<slha>...</slha> tags in the header of such 
files. When running Pythia without LHEF input (or if reading an LHEF 
file that does not contain SLHA information in the header), a separate 
file containing SLHA information may be specified using 
SLHA:file (see below). 
 
 
Normally the LHEF would be in uncompressed format, and thus human-readable 
if opened in a text editor. A possibility to read gzipped files has 
been added, based on the Boost and zlib libraries, which therefore 
have to be linked appropriately in order for this option to work. 
See the README file in the main directory for details 
on how to do this. 
 
 
Finally, the SLHA input capability can of course also be used to input 
SLHA-formatted MASS and DECAY tables for 
other particles, such as the Higgs boson, furnishing a less 
sophisticated but more universal complement to the 
standard PYTHIA 8-specific methods for inputting such information (for the 
latter, see the section on Particle Data 
and the scheme to modify it). This 
may at times not be desirable, so a few options can be used to curb the right 
of SLHA to overwrite particle data. 
Conversely, it is sometimes useful to allow the user to modify 
eg a mass parameter relative to its value in the SLHA spectrum. 
This is normally not permitted (the SLHA spectrum is normally self-consistent 
and should not be modified), but an option for allowing it is provided. 
 
 
The reading-in of information from SLHA or LHEF files is handled by the 
SusyLesHouches class, while the subsequent calculation of 
derived quantities of direct application to SUSY processes is done in the 
CoupSUSY, SigmaSUSY, 
and SUSYResonanceWidths classes. 
 
 
minMassSM, 
and SLHA:allowUserOverride) provide some safety against 
unintentionally overwriting PYTHIA's Standard-Model information. 
These parameters can be altered to hand over more or less control to the SLHA 
interface. In particular, 
a lot of mass and decay-table information may be included by default in some 
SLHA files, without it being the explicit intention of the user to overwrite 
the corresponding PYTHIA information. The default 
values of the SLHA safety parameters have been chosen so as to eliminate 
at least the most obvious causes of Garbage In Garbage Out. 
(E.g., there is usually no reason to modify the masses of well-measured SM 
particles, like the W and Z bosons, nor to replace their sophisticated 
internal decay treatments by the simplified isotropic treatment used for 
SLHA DECAY tables.) 
mMin and mMax cutoffs 
will normally be placed at 5 times the width or the on-shell mass divided by 
two, whichever is smaller, so that the default gives a decent sampling of the 
shape without straying too far from the on-shell value. The user is allowed to 
use the mMin and mMax parameters to choose a 
different sampling range if so desired (still subject to the constraint of at 
least one decay channel remaining open for on-shell decay products). 
SLHA:meMode. If the channel is off shell, then an 
meMode of 100 is always used. As a further protection against 
GIGO, if the channel appears to be physically impossible (defined as requiring 
fluctuations of more than more than 100 times the effective combined widths), 
it is switched of and a warning message is printed. 
mode   SLHA:readFrom   
 (default = 1; minimum = 0; maximum = 2)option  0 : is not read at all. Useful when SUSY is not simulated 
and normal particle properties should not be overwritten.   
option  1 : read in from the <slha>...</slha> 
block of a LHEF, if such a file is read during initialization, and else 
from the SLHA:file below.   
option  2 : read in from the SLHA:file below.   
   
 
word   SLHA:file   
 (default = void)void 
signals that no such file has been assigned. 
   
 
parm   SLHA:minMassSM   
 (default = 100.0)flag   SLHA:allowUserOverride   
 (default = off)off to preserve the 
internal self-consistency of SLHA spectra. If this flag is switched 
on, the mass values read from the SLHA block MASS are 
allowed to be modified by the user, using PYTHIA's standard 
readString and related methods. 
   
 
 
1→8 decays. 
 
flag   SLHA:useDecayTable   
 (default = on)DECAY tables or not. 
If this switch is set to off, PYTHIA will ignore any decay tables found 
in the SLHA file, and all decay widths will be calculated internally by 
PYTHIA. If switched on, SLHA decay tables will be read in, and will 
then supersede PYTHIA's internal calculations, with PYTHIA only 
computing the decays for particles for which no SLHA decay table is 
found. (To set a particle stable, you may either omit an SLHA 
DECAY table for it and then 
use PYTHIA's internal id:MayDecay switch for that 
particle, or you may include an SLHA DECAY table for it, 
with the width set explicitly to zero.) 
   
 
flag   SLHA:allowOnlyOffShell   
 (default = off)mode   SLHA:meMode   
 (default = 100; minimum = 100; maximum = 103)meMode = 100 and should be 
switched off by hand if so desired. 
It is up 
to the user to ensure that the final behaviour is consistent with what 
is desired (and/or to apply suitable post facto reweightings). 
Plotting the generator-level resonance and decay-product mass 
distributions (and e.g., mass differences), effective branching 
fractions, etc, may be of assistance to validate the behaviour of the 
program. 
   
 
 
mode   SLHA:verbose   
 (default = 1; minimum = 0; maximum = 3)flag   SLHA:NMSSM   
 (default = off) 
Using the QNUMBERS extension [Alw07], the SLHA 
can also be used to define new particles, with arbitrary quantum 
numbers. This already serves as a useful way to introduce new 
particles and can be combined with MASS and 
DECAY tables in the usual 
way, to generate isotropically distributed decays or even chains of 
such decays. (If you want something better than isotropic, sorry, you'll 
have to do some actual work ...) 
A more advanced further option is to make use of the possibility in the SLHA to include user-defined blocks with arbitrary names and contents. Obviously, standalone PYTHIA 8 does not know what to do with such information. However, it does not throw it away either, but instead stores the contents of user blocks as strings, which can be read back later, with the user having full control over the format used to read the individual entries.
 
The contents of both standard and user-defined SLHA blocks can be accessed 
in any class inheriting from PYTHIA 8's SigmaProcess 
class (i.e., in particular, from any semi-internal process written by 
a user), through its SLHA pointer, slhaPtr, by using the 
following methods: 
bool slhaPtr->getEntry(string blockName, double& val); bool slhaPtr->getEntry(string blockName, int indx, double& val); bool slhaPtr->getEntry(string blockName, int indx, int jndx, double& val); bool slhaPtr->getEntry(string blockName, int indx, int jndx, int kndx, double& val);
 
This particular example assumes that the user wants to read the 
entries (without index, indexed, matrix-indexed, or 3-tensor-indexed, 
respectively) in the user-defined block blockName, 
and that it should be interpreted as 
a double. The last argument is templated, and hence if 
anything other than a double is desired to be read, the 
user has only to give the last argument a different type. 
If anything went wrong (i.e., the block doesn't 
exist, or it doesn't have an entry with that index, or that entry 
can't be read as a double), the method returns false; true 
otherwise. This effectively allows to input completely arbitrary 
parameters using the SLHA machinery, with the user having full control 
over names and conventions. Of course, it is then the user's 
responsibility to ensure complete consistency between the names and 
conventions used in the SLHA input, and those assumed in any 
user-written semi-internal process code. 
 
Note that PYTHIA 8 always initializes at least 
the SLHA blocks MASS and SMINPUTS, starting from its internal 
SM parameters and particle data table values (updated to take into 
account user modifications). These blocks can therefore be accessed 
using the slhaPtr->getEntry() methods even in the absence 
of SLHA input. 
Note: in the SMINPUTS block, PYTHIA outputs physically correct 
(i.e., measured) values of GF, m_Z, and 
alpha_EM(m_Z). However, if one attempts to compute, e.g., 
the W mass, at one loop from these quantities, a value of 79 GeV results, 
with a corresponding value for the weak mixing angle. We advise to 
instead take the physically measured W mass from block MASS, and 
recompute the EW parameters as best suited for the application at hand.