1 / 19

Simulation of multi-jet processes using the BFKL event generator

Simulation of multi-jet processes using the BFKL event generator. Rasmus Mackeprang. Conventional picture of collision. Full matrix element for each final state incalculable Parton showers Parton showers effectively resums part of the full perturbative series (all orders).

yered
Download Presentation

Simulation of multi-jet processes using the BFKL event generator

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. Simulation of multi-jet processes using the BFKL event generator Rasmus Mackeprang A partridge in a pear tree

  2. Conventional picture of collision • Full matrix element for each final state incalculable • Parton showers • Parton showers effectively resums part of the full perturbative series (all orders). • Standard (DGLAP) showering treats collinear part of phase space Matrix element Collinear emissions Normally 22 Two turtle doves

  3. Consequences • Number of hard jets limited by the order to which the matrix element is calculated. • At the LHC there is a non-vanishing phase space for non-collinear emissions • Are we under-estimating our SM background in the multijet channels? Matrix element Collinear emissions Three French hens

  4. Alternative approach • BFKL formalism resums to all orders terms of • Sij is the invariant mass of emissions i and j • ti is a time-like momentum between them We can investigate to all orders the probability of hard jet emissions.  Large rapidity differences enhance dynamics. i j Four calling birds

  5. Jet production • Count “hard” jets in the event • Pick the two rapidity-wise extreme jets • Fixed order can only give you jets according to the order of the calculation • At high rapidities BFKL will give more hard jets njets BFKL 4 NLO 3 2 Δy 0 Five golden rings

  6. Angular decorrelation • Dijet events to LO will have Δφ=0 • Parton showers will smear this • Look at hard jets only • BFKL should show larger decorrelation at high rapidity differences Δφ <cos(Δφ)> 1 0 Δy 0 Six geese a-laying

  7. Multijet rates • With fixed order calculations you typically show 3/2 jets rates because you cannot treat higher orders. • Multijet rates at high rapidity differences should show differences between standard approach and BFKL. Seven swans a-swimming

  8. Parton level results • BFKL MC generator developed by Jeppe Andersen (CERN) • Weighted MC • No hadronization • Kt jets with R=0.6 • Pythia8 vs BFKL (easy to run on a laptop) • Looked at dijets and W+jets (We ν) Well, Pythia only really does W+jet… Eight maids a-milking

  9. Jet production Dijets • Used pseudo-rapidity • Hard jet has • Et > 40 GeV • |η| < 4.5 • ME cut is 20 GeV • Little difference in dijet events • W+jets an unfair comparison W+jets Nine ladies dancing

  10. Angular decorrelation Dijets • Low rapidity differences favour Pythia’s collinear emissions • Otherwise compatible for dijets • As for W+jets… W+jets Ten lords a-leaping

  11. Multijet ratios 3j/2j Dijets • Rates are “n or higher” • Slightly higher BFKL multijet rates • Effect not stronger at high eta gaps, though. 4j/2j W+jets Eleven pipers piping

  12. Exclusive rate ratios 3j/2j Dijets • Ratios are “n/(2 or higher)” • Largely the same conclusions 4j/2j W+jets Twelve drummers drumming

  13. Step back… • Seems BFKL is rather close to Pythia for dijets • DGLAP in turn seems to do a decent job • ATLAS uses Pythia6. This was Pythia8 • Taking Kt4H1TopoJets in J-samples we can make a (very) rough comparison A dozen and a partridge in a pear tree

  14. Pythia8 vs Pythia6 A dozen and two turtle doves

  15. Jet production in Alpgen • Order by order more jets are produced (well, duh…) • Samples are MLM matched  Can be added by integrated luminosity. W+2j W+3j W+4j W+5j A dozen and three French hens

  16. Accentuating the matrix element 3j/2j • Exclusive rate-ratios order by order • One sees clearly the extra jets entering W+2j W+3j 4j/2j W+4j W+5j A dozen and four calling birds

  17. Grand finale… • Adding Alpgen samples by integrated luminosity • The Alpgen prediction • Some agreement between BFKL and Alpgen • BFKL produces more jets, though • Consistent with missing virtual corrections in Alpgen • An order of magnitude more Alpgen stats after christmas… A dozen and five golden rings

  18. Last comments • BFKL is fast (1000 times faster than Pythia) • It reproduces dijets and agrees with parts of the pQCD W+jets predictions • W+jets is important background to BSM • The harder we kick Jeppe the faster he works (LSHA interface, unweighted events) • So, am I the only one who thinks this is interesting? • What if I say Higgs+jets? • LSHA and unweighting done there A dozen and six geese a-laying

  19. Technicalities and references • BFKL PDF: MRST 2004 NLO • On the BFKL MC Method: • hep-ph/0602182 (Phys.Lett. B639 (2006) 290) • hep-ph/0101180 (JHEP 0102:007,2001) • hep-ph/9706529 (Phys.Rev. D56 (1997) 5875-5884) • hep-ph/0305236 (Phys.Lett.B567:116-124,2003) • hep-ph/0309331 (Nucl.Phys.B679:345-362,2004) • On Parton Density Functions: • hep-ph/0410230 (Phys.Lett. B604 (2004) 61-68) A dozen and seven swans a-swimming

More Related