ZGOUBI USERS' GUIDE - HEP

52 4 DESCRIPTION OF THE AVAILABLE PROCEDURES$7$'q7$qÁ Ÿ > € > >'q7Ÿ > Á2 >q@2wwŠq 7Áüq >s ÁqÁ>É7>ÉÁ7 w >7$‘ >q7Tq7"7î'É>Š'77ÁÉTÉŠŠ77MCDESINT: Monte-Carlo simulation of in-flight decay[16]As soon as MCDESINT appears in a structure (normally, after OBJET or after CIBLE), in-flight decay simulation starts.It must be preceded by PARTICUL for the definition of mass and COM lifetime .The two-body decay simulated is2 7@ UThe decay is isotropic in the center of mass. 1 is the incoming particle, with mass(relative O momentumframe. 2 and 3 are decay products with respective masses and momentaThe decay length'Z7€]7and2K7, momentum! ! with = reference rigidity, see OBJET), and position 2 ¡ 2 1 € ¡ ! 1 ! 2 > > Á > T >of particle 1 is related to its center of mass lifetime by,€O7in the zgoubi€ "[ Á T 2 T T7 297.' 77 2The path lengthdecay formulaup to the decay point is then calculated from a random number \Üï 1 7q by using the exponential w 'L7ž ž 1 7After decay, particle 2 will be ray-traced with assumed positive charge, while particle 3 is discarded. Its scattering anglesin the center ÅCŸ of mass and are generated from two other random numbers and .1 T > 1is a relativistic invariant, and Å in the laboratory frame (Fig. 7) is given by(*)Å`Ÿ¡ )Å &`Å Ÿ€ >and momentumare given byÉ ŸÉ Ÿ >2>> w 2>7 2>@ 297‡2 >É Ÿ > poÉ Ÿ >&`Á > ÁÅ Ÿ > w >Finally, Å andvalue O€ >are transformed into the anglesand ¥in the zgoubi frame, and the relative momentum takes the! ! (where ¡ ! 1 ! 2is the reference rigidity, see OBJET), while the starting position of>1 ¡is > and > .> The decay simulation by zgoubi obeys the following procedures. In optical elements and field maps, after each integrationstep XPAS, the actual path length of the "k¤ particle, , is compared to its limit path length . If is passed, then the