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Higher Order γγ Generator Studies at 7 TeV

This study focuses on the reweighting and K-factors for γγ+X processes at the LHC, including Box, Born, One Frag, Two Frag, and Direct Fragmentation. It explores the impact of reweighting on the kinematics of prompt diphoton production and provides insights for background predictions in H->γγ searches.

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Higher Order γγ Generator Studies at 7 TeV

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  1. Higher order  Generator Studies at 7 TeV N. Chanon, S. Gascon-Shotkin, M. Lethuillier (IPNL)‏ 06/04/10, Theorie LHC France / GDR Terascale Tools / FCPPL Hadron Satellite I – Reweighting of Born + Frag processes with Resbos/Diphox II – Reweighting of Box processes with Resbos/Gamma2MC III – Reweighting of gg->H-> with HNNLO Why studying +X processes ? - Interesting from a theoretical point of view because prompt diphoton production is between pure QCD and QED - It's a background to light Higgs searches (and new physics in some BSM models)‏

  2. Higher order  Generator Studies Generators of +X processes BOX BORN ONE FRAG TWO FRAG DIRECT FRAGMENTATION

  3. Higher order  Generator Studies Generators of +X processes GAMMA2MC, NLO Bern, Dixon, Schmidt, hep-ph/0211216, 2002 RESBOS Balazs, Berger, Mrenna, Yuan, hep-ph/9712471, 1997 DIPHOX Binoth, Guillet, Pilon, Werlen, hep-ph/9911340, 2000 FIXED ORDER : NLO NLO with NNLL Resummation FIXED ORDER : NLO BORN + FRAG (and NLO corrections)‏ BOX (and NLO corrections)‏ Resbos only 1-frag : - LO, effectively in Resbos - NLO in Diphox 2-frag : DIPHOX only (NLO)‏

  4. Higher order  Generator Studies Generators of +X processes Energy scales :F = R = Mf = Mfactorization, renormalisation, fragmentation scale)‏ Sequential cuts : (designed for +X cross-section measurement with first data)‏ - M > 40 GeV, Photon || < 2.5 and Pt > 20 GeV - Sum of partonic transverse energy in a cone R<0.35 should be less than 5 GeV. Born Cross-sections Box Cross-sections K-factors

  5. Higher order  Generator Studies K-factors and reweighting - We do not have yet a PS generator at NLO for prompt photon production processes. - To measure +X process, we can either apply corrections due to all detector effects to data and compare corrected data distributions to partonic one, or reweight PS events with higher order ME distributions, and compare the result to data. - For the H-> searches, we need the most precise prediction of the background (including higher order kinematics) in a MC with showered/hadronized events. => Reweighting. Reweighting For analysis which don't use too much kinematics, using inclusive K-factors is fine. If we want the PS signal to have higher order kinematics, the K-factor should depend on - At least 1 variable (usually qT) Davatz, Dissertori, Dittmar, Grazzini, Pauss, hep-ph/0402218 - Or better, 2 variables (for the Higgs, qT, YH) Davatz, Stoeckli, Anastasiou, Dissertori, Dittmar, Melnikov, Petriello, hep-ph/0604077. For the  processes, the relevant variables would be qT and M, since they are sensitive to the resummation and fragmentation.

  6. Higher order  Generator Studies K-factors for Born + Frag processes : 1 Dim Diphox uses Fixed order calculation : divergences near boundaries qT, log M M Divergence at low qT for Diphox => 2 first bins merged Divergence at low M for Diphox => 2 first bins merged K-factors far from flat ! qT M M

  7. Higher order  Generator Studies K-factors for Born + Frag processes : 2-Dim K-factor RESBOS / Madgraph Born K-factor DIPHOX / Madgraph Born M M M M qT qT Resbos more accurate at low qT (resumation). Around qT~Mgg, the cross-section switches to the NLO without resum Diphox more accurate than Resbos at large Mgg (more fragmentation contributions)‏ - Madgraph phase space is correctly populated => it's possible to use it to compute 2D K-factors (contrary to Pythia)‏ - Lack of few events at large qT, large M

  8. Higher order  Generator Studies Impact of reweighting on Born kinematics DIPHOX RESBOS - Red (ME) and Green (reweighting with K(qT)) are superimposed - K(qT,M) reproduces well the ME spectrum until 170 GeV (small lack of stat in Madgraph sample at large qT and M, see previous slide)‏ - Inclusive K-factor and K(M) does not reweight properly the qT distribution

  9. Higher order  Generator Studies Impact of reweighting on Born kinematics DIPHOX RESBOS - Red (ME) and Green (reweighting with K(M)) are superimposed - K(qT) alone predicts a reasonable spectrum, but K(qT,M) is more accurate - Inclusive K-factor overestimate the rate

  10. Higher order  Generator Studies Impact of reweighting on Born kinematics RESBOS DIPHOX - Reweighting of cos(*) is almost fine for Diphox with any reweighting. The best fit is obtained with K(qT)‏ - Difficulties to reproduce cos(*) with the variables shown here for Resbos at low and large cos(*)‏

  11. Higher order  Generator Studies K-factors for Box processes : 1-Dim M qT K-factor~1 for Resbos below 60 GeV : due to ISR treatment in Pythia As Diphox, Gamma2MC has divergences near boundaries (fixed order calculation)‏ Unphysical lack of events with Resbos K-factor far from flat (orders of magnitude at large qT !)‏ K-factor near 1, increasing with M M qT

  12. Higher order  Generator Studies K-factors for Box processes : 2-Dim K-factor RESBOS / Pythia Box K-factor GAMMA2MC / Pythia Box M M M M qT qT Pythia phase space is under populated ! - For large qT, Mgg, K-factor is null because there is no Pythia event ! - Here no Madgraph sample overseen

  13. Higher order  Generator Studies Impact of reweighting on Box kinematics GAMMA2MC RESBOS - Red (ME) and Green (reweighting with K(qT)) are superimposed - K(qT,M) reproduces well the ME spectrum at low qT only, because of a lack of stat in Pythia sample at large qT and M (see previous slide) => can not be used as it is. - Inclusive K-factor and K(M) does not reweight properly the qT distribution

  14. Higher order  Generator Studies Impact of reweighting on Box kinematics GAMMA2MC RESBOS - Red (ME) and Green (reweighting with K(M)) are superimposed - Resbos almost fine for any reweighting here - Contrary to the Born, K(qT) predicts here a spectrum too soft for M>70 GeV with Gamma2MC, and too many events at low qT. The same for inclusve K-factor. - K(qT,M) is fine here

  15. Higher order  Generator Studies Impact of reweighting on Box kinematics GAMMA2MC RESBOS - Reweighting of cos(*) is almost fine for Gamma2MC with K(qT) but serious problems with K(M) and K(qT,M) ! - Difficulties to reproduce cos(*) with the K(qT) for Resbos, and the other reweighting are worst.

  16. Higher order  Generator Studies Generators of gg->H-> processes Energy scales :F = R = MHfactorization, renormalisation scale)‏ Sequential cuts : (designed for +X cross-section measurement with first data)‏ - Photon || < 2.5 and Pt > 20 GeV - Sum of partonic transverse energy in a cone R<0.35 should be less than 5 GeV.

  17. Higher order  Generator Studies K-factors for gg->H->processes : 1-Dim Higgs rapidity qT, log K-factor ~3 Divergence at low qT for HNNLO => 2 first bins merged

  18. Higher order  Generator Studies K-factors for gg->H->processes : 2-Dim Y qT - No problem of under-population in Pythia sample here, Y is well populated in the whole qT range

  19. Higher order  Generator Studies Impact of reweighting on gg->H-> kinematics - Red and green curve are superimposed - Reweighting with inclusive K-factor or with K(Ygg) overestimates the rate at large qT - K(qT) and K(qT, Ygg) agrees perfectly

  20. Higher order  Generator Studies Impact of reweighting on gg->H-> kinematics - Red and rose curves are superimposed - K(qT) and inclusive K-factors overestimate the rate in central regions, and underestimate slightly in forward refions - K(Ygg) and K(qT, Ygg) agrees very well (with statistical uncertainties)‏

  21. Higher order  Generator Studies Impact of reweighting on gg->H-> kinematics - Reweighting with any kind of variable would be almost fine

  22. Higher order  Generator Studies Conclusions / Perspectives  +X processes : - Diphox / Resbos are in agreement in a large region of the phase space. Diphox has fragmentation and describes better the large M region, whereas Resbos has resumation and describes better the low qT region. Unphysical lack of events not well understood in some regions with Resbos for qT>M - Born : Reweighting Madgraph with DIPHOX K(qT) or better K(qT,M) is fine even for cos(*)‏ - Box : Reweighting with K(qT,M) hard to perform because of a lack of events in Pythia gg->H-> processes : - Reweighting with K(qT) would be sufficient, but for more accuracy on Y, using K(qT,Y) would be better. Perspectives : - Produce Pythia Box sample with more events to get the K-factors on the whole phase space. - Try to reweight the Box with (qT, cos(*)) and (qT, )‏ - Integrate the reweighting in +X measurement and H-> analysis

  23. BACK-UP SLIDES

  24. Higher order  Generator Studies Born + frag contributions : Resbos/Diphox comparison Single differential distributions First bin is negative... LOG (Diphox : 2 first bins added)‏ - In DIPHOX, IR divergences cancels between born NLO and 1-frag LO : it is possible (although not completely accurate) to compare these contributions to the total contributions of DIPHOX and RESBOS. - DIPHOX born, 1-,2-frag : divergence at low qT - RESBOS : soft gluon resummation => no divergence at low qT

  25. Higher order  Generator Studies Born + frag contributions : Resbos/Diphox comparison Single differential distributions The slope with Resbos is a bit higher than with Diphox Resbos predicts more events at low  masses

  26. Higher order  Generator Studies Born + frag contributions : Resbos/Diphox comparison Double differential distributions unphysical lack of event Lack of events for qT > M

  27. Higher order  Generator Studies Born + frag contributions : Resbos/Diphox comparison Single differential distributions  Rapidity Usual  angular distributions have the same shape : not sensitive to resummation or fragmentation specificities

  28. Higher order  Generator Studies Born + frag contributions : Resbos/Diphox comparison Double differential distributions Diphox more equally populated over the phase space Lack of events at low qT, high Mand high M low qT qT qT qT M M M RESBOS : - Physical spectrum at low qT (resummation)‏ - Born NLO + 1-frag LO only - Lack of stat at some regions of the phase space DIPHOX : - Unhysical spectrum at low qT (fixed order)‏ - Born NLO + 1-frag, 2-frag NLO - Give contribution greater than Resbos at large M

  29. Higher order  Generator Studies Box contributions : Resbos/Gamma2MC comparison Single differential distributions LOG (Gamma2MC : 2 first bins added)‏ First bin is negative... unphysical lack of event - GAMMA2MC : no resummation => divergence at low qT - RESBOS : resummation => no divergence at low qT. But unphysical rate near M~60-70 GeV (can be due to the switching point resummation/NLO)‏

  30. Higher order  Generator Studies Box contributions : Resbos/Gamma2MC comparison Single differential distributions First bin is negative...

  31. Higher order  Generator Studies Box contributions : Resbos/Gamma2MC comparison Double differential distributions Resummation rate grows logarithmically with M

  32. Higher order  Generator Studies Box contributions : Resbos/Gamma2MC comparison Single differential distributions Rapidity

  33. Higher order  Generator Studies Box contributions : Resbos/Gamma2MC comparison Double differential distributions Gamma2MC very well populated (one run per M bin => I should try the same for the other generators)‏ Lack of event at medium qT, low Mgg qT qT M M M RESBOS : - Physical spectrum at low qT (resummation)‏ - Box NLO with one more diagram than Gamma2MC - Lack of stat at some regions of the phase space GAMMA2MC : - Unphysical spectrum at low qT (fixed order)‏ and low M Add positive bins close to negative ones - Statistics is not a problem

  34. Higher order  Generator Studies Born + frag contributions : Resbos/Diphox comparison Single differential distributions qT<M qT>M Diphox has a higher rate due to more frag contributions taken in account effect of resummation in Resbos : slightly higher rate than Diphox - But a non negligible amount of events have qT>Mgg : region where fragmentation is important and is better described in Diphox, and where no resum is performed in Resbos. - Most of the events have qT<Mgg : region where resummation effect is significant and is performed in Resbos

  35. Higher order  Generator Studies Reweighting issues Isolation issue : - At partonic level, ME NLO generators define isolation as the partonic energy in a cone. Either the whole partons is inside the cone, either not (because there is no hadronization)‏ - At particle level, inside PS generators, part of the parton energy can be in the cone if we loop on the final states particles. We should be careful of not saturating the isolation cone with underlying event. => Several ways to define isolation. Binning issue : - Use of binned distribution => K-factor is the same inside each bin. There could be bias especially for high slope curves (slope very high in Resbos at low qT)‏ - Use of unbinned distribution => Either fit the unbinned distributions and get the K-factor, or fit the binned K-factor. Anyway, small bias due to fit.

  36. Higher order  Generator Studies II – Generators of +X processes Plans for reweighting : isolation issue Pythia Box Pythia Box Pythia Box - Charged particles without e,m : mimic HCAL. But no such status 1 particles, therefore nul energy in the cone. - All status 1 particles in the cone : seems to be a reasonable estimate of partonic energy. All NLO analysis of CDF data use the same Et threshold as the experimental one. - GenJet collection : threshold at 5 GeV for the hadrons clustering => not used.

  37. Higher order  Generator Studies K-factors for Born + Frag processes : 1 Dim K-factors far from flat !

  38. Higher order  Generator Studies K-factors for Born + Frag processes : 2-Dim K-factor RESBOS / Pythia Born K-factor DIPHOX / Pythia Born M M M M qT qT Pythia phase space is under populated ! - For large qT, K-factor is null because there is no Pythia event ! - Looking for Madgraph sample (ME-PS matching allows harder spectra)‏

  39. Higher order  Generator Studies Impact of reweighting on Born kinematics DIPHOX RESBOS - Red (ME) and Green (reweighting with K(qT)) are superimposed - K(qT,M) reproduces well the ME spectrum at low qT only, because of a lack of stat in Pythia sample at large qT and M (see previous slide) => can not be used as it is. - Inclusive K-factor and K(M) does not reweight properly the qT distribution

  40. Higher order  Generator Studies Impact of reweighting on Born kinematics DIPHOX RESBOS - Red (ME) and Green (reweighting with K(M)) are superimposed - K(qT) predicts a spectrum too hard at large M - K(qT,M) is fine here (just below the expectation)‏ - Inclusive K-factor do a reasonnable job (too much events are large M)‏

  41. Higher order  Generator Studies Impact of reweighting on Born kinematics DIPHOX RESBOS - Reweighting of cos(*) is almost fine for Diphox with any reweighting. The best fit is obtained with K(qT)‏ - Difficulties to reproduce cos(*) with the variables shown here for Resbos at low and large cos(*)‏

  42. Higher order  Generator Studies Impact of reweighting on Born kinematics

  43. Higher order  Generator Studies Impact of reweighting on Born kinematics

  44. Higher order  Generator Studies Impact of reweighting on Box kinematics

  45. Higher order  Generator Studies Impact of reweighting on Box kinematics

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