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 Università 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, Università 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, Università 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 4.1 Common configuration file: powheg.input 4.2 General considerations 2 2 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 production 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 H and 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 quarksquark-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 → H in 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 αS is 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 B̃ instead of B̄. 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 pT distribution 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 N 3 myth for the kT jet-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. Sjöstrand 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–
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