Impossible choice of quarks

[KajetanNiewczas]

Impossible choice of quark 3
Implsible flavour transition

The problem I found while running only NC DIS, especially on smaller targets. I thought it was irreproducible, but I managed to get 3 times in a row for deuterium (with free nucleon target).

We have already agreed that it is impossible (by construction) for the code to get to this point.

I attach the parameters of the simulation (I think the number of real events can be reduced :) ).
nuwro.root.par.txt

[karuboniru]

I got some report when people doing some production for JUNO NC sample. The reproduction was on NuWro 19.02.2. Same input can not reproduce the issue on lastest git version (is this due to any effort of fixing this or from just some random sampling in phase space has changed?)

But anyway, within NuWro 19.02.2, I tried to traceback the event generation process, it seems to be due to a nan is returned at cr_sec_nc_nu_n_d (E=E@entry=940.95209946452928, W=W@entry=1620.6293325514189, nu=nu@entry=929.19219500084807, m=m@entry=0)

nuwro/src/dis/fragmentation_nc.cc

Line 45 in de2172d

double kwark_d = cr_sec_nc_nu_p_d(E,W,nu,m);

The nan will trace back to sigma_xy_nc_nu_n_d(E=941., xx=0.00, yy=0.988, m=0.00) = -nan which return nan.

nuwro/src/dis/dis_nc.cc

Line 641 in de2172d

return sigma_xy_nc_nu_n_d (E, xx, yy, m) * W / (M12 * E * nu) //jakobian

I think the issue is the xx here, which is calculated at

nuwro/src/dis/dis_nc.cc

Line 639 in de2172d

double xx = (M2 - W * W + 2 * M12 * nu) / (2 * M12 * nu);

Might be just a coincindene that gdb suggested that the formula do equal to zero

>>> p M2 - W * W + 2 * M12 * nu
$11 = 0

I checked and saw a lot of /xx thoughout the chain and the fact that xx can equal to zero is a problem. And in case M2 - W * W + 2 * M12 * nu can reach 0 by pure chance, I'd say this can explain why this kind of error is hard to reproduce. In this case should we investgate why xx = 0 or do we handle the situation when xx = 0

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Tried to plot sigma_xy_nc_nu_n_d as a function of xx and the result suggest that the xx = 0 limit should be 0