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--- 2015:conveners_april2-pre-meeting [2015/04/02 00:48]
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+++ 2015:conveners_april2-pre-meeting [2015/04/02 01:22]
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- ====== April 2, Pre-Meeting ======
+ Moved to [[2015:conveners:conveners_april2-pre-meeting]]  (Restricted Access)
-====== 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.

Les Houches

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