<chapter name="User Hooks"> 
 
<h2>User Hooks</h2> 
 
Sometimes it may be convenient to step in during the generation 
process: to modify the built-in cross sections, to veto undesirable 
events or simply to collect statistics at various stages of the 
evolution. There is a base class <code>UserHooks</code> that gives 
you this access at a few selected places. This class in itself does 
nothing; the idea is that you should write your own derived class 
for your task. One simple derived class (<code>SuppressSmallPT</code>) 
comes with the program, mainly as illustration, and the 
<code>main10.cc</code> program provides a complete (toy) example how 
a derived class could be set up and used. 
 
<p/> 
There are ten sets of routines, that give you different kinds of 
freedom. They are, in no particular order: 
<br/>(i) Ones that give you access to the event record in between 
the process-level and parton-level steps, or in between the 
parton-level and hadron-level ones. You can study the event record 
and decide whether to veto this event. 
<br/>(ii) Ones that allow you to set a scale at which the combined 
parton-level MPI+ISR+FSR downwards evolution in <ei>pT</ei> is 
temporarily interrupted, so the event can be studied and either 
vetoed or allowed to continue the evolution. 
<br/>(iii) Ones that allow you to to study the event after the first 
few ISR/FSR emissions, or first few MPI, so the event can be vetoed 
or allowed to continue the evolution. 
<br/>(iv) Ones that allow you to study the latest initial- or 
final-state emission and veto that emission, without vetoing the 
event as a whole. 
<br/>(v) Ones that give you access to the properties of the trial 
hard process, so that you can modify the internal Pythia cross section, 
alternatively the phase space sampling, by your own correction factors. 
<br/>(vi) Ones that allow you to reject the decay sequence of resonances 
at the process level. 
<br/>(vii) Ones that let you set the scale of shower evolution, 
specifically for matching in resonance decays. 
<br/>(viii) Ones that allow colour reconnection, notably in connection 
with resonance decays. 
<br/>(ix) Ones that allow an enhanced rate of rare shower splittings. 
<br/>(x) Ones that give you access to to hadronization parameters 
in each step in the hadronization process, and allows for a veto 
of individual hadrons. 
 
<br/>They are described further in the following numbered subsections. 
 
<p/> 
All the possibilities above can be combined freely and also be combined 
with the standard flags. An event would then survive only if it survived 
each of the possible veto methods. There are no hidden interdependencies 
in this game, but of course some combinations may not be particularly 
meaningful. For instance, if you set <code>PartonLevel:all = off</code> 
then the <code>doVetoPT(...)</code> and <code>doVetoPartonLevel(...)</code> 
locations in the code are not even reached, so they would never be called. 
 
<p/> 
Normally you would gather all your changes into one derived class. 
In some cases it may be more convenient to separate different 
functionalities. Therefore it is possible to hand in several user hooks. 
See the section on "Multiple user hooks" at the bottom of this page for 
further information on this possibility. 
 
<p/> 
The effect of the vetoes of types (i), (ii) and (iii) can be studied 
in the output of the 
<code><aloc href="EventStatistics">Pythia::stat()</aloc></code> 
method. The "Selected" column represents the number of events that were 
found acceptable by the internal Pythia machinery, whereas the "Accepted" 
one are the events that also survived the user cuts. The cross section 
is based on the latter number, and so is reduced by the amount associated 
by the vetoed events. Also type (v) modifies the cross section, while 
types (iv), (vi) and (vii) do not. 
 
<p/> 
A warning. When you program your own derived class, do remember that you 
must exactly match the arguments of the base-class methods you overload. 
If not, your methods will be considered as completely new ones, and 
compile without any warnings, but not be used inside <code>Pythia</code>. 
So, at the debug stage, do insert some suitable print statements to check 
that the new methods are called (and do what they should). 
 
<h3>The basic components</h3> 
 
For a derived <code>UserHooks</code> class to be called during the 
execution, a pointer to an object of this class should be handed in 
with the 
<br/><code><aloc href="ProgramFlow"> 
Pythia::setUserHooksPtr( UserHooks*)</aloc></code> 
<br/>method. The first step therefore is to construct your own derived 
class, of course. This must contain a constructor and a destructor. The 
<code>initPtr</code> method comes "for free", and is set up without 
any intervention from you. 
 
<method name="UserHooks::UserHooks()"> 
</method> 
<methodmore name="virtual UserHooks::~UserHooks()"> 
The constructor and destructor do not need to do anything. 
</methodmore> 
 
<method name="void UserHooks::initPtr( Info* infoPtr, 
Settings* settingsPtr, ParticleData* particleDataPtr, 
Rndm* rndmPtr, BeamParticle* beamAPtr, BeamParticle* beamBPtr, 
BeamParticle* beamPomAPtr, BeamParticle* beamPomBPtr, 
CoupSM* coupSMPtr, PartonSystems* partonSystemsPtr, 
SigmaTotal* sigmaTotPtr)"> 
this (non-virtual) method is automatically called during the 
initialization stage to set several useful pointers, and to set up 
the <code>workEvent</code> below. The corresponding objects can 
later be used to extract some useful information. 
<br/><aloc href="EventInformation">Info</aloc>: 
general event and run information, including some loop counters. 
<br/><aloc href="SettingsScheme">Settings</aloc>: 
the settings used to determine the character of the run. 
<br/><aloc href="ParticleDataScheme">ParticleData</aloc>: 
the particle data used in the event record 
(including <code>workEvent</code> below). 
<br/><aloc href="RandomNumbers">Rndm</aloc>: the random number 
generator, that you could also use in your code. 
<br/><aloc href="BeamRemnants">BeamParticle</aloc>: 
the <code>beamAPtr</code> and <code>beamBPtr</code> beam particles 
contain info on partons extracted from the two incoming beams, 
on the PDFs used, and more. In cases when diffraction is simulated, 
also special Pomeron beams <code>beamPomAPtr</code> and 
<code>beamPomBPtr</code> are introduced, for the Pomerons residing 
inside the respective proton. 
<br/><aloc href="StandardModelParameters">CoupSM</aloc>: 
Standard Model couplings. 
<br/><aloc href="AdvancedUsage">PartonSystems</aloc>: 
the list of partons that belong to each individual subcollision system. 
<br/><aloc href="TotalCrossSections">SigmaTotal</aloc>: 
total/elastic/diffractive cross section parametrizations. 
</method> 
 
<p/> 
Next you overload the desired methods listed in the sections below. 
These often come in pairs or triplets, where the first must return 
true for the last method to be called. This latter method typically 
hands you a reference to the event record, which you then can use to 
decide whether or not to veto. Often the event record can be quite 
lengthy and difficult to overview. The following methods and data member 
can then come in handy. 
 
<method name="void UserHooks::omitResonanceDecays(const Event& process, 
bool finalOnly = false)"> 
is a protected method that you can make use of in your own methods to 
extract a simplified list of the hard process, where all resonance decay 
chains are omitted. Intended for the <code>can/doVetoProcessLevel</code> 
routines. Note that the normal process-level generation does include 
resonance decays. That is, if a top quark is produced in the hard process, 
then also decays such as <ei>t &rarr; b W+, W+ &rarr; u dbar</ei> will be 
generated and stored in <code>process</code>. 
The <code>omitResonanceDecays</code> 
routine will take the input <code>process</code> and copy it to 
<code>workEvent</code> (see below), minus the resonance decay chains. 
All particles produced in the hard process, such as the top, will be 
considered final-state ones, with positive status and no daughters, 
just as it is before resonances are allowed to decay. 
<br/>(In the <code>PartonLevel</code> routines, these decay chains will 
initially not be copied from <code>process</code> to <code>event</code>. 
Instead the combined MPI, ISR and FSR evolution is done with the top 
above as final particle. Only afterwards will the resonance decay chains 
be copied over, with kinematics changes reflecting those of the top, and 
showers in the decays carried out.) 
<br/>For the default <code>finalOnly = false</code> the beam particles 
and incoming partons are retained, so the event looks like a normal 
event record up to the point of resonance decays, with a normal history 
setup. 
<br/>With <code>finalOnly = true</code> only the final-state partons 
are retained in the list. It therefore becomes similar in functionality 
to the <code>subEvent</code> method below, with the difference that 
<code>subEvent</code> counts the decay products of the resonances 
as the final state, whereas here the resonances themselves are the 
final state. Since the history has been removed in this option, 
<code>mother1()</code> and <code>mother2()</code> return 0, while 
<code>daughter1()</code> and <code>daughter2()</code> both return the 
index of the same parton in the original event record. 
</method> 
 
<method name="void UserHooks::subEvent(const Event& event, 
bool isHardest = true)"> 
is a protected method that you can make use of in your own methods to 
extract a brief list of the current partons of interest, with all 
irrelevant ones omitted. It is primarily intended to track the evolution 
at the parton level, notably the shower evolution of the hardest 
(i.e. first) interaction. 
<br/>For the default <code>isHardest = true</code> only the outgoing partons 
from the hardest interaction (including the partons added to it by ISR and 
FSR) are extracted, as relevant e.g. for <code>doVetoPT( iPos, event)</code> 
with <code>iPos = 0 - 4</code>. With <code>isHardest = false</code> instead 
the outgoing partons of the latest "subprocess" are extracted, as relevant 
when <code>iPos = 5</code>, where it corresponds to the outgoing partons 
in the currently considered decay. 
<br/>The method also works at the process level, but there simply extracts 
all final-state partons in the event, and thus offers no extra functionality. 
<br/>The result is stored in <code>workEvent</code> below. Since the 
history has been removed, <code>mother1()</code> and <code>mother2()</code> 
return 0, while <code>daughter1()</code> and <code>daughter2()</code> both 
return the index of the same parton in the original event record 
(<code>event</code>; possibly <code>process</code>), so that you can 
trace the full history, if of interest. 
</method> 
 
<method name="Event UserHooks::workEvent"> 
This protected class member contains the outcome of the above 
<code>omitResonanceDecays(...)</code> and 
<code>subEvent(...)</code> methods. Alternatively you can use it for 
whatever temporary purposes you wish. You are free to use standard 
operations, e.g. to boost the event to its rest frame before analysis, 
or remove particles that should not be analyzed. 
The <code>workEvent</code> can also be sent on to a 
<aloc href="EventAnalysis">jet clustering algorithm</aloc>. 
</method> 
 
<h3>(i) Interrupt between the main generation levels</h3> 
 
<method name="virtual bool UserHooks::initAfterBeams()"> 
This routine is called by Pythia::init(), after the beams have been 
set up, but before any other initialisation. Therefore, at this stage, 
it is still possible to modify settings (apart from 
<code>Beams:*</code>) and particle data. This is mainly intended 
to be used in conjunction with Les Houches Event files, where 
headers are read in during beam initialisation, see the header 
functions in the <aloc href="EventInformation">Info</aloc> class. 
In the base class this method returns true. By returning false, 
PYTHIA initialisation will be aborted. 
</method> 
 
<method name="virtual bool UserHooks::canVetoProcessLevel()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoProcessLevel(...)</code> 
will be called immediately after a hard process (and associated 
resonance decays) has been selected and stored in the 
<code><aloc href="EventRecord">process</aloc></code> event record. 
<br/>At this stage, the <code>process</code> record typically contains 
the two beams in slots 1 and 2, the two incoming partons to the hard 
process in slots 3 and 4, the N (usually 1, 2 or 3) primary produced 
particles in slots 5 through 4 + N, and thereafter recursively the 
resonance decay chains, if any. Use the method 
<code>omitResonanceDecays(...)</code> if you want to skip these 
decay chains. There are exceptions to this structure, 
for <aloc href="QCDProcesses">soft QCD processes</aloc> (where 
the partonic process may not yet have been selected at this stage), 
and when a <aloc href="SecondHardProcess">second hard process</aloc> has 
been requested (where two hard processes are bookkept). In general 
it is useful to begin the development work by listing a few 
<code>process</code> records, to clarify what the structure is for 
the cases of interest. 
</method> 
 
<method name="virtual bool UserHooks::doVetoProcessLevel(Event& process)"> 
can optionally be called, as described above. You can study the 
<code>process</code> event record of the hard process. 
Based on that you can decide whether to veto the event, true, or let 
it continue to evolve, false. If you veto, then this event is not 
counted among the accepted ones, and does not contribute to the estimated 
cross section. The <code>Pytha::next()</code> method will begin a 
completely new event, so the vetoed event will not appear in the 
output of <code>Pythia::next()</code>. 
<note>Warning:</note> Normally you should not modify the <code>process</code> 
event record. However, for some matrix-element-matching procedures it may 
become unavoidable. If so, be very careful, since there are many pitfalls. 
Only to give one example: if you modify the incoming partons then also 
the information stored in the beam particles may need to be modified. 
<note>Note:</note> the above veto is different from setting the flag 
<code><aloc href="MasterSwitches">PartonLevel:all = off</aloc></code>. 
Also in the latter case the event generation will stop after the process 
level, but an event generated up to this point is considered perfectly 
acceptable. It can be studied and it contributes to the cross section. 
That is, <code>PartonLevel:all = off</code> is intended for simple studies 
of hard processes, where one can save a lot of time by not generating 
the rest of the story. By contrast, the <code>doVetoProcessLevel()</code> 
method allows you to throw away uninteresting events at an early stage 
to save time, but those events that do survive the veto are allowed to 
develop into complete final states (unless flags have been set otherwise). 
</method> 
 
<method name="virtual bool UserHooks::canVetoPartonLevel()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoPartonLevel(...)</code> 
will be called immediately after the parton level has been generated 
and stored in the <code><aloc href="EventRecord">event</aloc></code> 
event record. Thus showers, multiparton interactions and beam remnants 
have been set up, but hadronization and decays have not yet been 
performed. This is already a fairly complete event, possibly with quite 
a complex parton-level history. Therefore it is usually only meaningful 
to study the hardest interaction, e.g. using <code>subEvent(...)</code> 
introduced above, or fairly generic properties, such as the parton-level 
jet structure. 
</method> 
 
<method name="virtual bool UserHooks::doVetoPartonLevel(const Event& event)"> 
can optionally be called, as described above. You can study, but not 
modify, the <code>event</code> event record of the partonic process. 
Based on that you can decide whether to veto the event, true, or let 
it continue to evolve, false. If you veto, then this event is not 
counted among the accepted ones, and does not contribute to the estimated 
cross section. The <code>Pytha::next()</code> method will begin a 
completely new event, so the vetoed event will not appear in the 
output of <code>Pythia::next()</code>. 
<note>Note:</note> the above veto is different from setting the flag 
<code><aloc href="MasterSwitches">HadronLevel:all = off</aloc></code>. 
Also in the latter case the event generation will stop after the parton 
level, but an event generated up to this point is considered perfectly 
acceptable. It can be studied and it contributes to the cross section. 
That is, <code>HadronLevel:all = off</code> is intended for simple 
studies of complete partonic states, where one can save time by not 
generating the complete hadronic final state. By contrast, the 
<code>doVetoPartonLevel()</code> method allows you to throw away 
uninteresting events to save time that way, but those events that 
do survive the veto are allowed to develop into complete final states 
(unless flags have been set otherwise). 
</method> 
 
<method name="virtual bool UserHooks::canVetoPartonLevelEarly()"> 
is very similar to <code>canVetoPartonLevel()</code> above, except 
that the chance to veto appears somewhat earlier in the generation 
chain, after showers and multiparton interactions, but before the 
beam remnants and resonance decays have been added. It is therefore 
somewhat more convenient for many matrix element strategies, where 
the primordial <ei>kT</ei> added along with the beam remnants should 
not be included. 
</method> 
 
<method name="virtual bool UserHooks::doVetoPartonLevelEarly(const 
Event& event)"> 
is very similar to <code>doVetoPartonLevel(...)</code> above, but 
the veto can be done earlier, as described for 
<code>canVetoPartonLevelEarly()</code>. 
</method> 
 
<h3>(ii) Interrupt during the parton-level evolution, at a 
<ei>pT</ei> scale</h3> 
 
During the parton-level evolution, multiparton interactions (MPI), 
initial-state radiation (ISR) and final-state radiation (FSR) 
are normally evolved downwards in 
one interleaved evolution sequence of decreasing <ei>pT</ei> values. 
For some applications, e.g  matrix-element-matching approaches, it 
may be convenient to stop the evolution temporarily when the "hard" 
emissions have been considered, but before continuing with the more 
time-consuming soft activity. Based on these hard partons one can make 
a decision whether the event at all falls in the intended event class, 
e.g. has the "right" number of parton-level jets. If yes then, as for 
the methods above, the evolution will continue all the way up to a 
complete event. Also as above, if no, then the event will not be 
considered in the final cross section. 
 
<p/> 
Recall that the new or modified partons resulting from a MPI, ISR or FSR 
step are always appended to the end of the then-current event record. 
Previously existing partons are not touched, except for the 
<aloc href="ParticleProperties">status, mother and daughter</aloc> 
values, which are updated to reflect the modified history. It is 
therefore straightforward to find the partons associated with the most 
recent occurrence. 
<br/>An MPI results in four new partons being appended, two incoming 
and two outgoing ones. 
<br/>An ISR results in the whole affected system being copied down, 
with one of the two incoming partons being replaced by a new one, and 
one more outgoing parton. 
<br/>An FSR results in three new partons, two that come from the 
branching and one that takes the recoil. 
<br/>The story becomes more messy when rescattering is allowed as part 
of the MPI machinery. Then there will not only be a new system, as 
outlined above, but additionally some existing systems will undergo 
cascade effects, and be copied down with changed kinematics. 
 
<p/> 
In this subsection we outline the possibility to interrupt at a given 
<ei>pT</ei> scale, in the next to interrupt after a given number of 
emissions. 
 
<method name="virtual bool UserHooks::canVetoPT()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoPT(...)</code> will 
interrupt the downward evolution at <code>scaleVetoPT()</code>. 
</method> 
 
<method name="virtual double UserHooks::scaleVetoPT()"> 
In the base class this method returns 0. You should redefine it 
to return the <ei>pT</ei> scale at which you want to study the event. 
</method> 
 
<method name="virtual bool UserHooks::doVetoPT(int iPos, const Event& event)"> 
can optionally be called, as described above. You can study, but not 
modify, the <code>event</code> event record of the partonic process. 
Based on that you can decide whether to veto the event, true, or let 
it continue to evolve, false. If you veto, then this event is not 
counted among the accepted ones, and does not contribute to the estimated 
cross section. The <code>Pytha::next()</code> method will begin a 
completely new event, so the vetoed event will not appear in the 
output of <code>Pythia::next()</code>. 
<argument name="iPos"> is the position/status when the routine is 
called, information that can help you decide your course of action: 
<argoption value="0"> when no MPI, ISR or FSR occurred above the veto scale; 
</argoption> 
<argoption value="1"> when inside the interleaved MPI + ISR + FSR evolution, 
after an MPI process; 
</argoption> 
<argoption value="2"> when inside the interleaved MPI + ISR + FSR evolution, 
after an ISR emission; 
</argoption> 
<argoption value="3"> when inside the interleaved MPI + ISR + FSR evolution, 
after an FSR emission; 
</argoption> 
<argoption value="4"> for the optional case where FSR is deferred from the 
interleaved evolution and only considered separately afterward (then 
alternative 3 would never occur); 
</argoption> 
<argoption value="5"> is for subsequent resonance decays, and is called once 
for each decaying resonance in a chain such as 
<ei>t &rarr; b W, W &rarr; u dbar</ei>. 
</argoption> 
</argument> 
<argument name="event"> the event record contains a list of all partons 
generated so far, also including intermediate ones not part of the 
"current final state", and also those from further multiparton interactions. 
This may not be desirable for comparisons with matrix-element calculations. 
You may want to make use of the <code>subEvent(...)</code> method below to 
obtain a simplified event record <code>workEvent</code>. 
</argument> 
</method> 
 
<h3>(iii) Interrupt during the parton-level evolution, after a step</h3> 
 
These options are closely related to the ones above in section (ii), so 
we do not repeat the introduction, nor the possibilities to study the 
event record, also by using <code>subEvent(...)</code> and 
<code>workEvent</code>. 
What is different is that the methods in this section give access to the 
event as it looks like after each of the first few steps in the downwards 
evolution, irrespective of the <ei>pT</ei> scales of these branchings. 
Furthermore, it is here assumed that the focus normally is on the hardest 
subprocess, so that ISR/FSR emissions associated with additional MPI's 
are not considered. For MPI studies, however, a separate simpler 
alternative is offered to consider the event after a given number 
of interactions. 
 
<method name="virtual bool UserHooks::canVetoStep()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoStep(...)</code> will 
interrupt the downward ISR and FSR evolution the first 
<code>numberVetoStep()</code> times. 
</method> 
 
<method name="virtual int UserHooks::numberVetoStep()"> 
Returns the number of steps <ei>n</ei> each of ISR and FSR, for the 
hardest interaction, that you want to be able to study. That is, 
the method will be called after the first <ei>n</ei> ISR emissions, 
irrespective of the number of FSR ones at the time, and after the 
first <ei>n</ei> FSR emissions, irrespective of the number of ISR ones. 
The number of steps defaults to the first one only, but you are free 
to pick another value. Note that double diffraction is handled as two 
separate Pomeron-proton collisions, and thus has two sequences of 
emissions. 
</method> 
 
<method name="virtual bool UserHooks::doVetoStep(int iPos, 
int nISR, int nFSR, const Event& event)"> 
can optionally be called, as described above. You can study, but not 
modify, the <code>event</code> event record of the partonic process. 
Based on that you can decide whether to veto the event, true, or let 
it continue to evolve, false. If you veto, then this event is not 
counted among the accepted ones, and does not contribute to the estimated 
cross section. The <code>Pytha::next()</code> method will begin a 
completely new event, so the vetoed event will not appear in the 
output of <code>Pythia::next()</code>. 
<argument name="iPos"> is the position/status when the routine is 
called, information that can help you decide your course of action. 
Agrees with options 2 - 5 of the <code>doVetoPT(...)</code> routine 
above, while options 0 and 1 are not relevant here. 
</argument> 
<argument name="nISR"> is the number of ISR emissions in the hardest 
process so far. For resonance decays, <code>iPos = 5</code>, it is 0. 
</argument> 
<argument name="nFSR"> is the number of FSR emissions in the hardest 
process so far. For resonance decays, <code>iPos = 5</code>, it is the 
number of emissions in the currently studied system. 
</argument> 
<argument name="event"> the event record contains a list of all partons 
generated so far, also including intermediate ones not part of the 
"current final state", and also those from further multiparton interactions. 
This may not be desirable for comparisons with matrix-element calculations. 
You may want to make use of the <code>subEvent(...)</code> method above to 
obtain a simplified event record. 
</argument> 
</method> 
 
<method name="virtual bool UserHooks::canVetoMPIStep()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoMPIStep(...)</code> will 
interrupt the downward MPI evolution the first 
<code>numberVetoMPIStep()</code> times. 
</method> 
 
<method name="virtual int UserHooks::numberVetoMPIStep()"> 
Returns the number of steps in the MPI evolution that you want to be 
able to study, right after each new step has been taken and the 
subcollision has been added to the event record. The number of steps 
defaults to the first one only, but you are free to pick another value. 
Note that the hardest interaction of an events counts as the first 
multiparton interaction. For most hard processes it thus at the first 
step offers nothing not available with the <code>VetoProcessLevel</code> 
functionality above. For the minimum-bias and diffractive systems the 
hardest interaction is not selected at the process level, however, so 
there a check after the first multiparton interaction offers new 
functionality. Note that double diffraction is handled as two separate 
Pomeron-proton collisions, and thus has two sequences of interactions. 
Also, if you have set up a second hard process then a check is made 
after these first two, and the first interaction coming from the MPI 
machinery would have sequence number 3. 
</method> 
 
<method name="virtual bool UserHooks::doVetoMPIStep(int nMPI, 
const Event& event)"> 
can optionally be called, as described above. You can study, but not 
modify, the <code>event</code> event record of the partonic process. 
Based on that you can decide whether to veto the event, true, or let 
it continue to evolve, false. If you veto, then this event is not 
counted among the accepted ones, and does not contribute to the estimated 
cross section. The <code>Pytha::next()</code> method will begin a 
completely new event, so the vetoed event will not appear in the 
output of <code>Pythia::next()</code>. 
<argument name="nMPI"> is the number of MPI subprocesses has occurred 
so far. 
</argument> 
<argument name="event"> the event record contains a list of all partons 
generated so far, also including intermediate ones not part of the 
"current final state", e.g. leftovers from the ISR and FSR evolution 
of previously generated systems. The most recently added one has not 
had time to radiate, of course. 
</argument> 
</method> 
 
<h3>(iv) Veto emissions</h3> 
 
The methods in this group are intended to allow the veto of an emission 
in ISR, FSR or MPI, without affecting the evolution in any other way. 
If an emission is vetoed, the event record is "rolled back" to the 
way it was before the emission occurred, and the evolution in <ei>pT</ei> 
is continued downwards from the rejected value. The decision can be 
based on full knowledge of the kinematics of the shower branching or MPI. 
 
<p/> 
To identify where shower emissions originated, the ISR/FSR veto 
routines are passed the system from which the radiation occurred, according 
to the Parton Systems class (see <aloc href="AdvancedUsage">Advanced 
Usage</aloc>). Note, however, that inside the veto routines only the event 
record has been updated; all other information, including the Parton 
Systems, reflects the event before the shower branching or MPI has 
taken place. 
 
<method name="virtual bool UserHooks::canVetoISREmission()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoISREmission(...)</code> 
will interrupt the initial-state shower immediately after each 
emission and allow that emission to be vetoed. 
</method> 
 
<method name="virtual bool UserHooks::doVetoISREmission( 
int sizeOld, const Event& event, int iSys)"> 
can optionally be called, as described above. You can study, but not 
modify, the <code>event</code> event record of the partonic process. 
Based on that you can decide whether to veto the emission, true, or 
not, false. If you veto, then the latest emission is removed from 
the event record. In either case the evolution of the shower will 
continue from the point where it was left off. 
<argument name="sizeOld"> is the size of the event record before the 
latest emission was added to it. It will also become the new size if 
the emission is vetoed. 
</argument> 
<argument name="event"> the event record contains a list of all partons 
generated so far. Of special interest are the ones associated with the 
most recent emission, which are stored in entries from <code>sizeOld</code> 
through <code>event.size() - 1</code> inclusive. If you veto the emission 
these entries will be removed, and the history info in the remaining 
partons will be restored to a state as if the emission had never occurred. 
</argument> 
<argument name="iSys"> the system where the radiation occurs, according 
to Parton Systems. 
</argument> 
</method> 
 
<method name="virtual bool UserHooks::canVetoFSREmission()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoFSREmission(...)</code> 
will interrupt the final-state shower immediately after each 
emission and allow that emission to be vetoed. 
</method> 
 
<method name="virtual bool UserHooks::doVetoFSREmission( 
int sizeOld, const Event& event, int iSys, bool inResonance = false)"> 
can optionally be called, as described above. You can study, but not 
modify, the <code>event</code> event record of the partonic process. 
Based on that you can decide whether to veto the emission, true, or 
not, false. If you veto, then the latest emission is removed from 
the event record. In either case the evolution of the shower will 
continue from the point where it was left off. 
<argument name="sizeOld"> is the size of the event record before the 
latest emission was added to it. It will also become the new size if 
the emission is vetoed. 
</argument> 
<argument name="event"> the event record contains a list of all partons 
generated so far. Of special interest are the ones associated with the 
most recent emission, which are stored in entries from <code>sizeOld</code> 
through <code>event.size() - 1</code> inclusive. If you veto the emission 
these entries will be removed, and the history info in the remaining 
partons will be restored to a state as if the emission had never occurred. 
</argument> 
<argument name="iSys"> the system where the radiation occurs, according 
to Parton Systems. 
</argument> 
<argument name="inResonance"> <code>true</code> if the emission takes 
place in a resonance decay, subsequent to the hard process. 
</argument> 
</method> 
 
<method name="virtual bool UserHooks::canVetoMPIEmission()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoMPIEmission(...)</code> 
will interrupt the MPI machinery immediately after each multiparton 
interaction and allow it to be vetoed. 
</method> 
 
<method name="virtual bool UserHooks::doVetoMPIEmission( 
int sizeOld, const Event& event)"> 
can optionally be called, as described above. You can study, but not 
modify, the <code>event</code> event record of the partonic process. 
Based on that you can decide whether to veto the MPI, true, or 
not, false. If you veto, then the latest MPI is removed from 
the event record. In either case the interleaved evolution will 
continue from the point where it was left off. 
<argument name="sizeOld"> is the size of the event record before the 
latest MPI was added to it. It will also become the new size if 
the MPI is vetoed. 
</argument> 
<argument name="event"> the event record contains a list of all partons 
generated so far. Of special interest are the ones associated with the 
most recent MPI, which are stored in entries from <code>sizeOld</code> 
through <code>event.size() - 1</code> inclusive. If you veto the MPI 
these entries will be removed. 
</argument> 
</method> 
 
<h3>(v) Modify cross-sections or phase space sampling</h3> 
 
This section addresses two related but different topics. In both 
cases the sampling of events in phase space is modified, so that 
some regions are more populated while others are depleted. 
In the first case, this is assumed to be because the physical 
cross section should be modified relative to the built-in Pythia 
form. Therefore not only the relative population of phase space 
is changed, but also the integrated cross section of the process. 
In the second case the repopulation is only to be viewed as a 
technical trick to sample some phase-space regions better, so as 
to reduce the statistical error. There each event instead obtains 
a compensating weight, the inverse of the differential cross section 
reweighting factor, in such a way that the integrated cross section 
is unchanged. Below these two cases are considered separately, 
but note that they share many points. 
 
<method name="virtual bool UserHooks::canModifySigma()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>multiplySigmaBy(...)</code> will 
allow you to modify the cross section weight assigned to the current 
event. 
</method> 
 
<method name="virtual double UserHooks::multiplySigmaBy( 
const SigmaProcess* sigmaProcessPtr, const PhaseSpace* phaseSpacePtr, 
bool inEvent)"> 
when called this method should provide the factor by which you want to 
see the cross section weight of the current event modified. If you 
return unity then the normal cross section is obtained. Note that, unlike 
the methods above, these modifications do not lead to a difference between 
the number of "selected" events and the number of "accepted" ones, 
since the modifications occur already before the "selected" level. 
The integrated cross section of a process is modified, of course. 
Note that the cross section is only modifiable for normal hard processes. 
It does not affect the cross section in further multiparton interactions, 
nor in elastic/diffractive/minimum-bias events. 
<argument name="sigmaProcessPtr, phaseSpacePtr">: 
what makes this routine somewhat tricky to write is that the 
hard-process event has not yet been constructed, so one is restricted 
to use the information available in the phase-space and cross-section 
objects currently being accessed. Which of their  methods are applicable 
depends on the process, in particular the number of final-state particles. 
The <code>multiplySigmaBy</code> code in <code>UserHooks.cc</code> 
contains explicit instructions about which methods provide meaningful 
information, and so offers a convenient starting point. 
</argument> 
<argument name="inEvent">: this flag is true when the method is 
called from within the event-generation machinery and false 
when it is called at the initialization stage of the run, when the 
cross section is explored to find a maximum for later Monte Carlo usage. 
Cross-section modifications should be independent of this flag, 
for consistency, but if <code> multiplySigmaBy(...)</code> is used to 
collect statistics on the original kinematics distributions before cuts, 
then it is important to be able to exclude the initialization stage 
from comparisons. 
</argument> 
</method> 
 
<p/> 
One derived class is supplied as an example how this facility can be used 
to reweight cross sections in the same spirit as is done with QCD cross 
sections for the minimum-bias/underlying-event description: 
 
<class name="SuppressSmallPT : public UserHooks"> 
suppress small-<ei>pT</ei> production for <ei>2 &rarr; 2</ei> processes 
only, while leaving other processes unaffected. The basic suppression 
factor is <ei>pT^4 / ((k*pT0)^2 + pT^2)^2</ei>, where <ei>pT</ei> 
refers to the current hard subprocess and <ei>pT0</ei> is the same 
energy-dependent dampening scale as used for 
<aloc href="MultipartonInteractions">multiparton interactions</aloc>. 
This class contains <code>canModifySigma()</code> and 
<code>multiplySigmaBy()</code> methods that overload the base class ones. 
 
<method name="SuppressSmallPT::SuppressSmallPT( double pT0timesMPI = 1., 
int numberAlphaS = 0, bool useSameAlphaSasMPI = true)"> 
 The optional arguments of the constructor provides further variability. 
<argument name="pT0timesMPI"> 
corresponds to the additional factor <ei>k</ei> in the above formula. 
It is by default equal to 1 but can be used to explore deviations from 
the expected value. 
</argument> 
<argument name="numberAlphaS"> 
if this number <ei>n</ei> is bigger than the default 0, the 
corresponding number of <ei>alpha_strong</ei> factors is also 
reweighted from the normal renormalization scale to a modified one, 
i.e. a further suppression factor 
<ei>( alpha_s((k*pT0)^2 + Q^2_ren) / alpha_s(Q^2_ren) )^n</ei> 
is introduced. 
</argument> 
<argument name="useSameAlphaSasMPI"> 
regulates which kind of new <ei>alpha_strong</ei> value is evaluated 
for the numerator in the above expression. It is by default the same 
as set for multiparton interactions (i.e. same starting value at 
<ei>M_Z</ei> and same order of running), but if <code>false</code> 
instead the one for hard subprocesses. The denominator 
<ei>alpha_s(Q^2_ren)</ei> is always the value used for the "original", 
unweighted cross section. 
</argument> 
</method> 
</class> 
 
<p/> 
The second main case of the current section involves three methods, 
as follows. 
 
<method name="virtual bool UserHooks::canBiasSelection()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>biasSelectionBy(...)</code> will 
allow you to modify the phase space sampling, with a compensating 
event weight, such that the cross section is unchanged. You cannot 
combine this kind of reweighting with the selection of a 
<aloc href="SecondHardProcess">second hard process</aloc>. 
</method> 
 
<method name="virtual double UserHooks::biasSelectionBy( 
const SigmaProcess* sigmaProcessPtr, const PhaseSpace* phaseSpacePtr, 
bool inEvent)"> 
when called this method should provide the factor by which you want to 
see the phase space sampling of the current event modified. Events are 
assigned a weight being the inverse of this, such that the integrated 
cross section of a process is unchanged. Note that the selection 
is only modifiable for normal hard processes. It does not affect the 
selection in further multiparton interactions, nor in 
elastic/diffractive/minimum-bias events. 
<argument name="sigmaProcessPtr, phaseSpacePtr">: 
what makes this routine somewhat tricky to write is that the 
hard-process event has not yet been constructed, so one is restricted 
to use the information available in the phase-space and cross-section 
objects currently being accessed. Which of their  methods are applicable 
depends on the process, in particular the number of final-state particles. 
The <code>biasSelectionBy</code> code in <code>UserHooks.cc</code> 
contains explicit instructions about which methods provide meaningful 
information, and so offers a convenient starting point. 
</argument> 
<argument name="inEvent">: this flag is true when the method is 
called from within the event-generation machinery and false 
when it is called at the initialization stage of the run, when the 
cross section is explored to find a maximum for later Monte Carlo usage. 
Cross-section modifications should be independent of this flag, 
for consistency, but if <code>biasSelectionBy(...)</code> is used to 
collect statistics on the original kinematics distributions before cuts, 
then it is important to be able to exclude the initialization stage 
from comparisons. 
</argument> 
</method> 
 
<method name="virtual double UserHooks::biasedSelectionWeight()"> 
Returns the weight you should assign to the event, to use e.g. when 
you histogram results. It is the exact inverse of the weight you 
used to modify the phase-space sampling, a weight that must be stored 
in the <code>selBias</code> member variable, such that this routine 
can return <code>1/selBias</code>. The weight is also returned by the 
<code>Info::weight()</code> method, which may be more convenient to use. 
</method> 
 
<h3>(vi) Reject the decay sequence of resonances</h3> 
 
Resonance decays are performed already at the process level, as 
an integrated second step of the hard process itself. One reason is 
that the matrix element of many processes encode nontrivial decay 
angular distributions. Another is to have equivalence with Les Houches 
input, where resonance decays typically are provided from the onset. 
The methods in this section allow you to veto that decay sequence and 
try a new one. Unlike the veto of the whole process-level step, 
in point (i), the first step of the hard process is retained, i.e. 
where the resonances are produced. For this reason the cross section 
is not affected here but, depending on context, you may want to introduce 
your own counters to check how often a new set of decay modes and 
kinematics is selected, and correct accordingly. 
 
<p/>The main method below is applied after all decays. For the production 
of a <ei>t tbar</ei> pair this typically means after four decays, 
namely those of the <ei>t</ei>, the <ei>tbar</ei>, the <ei>W+</ei> 
and the <ei>W-</ei>. If Les Houches events are processed, the rollback 
is to the level of the originally read events. For top, that might mean 
either to the tops, or to the <ei>W</ei> bosons, or no rollback at all, 
depending on how the process generation was set up. 
 
<method name="virtual bool UserHooks::canVetoResonanceDecays()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doVetoResonanceDecays(...)</code> 
will be called immediately after the resonance decays have been 
selected and stored in the <code>process</code> event record, 
as described above for <code>canVetoProcessLevel()</code>. 
</method> 
 
<method name="virtual bool UserHooks::doVetoResonanceDecays(Event& process)"> 
can optionally be called, as described above. You can study the 
<code>process</code> event record of the hard process. 
Based on that you can decide whether to reject the sequence of 
resonance decays that was not already fixed by the production step 
of the hard process (which can vary depending on how a process has 
been set up, see above). If you veto, then a new resonance decay 
sequence is selected, but the production step remains unchanged. 
The cross section remains unaffected by this veto, for better or worse. 
<note>Warning:</note> Normally you should not modify the <code>process</code> 
event record. However, as an extreme measure, parts or the complete decay 
chain could be overwritten. If so, be very careful. 
</method> 
 
<h3>(vii) Modify scale in shower evolution</h3> 
 
The choice of maximum shower scale in resonance decays is normally not a 
big issue, since the shower here is expected to cover the full phase 
space. In some special cases a matching scheme is intended, where hard 
radiation is covered by matrix elements, and only softer by showers. The 
below two methods support such an approach. Note that the two methods 
are not used in the <code>TimeShower</code> class itself, but when 
showers are called from the <code>PartonLevel</code> generation. Thus 
user calls directly to <code>TimeShower</code> are not affected. 
 
<method name="virtual bool UserHooks::canSetResonanceScale()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>scaleResonance(...)</code> 
will set the initial scale of downwards shower evolution. 
</method> 
 
<method name="virtual double UserHooks::scaleResonance( int iRes, 
const Event& event)"> 
can optionally be called, as described above. You should return the maximum 
scale, in GeV, from which the shower evolution will begin. The base class 
method returns 0, i.e. gives no shower evolution at all. 
You can study, but not modify, the <code>event</code> event record 
of the partonic process to check which resonance is decaying, and into what. 
<argument name="iRes"> is the location in the event record of the 
resonance that decayed to the particles that now will shower. 
</argument> 
<argument name="event"> the event record contains a list of all partons 
generated so far, specifically the decaying resonance and its immediate 
decay products. 
</argument> 
</method> 
 
<h3>(viii) Allow colour reconnection</h3> 
 
PYTHIA contains only a limites set of possibilities for 
<aloc href="BeamRemnants">colour reconnection</aloc>, and none of them 
are geared specifically towards rapidly decaying resonances. Notably, 
with the default 
<code><aloc href="MasterSwitches">PartonLevel:earlyResDec = off</aloc></code>, 
resonances will only decay after colour reconnection has already been 
considered. Thus a coloured parton like the top may be reconnected 
but, apart from this external connection with the rest of the event, 
the top decay products undergo no colour reconnection. 
For <code>PartonLevel:earlyResDec = on</code> the resonance will decay 
earlier, and thus the decay products may undergo reconnections, but not 
necessarily by models that are specifically geared towards this kind of 
events. For tryout purposes, a user hook can be called directly after the 
resonance decays, and there modify the colour flow. This holds whether the 
resonance decay is handled early or late, but is especially appropriate 
for the latter default possibility. While intended specifically for 
resonance decays, alternatively it is possible to switch off the built-in 
colour reconnection and here implement your own reconnection model 
for the whole event. 
 
<method name="virtual bool UserHooks::canReconnectResonanceSystems()"> 
In the base class this method returns false. If you redefine it 
to return true then the method <code>doReconnectResonanceSystems(...)</code> 
will be called immediately after the resonance decays and their 
associated final-state showers have been added to the event record. 
</method> 
 
<method name="virtual bool UserHooks::doReconnectResonanceSystems( 
int oldSizeEvent, Event& event)"> 
can optionally be called, as described above, to reconnect colours 
in the event. The method should normally return true, but false if the 
colour-reconnected event is unphysical and to be rejected. 
(If this is likely to happen, having a safety copy to restore to 
is a good idea, so that false can be avoided to the largest extent 
possible.) 
<argument name="oldSizeEvent"> the size of the event record before 
resonance decay products and their associated final-state showers 
have been added to the event. Can thus be used to easily separate 
the resonance-decay partons from those in the rest of the event. 
</argument> 
<argument name="event"> the event record contains a list of all particles 
generated so far. You are allowed to freely modify this record, but with 
freedom comes responsibility. Firstly, it is only meaningful to modify 
the colour indices of final-state (at the time) partons; all other event 
properties had better be left undisturbed. Secondly, it is up to you to 
ensure that the new colour topology is meaningful, e.g. that no gluon 
has obtained the same colour as anticolour index and thereby forms 
a singlet on its own. 
</argument> 
</method> 
 
<h3>(ix) Enhanced rate of rare shower splittings</h3> 
 
PYTHIA also offers possibilities to enhance the frequency of rare 
splittings.  This is not a trivial task, since a simple "upweighting" of 
splittings would produce a mismatch between emission and no-emission 
probabilities, leading to a violation of the principle that the parton 
shower should not change the inclusive (input) cross section. 
Nevertheless, a general algorithm that allows for increased emission 
probabilities, while keeping no-emission factors intact, was presented 
in <ref>Lon13a</ref>. 
 
<p/>In <ref>Lon13a</ref> two types of enhancements are proposed: those 
of "regular" shower emissions, and those of trial shower emissions, the 
latter as part of the mandatory Sudakov reweighting in ME+PS merging 
schemes. Both of these possibilities are accessible through 
<code>UserHooks</code>, but cannot be used at the same time. 
 
<p/>The price to pay for these enhancements is that events come with a 
compensatory weight. The advantages of obtaining higher statistics 
for rare branchings thus is mitigated, and the usefulness has to be 
evaluated case by case. 
 
<p/>Currently enhancements of ISR and FSR branchings have been 
included. These enhancements are currently not phase-space dependent, 
i.e. emissions will be enhanced uniformly in phase space. 
It should also be noted that the threshold 
region for gluon branchings to a pair of heavy quarks, specifically 
<ei>g &rarr; c + cbar</ei> and <ei>g &rarr; b + bbar</ei>, are not 
enhanced in this algorithm. Technically it is because this case is 
handled separately in the code, but there also remain some physics issues 
to understand. 
 
<p/>To increase statistics of rare emissions in the showers, e.g. QED 
or weak radiation, Pythia supplies the following functions implementing 
the strategy of section 4 in <ref>Lon13a</ref>. 
 
<method name="virtual bool UserHooks::canEnhanceEmission()"> 
In the base class this method returns false. If you redefine it to 
return true then the method <code>enhanceFactor(...)</code> will be 
used to rescale an initial-state splitting probability. Both 
<code>enhanceFactor(...)</code> and <code>vetoProbability(...)</code> 
will have to be derived to allow for a branching enhancement. 
</method> 
 
<method name="virtual double UserHooks::enhanceFactor( string name)"> 
This function should return the enhancement factor for the splitting 
probability with identifier <code>name</code>. It should return 1. 
by default, i.e. for all input strings it does not propose to handle. 
<argument name="name"> the name of the splitting that can enhanced. 
Currently, the following input names are recognized by the PYTHIA 
showers, and can thus be used to enhance the respective 
splittings. 
<p/> 
ISR QCD branchings: 
<br/><code>isr:G2GG</code> for <ei>g &rarr; g + g</ei>, 
<br/><code>isr:G2QQ</code> for <ei>g &rarr; q + qbar</ei> 
with <ei>q</ei> a light quark, 
<br/><code>isr:G2QQ:cc</code> for <ei>g &rarr; c + cbar</ei> 
<br/><code>isr:G2QQ:bb</code> for <ei>g &rarr; b + bbar</ei> 
<br/><code>isr:Q2QG</code> for <ei>q &rarr; q + g</ei>, 
<br/><code>isr:Q2GQ</code> for <ei>q &rarr; g + q</ei>; 
<p/> 
ISR QED branchings: 
<br/><code>isr:Q2QA</code> for <ei>q &rarr; q + photon</ei>, 
<br/><code>isr:Q2AQ</code> for <ei>q &rarr; photon + q</ei>; 
<p/> 
ISR weak shower branchings: 
<br/><code>isr:Q2QW</code> for <ei>q &rarr; q + W</ei> or 
<ei>q &rarr; q + Z</ei>; 
<p/> 
FSR QCD branchings: 
<br/><code>fsr:G2GG</code> for <ei>g &rarr; g + g</ei>, 
<br/><code>fsr:G2QQ</code> for <ei>g &rarr; q + qbar</ei> 
with <ei>q</ei> a light quark, 
<br/><code>fsr:G2QQ:cc</code> for <ei>g &rarr; c + cbar</ei> 
<br/><code>fsr:G2QQ:bb</code> for <ei>g &rarr; b + bbar</ei> 
<br/><code>fsr:Q2QG</code> for <ei>q &rarr; q + g</ei>; 
<p/> 
FSR QED branchings: 
<br/><code>fsr:Q2QA</code> for <ei>q &rarr; q + photon</ei>, 
<br/><code>fsr:A2QQ</code> for <ei>photon &rarr; q + qbar</ei>, 
<br/><code>fsr:A2LL</code> for <ei>photon &rarr; lepton + antilepton</ei>, 
<p/> 
FSR weak shower branchings: 
<br/><code>fsr:Q2QW</code> for <ei>q &rarr; q + W</ei> or 
<ei>q &rarr; q + Z</ei>; 
<p/> 
FSR hidden valley branchings: 
<br/><code>fsr:Q2QHV</code> for all hidden valley branchings. 
<br/> 
<br/>Charge-conjugated branchings are included whenever relevant. 
Note that the order of the daughters matters: in the backwards 
evolution machinery the step is from the first daughter to the mother 
by the emission of the second daughter. 
 
<p/> 
Let's consider some examples. 
The evolution step changing the partonic state from 
<ei>q qbar &rarr; e+ e-</ei> to <ei>g qbar &rarr; e+ e- qbar</ei> 
through an initial state splitting can be enhanced 
by allowing a non-unity return value for the splitting with 
<code>name</code> equalling <code>isr:G2QQ</code>. Another evolution 
step changing <ei>g qbar &rarr; e+ e- qbar</ei> to 
<ei>g qbar &rarr; e+ e- qbar gluon</ei> through FSR (ISR) can be 
enhanced by allowing a non-unity return value for the splitting with 
<code>name</code> equalling <code>fsr:Q2QG</code> 
(<code>isr:Q2QG</code>). Yet another ISR branching converting 
<ei>g qbar &rarr; e+ e- qbar</ei> to <ei>q qbar &rarr; e+ e- qbar q</ei> can 
be enhanced by non-unity return value for the splitting with 
<code>name</code> equalling <code>isr:Q2GQ</code> 
(note the ordering in the branching name). 
</argument> 
</method> 
 
<method name="virtual double UserHooks::vetoProbability( string name)"> 
This function should return the probability of an emission that has 
been enhanced with the help of <code>enhanceFactor(...)</code>, for the 
same <code>name</code> arguments as above. An optimal choice depends on 
your personal needs. Using 0.5 as a baseline is perfectly acceptable. 
It should return 0. by default, i.e. for all input strings it does not 
propose to handle. 
</method> 
 
<method name="double getEnhancedEventWeight()"> 
If you use enhanced emissions, it is paramount to attribute a corrective 
weight to each event containing enhanced emissions. This function returns 
this weight. In the presence of enhancements, all histograms and output 
events must account for this weight. 
</method> 
 
<p/>In the context of merging, it can be beneficial to allow for enhanced 
trial emissions. As discussed in section 3 of <ref>Lon13a</ref>, this 
means that the Sudakov factors that are commonly generated by event 
vetoes based on trial emissions (see e.g. <ref>Lon11</ref>) are 
instead given by small but non-vanishing event weights. This can have 
advantages, since all events of an input sample will be retained. 
Pythia allows users to enhance trial emissions by using the following 
functions. 
 
<method name="virtual bool UserHooks::canEnhanceTrial()"> 
In the base class this method returns false. If you redefine it to 
return true then the method <code>enhanceFactor(...)</code> (see above) 
will be used to rescale an initial-state trial splitting probability. 
</method> 
 
<p/>Trial emission enhancements also necessitate corrective weights. 
These are handled internally, so that users only have to ensure that 
<code>Info::mergingWeight()</code> or 
<code>Info::mergingWeightNLO()</code> are correctly taken into account. 
 
<p/>A simple example of enhanced regular emissions is provided in 
<code>main63.cc</code>. 
 
<h3>(x) Modified hadronization</h3> 
 
The methods in this group are intended to allow for modifications of 
the string hadronization model, involving changes to hadronization 
parameters based on local properties of the string. Given information 
on the hadronization history of a single string,  parameters can be 
changed to control the selection of quark flavour, <ei>z</ei> and 
<ei>pT</ei> in string breaks. Furthermore there is a option to veto 
the creation of a hadron before it is added to the event record, 
thus repeating a step in the hadronization procedure. 
 
<method name="virtual bool canChangeFragPar()"> 
In the base class this method returns false. If you redefine it to 
return true, it will enable the methods <code>doChangeFragPar(...)</code>, 
<code>doVetoFragmentation(...)</code> and 
<code>setStringEnds(...)</code>. 
</method> 
 
<method name="virtual bool doChangeFragPar(StringFlav* flavPtr, StringZ* zPtr, 
StringPT* pTPtr, int idEnd, double m2Had, vector&lt;int&gt; iParton, 
const StringEnd * sEnd)"> 
This is the method for changing fragmentation parameters. If all 
parameters are changed as they should, the method should return true. 
In case of errors the method returns false and a warning is printed. 
The method takes as argument pointers to three objects that hold the 
fragmentation parameters, and three arguments that give information 
about the string currently being hadronized. 
<argument name="flavPtr"> is a pointer to a <code>StringFlav</code> 
object, which selects quark and diquark flavours and hadron species 
formed in the string breaks. The parameters can be changed by setting 
new parameters in a <code>Settings</code> object and reinitializing 
with <code>flavPtr->init(settings,randomPtr)</code>, where 
<code>settings</code> is the settings object and <code>randomPtr</code> 
is a pointer to the desired random number generator. Note that the 
<code>UserHooks</code> base class already holds pointers to the 
<code>Settings</code> and <code>Random</code> objects. 
</argument> 
<argument name="zPtr"> is a pointer to a <code>StringZ</code> object, 
which selects the hadron momentum fraction <ei>z</ei> using the Lund 
fragmentation function. Parameters are changed by 
<code>zPtr->init(settings,particleData,randomPtr)</code> 
in the same way as above. 
</argument> 
<argument name="pTPtr"> is a pointer to a <code>StringPT</code> object 
which selects the hadron <ei>pT</ei> from a Gaussian distribution. 
Parameters are changed by 
<code>pTPtr->init(settings,particleData,randomPtr)</code> 
in the same way as above. 
</argument> 
<argument name="idEnd"> gives the code of the parton (quark, diquark or 
gluon) in the string end that we are currently hadronizing from. 
</argument> 
<argument name="m2Had"> gives the invariant mass squared of all hadrons 
produced from the current end of the string up to this point. Used to 
keep track of where we are on the string. 
</argument> 
<argument name="iParton"> contains the indices in the standard event 
record for all the partons in the string currently being hadronized. 
</argument> 
<argument name="sEnd"> is a pointer to a <code>StringEnd</code> object 
 containing additional information about the string break. 
</argument> 
</method> 
 
<method name="virtual bool doVetoFragmentation(Particle had, 
const StringEnd * sEnd)"> 
This method can veto the production of a hadron, whereby the current 
string break is redone. 
<argument name="had"> is a (copy of) the hadron being produced, 
just before it is added to the event record. 
</argument> 
<argument name="sEnd"> is a pointer to a <code>StringEnd</code> object 
containing additional information about the string break. 
</argument> 
</method> 
 
<method name="virtual bool doVetoFragmentation(Particle had1, Particle had2, 
const StringEnd * sEnd1, const StringEnd * sEnd1)"> 
This method can veto the production of the final hadrons produced in the 
fragmentation of a string. 
<argument name="had1,had2"> are (copies of) the two final hadrons being 
produced, just before they are added to the event record. 
</argument> 
<argument name="sEnd1,sEnd2"> are pointers to <code>StringEnd</code> objects 
containing additional information about the string break. 
</argument> 
</method> 
 
<method name="virtual bool canSetImpactParameter()"> 
In the base class this method returns false. If you redefine it to 
return true, it will enable the method <code>doSetImpactParameter()</code> . 
</method> 
 
<method name="virtual bool doSetImpactParameter()"> 
This method allows the user to select a particular impact parameter in the 
MPI machinery in the generation of minimum-bias events. The function takes no 
arguments and is intended for situations where the impact parameter is 
generated from the outside, as is the case in the Angantyr model for Heavy Ion 
collisions. Note that it does not modify the 
<aloc href="MultipartonInteractions"> 
  <code>MultipartonInteractions:bProfile</code></aloc> used in the MPI 
  machinery, and that currently it will only affect the 
  <aloc href="QCDProcesses"> 
  <code>SoftQCD</code></aloc> processes. 
</method> 
 
 
<h3>Multiple user hooks</h3> 
 
In addition to the 
<br/><code><aloc href="ProgramFlow"> 
Pythia::setUserHooksPtr( UserHooks*)</aloc></code> 
<br/>method there is a second 
<br/><code><aloc href="ProgramFlow"> 
Pythia::addUserHooksPtr( UserHooks*)</aloc></code> 
<br/>method that works almost like the former, but it allows the 
addition of further user hooks. These are stored as a vector and 
all of them will be called consecutively at the respective locations 
where they are set up to be active. 
 
<p/> 
If two or more of them are active at the same location it is up to 
the user to ensure that the joint action is the one intended. 
In cases where weights are assigned the net result will be a weight 
that is is the product of them. In cases where vetoes are involved, 
a veto will be returned if either hook wants to veto, i.e. the 
no-veto survival probability is combined multiplicatively. 
It is not meaningful to let two hooks set the resonance scale or 
change fragmentation parameters, so warnings will be issued if this 
occurs. 
 
</chapter> 
 
<!-- Copyright (C) 2020 Torbjorn Sjostrand --> 
