LOGO A partridge in a pear tree 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).  Standard (DGLAP) showering treats collinear part of phase space Collinear emissions Matrix element Normally 2  2 Two turtle doves

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

Alternative approach  BFKL formalism resums to all orders terms of  S ij is the invariant mass of emissions i and j  t i 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

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 Δy n jets NLO BFKL Five golden rings

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 Δy Six geese a-laying

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

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) Eight maids a-milking Well, Pythia only really does W+jet…

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

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

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

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

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

Pythia8 vs Pythia6 A dozen and two turtle doves

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

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

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

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

Technicalities and references  BFKL PDF: MRST 2004 NLO  On the BFKL MC Method:  hep-ph/ (Phys.Lett. B639 (2006) 290)  hep-ph/ (JHEP 0102:007,2001)  hep-ph/ (Phys.Rev. D56 (1997) )  hep-ph/ (Phys.Lett.B567: ,2003)  hep-ph/ (Nucl.Phys.B679: ,2004)  On Parton Density Functions:  hep-ph/ (Phys.Lett. B604 (2004) 61-68) A dozen and seven swans a-swimming