Show that momentum conservation at the QCD vertex requires...

Show that momentum conservation at the QCD vertex requires (for $z<1$ )
$$
\begin{aligned}
& P_{q q}(z)=P_{g q}(1-z), \\
& P_{q g}(z)=P_{q g}(1-z), \\
& P_{g g}(z)=P_{g g}(1-z) .
\end{aligned}
$$

Check that the explicit formulas for the "splitting" functions satisfy these relations.

Key Concepts

DGLAP Evolution Equations

The Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equations use splitting functions to describe how parton distribution functions change with the energy scale. These equations incorporate the probabilities of various splitting processes, ensuring consistency with conservation laws and providing predictions for the scale evolution of hadronic structure in high-energy collisions.

Light-Cone Momentum Fraction

In high-energy processes, it is convenient to express the momentum of partons in terms of the light-cone momentum fraction z. This variable, which ranges between 0 and 1, specifies the fraction of the parent parton’s momentum carried by one of the daughters after the splitting. The behavior of the splitting functions as functions of z reflects the underlying dynamics of parton emissions in QCD.

Momentum Conservation

Momentum conservation is a fundamental principle that requires the total momentum before and after a splitting process to remain the same. At a QCD vertex, the sum of the momentum fractions carried by the resulting partons must equal the momentum fraction of the original parton. This constraint imposes specific symmetry relations on the splitting functions, such as the transformation z -> 1-z in some cases.

QCD Splitting Functions

In Quantum Chromodynamics (QCD), splitting functions describe the probability for a parton (such as a quark or gluon) to radiate another parton and hence divide its momentum among the resulting particles. These functions appear in the evolution equations (such as the DGLAP equations) that govern the scale-dependence of parton distribution functions and fragmentation functions in high-energy processes.

Symmetry of Splitting Functions

Under the condition of momentum conservation at the vertex, the splitting functions must satisfy certain symmetry properties when the momentum fraction variable is exchanged with its complement (z to 1-z). This ensures that the splitting processes are properly balanced with respect to the distribution of momentum among the outgoing partons. Such relations are essential for verifying the consistency of the theoretical framework.

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