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src/EvtGenModels/EvtFourBodyPhsp.cpp

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#include <cmath>
#include "EvtGenBase/EvtKine.hh"
#include "EvtGenBase/EvtPDL.hh"
#include "EvtGenBase/EvtParticle.hh"
#include "EvtGenBase/EvtRandom.hh"
#include "EvtGenBase/EvtReport.hh"
#include "EvtGenModels/EvtFourBodyPhsp.hh"
std::string EvtFourBodyPhsp::getName()
{
return "FOURBODYPHSP";
}
EvtDecayBase* EvtFourBodyPhsp::clone()
{
return new EvtFourBodyPhsp;
}
std::array<double, 4> EvtFourBodyPhsp::phspFactor(
const double mM, const double m12, const double m34,
jbackUnsubmitted
Done
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I don't think we need to use double pointers here. Just define two phspFactor() functions, where the 2nd one includes q, p1 and p3 as extra arguments?

jback: I don't think we need to use double pointers here. Just define two phspFactor() functions…
krepsAuthorUnsubmitted
Done
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I'm not sure I understand. The reason why I use pointers here is that I'm actually returning also individual momenta so I do not have to calculate them twice. An alternative would be to avoid this function completely as it is used just in two places (when searching for maximum probability and at one point in decay).

kreps: I'm not sure I understand. The reason why I use pointers here is that I'm actually returning…
jbackUnsubmitted
Done
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Ok, I see what you mean.

jback: Ok, I see what you mean.
krepsAuthorUnsubmitted
Done
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After a bit of thinking, I decided against removing this function completely as in the search for maximum probability it is used in a few places and that would get a bit messier by repeating several lines every time.

kreps: After a bit of thinking, I decided against removing this function completely as in the search…
tlathamUnsubmitted
Not Done
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I'm also not a fan of this sort of construct. There are some alternatives, such as returning an array of 4 doubles or a tuple of 4 doubles.
I note that similar arguments are present in generatePairMasses, generateRectangle and generateTrapezoid.
However, since these are all private functions, I would say that this is a sufficiently minor detail that we can revisit it in a future modernisation sweep, e.g. after we've adopted C++17 as a minimum so we can use structured bindings to simplify the unpacking of the multiple returns.

tlatham: I'm also not a fan of this sort of construct. There are some alternatives, such as returning…
std::array<double, 4>& daughters ) const
{
std::array<double, 4> result;
result[1] = twoBodyMomentum( mM, m12, m34 );
result[2] = twoBodyMomentum( m12, daughters[0], daughters[1] );
result[3] = twoBodyMomentum( m34, daughters[2], daughters[3] );
result[0] = result[1] * result[2] * result[3];
return result;
}
void EvtFourBodyPhsp::init()
{
// Check that we have right number of daughters
checkNDaug( 4 );
// Check whether mother is quasi-stable
auto parent = getParentId();
double width = EvtPDL::getWidth( parent );
if ( width > 1e-6 ) {
m_stableMother = false;
}
// Check whether all daughters are stable
for ( int i = 0; i < 4; ++i ) {
auto daughter = getDaug( i );
width = EvtPDL::getWidth( daughter );
if ( width > 1e-6 ) {
m_stableDaughters = false;
m_daughterMasses[i] = EvtPDL::getMinMass( daughter );
} else {
m_daughterMasses[i] = EvtPDL::getMass( daughter );
}
}
// check correct number of arguments
checkNArg( 0, 2, 4 );
double mass1 = m_daughterMasses[0];
double mass2 = m_daughterMasses[1];
double mass3 = m_daughterMasses[2];
double mass4 = m_daughterMasses[3];
double mMother = EvtPDL::getMaxMass( parent );
if ( getNArg() > 2 ) {
m_m12Min = getArg( 0 );
m_m12Max = getArg( 1 );
m_m34Min = getArg( 2 );
m_m34Max = getArg( 3 );
} else {
if ( getNArg() > 0 ) {
m_m12Min = getArg( 0 );
m_m12Max = getArg( 1 );
} else {
m_m12Min = mass1 + mass2;
m_m12Max = mMother - mass3 - mass4;
}
m_m34Min = mass3 + mass4;
m_m34Max = mMother - mass1 - mass2;
if ( m_stableDaughters == false || m_stableMother == false ) {
m_fixedBoundary = false;
}
}
// Make sure that we have correct boundaries
if ( m_m12Min < mass1 + mass2 ) {
m_m12Min = mass1 + mass2;
}
if ( m_m12Max > mMother - mass3 - mass4 ) {
m_m12Max = mMother - mass3 - mass4;
}
if ( m_m34Min < mass3 + mass4 ) {
m_m34Min = mass3 + mass4;
}
if ( m_m34Max > mMother - mass1 - mass2 ) {
m_m34Max = mMother - mass1 - mass2;
}
if ( m_stableDaughters && m_stableMother ) {
m_boundaryShape = determineBoundaryShape( m_m12Min, m_m12Max, m_m34Max,
mMother );
} else {
m_boundaryShape = Shape::variable;
}
// If we have fixed boundary, we can precalculate some variables for
// m12 and m34 generation
if ( m_fixedBoundary ) {
if ( m_boundaryShape == Shape::trapezoid ) {
const double m12Diff = m_m12Max - m_m12Min;
const double minSum = m_m12Min + m_m34Min;
m_trapNorm = ( mMother - m_m34Min ) * m12Diff -
0.5 * ( m12Diff * ( m_m12Max + m_m12Min ) );
m_trapCoeff1 = mMother - m_m34Min;
m_trapCoeff2 = mMother * mMother - 2 * mMother * minSum +
minSum * minSum;
}
if ( m_boundaryShape == Shape::pentagon ) {
m_pentagonSplit = mMother - m_m34Max;
const double area1 = ( m_pentagonSplit - m_m12Min ) *
( m_m34Max - m_m34Min );
const double pm12Diff = m_m12Max - m_pentagonSplit;
const double area2 = 0.5 * pm12Diff *
( mMother + m_m34Max - m_m12Max ) -
pm12Diff * m_m34Min;
m_pentagonFraction = area1 / ( area1 + area2 );
const double m12Diff = m_m12Max - m_pentagonSplit;
const double minSum = m_pentagonSplit + m_m34Min;
m_trapNorm = ( mMother - m_m34Min ) * m12Diff -
0.5 * ( m12Diff * ( m_m12Max + m_pentagonSplit ) );
m_trapCoeff1 = mMother - m_m34Min;
m_trapCoeff2 = mMother * mMother - 2 * mMother * minSum +
minSum * minSum;
}
}
}
void EvtFourBodyPhsp::initProbMax()
{
double startM12 = m_m12Min + ( m_m12Max - m_m12Min ) / 20.;
double startM34 = m_m34Min + ( m_m34Max - m_m34Min ) / 20.;
bool contCond = true;
int iteration = 0;
auto parent = getParentId();
double mMother = EvtPDL::getMaxMass( parent );
double funcValue = 0;
while (contCond){
++iteration;
double currentM12 = startM12;
double currentM34 = startM34;
funcValue = phspFactor( mMother, currentM12, currentM34,
m_daughterMasses )[0];
// Maximum along m12
double step = ( m_m12Max - m_m12Min ) / 100.;
while ( step > 1e-4 ) {
double point1 = currentM12 + step;
if ( point1 > m_m12Max ) {
point1 = m_m12Max;
}
if ( currentM34 > mMother - point1 ) {
point1 = mMother - currentM34;
}
double point2 = currentM12 - step;
if ( point2 < m_m12Min ) {
point2 = m_m12Min;
}
double value1 = phspFactor( mMother, point1, currentM34,
m_daughterMasses )[0];
double value2 = phspFactor( mMother, point2, currentM34,
m_daughterMasses )[0];
if ( value1 > funcValue && value1 > value2 ) {
currentM12 = point1;
funcValue = value1;
} else if ( value2 > funcValue ) {
currentM12 = point2;
funcValue = value2;
}
step /= 2.;
}
// Maximum along m34
step = ( mMother - currentM12 - m_m34Min ) / 100.;
while ( step > 1e-4 ) {
double point1 = currentM34 + step;
if ( point1 > m_m34Max ) {
point1 = m_m34Max;
}
if ( point1 > mMother - currentM12 ) {
point1 = mMother - currentM12;
}
double point2 = currentM34 - step;
if ( point2 < m_m34Min ) {
point2 = m_m34Min;
}
double value1 = phspFactor( mMother, currentM12, point1,
m_daughterMasses )[0];
double value2 = phspFactor( mMother, currentM12, point2,
m_daughterMasses )[0];
if ( value1 > funcValue && value1 > value2 ) {
currentM34 = point1;
funcValue = value1;
} else if ( value2 > funcValue ) {
currentM34 = point2;
funcValue = value2;
}
step /= 2.;
}
// Check termination condition
double m12Diff = currentM12 - startM12;
double m34Diff = currentM34 - startM34;
double distSq = m12Diff * m12Diff + m34Diff * m34Diff;
if (distSq < 1e-8 || iteration > 50){
contCond = false;
}
startM12 = currentM12;
startM34 = currentM34;
}
setProbMax( funcValue * 1.05 );
}
void EvtFourBodyPhsp::decay( EvtParticle* parent )
{
parent->makeDaughters( getNDaug(), getDaugs() );
bool massTreeStatus = parent->generateMassTree();
if ( !massTreeStatus ) {
EvtGenReport( EVTGEN_ERROR, "EvtGen" )
<< "Failed to generate daughters masses in EvtFourBodyPhsp."
<< std::endl;
::abort();
}
double mMother = parent->mass();
// Need to check whether boundaries are OK and whether we need to work
// out boundary shape
double cM12Min, cM12Max;
double cM34Min, cM34Max;
EvtFourBodyPhsp::Shape cShape;
if ( m_fixedBoundary ) {
cM12Min = m_m12Min;
cM12Max = m_m12Max;
cM34Min = m_m34Min;
cM34Max = m_m34Max;
cShape = m_boundaryShape;
} else {
// In this case at least one particle has non-zero width and thus
// boundaries and shape of the region can change
for ( int i = 0; i < 4; ++i ) {
auto daughter = parent->getDaug( i );
m_daughterMasses[i] = daughter->mass();
}
cM12Min = m_m12Min > ( m_daughterMasses[0] + m_daughterMasses[1] )
? m_m12Min
: m_daughterMasses[0] + m_daughterMasses[1];
cM12Max = m_m12Max <
( mMother - m_daughterMasses[2] - m_daughterMasses[3] )
? m_m12Max
: mMother - m_daughterMasses[2] - m_daughterMasses[3];
cM34Min = m_m34Min > ( m_daughterMasses[2] + m_daughterMasses[3] )
? m_m34Min
: m_daughterMasses[2] + m_daughterMasses[3];
cM34Max = m_m34Max <
( mMother - m_daughterMasses[0] - m_daughterMasses[1] )
? m_m34Max
: mMother - m_daughterMasses[0] - m_daughterMasses[1];
cShape = determineBoundaryShape( cM12Min, cM12Max, cM34Max, mMother );
}
// Generate m12 and m34
auto masses = generatePairMasses( cM12Min, cM12Max, cM34Min, cM34Max,
mMother, cShape );
const double m12 = masses.first;
const double m34 = masses.second;
// calculate probability, it will return array with 4 elements with
// probability, q, p1 and p3
auto probEval = phspFactor( mMother, m12, m34, m_daughterMasses );
setProb( probEval[0] );
// initialise kinematics
const double cosTheta1 = EvtRandom::Flat(-1.0, 1.0);
const double sinTheta1 = std::sqrt( 1 - cosTheta1 * cosTheta1 );
const double cosTheta3 = EvtRandom::Flat(-1.0, 1.0);
const double sinTheta3 = std::sqrt( 1 - cosTheta3 * cosTheta3 );
const double phi = EvtRandom::Flat( 0., EvtConst::twoPi );
// m12 and m34 are put along z-axis, 1 and 2 go to x-z plane and 3-4
// plane is rotated by phi compared to 1-2 plane. All momenta are set
// in 12 and 34 rest frames and then boosted to parent rest frame
const double p1x = probEval[2] * sinTheta1;
const double p1z = probEval[2] * cosTheta1;
const double p1Sq = probEval[2] * probEval[2];
const double en1 = std::sqrt( m_daughterMasses[0] * m_daughterMasses[0] +
p1Sq );
const double en2 = std::sqrt( m_daughterMasses[1] * m_daughterMasses[1] +
p1Sq );
const double p3T = probEval[3] * sinTheta3;
const double p3x = p3T * std::cos( phi );
const double p3y = p3T * std::sin( phi );
const double p3z = probEval[3] * cosTheta3;
const double p3Sq = probEval[3] * probEval[3];
const double en3 = std::sqrt( m_daughterMasses[2] * m_daughterMasses[2] +
p3Sq );
const double en4 = std::sqrt( m_daughterMasses[3] * m_daughterMasses[3] +
p3Sq );
EvtVector4R mom1( en1, p1x, 0.0, p1z );
EvtVector4R mom2( en2, -p1x, 0.0, -p1z );
EvtVector4R mom3( en3, p3x, p3y, p3z );
EvtVector4R mom4( en4, -p3x, -p3y, -p3z );
const double qSq = probEval[1] * probEval[1];
const double en12 = std::sqrt( m12 * m12 + qSq );
const double en34 = std::sqrt( m34 * m34 + qSq );
EvtVector4R q12( en12, 0.0, 0.0, probEval[1] );
EvtVector4R q34( en34, 0.0, 0.0, -probEval[1] );
mom1.applyBoostTo( q12 );
mom2.applyBoostTo( q12 );
mom3.applyBoostTo( q34 );
mom4.applyBoostTo( q34 );
// As final step, rotate everything randomly in space
const double euler1 = EvtRandom::Flat( 0., EvtConst::twoPi );
const double euler2 = std::acos( EvtRandom::Flat( -1.0, 1.0 ) );
const double euler3 = EvtRandom::Flat( 0., EvtConst::twoPi );
mom1.applyRotateEuler( euler1, euler2, euler3 );
mom2.applyRotateEuler( euler1, euler2, euler3 );
mom3.applyRotateEuler( euler1, euler2, euler3 );
mom4.applyRotateEuler( euler1, euler2, euler3 );
// Set momenta for daughters
auto daug = parent->getDaug( 0 );
daug->init( daug->getId(), mom1 );
daug = parent->getDaug( 1 );
daug->init( daug->getId(), mom2 );
daug = parent->getDaug( 2 );
daug->init( daug->getId(), mom3 );
daug = parent->getDaug( 3 );
daug->init( daug->getId(), mom4 );
}
EvtFourBodyPhsp::Shape EvtFourBodyPhsp::determineBoundaryShape(
const double m12Min, const double m12Max, const double m34Max,
const double mMother ) const
{
double maxY = mMother - m12Min;
const bool corner1 = m34Max < maxY;
maxY = mMother - m12Max;
const bool corner2 = m34Max < maxY;
if ( corner1 && corner2 ) {
return Shape::rectangle;
} else if ( !corner1 && !corner2 ) {
return Shape::trapezoid;
}
return Shape::pentagon;
}
std::pair<double, double> EvtFourBodyPhsp::generatePairMasses(
const double m12Min, const double m12Max, const double m34Min,
const double m34Max, const double mMother,
const EvtFourBodyPhsp::Shape shape ) const
{
switch ( shape ) {
case EvtFourBodyPhsp::Shape::rectangle:
return generateRectangle( m12Min, m12Max, m34Min, m34Max );
break;
case EvtFourBodyPhsp::Shape::trapezoid:
return generateTrapezoid( m12Min, m12Max, m34Min, mMother );
break;
case EvtFourBodyPhsp::Shape::pentagon:
double split, fraction;
if ( m_fixedBoundary ) {
split = m_pentagonSplit;
fraction = m_pentagonFraction;
} else {
split = mMother - m34Max;
const double area1 = ( split - m12Min ) * ( m34Max - m34Min );
const double pm12Diff = m12Max - split;
const double area2 = 0.5 * pm12Diff *
( mMother + m34Max - m12Max ) -
pm12Diff * m34Min;
fraction = area1 / ( area1 + area2 );
}
if ( EvtRandom::Flat() < fraction ) {
return generateRectangle( m12Min, split, m34Min, m34Max );
} else {
return generateTrapezoid( split, m12Max, m34Min, mMother );
}
break;
default:
return std::make_pair( m12Min, m34Min );
break;
}
}
std::pair<double, double> EvtFourBodyPhsp::generateRectangle(
const double m12Min, const double m12Max, const double m34Min,
const double m34Max ) const
{
return std::make_pair( EvtRandom::Flat( m12Min, m12Max ),
EvtRandom::Flat( m34Min, m34Max ) );
}
std::pair<double, double> EvtFourBodyPhsp::generateTrapezoid(
const double m12Min, const double m12Max, const double m34Min,
const double mMother ) const
{
double norm, coeff1, coeff2;
if ( m_fixedBoundary ) {
norm = m_trapNorm;
coeff1 = m_trapCoeff1;
coeff2 = m_trapCoeff2;
} else {
const double m12Diff = m12Max - m12Min;
const double minSum = m12Min + m34Min;
norm = ( mMother - m34Min ) * m12Diff -
0.5 * ( m12Diff * ( m12Max + m12Min ) );
coeff1 = mMother - m34Min;
coeff2 = mMother * mMother - 2 * mMother * minSum + minSum * minSum;
}
const double rnd = EvtRandom::Flat();
const double m12 = coeff1 - std::sqrt( -2.0 * rnd * norm + coeff2 );
const double m34 = EvtRandom::Flat( m34Min, mMother - m12 );
return std::make_pair( m12, m34 );
}