tunneling mechanism, but for the longitudinal momentum distribution it is essential to take into account how the produced quarks and antiquarks can fit into a mesonic wavefunction, and combine to form final-state hadrons. The Lund fragmentation model is here inspired by the area law for Wilson loop integrals for a confining theory [@Wilson:1974sk], in analogy with the Nambu–Goto action for a massless relativistic string. The boost invariance of the relativistic string (and of a linear homogenous electric field) has to result in a boost-invariant distribution of hadrons, produced around a hyperbola in space-time.

Consider, for simplicity, a model in one space dimension with only one quark flavour and a single mesonic state with mass $m$. In the Lund hadronization model the probability, $\mathcal{P}$, for the production of a specific state with $n$ mesons with momenta $p_i$ ($i=1,...,n$) is given by the relation [@Andersson:1983jt]: $$\mathcal{P} \propto \left\{\left[\prod_1^n Nd^2 p_i
    \delta(p_i^2-m^2)\right]\delta^{(2)}(\sum p_i-P_{tot})\right\} \exp(-bA).
\label{eq:Lund}$$ Here the term in curly parenthesis is a phase space factor, where the dimensionless constant $N$ determines the relative weight between states with different number of mesons. The term $bA$ in the exponent corresponds to the imaginary part of the action for the massless string, which is responsible 