`Vec4`

class gives a simple implementation of four-vectors.
The member function names are based on the assumption that these
represent four-momentum vectors. Thus one can get or set
`Vec4`

can equally well be used to store a space-time
four-vector.
The `Particle`

object contains a `Vec4 p`

that
stores the particle four-momentum, and another `Vec4 vProd`

for the production vertex. For the latter the input/output method
names are adapted to the space-time character rather than the normal
energy-momentum one. Thus a user would not normally access the
`Vec4`

classes directly, but only via the methods of the
`Particle`

class,
see Particle Properties.

Nevertheless you are free to use the PYTHIA four-vectors, e.g. as part of some simple analysis code based directly on the PYTHIA output, say to define the four-vector sum of a set of particles. But note that this class was never set up to allow complete generality, only to provide the operations that are of use inside PYTHIA. There is no separate class for three-vectors, since such can easily be represented by four-vectors where the fourth component is not used.

Four-vectors have the expected functionality: they can be created, copied, added, multiplied, rotated, boosted, and manipulated in other ways. Operator overloading is implemented where reasonable. Properties can be read out, not only the components themselves but also for derived quantities such as absolute momentum and direction angles.

**Vec4::Vec4(double x = 0., double y = 0., double z = 0., double t = 0.) **

creates a four-vector, by default with all components set to 0.

**Vec4::Vec4(const Vec4& v) **

creates a four-vector copy of the input four-vector.

**Vec4& Vec4::operator=(const Vec4& v) **

copies the input four-vector.

**Vec4& Vec4::operator=(double value) **

gives a four-vector with all components set to *value*.

**void Vec4::reset() **

sets all components to 0.

**void Vec4::p(double pxIn, double pyIn, double pzIn, double eIn) **

sets all components to their input values.

**void Vec4::p(Vec4 pIn) **

sets all components equal to those of the input four-vector.

**void Vec4::px(double pxIn) **

**void Vec4::py(double pyIn) **

**void Vec4::pz(double pzIn) **

**void Vec4::e(double eIn) **

sets the respective component to the input value.

**double Vec4::px() **

**double Vec4::py() **

**double Vec4::pz() **

**double Vec4::e() **

gets the respective component.

**double Vec4::mCalc() **

**double Vec4::m2Calc() **

the (squared) mass, calculated from the four-vectors.
If *m^2 < 0* the mass is given with a minus sign,
*-sqrt(-m^2)*. Note the possible loss of precision
in the calculation of *E^2 - p^2*; for particles the
correct mass is stored separately to avoid such problems.

**double Vec4::pT() **

**double Vec4::pT2() **

the (squared) transverse momentum.

**double Vec4::pAbs() **

**double Vec4::pAbs2() **

the (squared) absolute momentum.

**double Vec4::eT() **

**double Vec4::eT2() **

the (squared) transverse energy,
*eT = e * sin(theta) = e * pT / pAbs*.

**double Vec4::theta() **

the polar angle, in the range 0 through
*pi*.

**double Vec4::phi() **

the azimuthal angle, in the range *-pi* through *pi*.

**double Vec4::thetaXZ() **

the angle in the *xz* plane, in the range *-pi* through
*pi*, with 0 along the *+z* axis.

**double Vec4::pPos() **

**double Vec4::pNeg() **

the combinations *E+-p_z*.

**double Vec4::rap() **

**double Vec4::eta() **

true rapidity *y* and pseudorapidity *eta*.

`friend`

methods that take one, two
or three four-vectors as argument. Several of them only use the
three-vector part of the four-vector.
**friend ostream& operator<<(ostream&, const Vec4& v) **

writes out the values of the four components of a `Vec4`

and,
within brackets, a fifth component being the invariant length of the
four-vector, as provided by `mCalc()`

above, and it all
ended with a newline.

**friend double m(const Vec4& v1, const Vec4& v2) **

**friend double m2(const Vec4& v1, const Vec4& v2) **

the (squared) invariant mass.

**friend double dot3(const Vec4& v1, const Vec4& v2) **

the three-product.

**friend double cross3(const Vec4& v1, const Vec4& v2) **

the cross-product.

**friend double theta(const Vec4& v1, const Vec4& v2) **

**friend double costheta(const Vec4& v1, const Vec4& v2) **

the (cosine) of the opening angle between the vectors,
in the range 0 through *pi*.

**friend double phi(const Vec4& v1, const Vec4& v2) **

**friend double cosphi(const Vec4& v1, const Vec4& v2) **

the (cosine) of the azimuthal angle between the vectors around the
*z* axis, in the range 0 through *pi*.

**friend double phi(const Vec4& v1, const Vec4& v2, const Vec4& v3) **

**friend double cosphi(const Vec4& v1, const Vec4& v2, const Vec4& v3) **

the (cosine) of the azimuthal angle between the first two vectors
around the direction of the third, in the range 0 through *pi*.

**friend double RRapPhi(const Vec4& v1, const Vec4& v2) **

**friend double REtaPhi(const Vec4& v1, const Vec4& v2) **

the *R* distance measure, in *(y, phi)* or
*(eta, phi)* cylindrical coordinates, i.e.
*R^2 = (y_1 - y_2)^2 + (phi_1 - phi_2)^2* and equivalent.

**Vec4 Vec4::operator-() **

return a vector with flipped sign for all components, while leaving
the original vector unchanged.

**Vec4& Vec4::operator+=(const Vec4& v) **

add a four-vector to an existing one.

**Vec4& Vec4::operator-=(const Vec4& v) **

subtract a four-vector from an existing one.

**Vec4& Vec4::operator*=(double f) **

multiply all four-vector components by a real number.

**Vec4& Vec4::operator/=(double f) **

divide all four-vector components by a real number.

**friend Vec4 operator+(const Vec4& v1, const Vec4& v2) **

add two four-vectors.

**friend Vec4 operator-(const Vec4& v1, const Vec4& v2) **

subtract two four-vectors.

**friend Vec4 operator*(double f, const Vec4& v) **

**friend Vec4 operator*(const Vec4& v, double f) **

multiply a four-vector by a real number.

**friend Vec4 operator/(const Vec4& v, double f) **

divide a four-vector by a real number.

**friend double operator*(const Vec4& v1, const Vec4 v2) **

four-vector product.

There are also a few related operations that are normal member methods:

**void Vec4::rescale3(double f) **

multiply the three-vector components by *f*, but keep the
fourth component unchanged.

**void Vec4::rescale4(double f) **

multiply all four-vector components by *f*.

**void Vec4::flip3() **

flip the sign of the three-vector components, but keep the
fourth component unchanged.

**void Vec4::flip4() **

flip the sign of all four-vector components.

`RotBstMatrix`

can be used to speed up operations.
**void Vec4::rot(double theta, double phi) **

rotate the three-momentum with the polar angle *theta*
and the azimuthal angle *phi*.

**void Vec4::rotaxis(double phi, double nx, double ny, double nz) **

rotate the three-momentum with the azimuthal angle *phi*
around the direction defined by the *(n_x, n_y, n_z)*
three-vector.

**void Vec4::rotaxis(double phi, Vec4& n) **

rotate the three-momentum with the azimuthal angle *phi*
around the direction defined by the three-vector part of *n*.

**void Vec4::bst(double betaX, double betaY, double betaZ) **

boost the four-momentum by *beta = (beta_x, beta_y, beta_z)*.

**void Vec4::bst(double betaX, double betaY, double betaZ, double gamma) **

boost the four-momentum by *beta = (beta_x, beta_y, beta_z)*,
where the *gamma = 1/sqrt(1 - beta^2)* is also input to allow
better precision when *beta* is close to unity.

**void Vec4::bst(const Vec4& p) **

boost the four-momentum by *beta = (p_x/E, p_y/E, p_z/E)*.

**void Vec4::bst(const Vec4& p, double m) **

boost the four-momentum by *beta = (p_x/E, p_y/E, p_z/E)*,
where the *gamma = E/m* is also calculated from input to allow
better precision when *beta* is close to unity.

**void Vec4::bstback(const Vec4& p) **

boost the four-momentum by *beta = (-p_x/E, -p_y/E, -p_z/E)*.

**void Vec4::bstback(const Vec4& p, double m) **

boost the four-momentum by *beta = (-p_x/E, -p_y/E, -p_z/E)*,
where the *gamma = E/m* is also calculated from input to allow
better precision when *beta* is close to unity.

**void Vec4::rotbst(const RotBstMatrix& M) **

perform a combined rotation and boost; see below for a description
of the `RotBstMatrix`

.

For a longer sequence of rotations and boosts, and where several
`Vec4`

are to be rotated and boosted in the same way,
a more efficient approach is to define a `RotBstMatrix`

,
which forms a separate auxiliary class. You can build up this
4-by-4 matrix by successive calls to the methods of the class,
such that the matrix encodes the full sequence of operations.
The order in which you do these calls must agree with the imagined
order in which the rotations/boosts should be applied to a
four-momentum, since in general the operations do not commute.

**RotBstMatrix::RotBstMatrix() **

creates a diagonal unit matrix, i.e. one that leaves a four-vector
unchanged.

**RotBstMatrix::RotBstMatrix(const RotBstMatrix& Min) **

creates a copy of the input matrix.

**RotBstMatrix& RotBstMatrix::operator=(const RotBstMatrix4& Min) **

copies the input matrix.

**void RotBstMatrix::rot(double theta = 0., double phi = 0.) **

rotate by this polar and azimuthal angle.

**void RotBstMatrix::rot(const Vec4& p) **

rotate so that a vector originally along the *+z* axis becomes
parallel with *p*. More specifically, rotate by *-phi*,
*theta* and *phi*, with angles defined by *p*.

**void RotBstMatrix::bst(double betaX = 0., double betaY = 0., double betaZ = 0.) **

boost by this *beta* vector.

**void RotBstMatrix::bst(const Vec4&) **

**void RotBstMatrix::bstback(const Vec4&) **

boost with a *beta = p/E* or *beta = -p/E*, respectively.

**void RotBstMatrix::bst(const Vec4& p1, const Vec4& p2) **

boost so that *p_1* is transformed to *p_2*. It is assumed
that the two vectors obey *p_1^2 = p_2^2*.

**void RotBstMatrix::toCMframe(const Vec4& p1, const Vec4& p2) **

boost and rotate to the rest frame of *p_1* and *p_2*,
with *p_1* along the *+z* axis.

**void RotBstMatrix::fromCMframe(const Vec4& p1, const Vec4& p2) **

rotate and boost from the rest frame of *p_1* and *p_2*,
with *p_1* along the *+z* axis, to the actual frame of
*p_1* and *p_2*, i.e. the inverse of the above.

**void RotBstMatrix::rotbst(const RotBstMatrix& Min); **

combine the current matrix with another one.

**void RotBstMatrix::invert() **

invert the matrix, which corresponds to an opposite sequence and sign
of rotations and boosts.

**void RotBstMatrix::reset() **

reset to no rotation/boost; i.e. the default at creation.

**double RotBstMatrix::deviation() **

crude estimate how much a matrix deviates from the unit matrix:
the sum of the absolute values of all non-diagonal matrix elements
plus the sum of the absolute deviation of the diagonal matrix
elements from unity.

**friend ostream& operator<<(ostream&, const RotBstMatrix& M) **

writes out the values of the sixteen components of a
`RotBstMatrix`

, on four consecutive lines and
ended with a newline.