SUBROUTINE QIHPAR - Generate identity of current quark $q^{\prime\prime}$ and virtual quark $q^\prime$

In this routine the HERWIG [1] identities of the current quark $q^{\prime\prime}$ as well as of the virtual quark $q^\prime$ are generated. This proceeds as follows:

• By means of the rejection method, the identity of the current quark is chosen randomly among $\{d,u,s\}$ (for $n_f=3$) or $\{d,u,s,c\}$ (for $n_f=4$), according to the probability distribution
  

  $$\frac{e_i^2}{\sum\limits_{q=d,u,\ldots} e_q^2}\, ; \hspace{6ex} i=d,u,\ldots$$ (1)

  
  
  Here, $e_i$ is the fractional electric charge of the quark flavour $i$.
• In the next step, the accepted identity of the current quark is with 50 % probability swapped with the identity of the corresponding anti-particle. In this way we end up with the following probability distribution for the identity of the current quark,
  

  $$\frac{e_i^2}{\sum\limits_{q=\left\{ \begin{array}{l} d,u,... ... ;\\ \overline{d},\overline{u}.\ldots \end{array} \right.$$ (2)

  
  

• In the final step, the identity of the virtual quark $q^\prime$ is set to the identity of the anti-particle corresponding to the current quark $q^{\prime\prime}$.

See Table 1 for a list of variables set in QIHPAR.

Table 1: Variables set in QIHPAR.

Name	Description
QINFAM	Number of (generated) quark-antiquark pairs, $n_f$
EQ2SUM	Sum over fractional charge squared of all $n_f$ quarks, $\sum\limits_{q=d,u,\ldots}e_q^2$
QIPINT(1)	HERWIG identity (IDHW) of current quark $q^{\prime\prime}$
QIPINT(2)	HERWIG identity (IDHW) of virtual quark $q^\prime$
QICQM2	Mass squared of current quark