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

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.

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

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  1. DIPHOX event generation“direct” process onlyan analysis of events processed by PYTHIA Shigeru Odaka KEK/IPNS 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. KEK-LAPP meeting

  3. 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 PDF Common brock for “break down” For gg KEK-LAPP meeting

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

  5. New routines for event generation Fill the common HEPRUP (run info.) Open the output file Subroutine dir_sub in src/dir/src/twophotdn.f The part calling SPRING for the 1st phase-space region 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. 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 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. 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. • 27 = 14, but some are empty (cross section = 0). • Mix the events randomly in proportion to the “absolute” cross sections evaluated by DIPHOX. KEK-LAPP meeting

  8. First test KEK-LAPP meeting

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

  11. PS effect LO LO (two entries/event) NLO NLO KEK-LAPP meeting

  12. PS effect LO LO NLO NLO 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, 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. KEK-LAPP meeting

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