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2<head>
3<title>SUSY Les Houches Accord</title>
4<link rel="stylesheet" type="text/css" href="pythia.css"/>
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6</head>
7<body>
8
9<h2>SUSY Les Houches Accord</h2>
10
11The PYTHIA 8 program does not contain an internal spectrum calculator
12(a.k.a. RGE package) to provide supersymmetric couplings, mixing angles,
13masses and branching ratios. Thus the SUSY Les Houches Accord (SLHA)
14[<a href="Bibliography.html" target="page">Ska04</a>][<a href="Bibliography.html" target="page">All08</a>] is the only way of
15inputting SUSY models, and SUSY processes (see
16the <a href="SUSYProcesses.html" target="page">SUSYProcesses</a> page)
17cannot be run unless such an input has taken place.
18
19<p/>
20The SLHA input format can also be extended for use with more general BSM
21models, beyond SUSY. Information specific to  how to use the SLHA
22interface for generic BSM models is collected below,
23under <a href="#generic">Using SLHA for generic BSM Models</a>, with
24more elaborate explanations and examples in [<a href="Bibliography.html" target="page">Des11</a>].
25
26<p/>
27Most of the SUSY implementation in PYTHIA 8 is compatible with both the
28SLHA1 [<a href="Bibliography.html" target="page">Ska04</a>] and SLHA2 [<a href="Bibliography.html" target="page">All08</a>]
29conventions (with some limitations for the NMSSM
30in the latter case). Internally, PYTHIA 8 uses the
31SLHA2 conventions and translates SLHA1 input to these when necessary.
32See the section on SUSY Processes and [<a href="Bibliography.html" target="page">Des11</a>] for more
33information.
34
35<p/>
36When reading LHEF files, Pythia automatically looks for SLHA information
37between <code>&lt;slha&gt;...&lt;/slha&gt;</code> tags in the header of such
38files. When running Pythia without LHEF input (or if reading an LHEF
39file that does not contain SLHA information in the header), a separate
40file containing SLHA information may be specified using
41<code>SLHA:file</code> (see below).
42
43<p/>
44Normally the LHEF would be in uncompressed format, and thus human-readable
45if opened in a text editor. A possibility to read gzipped files has
46been added, based on the Boost and zlib libraries, which therefore
47have to be linked appropriately in order for this option to work.
48See the <code>README</code> file in the main directory for details
49on how to do this.
50
51<p/>
52Finally, the SLHA input capability can of course also be used to input
53SLHA-formatted <code>MASS</code> and <code>DECAY</code> tables for
54other particles, such as the Higgs boson, furnishing a less
55sophisticated but more universal complement to the
56standard PYTHIA 8-specific methods for inputting such information (for the
57latter, see the section on <a href="ParticleData.html" target="page">Particle Data</a>
58and the <a href="ParticleDataScheme.html" target="page">scheme</a> to modify it). This
59may at times not be desirable, so a few options can be used to curb the right
60of SLHA to overwrite particle data.
61
62<p/>
63The reading-in of information from SLHA or LHEF files is handled by the
64<code>SusyLesHouches</code> class, while the subsequent calculation of
65derived quantities of direct application to SUSY processes is done in the
66<code>CoupSUSY</code>, <code>SigmaSUSY</code>,
67and <code>SUSYResonanceWidths</code> classes.
68
69<h3>SLHA Switches and Parameters</h3>
70
71<p/><code>mode&nbsp; </code><strong> SLHA:readFrom &nbsp;</strong> 
72 (<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 2</code>)<br/>
73Controls from where SLHA information is read.
74<br/><code>option </code><strong> 0</strong> : is not read at all. Useful when SUSY is not simulated
75and normal particle properties should not be overwritten. 
76<br/><code>option </code><strong> 1</strong> : read in from the <code>&lt;slha&gt;...&lt;/slha&gt;</code> 
77block of a LHEF, if such a file is read during initialization, and else
78from the <code>SLHA:file</code> below. 
79<br/><code>option </code><strong> 2</strong> : read in from the <code>SLHA:file</code> below. 
80 
81
82<p/><code>word&nbsp; </code><strong> SLHA:file &nbsp;</strong> 
83 (<code>default = <strong>void</strong></code>)<br/>
84Name of an SLHA (or LHEF) file containing the SUSY/BSM model definition,
85spectra, and (optionally) decay tables. Default <code>void</code>
86signals that no such file has been assigned.
87 
88
89<p/><code>flag&nbsp; </code><strong> SLHA:keepSM &nbsp;</strong> 
90 (<code>default = <strong>on</strong></code>)<br/>
91Some programs write SLHA output also for SM particles where normally
92one would not want to have masses and decay modes changed unwittingly.
93Therefore, by default, known SM particles are ignored in SLHA files.
94To be more specific, particle data for identity codes in the ranges
951 - 24 and 81 - 999,999 are ignored. Notably this includes <i>Z^0</i>,
96<i>W^+-</i> and <i>t</i>. The SM Higgs is modified by the SLHA input,
97as is other codes in the range 25 - 80 and 1,000,000 - . If you
98switch off this flag then also SM particles are modified by SLHA input.
99 
100
101<p/><code>parm&nbsp; </code><strong> SLHA:minMassSM &nbsp;</strong> 
102 (<code>default = <strong>100.0</strong></code>)<br/>
103This parameter provides an alternative possibility to ignore SLHA input
104for all particles with identity codes below 1,000,000 (which mainly
105means SM particle, but also includes e.g. the Higgses in
106two-Higgs-doublet scenarios) whose default masses in PYTHIA lie below
107some threshold value, given by this parameter. The default value of
108100.0 allows SLHA input to modify the top quark, but not, e.g., the
109<i>Z^0</i> and <i>W^+-</i> bosons.
110 
111
112<h3>SLHA DECAY Tables</h3>
113
114<p/><code>flag&nbsp; </code><strong> SLHA:useDecayTable &nbsp;</strong> 
115 (<code>default = <strong>on</strong></code>)<br/>
116Switch to choose whether to read in SLHA <code>DECAY</code> tables or not.
117If this switch is set to off, PYTHIA will ignore any decay tables found
118in the SLHA file, and all decay widths will be calculated internally by
119PYTHIA. If switched on, SLHA decay tables will be read in, and will
120then supersede PYTHIA's internal calculations, with PYTHIA only
121computing the decays for particles for which no SLHA decay table is
122found. (To set a particle stable, you may either omit an SLHA
123<code>DECAY</code> table for it and then 
124use PYTHIA's internal <code>id:MayDecay</code> switch for that
125particle, or you may include an SLHA <code>DECAY</code> table for it,
126with the width set explicitly to zero.)
127 
128
129<p/><code>parm&nbsp; </code><strong> SLHA:minDecayDeltaM &nbsp;</strong> 
130 (<code>default = <strong>1.0</strong></code>)<br/>
131This parameter sets the smallest allowed mass difference (in GeV,
132between the mass of the mother and the sum of the daughter masses)
133for a decay mode in a DECAY table to be switched on inside PYTHIA. The
134default is to require at least 1 GeV of open phase space, but this can
135be reduced (at the user's risk) for instance to be able to treat
136decays in  models with very small mass splittings.
137 
138
139<h3>Internal SLHA Variables</h3>
140
141<p/><code>mode&nbsp; </code><strong> SLHA:verbose &nbsp;</strong> 
142 (<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 3</code>)<br/>
143Controls amount of text output written by the SLHA interface, with a
144value of 0 corresponding to the most quiet mode.
145 
146
147The following variables are used internally by PYTHIA as local copies
148of SLHA information. User changes will generally have no effect, since
149these variables will be reset by the SLHA reader during initialization.
150
151<p/><code>flag&nbsp; </code><strong> SLHA:NMSSM &nbsp;</strong> 
152 (<code>default = <strong>off</strong></code>)<br/>
153Corresponds to SLHA block MODSEL entry 3.
154 
155
156<a name="generic"></a>
157<h2>Using SLHA for generic BSM Models</h2>
158
159</p>
160Using the <code>QNUMBERS</code> extension [<a href="Bibliography.html" target="page">Alw07</a>], the SLHA
161can also be used to define new particles, with arbitrary quantum
162numbers. This already serves as a useful way to introduce new
163particles and can be combined with <code>MASS</code> and
164<code>DECAY</code> tables in the usual
165way, to generate isotropically distributed decays or even chains of
166such decays. (If you want something better than isotropic, sorry, you'll
167have to do some actual work ...)
168</p>
169
170</p>
171A more advanced further option is to make use of the possibility
172in the SLHA to include user-defined blocks with arbitrary
173names and contents. Obviously, standalone
174PYTHIA 8 does not know what to do with such information. However, it
175does not throw it away either, but instead stores the contents of user
176blocks as strings, which can be read back later, with the user
177having full control over the format used to read the individual entries.
178</p>
179
180<p>
181The contents of both standard and user-defined SLHA blocks can be accessed
182in any class inheriting from PYTHIA 8's <code>SigmaProcess</code>
183class (i.e., in particular, from any semi-internal process written by
184a user), through its SLHA pointer, <code>slhaPtr</code>, by using the
185following methods:
186<a name="method1"></a>
187<p/><strong> &nbsp;</strong> <br/>
188  bool slhaPtr->getEntry(string blockName, double& val);
189 
190<strong> &nbsp;</strong> <br/>
191  bool slhaPtr->getEntry(string blockName, int indx, double& val);
192 
193<strong> &nbsp;</strong> <br/>
194  bool slhaPtr->getEntry(string blockName, int indx, int jndx, double& val);
195 
196<strong> &nbsp;</strong> <br/>
197  bool slhaPtr->getEntry(string blockName, int indx, int jndx, int
198  kndx, double& val);
199 
200</p>
201
202<p>
203This particular example assumes that the user wants to read the
204entries (without index, indexed, matrix-indexed, or 3-tensor-indexed,
205respectively) in the user-defined block <code>blockName</code>,
206and that it should be interpreted as
207a <code>double</code>. The last argument is templated, and hence if
208anything other than a <code>double</code> is desired to be read, the
209user has only to give the last argument a different type.
210If anything went wrong (i.e., the block doesn't
211exist, or it doesn't have an entry with that index, or that entry
212can't be read as a double), the method returns false; true
213otherwise. This effectively allows to input completely arbitrary
214parameters using the SLHA machinery, with the user having full control
215over names and conventions. Of course, it is then the user's
216responsibility to ensure complete consistency between the names and
217conventions used in the SLHA input, and those assumed in any
218user-written semi-internal process code.
219</p>
220
221<p>
222Note that PYTHIA 8 always initializes at least
223the SLHA blocks MASS and SMINPUTS, starting from its internal
224SM parameters and particle data table values (updated to take into
225account user modifications). These blocks can therefore be accessed
226using the <code>slhaPtr->getEntry()</code> methods even in the absence
227of SLHA input.
228Note: in the SMINPUTS block, PYTHIA outputs physically correct
229(i.e., measured) values of <i>GF</i>, <i>m_Z</i>, and
230<i>alpha_EM(m_Z)</i>. However, if one attempts to compute, e.g.,
231the W mass, at one loop from these quantities, a value of 79 GeV results,
232with a corresponding value for the weak mixing angle. We advise to
233instead take the physically measured W mass from block MASS, and
234recompute the EW parameters as best suited for the application at hand.
235</p>
236
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