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2015:conveners_april2-pre-meeting [2015/04/02 00:52]
2015:conveners_april2-pre-meeting [2015/04/02 01:22]
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- ====== April 2 (2015), Pre-Meeting ====== + Moved to [[2015:conveners:​conveners_april2-pre-meeting]] ​ (Restricted Access)
- +
-====== Issues to be discussed ====== +
- +
-**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 correctionsdid 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://​​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. +
2015/conveners_april2-pre-meeting.txt · Last modified: 2015/04/02 01:22 by fawzi.boudjema