LOGOA partridge in a pear tree

Simulation of multi-jet processes using the BFKL event generatorRasmus Mackeprang

Conventional picture of collision

Full matrix element for each final state incalculable

Parton showersParton showers effectively

resums part of the full perturbative series (all orders).

Standard (DGLAP) showering treats collinear part of phase space

Collinearemissions

Matrix element

Normally 22

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

Collinearemissions

Three French hens

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.

€

αS logSijti

+K ⎛

⎝ ⎜

⎞

⎠ ⎟~ α SΔy ij

i

j

Four calling birds

Jet production

Count “hard” jets in the eventPick the two rapidity-wise extreme jets Fixed order can only give you jets according

to the order of the calculationAt high rapidities BFKL will give more hard

jets

Δy

njets

3

20

4

NLO

BFKL

Five golden rings

Angular decorrelation

Dijet events to LO will have Δφ=0Parton showers will smear this Look at hard jets onlyBFKL should show larger

decorrelation at high rapidity differences

Δφ

Δy

<cos(Δφ)>

1

00

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 MCNo hadronizationKt jets with R=0.6Pythia8 vs BFKL (easy to run on a laptop) Looked at dijets and W+jets (We ν)

Eight maids a-milking

Well, Pythia only really

does W+jet…

Jet production

Used pseudo-rapidityHard jet has

Et > 40 GeV

|η| < 4.5ME cut is 20 GeV Little difference in

dijet eventsW+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

Dijets3j/2j

4j/2j

Exclusive rate ratios

Ratios are“n/(2 or higher)”

Largely the same conclusions

W+jets

Dijets3j/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 jobATLAS 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+2j W+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+2j W+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 predictionsW+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 NLOOn 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