1. 21 Jan 2008KEK-LAPP meeting1 DIPHOX event generation “direct” process only an analysis of events processed by PYTHIA Shigeru Odaka KEK/IPNS

2. 21 Jan 2008KEK-LAPP meeting2 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.

3. 21 Jan 2008KEK-LAPP meeting3 Break down for event generation 6  6 break-down to separate 6 flavor (d, u, s, c, b, (t)) contributions in 6 sub-processes. Subroutine STRFRAD in src/dir/hcoeff/htermdir.f Common brock for “break down” For gg   PDF

4. 21 Jan 2008KEK-LAPP meeting4 Function f4dimd3 in src/dir/src/twophotdn.fFunction ssy3r in src/dir/src/dir.f “break down” calculations in DIPHOX routines Checking the consistency

5. 21 Jan 2008KEK-LAPP meeting5 New routines for event generation Subroutine 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.

6. 21 Jan 2008KEK-LAPP meeting6 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 weight 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 (unsatisfactory), 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” flow ICOLUP(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) = 501 ICOLUP(2,1) = 0 ICOLUP(1,2) = 502 ICOLUP(2,2) = 501 ICOLUP(1,5) = 502 ICOLUP(2,5) = 0 The color-flow is trivial for the “direct” processes.

7. 21 Jan 2008KEK-LAPP meeting7 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. –2  7 = 14, but some are empty (cross section = 0). Mix the events randomly in proportion to the “absolute” cross sections evaluated by DIPHOX.

8. 21 Jan 2008KEK-LAPP meeting8 First test

9. 21 Jan 2008KEK-LAPP meeting9 Generation condition DIPHOX “default” Additional cuts in the histogramming Also, DIPHOX “default” DIPHOX “default”: preset condition in the version 1.2 distribution

10. 21 Jan 2008KEK-LAPP meeting10 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

11. 21 Jan 2008KEK-LAPP meeting11 LO NLO LO NLO (two entries/event) PS effect

12. 21 Jan 2008KEK-LAPP meeting12 LO NLO LO NLO PS effect

13. 21 Jan 2008KEK-LAPP meeting13 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, but 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 detector simulation in order to derive realistic predictions.