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About ihixs 1.* (legacy code)

iHixs is a (now superseded) program which computes inclusive Higgs boson cross-sections at hadron colliders, published in 2011. It incorporates QCD corrections through NNLO, real and virtual electroweak corrections, mixed QCD-electroweak corrections, quark-mass effects through NLO in QCD, and finite width effects for the Higgs boson and heavy quarks. iHixs can be used to obtain the most precise cross-section values in fixed order perturbation theory in the Standard Model. In addition, it allows for a consistent evaluation of the cross-section in modified Higgs boson sectors with anomalous Yukawa and electroweak interactions as required in extensions of the Standard Model. iHixs is interfaced with the LHAPDF library and can be used with all available NNLO sets of parton distribution functions.

For further details please check the arxiv pre-print published together with the code.

About the authors if ihixs 1.*

• B. Anastasiou
• S. Buehler
• F. Herzog
• A. Lazopoulos

ihixs1.* version history (legacy code)

• ihixs.1.4: Fixed a bug affecting the quark gluon NLO exact coefficient. The numerical impact is 0.5% on the NNLO cross section for the input configurations tested. Many thanks to Raoul Rontsch and Elisabetta Furlan for discovering this!
• ihixs.1.3.3: Bugfixes affecting the virtual NLO corrections in effective and exact formulation. Added two new PDF sets (CT10 and NNPDF23). Added the choice of whether to use the MS-bar or the On-shell scheme for quark masses.
• ihixs.1.3.1: A bug appearing when running ihixs with factorization scale different from renormalization scale was removed. The differences with respect to the old versions are at the percent level. Any value calculated with equal scales remains unchanged.
• ihixs.1.3: The PDF sets ABM11 and NNPDF21_as_00i (with i from 14 to 24) were added. Two more schemes for the treatment of the Higgs width were added (Running width scheme and Valencia-Willenbrock scheme). See the new default runcard on how to choose the new options.
• ihixs.1.2: bug related to invariant mass distribution histograms for certain higgs masses corrected. Those histograms also work with PDF error on now and in combination with all decays.
• ihixs.1.1: NNPDFs support added (only for one value of a_s, the one that is included in LHAPDF as of now - November 2011). To choose NNPDFs set pdf_provider = NNPDF . Bug (segmentation fault) appearing when compiling with gcc version 4.5.x corrected.
• ihixs.1.0: Initial release.

ihixs 1.* installation (legacy code)

The program uses the LHAPDF library to which it links dynamically. If you have LHAPDF installed globally, you can happily jump to step 6.

If you don't have the LHAPDF library installed globally in your system, you can install it locally (without root privileges) at a directory of your choice, denoted below by <path to LHAPDF directory>:

1. download the package from the LHAPDF web site.
2. untar it (tar -zxvf lhapdf-*.*.*.tar.gz) and do
   
   ./configure --prefix=<path to LHAPDF directory> --enable-low-memory
   make
   make install
   Note that the <path to LHAPDF directory> is an absolute path, not a relative one. See the LHAPDF installation guidelines for more details on how to install the LHAPDF library.
3. After having installed the library you have to tell the system where to find it, by fixing the LD_LIBARY_PATH
   UNIX/LINUX : export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:<path to LHAPDF directory>/lib
   MAC OSX : export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:<path to LHAPDF directory>/lib
   Placing this export command in your .bashrc (unix/linux) or .bash_profile (MAC OSX) will save you having to retype it after every logout.
4. Next, you have to fetch the actual pdf grids. In the directory of LHAPDF there is a bin/lhapdf-getdata executable, which can fetch the pdfs you need and place them in a local directory of your choice, here denoted by <path to pdf grids>.

   For example

   bin/lhapdf-getdata MSTW2008 --dest=<path to pdf grids>
   The full functionality of that script can be seen by
   bin/lhapdf-getdata --help
   See the LHAPDF installation guidelines for more details.

5. Finally you have to tell the system where to find the grids, by setting the environment variable
   export LHAPATH = <path to pdf grids>
   Placing this export command in your .bashrc (unix/linux) or .bash_profile (MAC OSX) will save you having to retype it after every logout.
6. Assuming that the LHAPDF library is available, the installation of iHixs proceeds simply through
   • If LHAPD is installed locally do
     configure LHAPDFDIR=<path to LHAPDF directory>/lib
     make
     in the main directory of iHixs.
   • If LHAPDF is installed globally you don't need to set the LHAPDFDIR variable, so just do
     configure
     make

We have checked that the program is compatible with the gcc compiler set, using gfortran on debian and mac OSX. Intel compilers will soon be supported as well.

ihixs 1.* documentation (legacy code)

Running the code

The various features of ihixs are controlled by an input runcard, an ascii file than is edited by the user. To run with a given runcard as input type in the installation directory:

In case no runcard is given, the program runs on the default card (called `runcard') in the installation directory. When no output filename is given, the program writes the output in runcard_name.out.

The output consists of the total cross sections per perturbative order in QCD, together with the corresponding Monte-Carlo errors achieved and the PDF errors. Those are set to zero if no PDF uncertainty is requested in the runcard. The input runcard is also appended.

Setting options and variables

In the runcard anything after a hash symbol,`#', is considered as a comment and is ignored. The following options are available:

• pdf_provider : sets the PDF grid used. The user can choose between MSTW08, ABKM09, JR09, NNPDF21 (since version 1.1), ABM11, NNPDF21_as_00i (with i from 14 to 24, denoting the fixed value of the strong coupling constant that was used in the fit) (since version 1.3), CT10 and NNPDF23 (since version 1.3.3). Within the MSTW PDFs there is also the option to switch confidence level from 68% to 90% and to use the MSTW grids with the strong coupling constant varied by one standard deviation from the best fit value. The exact filenames of the grids available are stated in the default runcard.
• effective_theory_flag : set to 0 for the exact LO and NLO QCD effects and HQET approximation for NNLO. Set to 1 for the improved HQET approximation through LO, NLO, NNLO.
• no_error_flag: Set to 0 to calculate with PDF uncertainty, set to 1 to calculate without PDF uncertainty.
• collider: Set to `LHC' or `TEVATRON'
• Etot: The total center of mass collider energy. This option is ignored if the collider chosen above is Tevatron.
• mhiggs: The nominal mass of the Higgs boson.
• higgs_width_scheme : Set to 0 for the default finite width scheme (naive Breit-Wigner distribution). Set to 1 for the Seymour scheme. Set to 2 for the running width scheme and set to 3 for the Valencia-Willenbrock scheme (Phys. Rev. D 46, 2247–2251). For a description of the other schemes see section 4 of 1202.3638.
• higgs_width_grid: = The path (absolute or relative to the run directory) of the file with the grid for the width of the Higgs, and the branching ratios to diphoton, WW, WW and bottom quarks as a function of the Higgs mass. If no path is set the default grid is used, HdecayGrid.dat, constructed with Hdecay v.3.532 with arguments that can be read in the header of the file. If the user supplies a grid file of his own, operating requirements are that the maximum number of grid points cannot exceed 16200, that the first three lines of the file are reserved for comments (so they are not read) and that the format of each line is respected, i.e. that the data is given in the order Higgs mass, Total width,BR to diphoton, BR to WW, BR to ZZ, BR to bottom quarks.
• min_mh : Setting a minimum in the invariant mass of the Higgs boson. This allows the user to study the total cross section in the presence of kinematical cuts.
• max_mh : Setting a maximum in the invariant mass of the Higgs boson.
• bin_flag : Set to 1 to produce files with the bin-integrated Higgs invariant mass. Set to 0 not to produce it. The data files produced contain the cross section per bin, with the bin size set to 1 GeV, from 30 to 2000 GeV at LO, NLO and NNLO. The files are named ` masshisto\m_H.\order', so e.g. for m_H=200GeV the NLO file will be ` masshisto200.1 '.
• muf/mhiggs : The ratio of the factorization scale to the Higgs mass.
• mur/mhiggs : The ratio of the renormalization scale to the Higgs mass.
• DecayMode: Set to no_width for the zero width approximation total cross section, to ` total' for finite width total cross section, or to the decay modes ` gamma gamma', ` ZZ', ` WW', ` b b-bar'.
• ProductionMode: Set to ` gg' for gluon fusion or to ` bb' for bottom quark annihilation.
• K_ewk: This is a global rescaling factor for all electroweak corrections. Set to 0.0 to switch them off.
• K_ewk_real: Set to 0.0 to switch the electroweak corrections to H+j off.
• K_ewk_real_b: Set to 0.0 to switch the electroweak corrections to H+j that include diagrams with massive quarks or Higgs boson in the loop, off.
• m_top: the mass of the top quark.
• Gamma_top: the width of the top quark.
• Y_top : rescaling factor for the SM Yukawa coupling of the top. Note that this can be set to an arbitrarily small positive value, but not to 0.0 exactly.
• scheme_top : 0: The MSbar mass is used in all places where the top mass enters. m_top from above is interpreted as the 'MSbar mass at the MSbar mass' (usually about 10 GeV smaller than the top pole mass). 1: The pole scheme is used, and m_top from above is not run.
• m_bot: the mass of the bottom quark.
• Gamma_bot: the width of the bottom quark.
• Y_bot: rescaling factor for the SM Yukawa coupling of the bottom quark.
• scheme_bottom : 0: The MSbar mass is used in all places where the bottom mass enters. m_bot from above is interpreted as the MSbar mass at 10 GeV. 1: The pole scheme is used, and m_bot from above is not run.
• heavy quark: Optional extra quarks in the model. The argument of this option should be formatted as m_Q:Width_Q:Y_Q:scheme where Y_Q is the rescaling factor from a SM-like Yukawa coupling m_Q/v. For example, adding an extra 300GeV quark with width 1.2GeV and a Yukawa coupling that is 5.7m_Q/v, with the mass treated in the on-shell scheme, the user should type:
  ` heavy quark = 300.0 : 1.2 : 5.7 : 1'
• m_Z: the mass of the Z boson.
• Gamma_Z: the width of the Z boson
• m_W: the mass of the W boson
• Gamma_W: the width of the W boson
• epsrel: Sets the relative Monte-Carlo integration error.
• epsabs: Sets the absolute Monte-Carlo integration error.
• nstart : Sets the number of points per Vegas iteration.
• nincrease: Set the number of points by which the number of points per iteration increases
• mineval: Set the minimum number of points before ending the Monte-Carlo integration
• maxeval: Set the maximum number of points after which the integration ends.
• adapt to central only: Set to 0 to force Vegas to adapt to all integrand. Set to 1 to adapt to the central integrand only. This is useful when running with PDF errors. Then each member of the PDF grid is treated as a separate integral. Adapting to the central only assumes that the peak structures of all integrals is similar which is a good approximation, and saves some CPU time.
• vegas_verbose: Set to 0 for silent Vegas output. Set to 2 to have information about each iteration printed in the standard output (the console).