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Manual for Wand ZProduction via Vector Boson Fusion in the
POWHEG BOX
This document describes the settings and input parameters that are specific to the implemen-
tation of Wand Zproduction via vector boson fusion at the LHC within the POWHEG BOX
framework. The parameters that are common to all POWHEG BOX implementations are given in
the manual-BOX.pdf document in the POWHEG-BOX/Docs directory. Since there are singular con-
figurations in the Born contributions, due care has to be exercised to avoid these regions when
generating the Born phase space. Two available options are described in the Input parameters
section. On this account, the code is not expected to give reliable results when used with very
inclusive cuts. If you use our program, please cite Refs. [?,?,?].
Running the program
Go to the directory $ cd POWHEG-BOX/VBF W-Z and create a directory where you want to run
the code:
$ mkdir testrun
The directory must contain the powheg.input and vbfnlo.input file. To compile the code with
gfortran, type
$ make
The code needs fastjet and lhapdf, so make sure that fastjet-config and lhapdf-config
are in the path.
After compiling, enter the testrun directory:
$ cd testrun
The program can be run sequentially by deactivating the variable manyseeds in powheg.input.
Execute the program with
$../pwhg main
The program can also be run in parallel by setting manyseeds to 1in powheg.input and in
several steps as for instance described in the manual for VBF Z production [?].
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Input parameters
To have more flexible choices for the input parameters of the calculation, a new input file
(vbfnlo.input) was added in addition to the standard powheg.input file. The use of this new
file and some special parameters for powheg.input will be explained in the following.
powheg.input
•mll gencut : cut on the invariant dilepton mass for Zjj production, set to 15 GeV if
lower than 15 GeV. This cut is needed to avoid singularities when a virtual photon decays
into two massless leptons.
•bornsuppfact:
0No Born suppression factor. You should use generation cuts via ptj gencut when
using this option.
1Use a Born suppression factor F(Φn) with
F(Φn) = p2
T,j1
p2
T,j1+ Λ2
pT j !n p2
T,j2
p2
T,j2+ Λ2
pT j !n
.
ΛpT j is set to the value given in ptsuppfact and F(Φn) vanishes whenever a singular
region in the Born phase space Φnis reached. The underlying Born kinematics are
then generated in the Powheg-Box according to a modified Bfunction,
Bsupp =B(Φn)F(Φn).
•ptj gencut: Generation cut on the jets’ pTin the phase space generator. Should be used
when bornsuppfact is set to 0.
•Phasespace:
1Default value, use the standard phase space (with bornsuppfact or ptj gencut).
2Use unweighted events as phase space generator. To use this option, unweighted
events with very inclusive cuts have to be generated with Vbfnlo in a seperate
run. Each event which survives the unweighting procedure has to be reweighted by
the factor Ji=σLO/(|MB(Φi)|2pdf(Φi)), the Born cross section over the respective
numerical value of the squared Born matrix element including pdfs. This factor Jiis
exactly the Jacobian factor of the Born phase space. Place the file with the reweighted
events called event.total.lhe in the running directory.
•bornxsec: Born cross section in nb of the unweighted events used for Phasespace 2. Not
needed otherwise.
•fakevirt : If the program is run in several steps, the generation of the grid can be
performed with fakevirt 1, where the virtual amplitude is set to a factor proportional
to the Born amplitude.
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vbfnlo.input
The input parameters which can be changed by the user are explained in the Vbfnlo Manual [?]
in detail. Here we give a list of the supported parameters with a short description.
•PROC ID:
120 pp →Z jj →`+`−jj
130 pp →W+jj →`+ν`jj
140 pp →W−jj →`−¯ν`jj
•DECAYMODE:
11 `=e(electron)
13 `=µ(muon)
•HMASS: Standard Model Higgs boson mass in GeV. Default value is 126 GeV.
•HWIDTH: It is possible to set the Higgs boson width with this input parameter. Default is
-999 GeV, which means that the internally calculated value of the width is used.
•EWSCHEME: Sets the scheme for the calculation of electroweak parameters. A summary of
the four available options is given in Table ??. Note that if EWSCHEME = 4 is chosen, all
variables in Table ?? are taken as inputs. As the parameters are not independent, this
can lead to problems if the input values are not consistent. In this scheme, all photon
couplings are set according to the input variable INVALFA and all other couplings are set
according to FERMI CONST. Default value is 3.
•ANOM CPL: If set to 1, anomalous gauge boson couplings are used if available for the selected
process. Anomalous coupling parameters are set via the file anomV.dat which has to be
located in the running directory. For details on the possible choices and parameterisations
of the anomalous couplings see the Vbfnlo Manual [?]. This is not a tested feature and
should always be compared to the NLO result of Vbfnlo. Default value is 0.
•TOPMASS: Top-quark mass in GeV. Default value is 172.4 GeV.
•TAU MASS: Tau mass in GeV used in the calculation of the Higgs boson width and branching
ratios. Default value is 1.77684 GeV.
•BOTTOMMASS: Bottom-quark pole mass in GeV, used in the calculation of the Higgs boson
width and branching ratios. Default value is 4.855 GeV, which corresponds to mMS
b(mb) =
4.204 GeV.
•CHARMMASS: Charm-quark pole mass in GeV used in the calculation of the Higgs boson
width and branching ratios. Default value is 1.65 GeV, corresponding to mM S
c(mc) =
1.273 GeV.
•FERMI CONST: Fermi constant, used as input for the calculation of electroweak parameters
in EWSCHEME = 1-4. Default value is 1.16637 ×10−5GeV−2.
•INVALFA: One over the fine structure constant, used as input for EWSCHEME = 1 and 4.
Within the other schemes this parameter is calculated. The default value depends on the
choice of EWSCHEME, as given in Table ??.
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EWSCHEME Parameter Default Value Input/Calculated
FERMI CONST 1.16637 ×10−5GeV−2Input
INVALFA 128.944341122 Input
1SIN2W 0.230990 Calculated
WMASS 79.9654 GeV Calculated
ZMASS 91.1876 GeV Input
FERMI CONST 1.16637 ×10−5GeV−2Input
INVALFA 132.340643024 Calculated
2SIN2W 0.222646 Input
WMASS 80.3980 GeV Calculated
ZMASS 91.1876 GeV Input
FERMI CONST 1.16637 ×10−5GeV−2Input
INVALFA 132.340705199 Calculated
3SIN2W 0.222646 Calculated
WMASS 80.3980 GeV Input
ZMASS 91.1876 GeV Input
FERMI CONST 1.16637 ×10−5GeV−2Input
INVALFA 137.035999679 Input
4SIN2W 0.222646 Input
WMASS 80.3980 GeV Input
ZMASS 91.1876 GeV Input
Table 1: Electroweak input parameter schemes.
To compare the code with [?], the flag consistency in the qqbqq.f and qqZWjqqi.f file in the
vbfnlo-files directory has to be set to .true.. Than, all amplitudes with Q2<4 GeV are
provided with a large damping factor, not only the ones with photons in the t-channel. After
that, the code has to be recompiled.
References
[1] F. Schissler, and D. Zeppenfeld, Parton Shower Effects on W and Z Production via Vector
Boson Fusion at NLO QCD, arXiv:1302.2884.
[2] C. Oleari and D. Zeppenfeld, QCD corrections to electroweak nu(l) j j and l+ l- j j produc-
tion, Phys. Rev. D 69, 093004 (2004) [hep-ph/0310156].
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[3] S. Alioli, P. Nason, C. Oleari, and E. Re, A general framework for implementing NLO
calculations in shower Monte Carlo programs: the POWHEG BOX, JHEP 1006 (2010)
043 [arXiv:1002.2581 [hep-ph]].
[4] B. J¨ager, S. Schneider, G. Zanderighi, Next-to-leading order QCD corrections to electroweak
Zjj production in the POWHEG BOX, arXiv:1207.2626 [hep-ph].
[5] K. Arnold, M. B¨ahr, G. Bozzi, F. Campanario, C. Englert, T. Figy, N. Greiner and C. Hack-
stein et al., Comput. Phys. Commun. 180, 1661 (2009) [arXiv:0811.4559 [hep-ph]];
K. Arnold, J. Bellm, G. Bozzi, F. Campanario, C. Englert, B. Feigl, J. Frank and T. Figy
et al., [arXiv:1207.4975 [hep-ph]];
K. Arnold, J. Bellm, G. Bozzi, M. Brieg, F. Campanario, C. Englert, B. Feigl and J. Frank
et al., [arXiv:1107.4038 [hep-ph]].
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