Les Houches Accord
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 LHAup
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 (LHEF), 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::init( 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.
Inside setInit()
, such information can be set by the following
methods:
void LHAup::setBeamA( int identity, double energy, int pdfGroup, int pdfSet)
void LHAup::setBeamB( int identity, double energy, int pdfGroup, int pdfSet)
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). The parton distribution information
defaults to zero. 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.
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(i)
double LHAup::xSec(i)
double LHAup::xErr(i)
double LHAup::xMax(i)
for process i
in the range 0 <= i <
sizeProc()
.
void LHAup::listInit(ostream& os = cout)
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 occured, 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)
gives the properties of the next particle handed in (cf. IDUP, ISTUP,
MOTHUP(1,..), MOTHUP(2,..), ICOLUP(1,..), ICOLUP(2,..), PUP(J,..),
VTIMUP, SPINUP
) .
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)
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.)
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 id1, int id2, double x1, double x2, double scalePDF, double xpdf1, double xpdf2)
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.
This information is returned by the methods
bool LHAup::pdfIsSet()
int LHAup::id1()
int LHAup::id2()
double LHAup::x1()
double LHAup::x2()
double LHAup::scalePDF()
double LHAup::xpdf1()
double LHAup::xpdf2()
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.)
void LHAup::listEvent(ostream& os = cout)
prints the above information for the current event. In cases where the
LHAup
object is not available to the user, the
Pythia::LHAeventList(ostream& os = cout)
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.)
An interface to Les Houches Event Files
The LHEF standard [Alw06] 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. Therefore a special
Pythia::init(fileName)
initialization option exists, where the LHEF name is provided as input.
Internally this name is then used to create an instance of the derived
class LHAupLHEF
, which can do the job of reading an LHEF.
An example how to generate events from an LHEF is found in
main12.cc
. Note the use of
Info::atEndOfFile()
to find out when the whole
LHEF has been processed.
To allow the sequential use of several event files the
Pythia::init(...)
method has an optional second argument:
Pythia::init(fileName, bool skipInit = false)
.
If called with this argument true
then there will be no
initialization, except that the existing LHAupLHEF
class
instance will be deleted and replaced by ones pointing to the new file.
It is assumed (but never checked) that the initialization information is
identical, and that the new file simply contains further events of
exactly the same kind as the previous one. An example of this possibility,
and the option to mix with internal processes, is found in
main13.cc
.
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. Specifically, it allows derived classes where one parton-level
configuration can be reused several times, e.g. in the context of
matrix-element-to-parton-shower matching (example in preparation).
To begin with also a small utility routine.
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::setInitLHEF(ifstream& is)
read in and set all required initialization information from the
specified stream. 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.
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
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, together with 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
the support has been reduced in the current version. Only the
main51.cc
example remains as an illustration, where the
previously separate interface code
(include/Pythia6Interface.h
) has been inserted in the
beginning. You also need to modify the examples/Makefile
to link main51.cc
properly also to a PYTHIA 6.4 library
version, see commented-out section for ideas how to to this.
Methods for LHEF output
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. Specifically, there are four routines in the base class
that can be called to write a Les Houches Event File. These should be
called in sequence in order 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()
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.
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.
PYTHIA 8 output to an LHEF
The above methods could be used by any program to write an LHEF.
For PYTHIA 8 to do this, a derived class already exists,
LHAupFromPYTHIA8
. In order for it to do its job,
it must gain access to the information produced by PYTHIA,
specifically the process
event record and the
generic information stored in info
. Therefore, if you
are working with an instance pythia
of the
Pythia
class, you have to instantiate
LHAupFromPYTHIA8
with pointers to the
process
and info
objects of
pythia
:
LHAupFromPYTHIA8 myLHA(&pythia.process, &pythia.info);
The method setInit()
should be called to store the
pythia
initialization information in the LHA object,
and setEvent()
to store event information.
Furthermore, updateSigma()
can be used at the end
of the run to update cross-section information, cf.
closeLHEF(true)
above. An example how the
generation, translation and writing methods should be ordered is
found in main20.cc
.
Currently there are some limitations, that could be overcome if
necessary. Firstly, you may mix many processes in the same run,
but the cross-section information stored in info
only
refers to the sum of them all, and therefore they are all classified
as a common process 9999. Secondly, you should generate your events
in the CM frame of the collision, since this is the assumed frame of
stored Les Houches events, and no boosts have been implemented
for the case that Pythia::process
is not in this frame.