21 Jan 2008 KEK-LAPP meeting 1
DIPHOX event generation“direct” process only
an analysis of events processed by PYTHIA
Shigeru Odaka
KEK/IPNS
21 Jan 2008 KEK-LAPP meeting 2
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
qg (q g)
21 Jan 2008 KEK-LAPP meeting 3
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
21 Jan 2008 KEK-LAPP meeting 4
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
21 Jan 2008 KEK-LAPP meeting 5
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.
21 Jan 2008 KEK-LAPP meeting 6
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.
21 Jan 2008 KEK-LAPP meeting 7
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.
qg /q g
qg /q g
gg
21 Jan 2008 KEK-LAPP meeting 8
First test
21 Jan 2008 KEK-LAPP meeting 9
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
21 Jan 2008 KEK-LAPP meeting 10
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
21 Jan 2008 KEK-LAPP meeting 11
m (GeV/c2 )
pT (GeV/c)
LO
NLO
LO
NLO
(tw
o en
trie
s/ev
ent)
PS effect
21 Jan 2008 KEK-LAPP meeting 12
(rad)
pT (GeV/c)
LO
NLO
LO
NLO
PS effect
21 Jan 2008 KEK-LAPP meeting 13
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.