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====== H+dijets (ggF) uncertainty study ======

This study aims at quantifying the gluon fusion contribution (and its uncertainty) to typical VBF Higgs plus two jets event selections. A Rivet analysis implementing the observables for the event selections below can be found {{:2013:hdijets-analysis.tgz|here}}.

The results of this study have been collected on a separate page: [[2013:groups:sm:higgs:hdijetsresults|Results of the h+dijets (ggF) uncertainty study]].

===== Foreseen contributions =====

**Korinna Zapp, Marek Schoenherr** Sherpa MEPS@NLO (0,1,2@NLO) 

**Jeppe R. Andersen, Jennifer M. Smillie,...** HEJ

**Stefan Prestel, Leif Lönnblad** Pythia8 UNLOPS, HJJ POWHEG
===== Setup =====

Five flavour calculation, PDF is CT10nlo (old fortran LHAGLUE-ID 11000). Masless b-quarks, no bbh-coupling (Yukawa_b=0). Thus, only the effective coupling of gluons to the Higgs is present and calculated in the infinte top mass limit. Perturbative QCD calculation (e.g. no multiparton interaction, no hadronisation, no intrinsic kperp). Stable Higgs with mH = 125 GeV (wH = 4.07 MeV, only if needed for extrapolation from one decay mode). E_CMS = 8 TeV.
===== Event selections =====

i) Inclusive Higgs production

Histogram the respective observable for inclusive Higgs production and Higgs production in association with at least one jet.

ii) Higgs plus dijet production

There are multiple ways of defining the two tagging jets/dijet system for this process. Two such selection which are expected to exhibit a somewhat different behaviour wrt. perturbative stability are (in close resemblance to the dijet gap fraction analysis of arXiv:1107.1641):

A) leading jet selection: use the two highest pT jets

B) forward-backward selection: use the two jets with highest and lowest rapidity


Further, within each definition of the dijet system, all observables are studied inclusively and after the application of typical VBF topological cuts to quantify their event selection efficiency. Thus, there are two levels of cuts

1) dijet inclusive event selection
   two tagging jets (according to A or B): anti-kT, R = 0.4, pT > 30 GeV, |eta| < 5
   the 3rd jet is defined as the non-tagjet with largest pT
   3rd jet: anti-kT, R = 0.4, pT > 30 GeV, |eta| < 5

2) VBF event selection
   VBF topological cuts applied on tagging jets : m(jj) > 400 GeV, dy(jj) > 2.8
   the 3rd jet is defined as the non-tagjet with largest pT in between the tagging jets in rapidity
   3rd jet: anti-kT, R = 0.4, pT > 30 GeV, |eta| < 5

===== Observables and binnings =====

The following observables were agreed upon:
(h Higgs, j1 leading tag jet, j2 subleading tagjet, j3 as defined above)

|}

|-
| pT(h)        || [0,300]   || 60 bins ||
|-
| y(h)         || [-5,5]    || 20 bins ||
|-
| pT(j1)       || [0,300]   || 60 bins ||
|-
| pT(j2)       || [0,300]   || 60 bins ||
|-
| y(j1)        || [-5,5]    || 20 bins ||
|-
| y(j2)        || [-5,5]    || 20 bins ||
|-
| m(j1,j2)     || [0,1000]  || 25 bins ||
|-
| dy(j1,j2)    || [0,10]    || 20 bins ||
|-
| dphi(j1,j2)  || [0,pi]    || 20 bins ||
|-
| pT(j3)       || [0,300]   || 60 bins ||
|-
| y(j3)        || [-5,5]    || 20 bins ||
|-
| y*           || [-5,5]    || 20 bins ||
|-
| dphi(h,j1j2) || [0,pi]    || 20 bins ||
|-
| pT(hj1j2)    || [0,300]   || 60 bins ||
|-
| abs(y*(h))   || [0,10]    || 20 bins ||     
|-
| Njet(incl)   || [0,4]     ||  5 bins ||
|-
| sigma(h in between j1 and j2)/sigma(dijet) ||
|}

===== Uncertainty study =====

Study dependence on all perturbative scales. How many there are, what are their preferred choices and what is a sensible range to vary them depends on the calculation. Examples are

* fixed-order uncertainties: muR, muF: [1,1], [0.5,1], [2,1], [1,0.5], [1,2]

* resummation uncertainties: sensible range, e.g muQ 0.5,1,2, or hfac and SCALEUP

* merging parameter: Qcut
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