H /4 < µ R,F < m H MSTW2008 NNLO PDFs anti-k t, R = 0.5 ε(p t,veto ) scheme a scheme b scheme c 10 p t,veto [GeV] 100

45 Consequences of a bad series Everything else you try about (gluon-fusion) Higgs production suffers as a result of the original bad series: e.g. jet veto efficiency FIXED ORDER PREDICTION Higgs production (m H = 125 GeV), NNLO pp, 7 TeV m H /4 < µ R,F < m H MSTW2008 NNLO PDFs anti-k t, R = 0.5 ε(p t,veto ) RESUMMED PREDICTION gg H, m H = 125 GeV pp, 8 TeV m H /4 < µ R,F, Q < m H, schemes a,b,c MSTW2008 NNLO PDFs anti-k t, R = 0.5 Pythia partons, Perugia 2011 tune ε(p t,veto ) scheme a scheme b scheme c p t,veto [GeV] ε(p t,veto ) / ε central (p t,veto ) NNLO NNLL+NNLO HqT-rescaled POWHEG + Pythia p t,veto [GeV] Banfi, Monni, GPS & Zanderighi 12 see also Becher & Neubert 12 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35

46 Looking at data differently Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35

47 H b b (57% of decays) v. hard to see Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Best hope is pp W ± H (and ZH), W ± l ± ν, H b b. H b W e,µ b ν

48 H b b (57% of decays) v. hard to see Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Best hope is pp W ± H (and ZH), W ± l ± ν, H b b. pp WH lνb b + bkgds ATLAS TDR 1999 Conclusion (ATLAS TDR): Theextraction ofasignalfrom H b b decays in the WH channel will be very difficultat thelhc, even under the most optimistic assumptions [...] Low efficiency, huge backgrounds, e.g. t t NB: Evidence of this channel seen recently at Tevatron, but similar difficulties H b W e,µ b ν

49 H b b (57% of decays) v. hard to see Best hope is pp W ± H (and ZH), W ± l ± ν, H b b. Conclusion (ATLAS TDR): Theextraction ofasignalfrom H b b decays in the WH channel will be very difficultat thelhc, even under the most optimistic assumptions [...] Low efficiency, huge backgrounds, e.g. t t pp WH lνb b + bkgds ATLAS TDR 1999 b b Analysis of signal/bkgd suggests: Go to high p t (p th,p tw > 200 GeV) Lose 95% of signal, but more efficient? Maybe kill t t & gain clarity? Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 e,µ H W ν

50 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 Cluster event, C/A, R=1.2 Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

51 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 Fill it in, show jets more clearly Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

52 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 Consider hardest jet, m = 150 GeV Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

53 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 split: m = 150 GeV, max(m 1,m2) m = 0.92 repeat Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

54 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 split: m = 139 GeV, max(m 1,m2) m = 0.37 mass drop Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

55 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 check: y 12 p t2 p t1 0.7 OK + 2 b-tags (anti-qcd) Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

56 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 R filt = 0.3 Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

57 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 R filt = 0.3: take 3 hardest, m = 117 GeV Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

58 pp ZH ν νb m H =115GeV Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Herwig Jimmy FastJet 2.3 R filt = 0.3: take 3 hardest, m = 117 GeV Butterworth, Davison, Rubin & GPS 08 also earlier work by Seymour; Butterworth et al

59 ATLAS and CMS H b b are high-p t, but 2-jet based Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35

60 Some taggers and jet-substructure observables Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Jet Declustering Seymour93 YSplitter Jet Shapes Matrix Element Mass Drop+Filter ATLASTopTagger JHTopTagger TW Planar Flow Templates CMSTopTagger Pruning N jettiness HEPTopTagger (+ dipolarity) Trimming Twist CoM N subjettiness (Kim) N subjettiness (TvT) EEC Shower Deconstruction Multi variate tagger Qjets apologies for omitted taggers, arguable links, etc. [NB: many of the tools available in FastJet & SpartyJet]

61 Handles for distinguishing signal v. background Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 softer prong mom. fraction z boosted X z (1 z) radiation off prongs sensitive to their colour (q v. g) large angle (>> 2m/p t ) radiation off X sensitive to its colour charge g gg(g) q qg(g) g b b H b b t qq q softer prong z soft soft hard hard hard prong colour factors 2 C A C F +C A 2 C F 2 C F 3 C F system colour factor C A C F C A 0 C F

62 Boosted Ws and tops in single jets: data! W s in a single jet tops in a single jet with Pruning + Mass Drop requirement NB: combined in IR unsafe way... with HEPTopTagger Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35

63 Some BSM searches with jet-substructure techniques Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 A range of techniques being used for varied BSM scenarios

64 Closing Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35

65 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 fraction of ATLAS & CMS papers that cite them Pythia 6.4 MC GEANT4 Anti k t jet alg. CTEQ6 PDFs Herwig 6 MC ALPGEN CTEQ6.6 PDFs MSTW2008 PDFs MC@NLO JIMMY Papers commonly cited by ATLAS and CMS as of , from papers, excluding self citations RPP2010 LO* PDFs POWHEG (2007) MC@NLO heavy flavour FastJet Herwig++ MC CT10 PDFs MadGraph4 FEWZ NNLO CL(s) technique LHC Machine Z1 UE Tune Pythia 8.1 PDF4LHC Plot by GP Salam based on data from ATLAS, CMS and INSPIREHEP

66 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 fraction of ATLAS & CMS papers that cite them Pythia 6.4 MC GEANT4 Anti k t jet alg. CTEQ6 PDFs Herwig 6 MC ALPGEN CTEQ6.6 PDFs MSTW2008 PDFs MC@NLO JIMMY Papers commonly cited by ATLAS and CMS as of , from papers, excluding self citations RPP2010 LO* PDFs POWHEG (2007) QCD MC@NLO heavy flavour FastJet Herwig++ MC CT10 PDFs MadGraph4 FEWZ NNLO CL(s) technique LHC Machine Z1 UE Tune Pythia 8.1 PDF4LHC Plot by GP Salam based on data from ATLAS, CMS and INSPIREHEP today s progress in QCD = tomorrow s workhorse

67 EXTRAS Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35

68 NLO bottleneck: 1-loop part Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Traditional Draw all Feynman diagrams with 1 loop. Work out formulae for them. Work hard to reduce integrals to known forms (+ tricks). Recursive/unitarity methods Assemble loop-diagrams from individual tree-level diagrams. Build trees by sticking together simpler tree-level diagrams

69 NLO bottleneck: 1-loop part Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Traditional Draw all Feynman diagrams with 1 loop. Work out formulae for them. Work hard to reduce integrals to known forms (+ tricks). Recursive/unitarity methods Assemble loop-diagrams from individual tree-level diagrams. Build trees by sticking together simpler tree-level diagrams Some main ideas: Bern, Dixon & Kosower 93 [sewing together trees] Britto, Cachazo & Feng 04 [on-shell complex loop momenta] Ossola, Pittau & Papadopoulos 06 [handful of loop momentum choices give full amplitude]

70 CDF Wjj: difference wrt MC v. ratio to MC Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 Data - MC (Events/(8 GeV/c 2 )) CDF difference CDF lνjj data (7.3 fb -1 ) M jj [GeV/c 2 ] Data extracted from CDF plots with aid of g3data Data / MC CDF ratio CDF lνjj data (7.3 fb -1 ) MC = VV, V+j, ttbar, QCD M jj [GeV/c 2 ] Data extracted from CDF plots with aid of g3data

71 Wjj ratio to MC, DØ v. CDF Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / DØ ratio D0 (4.3 fb -1 ) MC = VV, V+j, ttbar, QCD CDF ratio CDF lνjj data (7.3 fb -1 ) MC = VV, V+j, ttbar, QCD Data / MC M jj [GeV/c 2 ] Data extracted from CDF plots with aid of g3data Data / MC M jj [GeV/c 2 ] Data extracted from CDF plots with aid of g3data

72 Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE / 35 1 Papers commonly cited by ATLAS and CMS as of , from papers, excluding self-citations fraction of ATLAS & CMS papers that cite them Pythia 6.4 MC GEANT4 Anti-k t jet alg. CTEQ6 PDFs MSTW2008 PDFs CTEQ6.6 PDFs Herwig 6 MC RPP2010 ALPGEN LO* PDFs MC@NLO JIMMY MadGraph4 POWHEG (2007) FEWZ NNLO CT10 PDFs MC@NLO heavy-flavour ATLAS CMS Herwig++ MC FastJet Z1 UE Tune Pythia 8.1 Plot by GP Salam based on data from ATLAS, CMS and INSPIREHEP