News in jet algorithms:

Transcription

1 News in jet algorithms: SISCone and anti-k t Grégory Soyez Brookhaven National Laboratory G.P. Salam, G. Soyez, JHEP 5 (7) 86 [arxiv:74.292] M. Cacciari, G.P. Salam, G. Soyez, JHEP 4 (8) 63 [arxiv: ] Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 1/15

2 Jet definitions Aim: Study hard processes QCD backgrounds, top quark physics Higgs, physics beyond the standard model Define jets: parton jet But: partons are ambiguous Hence: Multiple definitions of a jet? Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 2/15

3 Two classes of algorithms Class 1: recombination Cass 2: cone Successive recombinations of the find directions of energy flow closest (a) pair of particle stable cones (b) Nice perturbative behaviour Small sensitivity to soft radiation (UE,PU) Often used in e ± e ±, e ± p Often used in pp (a) Distance: k t : d i,j = min(kt,i 2, k2 t,j )( φ2 i,j + y2 i,j ) Aachen/Cam.: d i,j = φ 2 i,j + y2 i,j (b) stable cones (radius R) such that: the total momentum of its contents points in the direction of its centre Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 3/15

4 How the cone works... Seeded (iterative) approaches: iterate from an initial position until stable seed = initial particle seed = midpoint between stable cones found at first step One has to deal with overlapping stable cones: 2 subclasses Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 4/15

5 How the cone works... Seeded (iterative) approaches: iterate from an initial position until stable seed = initial particle seed = midpoint between stable cones found at first step Class 2(a): cone with split-merge (ex.: JetClu, Atlas, MidPoint): p t,shared > f p t,min p t,shared f p t,min Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 4/15

6 How the cone works... Seeded (iterative) approaches: iterate from an initial position until stable seed = initial particle seed = midpoint between stable cones found at first step Class 2(a): cone with split-merge (ex.: JetClu, Atlas, MidPoint): p t,shared > f p t,min p t,shared f p t,min Class 2(b): cone with progressive removal (ex.: Iterative Cone) iterate from the hardest seed remove the stable cone as a jet and start again Idea: regular/circular jets Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 4/15

7 SNOWMASS accords SNOWMASS, Tevatron 199 (i.e. old!): i.e. usable by theoreticians (e.g. finite perturbative results) and experimentalists (e.g. fast enough) This talk: Seeded cone algorithms miss stable cones theoretical problems That can be solved keeping experimental usefulness Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 5/15

8 QCD divergences QCD probability for gluon bremsstrahlung at angle θ and -mom. k t : Two divergences: θ dp α s dθ θ dk t k t p t Collinear Soft k t p t Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 6/15

9 QCD divergences QCD probability for gluon bremsstrahlung at angle θ and -mom. k t : Two divergences: θ dp α s dθ θ dk t k t p t Collinear Soft k t p t For pqcd to make sense, the (hard) jets (or stable cones) should not change when one has a collinear splitting i.e. replaces one parton by two at the same place (η, φ) one has a soft emission i.e. adds a very soft gluon Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 6/15

10 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

11 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

12 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

13 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

14 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

15 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

16 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

17 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

18 IR unsafety of the Midpoint alg φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

19 IR unsafety of the Midpoint alg φ φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

20 IR unsafety of the Midpoint alg φ φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

21 IR unsafety of the Midpoint alg φ φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

22 IR unsafety of the Midpoint alg φ φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

23 IR unsafety of the Midpoint alg φ Stable cones: Midpoint: {1,2} & {3} {1,2} & {3} & {2,3} Seedless: {1,2} & {3} & {2,3} {1,2} & {3} & {2,3} φ Jets: (f =.5) Midpoint: {1,2} & {3} {1,2,3} Seedless: {1,2,3} {1,2,3} Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

24 IR unsafety of the Midpoint alg φ Stable cones: Midpoint: {1,2} & {3} {1,2} & {3} & {2,3} Seedless: {1,2} & {3} & {2,3} {1,2} & {3} & {2,3} φ Jets: (f =.5) Midpoint: {1,2} & {3} {1,2,3} Seedless: {1,2,3} {1,2,3} Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

25 IR unsafety of the Midpoint alg φ Stable cones: Midpoint: {1,2} & {3} {1,2} & {3} & {2,3} Seedless: {1,2} & {3} & {2,3} {1,2} & {3} & {2,3} φ Jets: (f =.5) Midpoint: {1,2} & {3} {1,2,3} Seedless: {1,2,3} {1,2,3} Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

26 IR unsafety of the Midpoint alg φ Stable cones: Midpoint: {1,2} & {3} {1,2} & {3} & {2,3} Seedless: {1,2} & {3} & {2,3} {1,2} & {3} & {2,3} φ Jets: (f =.5) Midpoint: {1,2} & {3} {1,2,3} Seedless: {1,2,3} {1,2,3} Stable cone missed IR unsafety of the midpoint algorithm Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 7/15

27 Solution: SISCone Solution: use a seedless approach, find ALL stable cones Naive approach: check stability of each subset of particle Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 8/15

28 Solution: SISCone Solution: use a seedless approach, find ALL stable cones Naive approach: check stability of each subset of particle Complexity is O ( N2 N) definitely unrealistic: 1 17 years for N = Midpoint complexity: O ( N 3) Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 8/15

29 Solution: SISCone Solution: use a seedless approach, find ALL stable cones Midpoint complexity: O ( N 3) Idea: use geometric arguments Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 8/15

30 Solution: SISCone Solution: use a seedless approach, find ALL stable cones Midpoint complexity: O ( N 3) Idea: use geometric arguments (a) (b) (c) Each enclosure can be moved (in any direction) until it touches a point... then rotated until it touches a second one Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 8/15

31 Solution: SISCone Solution: use a seedless approach, find ALL stable cones Midpoint complexity: O ( N 3) Idea: use geometric arguments Enumerate all pairs of particles with 2 circle orientations and 4 possible inclusion/exclusion find all enclosures Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 8/15

32 Solution: SISCone Solution: use a seedless approach, find ALL stable cones Midpoint complexity: O ( N 3) Idea: use geometric arguments Enumerate all pairs of particles with 2 circle orientations and 4 possible inclusion/exclusion find all enclosures Complexity: O ( N 3), with improvements: O ( N 2 log(n) ) Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 8/15

33 Solution: SISCone Solution: use a seedless approach, find ALL stable cones Midpoint complexity: O ( N 3) Idea: use geometric arguments Enumerate all pairs of particles with 2 circle orientations and 4 possible inclusion/exclusion find all enclosures Complexity: O ( N 3), with improvements: O ( N 2 log(n) ) C++ implementation: Seedless Infrared-Safe Cone algorithm (SISCone) G.Salam, G.S., JHEP 4 (7) 86; NB.: also available from FastJet [M.Cacciari, G.Salam, G.S.]; salam/fastjet Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 8/15

34 Physical impact Execution timings: 1 CDF midpoint (s= GeV) CDF midpoint (s=1 GeV) PxCone SISCone Random hard & soft partons fraction with hard hard+soft 1 JetClu 5.1% run time (s).1 MidPoint 16.4% PxCone 9.3%.1 < 1-9 Seedless [SM-p t ] SISCone 1.6%.1 N at least as fast as midpoint cones Fraction of hard events failing IR safety test IR safe JetClu,ATLAS cone: 5% failure MidPoint: 15% failure Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 9/15

35 Physical impact (2) Inclusive (midpoint/siscone-1) Masses in 3-jet events pp s = 14 TeV dσ midpoint(1) /dp t / dσ SISCone /dp t (b) Pythia 6.4 hadron-level (with UE) hadron-level (no UE) parton-level R=.7, f=.5, y <.7 rel. diff. for dσ/dm NLOJet R=.7, f=.5 R 23 < 1.4 Mass spectrum of jet 2 midpoint() -- SISCone SISCone M (GeV) 5 15 p t [GeV] Inclusive cross-section: Effect of a few percents Less sensiticity to the UE More exclusive processes: effects 45% (Important for LHC!) Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 1/15

36 Coll. unsafety of the iterative cone φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 11/15

37 Coll. unsafety of the iterative cone φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 11/15

38 Coll. unsafety of the iterative cone φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 11/15

39 Coll. unsafety of the iterative cone φ φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 11/15

40 Coll. unsafety of the iterative cone φ φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 11/15

41 Coll. unsafety of the iterative cone φ φ Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 11/15

42 Coll. unsafety of the iterative cone φ φ Before collinear spliting: 1 jet After collinear spliting: 2 jets collinear unsafety of the iterative cone algorithm Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 11/15

43 Anti-k t Come back to recombination-type algorithms: d ij = min(k 2p t,i, k2p t,j ) ( ) φ 2 ij + ηij 2 p = 1: k t algorithm p = : Aachen/Cambridge algorithm Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 12/15

44 Anti-k t Come back to recombination-type algorithms: d ij = min(k 2p t,i, k2p t,j ) ( ) φ 2 ij + ηij 2 p = 1: k t algorithm p = : Aachen/Cambridge algorithm p = 1: anti-k t algorithm [M.Cacciari, G.Salam, G.S.,JHEP 4 (8) 63] Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 12/15

45 Anti-k t Come back to recombination-type algorithms: d ij = min(k 2p t,i, k2p t,j ) ( ) φ 2 ij + ηij 2 p = 1: k t algorithm p = : Aachen/Cambridge algorithm p = 1: anti-k t algorithm [M.Cacciari, G.Salam, G.S.,JHEP 4 (8) 63] Why should that be related to the iterative cone?!? large k t small distance i.e. hard partons eat everything up to a distance R i.e. circular/regular jets, jet borders unmodified by soft radiation infrared and collinear safe Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 12/15

46 anti-k t Hard event + homogeneous soft background anti-k t is soft-resilient more in Matteo Cacciari s talk... Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 13/15

47 anti-k t Execution timings: run time (s) CDF midpoint (s= GeV) CDF midpoint (s=1 GeV) PxCone SISCone anti-k t (fastjet).1 N Distribution of hard jets area (π R 2 )/N dn/da Pythia 6.4 p t,min =1 TeV 2 hardest jets y <2, R=1 SISCone Cambridge k t anti-k t (/1) area/(πr 2 ) hadron-level + pileup active area As fast as the (fast) k t ([M. Cacciari, G. Salam, 6]) Regular hard jets of area πr 2 Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 14/15

48 Conclusions Midpoint and the iterative cone IR or Collinear unsafe (at O(α 4 s)) Observable 1st miss cones at Last meaningful order Inclusive jet cross section NNLO NLO 3 jet cross section NLO LO (NLO in NLOJet) W/Z/H + 2 jet cross sect. NLO LO (NLO in MCFM) jet masses in 3 jets LO none (LO in NLOJet) The IR-unsafety issue will matter at LHC + We do not want the theoretical efforts to be wasted Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 15/15

49 Conclusions Midpoint and the iterative cone IR or Collinear unsafe (at O(α 4 s)) Observable 1st miss cones at Last meaningful order Inclusive jet cross section NNLO NLO 3 jet cross section NLO LO (NLO in NLOJet) W/Z/H + 2 jet cross sect. NLO LO (NLO in MCFM) jet masses in 3 jets LO none (LO in NLOJet) The IR-unsafety issue will matter at LHC + We do not want the theoretical efforts to be wasted JetClu ATLAS Cone MidPoint Iterative Cone SISCone Anti-k t as fast IRC safe regular IRC safe Both available from FastJet ( salam/fastjet) Grégory Soyez HERA-LHC 8, CERN, Geneva, Switzerland, May SISCone and the anti-k t p. 15/15