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- | ====== April 2, Pre-Meeting ====== | + | Moved to [[2015:conveners:conveners_april2-pre-meeting]] (Restricted Access) |
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- | ====== Issues to be disccussed ====== | + | |
- | + | ||
- | **Joey** | + | |
- | + | ||
- | Hi. I wanted to share with the conveners some ideas for studies to be done before, at and after Les | + | |
- | Houches | + | |
- | that will have an impact not only on SM physics but on BSM as well. For that reason, and for the | + | |
- | reason that we would like to have more continuity between the 1st and 2nd sessions, I am sending this | + | |
- | to the complete list of conveners. The list of ideas below is incomplete, so other ideas are welcome. | + | |
- | + | ||
- | A key study that can be carried out at this year's Les Houches is a detailed investigation of | + | |
- | where fixed order predictions can be considered reliable, where resummation/parton | + | |
- | shower effects are important, and how the implementations of the resummation/parton shower effects may | + | |
- | differ. This is a source of great confusion among both experimentalists and | + | |
- | theorists I believe. There has not been to date (as far as I know) quantitative | + | |
- | comparisons of fixed order predictions to ME+PS predictions (with varied levels of | + | |
- | merging). In Powheg inclusive jet production, the | + | |
- | addition of parton showering seems to have a noticeable impact in almost all | + | |
- | kinematic regions. This has influenced at least the opinions of the experimental community. If this | + | |
- | is true, then we must understand why resummation effects are | + | |
- | important for regions which don't (naively) have large logs. I don't know to what extent | + | |
- | that the impact of parton showers can be varied within Powheg. Sherpa gives some | + | |
- | understanding of resummation effects by allowing a variation of the scale mu_Q; if | + | |
- | resummation effects are not important, then the variation of an observable with | + | |
- | mu_Q should be minimal. | + | |
- | + | ||
- | At Les Houches, we should be able to test Sherpa, Madgraph5_aMC@NLO and | + | |
- | Powheg. Each should be run without(/with) any non-perturbative effects, since we are trying to | + | |
- | understand perturbative effects. Sherpa and Madgraph5_aMC@NLO can be run with merged samples of 1 or 2 | + | |
- | additional jets at NLO. We can examine the impacts of varying the renormalization, factorization and | + | |
- | (where possible) the resummation scales as a function of how much merging has been done. | + | |
- | + | ||
- | One testbed for studying these effects are for the case of Higgs(+jets), where gosam | + | |
- | has provided fixed order ntuples for 1,2 and 3 jets at NLO. In addition, we should have the complete | + | |
- | Higgs+>=1 jet NNLO prediction. Frank Petriello will be at Les Houches during the first week. | + | |
- | We should make sure that we have NNLO predictions that can be directly compared (where relevant) to | + | |
- | the framework we are setting up at NLO. We can consider similar | + | |
- | distributions as were used for the ATLAS Higgs+jets paper (and for which a Rivet | + | |
- | routine exists, and which I will distribute). Take for example, Higgs+>=3 jets at NLO. Compare the pT | + | |
- | distributions for jet 1, 2 and 3 to the ME+PS predictions with merging information | + | |
- | from 0, 1 and 2 jets at NLO (0 being the inclusive Higgs cross section). Where are | + | |
- | there deviations for ME+PS predictions from the fixed order predictions? What are the relative | + | |
- | resummation scale dependences? Is jet 3 (or 2) affected differently than jet 1? If PS effects are | + | |
- | important, does the size of the | + | |
- | effects agree within the different frameworks? If not, why not? We're aiming for precision physics | + | |
- | measurements, so that level of precision must be present in the theoretical predictions. | + | |
- | + | ||
- | Can we also use the large cross section W/Z+jets for comparison to better understand these questions? | + | |
- | + | ||
- | Another testbed is inclusive jet production. There, it appears that in Sherpa there are | + | |
- | no major changes from fixed order predictions, but no quantitative comparisons have been | + | |
- | carried out yet. The resummation scale uncertainty, though, seems relatively large. This should be | + | |
- | understood. Are we just lucky (if fixed order = MEPS@NLO) that the central resummation scale | + | |
- | gives good agreement? Does the resummation scale uncertainty change as more matrix | + | |
- | element information is included? Inclusive jet production is very important to global PDF fits; so far | + | |
- | we are assuming that fixed order (+non-pert effects) is all that is needed to describe the data. | + | |
- | + | ||
- | A Sherpa study (probably there are others as well, but I don't know them off-hand) has indicated that | + | |
- | fixed order NLO does not do a good job of describing dijet cross sections where different cuts are placed on the | + | |
- | leading and next-to-leading jet. Of course, some effects are expected at threshold, but the difference between NLO | + | |
- | and NLO+PS exists out to fairly high jet pT values. On the other hand, NLO closely approximates NLO+PS for | + | |
- | inclusive jet measurements. (This agrees with the original studies performed by both CDF and QCD | + | |
- | theorists in the 90's that indicated that NLO(+non-perturbative corrections) did a reasonable job of describing | + | |
- | the jet shape for inclusive jets.) For boson+jet(s) (W/Z/Higgs), almost always the jets are ranked-ordered, so | + | |
- | that we look at the pT of the lead jet, the next-to-leading jet, and so on. Studies have also indicated that | + | |
- | PS corrections to fixed order predictions are limited in this case, perhaps because of the less steeply falling pT | + | |
- | spectra. This has to be quantitatively understood. | + | |
- | + | ||
- | A related question(s) is in regards to vetoed cross sections. If I look at the cross section for | + | |
- | Higgs+>=1 jet, where the jet pT cut is 30 GeV/c, do I have to treat this as a vetoed cross section? | + | |
- | What are the size of the effects? If this is an exclusive cross section, then there aren't very many inclusive | + | |
- | cross sections left. How can we test this? If some of the pi^2 terms are resummed, a la Tackmann, does | + | |
- | this really represent the NNLO correction? | + | |
- | + | ||
- | Another related question is with regards to scale dependence. A (universally used) central scale of | + | |
- | HT/2 seems to work reasonably well, but perhaps does not have a rigorous understanding of why it | + | |
- | should be the central scale (and why a variation of a factor of 2 around it is also reasonable). Here | + | |
- | we may be able to use CKKW/MINLO tools to shed some light on this issue. | + | |
- | + | ||
- | We should have a good start on these comparisons before Les Houches, so that the discussion at Les | + | |
- | Houches can be informed. We also need Rivet routines for the data analyses that we would like to test these | + | |
- | theory variations on. | + | |
- | + | ||
- | **Sasha** | + | |
- | I would add couple of more related issues might be important for | + | |
- | the Higgs production generation: | + | |
- | + | ||
- | 1. we know from the last Les Houches 2013 how big was a difference between | + | |
- | generator predictions of gg->h+2jets passing VBF h selections. The | + | |
- | proposal they made was to use in data the gluon initiated process and | + | |
- | check data with the different MC. I have discussed it a bit with | + | |
- | Frixione and looks like tt+jj with VBF selected jets may be a good | + | |
- | benchmark for Monte-Carlos. | + | |
- | + | ||
- | 2. Z+b(b) production important benchmark for SUSY H+b(b). Both ATLAS and | + | |
- | CMS 8 TeV analyses see the pTZ a bit harder than LO/NLO+PS predictions | + | |
- | see slide 22 of this talk for example: | + | |
- | + | ||
- | http://indico.cern.ch/event/331452/session/4/contribution/25/material/slides/1.pdf | + | |
- | + | ||
- | We see with MG5_aMC@NLO that Higgs pT in H+b(b) is sensitive to the | + | |
- | shower scale. So tuning it with Z+b(b) data may help to descrive properly | + | |
- | Higgs pT in H+b(b). | + | |
- | + | ||
- | **Josh** | + | |
- | CMS has produced/is producing large sets of events for ttbar+0,1,2j NLO with mg_aMC@NLO+pythia8 (FXFX merging) at both 8 and 13TeV. Since that is quite CPU intensive we can provide the lhe files if we have some common space somewhere where I can put them. | + |