Higher order Generator Studies at 7 TeV N. Chanon, S. Gascon-Shotkin, M. Lethuillier (IPNL) 31/03/10, GDR Terascale I – Comparison Resbos/Diphox for Born + Frag processes II – Comparison Resbos/Gamma2MC for Box processes III – K-factors and their impact on kinematics 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)
Higher order Generator Studies Generators of +X processes BORN BOX ONE FRAG TWO FRAG DIRECT FRAGMENTATION
Higher order Generator Studies Generators of +X processes DIPHOX Binoth, Guillet, Pilon, Werlen, hep-ph/9911340, 2000 RESBOS Balazs, Berger, Mrenna, Yuan, hep-ph/9712471, 1997 GAMMA2MC, NLO Bern, Dixon, Schmidt, hep-ph/0211216, 2002 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)
Higher order Generator Studies Generators of +X processes Energy scales : F = R = Mf = Mfactorization, 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
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
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
Higher order Generator Studies Born + frag contributions : Resbos/Diphox comparison Double differential distributions unphysical lack of event Lack of events for qT > M
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)
Higher order Generator Studies Box contributions : Resbos/Gamma2MC comparison Single differential distributions First bin is negative...
Higher order Generator Studies Box contributions : Resbos/Gamma2MC comparison Double differential distributions Resummation rate grows logarithmically with M
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.
Higher order Generator Studies K-factors for Born + Frag processes : 1 Dim K-factors far to be flat !
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)
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
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)
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(*)
Higher order Generator Studies K-factors for Box processes : 1-Dim K-factor~1 for Resbos below 60 GeV : due toISR treatment in Pythia K-factor near 1, increasing with M K-factor far to be flat
Higher order Generator Studies K-factors for Box processes : 2-Dim 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
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
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
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.
Higher order Generator Studies Conclusions / Perspectives - 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 - Reweighting of Pythia samples with qT, M, (qT,M) distributions has been performed. - Resbos/Diphox Born, Gamma2MC Box : reweighting with K(qT) fails to reproduce M. At the contrary, it is fine for Resbos Box. - All processes : K(qT) is almost fine for cos(*) but fails near boundaries 0 and 1. - Reweighting with K(qT,M) hard to perform because of a lack of events in Pythia samples in some regions of the phase space. Perspectives : - Produce Pythia sample with more events to get the K-factors on the whole phase space - Try the reweighting with (qT, cos(*)) and (qT, *) - Integrate the reweighting in +X measurement and H-> analysis
BACK-UP SLIDES
Higher order Generator Studies Generators of +X processes Generators choice : - DIPHOX Born + 1-,2-fragmentation contributions NLO - GAMMA2MC Box NLO - RESBOS Born NLO + 1-fragmentation LO + NLO Box with soft gluon resumation Energy scales : F = R = Mf = M F : factorization scale R : renormalisation scale Mf : fragmentation scale Sequential cuts : (designed for +X cross-section measurement with first data) - 40 < M < 1500 GeV for Born+frags, 40 < M < 350 GeV for Box (RESBOS cannot calculate the Box contribution for M > 350 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.
Higher order Generator Studies Generators of +X processes Born Cross-sections Box Cross-sections K-factors - At LO after cuts, Pythia Born and Box are at the same order of magnitude - Resbos rate is higher than Diphox for the born and than Gamma2MC for the Box - Kfactor is ~2.5-2.9 for the Born, 0.7-0.9 for the Box - Adding Born and Box, the K-factor is 1.6-1.9 for the +X process
Higher order Generator Studies Born and fragmentation +X processes Double frag from one parton : not included in Resbos and Diphox - Born LO (a), NLO corrections to Born (b)-(e) included in Diphox and Resbos - Born LO (a) included in ME of Pythia, which includes also (b) in the showering - 1-frag LO (f) included in Resbos only in an effective way - 1-,2-frag (f)-(g) included with their corrections in Diphox Resbos includes also resumation of soft partons in the initial state at NNLL : - At low qT, the cross-section includes resumation - At large qT, the cross-section switches to the NLO without resumation => The crossing point is near qT~M
Higher order Generator Studies Box +X processes - Box LO (a), and NLO corrections to Box (b),(c),(e) are included in Resbos and Gamma2MC - Resbos includes also the NNLO diagram (d) and resumation of soft ISR partons in a similar way as for the Born diagram.
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
Higher order Generator Studies Born + frag contributions : Resbos/Diphox comparison Double differential distributions Lack of events at low qT, high Mand high M low qT Diphox more equally populated over the phase space 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
Higher order Generator Studies Box contributions : Resbos/Gamma2MC comparison Single differential distributions Rapidity
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
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 - Most of the events have qT<Mgg : region where resummation effect is significant and is performed in Resbos - 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.
Higher order Generator Studies K-factors and reweighting - Davatz et. al, hep-ph/0604077, reweighting of Higgs samples with 2 variables : qT and Y have been used, but they claim that Y plays a minor role. - For the diphoton processes, the relevant variables would be qT and M, since they are sensitive to the resumation and fragmentation. - For +X processes, the rapidity (Yand rapidity difference (Y*) can be well reproduced by any kind of reweighting (see back-up) and won't be shown here For each sample/generator, we will compare reweighting with : - Inclusive K-factor depending only on the cross-sections - K-factor depending on single differential qT cross-sections - K-factor depending on single differential M cross-sections - K-factor depending on double differential (qT,M) cross-sections
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.
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.
Higher order Generator Studies Impact of reweighting on Born kinematics
Higher order Generator Studies Impact of reweighting on Born kinematics
Higher order Generator Studies Impact of reweighting on Box kinematics
Higher order Generator Studies Impact of reweighting on Box kinematics