DIPHOX event generation “direct” process only an analysis of events processed by PYTHIA

21 Jan 2008 KEK-LAPP meeting 1

DIPHOX event generation“direct” process only

an analysis of events processed by PYTHIA

Shigeru Odaka

KEK/IPNS


Calculation in DIPHOX• The cross section is calculated for 7 phase space regions, separately.• For each “event”, flavor contributions are summed when referring to PDF.• The matrix element is calculated for and initial states (4 sub-

processes) simultaneously, and the results are added to evaluate the differential cross section of the “event”.

• Need further break down of the processes and separation of flavors, in order to generate events which can be filled in the LHA common.

d i

d (i 1,,4)

i 1 for d (qq (g))

Qq4d

Qk4 fh1 qk

(x1) fh2 q k(x2 )

k

2 for d (qg (q))

Qq4d

Qk4 fh1 qk

(x1) fh1 q k(x1) fh2 g (x2 )

k

3 for d (q q (g))

Qq4d

Qk4 fh1 q k

(x1) fh2 qk(x2 )

k

4 for d (gq (q))

Qq4d

Qk4 fh1 g (x1) fh2 qk

(x2 ) fh2 q k(x2 )

k

qq

qg (q g)


Break down for event generation

• 66 break-down to separate 6 flavor (d, u, s, c, b, (t)) contributions in 6 sub-processes.

1 for qq (g)

2 for qg (q)

4 for q q (g)

5 for gq (q)

3 for q g (q )

6 for gq (q )

sub - processesSubroutine STRFRAD

in src/dir/hcoeff/htermdir.f

Common brock for “break down”

For gg

PDF


Function f4dimd3 in src/dir/src/twophotdn.f Function ssy3r in src/dir/src/dir.f

“break down” calculations in DIPHOX routines

Checking the consistency


New routines for event generationSubroutine dir_sub in src/dir/src/twophotdn.f

The part calling SPRING for the 1st phase-space region

Fill the common HEPRUP (run info.)

Open the output file

Write the HEPRUP contents to the output file using an LhaExt utility

Clear the break-down array for safety

Fill the common HEPEUP (event info.)

Write the HEPEUP contents to the output file using an LhaExt utility

Close the output file

The same procedure is repeated for all phase-space regions.

The output is an ASCII file in this example.

Binary files are also supported by LhaExt.

All new routines are attached at the bottom of src/main/twophot_histo.f and src/dir/src/twophotdn.f.


To fill the LHA commons

• The weighting strategy = -3 is chosen.– No selection should be done in the programs to read the events, and the we

ight may be negative; i.e., event weight = ±1.

• Four-momenta of the particles can be extracted as calculated in src/histo/doubletosingled_histo.f or src/ntuple/doubletosingled.f.

– My comment at the last meeting was incorrect.– The treatment of the final-state quark mass is still very primitive (unsatisfact

ory), though it is a minor issue.

• Subprocess/flavor is chosen randomly in proportion to the absolute value of the “break-down”.

– No problem to fill other parameters.

• Note on the color-flow information in the following.

Definition:

ICOLUP(1,I): ID of a “color” flowICOLUP(2,I): ID of an “anti-color” flow

A unique ID must be assigned for each color flow; numbers > 500 are recommended.

ICOLUP(1,1) = 501ICOLUP(2,1) = 0ICOLUP(1,2) = 502ICOLUP(2,2) = 501ICOLUP(1,5) = 502ICOLUP(2,5) = 0

For qg q

The color-flow is trivial for the “direct” processes.


Event generation and mixing

• Separate event files for 7 phase-space regions.• We have to execute and initial-state runs

separately, in order to avoid positive/negative mixing.– All “break-downs” have to have an identical sign in each

“event”, in order to evaluate proper weights of “break-down” components.

– LO has to be turned off when executing the initial-state run. Otherwise, (box) is double counted.

• As a result, we have 10 output files.– 27 = 14, but some are empty (cross section = 0).

• Mix the events randomly in proportion to the “absolute” cross sections evaluated by DIPHOX.

qq

qg /q g

qg /q g

gg


First test


Generation condition

DIPHOX “default”

Additional cuts in the histogramming

Also, DIPHOX “default”

pT(higher) 40 GeV/c, pT

(lower) 25 GeV/c

2.5 for both 's

80 m 140 GeV/c2

DIPHOX “default”:

preset condition in the version 1.2 distribution

gamma acceptance : 2.5, pT 25 GeV/c, m 80 GeV/c2

pp collisions at s 14 TeV

CTEQ6 for PDF, ren fac frag 0.5m

constant em , NLO s

num. of active flavors = 5 (up to b)

phase -space slice : pTm 0.1 GeV/c, Rth 0.1

gamma isolation: ET 15 GeV within R 0.4

including gg (box) at LO


Event Generation:

14,526 events generated for abs = 181.9 pb

8,077 positive-weight (+1) events

6,449 negative-weight (–1) events

= 20.4 ± 1.5 pb, while 22.2 pb by DIPHOX

Open triangles: generated events

Filled circles: after processed by PYTHIA 6.4

to be compared with

Histograms: produced by the original DIPHOX 1.2

abs d i

di

dii


m (GeV/c2 )

pT (GeV/c)

LO

NLO

LO

NLO

(tw

o en

trie

s/ev

ent)

PS effect


(rad)

pT (GeV/c)

LO

NLO

LO

NLO

PS effect


Summary

• Now we can generate NLO “di-photon” production events using DIPHOX.– Still limited to the “direct” process; i.e., yet to be a complete set.– Similar modification can be applied to “fragmentation” processes, bu

t it will be much more complicated.

• The generated events can be fed to PYTHIA/HERWIG through the LHA user-process interface (LhaExt), to add PS, hadronization and decays.– Namely, no problem to feed them to ATHENA (the ATLAS analysis/

simulation environment). An interface is already installed in ATHENA.

• The PYTHIA simulation gives a big change to LO events, while a small change to NLO events.– The analysis is still at a very primitive level. We have to apply a dete

ctor simulation in order to derive realistic predictions.

DIPHOX event generation “direct” process only an analysis of events processed by PYTHIA

Documents

Transcript of DIPHOX event generation “direct” process only an analysis of events processed by PYTHIA