Neutral kaons propagation in matter

Dear FLUKA colleagues,
I write you to ask about the possibility to simulate kaons regeneration in FLUKA. Specifically, in the setup that I have in mind, a ~ 50 GeV neutral kaon beam, with almost equal mixture of KL and KS, impinges on a thick target.

I am interested in studying the decay of these neutral kaons in the target itself. Since in my setup kaons propagate in matter, I need to account for the KS - KL regeneration process. Specifically, for this energy range, I think that incoherent regeneration is the dominant mechanism.

Is FLUKA capable of handling this? If so, is there any reference or document where the corresponding details are provided?

Andrea Celentano

Dear Andrea,

Starting with a (custom-source) mixture of a Klong and Kshort beam in FLUKA, their relative populations as the shower develops should not just follow the decay of the Kshort (quick) and of Klong (slower), but in addition be re-populated by neutral kaons generated during nuclear inelastic interactions. The latter will retain their K0 and K0bar identity, but will keep track of their time-dependent Kshort and Klong components: starting with a 1/sqrt(2) amplitude for both Klong and Kshort (thus replenishing the Klong and especially the Kshort population at production!), which will subsequently evolve with time according to their respective half-lives.

If this replenshing of the Kshort (and Klong) population via K0/K0bar secondaries from nuclear inelastic interactions is what was implied by incoherent regeneration, the code accounts for it. No coherent effects, though. Otherwise, the transport, decay, and re-interaction of neutral kaons is naturally accounted for.

To witness the Kshort vs Klong population, say, accross the boundary of your target region, for the time being you should add USRBDX scorings not only for Kshort and Klong, but also for K0 and K0bar in transport. For the last two, you’ll need 2 scorings each, weighted as follows with fluscw.f:

  • K0 weighted with AKSHRT*AKSHRT, i.e. the squared amplitude for the Kshort component from (which you’ll have to add in the INCLUDE lines of fluscw.f), in order to obtain the Kshort component
  • K0 weighted with AKLONG*AKLONG to obtain the Klong component
  • K0bar weighted with AKSHRT*AKSHRT to obtain the Kshort component
  • K0bar weighted with AKLONG*AKLONG to obtain the Klong component

A final accumulation of Kshort + Kshort(K0) + Kshort(K0bar) spectra should give you the total Kshort component, and analogously for the Klong one.

With kind regards,