Les Houches Accord
 
  - Initialization
- Event input
- A LHEF reader class
- A runtime Fortran interface
- A LHEF writer class
The Les Houches Accord (LHA) for user processes [Boo01] is the 
standard way to input parton-level information from a 
matrix-elements-based generator into PYTHIA. The conventions for 
which information should be stored has been defined in a Fortran context, 
as two commonblocks. Here a C++ equivalent is defined, as a single class. 
The most common application is to read input from a Les Houches Event File 
(LHEF) [Alw06], but it is also possible to have a runtime 
interface to another program. This page will discuss runtime interface. For 
LHEF handling, please refer to 
Les Houches Event Files. 
 
 
TheLHAup class is a base class, containing reading and 
printout functions, plus two pure virtual functions, one to set 
initialization information and one to set information on each new event. 
Derived classes have to provide these two virtual functions to do 
the actual work. The existing derived classes are for reading information 
from a Les Houches Event File, from the respective Fortran 
commonblocks, or from PYTHIA 8 itself. 
 
 
You are free to write your own derived classes, using the rules and 
methods to be described below. Normally, pointers to objects of such 
derived classes should be handed in with the 
Pythia::setLHAupPtr( LHAup*) 
method. However, with the LHEF format a filename can replace the 
pointer, see further below. 
 
 
Let us now describe the methods at your disposal to do the job. 
 
 LHAup::LHAup( int strategy = 3)   
the base class constructor takes the choice of mixing/weighting 
strategy as optional input argument, and calls setStrategy, 
see below. It also reserves some space for processes and particles. 
   
 
 virtual LHAup::~LHAup()   
the destructor does not need to do anything. 
   
 
 void LHAup::setPtr(Info* infoPtr)   
this method only sets the pointer that allows some information 
to be accessed, and is automatically called by 
Pythia::init(). 
   
 
 
Initialization
 
 
The LHAup class stores information equivalent to the 
/HEPRUP/ commonblock, as required to initialize the event 
generation chain. The main difference is that the vector container 
now allows a flexible number of subprocesses to be defined. For the 
rest, names have been modified, since the 6-character-limit does not 
apply, and variables have been regrouped for clarity, but nothing 
fundamental is changed. 
 
 virtual bool LHAup::setInit()   
this pure virtual method has to be implemented in the derived class, 
to set relevant information when called. It should return false if it 
fails to set the info. In the no-beams extension this method need not 
do anything, since by default strategy 3 is chosen and the rest is set 
vanishing, but the method must exist. 
   
 
 
Inside setInit(), such information can be set by the following 
methods: 
 void LHAup::setBeamA( int identity, double energy, int pdfGroup = 0, int pdfSet = 0)   
   
 void LHAup::setBeamB( int identity, double energy, int pdfGroup = 0, int pdfSet = 0)   
sets the properties of the first and second incoming beam, respectively 
(cf. the Fortran IDBMUP(1), EBMUP(i), PDFGUP(i), PDFSUP(i), 
with i 1 or 2). These numbers can be used to tell which PDF 
sets were used when the hard process was generated, while the normal 
PDF Selection is used for the further 
event generation in PYTHIA. 
   
 
 void LHAup::setStrategy( int strategy)   
sets the event weighting and cross section strategy. The default, 
provided in the class constructor, is 3, which is the natural value 
e.g. for an LHEF. 
argument strategy   :  
chosen strategy (cf. IDWTUP; see [Sjo06] 
section 9.9.1 for extensive comments). 
argumentoption  1 :  events come with non-negative weight, given in units 
of pb, with an average that converges towards the cross section of the 
process. PYTHIA is in charge of the event mixing, i.e. for each new 
try decides which process should be generated, and then decides whether 
is should be kept, based on a comparison with xMax. 
Accepted events therefore have unit weight.   
argumentoption  -1 :  as option 1, except that cross sections can now be 
negative and events after unweighting have weight +-1. You can use 
Info::weight() 
to find the weight of the current event. A correct event mixing requires 
that a process that can take both signs should be split in two, one limited 
to positive or zero and the other to negative or zero values, with 
xMax chosen appropriately for the two.   
argumentoption  2 :  events come with non-negative weight, in unspecified 
units, but such that xMax can be used to unweight the events 
to unit weight. Again PYTHIA is in charge of the event mixing. 
The total cross section of a process is stored in 
xSec.   
argumentoption  -2 :  as option 2, except that cross sections can now be 
negative and events after unweighting have weight +-1. As for option -1 
processes with indeterminate sign should be split in two.   
argumentoption  3 :  events come with unit weight, and are thus accepted 
as is. The total cross section of the process is stored in 
xSec.   
argumentoption  -3 :  as option 3, except that events now come with weight 
+-1. Unlike options -1 and -2 processes with indeterminate sign need not be 
split in two, unless you intend to mix with internal PYTHIA processes 
(see below).   
argumentoption  4 :  events come with non-negative weight, given in units 
of pb, with an average that converges towards the cross section of the 
process, like for option 1. No attempt is made to unweight the events, 
however, but all are generated in full, and retain their original weight. 
For consistency with normal PYTHIA units, the weight stored in 
Info::weight() has been converted to mb, however. 
   
argumentoption  -4 :  as option 4, except that events now can come 
either with positive or negative weights.   
Note 1: if several processes have already been mixed and 
stored in a common event file, either LHEF or some private format, it 
would be problematical to read back events in a different order. Since it 
is then not feasible to let PYTHIA pick the next process type, strategies 
+-1 and +-2 would not work. Instead strategy 3 would be the recommended 
choice, or -3 if negative-weight events are required. 
Note 2: it is possible to switch on internally implemented 
processes and have PYTHIA mix these with LHA ones according to their relative 
cross sections for strategies +-1, +-2 and 3. It does not work for strategy 
-3 unless the positive and negative sectors of the cross sections are in 
separate subprocesses (as must always be the case for -1 and -2), since 
otherwise the overall mixture of PYTHIA and LHA processes will be off. 
Mixing is not possible for strategies +-4, since the weighting procedure 
is not specified by the standard. (For instance, the intention may be to 
have events biased towards larger pT values in some particular 
functional form.) 
   
   
 
 void LHAup::addProcess( int idProcess, double xSec, double xErr, double xMax)   
sets info on an allowed process (cf. LPRUP, XSECUP, XERRUP, 
XMAXUP). 
Each new call will append one more entry to the list of processes. 
The choice of strategy determines which quantities are mandatory: 
xSec for strategies +-2 and +-3, 
xErr never, and 
xMax for strategies +-1 and +-2. 
   
 
Note: PYTHIA does not make active use of the (optional) 
xErr values, but calculates a statistical cross section 
error based on the spread of event-to-event weights. This should work 
fine for strategy options +-1, but not for the others. Specifically, 
for options +-2 and +-3 the weight spread may well vanish, and anyway 
is likely to be an underestimate of the true error. If the author of the 
LHA input information does provide error information you may use that - 
this information is displayed at initialization. If not, then a relative 
error decreasing like 1/sqrt(n_acc), where n_acc 
is the number of accepted events, should offer a reasonable estimate. 
 
 void LHAup::setXSec( int i, double xSec)   
update the xSec value of the i'th process 
added with addProcess method (i.e. i runs 
from 0 through sizeProc() - 1, see below). 
   
 
 void LHAup::setXErr( int i, double xErr)   
update the xErr value of the i'th process 
added with addProcess method. 
   
 
 void LHAup::setXMax( int i, double xMax)   
update the xMax value of the i'th process 
added with addProcess method. 
   
 
 void LHAup::setInfoHeader(string &key, string &val)   
set the header key to have value val. 
This is a wrapper function to the 
Info::setHeader function that 
should be used in any classes derived from LHAup. 
   
 
 
Information is handed back by the following methods 
(that normally you would not need to touch): 
 int LHAup::idBeamA()   
   
 int LHAup::idBeamB()   
   
 double LHAup::eBeamA()   
   
 double LHAup::eBeamB()   
   
 int LHAup::pdfGroupBeamA()   
   
 int LHAup::pdfGroupBeamB()   
   
 int LHAup::pdfSetBeamA()   
   
 int LHAup::pdfSetBeamB()   
for the beam properties. 
   
 int LHAup::strategy()   
for the strategy choice. 
   
 int LHAup::sizeProc()   
for the number of subprocesses. 
   
 int LHAup::idProcess(int i)   
   
 double LHAup::xSec(int i)   
   
 double LHAup::xErr(int i)   
   
 double LHAup::xMax(int i)   
for process i in the range 0 <= i < 
sizeProc(). 
   
 double LHAup::xSecSum()   
 double LHAup::xErrSum()   
the sum of the cross sections and errors (the latter added quadratically). 
Note that cross section errors are only meaningful for strategies +-3. 
   
 
 void LHAup::listInit()   
prints the above initialization information. This method is 
automatically called from Pythia::init(), 
so would normally not need to be called directly by the user. 
   
 
 
 
 
 
Event input
 
 
The LHAup class also stores information equivalent to the 
/HEPEUP/ commonblock, as required to hand in the next 
parton-level configuration for complete event generation. The main 
difference is that the vector container now allows a flexible number 
of partons to be defined. For the rest, names have been modified, 
since the 6-character-limit does not apply, and variables have been 
regrouped for clarity, but nothing fundamental is changed. 
 
 
The LHA standard is based on Fortran arrays beginning with 
index 1, and mother information is defined accordingly. In order to 
be compatible with this convention, the zeroth line of the C++ particle 
array is kept empty, so that index 1 also here corresponds to the first 
particle. One small incompatibility is that the sizePart() 
method returns the full size of the particle array, including the 
empty zeroth line, and thus is one larger than the true number of 
particles (NUP). 
 
 virtual bool LHAup::setEvent(int idProcess = 0)   
this pure virtual method has to be implemented in the derived class, 
to set relevant information when called. For strategy options +-1 
and +-2 the input idProcess value specifies which process 
that should be generated, while idProcess is irrelevant 
for strategies +-3 and +-4. The method should return 
false if it fails to set the info, i.e. normally that the supply of 
events in a file is exhausted. If so, no event is generated, and 
Pythia::next() returns false. You can then interrogate 
Info::atEndOfFile() 
to confirm that indeed the failure is caused in this method, and decide 
to break out of the event generation loop. 
   
 
 
Inside a normal setEvent(...) call, information can be set 
by the following methods: 
 void LHAup::setProcess( int idProcess, double weight, double scale, double alphaQED, double alphaQCD)   
tells which kind of process occurred, with what weight, at what scale, 
and which alpha_EM and alpha_strong were used 
(cf. IDPRUP, XWTGUP, SCALUP, AQEDUP, AQCDUP). This method 
also resets the size of the particle list, and adds the empty zeroth 
line, so it has to be called before the addParticle method below. 
   
 void LHAup::addParticle( int id, int status, int mother1, int mother2, int colourTag1, int colourTag2, double p_x, double p_y, double p_z, double e, double m, double tau, double spin, double scale)   
gives the properties of the next particle handed in (cf. IDUP, ISTUP, 
MOTHUP(1,..), MOTHUP(2,..), ICOLUP(1,..), ICOLUP(2,..),  PUP(J,..), 
VTIMUP, SPINUP; while scale is a new optional property, 
see further below) . 
   
 
 
Information is handed back by the following methods: 
 int LHAup::idProcess()   
process number. 
   
 
 double LHAup::weight()   
Note that the weight stored in Info::weight() as a rule 
is not the same as the above weight(): the method here gives 
the value before unweighting while the one in info gives 
the one after unweighting and thus normally is 1 or -1. Only with strategy 
options +-3 and +-4 would the value in info be the same as 
here, except for a conversion from pb to mb for +-4. 
   
 
 double LHAup::scale()   
   
 double LHAup::alphaQED()   
   
 double LHAup::alphaQCD()   
scale and couplings at that scale. 
   
 
 int LHAup::sizePart()   
the size of the particle array, which is one larger than the number 
of particles in the event, since the zeroth entry is kept empty 
(see above). 
   
 
 int LHAup::id(int i)   
   
 int LHAup::status(int i)   
   
 int LHAup::mother1(int i)   
   
 int LHAup::mother2(int i)   
   
 int LHAup::col1(int i)   
   
 int LHAup::col2(int i)   
   
 double LHAup::px(int i)   
   
 double LHAup::py(int i)   
   
 double LHAup::pz(int i)   
   
 double LHAup::e(int i)   
   
 double LHAup::m(int i)   
   
 double LHAup::tau(int i)   
   
 double LHAup::spin(int i)   
   
 double LHAup::scale(int i)   
for particle i in the range 
0 <= i < sizePart(). (But again note that 
i = 0 is an empty line, so the true range begins at 1.) 
   
 
 
From the information in the event record it is possible to set 
the flavour and x values of the initiators 
 void LHAup::setIdX(int id1, int id2, double x1, double x2)   
   
 
 
This information is returned by the methods 
 int LHAup::id1()   
   
 int LHAup::id2()   
   
 double LHAup::x1()   
   
 double LHAup::x2()   
the flavour and x values of the two initiators. 
   
 
 
In the LHEF description [Alw06] an extension to 
include information on the parton densities of the colliding partons 
is suggested. This optional further information can be set by 
 void LHAup::setPdf( int id1pdf, int id2pdf, double x1pdf, double x2pdf, double scalePDF, double pdf1, double pdf2, bool pdfIsSet)   
which gives the flavours , the x and the Q scale 
(in GeV) at which the parton densities x*f_i(x, Q) have been 
evaluated. The last argument is normally true. 
   
 
 
This information is returned by the methods 
 bool LHAup::pdfIsSet()   
   
 int LHAup::id1pdf()   
   
 int LHAup::id2pdf()   
   
 double LHAup::x1pdf()   
   
 double LHAup::x2pdf()   
   
 double LHAup::scalePDF()   
   
 double LHAup::pdf1()   
   
 double LHAup::pdf2()   
where the first one tells whether this optional information has been set 
for the current event. (setPdf(...) must be called after the 
setProcess(...) call of the event for this to work.) 
Note that the flavour and x values usually but not always 
agree with those obtained by the same methods without pdf 
in their names, see explanation in the 
Event Information description. 
   
 
 
The maximum scale for parton-shower evolution of a Les Houches event is 
regulated by the 
TimeShower:pTmaxMatch 
and 
SpaceShower:pTmaxMatch 
modes. If you want to guarantee that the input scale value 
is respected, as is often the case in matching/merging procedures, you 
should set both of these modes to 1. That only affects the hard process, 
while resonance decays are still processed using the resonance mass to 
set the upper limit. However, the optional 
Beams:strictLHEFscale = on 
setting restricts also resonance-decay emissions to be below the input 
scale value. 
 
 
As a further non-standard feature, it is also possible to read in the 
separate scale values of all final particles. Such scale values could be used 
e.g. to restrict the maximum scale for shower evolutions for each parton 
separately. This reading will only be applied if the  
Beams:setProductionScaleFromLHEF switch is true (see  
Beam Parameters for details). 
This information is returned by the method 
double LHAup::scale(int i). When no such information 
has been read from the LHEF, the scale defaults to -1. 
 
 
 void LHAup::listEvent()   
prints the above information for the current event.  In cases where the 
LHAup object is not available to the user, the 
Pythia::LHAeventList() method can 
be used, which is a wrapper for the above. 
   
 
 virtual bool LHAup::skipEvent(int nSkip)   
skip ahead nSkip events in the Les Houches generation 
sequence, without doing anything further with them. Mainly 
intended for debug purposes, e.g. when an event at a known 
location in a Les Houches Event File is causing problems. 
Will return false if operation fails, specifically if the 
end of an LHEF has been reached. The implementation in the base class 
simply executes setEvent() the requested number of times. 
The derived LHAupLHEF class (see below) only uses the 
setNewEventLHEF(...) part of its setEvent() 
method, and other derived classes could choose other shortcuts. 
   
 
 
The LHA expects the decay of resonances to be included as part of the 
hard process, i.e. if unstable particles are produced in a process then 
their decays are also described. This includes Z^0, W^+-, H^0 
and other short-lived particles in models beyond the Standard Model. 
Should this not be the case then PYTHIA will perform the decays of all 
resonances it knows how to do, in the same way as for internal processes. 
Note that you will be on slippery ground if you then restrict the decay of 
these resonances to specific allowed channels since, if this is not what 
was intended, you will obtain the wrong cross section and potentially the 
wrong mix of different event types. (Since the original intention is 
unknown, the cross section will not be corrected for the fraction of 
open channels, i.e. the procedure used for internal processes is not 
applied in this case.) 
 
 
Even if PYTHIA can select resonance decay modes according to its 
internal tables, there is normally no way for it to know which 
decay angular correlations should exist in the simulated process. 
Therefore almost all decays are isotropic. The exceptions are Higgs and 
top decays, in the decay chains H → WW/ZZ → f fbar f' fbar' 
and t → b W → b f fbar, where the process-independent 
correlations implemented for internal processes are used. If part of 
the decay chain has already been set, however (e.g. H → WW/ZZ 
or t → b W), then decay is still isotropic. 
 
 
The LHA standard only allows for one hard subcollision in an event. 
Further multiparton interactions are supposed to be handled by the 
internal MPI machinery. As a nonstandard feature, it is possible 
to input two hard subcollisions in the same event, to match the internal 
second hard process machinery. 
In such cases two partons are extracted from each of the two incoming 
hadrons. A restriction is that, unlike the single-subprocess case, 
it is important that the partons are input in the order that PYTHIA 
later would need. That is, the two subcollisions should follow each 
other, with instate preceding outstate. Any resonance decay chain 
should be put at the end, after both interactions. As illustration, 
consider double W production. With 1 and 2 
labelling the two subcollisions, and A and B the two 
incoming hadron beams, the event record ordering should be 
in_A1 - in_B1 - W_1 - in_A2 - in_B2 - W_2 - f_1 - fbar_1 - f_2 - 
fbar_2, where f fbar is the fermion decay products of 
the respective W. A limitation is that currently only one 
input scale is used, that thereby limits all further partonic activity 
in the same way for both processes. 
 
 
When transferring events through the runtime interface, it is worth 
noting that PYTHIA offers some settings to ensure the consistency of 
momenta, e.g. reshuffling of particle momenta to guarantee on-shell 
conditions,  or matching the sum of incoming to the sum of outgoing momenta. 
The documentation of settings related to this can be found under the 
Transfer to the PYTHIA process record heading of 
Les Houches Event Files . 
 
 
A LHEF reader class
 
 
The LHEF standard ([Alw06], [But14]) specifies a format 
where a single file packs initialization and event information. This has 
become the most frequently used procedure to process external parton-level 
events in Pythia, and is discussed in detail in the 
Les Houches Event Files  section. 
Internally the file handling and reading is us handled by instance of the 
derived class LHAupLHEF. 
 
 
The workhorses of the LHAupLHEF class are three methods 
found in the base class, so as to allow them to be reused in other 
contexts. 
 
 bool LHAup::setInitLHEF(ifstream& is, bool readHeaders = false)   
read in and set all required initialization information from the 
specified stream. With second argument true it will also read and store 
header information, as described above. Return false if it fails. 
   
 
 bool LHAup::setNewEventLHEF(ifstream& is)   
read in event information from the specified stream into a staging area 
where it can be reused by setOldEventLHEF. 
   
 
 bool LHAup::setOldEventLHEF()   
store the event information from the staging area into the normal 
location. Thus a single setNewEventLHEF call can be 
followed by several setOldEventLHEF ones, so as to 
process the same configuration several times. This method currently 
only returns true, i.e. any errors should be caught by the preceding 
setNewEventLHEF call. 
   
 
 
These three main methods build on a number of container classes and a 
generic LHEF reader class (called Reader) found in 
LHEF3.h and LHEF3.cc. The Reader 
handles all the parsing and storage necessary to adhere with 
[But14]. (A matching Writer class is also 
available; see documentation in LHEF3.h how it can be 
used.) All parsing that is not strictly part of the LHEF format 
(e.g. the reading of header information) is instead performed directly in 
the LHAupLHEF methods. 
 
 
Some other small utility routines are: 
 
 bool LHAup::fileFound()   
always returns true in the base class, but in LHAupLHEF 
it returns false if the LHEF provided in the constructor is not 
found and opened correctly. 
   
 
 bool LHAup::useExternal()   
always returns false in the base class, but in LHAupLHEF 
it returns false if the LHAupLHEF instance is constructed to 
work on an input LHE file, while it returns true if the LHAupLHEF 
instance is constructed to use externally provided input streams instead. 
For the latter, the LHAupLHEF instance should have been 
constructed with the class constructor 
LHAupLHEF(Info* infoPtrIn, istream* isIn, istream* isHeadIn, 
bool readHeadersIn, bool setScalesFromLHEFIn). 
   
 
 void LHAup::setInfoHeader(const string &key, const string &val)   
is used to send header information on to the Info class. 
   
 
 
A few other methods, most of them derived from the base class, 
streamlines file opening and closing, e.g. if several LHE files are 
to be read consecutively, without the need for a complete 
reinitialization. This presupposes that the events are of the same 
kind, only split e.g. to limit file sizes. 
 
 bool LHAup::newEventFile(const char* fileIn)   
close current event input file/stream and open a new one, to 
continue reading events of the same kind as before. 
   
 
 istream* LHAup::openFile(const char *fn, ifstream &ifs)   
   
 void LHAup::closeFile(istream *&is, ifstream &ifs)   
open and close a file, also gzip files, where an intermediate 
decompression layer is needed. 
   
 
 void LHAupLHEF::closeAllFiles()   
close main event file (LHEF) and, if present, separate header file. 
   
 
 
A runtime Fortran interface
 
 
The runtime Fortran interface requires linking to an external Fortran 
code. In order to avoid problems with unresolved external references 
when this interface is not used, the code has been put in a separate 
include/Pythia8Plugins/LHAFortran.h file, that is not 
included in any of the other library files. Instead it should be included 
in the user-supplied main program, and used to create a derived class that 
contains the implementation of two methods below that call the Fortran 
program to do its part of the job. 
 
 
The LHAupFortran class derives from LHAup. 
It reads initialization and event information from the LHA standard 
Fortran commonblocks, assuming these commonblocks behave like two 
extern "C" struct named heprup_ and 
hepeup_. (Note the final underscore, to match how the 
gcc compiler internally names Fortran files.) 
 
 
The instantiation does not require any arguments. 
 
 
The user has to supply implementations of the fillHepRup() 
and fillHepEup() methods, that is to do the actual calling 
of the external Fortran routines that fill the HEPRUP and 
HEPEUP commonblocks. The translation of this information to 
the C++ structure is provided by the existing setInit() and 
setEvent() code. 
 
 
Up to and including version 8.125 the LHAupFortran class 
was used to construct a runtime interface to PYTHIA 6.4. This was 
convenient in the early days of PYTHIA 8 evolution, when this program 
did not yet contain hard-process generation, and the LHEF standard 
did not yet exist. Nowadays it is more of a bother, since a full 
cross-platform support leads to many possible combinations. Therefore 
this support has been removed, but can still be recuperated from 
previous code versions, in a reduced form up to version 8.176. 
 
 
A LHEF writer class
 
 
The main objective of the LHAup class is to feed information 
from an external program into PYTHIA. It can be used to export information 
as well, however. The main documentation of these features is found in 
the Les Houches Event Files  section. Nevertheless, 
we quickly summarize the main workhorse functions here. There are four 
routines in the base class that should be called in sequence to build up the 
proper file structure. 
 
 bool LHAup::openLHEF(string filename)   
Opens a file with the filename indicated, and writes a header plus a brief 
comment with date and time information. 
   
 
 bool LHAup::initLHEF()   
Writes initialization information to the file above. Such information should 
already have been set with the methods described in the "Initialization" 
section above. 
   
 
 bool LHAup::eventLHEF(bool verbose = true)   
Writes event information to the file above. Such information should 
already have been set with the methods described in the "Event input" 
section above. This call should be repeated once for each event to be 
stored. By default the event information is lined up in columns. 
To save space, the alternative verbose = false only 
leaves a single blank between the information fields. 
   
 
 bool LHAup::closeLHEF(bool updateInit = false)   
Writes the closing tag and closes the file. Optionally, if 
updateInit = true, this routine will reopen the file from 
the beginning, rewrite the same header as openLHEF() did, 
and then call initLHEF() again to overwrite the old 
information. This is especially geared towards programs, such as PYTHIA 
itself, where the cross section information is not available at the 
beginning of the run, but only is obtained by Monte Carlo integration 
in parallel with the event generation itself. Then the 
setXSec( i, xSec), setXErr( i, xSec) and 
setXMax( i, xSec) can be used to update the relevant 
information before closeLHEF is called. 
Warning: overwriting the beginning of a file without 
upsetting anything is a delicate operation. It only works when the new 
lines require exactly as much space as the old ones did. Thus, if you add 
another process in between, the file will be corrupted. 
   
 
 string LHAup::getFileName()   
Return the name of the LHE file above.