Manual BOX Gg H
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Preprint typeset in JHEP style - PAPER VERSION December 18, 2013
POWHEG BOX 1.99-beta
The POWHEG BOX user manual:
Higgs boson production through gluon fusion in the
2HDM
Simone Alioli
Deutsches Elektronen-Synchrotron DESY
Platanenallee 6, D-15738 Zeuthen, Germany
E-mail: simone.alioli@desy.de
Paolo Nason
INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milan, Italy
E-mail: Paolo.Nason@mib.infn.it
Carlo Oleari
Universit`a di Milano-Bicocca and INFN, Sezione di Milano-Bicocca
Piazza della Scienza 3, 20126 Milan, Italy
E-mail: Carlo.Oleari@mib.infn.it
Emanuele Re
Institute for Particle Physics Phenomenology, Department of Physics
University of Durham, Durham, DH1 3LE, UK
E-mail: emanuele.re@durham.ac.uk
Emanuele Angelo Bagnaschi, Pietro Slavich
LPTHE, 4, Place Jussieu, F-75252 Paris, France
E-mail: bagnaschi@lpthe.jussieu.fr, slavich@lpthe.jussieu.fr
Giuseppe Degrassi
Dipartimento di Fisica, Universit`a di Roma Tre and INFN, sezione di Roma Tre
Via della Vasca Navale 84, I-00146, Rome, Italy
E-mail: degrassi@fis.uniroma3.it
Alessandro Vicini
Dipartimento di Fisica, Universit`a di Milano and INFN, sezione di Milano
Via Celoria 16, I-20133 Milano, Italy
E-mail: alessandro.vicini@mi.infn.it
Abstract: This note documents the use of the package POWHEG BOX for Higgs boson
production trough gluon fusion in the 2HDM. Results can be easily interfaced to shower
Monte Carlo programs, in such a way that both NLO and shower accuracy are maintained.
Keywords: POWHEG, Shower Monte Carlo, NLO.
Contents
1. Introduction 1
2. System requirements 2
3. Generation of events 2
4. Process specific input parameters 2
4.1 Common configuration file: powheg.input 2
4.2 General considerations 3
1. Introduction
The POWHEG BOX program is a framework for implementing NLO calculations in Shower
Monte Carlo programs according to the POWHEG method. An explanation of the method
and a discussion of how the code is organized can be found in refs. [1, 2, 3]. The code is
distributed according to the “MCNET GUIDELINES for Event Generator Authors and
Users” and can be found at the web page
http://powhegbox.mib.infn.it.
This program is an implementation of the NLO cross section for Higgs boson produc-
tion via gluon fusion process, in the MSSM, in the POWHEG formalism of refs. [1, 2]. It is
based over the work done by Alioli et al [9]. The supported processes are the production
of the light CP even scalar h, of the heavy CP even scalar Hand of the pseudoscalar A.
This implementation provides the exact treatment in the amplitude of the quark and of
the squark masses; it also provides the evaluation of the diagrams involving the quark-
squark-gluino vertices, which are approximated by different mass expansions, as described
in refs.[?,?,?]. MSSM parameters are A detailed description of the implementation can
be found on ref. [17].
Spin correlations of Higgs boson decay products are not included, being it a scalar.
This issue can be safely left to the subsequent Shower Monte Carlo program. Finite Higgs
boson width effects are accounted for.
The code, that can be found in the POWHEG-BOX/gg H MSSM subdirectory is based on
the subtraction scheme by Frixione, Kunszt and Signer implemented in the POWHEG BOX,
– 1 –
rather than on the scheme discussed in the paper [9]. Please cite it anyhow if you use the
program.
In order to run the POWHEG BOX program, we recommend the reader to start from the
POWHEG BOX user manual, which contains all the information and settings that are common
between all subprocesses. In this note we focus on the settings and parameters specific to
gg →φ(φ=h, H, A) implementation.
2. System requirements
Chaplin ¿= 1.2 is required to compile and use this MC event generator.
3. Generation of events
Build the executable
$ cd POWHEG-BOX/gg H 2HDM
$ make pwhg main
Then do (for example)
$ cd testrun-lhc
$ ../pwhg main
At the end of the run, the file pwgevents.lhe will contain 100000 events for gg →Hin
the Les Houches format.
In order to shower them with PYTHIA (for example) do
$ cd POWHEG-BOX/gg H 2HDM
$ make main-PYTHIA-lhef
$ cd testrun-lhc
$ ../main-PYTHIA-lhef
4. Process specific input parameters
The input parameters for POWHEG are separated into two files: one is the standard
powheg.input and the others differs according to the renormalization scheme selected for
the MSSM parameters. Currently we implement an On-Shell and a DR scheme.
4.1 Common configuration file: powheg.input
The common parameters, defined in ’powheg.input’ are:
zerowidth 1 ! Control if the Higgs boson is to be produced on-shell or not:
1 = On-Shell; 0 = Off-shell with Breit-Wigner
hwidth 10 ! Higgs width, dummy value, you should change it if you do off-shell
production
bwhape 1 ! BW shape: 1 = running width; 2 = fixed width
masswindow 10d0 !(default 10d0) number of widths around hmass in the BW for
an off-shell Higgs boson
– 2 –
ew 0 ! ew = 0 disable EW corrections, ew = 1 enable EW corrections
fastew 1 ! fast ew corrections evaluations by mass sampling
gfermi 0.116637D-04 ! GF
hdecaywidth 0 ! use HDECAY computed width
hdecaymode -1 ! PDG code for first decay product of the higgs
! allowed values are: 0 all decay channels open
! 1-6 d dbar, u ubar,..., t tbar (as in HERWIG)
! 7-9 e+ e-, mu+ mu-, tau+ tau-
! 10 W+W-
! 11 ZZ
! 12 gamma gamma
runningscale 0 ! Use running scale
! **** Mandatory parameters for the 2HDM model ****
higgstype 1 ! CP-even, neutral, higgs type: 1 = light higgs; 2 = heavy higgs
scheme 0 ! Renormalization scheme. 0 = OS , 1 = MSBAR; 2 = DRBAR topmass 172.5
! top quark mass bottommass 4.75d0 ! bottom quark mass - if defined it enables
the bottom quark !charmmass 1.5d0 ! char quark mass - if defined it enables
the charm quark ! Optional ! 2HDM parameter 2HDMtype 1 ! 2HDM type; 1=type
I; 2=type II; 3=Lepton specific; 4 =Flipped ! decoupling tanb 10d0 ! Tan beta
alpha -0.09887d0 ! Higgs mixing angle
4.2 General considerations
The running of αSis evaluated at two loop order, correctly matching, at flavour thresholds,
different definitions that depends on the number of flavours that can be considered light
at the renormalization scale.
An example of powheg.input files is given in the subdirectory gg H 2HDM/testrun-lhc.
In all examples, the choice of the parameters that control the grid generation is such that a
reasonably small fraction of negative weights is generated, so they can be run as they are.
We remind the reader that these negative weights are only due to our choice of generating
˜
Binstead of ¯
B. They indeed correspond to phase space points where NLO corrections are
bigger than LO contributions. Had we performed the integration over the full radiation
phase space these negative weights would have disappeared completely.
In case one is interfacing to HERWIG or PYTHIA SMC programs, we provide a facility to
select the Higgs boson decay products in these programs :
hdecaymode 12 ! code for selection of Higgs boson decay products:
! -1 the Higgs boson is left undecayed by the SMC
! 0 all decay channels are open
! 1-6 d dbar, u ubar,..., t tbar (as in HERWIG)
! 7-9 e+ e-, mu+ mu-, tau+ tau-
! 10, 11, 12 W+W-, ZZ, gamma gamma
– 3 –
Together with the mandatory parameters, the POWHEG BOX input facility allows for an
easy setting of run parameters, by explicitly adding the relevant lines to the input card.
In case one of the following entries is not present in the input card the reported default
value is assumed. In any case, these parameters are printed in the output of the program,
so their values can be easily tracked down.
masswindow 10d0 !(default 10d0) number of widths around hmass in the BW for
an off-shell Higgs boson
runningscale 0 ! choice for ren and fac scales in Bbar integration
0: fixed scale M H
1: running scale inv mass H
Of particular importance are the following parameters:
•hhfact 100d0 ! (default no dumping factor) dump factor for high-pt radiation:
> 0 dumpfac=h**2/(pt2+h**2) controls how much of real contribution enters in
the POWHEG Sudakov form factor. By default all real contributions are included, but
this may lead to a NNLO mismatch in the higher Higgs boson pTdistribution tail,
with respect to fixed order NLO results. This actually brings POWHEG BOX results
closer to NNLO ones, but if one want to switch-off this feature it’s possible to use a
reduced real contribution Rred =R×dumpfact in the Sudakov and to generate the
remaining R×(1-dumpfact) part without suppression, as documented in Sec. 4.3 of
ref. [9].
•zerowidth 1 (default 0 = false) enforce the calculation in the Higgs zero width
approximation.
•bwshape 1 (default 1) choose the functional form of the Breit-Wigner along which
the Higgs virtuality is distributed, in case of the zero width approximation has not
been chosen. Allowed values are 1 for a BW with a running width, 2 for a fixed
width.
References
[1] P. Nason, “A new method for combining NLO QCD with shower Monte Carlo algorithms,”
JHEP 0411 (2004) 040 [arXiv:hep-ph/0409146].
[2] S. Frixione, P. Nason and C. Oleari, “Matching NLO QCD computations with Parton Shower
simulations: the POWHEG method,” JHEP 0711 (2007) 070 [arXiv:0709.2092 [hep-ph]].
[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,” [arXiv:1002.2581
[hep-ph]].
[4] S. Frixione and B. R. Webber, “Matching NLO QCD computations and parton shower
simulations,” JHEP 0206 (2002) 029 [arXiv:hep-ph/0204244].
[5] S. Frixione and B. R. Webber, “The MC@NLO 3.3 event generator,” arXiv:hep-ph/0612272.
– 4 –
[6] S. Dawson, Radiative corrections to Higgs boson production,Nucl. Phys. B359 (1991)
283–300.
[7] A. Djouadi, M. Spira, and P. M. Zerwas, Production of Higgs bosons in proton colliders:
QCD corrections,Phys. Lett. B264 (1991) 440–446.
[8] M. Spira, A. Djouadi, D. Graudenz, and P. M. Zerwas, Higgs boson production at the LHC,
Nucl. Phys. B453 (1995) 17–82, [hep-ph/9504378].
[9] S. Alioli, P. Nason, C. Oleari and E. Re, “NLO Higgs boson production via gluon fusion
matched with shower in POWHEG,” arXiv:0812.0578 [hep-ph].
[10] http://mcfm.fnal.gov/
[11] M. Cacciari and G. P. Salam, Dispelling the N3myth for the kTjet-finder,Phys. Lett. B641
(2006) 57–61, [hep-ph/0512210].
[12] E. Boos et al., “Generic user process interface for event generators,” arXiv:hep-ph/0109068.
[13] J. Alwall et al., “A standard format for Les Houches event files,” Comput. Phys. Commun.
176 (2007) 300 [arXiv:hep-ph/0609017].
[14] T. Sj¨ostrand et al., in “Z physics at LEP1: Event generators and software,”, eds. G. Altarelli,
R. Kleiss and C. Verzegnassi, Vol 3, pg. 327.
[15] M. R. Whalley, D. Bourilkov and R. C. Group, “The Les Houches accord PDFs (LHAPDF)
and LHAGLUE,” arXiv:hep-ph/0508110.
[16] S. Alioli, P. Nason, C. Oleari and E. Re, “NLO vector-boson production matched with
shower in POWHEG,” JHEP 0807, 060 (2008) [arXiv:0805.4802 [hep-ph]].
[17] E. Bagnaschi, G. Degrassi, P. Slavich and A. Vicini, arXiv:1111.2854 [hep-ph].
– 5 –
