New pythia8 version

[u/mrichter/AliRoot.git] / PYTHIA8 / pythia8145 / xmldoc / LesHouchesAccord.xml

<chapter name="Les Houches Accord">

<h2>Les Houches Accord</h2>

The Les Houches Accord (LHA) for user processes <ref>Boo01</ref> 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.

<p/>
The <code>LHAup</code> 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. 

<p/>
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 
<code><aloc href="ProgramFlow">Pythia::init( LHAup*)</aloc></code> 
method. However, with the LHEF format a filename can replace the 
pointer, see further below. 

<p/>
Let us now describe the methods at your disposal to do the job.

<method name="LHAup::LHAup( int strategy = 3)"> 
the base class constructor takes the choice of mixing/weighting 
strategy as optional input argument, and calls <code>setStrategy</code>,
see below. It also reserves some space for processes and particles.
</method>

<method name="virtual LHAup::~LHAup()"> 
the destructor does not need to do anything.
</method>

<method name="void LHAup::setPtr(Info* infoPtr)"> 
this method only sets the pointer that allows some information
to be accessed, and is automatically called by 
<code>Pythia::init(...)</code>. 
</method>

<h3>Initialization</h3>

The <code>LHAup</code> class stores information equivalent to the 
<code>/HEPRUP/</code> 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.

<method name="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.
</method>

<p/>
Inside <code>setInit()</code>, such information can be set by the following 
methods:
<method name="void LHAup::setBeamA( int identity, double energy, 
int pdfGroup, int pdfSet)"> 
</method>
<methodmore name="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 <code>IDBMUP(1), EBMUP(i), PDFGUP(i), PDFSUP(i)</code>,
with <code>i</code> 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 
<aloc href="PDFSelection">PDF Selection</aloc> is used for the further 
event generation in PYTHIA.
</methodmore>

<method name="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 name="strategy">
chosen strategy (cf. <code>IDWTUP</code>; see <ref>Sjo06</ref> 
section 9.9.1 for extensive comments).
<argoption value="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 <code>xMax</code>.
Accepted events therefore have unit weight.</argoption>
<argoption value="-1"> as option 1, except that cross sections can now be
negative and events after unweighting have weight +-1. You can use 
<code><aloc href="EventInformation">Info::weight()</aloc></code> 
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 
<code>xMax</code> chosen appropriately for the two.</argoption>
<argoption value="2"> events come with non-negative weight, in unspecified 
units, but such that <code>xMax</code> 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 
<code>xSec</code>.</argoption>
<argoption value="-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.</argoption>
<argoption value="3"> events come with unit weight, and are thus accepted 
as is. The total cross section of the process is stored in 
<code>xSec</code>.</argoption>
<argoption value="-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).</argoption>
<argoption value="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 
<code>Info::weight()</code> has been converted to mb, however.
</argoption>
<argoption value="-4"> as option 4, except that events now can come 
either with positive or negative weights.</argoption> 
<note>Note 1</note>: 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>Note 2</note>: 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 <ei>pT</ei> values in some particular 
functional form.)
</argument>
</method>

<method name="void LHAup::addProcess( int idProcess, double xSec, 
double xErr, double xMax)"> 
sets info on an allowed process (cf. <code>LPRUP, XSECUP, XERRUP, 
XMAXUP</code>). 
Each new call will append one more entry to the list of processes.
The choice of strategy determines which quantities are mandatory:
<code>xSec</code> for strategies +-2 and +-3, 
<code>xErr</code> never, and
<code>xMax</code> for strategies +-1 and +-2. 
</method>

<note>Note</note>: PYTHIA does not make active use of the (optional)
<code>xErr</code> 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 <ei>1/sqrt(n_acc)</ei>, where <ei>n_acc</ei>     
is the number of accepted events, should offer a reasonable estimate.

<method name="void LHAup::setXSec( int i, double xSec)"> 
update the <code>xSec</code> value of the <code>i</code>'th process
added with <code>addProcess</code> method (i.e. <code>i</code> runs
from 0 through <code>sizeProc() - 1</code>, see below).
</method>

<method name="void LHAup::setXErr( int i, double xErr)"> 
update the <code>xErr</code> value of the <code>i</code>'th process
added with <code>addProcess</code> method.
</method>

<method name="void LHAup::setXMax( int i, double xMax)"> 
update the <code>xMax</code> value of the <code>i</code>'th process
added with <code>addProcess</code> method.
</method>

<p/>
Information is handed back by the following methods 
(that normally you would not need to touch):
<method name="int LHAup::idBeamA()">
</method>
<methodmore name="int LHAup::idBeamB()">
</methodmore>
<methodmore name="double LHAup::eBeamA()">
</methodmore>
<methodmore name="double LHAup::eBeamB()">
</methodmore>
<methodmore name="int LHAup::pdfGroupBeamA()">
</methodmore>
<methodmore name="int LHAup::pdfGroupBeamB()">
</methodmore>
<methodmore name="int LHAup::pdfSetBeamA()">
</methodmore>
<methodmore name="int LHAup::pdfSetBeamB()">
for the beam properties.
</methodmore>
<method name="int LHAup::strategy()">
for the strategy choice.
</method>
<method name="int LHAup::sizeProc()"> 
for the number of subprocesses.
</method>
<method name="int LHAup::idProcess(i)"> 
</method>
<methodmore name="double LHAup::xSec(i)"> 
</methodmore>
<methodmore name="double LHAup::xErr(i)"> 
</methodmore>
<methodmore name="double LHAup::xMax(i)"> 
for process <code>i</code> in the range <code>0 &lt;= i &lt; 
sizeProc()</code>.   
</methodmore>

<method name="void LHAup::listInit(ostream& os = cout)"> 
prints the above initialization information. This method is
automatically called from <code>Pythia::init(...)</code>, 
so would normally not need to be called directly by the user.
</method>

<h3>Event input</h3>

The <code>LHAup</code> class also stores information equivalent to the 
<code>/HEPEUP/</code> 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.

<p/>
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 <code>sizePart()</code> 
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 (<code>NUP</code>). 

<method name="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 <code>idProcess</code> value specifies which process 
that should be generated, while <code>idProcess</code> 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 <code>Pythia::next()</code> 
returns false. You can then interrogate 
<code><aloc href="EventInformation">Info::atEndOfFile()</aloc></code> 
to confirm that indeed the failure is caused in this method, and decide 
to break out of the event generation loop.

<p/>
Inside a normal <code>setEvent(...)</code> call, information can be set 
by the following methods:
<method name="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 <ei>alpha_EM</ei> and <ei>alpha_strong</ei> were used
(cf. <code>IDPRUP, XWTGUP, SCALUP, AQEDUP, AQCDUP</code>). This method 
also resets the size of the particle list, and adds the empty zeroth 
line, so it has to be called before the <code>addParticle</code> method below.
</method>
<method name="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. <code>IDUP, ISTUP, 
MOTHUP(1,..), MOTHUP(2,..), ICOLUP(1,..), ICOLUP(2,..),  PUP(J,..), 
VTIMUP, SPINUP</code>) .
</method>

<p/>
Information is handed back by the following methods:
<method name="int LHAup::idProcess()">
process number.
</method>

<method name="double LHAup::weight()">.
Note that the weight stored in <code>Info::weight()</code> as a rule
is not the same as the above <code>weight()</code>: the method here gives
the value before unweighting while the one in <code>info</code> gives
the one after unweighting and thus normally is 1 or -1. Only with strategy
options +-3 and +-4 would the value in <code>info</code> be the same as 
here, except for a conversion from pb to mb for +-4. 
</method>

<method name="double LHAup::scale()">
</method>
<methodmore name="double LHAup::alphaQED()">
</methodmore>
<methodmore name="double LHAup::alphaQCD()">
scale and couplings at that scale.
</methodmore>

<method name="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). 
</method>

<method name="int LHAup::id(int i)"> 
</method>
<methodmore name="int LHAup::status(int i)"> 
</methodmore>
<methodmore name="int LHAup::mother1(int i)"> 
</methodmore>
<methodmore name="int LHAup::mother2(int i)"> 
</methodmore>
<methodmore name="int LHAup::col1(int i)"> 
</methodmore>
<methodmore name="int LHAup::col2(int i)"> 
</methodmore>
<methodmore name="double LHAup::px(int i)"> 
</methodmore>
<methodmore name="double LHAup::py(int i)"> 
</methodmore>
<methodmore name="double LHAup::pz(int i)"> 
</methodmore>
<methodmore name="double LHAup::e(int i)"> 
</methodmore>
<methodmore name="double LHAup::m(int i)"> 
</methodmore>
<methodmore name="double LHAup::tau(int i)"> 
</methodmore>
<methodmore name="double LHAup::spin(int i)"> 
for particle <code>i</code> in the range 
<code>0 &lt;= i &lt; sizePart()</code>. (But again note that 
<code>i = 0</code> is an empty line, so the true range begins at 1.)   
</methodmore>

<p/>
In the LHEF description <ref>Alw06</ref> an extension to 
include information on the parton densities of the colliding partons
is suggested. This optional further information can be set by
<method name="void LHAup::setPdf( int id1, int id2, double x1, double x2, 
double scalePDF, double xpdf1, double xpdf2)">
which gives the flavours , the <ei>x</ei> and the <ie>Q</ei> scale 
(in GeV) at which the parton densities <ei>x*f_i(x, Q)</ei> have been
evaluated.
</method>

<p/>
This information is returned by the methods
<method name="bool LHAup::pdfIsSet()">
</method>
<methodmore name="int LHAup::id1()">
</methodmore>
<methodmore name="int LHAup::id2()">
</methodmore>
<methodmore name="double LHAup::x1()">
</methodmore>
<methodmore name="double LHAup::x2()">
</methodmore>
<methodmore name="double LHAup::scalePDF()">
</methodmore>
<methodmore name="double LHAup::xpdf1()">
</methodmore>
<methodmore name="double LHAup::xpdf2()">
where the first one tells whether this optional information has been set
for the current event. (<code>setPdf(...)</code> must be called after the
<code>setProcess(...)</code> call of the event for this to work.)
</methodmore>

<p/>
<method name="void LHAup::listEvent(ostream& os = cout)"> 
prints the above information for the current event.  In cases where the
<code>LHAup</code> object is not available to the user, the 
<code>Pythia::LHAeventList(ostream& os = cout)</code> method can 
be used, which is a wrapper for the above. 
</method>

<method name="virtual bool LHAup::skipEvent(int nSkip)"> 
skip ahead <code>nSkip</code> 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 <code>setEvent()</code> the requested number of times.
The derived <code>LHAupLHEF</code> class (see below) only uses the
<code>setNewEventLHEF(...)</code> part of its <code>setEvent()</code> 
method, and other derived classes could choose other shortcuts.
</method>

<p/>
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 <ei>Z^0, W^+-, H^0</ei> 
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.) 

<h3>An interface to Les Houches Event Files</h3>

The LHEF standard <ref>Alw06</ref> 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 
<code><aloc href="ProgramFlow">Pythia::init(fileName)</aloc></code>
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 <code>LHAupLHEF</code>, which can do the job of reading an LHEF.

<p/>
An example how to generate events from an LHEF is found in 
<code>main12.cc</code>. Note the use of 
<code>Info::atEndOfFile()</code> to find out when the whole
LHEF has been processed. 

<p/>
To allow the sequential use of several event files the 
<code>Pythia::init(...)</code> method has an optional second argument: 
<code>Pythia::init(fileName, bool skipInit = false)</code>.
If called with this argument <code>true</code> then there will be no 
initialization, except that the existing <code>LHAupLHEF</code> 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 
<code>main13.cc</code>.

<p/>
The workhorses of the <code>LHAupLHEF</code> 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.

<method name="bool LHAup::fileFound()"> 
always returns true in the base class, but in <code>LHAupLHEF</code>
it returns false if the LHEF provided in the constructor is not
found and opened correctly.
</method>

<method name="bool LHAup::setInitLHEF(ifstream& is)">
read in and set all required initialization information from the 
specified stream. Return false if it fails.
</method>

<method name="bool LHAup::setNewEventLHEF(ifstream& is)">
read in event information from the specified stream into a staging area 
where it can be reused by <code>setOldEventLHEF</code>.
</method>

<method name="bool LHAup::setOldEventLHEF()">
store the event information from the staging area into the normal 
location. Thus a single <code>setNewEventLHEF</code> call can be 
followed by several <code>setOldEventLHEF</code> 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 
<code>setNewEventLHEF</code> call.
</method>
 

<h3>A runtime Fortran interface</h3>

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
<code>LHAFortran.h</code> 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. 

<p/>
The <code>LHAupFortran</code> class derives from <code>LHAup</code>. 
It reads initialization and event information from the LHA standard 
Fortran commonblocks, assuming these commonblocks behave like two 
<code>extern "C" struct</code> named <code>heprup_</code> and
<code>hepeup_</code>. (Note the final underscore, to match how the 
gcc compiler internally names Fortran files.) 

<p/>
The instantiation does not require any arguments. 

<p/>
The user has to supply implementations of the <code>fillHepRup()</code>
and <code>fillHepEup()</code> methods, that is to do the actual calling 
of the external Fortran routines that fill the <code>HEPRUP</code> and 
<code>HEPEUP</code> commonblocks. The translation of this information to 
the C++ structure is provided by the existing <code>setInit()</code> and
<code>setEvent()</code> code. 

<p/>
Up to and including version 8.125 the <code>LHAupFortran</code> 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 
<code>main51.cc</code> example remains as an illustration, where the
previously separate interface code 
(<code>include/Pythia6Interface.h</code>) has been inserted in the 
beginning. You also need to modify the <code>examples/Makefile</code> 
to link <code>main51.cc</code> properly also to a PYTHIA 6.4 library
version, see commented-out section for ideas how to to this.

<h3>Methods for LHEF output</h3>

The main objective of the <code>LHAup</code> 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. 

<method name="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.
</method>

<method name="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.
</method>

<method name="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. 
</method>

<method name="bool LHAup::closeLHEF(bool updateInit = false)">
Writes the closing tag and closes the file. Optionally, if 
<code>updateInit = true</code>, this routine will reopen the file from
the beginning, rewrite the same header as <code>openLHEF()</code> did,
and then call <code>initLHEF()</code> 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 
<code>setXSec( i, xSec)</code>, <code>setXErr( i, xSec)</code> and
<code>setXMax( i, xSec)</code> can be used to update the relevant 
information before <code>closeLHEF</code> is called.
<note>Warning:</note> 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. 
</method>

<h3>PYTHIA 8 output to an LHEF</h3>

The above methods could be used by any program to write an LHEF. 
For PYTHIA 8 to do this, a derived class already exists,
<code>LHAupFromPYTHIA8</code>. In order for it to do its job,
it must gain access to the information produced by PYTHIA, 
specifically the <code>process</code> event record and the
generic information stored in <code>info</code>. Therefore, if you
are working with an instance <code>pythia</code> of the 
<code>Pythia</code> class, you have to instantiate
<code>LHAupFromPYTHIA8</code> with pointers to the 
<code>process</code> and <code>info</code> objects of 
<code>pythia</code>:
<br/><code>LHAupFromPYTHIA8 myLHA(&pythia.process, &pythia.info);</code>

<p/>
The method <code>setInit()</code> should be called to store the
<code>pythia</code> initialization information in the LHA object,
and <code>setEvent()</code> to store event information. 
Furthermore, <code>updateSigma()</code> can be used at the end 
of the run to update cross-section information, cf.
<code>closeLHEF(true)</code> above. An example how the 
generation, translation and writing methods should be ordered is
found in <code>main20.cc</code>.

<p/>
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 <code>info</code> 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 <code>Pythia::process</code> is not in this frame. 
 
</chapter>

<!-- Copyright (C) 2010 Torbjorn Sjostrand -->