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<title>SUSY Les Houches Accord</title>
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<h2>SUSY Les Houches Accord</h2>
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The PYTHIA 8 program does not contain an internal spectrum calculator
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(a.k.a. RGE package) to provide supersymmetric couplings, mixing angles,
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masses and branching ratios. Thus the SUSY Les Houches Accord (SLHA)
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[<a href="Bibliography.php" target="page">Ska04</a>][<a href="Bibliography.php" target="page">All08</a>] is the only way of
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inputting SUSY models, and SUSY processes (see
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the <?php $filepath = $_GET["filepath"];
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echo "<a href='SUSYProcesses.php?filepath=".$filepath."' target='page'>";?>SUSYProcesses</a> page)
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cannot be run unless such an input has taken place.
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The SLHA input format can also be extended for use with more general BSM
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models, beyond SUSY. Information specific to how to use the SLHA
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interface for generic BSM models is collected below,
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under <a href="#generic">Using SLHA for generic BSM Models</a>, with
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more elaborate explanations and examples in [<a href="Bibliography.php" target="page">Des11</a>].
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Most of the SUSY implementation in PYTHIA 8 is compatible with both the
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SLHA1 [<a href="Bibliography.php" target="page">Ska04</a>] and SLHA2 [<a href="Bibliography.php" target="page">All08</a>]
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conventions (with some limitations for the NMSSM
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in the latter case). Internally, PYTHIA 8 uses the
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SLHA2 conventions and translates SLHA1 input to these when necessary.
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See the section on SUSY Processes and [<a href="Bibliography.php" target="page">Des11</a>] for more
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When reading LHEF files, Pythia automatically looks for SLHA information
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between <code><slha>...</slha></code> tags in the header of such
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files. When running Pythia without LHEF input (or if reading an LHEF
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file that does not contain SLHA information in the header), a separate
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file containing SLHA information may be specified using
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<code>SLHA:file</code> (see below).
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Normally the LHEF would be in uncompressed format, and thus human-readable
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if opened in a text editor. A possibility to read gzipped files has
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been added, based on the Boost and zlib libraries, which therefore
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have to be linked appropriately in order for this option to work.
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See the <code>README</code> file in the main directory for details
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Finally, the SLHA input capability can of course also be used to input
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SLHA-formatted <code>MASS</code> and <code>DECAY</code> tables for
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other particles, such as the Higgs boson, furnishing a less
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sophisticated but more universal complement to the
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standard PYTHIA 8-specific methods for inputting such information (for the
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latter, see the section on <?php $filepath = $_GET["filepath"];
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echo "<a href='ParticleData.php?filepath=".$filepath."' target='page'>";?>Particle Data</a>
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and the <?php $filepath = $_GET["filepath"];
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echo "<a href='ParticleDataScheme.php?filepath=".$filepath."' target='page'>";?>scheme</a> to modify it). This
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may at times not be desirable, so a few options can be used to curb the right
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of SLHA to overwrite particle data.
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The reading-in of information from SLHA or LHEF files is handled by the
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<code>SusyLesHouches</code> class, while the subsequent calculation of
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derived quantities of direct application to SUSY processes is done in the
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<code>CoupSUSY</code>, <code>SigmaSUSY</code>,
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and <code>SUSYResonanceWidths</code> classes.
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<h3>SLHA Switches and Parameters</h3>
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<p/><code>mode </code><strong> SLHA:readFrom </strong>
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(<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 2</code>)<br/>
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Controls from where SLHA information is read.
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<br/><code>option </code><strong> 0</strong> : is not read at all. Useful when SUSY is not simulated
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and normal particle properties should not be overwritten.
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<br/><code>option </code><strong> 1</strong> : read in from the <code><slha>...</slha></code>
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block of a LHEF, if such a file is read during initialization, and else
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from the <code>SLHA:file</code> below.
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<br/><code>option </code><strong> 2</strong> : read in from the <code>SLHA:file</code> below.
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<br/><br/><table><tr><td><strong>SLHA:file </td><td></td><td> <input type="text" name="1" value="void" size="20"/> (<code>default = <strong>void</strong></code>)</td></tr></table>
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Name of an SLHA (or LHEF) file containing the SUSY/BSM model definition,
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spectra, and (optionally) decay tables. Default <code>void</code>
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signals that no such file has been assigned.
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<br/><br/><strong>SLHA:keepSM</strong> <input type="radio" name="2" value="on" checked="checked"><strong>On</strong>
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<input type="radio" name="2" value="off"><strong>Off</strong>
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(<code>default = <strong>on</strong></code>)<br/>
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Some programs write SLHA output also for SM particles where normally
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one would not want to have masses and decay modes changed unwittingly.
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Therefore, by default, known SM particles are ignored in SLHA files.
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To be more specific, particle data for identity codes in the ranges
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1 - 24 and 81 - 999,999 are ignored. Notably this includes <i>Z^0</i>,
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<i>W^+-</i> and <i>t</i>. The SM Higgs is modified by the SLHA input,
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as is other codes in the range 25 - 80 and 1,000,000 - . If you
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switch off this flag then also SM particles are modified by SLHA input.
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<br/><br/><table><tr><td><strong>SLHA:minMassSM </td><td></td><td> <input type="text" name="3" value="100.0" size="20"/> (<code>default = <strong>100.0</strong></code>)</td></tr></table>
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This parameter provides an alternative possibility to ignore SLHA input
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for all particles with identity codes below 1,000,000 (which mainly
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means SM particle, but also includes e.g. the Higgses in
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two-Higgs-doublet scenarios) whose default masses in PYTHIA lie below
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some threshold value, given by this parameter. The default value of
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100.0 allows SLHA input to modify the top quark, but not, e.g., the
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<i>Z^0</i> and <i>W^+-</i> bosons.
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<h3>SLHA DECAY Tables</h3>
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<br/><br/><strong>SLHA:useDecayTable</strong> <input type="radio" name="4" value="on" checked="checked"><strong>On</strong>
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<input type="radio" name="4" value="off"><strong>Off</strong>
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(<code>default = <strong>on</strong></code>)<br/>
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Switch to choose whether to read in SLHA <code>DECAY</code> tables or not.
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If this switch is set to off, PYTHIA will ignore any decay tables found
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in the SLHA file, and all decay widths will be calculated internally by
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PYTHIA. If switched on, SLHA decay tables will be read in, and will
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then supersede PYTHIA's internal calculations, with PYTHIA only
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computing the decays for particles for which no SLHA decay table is
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found. (To set a particle stable, you may either omit an SLHA
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<code>DECAY</code> table for it and then
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use PYTHIA's internal <code>id:MayDecay</code> switch for that
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particle, or you may include an SLHA <code>DECAY</code> table for it,
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with the width set explicitly to zero.)
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<br/><br/><table><tr><td><strong>SLHA:minDecayDeltaM </td><td></td><td> <input type="text" name="5" value="1.0" size="20"/> (<code>default = <strong>1.0</strong></code>)</td></tr></table>
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This parameter sets the smallest allowed mass difference (in GeV,
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between the mass of the mother and the sum of the daughter masses)
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for a decay mode in a DECAY table to be switched on inside PYTHIA. The
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default is to require at least 1 GeV of open phase space, but this can
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be reduced (at the user's risk) for instance to be able to treat
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decays in models with very small mass splittings.
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<h3>Internal SLHA Variables</h3>
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<p/><code>mode </code><strong> SLHA:verbose </strong>
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(<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 3</code>)<br/>
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Controls amount of text output written by the SLHA interface, with a
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value of 0 corresponding to the most quiet mode.
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The following variables are used internally by PYTHIA as local copies
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of SLHA information. User changes will generally have no effect, since
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these variables will be reset by the SLHA reader during initialization.
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<br/><br/><strong>SLHA:NMSSM</strong> <input type="radio" name="6" value="on"><strong>On</strong>
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<input type="radio" name="6" value="off" checked="checked"><strong>Off</strong>
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(<code>default = <strong>off</strong></code>)<br/>
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Corresponds to SLHA block MODSEL entry 3.
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<a name="generic"></a>
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<h2>Using SLHA for generic BSM Models</h2>
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Using the <code>QNUMBERS</code> extension [<a href="Bibliography.php" target="page">Alw07</a>], the SLHA
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can also be used to define new particles, with arbitrary quantum
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numbers. This already serves as a useful way to introduce new
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particles and can be combined with <code>MASS</code> and
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<code>DECAY</code> tables in the usual
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way, to generate isotropically distributed decays or even chains of
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such decays. (If you want something better than isotropic, sorry, you'll
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have to do some actual work ...)
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A more advanced further option is to make use of the possibility
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in the SLHA to include user-defined blocks with arbitrary
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names and contents. Obviously, standalone
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PYTHIA 8 does not know what to do with such information. However, it
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does not throw it away either, but instead stores the contents of user
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blocks as strings, which can be read back later, with the user
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having full control over the format used to read the individual entries.
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The contents of both standard and user-defined SLHA blocks can be accessed
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in any class inheriting from PYTHIA 8's <code>SigmaProcess</code>
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class (i.e., in particular, from any semi-internal process written by
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a user), through its SLHA pointer, <code>slhaPtr</code>, by using the
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<a name="method1"></a>
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<p/><strong> </strong> <br/>
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bool slhaPtr->getEntry(string blockName, double& val);
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<strong> </strong> <br/>
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bool slhaPtr->getEntry(string blockName, int indx, double& val);
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<strong> </strong> <br/>
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bool slhaPtr->getEntry(string blockName, int indx, int jndx, double& val);
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<strong> </strong> <br/>
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bool slhaPtr->getEntry(string blockName, int indx, int jndx, int
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This particular example assumes that the user wants to read the
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entries (without index, indexed, matrix-indexed, or 3-tensor-indexed,
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respectively) in the user-defined block <code>blockName</code>,
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and that it should be interpreted as
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a <code>double</code>. The last argument is templated, and hence if
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anything other than a <code>double</code> is desired to be read, the
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user has only to give the last argument a different type.
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If anything went wrong (i.e., the block doesn't
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exist, or it doesn't have an entry with that index, or that entry
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can't be read as a double), the method returns false; true
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otherwise. This effectively allows to input completely arbitrary
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parameters using the SLHA machinery, with the user having full control
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over names and conventions. Of course, it is then the user's
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responsibility to ensure complete consistency between the names and
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conventions used in the SLHA input, and those assumed in any
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user-written semi-internal process code.
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Note that PYTHIA 8 always initializes at least
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the SLHA blocks MASS and SMINPUTS, starting from its internal
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SM parameters and particle data table values (updated to take into
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account user modifications). These blocks can therefore be accessed
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using the <code>slhaPtr->getEntry()</code> methods even in the absence
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Note: in the SMINPUTS block, PYTHIA outputs physically correct
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(i.e., measured) values of <i>GF</i>, <i>m_Z</i>, and
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<i>alpha_EM(m_Z)</i>. However, if one attempts to compute, e.g.,
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the W mass, at one loop from these quantities, a value of 79 GeV results,
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with a corresponding value for the weak mixing angle. We advise to
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instead take the physically measured W mass from block MASS, and
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recompute the EW parameters as best suited for the application at hand.
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<input type="hidden" name="saved" value="1"/>
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<table width="100%"><tr><td align="right"><input type="submit" value="Save Settings" /></td></tr></table>
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$handle = fopen($filepath, 'a');
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if($_POST["1"] != "void")
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$data = "SLHA:file = ".$_POST["1"]."\n";
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if($_POST["2"] != "on")
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$data = "SLHA:keepSM = ".$_POST["2"]."\n";
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fwrite($handle,$data);
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if($_POST["3"] != "100.0")
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$data = "SLHA:minMassSM = ".$_POST["3"]."\n";
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fwrite($handle,$data);
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if($_POST["4"] != "on")
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$data = "SLHA:useDecayTable = ".$_POST["4"]."\n";
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fwrite($handle,$data);
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if($_POST["5"] != "1.0")
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$data = "SLHA:minDecayDeltaM = ".$_POST["5"]."\n";
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fwrite($handle,$data);
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if($_POST["6"] != "off")
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$data = "SLHA:NMSSM = ".$_POST["6"]."\n";
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fwrite($handle,$data);
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